DESIGN, ACTUATION, AND CONTROL

OF A COMPLEX HAND MECHANISM

by

Jason Dean Potratz

A thesis submitted in partial fulfillment of the requirements for the Master of Science degree

in Mechanical Engineering in the Graduate College of The University of Iowa

July 2005

Thesis Supervisor: Professor Karim Abdel-Malek

Copyright by

JASON DEAN POTRATZ

2005

All Rights Reserved

Graduate College The University of Iowa

Iowa City, Iowa

CERTIFICATE OF APPROVAL

_________________________

MASTER’S THESIS

_____________

This is to certify that the Master’s thesis of

Jason Dean Potratz

has been approved by the Examining Committee for the thesis requirement for the Master of Science degree in Mechanical Engineering at the July 2005 graduation.

Thesis Committee: ___________________________________________________

Karim Abdel-Malek, Thesis Supervisor

___________________________________________________

Jasbir Arora

___________________________________________________

Shaoping Xiao

ii

“Sorry” doesn’t put thumbs on the hand, Marge.

- Homer J. Simpson

iii

ACKNOWLEDGMENTS

I would like to that Professor Karim Abdel-Malek for supervising this research

and serving on my thesis committee. I would like to thank Professors Jasbir Arora and

Shaoping Xiao for also serving on my thesis committee. I would like to thank Dr.

Timothy Marler for thesis editing and expert writing advice. I would like to thank Dr.

Jingzhou Yang for collaborating on this work. Finally I would like to thank my friends

and family for their support.

iv

TABLE OF CONTENTS

LIST OF TABLES v

LIST OF FIGURES vi

CHAPTER

I. INTRODUCTION 1

1.1 Literature review 3 1.2 Motivation and objectives 7 1.3 Overview of thesis 9

II. MECHANICAL DESIGN 10

2.1 Design of the spring deflection joint mechanism 10 2.2 Design of the actuation system 19 2.3 Finger deflection 25 2.4 Aesthetics 29

III. ELECTRICAL DESIGN 31

3.1 Performance requirements 31 3.2 Stepper motors versus servomotors 34 3.3 Stepper motor theory and operation 36 3.4 Stepper motor, drive, and power supply setup 45

IV. MOTION CONTROL 48

4.1 Motion control setup 48 4.2 Programming 55

V. APPLICATIONS AND RESULTS 60

5.1 Applications 60 5.2 Results 65

VI. CONCLUSION 70

6.1 Contributions 70 6.2 Shortcomings and future work 73

REFERENCES 77

v

LIST OF TABLES

Table 2.1 Spring properties 18

Table 2.2 Joint range of motion 27

Table 3.1 Force and torque requirements for each joint 33

vi

LIST OF FIGURES

Figure 2.1 Finger segment with cable and conduit 11

Figure 2.2 Degrees of freedom of fingers 12

Figure 2.3 Completed finger assembly 13

Figure 2.4 Cable routing 14

Figure 2.5 Hand anatomy 15

Figure 2.6 Entire hand assembly 16

Figure 2.7 Actuation system 19

Figure 2.8 Pulley assembly with shaft extension, coupler, pulleys, bearing, and nut 22

Figure 2.9 Fixture assembly with motors, motor drives, and pulley assemblies 24

Figure 2.10 Trajectory and range of motion experimental setup 25

Figure 2.11 Fingertip trajectory and orientation 28

Figure 2.12 Artificial and human hand, palm 30

Figure 2.13 Artificial and human hand, back of hand 30

Figure 3.1 Unipolar stepper motor 38

Figure 3.2 Bipolar stepper motor 39

Figure 3.3 Torque versus position for a single winding and two winding motors 41

Figure 3.4 Torque versus position for two windings using micro stepping 41

Figure 3.5 Current through winding versus time 42

Figure 3.6 Torque versus speed 43

Figure 3.7 Current versus time with and without current limiting 45

Figure 3.8 Manufacturer’s torque rating (oz-in) of motor versus speed (revolutions/second) 46

vii

Figure 4.1 Actuation and control setup 50

Figure 4.2 Wiring diagram 52

Figure 4.3 State activation chart 56

Figure 5.1 Cylindrical grasp without glove 66

Figure 5.2 Spherical grasp with glove, one 66

Figure 5.3 Spherical grasp with glove, two 67

Figure 5.4 Pinch grasp with glove, one 67

Figure 5.5 Pinch grasp with glove, two 68

1

CHAPTER I

INTRODUCTION

An artificial human-like hand based on a new design would fulfill different needs.

There are numerous robotics applications that would benefit from an anthropometrically

shaped devise, a “gripper”, capable of grasping and manipulating objects. Other grippers,

often as simple as a one degree of freedom (DOF) pinching device, are used successfully

in many applications, but a gripper that accurately reproduces human motion and

grasping postures is novel and would be beneficial. For instance, in cases where the

environment prevented or inhibited a human from completing a task, a robotic hand could

be used as part of a system to complete the task. If humanoid robots were ever to become

a reality, then a human-like hand would become a necessity. These robots would interact

in the same environment as humans and therefore manipulate the same objects that would

be designed with the intention of being manipulable by human hands.

Another central need that would be fulfilled by a new artificial human-like hand

would be advanced prosthetics. The ideal replacement for a missing human hand that

accurately mimics a human hand in appearance, dexterity, tactile feedback to the user,

and simplicity of use, remains far from a reality. Currently available myoelectric robotic

prosthetic hands, which typically only have a few DOFs fall far short of this ideal

prosthetic hand in terms of appearance, functionality, and easy of use. Surveys

conducted on user satisfaction of myoelectric hands show that 30-50 percent of users do

not use their hands regularly. The reasons for this lack of use are: low functionality,

unsatisfactory cosmetics partly caused by unnatural motion, and low controllability.

2

From a mechanics perspective, even these relatively simple hands are not natural to

control; users have to exert an enormous amount of concentration for even simple tasks

(Massa et al, 2002).

In this work we developed a five-fingered hand with 15 DOFs that mimics a

human hand in terms of appearance, motion, and grasping ability. One key feature that

makes this hand more advanced than any other multi-fingered hands is the unique

mechanism used for the joints. This newly developed mechanism is based on the loading

of a compression spring in both the axial and transverse directions and offers several

advantages over a hand mechanism based on rigid links and revolute joints. Since the

structure of each finger is primarily made up of compression springs, the majority of the

volume of each finger is occupied by empty space. Thus, the potential for constructing a

very light hand exists if the proper materials are used along with an efficient design. A

lightweight design is a critical aspect of a comfortable, wearable prosthetic devise and

offers obvious advantages for robotics applications, such as smaller actuators and less

power consumption. The unique mechanism also allows the hand to be inherently

compliant, which enables the fingers to naturally conform to the shape of the object the

hand is grasping, making for a more secure grip without the necessity of deliberately

adjusting the defection of each joint to achieve the same shape. The joint mechanisms

are actuated by a series of cables and conduits that allow the motors that actuate the joints

to be located off the hand, also increasing the potential for a very light weight hand.

3

1.1 Literature review

Some of the first mechanical hands were part of body-powered artificial limbs

with a mechanical hook or claw type end effecter. Powered mechanical hands have seen

some significant advances since these hands were considered state of the art. These

advances have occurred primarily in prosthetics applications. An ideal solution for

replacing a human hand is still far from being realized. Possibly one of the reasons this is

true is that the two primary design criteria for prosthetic hands are often conflicting ones.

Because the hand is part of the body, it is unique to each person. Likewise, the ideal

replacement for the human hand would also be unique to each user. On the other hand,

any prototype device has to be a solution that is well accepted by enough users to warrant

developing it into a marketable product (Kyberd et al, 2003). Although there are many

advanced hands under development, none of them incorporates all of the advancements

that research has produced. Many of them implement advanced control schemes and

sensors to achieve a significant level of automation, but many of the most advanced

hands still do not have five powered fingers (see Butterfass et al, 1998, Tura et al, 1998,

Carrozza et al, 2003, Zecca et al, 2003, Kyberd and Chappell, 1994), and Kyberd et al,

2001). None of them has as many fully actuated DOFs as the human hand, an attribute

which of course, an ideal replacement for the human hand would have. These

deficiencies are probably due to compromises made to save weight and to reduce the

complexity of controlling a greater number of DOFs.

Many commercially available hands, such as the Otto Bock SensorHandTM, only

have three digits, usually the thumb, index, and middle fingers, and have two or three

4

DOFs, which are coupled together, and actuated by one motor. Hands such as these are

only capable of being used for a precision or power grasp and have only limited

functional use (Yang et al, 2005).

One of the projects that has been under development the longest is the

Southampton Hand at the University of Manchester Institute of Science and Technology.

The Southampton Hand generically refers to the concepts that have been implemented in

a series of hands, but the original was built in 1969 (Kyberd and Chappell, 1994). The

earliest designs were quite large and relied on very simple electronics. Great potential for

advancement in robotic hands came with the advent of the microprocessor. This was the

key to solving many of the problems that prevented early hands from being very

practical: size, weight, and power consumption. In the current state the Southampton

Hand has two independent degrees of freedom, flexion of the first two fingers and flexion

of the thumb. Combined force and slip sensors in the tips of the fingers along with a

force sensor in the palm are used to aid the user in grasping and holding an object with

minimal user input. The idea of minimal user input is one of the key concepts being

developed as part of the Southampton Hand project, or more specifically that of

hierarchical control. The user only has to provide higher-level commands like “open”,

“hold”, or “squeeze”, and the movements of individual finger joints are coordinated by

microprocessors using feedback from the sensors. This reduces the mental load on the

user, which is a common cause of rejection of a device by the user (Kyberd, 2000).

Another research project that has been under development since the mid 1980’s

and has been a test bed for many other research projects to develop control strategies is

the Utah/MIT Dexterous Hand. This hand has three fingers and a thumb with four

5

degrees of freedom each for a total of sixteen degrees of freedom. The joints are each

powered by opposing extensor and flexor tendons, which are actuated by pneumatic

actuators that are located in a remote actuator pack. There are Hall effect sensors to

provide angular position feedback and the tendons pass over force sensors in the wrist to

provide force feedback. The control system for the Utah/MIT hand “attempts to control

forces between the fingers and an object by recursive refinement of trajectories and by

varying stiffnesses of the joints. Ultimately, controlled compliance of the fingers

combined with a regulated position error produces appropriate forces to maintain the grip

and to move the object within the grasp” (Speeter, 1990). This enables the hand to grasp

an object and manipulate it within its grasp. Although this hand is capable of complex

manipulation and has served for a very effective test bed for developing other

components and control schemes the bulky apparatus required to control it, including the

pneumatic actuators, preclude it from being used as a prosthetic and give it a rather

limited potential for use in practical robotic applications.

A group in Italy is responsible for the development of a three fingered, two-

degree of freedom, underactuated prosthetic hand. The key feature of this hand is the

capability of an adaptive grasp due to the underactuation. Underactuation refers to a

mechanism that has fewer actuators than it has degrees of freedom. The other degrees of

freedom are then “actuated” by elastic elements, usually springs, and mechanical stops.

The basic mechanism to control an underactuated system is a differential gear system.

The mechanism employed in this hand is based on a tendon-like transmission system.

The tendons are actuated by electric motors. The tendons then wrap around a series of

three pulleys, one pulley being located at each of the three finger joints. The tension in

6

the tendon is then transformed into a moment that is proportional to the radius of each

pulley and actuates the corresponding joint in the finger. The three joints in each finger

are also fitted with springs to “actuate” the remaining degrees of freedom. The kinematic

relationship between each link in the fingers can be modified by changing either pulley

size or spring stiffness. This arrangement allows for an adaptive grasp that adjusts the

shape of each finger to conform to the shape of the object being grasped (Massa, et al,

2002). This concept of adaptive grasping has shown much potential for grasping tasks in

prosthetic applications but is not capable of manipulation tasks and is therefore unsuitable

for most robotics applications. It will also have to be abandoned in order to someday

achieve the perfect solution for a human hand replacement, which of course, would not

be capable of just performing grasping tasks but also performing manipulation tasks.

This would, of course, only be possible provided that a user interface advanced enough to

be capable of interpreting the users intentions with minimal conscious effort is

developed.

The TUAT/Karlsruhe Humanoid Hand, which is being developed with both

prosthetics and robotics applications in mind, is a five-fingered hand with twenty-one

degrees of freedom that are driven by one actuator. This hand has a unique link

mechanism to couple all of the joints together to allow for an adaptive grasp. Basically,

all of the fingers begin to move as the actuator begins to move and the grasp begins to

close. As one finger comes into contact with the grasped object that finger stops moving

and the fingers not in contact with the object yet, continue to displace until they too come

into contact with the object. In this way the individual fingers conform to the shape of

the object, contact force is balanced between the fingers, and a truly adaptive grasp is

7

achieved. The fingers even tend to automatically readjust if the object inadvertently

shifts within the grasp. Because of this, the researchers plan not to rely on any sort of

sensors for feedback (Fukaya, et al, 2000). Again, using a hand with an adaptive grasp

offers an advantage in the fact that it can achieve a stable grasp without the use of a

complex control system, but also has a major disadvantage in the fact that this kind of

hand can only ever be used for grasping and is not capable of performing any kind of

manipulation tasks.

1.2 Motivation and objectives

There remains a need to have a much more human-like artificial hand for many

applications. Patients who lack part of an upper limb desire to not only restore the

functionality of that limb but just as importantly any device used should be life-like and

aesthetically pleasing. Currently available prosthetic devices fall short in many ways.

They are hard to control and do not result in a natural motion. Due in part to their limited

number of degrees of freedom, currently available prosthetic hands have limited

practicality. They cannot perform a variety of tasks that would normally be completed

using a human hand. Lastly, they are not aesthetically pleasing. Ideally the device would

perfectly mimic the human hand in strength, dexterity, appearance, and easy of use. It

should also be so natural to operate that the user would not have to put any more

conscious effort into it than a real hand. This idealization is a far from being realized and

there is much room for improvement in models currently available and under

development. Even when considering projects under development, compromises have

8

been made in every prototype. Many still do not have five digits. Some are relatively

simple to operate due to their adaptive grasp designs, but these can only be used in

grasping type applications and cannot be used in any sort of application that requires

complex manipulation. Currently available prosthetic devices lack degrees of freedom

and powered digits. They cannot perform the same manipulation tasks and grasping

postures as a human hand. They are cumbersome and impractical because they require a

substantial amount of concentration from the user to operate and can only be used for a

limited number of basic tasks. There is also much room to improve the aesthetics of

these devises, which is an important part of overall comfort for the user.

There is also a need to have a very human like manipulation ability in many

robotics applications. For instance, in the remote operation of a robotic hand in situations

where the environment either inhibits a human to do the same task unaided or prevents it

all together. One specific example of this is the ADAH Project that involves developing

a robotic hand to assist astronauts during extra vehicular activities in space who are

normally hindered by the need to wear pressurized gloves (Carrozza, et al, 2002).

Another instance would be in the eventual development of a humanoid robot which

would likely be required to interact in the same environments as humans do on a regular

basis, and therefore manipulate the same objects that humans do. The most logical end

effecter for these kinds of tasks would, of course, be a hand that mimics the human hand.

The objectives to be demonstrated as fulfilled by this research are:

1. Design and produce a five-fingered, fifteen degree-of-freedom prototype hand

that implements a novel joint mechanism based on the deflection of a

9

compression spring. Mimic a human hand in size, appearance, and motion.

Design the hand to accommodate a cosmetic glove.

2. Create a bench-top actuation and motion control system to explore, develop, and

demonstrate the capabilities of the hand.

3. Develop software for a user interface to coordinate hand movement.

1.3 Overview of thesis

Chapter 2 covers the mechanical design of the hand, including the following:

design of the spring deflection joint mechanism, design of the actuation system, results of

experimentation, and overall esthetics. Chapter 3 discusses the electrical design,

including: performance requirements, how the stepper motors used in this application

fulfill those requirements and general theory of operation of stepper motors, and will

discuss the other electronic components used. Chapter 4 discusses computer control of

the hand including the motion controller card used and the program developed for control

of the hand and simulation of a user interface scheme under development that uses

myoelectric signals to control a prosthetic device. Chapter 5 discusses use of the hand.

The first section in this chapter covers potential applications of the hand, such as an

improvement over current designs in prosthetic hands or the possibility of its use in

robotics applications. The second section will discuss examples of what the hand is

capable of. Chapter 6 provides a summary of contributions and a discussion of potential

future work.

10

CHAPTER II

MECHANICAL DESIGN

Chapter two will cover aspects of the mechanical design of the hand and the

system that actuates it. More specifically, it covers the novel flexing elements used to

make up the joints of the hand, the actuation system, experimentation on the hand to

determine the kinematic relationship between the orientation of the motor rotor and the

position of the end effecter, and how the components were designed to be esthetically

pleasing. These flexible joint elements are the basis for hand motion and at the same

time make up the majority of the structure of the fingers. The purpose of the actuation

system is to move the joints of the hand in response to computer-generated commands. It

includes five stepper motors, fifteen flexible cable and conduit sets to transmit motion

from the motors to the joints of the hand, and five pulley assemblies.

2.1 Design of the spring deflection joint mechanism

The key feature of this hand mechanism that makes it unique is its joint

mechanism used for the finger joints. Unlike all other mechanical hands, which employ

solid links with revolute joints or another conventional mechanism, this one makes use of

a flexible element made up primarily of a compression spring. Most of the structure for

each finger segment is made up by this spring, Figure 2.1. It is also this flexible element

that allows for the motion for the flexion and extension DOF of the finger, Figure 2.2.

Each finger is made up of a series of three springs connected in series that are held in

11

place by short aluminum finger segment links. Each segment, in the current design, has

one DOF and is actuated by a single cable. Although each joint currently has only one

DOF to replicate the extension and flexion of the human finger, more cables could be

added at appropriate orientations around certain finger segments to add additional DOF’s;

the DOF’s would replicate the abduction and adduction motion of the human finger.

Figure 2.1 depicts a CAD model of a typical joint segment that would be used to

make up a finger. This segment includes a spring and the cable and conduit used to

actuate the segment. Two aluminum finger segment links at either end are used to

connect the spring to the springs in the preceding and following segments and to retain

the ends of the cable and conduit. The finger segment links have a thread-like structure

Figure 2.1 Finger segment with cable and conduit

12

at each end that screw into the first coil of the compression spring, holding the spring

securely.

The joint segment also includes a rectangular rubber block, or stiffener, oriented

parallel to the axial direction of the compression spring, located at on the edge of the

segment opposite of the cable. The finger segment structure offers very little resistance

to lateral deflection but offers a much higher resistance to axial compression, which

ensures that segment will deflect laterally in the direction of the palm (flexion of the

finger) with very little resistance. The finger segment will require much more applied

force to deflect in the direction of the back of the hand (extension of the finger). In other

Figure 2.2 Degrees of freedom of fingers

_________________________________________________________________________ Source: Peña Pitarch, E., Yang, J., Abdel-Malek, K., (2005) “SANTOSTM Hand: A 25 Degree-of-Freedom Model” Proceedings of the Society of Automotive Engineers Digital Human Modeling for Design and Engineering Symposium, June 14-16, Iowa City, Iowa

13

words, it makes the finger segment stiffer when bent in one direction as apposed to the

other.

Figure 2.3 shows one entire finger unit with three finger segments and three

cable-and-conduit sets to actuate all the segments. The tip of the thumb is located

towards the right of the Figure 2.3. At the left of the Figure 2.3 is the thumb base, which

slips into the aluminum hand body fixture. The other four fingers are constructed in a

similar fashion.

The cables and conduits run from the finger segment that they actuate, through the

empty space in the center of the preceding elements, continue through the hollow finger

bases, and exit though the bottom of the hand body fixture, Figure 2.4 and Figure 2.5.

This arrangement allows for a single segment to be actuated without affecting the other

two segments located in that finger. Figure 2.4 shows how a set of cables and conduits

would be routed through a series of finger segments.

Figure 2.3 Completed finger assembly

14

The hand body is also constructed of aluminum and designed to resemble the

shape of a hand. The fingers are arranged in a configuration that resembles human

anthropometry and is suitable for grasping.

Fifteen finger segments similar to one shown in Figure 2.1 were developed to

comprise an entire five-fingered hand with three segments in each of the four fingers and

the thumb to make a total of 15 DOF’s. The thumb is capable of bending in extension

and flexion at the carpometacarpal (CMC), the metacarpophangeal (MCP), and the

interphangeal (IP) joints. Each finger is capable of bending in flexion and extension at

Figure 2.4 Cable routing

15

the metacarpophalangeal (MCP), the proximal interphalangeal (PIP), and the distal

interphalangeal (DIP) joints, Figure 2.5.

Each finger can be added to and removed from the rest of the hand as one unit,

which allows the entire hand to be assembled inside a cosmetic glove, which will be

discussed more in section 2.4. Figure 2.6 shows the complete hand with three DOF’s for

each of the five fingers, giving it 15 DOF’s.

Figure 2.5 Hand anatomy ______________________________________________________

Source: Peña Pitarch, E., Yang, J., Abdel-Malek, K., (2005) “SANTOSTM Hand: A 25 Degree-of-Freedom Model” Proceedings of the Society of Automotive Engineers Digital Human Modeling for Design and Engineering Symposium, June 14-16, Iowa City, Iowa

16

One of the most important objectives when considering the design of the entire

hand as a system is the ability to grip an object. The inclusion of a rubber stiffener in the

finger segment is the key to making it operate in a way that makes gripping possible.

Although the performance of the flexing element would be greatly reduced, it would be

possible to use it without the stiffener. Without the stiffener, the stiffness of the

compression spring itself represents an important tradeoff in the design of the finger

segment. The maximum normal force that can be generated between a finger and an

object grasped between the fingers and the palm or between the fingers and the thumb is

proportional to the stiffness of the springs in that finger. When the hand closes to grasp

the object, the cables shorten and pull the upper link towards the lower link, causing the

finger segments to bend and curl towards the palm. After a finger first comes into

contact with an object and more tension is placed on the cable, normal force is developed

Figure 2.6 Entire hand assembly

17

between the finger and the surface of the object. Given that the grasped object exerts a

force on the finger, there is a corresponding equal and opposite force for the force that the

finger exerts on the grasped object. This force will cause the flexing elements to

compress on the backside, opposite of the cable, causing the flexing element to

simultaneously straighten and shorten. This effect produces undesirable and unrealistic

motion and grasping postures and does not allow for sufficient force to be placed on the

grasped object. Increasing the stiffness of the springs will increase the maximum amount

of normal force that the finger is able to produce and also the maximum grasping force of

the entire hand. Greater tension for actuating the hand would be required and therefore

necessitate the use of large and more powerful motors for actuation. Using a rubber

stiffener to increase the stiffness of the compression spring in only one direction is an

innovative solution that simultaneously takes advantage of using both a stiffer spring

which can create greater grasping force and a softer spring which is easier to actuate.

This is because the material in between each link of the spring makes it much harder for

that side of the spring to compress. This design still allows the fingers to react naturally

to applied external forces.

More than one set of springs was tested on the hand. The first set proved to be

too stiff and would require using larger motors. A second set of springs that were softer

was tested. This set proved to be much more practical. They are sufficiently stiff enough

to provide structure for the finger and to produce normal force for gripping, at the same

time soft enough to require a reasonable size motor for actuation. Their dimensions and

spring constants are included in Table 2.1. The spring constant, k, is calculated from the

dimensions and the number of active coils, or the number of coils which separate the two

18

finger segment links, not the entire number of coils in the compression springs. The

spring constants are calculated according to the equation 4

38

Gdk

nD= , where G is the shear

modulus of steel which is approximately 611.6 10× pounds per square inch. In Table 2.1,

the individual joints are labeled with an i – j convention where i refers to the finger and j

refers to the joint. The thumb corresponds to i = 1 and the index finger to i = 2 and so

forth. The joint closest to the fingertip of each finger corresponds to j = 1, the middle

joint to j = 2, and the joint closest to the hand corresponds to j = 3.

Table 2.1 Spring properties

19

2.2 Design of the actuation system

The design of the hand mechanism calls for the joints to be actuated by cable and

conduit sets. Since the hand was designed to be incorporated into either a robotic or

prosthetic system, the most likely method of doing the work required for actuation is

using electric motors. The actuation system, Figure 2.7, links the motors to the cable

conduit sets and the individual motors to each other, in such a way that individual cables

can be displaced while holding the conduit in place.

Once the hand mechanism itself was developed, an actuation system was

necessary to explore its capabilities and refine its design. For the research and

Figure 2.7 Actuation system

20

development stage of the hand prototype, a bench-top actuation system serves well for

experimentation and further refinement. This bench-top system is designed to be simple,

cost effective, and configurable. It is not optimized to fit in a small, ergonomic package

similar to what would be necessary for integration into a robotics or prosthetics

application.

Included in the actuation system are the cables that actuate each finger segment

and the conduit that houses and routes the cables. As tension is applied to the free end of

the cable, an equal and opposite force must be applied to the same end of the

corresponding conduit to prevent movement of the entire hand. This force is transmitted

down the length of the conduit to the other end of the conduit and to the finger segment

where it is constrained. The equal and opposite force restricts the tension in a cable to

actuating only the intended finger segment.

The actuation system also contains five stepper motors which each turn three

pulleys; each pulley actuates one cable each. The rest of the actuation system consists of

assorted hardware to connect three pulleys to each motor and to connect one cable to

each pulley (Figure 2.8), and a fixture to hold all of the motors and pulleys (Figure 2.9).

Finally, 15 cable adjusters individually adjust the pretension in each cable in order for all

three cables in one finger to begin to displace all the finger segments simultaneously.

The cable adjusters screw in or out of the fixture, in effect lengthening or shortening the

conduit so the that the tension in a each cable can be adjusted with respect to the tension

of the other two cables for that finger. In this way the finger segments for a given finger

can all begin to move at a certain orientation of the stepper motor and can reach full

displacement at the same time at a second orientation of the motor. The cables are 3/64

21

of an inch diameter steel “wire rope” or “aircraft cable”. At first 1/16 of an inch diameter

cable was used but it was determined that it was too stiff to wrap around such small

diameter pulleys. Switching to 3/64 diameter cable solves this problem well. Currently

enough cable and conduit was included to locate the hand approximately three feet away

from the rest of the actuation system.

To reduce cost and complexity for research and development stage, the joints are

coupled to reduce the DOF’s of the actuation system for the hand. Joint coupling also

reduces the number of motors that would be potentially required. This is a big advantage

when considering that the hand mechanism is meant to be part of a robotic or prosthetic

system. The coupling is achieved by actuating all three DOF’s for each finger with one

stepper motor that rotates three pulleys. The circumferences of the pulleys were varied

with respect to each other according to the maximum displacement of the cable necessary

to draw the corresponding finger segment to its maximum defection. This way, the three

finger segments in a given finger would be at the same percentage of full deflection at all

times. All three segments would begin to move simultaneously. They would also reach

their maximum displacement simultaneously at one orientation of the motor. Coupling

the joints in this manner provides a motion that is similar to natural human flexion and

extension of the fingers because human DIP and PIP joints move in a coupled manner as

well.

The pulley system assembly is shown in Figure 2.8. This system incorporates

timing pulleys to wrap up the cables. Timing are simple, cogged pulleys that are

typically available in various sizes, but are generally relatively small and are designed to

be used with cogged belts that transfer power between two or more axes without slip.

22

This application is much different than the intended one but the pulleys perform well and

were a convenient choice. This is because they were readily available in the correct

sizes, were economical, and required little modification to work with the other

components.

To turn three pulleys with each motor, it was necessary to create an assembly to

lengthen the motor shafts. This was done by producing a shaft that was the same

diameter as the motor shaft, had a flat notch for a setscrew and was threaded at the other

end. This was connected to the shaft of the stepper motor by use of a coupler with two

setscrews to engage both the motor shaft and the shaft extension. This allowed room for

three pulleys, a bearing, and sufficient threads still exposed to hold a nut on top while

Figure 2.8 Pulley assembly with shaft extension, coupler, pulleys, bearing, and nut

23

leaving room in between the pulleys to pinch three cables. The pulleys are kept from

slipping around the surface of the shaft by use of a setscrew that sets on to same notch as

the coupler. The cables are gripped by the pulleys by means of a notch cut into the hubs

of the pulleys. A cable retainer, which is essentially a ring, then fits around the hub to

keep the cable from slipping out of the notch. A space is then left in between each pulley

and the pulley above it or in the case of the upper most pulley the space is left in between

the pulley and the bearing above it. This space allows the cable to be wrapped around the

shaft after it is run through the notch so that when the nut on top is tightened all three

cables are pinched and the cables are locked firmly to the pulleys.

A bench top fixture was constructed to hold several components including the

stepper motors, pulley assemblies and cable adjusters and performs several other

functions necessary to actuate the hand. The fixture also houses the stepper motor drives

and two switches all of that will be discussed in more detail in chapter three. See Figure

2.9. This fixture may not be a practical enough means of housing all the components to

be suitable for robotics or prosthetics applications, but serves its purpose well for the

developmental stage.

One of the other functions fulfilled by the fixture is to give the conduits

something to push against as the cables are tensioned allowing the actuation of the fingers

to occur. Recall, that as a finger is actuated the conduits push on one finger segment link

as the cables pull on another. The fixture also has a plate that can be slid horizontally and

then held in place with two bolts, adjusting its distance from the five stepper motors.

This plate provides support to the free end of the motor shafts by providing a surface to

support the normal forces that are transmitted through the bearings on the ends of the

24

shafts as the cables are tensioned. Transmitting forces through the bearing to the motor

fixture lessens the bending moment being applied to the end of the motor shaft by

changing the loading conditions from those similar to a cantilever beam to those of a

simply supported beam.

Figure 2.9 Fixture assembly with motors, motor drives, and pulley assemblies

25

2.3 Finger deflection

The change in shape of the fingers as they are deflected is important to understand

when planning motions that will be executed by a control system. The deformed shape

and fingertip trajectory determine where the finger will come into contact with an object

as it is grasped. Fingertip orientation as the finger is flexed towards full flexion is also

important to understand. This is so that the points on the fingers that contact an object as

it is grasped can be predicted. A series of experiments to measure these characteristics is

presented below (Figure 2.10).

Figure 2.10 Trajectory and range of motion experimental setup

26

Each finger was measured individually. They were removed from the rest of the

hand and fitted with an end effecter (needle) at the fingertip. The base of the finger was

then fixed in a clamp. The initial position and the orientation of the end effecter were

recorded on paper behind the finger. The finger was initially positioned at full extension

and then was stepped in small increments, 20 steps apart (see Chapter 3), until full

flexion was reached. Measurements were taken to determine the position of the end

effecter in the flexion / extension plane and orientation of the end effecter in the same

plane (Figure 2.11). The joint angles of each segment were measured at full extension

and full flexion to determine range of motion (Table 2.2). Joint angles are defined as the

angle of deflection from one end of the spring to the other. They are determined by the

orientation of one finger segment link with respect to the previous one.

These experiments point out that range of motion of the fingers is one area where

there is room for improvement in the design of the hand. It is suggested that the total

range of motion for the fingers of the human hand is 215 degrees for the thumb and

between 270 and 300 degrees for the index, middle, ring, and little fingers (Peña Pitarch

et al, 2005). Because of this deficiency, the hand may have trouble grasping small

objects in certain grasp types, for instance grabbing a pencil in a cylindrical grasp.

Results from these experiments, specifically the maximum range of motion, were

used to develop the motion control program discussed in Chapter 4. The maximum

rotation of the motor must be observed so that the motors do not try to move the fingers

past their maximum deflection point.

These experiments point out that range of motion of the fingers is one area where

there is room for improvement in the design of the hand. It is suggested that the total

27

range of motion for the fingers of the human hand is 215 degrees for the thumb and

between 270 and 300 degrees for the index, middle, ring, and little fingers (Peña Pitarch

et al, 2005). Because of this deficiency, the hand may have trouble grasping small

objects in certain grasp types, for instance grabbing a pencil in a cylindrical grasp.

Results from these experiments, specifically the maximum range of motion, were

used to develop the motion control program discussed in Chapter 4. The maximum

rotation of the motor must be observed so that the motors do not try to move the fingers

past their maximum deflection point.

Table 2.2 Joint range of motion

Finger JointMinimum joint angle (degrees)

Maximum joint angle (degrees)

Range of motion

(degrees)

Maximum roation of motor

(degrees)

1-1, interphangeal 22 49 271-2, metacarpophalangeal 12 44 321-3, carpometacarpal 18 49 31finger range of motion2-1, distal interphalangeal 12 83 712-2, proximal interphalangeal 12 62 502-3, metacarpophalangeal 9 66 57finger range of motion3-1, distal interphalangeal 4 76 723-2, proximal interphalangeal 12 72 603-3, metacarpophalangeal 10 84 74finger range of motion4-1, distal interphalangeal 13 75 624-2, proximal interphalangeal 19 87 684-3, metacarpophalangeal 0 56 56finger range of motion5-1, distal interphalangeal 17 76 595-2, proximal interphalangeal 17 77 605-3, metacarpophalangeal 15 63 48finger range of motion 167

90

178

206

186

Little

Thumb

Index

Middle

Ring

130

200

170

170

200

28

Thumb

0

10

20

30

40

50

60

70

0 20 40 60 80 100

X Position (mm)

Y P

ositi

on (

mm

)

Index

0

10

20

30

40

50

60

70

0 20 40 60 80 100

X Position (mm)

Y P

ositi

on (

mm

)

Middle

0

10

20

30

40

50

60

70

0 20 40 60 80 100

X Position (mm)

Y P

ositi

on (

mm

)

Ring

0

10

20

30

40

50

60

70

0 20 40 60 80 100

X Position (mm)

Y P

ositi

on (

mm

)

Little

0

10

20

30

40

50

60

70

0 20 40 60 80 100

X Position (mm)

Y P

ositi

on (

mm

)

Origin and fingertip location and orientation on finger.

Figure 2.11 Fingertip trajectory and orientation

29

2.4 Aesthetics

The need to have a realistic, esthetically pleasing device is especially necessary in

the field of prosthetics. For many prosthesis users, the aesthetics of a device is just as

important as the functionality. The ideal replacement for the human hand is one that

perfectly mimics not only the function of the human hand but also the appearance

including size, shape, weight, texture, color, and movement. The user of a prosthetic

device has to be very comfortable using it and the ideal comfort level naturally includes

appearance among other things (Kyberd et al, 2003). To help achieve this ideal, this hand

was intentionally designed for use with a cosmetic glove covering it. The glove resembles

a human hand and forearm, and this dictated the dimensions of the majority of the

components used in the construction of the hand. The diameters of the finger segment

links and of the springs were chosen so that they would closely match the inner

dimensions of the cosmetic glove. The lengths of springs were carefully chosen so that

middle of the spring lengthwise, and therefore the middle of the curve of the deflected

finger segment, would coincide with the location of the knuckles of the cosmetic glove.

In its current state, improvements in the hand’s structure could still be made to achieve a

closer resemblance to the shape of a human hand, especially in the case of the hand body.

To achieve the ideal shape for an entire hand assembly would necessitate a close focus on

human hand anthropometry when designing all the components.

Results for the developmental stage of this more realistic mechanical hand have

been satisfactory. Figure 2.12 and Figure 2.13 compare, respectively, the palm side and

the back of the hand of the mechanical hand with a human hand. Each photo shows the

30

human hand on the right, the artificial hand on the left. Although there is some difference

in color between the human and cosmetic glove, which is available in many skin tones,

the shape of artificial hand does mimic the human hand well, especially considering that

improvements could be made with a more anthropometrically driven design.

Figure 2.12 Artificial and human hand, palm

Figure 2.13 Artificial and human hand, back of hand.

31

CHAPTER III

ELECTRICAL DESIGN

This chapter discusses the design of the electrical components of the hand and the

requirements the design must satisfy. The requirements considered include the amount of

tension that must be produced in the cables to fully displace each finger and produce a

sufficient gripping force, the length of cable displacement necessary, and the resolution

of movement necessary to produce precise finger movements. Component selection is

critical to satisfying these requirements and defines the specific challenges that must be

overcome to achieve good performance of the hand. Small DC motors are the best choice

to actuate the hand. This decision leaves two options, servomotors or stepper motors.

The advantages and disadvantages of each as well as the reasoning for choosing stepper

motors to actuate the hand are discussed below. The operating principles and strategies

of stepper motors, five of which form the basis of the electrical system, are also

discussed. Understanding how a stepper motor works is background information but it is

also essential to selecting the proper components, which ultimately determines how the

hand will function. Finally, the entire electrical setup that actuates the hand is detailed.

The actual performance of the entire system is also discussed.

3.1 Performance requirements

The motors that will provide the best solution to actuate the hand, that is provide

the best torque, speed, acceleration, and positional accuracy characteristics, depend on

32

what kind of operating conditions they will experience. Therefore, when selecting

motors and other required electronic components to power the hand, it is necessary to

consider everything that will be required of them when displacing the fingers or grasping

an object. The major requirements considered include the amount of torque required to

fully deflect the fingers and exert gripping force on an object, the speed at which a finger

should move in order to mimic human motion, and positional accuracy of the motor

needed for fine manipulation.

The first requirement to consider is the amount of force it takes to displace the

fingers to full flexion and how much torque is required of the motors to exert this much

force on the cables. This is the minimum force required to produce the closed hand

posture. Additional force will need to be generated in order to produce significant

normal force on the surface of a grasped object. It is also just as important to consider

the pulley radii that will be used with the motors, as this will give a direct relationship

between the force necessary to pull the cables and the torque required to produce this

force. The smaller the radius used for each pulley the less torque required to generate the

same amount of tension in the cable and therefore the smaller motor required. On the

other hand, there is a limit to how small a pulley can be. The outside diameter, the

diameter at which the cable will contact the pulley, must be at least some minimum

amount greater than the bore diameter, or the diameter of the hole in the center of the

pulley that will fit the motor shaft. Therefore, the torque rating of the motor cannot be

considered independently, but must be considered at the same time as size of the pulley

and the diameter of the motor shaft. Table 3.1 lists the force necessary to displace each

joint to full displacement and the total force necessary to displace each finger. Also listed

33

is the required torque for the pulley sizes used. The joints are labeled according to the

same i, j notation as described in Chapter 2.

The sizes of the pulley used, more specifically the radii are based on several

considerations. First is the smallest pulley outside diameter possible to use with the shaft

diameter of the motor. Also it is necessary to have a suitable relationship between the

three pulley radii for one given finger so that the three finger segments will reach full

displacement at the same time. The third consideration is the standard size pulleys

readily available. The pulley radii for each joint are also listed in Table 3.1.

Another consideration to take into account is the maximum and minimum change

in orientation that the motor will be expected to make in one movement. Since the

average displacement required for each joint is less the one inch of cable (listed in Table

3.1) and the displacement for any individual finger joint is less than the circumference of

Table 3.1 Force and torque requirements for each joint

34

the corresponding pulley, no motor will be required to rotate the output shaft of the

gearbox an entire 360° . Therefore the movements that require the highest angular

velocities are very short in duration. Because of this, a motor that can accelerate and

decelerate relatively quickly is required for quick movements, whereas, grasping and

manipulation many times requires very fine movements. Therefore a motor that can be

precisely positioned is also necessary. If no other dedicated device, such as an

electrically triggered brake, is used to maintain cable tension in static loading situations,

an additional requirement of the actuation system is that the motors provide a means to

hold the position of the hand stably.

3.2 Stepper motors versus servomotors

Electric motors are the obvious choice to actuate the fingers of the hand when

considering either robotics or prosthetics applications. No other actuation method, such

as pneumatic or hydraulic, is as reliable or as easy to implement. Production of a

marketable product for either a prosthetics or robotics application will likely require a

device that can run on battery power. This naturally leads to the choice of using motors

that run on direct current (DC) rather than motors that are powered by alternating current

(AC) since power from the battery is already DC. Considering the requirement for a DC

motor, and torque and speed requirements there are two basic motor categories that could

actuate the hand, servomotors and stepper motors. This is a critical decision as it has a

large impact on overall performance and greatly dictates many of the other system

components to be used.

35

Servomotors in a size range suitable for this application are typically DC brush or

brushless motors. They use a position sensor, usually potentiometer based with an

analogue signal or an optical encoder, to provide position feedback for closed loop

control. Using an electronic controller, accurate positioning can be achieved similar to a

stepper motor, but a feedback signal is a necessity. Servomotors generally have a higher

rpm range than stepper motors and produce more torque at higher velocities than at lower

ones.

Stepper motors are electric DC motors with no commutators. In most other

electric motors the commutators switch the electromagnetic poles in the motor so that the

rotor is constantly made to turn. In the case of stepper motors all the commutation is

handled by external circuitry generally referred to as a stepper motor drive. The drive

will energize the correct magnetic pole or poles to advance the motor to the next “step”,

hence the term “stepper motor”. The windings are then energized in the proper sequence

and rate to make the motor rotate the desired direction and at the desired rate.

A stepper motor with the correct power supply and drive can be rotated at

relatively high speeds, although generally a stepper motor cannot easily be made to rotate

as fast as a servomotor. But stepper motors can accelerate or decelerate at relatively high

rates and can be turned to a precise position and then “hold” that position. One

advantage that stepper motors have is that in many applications they can be controlled

with open loop control, which doesn’t require any feedback from an encoder. The

encoder reports the position of the motor to the controller so that errors can be corrected.

As long as the stepper motor doesn’t “slip” or fail to advance to the next step when the

drive commands it to, then the stepper motor can be controlled in an open loop. This is

36

possible in applications where the loads and accelerations placed on the motor do not

exceed its maximum torque (Jones, 2004).

3.3 Stepper motor theory and operation

This section discusses the three basic types of stepper motors, permanent magnet,

variable reluctance, and hybrid, which is a cross between the previous two, advantages

and disadvantages of each, strategies for achieving the greatest performance from a

stepper motor, and the physics behind these concepts.

The basic difference has to do with the construction of their rotors and the

arrangement of their windings. The permanent magnet stepping motor relies on the

electromagnetic interaction of an energized stator winding and a permanent magnet rotor.

One thing that differs between a permanent magnet or a hybrid stepping motor and a

variable reluctance stepping motor is that fact that the permanent magnet and variable

reluctance motors maintain a fraction of their holding torque even the windings are not

energized. This is because the poles of the permanent magnet in either one of these

motors are attracted to the stator poles. The torque present when no windings are

energized is often referred to as detent torque. A permanent magnet motor can be

operated constructed as either a unipolar or a bipolar motor, both of which will be

explained shortly (Jones, 2004).

The variable reluctance stepping motor has no permanent magnet rotor, and

therefore relies on the “principle of minimizing the reluctance along the path of the

applied magnetic field”. The stator has a magnetic core and is constructed with a stack of

37

steel laminations and the rotor, which has teeth and slots, is made of soft steel that is not

magnetized (National Instruments, 2005). Because the variable reluctance stepper motor

does not have a magnet rotor it has no detent torque and will rotate freely when no

current is supplied to the windings.

Hybrid motors combine several characteristics of both variable reluctance motors

and permanent magnet motors. They combine a permanent magnet rotor and multi

toothed stator poles made of soft steel and are very similar to a permanent magnet motor

in terms of control (Jones, 2004). The stator of a hybrid motor is very similar to a

variable reluctance motor except for one aspect. In a variable reluctance motor, only one

coil of each phase is wound around each pole. Usually in hybrid motors, which have

what is known as a “bifilar” connection, there are two coils wound around each pole –

one from two different phases. Torque is then created by the magnetic interaction of the

permanent magnet rotor and the stator (National Instruments, 2005).

Permanent magnet and hybrid stepper motors can either be unipolar or bipolar.

The difference is defined by the arrangement of their windings and determines the kind of

drive that must be used to power them. A unipolar motor has two windings or phases.

The windings have a center tap which is usually connected to the positive supply. Then

one or the other of the other two ends is grounded (Figure 3.1). This reverses the current

flow though the winding and the direction of the magnetic field produced by the winding.

Unipolar motors can be operated by energizing either half of one winding at a time, or

half of both windings at a time (Jones, 2004). The advantage with a unipolar motor is that

it requires less sophisticated drive circuitry. The output current from the drive is always

in one direction. The disadvantage with comparison to a bipolar motor is that at any one

38

time, at best only fifty percent of the total windings in the motor are being used. This

results in less torque for a given size motor with comparison to a bipolar motor (Lin

Engineering, 2005).

A bipolar motor is constructed essentially the same as a unipolar motor except

that the windings are simpler. There are no center taps (Figure 3.2). The simpler

winding circuitry leads to a simpler overall motor design. Although the motor itself is

Figure 3.1 Unipolar stepper motor ____________________________________________________________________

Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)

39

simpler, it requires more advanced control circuitry because it is necessary to reverse the

direction of the current through the windings (Jones, 2004). The advantage a bipolar

motor has is that one hundred percent of its windings can be used at any given time. Also

more torque is produced. The amps per phase equals 1 2 of the amps per coil when

two coils are connected in series, as in a bipolar motor. If N represents the number of

turns per coil then N I× is proportional to the torque in a unipolar motor, then

( )2 1 2N I , or 2NI , is proportional to the torque in a bipolar motor, approximately

40% more then a unipolar motor (Lin Engineering, 2005).

Figure 3.2 Bipolar stepper motor ______________________________________________________________

Source: Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)

40

Although a typical hybrid stepping motors, due to the design of their rotors and

windings, have 200 steps per revolution, drive electronics that can utilize more advanced

electromagnetic control strategies can achieve much finer angular resolution. A motor

with 200 steps per revolution is said to have a step angle of 360°/200 or 1.8°. When a

drives divide a step in two, this is commonly referred to half stepping. Anything finer

than half stepping is commonly referred to as micro stepping.

As long as none of the magnetic circuit is saturated, powering both motor

windings simultaneously will produce a torque versus position curve that is the sum of

the torque versus position curves for the two motor windings taken independently (Figure

3.3). The two curves will be S radians out of phase for a two-winding permanent magnet

or hybrid motor, where S is the step angle. If the currents in the two windings are equal,

the peaks and valleys of the sum will be S/2 radians from the peaks of the single winding

curves. This is the key to half-stepping. The two-winding holding torque is the maximum

of the combined torque curve when both windings are carrying their maximum current.

For common two-winding permanent magnet or hybrid stepping motors, the two-winding

holding torque is, again, 2 times the single winding holding torque. This assumes that

the magnetic circuit is not saturated and that the torque versus position curves are ideal

sinusoids.

Micro stepping is an extension of this idea that uses two different current levels

through the two motor windings as in Figure 3.4. Common stepper motor drives divide

steps in to half steps, on quarter steps, and one eight steps. Some stepper motor drives

are capable of even finer precisions.

41

For a two-winding variable reluctance or permanent magnet motor, assuming

nonsaturating magnetic circuits, and making the same assumptions as before, the

following two formulas give the key characteristics of the composite torque curve:

2 2h a b= + and ( )1tan2

Sx b a

π−= . Where: a equals the torque applied by the

winding with equilibrium at 0 radians, b equals the torque applied by the winding with

equilibrium at S radians, h equals holding torque of composite curve, x equals

Figure 3.3 Torque verses position for a single winding and two winding motors ________________________________________________________________

Source: Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)

Figure 3.4 Torque versus position for two windings using micro stepping __________________________________________________________

Source: Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)

42

equilibrium position, in radians, and S again equals step angle, in radians. With no

saturation, the torques a and b are directly proportional to the currents through the

corresponding windings. The two currents are then varied and it is possible to achieve

1/4, 1/8, or smaller step sizes (Jones, 2004).

An important consideration in operating stepper motors at high-speed is the effect

of the inductance of the motor windings. Rise and fall time of the current through the

windings is a factor of inductance of the motor winding. Ideally the current versus time

would be a square-wave but the inductance of the winding causes it to be an exponential,

as illustrated in Figure 3.5

The exact characteristics of the curve of current through each winding versus time

depend as much on the drive circuitry as they do on the motor. These time constants of

these exponentials can easily differ. The rise time is a condition of the drive voltage and

drive circuitry, while the fall time is determined by the circuitry used to dissipate the

stored energy in the motor winding.

Figure 3.5 Current through winding versus time

__________________________________________________ Source: Jones, D., “Control of Stepping Motors”, (2004),

<http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)

43

At relatively low step rates, the rise and fall times have much less effect on the

motor’s torque than at high step rates, as shown in Figure 3.6. This is because the ratios

of rise and fall time to the duration of time a winding is energized is much lower at slow

step rates compared to higher ones. At low step rates the winding is conducting full

current for a greater percentage of the time that the winding is energized. This leads to

greater running torque at low speeds.

The motor's maximum speed is the speed at which the available torque goes to

zero. A curve of torque versus speed for a typical motor and control system can usually

be approximated by a horizontal line at low step rates and a line with negative slope

going to zero over the range of higher step rates. The cutoff speed is defined as the step

rate at which these two regions of the curve meet. A definite cutoff speed is rare,

therefore, statements of a motor's cutoff speed are approximate. The rise and fall times of

Figure 3.6 Torque versus speed

___________________________________________________ Source: Jones, D., “Control of Stepping Motors”, (2004),

<http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)

44

the current through the motor windings occupy a relatively small percent of each step

when the motor is operating at rates less than its cutoff speed. While at the cutoff speed,

the step duration is comparable to the sum of the rise and fall times. The exact torque

versus speed curve depends on the rise and fall times in the motor windings, and these

depend on the motor control system as well as the motor. Therefore, the control system

for a motor also has a large effect on the cutoff speed and maximum speed, not just the

motor itself (Jones, 2004).

One strategy to improve a motor’s cutoff speed, maximum speed, and high-speed

torque is current limiting. Increasing the voltage applied to the windings increases the

current through the windings in a simple V I R= × relationship, where V is the voltage, I

is the current, and R is the resistance of the windings. Increasing the voltage applied to

the winding to a level which results in a current that is significantly higher than the rated

current results in much quicker rise and fall time. Unfortunately current levels this high

result in damage to the motor; usually the thermal breakdown of the insulating material in

the motor windings. The idea behind current limiting is to use a voltage significantly

higher than necessary to achieve the rated current, but to use advanced circuitry to drop

the voltage applied to the windings, once the rated current level is reached. In this way,

the current versus time curve comes much closer to resembling a square wave, as in

Figure 3.7. The advanced circuitry techniques used to achieve current limiting include

resistive current limiters, linear current limiters, open loop current limiters, one-shot

feedback current limiters, or hysteresis feedback current limiting. Current limiting

technology is also required to achieve micro stepping since at times during micro

stepping one of the motor windings is run at less than the rated current (Jones, 2004).

45

3.4 Stepper motor, drive, and power supply setup

For the research and development stage of the hand mechanism, a system that is

easily reconfigurable and expandable has obvious advantages. The overall system that

converts electrical power to mechanical power consists of stepper motors, stepper motor

drives, and a power supply. This section describes how all the separate components that

were selected based on the performance requirements and operating principles discussed

above are combined in one system. It also discusses the performance of the system.

Five NEMA Size-17, bi-polar, hybrid, 1.8° , DC stepper motors equipped with

3.6:1 gear reduction via an offset spur gear provide mechanical power for the hand. Five

RMS Technologies, R208 drives power the motors. They have micro stepping and

current limiting technologies. Two variable voltages linear power supplies from BK

Precision, which run on 110 VAC and provide an output of 0 to 15 VDC at 40 Amps,

provide the electrical power. They are connected in series, providing variable voltage

over the input range of the drives, 12 to 24 Volts.

Figure 3.7 Current versus time with and without current limiting ____________________________________________________

Source: Jones, D., “Control of Stepping Motors”, (2004), <http://www.cs.uiowa.edu/~jones/step/> (19 November 2004)

46

This system provides excellent performance characteristics suitable for actuating

the hand mechanism. The angular resolution of the motors allows for very precise

control of the cable displacement, and therefore, hand posture. With the 3.6 to 1 gear

reduction, this setup is capable of step sizes as small as 0.5° without micro stepping, or

using the finest level of micro stepping, one eighth stepping, the drives are capable of

rotations as small as 0.0625° . Depending on the pulley size for a given joint, these

rotations correspond to cable displacements as small as approximately 0.036 mm and

0.0045 mm, respectively. This particular combination of drives, motors, and gearing

produces running torque of 1.06 N-m (150 oz-in) at low speeds and 0.81 N-m (115 oz-in)

at 2 revolutions per second, as reported by the manufacturer (Figure 3.8).

Figure 3.8 Manufacturer’s torque rating (oz-in) of motor versus speed (revolutions/second) _________________________________________________________________________

Source: Lin Engineering, “Lin Engineering FAQ”, (2005), <http://www.linengineering.com/site/resources/faq.html> (29 March 2005)

47

These torque and velocity figures refer to torque and velocity after gear reduction

and not torque and velocity of the motor itself. Holding torque, or the torque the motor

provides to keep the hand static, is critical to maintaining a grasp. The holding torque

produced when the maximum current is supplied to the windings while the motor is

stationary would be approximately equal to the maximum running torque, but the friction

associated with the gear reduction increases this amount. The amount of torque produced

provides tension on the cables that is quite sufficient for posturing the hand and providing

gripping force.

The mechanical aspects of the hand have dictated the design of the electrical

system that powers the hand. The most critical aspects were the amount of tension

required in the cables to displace the fingers fully and the corresponding displacement of

cable. Due to these mechanical aspects of the hand, five stepper motors with gear

reduction were determined to be the most suitable devises for converting electrical power

into mechanical movement. With suitable drives, which are capable of current limiting

and micro stepping, the stepper motors provide sufficient torque over the entire angular

velocity range, especially at the low end, and at the same time are capable of extremely

fine movements. Since stepper motors are relatively inexpensive it is wise to plan to size

a motor large enough to provide torque two or three times greater than the maximum

expected required torque. Of course, as the process of optimizing the design for a

prosthetics or robotics application is begun, the use of oversized motors may no longer be

practical. Once the appropriate stepper motors are selected, this decision determines the

other necessary electrical components: stepper motor drives, and a power supply.

48

CHAPTER IV

MOTION CONTROL

This chapter discusses the computerized motion control for the hand, why such

control necessary, various alternatives for achieving it, and specifics of the method used.

A user interface based on a hierarchical control scheme that simulates how the hand

could be controlled in the real world is discussed. New software, which is discussed in

the chapter, was developed to implement this control scheme. Also, further

advancements necessary to achieve a useable level of controllability of the hand are

discussed.

4.1 Motion control setup

To achieve coordinated motion of five DOF’s which mimics human motion,

requires computerized control. At a lower level of control, simple finger motions,

computerized control is necessary to manage simpler requirements such as maintaining

the joint limits of the hand and defining a single move, such as number of steps,

direction, maximum velocity, rate of acceleration, rate of deceleration, etc. At a higher

level of control, computerized control is also necessary for inter-coordination of five

independent DOF’s. Computerized control is especially necessary to implement a

hierarchical control scheme which can simplify the necessary input from a prosthesis user

to a level where managing a high number of DOF’s is comfortable.

49

The term hierarchical control scheme refers to one in which it is possible to

control a high number of degrees of freedom while the user only inputs a smaller number

of DOF’s, in prosthetic control typically one or two. For instance a user may only signal

a higher level command such as “ close grasp” and the control scheme would generate

corresponding lower level commands such as move the first axis 30° counter-clockwise,

move the second axis 45° counter-clockwise, etc. Additional information could also be

collected be external sensors in the hand to help make decisions automatically, such as

stop tightening the grasp when an object comes into contact with one or more fingers.

Since stepper motors have no commutators like other DC motors, all the

commutation, or continuous change in the electromagnetic field to turn the rotor, must be

handled by external electronics. The device that is responsible for this is referred to as a

drive. At the very least the stepper motor drives require two inputs signals to operate,

direction and pulse rate. The direction signal is a simple on - off signal of plus or minus

5V to indicate the direction of travel, while the pulse rate signal is a sine wave, the

frequency of which indicates the pulses per second which is directly proportional to the

number of steps per second. The drives then energize the appropriate windings of the

stepper motor at the appropriate times.

When considering the most basic aspect of control for this application, sending

five direction and pulse signals to five stepper motors drives, the are several categories of

electronics that could possibly perform this task as well as more complex tasks. These

categories include electronics specifically designed for this task such as PCI or ISA bus

motion controller cards that reside in a host PC, a range of stand alone motion controllers

that are an analogue of the PCI or ISA bus versions but operate removed from the PC, as

50

well as more general purpose electronics which could be adapted to perform the task such

as Programmable Logic Controllers (PLC’s), PC-104 based electronics, or a custom

designed electronics board.

For the current state of the project, research and development, a system that is

modular and easily configurable, both in terms of hardware and software, greatly aids in

experimentation and refinement by allowing the easy addition of new hardware

components or programming. After examining all the alternatives, a control system

based on a PCI bus motion controller was determined to be the most practical solution. It

is simple to implement, designed specifically to perform all of the necessary control

tasks, easily configurable in terms of software and hardware, and relatively cost effective.

The motion controller is a DMC-1850 five-axis motion controller that resides in the PCI

Bus of a standard desktop PC. The number of axes refers to the number of DOF’s or

motors that it can control. It has its own microprocessor and memory and performs all of

the motion commands internally without using the computers resources (Figure 4.1).

Figure 4.1 Actuation and control setup

51

The stepper motor drives used are RMS Technologies, R208 bipolar drives. They

have current limiting technology, so they can run on power anywhere from 12 to 24

VDC. They also have a peak current which is adjustable from .35 to 2 Amps peak, to suit

the particular motor and application. They are capable of step sizes of full, half, and 1/4

and 1/8 micro-stepping. Other features include optically isolated step, direction, and

enable/disable inputs, and a current cutback feature that can be disabled. When enabled,

the current cutback feature reduces the holding current to 23% of peak current to reduce

temperature buildup and energy consumption. They have a 9-pin input connection, the

other six that have not been mentioned previously include: main power and ground, a 5V

power and ground for the logic circuits, and two inputs which can be enabled and

disabled in four different combinations to select the step size. In this way the controller

can control all the functions of the drives. A detailed wiring diagram is included in

Figure 4.2.

The interconnect module is basically an extension of the motion controller and

provides terminals to handle all of the input/output for the motion controller. This

includes committed I/O for each axis for pulse rate, direction, and an enable signals

(which activates each drive), along with several digital and analog inputs and outputs,

which allows for a great deal of expandability ideal for this stage of development. The

particular motion controller used actually requires two interconnect modules because

each module can handle up to four axes. This setup has five motors. The interconnect

module terminals included in the figure are only the ones needed for to control the

functions of the drives, there are several more which can be used to add a variety of

hardware.

52

17

18

24

25

27

28

30

31

33

34

66

67

68

69

70

71

72

73

signal ground

+ 5VDC

A axis direction

A axis pulse output

B axis direction output

B axis pulse output

C axis direction

C axis pulse output D axis direction

D axis pulse output

output 1

output 2

output 3

output 4

output 5

output 6

output 7

output 8

37

38

39

40

A axis enable

B axis enable

C axis enable

D axis enable

Power Supply

+12 – 24 VDC ground

33

34

40

E axis direction

E axis pulse output

E axis enable

66output 9

67output 10

Interconnect

Interconnect

1. +12V

2. step

3. step

4. enable

5. direction

6. ground

7. logic ground

8. +5V

9. PWM

B

B

A

A

Drive, A

1. +12V

2. step

3. step

4. enable

5. direction

6. ground

7. logic ground

8. +5V

9. PWM

B

B

A

A

Drive, B

1. +12V

2. step

3. step

4. enable

5. direction

6. ground

7. logic ground

8. +5V

9. PWM

B

B

A

A

Drive, C

1. +12V

2. step

3. step

4. enable

5. direction

6. ground

7. logic ground

8. +5V

9. PWM

B

B

A

A

Drive, D

1. +12V

2. step

3. step

4. enable

5. direction

6. ground

7. logic ground

8. +5V

9. PWM

B

B

A

A

Drive, E

Figure 4.2 Wiring diagram

53

Explicit commands can be sent for execution by the motion controller from an

application running on the PC that is designed specifically for use with this motion

controller. This of course, is a very basic way of controlling the hand and is only useful

for developmental purposes. This method does not allow for the possibility of executing

a coordinated series of motions. The motion controller can also interact with an

executable running on the PC. In this way virtually any level of logic can be

incorporated in the control of the actuation system. The highest-level commands are

generated in the executable running on the PC. Explicit commands, such as specific

relative position moves with a defined acceleration, deceleration, and velocity, can then

be sent to the motion controller card. The motion controller then converts these

commands to the appropriate pulse rate and direction signals for each axis and then sends

them to the motor drives via the interconnect module. The motor drives then use these

pulse rate and direction signals to energize the windings of each stepper motor in the

appropriate sequence to complete the required motion.

The inputs can be used to read nearly any variety of external sensors, which can

then be monitored by the motion controller. The executable can then query the motion

controller for the state of these inputs. In this way, the external sensors can be used to

trigger events that happen in the executable.

For example, as part of a hierarchical control scheme, pressure sensors could be

incorporated into the palm and fingertips of the hand. Myoelectric signals from the user

could be monitored until the appropriate signal was detected to indicate that the user

intends to close the grasp of the hand. The hand would begin to close until a change in

the signal from the pressure sensors indicated that contact with an object had been

54

achieved causing the hand to hold its position after the appropriate pressure has been

applied.

The system also includes a main power switch in the positive wire (Figure 4.2) in

between the power supply and the motor drives that is installed on the motor fixture, a 20

Amp fuse in between the power supply and the switch, and an abort switch. The abort

switch is wired to a designated input, that when activated terminates any motions

currently being executed and prevents other motions from being executed until the switch

is reset.

One addition to the system, which would not only greatly improve performance of

the current system, but would also be a necessary element of the control system in any

form suitable for a consumer product, would be encoders or some other device which

could provide a position feedback signal in order to correct positional errors. Since when

stepper motors move they move a commanded number of steps, which corresponds to a

certain angular position, stepper motors work very well in some applications that do not

include positional feedback. This is true when the inertia of the load and attempted

accelerations do not exceed the available torque of the motor. If the load on the motor

does exceed the available torque, the stepper motor will “slip” or fail to advance to the

next step. In this particular application the end effectors purposely collide with a grasped

object, and therefore the maximum range of motion are, in effect, different for every

object the hand grasps. A positional feedback signal could detect a slip by the stepper

motors, which could signal contact with a grasped object or other cases in which the

motor slips. Without this feedback, once a slip occurs, all knowledge of position is lost.

55

4.2 Programming

This section discusses new software that was developed to implement a

hierarchical control scheme. A generic method for programming the hardware used and

basic capabilities are discussed first.

A software development kit is provided with the motion controller card. It

includes, dynamic link libraries, (DLL’s) which can be used with any Windows

programming environment that interfaces with DLL’s. The basic programming model

includes six steps, described as follows. The first step is to the declare functions. The

second step is to start a communication session between the executable and the controller.

The third step is to download a program. If necessary an entire predefined program can

be downloaded from the computer to the controller and then executed on command. Step

four is to send live commands including axis motions, input activation, and input

querying. The last step after the rest of the program is complete is to close

communication. The software developed to control the hand follows these basic steps.

One of the most difficult aspects of developing a hand mechanism that could

potentially be used as the major component of a prosthetic system with a large number of

DOF’s is a user interface and control scheme. Many currently available prosthetics use

myoelectric signals from the residual muscles to control grasping. However, to

consciously control more than one or two DOF’s individually is too mentally complex for

the user and would make the device too cumbersome to use. Control of a large number

of DOF’s is impossible unless some sort of control scheme can be developed to simplify

the commands the user must provide and coordinate many DOF’s.

56

To demonstrate the capabilities of the hand, a control scheme was developed

based on a control scheme developed for a different application. Knutson et al (2004)

developed a control scheme simulating state activation of a neuroprosthesis using two

myoelectric signals, from the wrist extensor and flexor muscles. Sensors were implanted

in patients’ wrist flexor and extensor muscles, and then both myoelectric signals were

monitored. The flexor myoelectric signal was then used to represent an x-coordinate; the

extensor myoelectric signal was used to represent a y-coordinate, which together

corresponded to a location on a state activation chart (Figure 4.3).

Figure 4.3 State activation chart

__________________________________________________________________ Source: Knutson, J., Hoyen, H., Kilgore, K., Peckham P., (2004), “Simulated Neuroprosthesis State Activation and Hand-Position Control Using Myoelectric Signals from Wrist Muscles”, Journal of Rehabilitation Research & Development, Vol. 41, Issue 3B, P461.

57

The signal space is divided into four regions, “Hold”, “Open”, “Close”, and

“Change Grasp Pattern”. When the Hold command is active, which corresponds to both

sets of muscles being at rest, the hand mechanism stays in its current posture. When the

extensor muscles are excited, the Close command is activated, which causes the hand to

tighten its grasp. Likewise, when the flexor muscles are excited, the Open command is

activated widening the grasp. When both sets of muscles are excited, the Change Grasp

Pattern command is activated; it can be used to toggle between different grasping

postures.

In order to apply this control scheme as a user interface to operate the hand

software was developed to use a joystick to simulate the myoelectric signals. One DOF

of the joystick simulates the extensor myoelectric signal, while the other simulates the

flexor myoelectric signal in the same manner as the above-mentioned neuroprosthesis. A

combination of the two signals determines the location of a control point that exists on a

two-dimensional state activation chart (Figure 4.3).

The joystick, which contains two linear potentiometers that are each manipulated

by one DOF of the joystick, is connected to the interconnect module. The potentiometers

are each connected to one of the analogue inputs and the voltage across the

potentiometers is read. After the program goes through a short calibration routine to find

the minimum and maximum voltages for both DOF’s, the program runs in a loop,

constantly monitoring the voltage of across the potentiometers, until a lack of input

causes the program to time out and terminate. The signals from both potentiometers are

normalized and converted into Cartesian coordinates. The joystick has been physically

modified so that the tension of internal springs cause the at-rest position of the joystick to

58

reside at the lower left corner of the range of motion, similar to the state activation chart.

The command regions of the program are also divided up in a similar manner as the

chart. Moving the control point out of the Hold region by varying one or both DOF’s

activates one of the other commands depending on which command region is entered,

Close, Open, or Change Grasp Pattern.

Proportional control is the ability for the user to determine how far the device

opens or closes by some means. Proportional control is incorporated into the control

scheme by correlating the how far the control point travels into either region before

returning back towards the Hold region to how far the hand opens or closes. Moving the

joystick to its maximum range of motion in a given DOF corresponds to either 100%

closure or opening. Since this is an open loop system, the program monitors position by

tracking the commanded direction and number of steps of each move. Joint limits are

maintained by adjusting any command that calls for a motion beyond the joint limits by

reducing the number of steps in the command so that the motion takes the particular DOF

up to the joint limit. Subsequent motions in the direction of the reached joint limit are

ignored.

The Change Grasp Pattern toggles between different sets of relationships between

the five DOF’s, to achieve different grasp postures. If the adduction / abduction DOF

were included in the fingers the hand would be capable of several kinds of grasps.

However in its current configuration the hand is still sufficiently capable to perform some

grasps types. It is programmed to do a pinch grasp with the thumb and forefinger and a

cylindrical grasp with all five fingers.

59

Use of a PCI 5-axis motion controller allows for computerized motion control

system that is ideal for developing the hand due to the great expandability. Software can

easily be rewritten or hardware added to experiment with new control strategies for the

hand. A user interface that simulates a real world hierarchical control scheme has been

completed. This allows for some rudimentary control of five DOF’s while only requiring

two DOF’s of input from the user. Ideally a level of control would be achieved that more

closely approaches a human hand in terms of intuitive use and performance. It would be

necessary to include encoders for positional feedback as well as additional sensors in the

hand to provide feedback to indicate contact with an object, slip of a grasped object, etc.

60

CHAPTER V

APPLICATIONS AND RESULTS

Previously, in Chapter I, potential applications of the hand mentioned are

mentioned, adaptation of the hand to be used as a prosthetics device and incorporation of

the hand into a robotics system as an anthropometrically based gripper. In addition to

these, there is a third application for which a advanced hand mechanism could be used, as

a physical model to develop a cognitive model that governs the grasping capability of a

digital human and verify its accuracy. Section 5.1 discusses how the hand mechanism

could be used for all three applications and what would have to be done to adapt it for

each. Section 5.2 discusses the results achieved thus far and how suitable the hand would

be for these applications.

5.1 Applications

Prosthetic devices to replace the hand can either be hook shaped or a more

anthropometrically shaped hand with a range of functionality from one or two simple

grasps to purely aesthetic device. Approximately 70 percent of users in the United States

use a hook over a hand devise (Doshi, et al. 1998). The fact that not all users choose to

use a mechanical hand prosthesis leads to the conclusion that the devices commercially

available are either too expensive or are not practical enough for all users to consider

them beneficial. Typically commercially available models only have three active fingers,

which are usually all activated by the same motor and therefore only have one DOF.

61

They are only capable of a cylindrical or pinch type grasp depending on how the hand

comes in to contact with the object.

Integration of the hand mechanism into a prosthetic system would require two

different user interfaces, a physical one and another one that allows the user to send

commands to the device. The purpose of the physical interface would be to connect the

devise to the user’s body. The physical interface would be customized to each user and

would include a socket or cup that would allow the device to join with the user’s residual

limb. The conventional command interface, which allows the user to interact with a

prosthetic devise, uses myoelectric sensors placed on the surface of the skin. Myoelectric

control has three main advantages, accuracy of command selection, intuitiveness of

control, and a quick response time of the system (Englehart and Hudgins, 2003).

Typically myoelectric control can be troublesome because the sensors can move or be

easily placed in slightly different locations every time they are applied which causes

inconsistency in the myoelectric signals, which are inherently weak and noisy (Davalli et

al, 2000). Because of this, many systems require an initialization routine to calibrate the

device to the changes in the signal every time the device is put on. Myoelectric control

would also require additional electronics to amplify and filter the signal.

This hand device, with further advancements such as additional DOF’s and the

appropriate sensors combined with an advanced control system based on a hierarchical

control scheme could be integrated into a prosthetic system. This system would have

several potential advantages over currently available devices and devices currently being

researched. Since the fingers are based on a compression spring and do not contain any

rigid links, complex revolute joint mechanism, or actuators, a considerable percentage of

62

they’re volume is occupied by empty space. This provides the potential for an extremely

lightweight hand if the design for the fingers can be combined with a design for the rest

of the hand that is optimized to be lightweight.

Since the forces to actuate the fingers are transmitted by a cable and conduit pair,

there are several options for the locations of the actuators. This could possibly present a

design trade off between minimization of the perceived weight of the device and

containing it in the smallest package possible. In any case the actuation system would

need to be made more efficient so that smaller, lighter motors could be used and the

entire actuation system could be contained in a smaller, more ergonomic package.

Depending on the size of the motors that are sufficiently powerful enough and therefore

the torque required to actuate the hand, there could be up to three options for the location

of the motors.

The first option would be in the palm of the hand. For this option the motors

would have to be extremely small, since there would still likely be at least five motors.

This would also increase the weight of the hand mechanism and locate the center of mass

of the device further away from the users shoulder, making it more cumbersome and

tiring to use.

The second option would take advantage of fact that the forces are transmitted

through flexible cable conduit pairs and locate the motors completely off of the limb.

The motors could be clustered in an ergonomic package that is worn somewhere else on

the body, possibly in a pack worn around the waist. The cables could then run from the

pack, under the clothes, to the hand. The advantage would be that much of the weight

could be located completely off of the limb, minimizing the burden on the user. The

63

disadvantage would be the addition of another component to the system that the user

would have to wear.

If the amount of residual limb allowed for it, the third option would be a

compromise between the first two options. This would be to locate the motors in the

forearm or wrist space. This would locate the weight further up the user’s limb reducing

the perceived weight of the device compared to locating the motors in the hand itself and

at the same time eliminating the need for an additional pack to be worn.

No matter where the motors are located it would be beneficial to spend effort on

optimizing the entire system so that it requires the smallest motors possible. The design

of the force transmission system could be optimized to reduce losses due to friction. The

design of the joints could be optimized so that a given deflection of the spring takes the

smallest amount of tension in the cable, while at the same time creating the most contact

force at the fingertips. These two design points are key to making the overall system

practical because they lead to reducing the minimum size of the motors necessary, which

is beneficial for several reasons. Smaller motors are easier to locate in an ergonomic

package and leave more options as to their location. Smaller motors would reduce the

total weight of the device, which reduces the amount of effort on the user’s part to utilize

it. They would also require less power, reducing the size of the batteries necessary to

power the device for a given amount of time, additionally reducing weight and bulk.

The hand could also be used as an anthropometrically based end effecter or

gripper for a robotics system. Applications that require this sort of gripper over a more

primitive pincher-like gripper are those that will model their gripping strategies on the

strategies that a human employs. The applications that are likely to use an

64

anthropometrically based gripper are the ones in which the system would be required to

perform operations that were unforeseen and varied in nature and therefore required a

gripper that would be versatile and easily adaptable for many different tasks instead of a

more primitive pincher-like gripper may only performs one kind of task well.

For example, humanoid robots, that one day could be part of everyday life, would

be interacting with the objects found in everyday life. Most of these objects, whether

deliberately or not, are all to one degree or another designed to be manipulated by human

hands. Therefore the most logical end effecter to manipulate these objects is one based

on the human hand.

In the more near future, a complex hand mechanism may be used as part of a

robotic system to complete tasks that would normally be completed by a human but the

task is more difficult or more dangerous to complete due to the environment.

Specifically this is being researched for applications in space (Farry et al, 1996, and

Carrozza et al, 2002). These projects investigate the possibility of allowing astronauts to

complete certain tasks that would normally be extra-vehicular activities by remotely

operating a hand mechanism mounted on a robotics arm system. This would eliminate

the difficulty associated with trying to manipulate objects while wearing a pressurized

glove and the elevated level of danger of an extra-vehicular activity.

Although more of a side effect than a direct application, another field that could

benefit from development of this hand mechanism is digital human modeling. Digital

human modeling tries to replicate what a human does in a virtual environment for the

purpose of studying how a human might interact with products in order to further refine

these products before they are built. Development of a control system that is sufficiently

65

advanced enough to replicate human motion when grasping an object, and do so with a

minimum of effort from the user, would necessarily require an intense study of all the

processes that happen when a human grasps and manipulates an object. Processes that

require better understanding include how the human brain coordinates individual finger

movements and inter-coordination of the fingers to work together without conscious

thought, how tactile feedback plays into this, and how humans unconsciously decided on

a grasping strategy based on the object’s shape, size, and weight, and the task that is to be

accomplished. A better understanding of these things would be important in developing a

control system for the hand. This would benefit the field of digital human modeling,

which also aims to mimic what humans do in real life, but tries to replicate it in a virtual

environment.

5.2 Results

A fifteen DOF hand has been developed. It has all the major DOF’s in the flexion

/ extension plane that a human hand has, which is many more than any commercially

available model and competitive with other complex hand mechanisms currently being

researched. A system to actuate it has been developed that couples the joints together in

sets of thee, reducing the number of motors necessary and therefore simplifying the

required electronics. Motion control and a hierarchical control scheme have also been

developed. Although the hand does not mimic the human hand perfectly, it is capable of

performing a cylindrical grasp, a spherical grasp, and a pinch grasp (Figure 5.1 through

Figure 5.5). In this section we show examples of the hand performing these grasps.

66

Figure 5.2 Spherical grasp with glove, one

Figure 5.1 Cylindrical grasp without glove

67

Figure 5.4 Pinch grasp with glove, one

Figure 5.3 Spherical grasp with glove, two

68

The grasps developed look natural can hold objects securely. The cylinder

shown in Figure 5.1 is a steel bar that weighs approximately 350 grams and is 19 mm in

diameter. The bar has two different surface finishes, a smooth one and a rough one.

When only the smooth surface is gripped, the bar will slip if it is held in the vertical

position. The bar stops slipping as soon as the rougher surface slides in between the

index finger and thumb. However this test was conducted with out the use of the

cosmetic glove, which has a much higher coefficient of friction than the aluminum

surface of the hand.

The device weighs approximately 388 grams. This includes the weight of the

fingers and hand body and does not include the glove, actuation system, or electronics.

The aluminum hand body consists of a large percentage of the weight of the hand,

Finger 5.5 Pinch grasp with glove, two

69

approximately 220 grams or about 57 percent of the total weight. The design of the

fingers gives the hand an advantage in terms of weight. Since most of the volume of the

fingers is empty space they are very lightweight. All five fingers weigh a total of about

118 grams. The hand body could be redesigned with emphasis place on weight savings

and be produced out of a lightweight material. A redesigned hand body and the fingers

would make for a lightweight combination putting the hand on par with or ahead of

competitive models, such as the prosthesis by (Doshi et al, 1998), 203 grams, the Otto

Bock hand, 390 grams, or the APRL hand, 421 grams (Yang et al, 2004).

70

CHAPTER VI

CONCLUSION

6.1 Contributions

This work has been focused on developing a complex anthropometrically based

hand mechanism. Such a mechanism has several useful applications. The first

application is use in any robotics system that requires a human like gripper. Such a

gripper has several advantages over simpler designs like simple pinching mechanisms. A

simple mechanism is limited in the number of grasps and manipulations it can perform

and therefore can only be used for a predetermined set of tasks. An anthropometrically

based gripper is highly configurable and adaptable. Its use is only limited in scope to any

grasping or manipulation tasks that a human might perform, and can therefore be used for

unforeseen tasks that arise. This would be an ideal grasping mechanism if humanoid

robots were to ever become part of everyday life. In the more near future, human like

grippers may be used as part of a remotely operated system in environments that make

work difficult or dangerous such as extra vehicular activities in space.

The second is as a prosthetic hand. Commercially available models typically only

have a few active fingers and one DOF. Although they restore some functionality to the

user’s limb, they lack realism and the ability to do more than a few basic grasps. This

leaves the user with less realism and function than a human hand. The ideal replacement

for the human hand would equal the human hand in realism, dexterity, easy of use, and

71

sensory feedback to the point where the user would forget that the hand is not a part of

their own body.

Certain shortcomings of current prosthetic hands, such as lack of realism and

functionality, leave users wanting improvement in their devices. These can be corrected

by adding DOF’s. This hand has fifteen DOF’s, significantly more than current models,

but still fewer than the human hand. This allows it to perform three types of grasps on an

object: cylindrical, spherical, and pinch type grasps.

The contribution that makes this work unique is the development of a novel

flexible bending segment based on a compression spring. Three of these segments make

up a finger that has three DOF’s in the flexion / extension plane. A finger of this type

offers several advantages over a finger made in a more conventional manner using rigid

links and revolute joints. The first advantage is that since the majority of the structure is

comprised of a compression spring, much of the volume is empty space. This gives the

hand the potential to be very lightweight. The other advantage is that the fingers are

inherently compliant. They deflect and conform to the shape of the object that they are

grasping, giving a more secure grip.

An actuation method has been developed that will serve to test and further

develop the hand. The three DOF’s in each finger are coupled together and actuated

through cable and conduit sets by a stepper motor. The stepper motors provide enough

torque to produce significant gripping force, enough to securely hold a 350 gram cylinder

19 mm in diameter without slipping. The stepper motors can be precisely positioned with

displacements of cable as small as 0.0045 mm if micro stepping is used.

72

Computer control is necessary to coordinate the five DOF’s of the hand. A

control system suitable for research and development purposes was developed based on a

motion controller card residing in a desktop PC. An executable can run on the PC and

send commands to be executed by the motion controller card. The motion controller

converts these commands into signals suitable for the stepper motor drives, which

energize the windings in the motor to carry out the motion. There are several available

inputs and outputs that can be used to trigger events in the executable and therefore

provide means to interact with the hand in nearly anyway imaginable. This

configuration works well for research and development because it is easily configurable,

allowing for easy testing of different system configurations.

A simulation of a control scheme was conducted to demonstrate one possibility

for interacting with the hand. The control scheme being simulated uses myoelectric

signals and a hierarchical command structure to return function to the hands of spinal

injury patients. In the actual tests with patients, two myoelectric signals from different

muscle groups in the forearm are monitored. These signals are then used to select

commands to control the hand. The simulation was conducted by using signals from a

two DOF joystick to simulate the two signals from the patients’ muscles. A hierarchical

control scheme such as this one, allows the user to actively control only two DOF’s while

coordinating five. This reduces the mental burden on the user. To consciously control all

the DOF’s in a complex hand mechanism individually would be far too mentally complex

for the user and make the hand completely impractical. An ideal replacement for the

human hand would be completely intuitive to use and require no more conscious thought

than a natural hand.

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This work is by no means completed to the point where the hand is a completed

product ready for integration into one of several applications. It is completed to the point

where the concept of a five fingered anthropometric hand made of fingers with

deformable bending segments is shown to have merit. Thus far, the hand is computer

controlled and remotely actuated. It is capable of using three different grasp styles to

securely hold objects of different shapes and sizes. It has also been developed enough to

discover its weakness and to see areas where further research can be done.

6.2 Shortcomings and future work

The first essential step in developing a hand mechanism based on the concepts

shown in this work to point where it is as capable as a human hand would be to replicate

all the DOF’s that a human hand has, 25 according to some models (Peña Pitarch et al,

2005). All these DOF’s would also need to have the same range of motion that the

human hand does. Only then will a hand mechanism be capable of all the same grasping

styles, gestures, and manipulations that the human hand is capable of. The first step

would be to extend the range of motion of the current DOF’s. This would involve careful

design of the segment links to ensure that the three cables in each finger are all parallel to

the axis of the finger and are coplanar, something that is only approximated in the current

design. These corrections would ensure a motion that is more purely flexion and

extension.

Also the cables and conduits should be retained in the segment links as far away

from center as possible, contacting the inside edge of each coil of the compression

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springs. Designs with the cable located outside of the compression spring should also be

considered. This could dramatically increase the range of motion of the joints, but at the

same time bring about new difficulties. The cable would come into contact with the

cosmetic glove and likely cause tearing. It would also be likely to be the part of the hand

that first come into contact with a grasped object, changing the stability of the grasp.

The next essential modification to increase the number of DOF’s of the hand

mechanism and the number of gestures and grasp styles it is capable of, is to add DOF’s

for the abduction / adduction range of motion in certain finger segments, specifically the

metacarpophangeal joints of the index, middle, ring, and little fingers. This could be

accomplished by adding two more cable and conduit sets to these bending segments and

then locating them at different orientations, ninety degrees either way of the cable used

for flexion of the fingers. A careful study would have to be performed to determine how

to coordinate cable movements of the three cables for both individual DOF movements,

and combined DOF movements of one of these segments.

Opposition of the thumb could possibly be accomplished in the same manner or it

may prove to be more practical to achieve this motion with a more conventional style

joint mechanism. In either case, conventional style joints would likely have to be used to

replicate the remaining DOF’s in the palm of the hand. One model of the human hand

(Peña Pitarch et al, 2005) includes four more DOF’s in the palm, one in the flexion /

extension plane, and one in the abduction / adduction plane at the base of the metatarsal

of both the ring and little finger.

When considering the design of the fingers and the palm for the purpose of adding

new DOF’s, it is not only necessary to consider the realism of the motion achieved, but

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the static shape of the hand as well. Whether developing the hand mechanism for a

robotics or a prosthetic application, overall shape is important. As a gripper for a

humanoid robot a realistic looking hand is more aesthetically pleasing and user friendly

for the people interacting with the robot. As a prosthetic hand a realistic shape is

essential. How the device discreetly blends in with the rest of their body is one of the

primary concerns of prosthetic users.

While additional DOF’s improve the functionality of the hand they also add to the

complexity of other challenges. The control system becomes inherently more complex,

as well as the actuation system. No matter what the application, the actuation system will

likely have to experience a great deal of refinement in order to fit in a small confined

package and to minimize power consumption. This is especially true if the hand is

developed into a prosthetic. Here, the need for the entire device be contained in a small,

ergonomic, lightweight package is probably most relevant. This includes the actuation

system, electronics, and power supply as well as the hand itself.

Perhaps the greatest challenge will be adding some level of intelligence to the

hand so that it will require the least amount of input possible by the user. This is

especially necessary if it is used as a prosthetic device. Reducing the mental burden on

the part of the user is one of the keys to making it an aid in everyday activities. Adding

intelligence to the hand and so it can automatically perform certain functions is one way

to achieve this.

A very robust and sophisticated control system would need to be developed if 25

DOF’s were incorporated into the hand. One strategy for making the control system

more intelligent is to in effect replicate the human sense of touch and use this information

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to control the hand. For instance, pressure sensors could be integrated into the fingers

and the palm of the hand and could be used to signal the control system when contact has

been made with an object and the magnitude and distribution of force on an object

(Butterfass et al, 1998 and Tura et al 1998). A method to best use this information to

securely hold an object needs to be developed as well.

Another way to make the grasp more reliable is to add accelerometers to the hand

to detect the vibrations from the slip of an object (Tura et al 1998). The idea is that the

hand would grasp an object with a small amount of pressure. If the object begins to slip,

the accelerometers detect the vibration and increase the pressure of the grasp.

All of the improvements suggested have had one central aim, to better mimic the

human hand, in shape, motion, sense of touch, and easy of use. The surest strategy to

achieving the development of the best possible hand mechanism is to strive to mimic the

best possible example, the human hand.

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