control of flexible pneumatic robot arm using...

CONTROL OF FLEXIBLE PNEUMATIC ROBOT ARM USING

MASTER DEVICE WITH PNEUMATIC BRAKE MECHANISM

Mohd ALIFF*, Shujiro DOHTA*, Tetsuya AKAGI and Takafumi MORIMOTO*

* Graduate School of Engineering, Okayama University of Science, 1-1, Ridai-cho, Kita-ku, Okayama 700-0005, Japan

(E-mail: [email protected])

ABSTRACT

In this paper, a novel structure for bilateral control in master-slave system is presented. From our previous study, it is

needed to apply the brake mechanism on the master arm and it perhaps can be a bilateral control by taking into account

the force factor in both master-slave arms. The compact and lightweight brake mechanism can avoid patients from

injury by limiting and stopping the movement of master arm when it moves in the dangerous area or angle during the

therapy. The control system consists of flexible pneumatic cylinders, potentiometers, quasi-servo valves, accelerometers

and a microcomputer. The proposed analytical model of the master-slave control will be explained and the control

performances of the brake system were investigated. The results from experiments show that the proposed model has a

potential for application on robot arm. Thus, this master-slave control with brake mechanism has a great potential for

application in rehabilitation field.

KEY WORDS

Pneumatic robot arm, Flexible pneumatic cylinder, Master-slave control, Rehabilitation device, Bilateral control

NOMENCLATURE

( 1 line space )

e : deviation of the cylinder length

KD : differential gain

KI : integral gain

KP : proportional gain

L : cylinder length (displacement)

r : distance from the center of the round stage

to the center of the slide stage in the

cylinder

R : radius of curvature of the cylinder

u : control input (duty ratio)

Vx , Vy , Vz : output differential voltages from the

accelerometer

α : bending direction angle from X-axis

β : bending angle from Y-axis

Subscripts:

m : master arm

s : slave arm

0 : central tube

1 , 2 , 3 : location number of the flexible cylinder

Copyright © 2014 JFPS. ISBN 4-931070-10-8

Proceedings of the 9th JFPS International Symposiumon Fluid Power, Matsue, 2014

Oct. 28 - 31, 2014

822

3C2-3

INTRODUCTION

One of the social problem in Japan is the percentage of

elderly population is continuously increasing while

birthrates continue its decline. The numbers continue to

increase year after year and are raising concerns of

professionals from a variety of fields including social

science, medical science and engineering. Nowadays,

many studies have shown that robots can be beneficial

to healthcare in a variety of ways from supporting in

lifting patients [1], to performing complex surgery [2].

Among the numerous robots designed to deliver arm

therapy, MIT-MANUS [3], ARM-GUIDE [4], and

MIME [5] are three representative devices that have

been tested extensively with patients. MIT-MANUS can

support patient in executing reaching movements in

horizontal plane. However, a better improvement in

shoulder strength and function needs arms to be trained

in vertical direction as well, so ARM-GUIDE and

MIME which can give training in a three-dimensional

workspace were developed. ARM-GUIDE that allows

the patient to exercise against gravity can be used as

diagnostic tool and a treatment tool for addressing arm

impairment. However, these robotic arms are heavy in

weight and must be fixed on walls and poles. This

causes the limited space in motion and patients are

easily to feel excess fatigue. Furthermore, these robots

are complex to set up by patients themselves and are not

suitable for rehabilitation training program at home [6].

The purpose of our study is to develop a flexible and

lightweight actuator and to apply it to a flexible robot

arm and rehabilitation device. Novel types of the

flexible pneumatic actuator that can be used even if the

actuators are deformed by the external forces have been

proposed and tested [7], [8]. We have also proposed and

tested the flexible robot arm with simple structure by

using the rod-less type flexible pneumatic cylinders [9].

This robot arm has three degrees-of-freedom that is

bending, extending and contracting. The master-slave

control is adopted into the robot arm as a control

method in order to be used in rehabilitation field. The

master-slave control is necessary when a physical

therapist wants to give a rehabilitation motion to a

patient. During rehabilitation, the therapist will control

the movement of the robot by holding the master arm

and move it repeatedly according to conditions and

needs of patient, and at the same time, the patient will

follow the movement by holding the slave arm.

In this paper, an analytical model of the master-slave

control will be explained and the control performances

of the brake system for a bilateral control were

investigated. The system consists of a flexible robot arm,

a microcomputer, accelerometers, small-sized

quasi-servo valves and potentiometers. The construction

of the robot arm is very lightweight and compact. The

size of the robot arm is Ø100 mm x 300 mm and the

mass is 380 g. From the viewpoint of safety,

human-friendly, precisely in attitude control, compact

construction and lightweight, this flexible mechanism

has a potential to be used in rehabilitation field.

FLEXIBLE PNEUMATIC CYLINDEFR

Flexible Pneumatic Cylinder The most extensive use of robotic technology for

medical applications has been in rehabilitation robotics,

which includes assistive robots and therapeutic robots.

The actuator to be used in rehabilitation robots has also

been studied. The actuator must be safe and does not

harm patients who use it during the therapy. The

proposed cylinder tube that is made from soft

polyurethane and is driven by pneumatic pressure is

suitable to be used in the robot arm. As the most parts of

the robot arm are air based, we can have less

complicated design and the robot can be made of

inexpensive material. These pneumatic actuators also

have long life and perform well with negligible

maintenance requirement throughout their life cycle.

Figure 1 shows the construction of a flexible pneumatic

cylinder [8]. It consists of a flexible tube as a cylinder

and gasket, one steel ball as a cylinder head and a slide

stage that can slide along the outside of the tube. The

steel ball is pinched by two pairs of brass rollers from

both sides of the ball. The operating principle of the

flexible pneumatic cylinder is as follows. When the

pressure is applied from one end of the flexible tube, the

9 mm steel ball which is in the middle of the slide stage

is pushed and moved accordingly. At the same time, the

steel ball pushes the brass rollers and then the slide

stage moves while it deforms the tube. We investigated

the minimum driving pressure of the tested flexible

pneumatic cylinder using various center distance D

(shown in Fig.1) and W between two pairs of rollers as

a design parameter. From the experiments, we found

that the distance D of 14.4 mm and the distance W of 10

mm were best [8]. In this combination between D and W,

the frictional force of the slide stage is small and the 9

mm steel ball does not get out from the slide stage.

Figure 1 Construction of flexible pneumatic cylinder

Copyright © 2014 JFPS. ISBN 4-931070-10-8 823

FLEXIBLE ROBOT ARM AND CONTROL

STSTEM

Flexible Robot Arm Figure 2 shows the construction of flexible robot arm.

The robot arm consists of two round stages: an upper

and a lower, three flexible pneumatic cylinders, an

accelerometer, a potentiometer and three slide stages.

The outer diameter of the upper and lower stage is 100

mm and the length of cylinder tube is 250. Each flexible

pneumatic cylinder is arranged so that the central angle

of two adjacent slide stages becomes 120 degrees on the

stage. An end of each flexible cylinder is fixed to the

upper stage. Two on/off control valves (Koganei Co.

Ltd., G010HE-1) or quasi-servo valves are used to drive

one flexible pneumatic cylinder. In order to control the

three flexible cylinders, six control valves are needed.

The basic operating principle of the flexible robot arm is

as follows. To make the arm extend or contract, pressure

must be applied on one end of the cylinder only. The

robot arm will extend or move upward if the pressure is

applied on the three upper ends of the cylinders. On the

other hand, the robot arm will contract or move

downward if the pressure is applied on the three bottom

ends of the cylinders. To perform a bending motion to

the right, for example, the right cylinder requires

pressure from both ends to limit movement while the

remaining two cylinders require pressure from the top to

enable movement. The robot arm will bend by

constricting one cylinder and extending the others. The

upper round stage moves in the range of 0 to 180 mm

with three cylinder tubes.

Figure 2 Construction of flexible robot arm

Analytical Model for Master-slave Control Figure 3 shows an analytical model for the master-slave

attitude control of the flexible robot arm. In this model,

we assume that the shape of each flexible pneumatic

cylinder becomes a circular arc when the arm bends. In

this figure, L, L1, L2 and L3 are the cylinder displacement

length for the central tube, cylinder 1, cylinder 2 and

cylinder 3 respectively.

Figure 3 Analytical model for attitude control

The bending direction angle α is the angle from X axis

and bending angle β is the angle from Y axis. From the

geometrical relationship shown in Fig.3, the desired

length (in a master arm) and the present length (in a

slave arm) of each cylinder L are calculated by the

following equations (1)-(4) by using the bending

direction angle α and the bending angle β.

iiii rRL cos1 (i=m,s) (1)

iiii rRL

)}3

2cos({2

(i=m,s) (2)

iiii rRL

)}3

4cos({3

(i=m,s) (3)

i

ii

LR

0 (i=m,s) (4)

Subscripts m and s indicate the desired (master arm) and

the present (slave arm), respectively. And subscript

number (1,2,3) indicates the location number of the

cylinder. R and r mean the radius of curvature of the

cylinder and the distance from the center of the round

stage to the center of the slide stage in the cylinder,

respectively. Radius r is 33mm. The bending direction

angle α, and the bending angle β are given by following

equations using the output voltage from the

accelerometer.

22

1cos

yixi

xii

VV

V

(i=m,s) (5)

)(cosmax

1

iz

zii

V

V (i=m,s) (6)

Where, Vx, Vy, and Vz mean the accelerometer output

differential voltage from the initial value. The voltages


Vx and Vy correspond to the angle from the horizontal

plane and Vz corresponds to the angle from the vertical

plane. Then, Vzmax means the difference of Vz between

the values in horizontal and vertical planes. By using

Eqs.(1)-(6), we can calculate the length of the three

cylinders for every bending state of the robot arm. In

our previous study [10], we confirmed that the

calculated angle agreed well with the measured value.

The error between both angles was less than 1 degree.

Master-Slave Control System We tried to construct the control system as compact and

inexpensive as possible. Figure 4 shows the schematic

diagram of the control system. The control system

consists of the slave arm, the master arm, six

quasi-servo valves and a high-speed SH/7125

microcomputer. The slave arm consists of two round

stages: an upper and a lower, three flexible pneumatic

cylinders, an accelerometer, a potentiometer and three

slide stages. The previous proposed master arm just had

an accelerometer sensor placed at the top [10]. The

attitude of the upper round stage of the slave arm is

detected with the accelerometer installed on atop of the

stage. The accelerometer can detect the bending angle

of the upper stage by measuring the change of gravity

for X,Y, and Z axis. From these values, the bending

direction angle α and the bending angle β are calculated

by using the Eq. (5) and Eq. (6).

Figure 4 Schematic diagram of control system

Figure 5 shows the block diagram of the master-slave

control system. The control is carried out as follows.

The microcomputer gets the output voltage from the

bending angles and calculates the desired length and the

present length of each flexible cylinder by using Eqs.

(1)-(4). The “length of the flexible cylinder” is defined

as a distance between the upper and the lower round

stage. By using this method, each length of the cylinder

can be controlled as a position control, and the

master-slave control can be realized.

Figure 5 Block diagram of the master-slave control

MASTER-SLAVE CONTROL USING BRAKE

MECHANISM

A bilateral system allows a human operator to perform

manipulation or sensing task in remote, hazardous or

confined environments. A bilateral system consists of a

slave robot and a master robot. The master is handled by

the human operator for controlling the slave robot and

sensing the slave environment contact forces. The

bilateral systems have many promising applications

especially in minimally invasive surgery and space and

underwater exploration, or hazardous environments

where human action is clearly restricted [12].

Commonly, the actuators used in most of bilateral

system are electrical direct-current motors. They are

easy to install, quiet and simple to control. However,

when gear-boxes are used to produce large actuation

torques, they may result in backlash and high inertia,

which are undesirable because they introduce

discontinuity and distortion in the force reflected to the

operator. In this study, we investigate the development

and control of pneumatic actuators in a bilateral system.

Compared to the electrical actuators, pneumatic

actuators have higher force-to-mass ratio and can

generate larger forces without the need for any

reduction mechanism such as a gear-box.

Pneumatic Brake Mechanism In this paper, a pneumatic brake mechanism is proposed

and tested to realize a simple bilateral control. Figure 6

shows the brake mechanism. It consist of supply port,

acrylic plate and silicon tube. This brake is very

lightweight, compact and safe to be used in robot arm

and rehabilitation field. For applying on the robot arm,

three brake mechanisms were developed and were

placed on the lower round stage of the master arm. The

operating principle of the brake mechanism is as

follows. When the pressure is applied to the supply port,

the silicon tube in the brake will inflate and at the same

time will fix the flexible pneumatic cylinder that is

placed in the middle of the brake. Transparency is the

principal performance goal in bilateral controller design.

It measures the quality of recreation of the mechanical


properties of the remote environment for human

operator. Abundant control theories have been proposed

to achieve high master-slave transparency when the

slave is free motion and or contact motion [13].

Figure 6 Brake mechanism

By using this brake mechanism, it can make the

transparency in master-slave control becomes better and

more acute by stopping the master arm when the

difference in attitude position between two arms are

high. Furthermore, the pneumatic brake will limit and

prevent the continuation of such dangerous motion to an

area or angle that can be harmful to a patient who holds

the slave arm. Moreover, the brake also can stop the

master arm from moving when there is an external force

that suddenly hit on a master arm during the therapy.

These all features will be discussed in detail at

experimental results session. By applying this brake

mechanism on our robot arm, perhaps it can be a simple

bilateral control by taking into account the force factor

in both master and slave arms.

Master Device with Pneumatic Brake Mechanism Figure 7 and Figure 8 shows the construction and

overview of the master arm with pneumatic brake

mechanism, respectively. The master arm consists of an

accelerometer sensor set on the top of the upper stage,

two round stages: an upper and a lower, three flexible

pneumatic cylinders, three brakes mechanism, a central

tube, three on/off control valves and a potentiometer.

The diameter of the upper and lower round stage is 100

mm. The upper round stage can moves in the range of 0

mm to 180 mm while the lower stage is fixed. The

previous proposed master arm is more compact and

lightweight because it has only an accelerometer on

atop to give the reference attitude value. However, the

structure and features are still not enough to be used as

rehabilitation tool. The previous master arm can support

the movement on the horizontal direction only. But the

rehabilitation in wrist needs arms to be trained in

vertical direction as well, so the new master arm which

can give training in three-dimensional workspace is

developed and proposed in this paper. Furthermore, the

previous master arm does not have the bilateral concept

and cannot sense the slave environment and contact

forces. It is feared when the arm is used in therapy, the

therapist cannot feel the resistance force of the slave

arm, and will continue until the movement will

endanger the patient. Thus, despite the previous master

arm can realize the attitude control of the robot arm

very well, inadequate structure and features cause the

concern on the safety of patients when it is used in

therapy.

Figure 7 Construction of master arm with brake system

Figure 8 Overview of master arm with brake system

Control Results Figure 9(a) shows a view of master-slave control with

brake mechanism. The left arm is the master arm with

three pneumatic brakes mechanism installed on it and

the right arm is the slave arm. During rehabilitation,

human hand is put on the slave arm as shown in Fig.

9(b). For the master-slave control method the following

PID control scheme was applied.


dt

deKdteKeKu i

DiIipi (i=1,2,3) (7)

isimi LLe (i=1,2,3) (8)

Where, ui means the control input and ei is the deviation

of each cylinder length. KP (=8.2 [%/mm]), KI (=0.0049

[%/mm]) and KD (=38.5 [%/mm]) are the proportional

gain, the integral gain and the differential gain,

respectively. These control parameters are adjusted

based on the ultimate sensitivity method. Using the

lower duty ratio as a control input to the PWM valve,

there are exists a dead zone for output flow rate of the

valve. Therefore, the input duty ratio of the PWM valve

is always added by 22.5% to the absolute value of the

control input which is calculated by Eq. (7).

Furthermore, the state of the switching valve is decided

by the sign (positive or negative) of the control input.

Since this control scheme is embedded into the

microcomputer, we can use the valve like a typical

servo valve without complex operations.

(a) Overview (b) Rehabilitation

Figure 9 Master-slave control with brake mechanism

Figure 10 shows the control results of the cylinder 1

with brake mechanism. The blue line is the length of the

cylinder 1 of the master arm, the red line is the length of

cylinder 1 of the slave arm, the purple line is the output

voltage of the brake mechanism and the orange line is

the deviation length of the cylinder 1. The pneumatic

brake mechanism will start working when the deviation

length of the two arms reaches at some value. The

maximum deviation value for the pneumatic brake

mechanism start working had been investigated at 5 mm,

10 mm, 15 mm and 20 mm as shown in Fig. 10. The

result shows that the 5 mm, 10 mm and 15 mm of the

deviation make the brakes always work. From these

experimental results, we can conclude that, when the

maximum deviation value is becomes smaller, the

chance of the pneumatic brake mechanism will work

becomes higher. This probability is very important to

the movement of the flexible cylinder because when the

brake is always braking, the flexible cylinder cannot

move smoothly. The friction between the flexible

tube and the brake also makes it more difficult to

move. From these experiments also, we can conclude

that the best maximum deviation value for the brake

mechanism is 20 mm because this value is just right

for the brake mechanism to work besides the

pneumatic cylinder can move naturally. Figure 11

shows the master-slave control results. During the

experiment, the master arm is operated by a human

hand to be bent, extended and contracted. The control

sampling period is 2.3ms, and the PWM period of the

quasi-servo valve is 10ms.

Figure 10 Control results of the cylinder 1 with

pneumatic brake mechanism for four kinds of

maximum deviation value; 5, 10, 15, 20 mm

Furthermore, in order to be used in aged society or

disabled persons, the robots needs to be simple to set up

by patients themselves. The robot also needs to be

compact and lightweight in order to be used for

rehabilitation training program at home. Therefore, we

try to constructs the simple, compact and lightweight

bilateral control system and apply it on our robot arm.

In these figures, the blue lines are the target values of

the virtual master flexible cylinder, the red lines are the

length of the slave arm which is calculated by using

high-speed microcomputer, the purple lines are the

voltage values of the brake mechanism and the orange

lines are the deviation length between the master and

the slave arm.


Figure 11 Master-slave control results

In this experiment, we set the maximum deviation value

for brake mechanism to work to 20 mm. Therefore, in

these figures we can see that when the deviation length

between the master and the slave arm up to 20 mm, the

pneumatic brake mechanism which is driven by on/off

valve will start working and braking each flexible

cylinder L1, L2 and L3 of the master arm. This can give

safety to the patients by stopping the therapist from

continuing to move when the slave arm cannot trace the

position of master arm because of error or others. Brake

mechanisms will also prevent the master arm from

continuing to move to the area or angle that will harm to

the patient's wrist. These brakes also will help that the

master-slave control becomes more accurate and

transparent because the brake will stop the master arm

from moving when the deviation between two arms

becomes high. Then, when the deviation between the

two arms become smaller, where the slave arm came to

follow the movement of the master and have the same

shape and angle like the master arm, the brake is

released and the master arm will be able to work as

normal movement. Figure 12 shows the external force

experiments on the slave arm. During the experiment,

we impose the large external force directly to the upper

round stage of the slave arm to investigate how the

pneumatic brake mechanism will respond. From Fig. 12,

we can see that there is a sudden change in the red line

of the cylinders 1, 2 and 3. This is caused by the

external forces applied on the slave arm. Based on these

changes, the deviation length also changes and when it

reaches 20 mm, the pneumatic brake mechanism which

is installed on the master arm begins to move to stop the

master arm from moving.

Figure 12 External force experiments on the slave arm

As a conclusion, the installed pneumatic brake

mechanism on the master arm not only can stop the

master arm if the external force is imposed on it, but

also can prevent the master arm from moving if the

external force suddenly hit the slave arm during the

therapy session. By using this brake mechanism on the

rehabilitation field especially on our robot arm, it can

prevent and stop the therapist from continuing to move

when there is a frictional force that exists in the slave

arm. And with the ability to take into account the

external forces on slave arm, we trust that this brake

mechanism can be a simple bilateral control in

rehabilitation field. From Fig.11 also we can find that

the slave arm can detect well the position of the master

arm though sometimes large deviation occurs during the

experiments. These deviation problems can be avoided

by using the proposed pneumatic brake mechanism by

setting the deviation value at 20 mm. Thereby, we can

confirm that the effectiveness of the attitude control can

be achieved by using the proposed master arm with

pneumatic brake mechanism and proposed analytical

model. From these experimental results also we can

confirm that the proposed pneumatic brake mechanism

is working well on our master arm. This proposed

pneumatic brake can provide safety to the patient and at


the same time can make our master-slave control

becomes more accurate. By the features such as safety,

compact, lightweight, precise control and able to take

into account the force factor in both master and slave

arms, the effectiveness of proposed pneumatic brake

mechanism is confirmed and can be used as a simple

bilateral control in the rehabilitation field.

CONCLUSIONS

In this paper, the pneumatic actuators with the flexible

cylinder and microcontroller are chosen for the

development of the robot arm. The study focusing on

the control of flexible pneumatic robot arm using master

device with pneumatic brake mechanism can be

summarized as follows.

1) The compact, lightweight and inexpensive control

system of the flexible robot arm for master-slave control

was briefly explained. The analytical model of the

master-slave control was also discussed. The system

consists of the microcomputer, compact and

inexpensive quasi-servo valves, accelerometers and the

tested robot arm.

2) The pneumatic brake mechanism which is

lightweight, compact and simple structure was proposed

and developed. The size is about 40 mm x 40 mm and it

is very suitable to be used in our robot arm. The

experimental results show that the best deviation length

value for the pneumatic brake to work is 20 mm by

taking into account the friction and the movement of the

master arm. This brake mechanism is able to realize the

simple bilateral control of the robot arm.

3) We also had proposed and tested the master arm with

three pneumatic brake mechanisms installed on it. The

proposed master arm is important in order to give

training in three-dimensional space and for the

realization of the simple bilateral system. The result

shows that the proposed master arm can control well the

position of the robot arm. Furthermore, by taking into

account the force factor on the slave arm, the proposed

master arm with pneumatic brake mechanism can be a

simple bilateral control of the robot arm.

ACKNOWLEDGMENTS

Finally, we would like to acknowledge that a part of this

research was supported by a research-aid fund from the

Ministry of Education, Culture, Sports, Science and

Technology of Japan through a Financial Assistance

Program for QOL Innovative Research (2012-). We

wish to express our gratitude to the Ministry for their

contribution.

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