research article research of a new 6-dof force ... - hindawi

Research ArticleResearch of a New 6-Dof Force Feedback HandController System

Xin Gao, Yifan Wang, Jingzhou Song, Qingxuan Jia, and Hanxu Sun

School of Automation, Beijing University of Posts and Telecommunications, Beijing 100876, China

Correspondence should be addressed to Xin Gao; [email protected]

Received 14 November 2013; Revised 29 January 2014; Accepted 6 February 2014; Published 16 April 2014

Academic Editor: Yangmin Li

Copyright © 2014 Xin Gao et al.This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

The field of teleoperation with force telepresence has expanded its scope to include manipulation at different scales and in virtualworlds, and the key component of which is force feedback hand controller. This paper presents a novel force feedback handcontroller system, including a 3-dof translational and 3-dof rotational hand controllers, respectively, to implement position andposture teleoperation of the robot end effector. The 3-dof translational hand controller adopts innovative three-axes decouplingstructure based on the linear motor; the 3-dof rotational hand controller adopts serial mechanism based on three-axes intersectingat one point, improving its overall stiffness. Based on the kinematics, statics, and dynamics analyses for two platforms separately, thesystem applies big closed-loop force control method based on the zero force/torque, improving the feedback force/torque accuracyeffectively. Experimental results show that self-developed 6-dof force feedback hand controller has good mechanical properties.The translational hand controller has the following advantages: simple kinematics solver, fast dynamic response, and better than0.05mm accuracy of three-axis end positioning, while the advantages of the rotational hand controller are wide turning space,larger than 1Nm feedback, greater than 180 degrees of operating space of three axes, respectively, and high operation precision.

1. Introduction

At present, the robot systems are widely used in hazardousoperations, disaster, nuclear environment, deep sea, space,and other areas, which can replace humans in dangerous andextreme conditions to complete the tasks [1, 2]. When robotsare working in special environment, the task usually requireshigh operating precision [3–5].

Due to the combination of human decision-makingcapacity and operational capability of robots in hazardousenvironment, teleoperation can complete complex and spe-cial tasks while making people away from the site [6, 7]. As animportant interface used to build a close dynamic couplingbetween operators and robots, hand controller can sendposition, posture, and other information to the operationobject. It can also accept the environmental informationfrom control system, such as the force/torque, providing theoperator with force telepresence and implementing effectiveinterventions and control for robots [8]. In the teleoperationsystem, hand controller’s performance has a direct impact

on the operating performance and reliability of the bilateralcontrol system [9, 10].

According to the structure form, hand controller isdivided into serial type, parallel type, and compound type[11]. Serial mechanism has obvious drawbacks. Firstly, theworkspace of serial mechanism contains multiple singular-ities where the transmission performance will deterioratequickly. Second, because of the cantilever structure, and inorder to get structural stiffness, serial mechanism makessystem structure cumbersome and increases the quality ofinstitutions and the moment of inertia. The third disadvan-tage is that it has low stiffness which is bad for effectiveforce feedback. The features of parallel mechanism are thattranslation and rotation can be decoupled which is helpfulto improve the motion precision, and the rigidity of insti-tutions gives rise to strong bearing capacity. But because ofthe complex structure, parallel mechanism not only bringsdifficulties to design the system but also results in the frictionof motion pairs increasing. Meanwhile, the friction which isoften difficult to eliminate in design cannot be compensated

Hindawi Publishing CorporationJournal of RoboticsVolume 2014, Article ID 646574, 9 pageshttp://dx.doi.org/10.1155/2014/646574

2 Journal of Robotics

by driving elements easily after design. Moreover, parallelmechanismhas littlemotion space. Compoundmechanism isthe complementation of serial mechanism and parallel mech-anism. But due to the difficult design and implementation,compound mechanism has not been widely used.

At first, hand controller is often designed for a specificapplication [12], which takes researchers much time to designdevices, so modern hand controllers pay more attention togeneralization. For the uncertainty of the task, mechanismsof the generic hand controller are much different from thoseof the slave, which also makes the manipulator controlcomplicated. It is difficult to model accurately for operator,dynamics of force feedback main hand and the environment,so current studies for the control problems of force sensinginterface equipment mainly focus on ensuring the stabilityof the system. Developing simple and reliable controller withhigher stability and transparency is still the key of masterhand control.

This paper presents a novel force feedback hand con-troller. The 6-dof hand controller can be divided into a 3-doforthogonal parallel translational platform and a 3-dof serialrotational platform.The former has large workspace and highstiffness, and the latter takes full account of the comfort whileoperating, the complexity of control algorithm, etc.The papermakes kinematics, statics, and dynamics analyses on the twoplatforms, respectively. On this basis, using servomotors andforce/torque sensors produce the detection/drive module.What is more, a big force closed-loop control method basedon the force sensor and a torque real time control methodbased on the speed adjustment are presented. In the end, theeffectiveness of the control method is verified by experimentresults which show that 6-dof force feedback hand controllerobtaining desirable force telepresence has good mechanicalproperties.

2. The Whole Scheme Design ofHand Controller

For meeting the basic requirements of the movement, handcontroller system should have 6 degrees of freedom [13].Meanwhile, in order to reduce difficulties of the institutionsdesign and control, this paper adopts the method that trans-lational and rotational degrees of freedom are decoupled,namely, to design a separate structure for 3 translationaland 3 rotational degrees of freedom and to be controlled bythe same PC control program in different threads (shown,respectively, in Figures 1 and 2).

Translational platform is a 3-dof orthogonal mechanism,which is composed of the base and three branched chains.Drive components are three linear motors installed on theorthogonal bases separately and detection units are threegrating rulers matched with linear motor platforms. Threemotors are pairwise orthogonal and each branched chainconsists of a sliding pair, three revolute pairs, and twomember bars. The axis of sliding pair is parallel with that ofrevolute pairs. This mechanism has the advantages of simplestructure, decoupling branched chains, no singularity inoperating space, simple kinematic analysis, and easy control.

XY

Z

Terminalposition

Figure 1: Picture of translational platform under SolidWorks.

Handle 1Baseline 1

Rotation 1

Handle 2Baseline 2

Rotation 2

Baseline 3

Rotation 3

Figure 2: Picture of rotational platform under SolidWorks.

Rotational platform is a 3-dof serial mechanism whichis composed of the base and three branched chains. Thehandle is installed in the center of an edge that is a part ofthe end of the branched chains. Drive components are threemotors which are in the coaxial direction with the revolutepairs separately and detection units are encoders installedcoaxially with the rotating direction of the branched chains.Three serial axes of the revolute pairs intersect at one pointvertically.The structure is simple and can control the rotationangle through large angle. Each branched chain is directlyinstalled with an encoder, so the clearance generated afterusing the planetary gear reducer can be eliminated and therotation angle can also be measured accurately.

In order to make the working range of hand controllercover that of hands as far as possible, the workspace oftranslational platform is designed to work between ±280mmand ±320mm. In theory rotational platform is able toachieve± 180∘ rotation with any one of the three degreesof freedom. The grating ruler and encoder should detectthe change of hand’s position and posture in real time. Themeasurement accuracy, in general, is higher than the control

Journal of Robotics 3

accuracy. But for hand controller system, as the result ofdetection is the change of hand’s position and posture, itis satisfactory that the measurement accuracy adapts tothe kinematic accuracy of hands sufficiently; namely, themillimeter is all right. Therefore, the detection accuracy of a3-dof translation is in the millimeter level and that of a 3-dofrotation can be ±0.1∘. The scope of force feedback is a veryimportant parameter because it directly affects the feeling ofoperators and the judgment on the slave environment. Thehand controller system adopts the force closed-loop controlmethod that makes the institutions to follow the movementof hands smoothly, so operators will not receive obvious forcefeelings in no force feedback stage. Finally, the force andtorque feedback are limited within the scope that hands haveobvious feelings and the long time operation will not producefatigue.

3. The Kinematics and Statics Analysis ofHand Controller

3.1. Translational Platform. The translational platform drivecomponents: linear motor is mounted on a base with whichthe𝑋,𝑌, 𝑍 three-axis coincide, and therefore sliding pairs areused as the mechanism drive. In the following, this paper willmake kinematic analysis about sliding pairs.

First, the positive and inverse kinematic solutions of thetranslational platform are calculated. Because of the exactlysame situation of the three-branched chains, this paper willanalyze the branched chain on the 𝑋 axis. The positivekinematic solution can be described as follows: knowingthat the linear motor displacement is 𝑆(𝑢, V, 𝑤), find thecoordinates of point 𝑃(𝑃

𝑥, 𝑃𝑦, 𝑃𝑧), which is the end of the

hand controller. As 𝑥𝑖, 𝑦𝑖, 𝑧𝑖(𝑖 = 1, 2, 3) are revolute pairs and

they parallel each other, 𝑥𝑖(𝑖 = 1, 2, 3) plane parallels 𝑌𝑂𝑍

plane. Therefore, it is obvious that 𝑃𝑥= 𝑢, 𝑃

𝑦= V, 𝑃

𝑧= 𝑤

which can be expressed as a vector form:

𝑃 = 𝑆. (1)

In the above equation, 𝑃 = [𝑃𝑥, 𝑃𝑦, 𝑃𝑧]𝑇

, 𝑆 = [𝑢, V, 𝑤]𝑇.

It is not necessary to analyze the position inverse solution ofthe mechanism because it resembles to the positive one.

As for the speed analysis of the translational platform, itis important to know the relationship between its Jacobianmatrix and the speed of the end and the drive. With thedifferential equations, the speed relationship between themotion platform and the linear motor block is deduced:

[

[

�̇�

V̇�̇�

]

]

= 𝐽[

[

�̇�𝑥

�̇�𝑦

�̇�𝑧

]

]

. (2)

In this equation, Jacobian matrix 𝐽 is a 3 × 3 unit matrix.When applying the speed 𝑉 on the handle, it is dividedinto 𝑉

𝑝𝑥, 𝑉𝑝𝑦, 𝑉𝑝𝑧

in 𝑋,𝑌, 𝑍 three directions. And then whentransporting the speed to the linear motor, the requiredspeeds are 𝑉

𝑝𝑥, 𝑉𝑝𝑦, 𝑉𝑝𝑧.

The last part is the statics analysis. Through the staticsanalysis, there is an overview of the mechanism force and

the capacity against load. Using the virtual work principle canelicit the relationship between the driving force𝑓

𝑖(𝑖 = 1, 2, 3)

of the sliding pairs and the generalized force vector𝑓of theend 𝑃 of the handle. Assuming that the virtual displacementvector of each linear motor block on 𝑋,𝑌, 𝑍 axis is 𝛿𝑆

1,

and the virtual displacement vector of the correspondingplatform is 𝛿𝑃, the virtual work of the sliding pairs and theend of the motion platform will be

𝑤 = 𝑓𝑇

𝑖

⋅ 𝛿𝑆1,

𝑤 = 𝑓𝑇

⋅ 𝛿𝑃.

(3)

According to the principle of virtual work, the virtualwork, which is produced by the virtual displacement of thesliding pairs caused by the output force of the drive parts, isequal to the one produced by the bottom motion platformvirtual displacement; namely,

𝑓𝑇

𝑖

⋅ 𝛿𝑆1= 𝑓𝑇

⋅ 𝛿𝑃. (4)

Through the kinematic analysis, there is a one-to-onerelationship between the displacements of the linear motorslider and the bottom motion platform:

𝑃 = 𝑆. (5)

So the statics equation of translational platform is asfollows:

[

[

𝑓𝑥

𝑓𝑦

𝑓𝑧

]

]

= 𝐽𝑇[

[

𝑓1

𝑓2

𝑓3

]

]

𝐽 = [

[

1

1

1

]

]

. (6)

The left of (6) is the output force vector of the movementplatform, while the right is the linear driving force vector.Theoutput and input forces in three directions are correspondingto each other, because the Jacobian matrix is the unit vector.

3.2. Rotational Platform Analysis. Rotational platformbelongs to a 3-dof serial mechanism, so it just provides theposture information of the slave robot. Meanwhile, due tothe decoupling design, the translational platform has noinfluence on the end posture of the slave robot, only theposture information of the rotational mechanism on thestatic platform is required. The positive solution is providedby theD-Hmethod. Set up the basic and the joints coordinatesystem of the serial mechanism just like Figure 3, assumingthat the three-rotary axis is orthogonal at the initial positionfor convenience.

A data table, as shown in Table 1, is established accordingto the D-H method.

Using the parameters shown in Table 1 can get thecoordinate transformation matrix:𝑖−1

𝑖𝑇

=

[[[

[

cos 𝜃𝑖

− sin 𝜃𝑖

0 𝑎𝑖−1

sin 𝜃𝑖cos𝛼𝑖−1

cos 𝜃𝑖cos𝛼𝑖−1

− sin𝛼𝑖−1

𝑑𝑖sin𝛼𝑖−1

sin 𝜃𝑖sin𝛼𝑖−1

cos 𝜃𝑖sin𝛼𝑖−1

cos𝛼𝑖−1

𝑑𝑖cos𝛼𝑖−1

0 0 0 1

]]]

]

.

(7)


1

2

3

X0

X1

X2X3

L1

L2

L3

L4

L5

L6

L7

Z0

Z1

Z2

Z3Y1

Y2

Y3Fy

Fy

Fz

Figure 3: Three-axis intersection serial mechanism.

Therefore,

0

1𝑇 =

[[[

[

cos 𝜃1

− sin 𝜃1

0 0

sin 𝜃1

cos 𝜃1

0 0

0 0 1 𝑑1

0 0 0 1

]]]

]

1

2𝑇 =

[[[

[

cos 𝜃2

− sin 𝜃2

0 0

0 0 −1 0

sin 𝜃2

cos 𝜃2

0 0

0 0 0 1

]]]

]

2

3𝑇 =

[[[

[

cos 𝜃3

− sin 𝜃3

0 0

0 0 −1 0

sin 𝜃3

cos 𝜃3

0 0

0 0 0 1

]]]

]

.

(8)

It is only necessary to analyze the inverse kinematics solu-tion of the rotational platform tomeet actual requirements, as(7) reveals

0

3𝑇 =

[[[

[

𝑛𝑥

𝑜𝑥

𝑎𝑥

𝑝𝑥

𝑛𝑦

𝑜𝑦

𝑎𝑦

𝑝𝑦

𝑛𝑧

𝑜𝑧

𝑎𝑧

𝑝𝑧

0 0 0 1

]]]

]

. (9)

In addition,

0

1

𝑇−1

⋅0

3𝑇 =1

2𝑇2

3

𝑇. (10)

Therefore,

[[[

[

𝑛𝑥𝑐1+ 𝑛𝑦𝑠1

𝑜𝑥𝑐1+ 𝑜𝑦𝑠1

𝑎𝑥𝑐1+ 𝑎𝑦𝑠1

𝑝𝑥𝑐1+ 𝑝𝑦𝑠1

𝑛𝑦𝑐1− 𝑛𝑥𝑠1

𝑜𝑦𝑐1− 𝑜𝑥𝑠1

𝑎𝑦𝑐1− 𝑎𝑥𝑠1

𝑝𝑦𝑐1− 𝑝𝑥𝑠1

𝑛𝑧

𝑜𝑧

𝑎𝑧

−𝑑1+ 𝑝𝑧

0 0 0 1

]]]

]

=

[[[

[

𝑐2𝑐3

−𝑐2𝑠3

𝑠2

0

−𝑠3

−𝑐3

0 0

𝑠2𝑐3

−𝑠2𝑠3

−𝑐2

0

0 0 0 1

]]]

]

.

(11)

Table 1: D-H parameters.

Member bars 𝑎𝑖−1

𝛼𝑖−1

𝑑𝑖

𝜃𝑖

1 0 0 𝑑1

02 0 90 0 903 0 90 0 0

Let (1, 2)2 + (3, 3)2 on the two sides of the equation be equal:

(𝑎𝑥𝑐1+ 𝑎𝑦𝑠1)

2

+ 𝑎2

𝑧

= 𝑠2

2

+ 𝑐2

2

= 1. (12)

Then,

𝑎𝑥𝑐1+ 𝑎𝑦𝑠1= ±√1 − 𝑎

2

𝑧

= 𝑡. (13)

It is known that 𝑎𝑥𝑐1+ 𝑎𝑦𝑠1= 𝑡. Solve the equation with

the universal formula, and make these equations set up:

𝑢 = tan 𝜃

2

; cos 𝜃 =1 − 𝑢2

1 + 𝑢2

; sin 𝜃 =2𝑢

1 + 𝑢2

. (14)

Bring it back to (12) and get this quadratic equation withone unknown 𝑢:

(𝑡 + 𝑎𝑥) 𝑢2

− 2𝑎𝑦𝑢 + 𝑡 − 𝑎

𝑥= 0,

𝑢 =

2𝑎𝑦± √4𝑎

𝑦

2

− 4 (𝑡2

− 𝑎𝑥

2

)

2 (𝑡 + 𝑎𝑥)

=

𝑎𝑦± √𝑎2

𝑥

+ 𝑎2

𝑦

− 𝑡2

𝑡 + 𝑎𝑥

.

(15)

So far there are four groups of solutions of 𝜃1; then 𝜃

2and

𝜃3should be worked out.In terms of the statics analysis of the rotational platform,

assuming that the output torques of the motors on the three-rotary directions are 𝜏

𝑖(1, 2, 3) and the microangle displace-

ments are 𝛿𝜃𝑖(1, 2, 3) while the output torques of the bottom

handle are 𝑇𝑖(1, 2, 3) and the microangle displacements are

𝛿𝜓𝑖(1, 2, 3), and using the virtual work principle can get the

following relationship:

𝜏𝑖⋅ 𝛿𝜃𝑖= 𝑇𝑖⋅ 𝛿𝜓𝑖. (16)

According to the kinematics, there is

𝜃𝑖= 𝜓𝑖. (17)

The statics equation of the rotational platform will be

[

[

𝑇1

𝑇2

𝑇3

]

]

= 𝐼[

[

𝜏1

𝜏2

𝜏3

]

]

𝐼 = [

[

1

1

1

]

]

. (18)

4. Design of Control System

4.1. Master-Slave Bilateral Control. Teleoperation is animportant human-machine interaction in the robotoperation, and transparency is the characteristic thatideal teleoperation system should have, which can makeoperators feel the real influence of the environment [14].At present, location-location type, force-location type, and


PIDcontroller Drive Drive

elements Platformoutput

Force/torquesensor

+

−

Fh(𝜏h)

E Δ ̇X(Δ ̇𝜃)F𝜀(𝜏𝜀)

Figure 4: Force closed-loop control structure.

force feedback-location type are three kinds of controlstructure used in the teleoperation [15]. Wherein, the forcefeedback-location type can produce better effect of forcetelepresence; accordingly this paper designs the hardwarestructure of hand controller as follows: without consideringthe slave, the force sensor installed at the end of the handlecan detect the actual force situation of operators and thenadjust the stress to achieve the best effect of force feedbackthrough the control system in real time.

4.2. Force Closed-Loop Control Strategy. Basic control meth-ods based on force control can be divided into the impedancecontrol, hybrid control, and explicit force control accordingto the relationship between the position, velocity, and forcecontrol variables [16–18]. In the above categories, the suitablestrategies for hand controller system in this paper are theimpedance control, admittance control, and explicit forcecontrol based on location. Comparing the robot system withthe force feedback hand controller system, it is obvious thatthe expected stress of the end in robot system is the sameas the force feedback in bilateral control of hand controllersystem.However, the force control of robot system is radicallydifferent from that of hand controller system. In robot system,the force signal of the end detected should be converted intospeed adjustment through the admittance model or positionadjustment through the force controller, and the controlsystem must have an integral speed loop or position loop,namely, the desired speed �̇�

𝐷or desired position𝑋

𝐷, on the

basis of which the adjustment amount could be regulated.Hand controller system is operated by operators who controlthe motion speed and the stop position of institutions, so itcannot be adjusted by the desired speed or position.

Considering the above problems and planning to inte-grate the teleoperation and bilateral control into one controlalgorithm, this paper puts forward a kind of force closed-loopcontrol method based on the force sensor.The core idea aimsto adjust the output of drive parts in real time and get thedesired output of the system according to the actual stress ofhands detected by the force sensor. Control structure diagramis as shown in Figure 4.

𝐹𝜀(𝜏𝜀) is the interactive force/torque measured or calcu-

lated when the slave robot or objects in virtual environmentcontact with the environment; 𝐹

ℎ(𝜏ℎ) is the actual stress of

hands detected by the force sensor. In the teleoperation stage𝐹𝜀(𝜏𝜀) = 0, the target output of system is 0N (Nm). If

hand controller can follow the movement of hands precisely,the value detected by the force sensor should be 0 as far

as possible; in the bilateral stage, 𝐹𝜀(𝜏𝜀) has a specific value

which is the force/torque that operators can actually feel.With this value as the desired output of system, the controllercan adjust the output of drive parts in real time to make theforce/torque detected by the force sensor remain in the sizeof 𝐹𝜀(𝜏𝜀).

Getting the difference 𝐸 = Δ𝐹(Δ𝜏) between the desiredvalue 𝐹

𝜀(𝜏𝜀) and the actual value 𝐹

ℎ(𝜏ℎ), speed variation

Δ�̇�(Δ ̇𝜃) could be acquired after Δ𝐹(Δ𝜏) is calculated by thePID arithmetic. The control law is shown as follows:

Δ ̇𝜃 = 𝑃 ⋅ Δ𝜏 + 𝐼 ⋅ ∫Δ𝜏 𝑑𝑡 + 𝐷 ⋅𝑑Δ𝜏

𝑑𝑡

. (19)

The reason for using the PID controller is that it does notneed to establish the corresponding model for translationalplatform and rotational platform. If using the impedancecontrol, it is necessary to confirm the inertia coefficient,damping coefficient, and stiffness coefficient, which is noteasy to obtain accurately and will increase the difficulty ofdesign and the complexity of system. PID control, however,because of its simple calculation and a good response speedguaranteed by all hardware devices, can achieve good controleffect after using a certain method to adjust and select thethree parameters of 𝑃, 𝐼, and𝐷.

After the PID controller calculates the speed variationΔ�̇�(Δ ̇𝜃), this speed value according to the required commandformat should be sent to the Pmac motion control cardwhich can parse the command and send analog signals todrive parts; after receiving the analog signals, drive partswill drive motors to achieve the speed control according tothe relationship between the voltage and speed collated inadvance. The part inside the dashed line in Figure 4 showsthat the driver and drive components (linear motors androtary motors) will be treated as a whole. After receivingthe desired speed value, the driver will complete the speedcontrol of motors on the basis of the parameters adjustedautomatically.

5. The Implementation of HandController System

5.1. The Hardware System Composition. The hardware ofhand controller system consists of the controller, detectioncomponents, and drive components. Wherein the controllerincludes IPC and Pmac motion control card; detectioncomponents include ATI six-dimensional force/torque sen-sors, grating rulers, and rotary encoders; drive componentsinclude Parker linear motor and Maxon rotary motors. 3-dof translational platform and 3-dof rotational platform areshown in Figure 5.

The hardware composition and relationship of the wholesystem are shown in Figure 6.

5.2. Principal Computer Control Software. This paper selectsWindows XP as the development platform because all thehardware devices including the force sensor, data acquisitioncard, Elmo driver, and Pmac motion control card have gooddrive support and development support for this operating


(a) (b)

Figure 5: Translational platform and rotational platform.

Motion control card

A/D data acquisition card

Linear motor grating ruler

Six-dimensional measurement

Rotary motor encoder

Motor driver Motor

Position

Assisted verification system

Force

Position

Force

IPC

Figure 6: System composition schematic diagram.

system, which will reduce unnecessary compatibility prob-lems. Meanwhile, Visual Studio 2010 and C++ are selectedto develop the control software. The workflow of the controlprogram is as follows.

(1) Hardware initialization: before starting, hand con-troller system should initialize the hardware, such as Pmacmotion control card and the force sensor. After successful ini-tialization, hand controller system waits for the teleoperationstage.

(2) Teleoperation stage: operators control the handleand the force/torque value collected should be sent to thecontrol program which will invoke the matrix calculationfunction to complete the calculation of force/torque value;the output of motors that drives the hand controller systemto follow the movement of hands can be calculated throughtranslational and rotational control algorithm; meanwhile,

communication thread will send the position and posturevariation that is collected and calculated to the slave.

(3) Bilateral control stage: when there is force/torquefeedback information from the slave back to the master,translational control thread and rotational control thread willinvoke the associated control algorithm to adjust the outputof motors and make operators feel the force feedback effect.

The flow chart of control software is shown in Figure 7.After the control program works, there will be three threadscalculating simultaneously in the background, which aretranslational platform control thread, rotational platformcontrol thread, and communication thread. The first twoare mainly responsible for completing the algorithm duringteleoperation stage and bilateral stage, calculating the outputof motors in real time, and controlling motors through thecommunication with Pmac card; the last one reads the data


MFC program start

P-mac initialization

sensor initialization

Click on connect button

and start operation

Acquire position

information from P-mac card

Receive

information from

Send position information to According to

agency model decompose

moves due to received position,if contacting with

environment,calculate the force

and return it to master

Drive motor to output

corresponding

through instructions from

P-mac card

force/torque

force/torque

force/torque

slave/virtual end

slave/virtual end

Slave/virtual end

6-d force/torque

Figure 7: PC control software flow chart.

of the grating ruler and the rotary encoder from the Pmaccard and then give it to the principal computer, calculate theposition and posture variation of hand controller, and controlthe data transmission between the master and the slave.

5.3. Experiments and Results. The precision of hand con-troller is very important to the robot teleoperation system;therefore, to design a high precision hand controller andcalibrate the precision of the main hand through scientificexperiments have a positive meaning. Precision calibrationcan improve the geometric accuracy of institutions effectivelyand have deep influence on the accuracy of the master-slaveoperation. In this paper, using laser tracking to detect theposition precision of hand controller, and the experimentdata, is shown in Table 2.

The force closed-loop control algorithm is tested onthe translational platform. Firstly, the performance in theteleoperation stage is tested. Because the desired force valueof the outer ring is 0N in the stage, when operators controlthe hand controller to move, the value measured by the forcesensor should also remain close to 0N.The experiment resultis shown in Figure 8.

Time

0.05

0.03

0.01

−0.01

−0.03

−0.05

(N)

Figure 8: The movement in teleoperation stage.

0.00

5.00

10.00

15.00

Time

−5.00

−10.00

−15.00

Measured value of force sensorOutput value of motor

Figure 9: Low-speed reverse.

In Figure 8, the coarse curve represents the real forceinformation measured by the force sensor, and the fine curverepresents the desired value. When operators control thehand controller, the control algorithm will drive institutionsto conform to themovement of hands according to themotorvelocity calculated by the deviation between desired valueandmeasured value, which can ensure that the value detectedby the force sensor remains at around 0N. As can be seenfrom Figure 8, ignoring the jitter of hands while operating,the feeling force of hands maintains at around 0N, while thefluctuating error is within the scope of 0.01N. So operatorscan control the hand controller easily and smoothly.

Operators may often change the operation direction, andwhether the control algorithm can drive institutions to followthe movement of hands very well or not will affect the handfeel. In the experiment, the commutation ability is testedunder both low speed and high speed, and the results areshown in Figures 9 and 10. The fine curve represents theanalog signal calculated by the algorithm and sent to themotor; the coarse curve represents the force informationdetected by the force sensor.

Through the conversion of semaphore, the velocity oflow-speedmovement is about 0.75m/s and that of high-speedmovement is about 2.4m/s. As can be seen from Figures 9and 10, when the operation direction of hands changes, thealgorithm can quickly adjust the output of the motor so as


Table 2: The position precision of hand controller.

3-Dof moving distance of motor platform (mm) 3-Dof tracking distance of target ball (mm)𝑋 axis precision test 𝑌 axis precision test 𝑍 axis precision test

30 29.983 30.184 29.88630 29.992 30.065 30.03530 29.943 30.044 30.01530 29.891 30.065 30.01430 29.846 30.124 30.01230 29.811 30.061 30.031Relative error 0.3% 0.3% 0.003%

0.00

10.00

20.00

30.00

Time

−20.00

−10.00

−30.00

Measured value of force sensorOutput value of motor

Figure 10: High-speed reverse.

2.00

2.50

3.00

3.50

4.00

Time

Force Expected value

(N)

Figure 11: The stress in static state.

to follow the movement of hands. Because the commutationdelay of the whole system is about 5ms, operators will feel noobvious delays in practice.

In bilateral control phase, a big closed-loop force controlmethod is used to make accurate control for the real forceoperators feel, and the force value measured by the forcesensor should be close to the desired feedback. During thebilateral stage, this paper adds 3N feedback forces to the 𝑋

axis andmeasures the data separately in the condition of staticand motion state, as shown in Figures 11 and 12.

2.00

2.50

3.00

3.50

4.00

4.50

5.00

Time

Force Expected value

(N)

Figure 12: The stress in motion state.

As can be seen from Figure 11, while joining 3N feedbackforces and keeping the handle in a quiescent state, the actualforce detected fluctuates around 3N and the error is about±0.1 N because of the jitter of hands; under the conditionthat the feedback force is still 3 N, the motion experimentagainst the feedback force direction at first and then along thefeedback force direction is carried out, and the result is shownin Figure 12. Because of the superposition of manpower andthe motor driving force, the motion platform will producean acceleration. If the motion direction is opposite to thatof feedback force, the value measured by the force sensor ismore than 3N. Conversely, if they are in the same direction,the value will be less than 3N. But on the whole, the actualstress still fluctuates in the vicinity of 3N.

In the stage of bilateral control, when the slave sendsfeedback force to the master, operators will immediately feelobvious force feedback and still be able to complete thescheduled operation stably and smoothly.

After the above tests, it shows that on the basis of achiev-ing the control target of teleoperation and bilateral controlstage, the two control methods integrate into one controlalgorithm perfectly. Teleoperation and bilateral control cantake control quickly according to the values measured bythe force sensor and the response time is only about 5ms.Moreover, switching between the two stages will have noobvious time delay.Therefore, the proposed force closed-loop


control algorithm based on the force sensor can meet thecontrol requirements of the proposed hand controller systemvery well.

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper.

Acknowledgments

The authors would like to thank their colleagues fromthe Robotics Research Group for helpful discussions andcomments on this paper. This work is supported in part bythe Fundamental Research Funds for the Central Universities(2012RC0504), Key Project of Chinese National Programs forFundamental Research andDevelopment (973 Program) (no.2013CB733005), and National Natural Science Foundation ofChina (no. 61175080).

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