Control experiments performed using an isotonic joystick connected to a six degree-of-freedom manipulator equipped with a six dimensional force-torque sensor at the base of the manipulator end effector are described. The preliminary control experiments were aimed at the investigation of the human operators' ability to command and control forces in different directions by varying the information conditions and the values of the feedforward and feedback command gains in the bilateral control loop. The main conclusions are: (1) a quantified graphic display of force-torque information can considerably enhance the operator's ability to perform a quantitatively sharp force-torque control, and (2) there seems to be a task dependent optimal combination of the feedforward and feedback command gain values which provide a dynamically smooth and stable bilateral control performance

Bejczy, A. K.

Handlykken, M.

English

NASA Technical Reports Server

, EXPERIMENTAL RESULTS WITH A SIX-DEGREE-OF-FREEDOM
FORCE-REFLECTING HAND CONTROLLER
A.K. Bejczy and M. Handlykken*)
Jet Propulsion Laboratory
California Institute of Technology
Pasadena, CA 91109
SUMMARY
The slx-degree-of-freedom force-reflecting hand controller under current
investigation at JPL is an isotonic joystick, its hand grip is able to follow
all the translational and orientational motions an operator's hand can comfor-
tably make within a 30 cm cube. Each degree-of-freedom of this joystick can be
backdriven by a motor commanded by the forces and torques sensed at the base
of the hand of a remote manipulator. Thus, the operator can "feel" the task
he is controlling when this Joystick is connected to a remote manipulator
through a computer. The use of this joystick for remote manipulator control
can generalize the bilateral force-reflecting control of manipulators. Gen-
eralization means that the "master arm" function can be performed by this
"universal" force-reflectlng hand controller which is dissimil=r to the "slave
arm" both klnematlcally and dynamically. This paper briefly summarizes and
evaluates a few preliminary control experiments performed by using this hand
controller connected to a six-degree-of-freedom manipulator equipped with a
slx-dlmensional force-torque sensor at the base of the manipulator end effec-
tor. The preliminary control experiments were aimed at the investigation of
the human operators' ability to command and concrol forces in different direc-
tions by varying (i) the information conditions and (ii) the values of the
feedforward and feedback command gains in the bilateral control loop. The
main conclusions are: (i) a quantified graphic display of force-torque infor-
mation can considerably enhance the operator's ability to perform a quantita-
tively sharp force-torque control, and (ii) there seems to be a task dependent
optimal combination of the feedforward and feedback command Bain values which
provide a dynmnically smooth and stable bilateral control performance.
I. INTRODUCTION
In the rurrent bilateral force-reflecting master-slave manipulator systems
widely and successfully employed in the nuclear industry the master and slave
arms are in essence identical and interchangeable (Refs. 1-5). A limiting
factor for broadening the application of the force-reflecting master-s!ave
manipulator systems is the nature of the master arm. Typically, the present
master arms are large and heavy, and require a large operating volume.
A pilot development system has been implemented at JPL recently. The
system utilizes a six-degree-of-freedom force-reflecting hand controller as a
master arm in combination with a slave arm which is totally dissimilar to the
*) On leave of absence from the Technical University of Norway,
Trondhelm, Norway.
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hand controller both kinematlcally and dynamlcally. The development system is
briefly described in Section II. The overall system is a klnesthetically cou-
pied man-machine system. The input-output characteristics of the human hand
play a key role in the bilateral control implementation which requires the
use of a computer. In Section III control experiments are described aimed at
evaluating the human operators' ability to control forces using this general
purpose hand controller in a bilateral control mode under varying information
and control conditions. The conclusions are summarized in Section IV.
II. EXPERIMENTAL SYSTEM
The main mechanical elements of the development systpm are shown in
Figure i. They are: a six-degree-of-freedom manipulator, a slx-dimenslonal
force-torque sensor mounted to the base of the end effector, and a slx-degree-
of freedom backdrlvable hand controller. A computer which performs the
coordinate transformations and closes the control loop between the hand con-
troller and manipulator, as well as the sensor, drive and interface electron-
ics are essential elements of the overall system.
The key mechanical elen,ent is the hand controller *) which acts here as a
generalized master arm. It is essentially a backdrlvable slx-dlmenslonal
isotonic Joystick which has been designed to conform to the motion range of
an operator seated at a console. Its hand grip is able to follow all the
translational and orlentational motions that the operator's hand can comfortably
make within a 30 cm (about I it) cube work space. The hand controller mechanism
is self-balanced, and can be mounted horizontally (as seen in Figure i) or
vertically. The self-balanced mechanism together with low backlash, low
friction and low effective inertia at the hand grip render this hand controller
a "transparent" interface between the human operator and a remote manipulator.
More on the hand controller mechanism can be found in Reference 6.
The hand controller performs a dual function. First, it provides posltion
and orientation commands to the manipulator. Second, it provides force and
torque feedback to the operator's hand from the manipulator. This hand con-
troller does not have any geometric and dynamic similarity to the manipulator
it controls. In that sense it is a general purpose device: it can be inter-
_aced to any manipulator through a computer. The computer reads the Joint
wlrlables measured at both the hand controller and manipulator. Based on
these measurements, real time computer algorithms establish the positional and
orientatlonal control relations between the hand controller and the manipulator.
Likewise, real-tlme computer algorithms determine the motor torques needed to
backdrive the hand controller Joints as a function of the forces and torques
sensed at the mechanical hand in order to provide a force-torque "feeling" to
the opera:or's hand that parallels the force-torque "feeling" of the mechanical
hand. The JPL/CURV manipulator, its kinematics, geometrical equations and
control system together with the force-torque sensor integrated with it are
described in detail in References 6-8.
_)The mechanism of the hand controller was designed by J.K. Salisbury, Jr.,
Design Division, Mechanical Engineering Department, Stanford University,
Stanford, CA.
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_ |
Figure 2 shows a s_plified linear model of the bilateral control system
dynamics referenced to one/one Joint of the hand controller and manipulator.
For simplicity, the geometric transformations are omitted from Figure 2. The
overall performance of the bilateral control system is highly dependent upon
the controller's ability of handling the interacting dynemlcs of the hand
controller and manipulator. Note in Figure 2 that these two devices dynami-
cally interact through the operator's hand. Note also in Figure 2 that the
force-torque feedback to the operator's hand consists of three parts:
(l) velocity damping, (ii) position error feedback, and (iii) feedback from
the force-torque sensor. More on the bilateral control system analysis and
synthesis can be found in References 9-10.
The simplified linear model shown in Figure 2 is only intended to illu-
strate two major points: (a) the general frame of the dynamic interaction
between the manipulator, hand controller and the operator's hand, and (b) the
meaning of the two control parameters, Ks and Kf, which were the key variables
in the control experiments described below.
III. EXPERIMENTS
The purpose of the prelimlnary experiments was (I) to check out the
overall performance and stability of the bilateral control system and (2) to
evaluate the kinesthetic ability of the operator's hand to control prescribed
forces in different directions when (i) the feedforward position scaling Ks
and the force feedback gain Kf were changed and (It) with or without using
graphic display of force-torque Information.
Four basic control experiments were performed:
i. Push down and hold 50N (~i0 ib) force.
2. Push down and glide laterally with 50N (~i0 Ib) force.
3. Push forward, hold 5CN (_i0 Ib) force, and zero out the lateral
and down forces.
4. Push forward and down at the same time, hold 50N (~10 Ib) force
in each direction and zero out the lateral force.
In experlments 1 and 2 the task was set up so that the operator's wrist
was free of lateral tension, ll_experiments 3 and 4 the task set-up required
that the operator's hand be in lateral tension during the force control test.
Note that the main force control action was (i) along the line of gravity field
in experiments I and 2, (il) perpendicular to the field of gravity in experi-
ment 3, and (ill) with 45 degree tilt relative to the field of gravity in
experiment 4. Note also that experiments 3 and 4 required the simultaneous
control of all three (Fx, Fy and Fz) force components explicitly. In experi-
ments i and 2 only one force component (Fx or Fz) control was required
explicitly; the control of the remalulng two (Fy, Fz or Fx, Fy) force compo-
nents was only required im,_llcitly.
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Figures 3 through 8 show a few representative samples of the more r_'_n
300 experimental data curves generated so far. The unit value of the force
feedback gain (Kf = I) was nearly equal to 5N (~i ib). The unit value of
the feedforward gain (Ks = i) signifies a one to one correspondence between the
hand controller and manipulator displacements; the value Ks = 0.5 means that a
I0 cm hand controller displacement causes only a 5 cm manipulator displacement.
The labels V and G at the performance curves in Figures 3 through 8 are
re] ed to two different information conditions. For the V curves, the
op ators had only visual feedback from the task scene together with the
manually nerceivable force feedback. For the G curves, the operators could
observe a real-time color graphic bar display of the Fx, Fy and Fz forces
acting on the mechanical hand in addition to the manually perceivable force
feedback.
The data to some extent are hardware and software dependent and influenced
by training, learning and other external conditions, and the total data base
is quite narrow. Therefore, a detailed data evaluation is not yet possible.
However, the data obtained so far allow a few general conclusions.
IV. CONCLUSIONS
i) Generalization of force-reflectlng bilateral control of "master-
slave" manipulators is feasible in the sense that the master arm does not
have to be a kinematic and dynamic replica of the slave arm.
2) There is a trade-off between Ks (position feedforward scaling) and
Kf (force feedback gain) parameter values: higher KS requires lower Kf, or
conversely, to obtain a dynamically smooth and stable performance.
3) There seems to be an optimal combination of the Ks and Kf parameter
values. The optimal combination may be task dependent.
4) The operator's body posture, including the posture of his arm and
hand holding the hand controller relative to his body, has a major influence
on the dynamic performance of the overall control system.
5) A quantified graphic display of force-torque information considerably
aids the operator in performing a quantitatively sharp force-torque control
through a bilateral force-reflectlng control system. Under certain gain
conditions and without graphic display of force-torque data the system can
becon_e unstable (Figure 8).
6) The speed and direction at which contact is established between the
mechanical hand and object have a major influence on the stability of ta_k
per formance.
7) It is deslr_,ble to have a stiffer control coupling between the hand
, controller a.d manipulator.
8) Higher force feedback capability is desirable in the hand controller.
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Acknowledgement
The research described in this paper was carried out at the Jet
Propulsion Laboratory, California Institute of Technology, under NASA
Contract NAS7-100.
References
i. R. Goertz, Manipulator Systems Development at ANL, Proceedings of the
12th Conference on Remote Systems Technology, ANS, November, 1964,
pp. 117-136.
2. L. Galblatl and C. Mancinl, A Compact and Flexible Servo System for
Master-Slave Electronic Manipulator, Proceedings of the 12th Conference
on Remote Systems Technology, ANS, November 1969, p. 73.
3. C.R. Flatau, Compact Servo Master-Slave Manlpulator with Optimized
Co_unlcatlons, Proceedings of the 17th Conference on Remote Systems
Technology, ANS, November, 1969, pp. 154-164.
4. J. Vertut, Experience and Remarks on Manipulator Evaluation, Perform-
ance Evaluation of Programmable Robots and Manipulators, NBS Special
Publication No. 459, Washington D.C., October 1975, pp. 97-112.
5. D.G. Jelatis, Characteristics and Evaluation of Master-Slave Manipula-
tors, Performance Evaluation of Programmable Robots and Manipulators,
NBS Special Publication No. 459, Washington D.C., October 1975,
pp. 141-145.
6. A.K. BeJczy and J.K. Salisbury, Jr., Kinesthetic Coupling Between
Operator and Remote Manipulator, Proceedings of the International
Computer Technology Conference ASME Century 2, Volume i, San Francisco,
CA, August, 1980, pp. 197-211.
7. A.K. BeJczy, Allocation of Control Between Man and Computer in Remote
Manipulation, Second Symposium on Theory and Practice of Robots and
Manipulators, Proceedings, Eds. A. Morecki and K. Kedzlor, Elsevier
Scientific Publishing Co., Amsterdam-Oxford-New York, 1977, pp. 417-430.
8. A.K. BeJczy, Manipulation of Large Objects, Third Symposium on Theory
and Practice of Robots and Manipulators, Proceedings, Eds. A. Moreckl,
G. Bianchl 8nd K. Kedzlor, Elsevier Scientific Publishing Co.,
Amsterdam-Oxford-New York, 1980, pp. 301-322.
9. M. Handlykken and T. Turner, Control System Analysis and Synthesis for
a Six-Degree-of-Freedom Universal Force-Reflectlng Hand Controller,
Proceedings of the 19th IEEE Conference on Decision and Control,
Albuquerque, NM, December, 1980, pp. 1197-1205.
10. M. Handlykken, Dynamic Stabl]ization tbthods for Bilateral Control of
Remote Manlpulation, Proceedings of the ISMM Symposium on Mini-and-Micro-
Computers in Measurement and Control, San Francisco, CA. May 20-22, 1981.
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1982005792-462
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TRANSLATIGN ROTATION
1. FORWARD-BACKWARO 4. YAW
2. VERTICALUP-0OWN 5. PITCH
3. LATERALLEFT-RIGHT 6. ROLL
i.
Figure i. Overall Exper_',_ental System and Hand
Controller Reference Frame
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• r,
Ks:l Ks:O_
Kf:l
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Kf : 0.25 - __i_.
VERTICAL AXES: FORCE,25 N PER MARK
HORIZONTAL AXES: TIME, 1 SECPER MARK
Ks: POSITION FORWAROGAIN
Kf : FORCE FEEDBACKGAIN
V : ONLY VISUAL SCENEOBSERVATION
G : ALSOGRAPHIC DISPLAY OF FORCE OATA
Figure 3. Push Down and Hold Experiments Data
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Ks: 1 Ks: 0.5
, Kf: 1 -_ ..........
Kf : 0.25
VERTICAL AXES: FORCE,25 N PER MARK
HORIZONTAL AXES: TIME, 1 SECPER MARK
Ks : POSITION FORWAROGAIN
K/: FORCE FEEDBACKGAIN
V : ONLY VISUAL SCENEOBSERVATION
G : ALSOGRAPHIC DISPLAY OF FORCE OATA
Figure 4. Push Down and Lateral Gllde
Experiments Data
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Experimental results with a six-degree-of-freedom force-reflecting hand controller

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