A major drawback of open loop masters is a lack of force feedback, limiting their ability to perform complex tasks such as assembly and repair. Researchers present a simple dextrous force feedback master for computer assisted telemanipulation. The device is compact, portable and can be held in the operator hand, without the need for a special joystick or console. The system is capable of both position feed forward and force feedback, using electronic position sensors and a pneumatic micro-actuator. The level of forces exercised by the pneumatic actuator is such that near rigidity may be attained. Experimental results showing good system linearity and small time lag are given

Burdea, Grigore C.

Speeter, Thomas H.

English

NASA Technical Reports Server

N90-29058
PORTABLE DEXTROUS FORCE FEEDBACK MASTER
FOR ROBOT TELEMANIPULATION (P.D.M.F.F.)
Grigore C. Burdea Thomas H. Speeter
Electrical and Computer Engineering Dept. A.T.&T. Bell Laboratories
Rutgers University Machine Perception Research Dept.
Piscataway, NJ 08855-0909 Holmdel, NJ 07733
Abstract
A major drawback of open loop masters is a lack of force feedback, limiting their ability
to perform complex tasks such as assembly and repair. We present a simple dextrous force
feedback master for computer assisted telemanipulation. The device is compact, portable
and can be held in the operator hand, without the need for a special joystick or console.
The system is capable of both position feed forward and force feedback, using electronic
position sensors and a pneumatic micro actuator. The level of forces exercised by the
pneumatic actuator is such that near rigidity may be attained. We present experimental
results showing good system linearity and small time lag.
1. Introduction
Present telemanipulation techniques include mechanical masters, open loop servo
masters, and to a lesser extent closed loop servo masters. Direct mechanical telemanip-
ulation is often the simplest method, but cannot be used in applications where the slave
is not in the immediate vicinity of the master, as may well be the case in space applica-
tions. Closed loop servo telemanipulation eliminates the proximity requirement, but when
force feedback is provided by servo encoders on the slave arm, it is necessary to have two
nearly identical devices to act as master and slave. This duplication of resources may be
prohibitive in terms of cost and payload weight.
Current research efforts aim at eliminating the duplicate master by replacing it with
force feedback joysticks or sensorized spheres[3]. These devices are less "natural" to use
by an operator since direct similitude does not exist between human hand and robot finger
motions.
Dextrous master control represents a recent addition to the field of telemanipulation.
A dextrous master can replace the classical manipulator arm, joystick or keypad master
with an operator's hand motions[6]. Use of the human hand is a natural form of control
and is applicable to both nondextrous and dextrous slave deviccs. ]3ecause the human
hand is used as master, duplication of hardware is not required, and weight, inertia and
friction are reduced. This can bring significant improvements in the time necessary to corn-
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plete a task. It is estimated [2] that an improvement on the order of 10 can be expected
on the time efficiency quotient[7] when a dextrous master is used in place of keypad control.
While an open-loop dextrous master creates a natural control environment, it lacks
the ability to bring force feedback to the operator hand, which in turn limits the utility of
the slave device. This paper describes work towards the development of a dextrous master
with force feedback. Such a device will allow the execution of complex tasks such as assem-
bly and repair, using the human hand as master in closed loop teleoperation environment.
2. Conceptual design
The design of the Portable Dextrous Master with Force Feedback (PDMFF) was
guided by the need to bring force feedback from a robot end effector to the human hand
serving as master. The aim is to produce a compact, hand held device that fits inside the
palm. It should function as position controller for the robot end effector (either conven-
tional gripper or dextrous hand), and provide force feedback to the operator.
We have developed an initial master device to control and exert feedback in one
degree of freedom. The position sensing device is an LVDT transducer and force feedback
is provided by a small pneumatic piston in parallel with the LVDT (Fig. 1). The LVDT
requires only two contact points to secure its ends, therefore two fingers are suffieient to
hold it. Use of the thumb and middle fingers assures a good grip and sufficient distance to
accommodate the LVDT and piston.
The first joint of the thumb has two degrees of freedom namely anteposition/retro-
position and abduction (Fig. 2) [1]. Human factor studies[5] show that the most com-
fortable thumb postures are those with little anteposition. The need to minimize fatigue
implies that such postures should be accommodated by the master. A sphere joint on the
middle finger mount allows for 70 degrees rotation. This combined with the translation
movement of the active element produces a conic work envelope as shown in Fig. 3 and
allows the user to comfortably position the PDMFF between his fingers.
The volume of the master work envelope is given by:
where:
z" sin2ct cosa d ( _-_ + Id -k i2)
a is the sphere joint angle,
d is the linear travel,
I is the length of the mechanical mount.
(1)
Due to limitations on the thumb's range of motion, the useful volume used to control
the slave gripper is about 25_ of the work envelope.
154
OF pOOR QUf-.oLiTY
Fig. 1 - PDMFF prototype
abduc_n
D..,_o r P.nt,_r
BadJa]
RadiaJ _ _ _'_abduction
abducz_on _-_-- i
Fig. 2- Terminology of thumb motion [1]
155
Lateral View
:/d
_ _/ _'f
t t
Top View
,/
/
:' ii /
rk volume
Fig. 3 - PDMFF work envelope
The desire for reduced dimensions and weight, while maintaining sufficient force
capability led to the selection of a pneumatic micro cylinder as the active element. The
cylinder spring is removed, allowing for free motion of the piston. The piston shaft is
coupled with a second shaft attached to a magnetic core traveling inside the LVDT sensor.
The resulting configuration is shown in Fig. 4.
FinBer support
_r shaft
Spherical Joint
Core LVDT tensor
tiiiiiiiiiiiiiii;:::!-:::::::::;:::i[[[[[[[[[[[[[![[[[[[[[[[[[[[[[[[[[[[[[[[[
/
Electrical connection
Air inlet
Thumb support
F|g. 4- PDMFF design
156
3. Experimental installation
The PDMFF is integrated with a host computer for control as shown in Fig. .
Potitior, feedforward loop
/_ !_ OPERATOR
PDMFF
Force feedback loop
Supply air
II0 V AC
bl LVDT Signal
Conditioning
PMZ Valve
Controller
l
110 V AC
AID
Convener
ROBOT SLAVE
T Position
I Force Feedback r / Feedf°rw"
HOST
COMPUTER
Fig. 5 - Experimental installation
Position signals from the LVDT axe passed through a signal conditioning unit, giving
an analog signal (voltage) proportional to the core's displacement. This voltage is digitized
by an analog to digital converter and sampled by a host computer at 500Hz. The host uses
this signal to position the slave robot, in this case the Utah-MIT hand[4]. Force feedback
from the hand is sampled by the host and used to drive the pneumatic cylinder. At this
point a scaling up or down may be implemented to adjust the gain in the feedback loop. A
voltage is sent to the valve controller which raises or lowers the air pressure in the cylinder
using an analog proportional control system.
4. Test results
PDMFF was tested both under static and dynamic conditions. The static test was
aimed at determining the force exercised by the cylinder as a function of the voltage con-
trolled by the host computer. Four tests were run with air supply pressures, P,.i,, of 60, 70,
80 and 90 psi. For each supply pressure voltages from 0 to 10 V were applied to the valve
controller. Under these conditions the force was measured with a load cell. The results
axe presented in Fig. 6. The force feedback is given by:
Fle_ = P_i, A - Ft,laio. (2)
157
where: A is the piston area,
P.i. is the air pressure after the valve regulator,
-Ptriaim_ is the piston friction.
e_
©
0.8
0.6
0.4
0.2
0.0
60 psi
70 psi
80 psi
90 psi
Pneumatic Cylinder Force Output
I I
I , , , I , , , I ,
2 4 6
Input Voltage
Fig. 6 - Micro cylinder force measurements
158
The linearity of equation (2) is reflected in the graph, with the exception of plateaus
appearing at higher voltages. At these plateaus maximum air i_ow is applied on the piston,
and force no longer varies with increased voltage. A supply pressure of 90 psi and a voltage
of 6V results in almost a pound force. This is a large enough force to produce near rigidity
in the master and is sufficient to simulate the sensation of a solid object in the grasp.
A second set of tests measured the system response to a step function applied to the
master. The same load cell was used to determine the force variation in time• Results
are presented in Fig. 7. Tests showed an approximately 50 msec rise time with almost no
overshoot or oscillation•
force (ibs) |
0.8
0.6
X
x
x
x
x
X
x
x
x
x
x
x
x
x
0
•.-j- • I • --|, • " i
• I • • • I • , , I , , , I , . . I
200 400 600 800 lO00
Time (ms)
Fig. 7 - Dynamic response for step function input
159
5. Integration with robotic slaves
The design of the PDMFF makes it compatible with both dextrous and nondextrous
end electors. When coupled with a paralld finger gripper, the LVDT signal controls the
gripper opening in a linear rdationship. Since the piston travel is fixed for a given cylinder,
the gripper opening is given by
(s)
where
G is the gripper maximum opening,
d is the piston maximum extension.
When teiemanipulating with a dextrous hand, we are faced with the problem of con-
trolling multiple degrees of freedom with a device with only one degree of freedom. As an
initial test we have used the PDMFF to control a three-fingered pinching motion of the
Uhah/MIT dextrous hand. An initial hand pose, with the fingers open and a final position
with the thumb, index and middle fingers in a pinching posture are established. The po-
sition of the core within the LVDT is used to linearly interpolate between the initial and
final pose and the hand is driven to the desired positions by the hand's resident position
servo system. The resulting motion, as the PDMFF is opened and dosed is a natural pinch
and release action of the hand.
Forces exerted on objects by the hand are determined from the tensions sensed in
the tendons of the hand. Each joint of the hand is operated by two tendons, one for flexion
and one for extension, and each tendon is gauged, allowing their tension to be monitored.
By observing the difference between flexion and extension tensions in the thumb, the net
force exerted on an object can be deduced. This value is scaled and used to directly drive
the valve of the pneumatic piston. The piston's internal pressure, therefore, is directly
proportional to the force being exerted by the hand on the grasped object.
Objects of various shapes and sizes can easily be grasped and held by the dextrous
hand using the PDMFF as master. Forces reflected from the hand to the user allow deft
control of the grasping force. After a short learning period, the user is able to grasp ob-
jects lightly, at the verge of dropping, or with great force, greater than capable with the
human hand alone by taking advantage of the strength of the Utah/MIT dextrous hand.
The rigidity or compliance of an object grasped by the dextrous hand can be felt by the
user in a very natural way, by squeezing the object and feeling the resulting reflected forces.
6. Conclusion
The force reflecting master described here allows the user to control the motion of,
and feel the force exerted on a slave robotic device. The PDMFF uses a simple parallel
arrangement of an LVDT position sensor and micropneumatic piston to provide a closed
160
loop telemanipulation system.
Interfaces to the PDMFF are simple, consisting of one voltage out, corresponding
to the LVDT's position, and one voltage in, driving the pneumatic cylinder. Control of
the slave will vary in complexity depending upon the device, however the simplicity of
the PDMFF may allow the use of discrete components in the interface between master
and slave rather than requiring the intervention of a processor. This can further reduce
the complexity, cost and size of a master-slave system using the PDMFF, and may allow
the development of compact, multidegree of freedom masters using several PDMFF-like
mechanisms in parallel.
7. Acknowledgements
Work on this paper has been supported by funds from Rutgers University Research
Council Grant and from AT&T Bell Laboratories.
References
1. American Society for Surgery of the Hand, The Hand: Ezamination and Diagnosis,
Churchill Livingstone, London (1983).
2. Burdea, Grigore C. and Kicha Ganapathy, "On Dextrous Robot Telemanipulation and
Learning," AT&T Bell Laboratories Technical Report 11352-881003-11TM.
3. Hirzinger, G., "The Space and Telerobotic Concepts of DFVLR Rote.x," 1987 Pro-
ceedin9s IEEE International Conference on Robotics and Automation, pp. 443-449, IEEE,
(1987).
4. Jacobsen, S.C., 3.E. Wood, D.F. Knutti, and K.B. Biggers, "The Utah-MIT Dextrous
Hand: Work in Progress," Journal of Robotics Research $ (4), pp. 21-50 (1984).
5. McCormick, E.J., Human Factors in Engineering and Design, McGraw-HiU (1976).
6. Pao, Lucy Y. and Thomas H. Speeter, "Transformation of Human Hand Positions for
Robotic Hand Control," 1989 IEEE International Conference on Robotics and Automa-
tion, IEEE, (1989). (submitted)
7. Vertut, Jean and Philippe Coiffet, "Teleoperation and Robotics: Applications and
Technology," Robot Technology 3B, 256 pp., Prentice Hall, (1986).
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Portable Dextrous Force Feedback Master for robot telemanipulation (PDMFF)

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