A smart robot hand developed at JPL for the Protoflight Manipulator Arm (PFMA) is described. The development of this smart hand was based on an integrated design and subsystem architecture by considering mechanism, electronics, sensing, control, display, and operator interface in an integrated design approach. The mechanical details of this smart hand and the overall subsystem are described elsewhere. The sensing and electronics components of the JPL/PFMA smart hand are summarized and it is described in some detail in control capabilities

Bejczy, Antal

Killion, Richard R.

Robinson, Lee R.

English

NASA Technical Reports Server

N89-10093
MULTIPLE SENSOR SMART ROBOT HAND WITH FORCE CONTROL
Richard R. Killion
Lee R. Robinson
Antal Bejczy
JET PROPULSION LABORATORY
California Institute of Technology
Pasadena, California
i
I. INTRODUCTION
Analysis of anticipated space assembly, servicing and
repair tasks to be performed by robot arms motivated the
work at the Jet Propulsion Laboratory (JPL) for the design
and development of multifunctional and smart robot hands.
Here the term "multifunctional" refers to the hand's
mechanical capabilities, while the term "smart" refers to
the hand's sensing and control capabilities. The analysis
also led to the conclusion that an evolutionary approach to
the design and development of robot hands can generate
important and needed capability increases. The first step
in this evolutionary development effort was the
consideration of one degree-of-freedom parallel claw end
effectors equipped with force/torque balance and grasp force
sensors, and capable of being servoed in position, rate, and
grasp force modes of control.
This paper describes a smart robot hand developed at
JPL for the Protoflight Manipulator Arm (PFMA) at the
Marshall Space Flight Center (MSFC). The development of
this smart hand was based on an integrated design and
subsystem architecture by considering mechanism,
electronics, sensing, control, display and operator
interface in an integrated design approach. The mechanical
details of this smart hand and the overall subsystem
integration are described elsewhere (Refs. 1 and 2). In
this paper we briefly summarize the sensing and electronics
components of the JPL/PFMA smart hand and describe in some
detail its control capabilities.
https://ntrs.nasa.gov/search.jsp?R=19890000722 2020-03-20T06:04:06+00:00Z
II. THE MECHANICAL HAND
I_I.l Requirements
The smart hand was designed for and integrated with the
PFMA to perform the following specific tasks:
Task I: Mate and demate a standard fluid coupling.
Task 2: Open and close an access panel by turning a wing
nut.
Task 3: Remove and replace a battery module by grasping a
square beam.
Task 4: Deploy and retrieve a telescoping vertical antenna.
" The gripper's intermeshing claws were designed to grasp
square beams (as attached in Orbital Replacement Units) as
well as round and oval beams. In addition, a graphics
display subsystem provides sensor information to the human
operator during task performance. Figure 1 shows the
mechanism of theend effector and the overall integrated
subsystem including electronics, data handling, display and
control input panel.
L_
2
ORIGINAL PAGE IS
OF POOR QUALrTY
!
Figure i. PFMA/JPL Smart End Effector Mechanism, Local Electronics
and Overall Subsystem
III. SMART HAND SENSORS
_F/T _nsor
During the grasping phase in a zero-g environment,
unwanted forces and torques can be detrimental to both the
success of the task and the satellite being serviced. To
monitor such forces and torques, a force/torque sensor is
mounted between the hand and wrist. Semiconductor strain
guages are mounted in the Maltese Cross design force/torque
sensor with a full-bridge configuration. This configuration
results in eight analog guage readings that are read and
converted to 12-bit digital values by the Sensor CPU for
eventual display for the human operator.
III.2 _g _o_xgm i_n_or
Mounted in the base of each of the two fingers are four
semiconductor strain guages in a full bridge configuration.
They have been designed to measure up to 120 pounds of
clamping force. These readings are converted to 12-bit
digital by both the Sensor CPU and Servo CPU. The Servo CPU
requires the digitized sensor data. in real-time (400 hz) for
servoing. The Sensor CPU requires the sensor data to be sent
to the Signal Processing CPU for the slower (30 hz) graphics
display.
III.3 Tachometer and Potentiometer
Rate and position information is required by the Servo
CPU for motor control of the hand closure. Position
information is required by the Sensor CPU for eventual
display for the human operator.
111.4 Tactile Sensing
For future use, there im a reserve of 32 additional
analog channels for force sensing of each of the individual
plates of the intermeshing fingers. The force profile along
the plates will give misalignment information to reduce
torque applied to satellites in a zero-g environment.
III.5 Optical Proximity Sengin__g
Future plans also include optical proximity sensing. A
proximity sensor consists of a photoemitter and a
photodector with are focused such that the optic axes of the
two converge at a focal point. Distance is determined by
the intensity of the light received by the photodetector.
This will reduce misalignment before contact, thus reducing
unwanted forces and torques during contact.
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DISPLAY
[ SYNCHRONIZATION SI(_WM_
H_:_C 'IV ELECTRONICS
O0
0 _:_,
=Pr-
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Figure 2. Overall Data Handling for PFMA/JPL Smart Hand
IV. LOCAL ELECTRONICS
The local electronics (electronics in the base of the
hand) consists of the Sensor Subsystem and the End Effector
Subsystem. See Figure 2.
IV.I Sensor Subsystem
The Sensor Subsystem consists of the Sensor CPU,
Multiplexer, Sample & Hold, A/D converter and
instrumentation amplifiers. The Sensor CPU controls the
analog multiplexing and the A/D conversion. The multiplexer
handles 8 force/torque channels, 2 clamp force channels, and
1 position channel. After the Sensor CPU receives a
conversion complete signal, it reads in the data and
transmits it on request to the Signal Processing Subsystem
via an RS-232 serial link through the Slip Rings.
IV.2 End Effector Subsystem
The End Effector Subsystem constists of two CPUs, the
Motorola MC68701 for communications and the Motorola MC68705
for motor servoing.
The Communication CPU receives commands from
the Control Computer in the Human Operator Subsystem via an
RS-232 serial link through the slip rings. It receives
serial data through its on-chip serial port and then checks
for transmission errors using a 16-bit checksum. If the
command is error-free, it passes the command to the Servo
CPU via fast (30 micro seconds) parallel communication.
The Servo CPU executes the command to control the 3-
phase D.C. brushless motor. Pulse width modulation and
commutation to the motor windings is also done by the Servo
CPU.
V. EXTERNA_ ELECTRONICS
The External Electronics (electronics not in the hand)
consist of the Slip Ring Subsystem, Human Operator Subsystem
and the Graphics Subsystem. See figure 2.
V.I SIiR Ring Subsystem
The interface between the smart hand and the PFMA is a
rotary slip ring joint. Seven slip ring connections were
allocated for power (24 VDC & 20khz 50 VAC), and data
communications for both the Sensor Subsystem and End
Effector Subsystem.
6
V.2 HLperato r Subsystem
rt of the test system, and also as an independent
input ce to the smart hand besides the MSFC Control
Compul Control Box was developed. This Control Box has
a joy_(l-degree of freedom), a slide switch(to command
posit_ 3-way toggle switch (to switch modes), 2 rotary
dial hes,(to set rate and force limits), a "hold"
buttord status display "LEDs". This box has a Control
Compu_ich is a Motorola MC68705. The joystick, along
with ode,clamping force limit and rate limit switches
is re,he MC68705. The proper command is generated and
sent he Communication CPU via RS-232 across the slip
rings e Communication CPU.
V.3 S_ro_essing Subsystem
_gnal Processing CPU reads the sensor data from
the _ serial data stream coming from the Sensor CPU.
It t_nds graphics commands via its Multibus to the
Graph_cessor in the Graphics Subsystem.
V.4 Grs Subsystem
_aphics processor receives graphics commands from
the [ Processing CPU and generates a graphical
repre_on of the force/torque, claw position, and
clampirce. In figure 3 the three-axis coordinate system
on thehics display shows the resultant forces. The bar
graphsag the periphery of the display shows the
resultorques due to roll, pitch and yaw. The vertical
bars o left indicate gripper opening and clamping force
sensedach of the two claws.
ORIGINAL PAGE IS
OF POOR QUALITY
i •
Figure 3. PFMA/JPL Smart End Effector Graphics Display
(Left Vertical: e/e Opening-Closing Status;
Second Vertical: Grasp Force Sensors Value;
Center 2 and a 1/2 Frame: Up-Down, In-Out
and Left-Right Forces; Top Horizontal: Roll
Torque; Bottom Horizontal: Yaw Torque;
Right Vertical: Pitch Torque)
8
VI. DESCRIPTION OF CONTROL MODES
Three primary control modes are designed into the smart
hand. These modes are as follows:
(I) A Position Mode wherein the claws may be positioned
to a specified differential opening and which incorporates
an autonomous default "backup" mode if a force is detected.
The hand reverses direction and stops after 1/8". It then
refuses to accept position commands that would cause the
same collision. This condition remains until the human
operator either changes to rate mode or commands a position
to move opposite the direction of the previous collision.
(2) A Rate Mode wherein the claws may be moved at a
specified differential rate and which automatically
transitions into a Force Mode Control configuration
when an object has been encountered which generates a
force opposing claw motion.
(3) A Hold Mode wherein a currently existing force which
has been generated in Force Mode may be commanded to
continuously control claw squeezing force without further
command input.
A complete control mode diagram for the smart hand is
shown in Figure 4. This diagram details the drive system
hardware selected for the final system configuration. In
addition, a computer simulation of this system has been
generated for use in prediction of control mode performance.
After optimizing the smart hand drive system hardware
selection for motor torque-speed characteristics, power
efficiency and feedback element resolution/dynamic range
tradeoffs, consideration of algorithm parameters was
undertaken.
The processor utilized as the smart hand servo
controller is the Motorola MC68705 operating at 2MHz. It was
considered imperative that all control algorithms (with
attendent feedback data sampling) operate rapidly compared
to the system response. A design target of less than 2.5
msec was considered desireable based upon the inherent need
to minimize the buildup of force and energy transfer to the
workpiece during conditions of unanticipated contact.
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10
VI.l POSITION MODE
As can be determined from Figure 4,
has a characteristic equation of
2
s + (42 + 42Kv)s + 36.3Kp.
the Position loop
Because the MC68705 processor being utilized has no hardware
multiply/divide capability, it was desireable to implement
algorithm gain factors as powers of 2 so that digital shift
techniques could be utilized. Therefore, desiring a
critically damped system, a set of appropriate gains would
be Kv=0.25 and Kp=38. To incorporate digital shifts gains of
Kv=0.25 and Kp=32 were selected. With data sampling at a 400
Hz rate and a control loop natural frequency of
approximately 6 Hz, linear analysis should be applicable.
Four included figures document the expected and actual
performance of the smart hand in Position mode. Figure 5 is
the result of a simulation run using the analytically
established gains, and shows slight overshoot. Figure 6
shows the results of a large position step test of the hand
using the established gains. Slightly more overshoot is
observed, however, this anomaly was traced to excessive
mechanical deadband in the test unit which could not be
readily remedied. In order to reduce the mechanical wear of
the drive mechanism during testing, the gain Kp was reduced
to 8. In addition, the rate feedback gain Kv was made
position dependent, increasing to 0.5 when the actual
position was within .I00 inches of the commanded position.
The performance with these changes is shown in Figure 7.
Even though the position loop static accuracy was
compromised by the gain reduction the increased smoothness
of operation indicated overall increased benefits. Figure 8
indicates the large step position performance where an
object is encountered prior to reaching the commanded
position. As can be seen when a force is detected the claws
stop trying to finish positioning and "back up" slightly to
eliminate any detection of force and the hand awaits a
subsequent valid command recognizing that it cannot complete
the last one received.
ii
VI. 2 RATE MODEand RATE TO FORCE TRANSITION MODE
The Rate Mode configuration from Figure 4
characteristic equation of
2
s + 42s + 42Kva*Kvl.
yields a
Selecting Kva=8 and Kvl=l as a proper gain set for the
loop constants, provides an adequately damped system
response as demonstrated by the simulator run output shown
in Figure 9. System tests have validated these gain
constants through demonstration of proper rate loop
performance over the full dynamic range of operation.
Now to the more difficult section of the design
requirements, the Force Mode control loop and the
transitioning to it from the Rate mode. Without some
predictor capability (i.e. proximity sensing) it is
important that the bandwidth of the Force loop be maximized
so that unplanned force transients imparted to the workpiece
are minimized. From Figure 4 it can be determined that the
characteristic equation of the Force mode configuration is
3 2
s + 42.08s + 2672.4s + 3752Kh*Kha.
Selecting Kh:l and Kha=0.5 yields factors of
2
(s + 0.7)(s + 41.3s + 2642).
These factors indicate a slightly underdamped control
response. A simulator run output using the selected
parameters is shown in Figure i0. The response is
considerably less damped than the linearized system
equations would indicate, however, this has been determined
to be due to a realistic motor power limit included in the
simulation. To compensate for the effect of power limiting
the damping was increased by raising Kha to i. Operational
testing of the smart hand in the Rate mode with Auto
Transitioning to Force indicated that the increased damping
was adequate. Photos showing performance in these control
regimes are included as Figures II thru 13. These tests were
run by starting with the claws completely open and giving a
full rate close command with obstacles of various
compliances set to interfere with the closing. Figure Ii
shows the transitioning region when a spring loaded
compliance of approximately .001 in/ib was used as the
target workpiece. Figure 12 shows the transitioning when a
J
12
solid aluminum bar (compliance less than .00001 in/ib) was
utilized as the workpiece. The observable stepping in the
force profile of relatively non-compliant loads is a
phenomena of the software integrator and the incremental
nature of the pulse width modulation resolution. Figure 13
is an expansion of the initial transient of Figure 12
demonstrating the transient energy transfer of the claw
dynamics to the workpiece under near worst case conditions.
vl.3 HQt_D
Figures 14 and 15 show simulated performance of the
control system to load disturbances when in the Hold Mode.
The gains of the loop were set to Kg=l and Kga=l based upon
the evaluation of the Force Mode response as previously
described.
13
POSITION
inche=
25
2 f_
/
/t
151 _
0.5
- POSITION
o f ........
o 6 lz_a 24o3:_.L=,85, o_,_;2;s
TIME
Figure 5. Smart Hand Position Mode
POSITION ;
TACH
VERT x 10 Kp" 32
Figure 6. Position Servo Underdamped
P(_ITION
TACH
VERT x 10
Figure 7.
Kp-8
Position Servo Behavi,_r
14
ORLGIt*,:AL *:,"_ _$
OF POOR QUALi'i"v
OF !:........_' ..... _ !"_
FORCE
TACH
PO61TION
VERTxlO
Figure 8. Position Mode With Interfering Force
1.2
0.96
0.72
POSITION
mchm
0.48
0.24
0
Figure 9.
- TACH-Amp
-- POSITION /
./
/
/
0 6 12 18 24 0.3 36 42 48 54 0.6 66 72 78
TiME - seconds
Smart Hand Rate Mode
FORCE
pounds
70"
56,
42,
28-
14-
0-
Figure
I Kha= 0.5
;_ ; ; _ 0'.',,i',_ 4,_o:2_2',_s0:3
TIME - seconds
I0. Smart Hand Force Without Hold
15
OF POOR QUALIFY
FORC E
VERT x 10 SPRING-LOADED FORCE TEST DEVICE
Figure ii. RaCe to Force Transition- Full Rate
FORCE
TACH
VERT x 10
Figure 12.
ALUMINUM BAR
Rate to Force Transition -
1 sec/cm Scale
FORCE
TACt'I
Figure 13.
VERT I 10 ALUMINUM BAR
Rate to Force Transition- Full Rate
Initial Transient Expanded 50 ms/cm Scale
16
FORCE
pounds
26
21
16
10
0
Kg=l
- GRIP FORCE Kga = 1
I I I _ I 1 214 | ! ! I I I I |0 4 8 1 16 0.2 28 32 36 0.4 44 48 52 56
TIME - seconds
Figure 14. Smart Hand Force With Hold
26
FORCE
pounds
24.8
23.6 -
22.4 -
21.2 -
20
0 4 8 1 1 0.2 24 28 32 36 0.4 44 48 52 56
TIM E - seconds
Figure 15. Smart Hand Force With Hold
17
OF F.'9OP, QL|_Lrl-_
®
Figure 16. PFMA/JPL Smart End Effector Integrated With
PFMA Arm at MSFC (Performing Connector
Coupling Operation)
18
VII. CONCLUSIONAND FUTUREPLANS
The smart hand has been mounted to the PFMA at MSFC
(see Fig. 16) and tested with human operators performing
simulated satellite servicing tasks. The tasks consisted of
connecting and disconnecting a fluid coupling device,
simulating module changeout, and attempting to exert
constant forces and monents on the environment. Operators
performed the tasks both with and without the force/torque
display. Testing consisted of recording forces and torques
from the wrist and jaw mounted sensors while operators
performed tasks in manual control mode from a remotely
located control station at MSFC. The tests and the results
are described and evaluated in detail in Ref. 3.
In general, the experienced operators accomplished the
tasks with lower levels of root-mean-square forces than
intermediate or naive subjects. However, the test results
have shown that improved display and manipulator control
modes will be required to take full advantage of the end
effector's sensing capabilities. The general conclusion is
that sensors, displays, actuators and control modes for
teleoperation cannot be designed or fully evaluated in
isolation. For improved and optimized performance, the full
teleoperation control loop, including the human operator,
must be considered as it was pointed out in Ref. 3.
Future development plans include:
(i) Implementing the automatic execution of grasp force
control
(ii) Implementing event driven displays
(iii) Designing a proximity ranging device integratable with
the existing smart hand system.
In an event driven display, the simultaneous presence
(or absence) of several force and torque component levels
will be monitored and automatically indicated on the display
with a distinct and easily perceivable symbol.
VIII. ACKNOWLEDGEMENT
This work was carried out at the Jet Propulsion
Laboratory, California Institute of Technology, under
contract to the National Aeronautics and Space
Administration.
19
IX. REFERENCES
I. A.K. Bejczy and B.M. Jau, Servicing with Smart End
Effectors on OMV Manipulator, Proceedings of Satellite
Servicing Workshop II, Johnson Space Center, Houston, TX,
November 6-8,1985.
2. A.K. Bejczy, E.P. Kan and R.E. Killion, Integrated
Multi-Sensory Control of Space Robot Hand, AIAA Guidance,
Navigation and Control Conference, Snowmass, CO, August 19-
w 21,1985,paper no.: AIAA-85-1882.
3. B. Hannaford, Task Level Testing of the JPL-OMV Smart
End Effector, Proceedings of Space Telerobotics Workshop,
JPL-CALTECH, January 20-22,1987
2O


Multiple sensor smart robot hand with force control

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