Inertial devices, including sensors and actuators, offer the potential of improving the tracking of telerobotic commands for space-based robots by smoothing payload motions and suppressing vibrations. In this paper, inertial actuators (specifically, torque-wheels and reaction-masses) are studied for that potential application. Batch simulation studies are presented which show that torque-wheels can reduce the overshoot in abrupt stop commands by 82 percent for a two-link arm. For man-in-the-loop evaluation, a real-time simulator has been developed which samples a hand-controller, solves the nonlinear equations of motion, and graphically displays the resulting motion on a computer workstation. Currently, two manipulator models, a two-link, rigid arm and a single-link, flexible arm, have been studied. Results are presented which show that, for a single-link arm, a reaction-mass/torque-wheel combination at the payload end can yield a settling time of 3 s for disturbances in the first flexible mode as opposed to 10 s using only a hub motor. A hardware apparatus, which consists of a single-link, highly flexible arm with a hub motor and a torque-wheel, has been assembled to evaluate the concept and is described herein

Ghosh, Dave

Kenny, Sean P.

Montgomery, Raymond C.

Shenhar, Joram

English

NASA Technical Reports Server

N93-18839
EVALUATION OF INERTIAL DEVICES FOR THE
CONTROL OF LARGE, FLEXIBLE,
SPACE-BASED TELEROBOTIC ARMS
Raymond C. Montgomery 1, Sean Kenny 1,
Dave Ghosh 2, and Joram Shenhar3
NASA Langley Research Center
Hampton, VA
ABSTRACT
Inertial devices, including sensors and actuators, offer the
potential of improving the tracking of telerobotic commands for space-
based robots by smoothing payload motions and suppressing vibrations.
In this paper, inertial actuators (specifically, torque-wheels and
reaction-masses) are studied for that potential application. Batch
simulation studies are presented which show that torque-wheels can
reduce the overshoot in abrupt stop commands by 82 percent for a two-
link arm. For man-in-the-loop evaluation, a real-time simulator has
been developed which samples a hand-controller, solves the nonlinear
equations of motion, and graphically displays the resulting motion on a
computer workstation. Currently, two manipulator models, a two-link,
rigid arm and a single-link, flexible arm, have been studied. Results are
presented which show that, for a single-link arm, a reaction-
mass/torque-wheel combination at the payload end can yield a settling
time of 3 s for disturbances in the first flexible mode as opposed to 10 s
using only a hub motor. A hardware apparatus, which consists of a
single-link, highly flexible arm with a hub motor and a torque-wheel,
has been assembled to evaluate the concept and is described herein.
1 Aero-Space Technologist, NASA Langley Research Center.
2 Principal Engineer, Lockheed Engineering and Sciences Co., Hampton, VA.
3 Staff Engineer, Lockheed Engineering and Sciences Co., Hampton, VA.
PRECEDING PhGE BLANK NOT FILMED
289
https://ntrs.nasa.gov/search.jsp?R=19930009650 2020-03-17T07:26:31+00:00Z
THE PROBLEM
The problem addressed in this research is illustrated in this chart.
A human operator moves a hand-controller according to what is
perceived as the correct input to maneuver a payload. This is based on
observation of the payload via an out-the-window view and closed-
circuit television monitors. Based on the hand-controller input and joint
sensors, the control system moves the payload via a kinematic linkage.
The motion, instead of being what the operator expects, is characterized
by unwanted motions which result from the inability to predict the
motion of the system based solely on sensors at the joints (typically
angle encoders) and from complex structural vibrations. The prediction
errors and the structural vibrations are related to the size of the
linkage. For the space shuttle remote manipulator (approximately 50 ft.
in length) the vibrations can be in the order of 6 in. peak-to-peak with
a frequency as low as 0.2 Hz. This behavior, therefore, limits precision
payload operations and results in loss-of-time in planning actions and
waiting for vibrations to settle after excitation.
\
Unpredicted Endpoint Res
Desir
_fl,.,17 Motion /
Correct
Input
290
SOLUTION CONCEPT
The idea is to place an inertial control unit at the interface
between the payload and the kinematic linkage. This unit would
possibly use torque-wheels, reaction-mass actuators, reaction-jets, and
motion isolation subsystems to isolate the payload from vibrations of
the kinematic linkage and still allow transmission of the payload
maneuvering loads eliminating the problems with non-collocation in the
design frequency range. The purpose of the unit is to isolate the
payload from structural vibrations of the kinematic linkage and to
reduce or eliminate lags in the response of the linkage which are caused
by structural vibrations and nonlinearities in joint motor response. The
sizing of the inertial components is, thus, a function primarily of the
characteristics of the linkage and, is believed, independent of the
payload.
INERTIAL SENSORS
AND REACTION -
DEVICES
MOTION
ISOLATION
SYSTEM
291
CONTROL WITH CONVENTIONAL JOINT MOTORS
One approach to the problem is to employ additional inertial
sensors to determine the track of the payload, and to develop a control
law that overcomes the difficulties in non-collocation of the control
actuators (at the joints) and the desired response variables (at the
payload-end of the arm). Over approximately the last fifteen years, this
approach has been researched with no adequate resolution of the
problem for precision operations with large, flexible robot arms. This
chart lists some of the difficulties. The major one is non-collocation of
the actuators with the point of interest. The phase of all vibrations in
the control system bandwidth must be predicted accurately in order to
get high gains in the control loop, or the control system must be gain
stabilized resulting in a low loop-gain with associated poor performance.
This difficulty has led us to another hardware-based approach of using
inertial actuators (torque-wheels, reaction-mass actuators, etc.) to solve
the problem.
FEEDFORWARD CONTROL-
• NO DISTURBANCE REJECTION
FEEDBACK CONTROL-
. NON-COLLOCATION CURRENTLY
LIMITS GAIN
ALTERNATIVE-- USE COLLOCATED DEVICES
292
BATCH SIMULATION
MODEL
A batch simulator has been used to study space-based arms of the
Space Shuttle Remote Manipulator System (RMS) class. The simulator
includes the dynamics of a planar model of a two-link arm and a torque
wheel attached to the free end. The diagram illustrates the arm model
used. Dynamic elements included in the simulator are two links, the
joint motors, and the torque-wheel. Parameters of the arm were
selected to be representative of a large, space-based, RMS-class robotic
arm. Specifically, the links are of equal length, 6.7 m, and each has a
mass of 204 kg. Parameters of the torque wheel are similar to those of
the Langley torque-wheels. The torque-wheel total mass was 38.6 kg
and its maximum output was 60 N-m at .5 Hz. The simulator
implements a digital joint motor controller as well as the torque-wheel
controller. The joint motor control scheme uses inverse kinematics to
generate joint angle commands from telerobotic translational command
inputs and a proportional-integral-derivative (PID) controller that
generates joint motor angular velocity command signals given the joint
angle commands. The torque-wheel controller uses collocated rate
feedback.
JOINT-MOTORS
6.7 m ARMS_, /
204 kg ...---" TORQUE-
-__ WHEEL,
x 38.4 kg .1 m/s
293
BATCH SIMULATION RESULTS
A simulation study was undertaken to suppress vibrations
following an abrupt stop input. The arm was given command inputs for
horizontal translation of the end-point at a constant velocity for 10
seconds followed by an abrupt stop The time history compares the
vertical motion responses, which are ideally zero, both with and without
the torque-wheel. The torque-wheel, while operating within its design
capability, substantially affects the second overshoot of the vertical
motion response. The conclusion is that, subject to the limitations of a
batch simulation, a torque-wheel of the size developed at the NASA
Langley Research Center can be of value in the control of the arm.
O
>
0
-5
-10
Stop cmd , _ ", Joint Motors Only
issued here t _ J
S_p _ -
II
onents
I and Joint Motors
]_ 82 % REDUCTION
_1 IN OVERSHOOT
-158 9 10 1'1 1_2 1'3 1'4
Time, s
15
294
REAL-TIME SIMULATOR
INTEGRATED SOFTWARE ENVIRONMENT
Modelling, control, and simulation of flexible link manipulators requires a
set of reliable and efficient software tools. Symbolic manipulation programs
represent one of the most versatile software environments currently available
for modelling complex dynamical systems. This versatility provides the
researcher a high degree of flexibility in terms of the ability to implement
theoretically different modelling methods. In addition to their versatility,
most of the symbolic manipulation programs have the ability to be integrated
with commercially available control design packages. The integration of
different software packages is primarily related to the sophisticated pre- and
post-processing capabilities available within "the respective packages. The
figure below presents an integrated software environment which utilizes two
commercially available packages for modelling and control, and an in-house
developed real-time simulator. The symbolic manipulation program is used
to generate executable "C" code for the flexible link manipulators system
models. This code is then the input to the control package's preprocessor
which converts the "C" code into executable script. Control design may
then be accomplished by uploading the preprocessed script. The output of
the control design, i.e., the gain matrices, may then be directly uploaded
into the real-time simulator.
CONTROL
MODELING
'_TTc°_g_"JJ_i_
// REAL TIM E 114"---_ __ "L','A'_'_-kr_._ll..I
l s,,.,,U,_Ar,o,jt J
SIMULATION vl , , , , , . , , , , , , , , ,
295
REAL-TIME SIMULATION ENVIRONMENT
To evaluate the usefulness of inertial actuators for maneuvering and
vibration control of single and multi-link flexible manipulators, a real-time
man-in-the-loop simulator has been developed. This simulator utilizes a SUN
workstation to graphically display the dynamic response of the manipulator
system as well as permit man-in-the-loop control through the use of an
external input device. The workstation serves as a computational platform
which is used to sample the input device, solve the nonlinear equations
of motion, and graphically display the resulting motion. Currently, several
manipulator models have been developed and successfully implemented in
this simulator. These include both a two-link rigid arm and a single link
flexible arm. In addition to simulating elastic and rigid body motions,
task scenarios are also simulated. A typical task involves maneuvering the
manipulator to a payload, capturing the payload, and then maneuvering the
payload/manipulator system to a specified target. This payload capture task
will facilitate the further evaluation of inertial actuators for man-in-the-loop
control of flexible manipulators. The simulator, as shown below, consists
of three processes which pass data back and fourth via the shared memory
UNIX interprocess communication facilities resident on a SUN workstation.
SUN
PROCESS 1 PROCESS 2
GRAPHICS ] [ DYNAMICSX Wlndow= & CONTROL& PIXRECT
SHARED MEMORY
UNIX Sys(em Calls
i,ii l i l l_ll iI II inl nllt
i SUBNITIAROU::NES: i
• l
: .A_B.
e CONTROL GNN$ •
: oo(sotwR :
: tORCO_.OL i
• SCREEN TRANS- •
FORMATIONS •i............. ;
296
REAL-TIME SIMULATIONS
MODEL
The model used in the real-time simulator is a long single link flexible
manipulator that is similar in physical dimensions to the Space Shuttle's
Remote Manipulator System (RMS). The flexible arm, as shown below, is
equipped with three actuators, one hub actuator and two inertial tip actuators.
The inertial actuators used for this model are a torque-wheel, which is used
to provide a torque input about the ann's bending axis, and a reaction-mass
actuator to provide an input force in the ann's plane of motion.
Model Parameters
II
HUB MOTOR
Tmax=2.8 N-M, Ulmax=213 rad/s, Emax-50 Volts
I II I I I I I
TORQUE-WHEEL MOTOR
Tmax=67.8 N-M, _max=6.5 rad/s, Emax=50 Volts
ir
REACTION-MASS MOTOR
Fmax=128 N, _rpmax=l M/s, Emax=50 Volts
III I
ARM
p=55.16 Kg/m, E=l.38ell N/M 2, L=13.42 M, I=2.08e-5 M 4
III I I i
REACTION- A
MASS (p TORQUE-
__WHEEL
ARM A y(x,t)
HUB _MOTOR
eBeeee eeee eee • m'ee ee eeeeel_l_
297
REAL-TIME SIMULATION RESULTS
The time history results, as shown below, present the disturbance re-
jection capabilities for both models, i.e., models with and without inertial
actuators. The disturbance considered herein was a perfect first mode dis-
placement initial condition. The analysis and control models both considered
three flexible modes, corresponding to the first three "cantilever type" modes
with the appropriate boundary conditions to account for the inertial devices.
For control system design, a full-state feedback law was obtained using Lin-
ear Quadratic Regulator (LQR) design theory. The selection of the Q and R
weighting matrices required several iterations to satisfy state constraints on
the inertial actuators, e.g., maximum torque-wheel velocity, reaction-mass
stroke, and reaction-mass velocity. The objective for the controller design
was to achieve operation of the inertial devices near their maximum spec-
ifications. The simulation results show that the model using the inertial
actuators (solid line), for this type of disturbance, reduced the tip position
settling time by more than sixty percent over conventional hub motor only
control (dotted line).
50
0
50[
-100 /
0
ttub Voltage
_j
5
Time (sec)
10
5 Tip Pos (ram)
//'-,'
',.,,
i I
| i
i t
I
-10
0 5 10
Time (sec)
298
HARDWARE TESTBED
FLEXIBLE ARM EXPERIMENTAL SETUP
The flexible arm test article consists of three major elements: a
base-mounted hub assembly (on the left), a flexible beam (at the
center), and a beam-tip sensor-actuator assembly (on the right). The
hub assembly is comprised of a gimbaled bracket (to provide a
rotational, one degree-of-freedom motion to the root of the beam about
the vertical axis), a torque motor (to drive the gimbaled bracket), a low
rate capability tachometer, and an angular position resolver (to provide
rate and position measurements, respectively). The design of the beam
was accomplished using constrained optimization. The constraints
consisted of maintaining 2-deg/ft-lbf in static torsional stiffness and a
0.8-Hz for the frequency of the lowest bending mode. The optimization
criteria was to minimize the end deflection of the beam as subject to
gravity loading. This produced an 891-mm long beam with a
75x2.38-mm rectangular cross-section made of A1 6061-T6. The tip
sensor-actuator assembly is comprised of a torque wheel with an optical
sensor for flywheel rate detection. Both manual and automatic control
tests can be conducted using a control computer that has A/D and D/A
converters and a timer for precise timing of data sampling processes. An
analog-output, hand-controller will be used to provide manual inputs.
A linear accelerometer will be mounted on the tip bracket to sense the
tip acceleration. A proximity sensor, mounted on an independent
pedestal near the tip of the beam, generates a signal for feedback
control, for driving an oscillograph display, and for performance
monitoring of manual and automatic control tasks.
299
BASE-MOUNTED HUB ASSEMBLY
The photograph depicts the gimbaled bracket with the mounted
beam. The hub torque motor appears in the lower part and also provides
the support for the hub tachometer, mounted underneath the torque
motor housing. The angular position resolver (sine-cosine wire-wound
potentiometer) is mounted on the top end of the gimbal axle.
HUB TORQUE MOTOR AND TACHOMETER ASSEMBLY
The photograph depicts a bottom view of the hub torque motor
showing a view of the low rate capability tachometer.
300
BEAM-TIP SENSOR-ACTUATOR ASSEMBLY
The photograph depicts the tip sensor-actuator assembly. The
torque wheel fork-bracket provides support for the flywheel axle as
well as the optical sensor for flywheel rate detection. The linear
accelerometer will be mounted on the tip bracket to sense tip
acceleration. Near the tip an independent pedestal holds a beam
proximity sensor. This sensor generates a signal for feedback control,
for driving an oscillograph display, and for performance monitoring of
manual and automatic control tasks.
301



Evaluation of inertial devices for the control of large, flexible, space-based telerobotic arms

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