Increasing research is being done into industrial uses for the microgravity environment aboard orbiting space vehicles. However, there is some concern over the effects of reaction forces produced by moving objects, especially motors, robotic actuators, and astronauts. Reaction forces produced by the movement of these objects may manifest themselves as undesirable accelerations in the space vehicle making the vehicle unusable for microgravity applications. It is desirable to provide compensation for such forces using active means. This paper presents the design and experimental evaluation of the NASA three degree of freedom reaction compensation platform, a system designed to be a testbed for the feasibility of active attenuation of reaction forces caused by moving objects in a microgravity environment. Unique 'linear motors,' which convert electrical current directly into rectilinear force, are used in the platform design. The linear motors induce accelerations of the displacer inertias. These accelerations create reaction forces that may be controlled to counteract disturbance forces introduced to the platform. The stated project goal is to reduce reaction forces by 90 percent, or -20 dB. Description of the system hardware, characterization of the actuators and the composite system, and design of the software safety system and control software are included

Birkhimer, Craig

Choi, Benjamin

Lawrence, Charles

Newman, Wyatt

English

NASA Technical Reports Server

N94- 33304
DESIGN, CHARACTERIZATION, AND CONTROL OF THE NASA THREE DEGREE
OF FREEDOM REACTION COMPENSATION PLATFORM
Craig Birkhimer and Wyatt Newman
Case Western Reserve University
Cleveland, Ohio
and
Benjamin Choi and Charles Lawrence
NASA Lewis Research Center
Cleveland, Ohio
Introduction
Increasing research is being done into industrial uses for the microgravity
environment aboard orbiting space vehicles. However, there is some concern over
the effects of reaction forces produced by moving objects, especially motors,
robotic actuators, and astronauts. Reaction forces produced by movement of these
objects may manifest themselves as undesirable accelerations in the space
vehicle, making the vehicle unusable for microgravity applications. It is desirable to
provide compensation for such forces using active means.
This paper presents the design and experimental evaluation of the NASA
three degree of freedom reaction compensation platform, a system designed to be
a testbed for the feasibility of active attenuation of reaction forces caused by
moving objects in a microgravity environment. Unique "linear motors", which
convert electrical current directly into rectilinear force, are used in the platform
design. The linear motors induce accelerations of the displacer inertias. These
accelerations create reaction forces that may be controlled to counteract
disturbance forces introduced to the platform. The stated project goal is to reduce
reaction forces by 90%, or -20 dB. Description of the system hardware,
characterization of the actuators and the composite system, and design of the
software safety system and control software are included.
System Hardware
Figure 1 shows the design of the platform system. The platform system
consists of a passive spring-mass-damper with added active components and
sensors. The passive system attenuates forces at frequencies greater than the
resonance, and passes forces at frequencies below the resonance. Figure 2
shows a Bode plot of the transfer function from the disturbance force applied to the
platform to the residual force felt at the mechanical ground. Since the passive
system provides at least -20 dB disturbance attenuation for frequencies above 88
rad/s, the active system design should be most concemed with disturbance rejection
below that frequency. The resonant frequency could be lowered by decreasing the
spring constant, at the expense of larger platform excursion, or by increasing the
system mass, which may not be desirable in a space-going system. Also, damping
could be added to reduce the effect of the resonance, but this may spread the
phase transition over an unacceptably large frequency range.
The displacers of the linear motors are constrained to vertical motions with
respect to the platform, and can thus react to vertical disturbance forces (along the
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https://ntrs.nasa.gov/search.jsp?R=19940028798 2020-06-16T12:10:07+00:00Z
z-axis) and moments about the x- and y-axes. The motors are each capable of 712
N maximum force. All have a displacer mass of 5.6 kg, and a stroke of 0.3 m.
Some insight can be gained by using the maximum force rating of the motors and
the stroke limit to plot force and position attainable as a function of frequency, as
shown in Figure 3. Below 4.8 Hz, the force available is limited by the position
constraint; above that frequency, the position amplitude is limited by the maximum
force constraint. Therefore, it is safe to attempt control at high frequencies, while
commanding a large-amplitude control signal at low frequencies may be unsafe or
ineffective. The switch frequency could be decreased by increasing the mass of
the motor displacer, which may be undesirable, or by increasing the displacement
limit, which would require replacing the motors. Increasing the motor mass would
have the added effect of decreasing the maximum velocity, which would decrease
forces due to friction and back-EMF.
All of the motors are equipped with optical incremental encoders accurate to
10 I_m, home switches, and limit overrun switches. In addition, each motor is
equipped with a compressed air "spring" support system to counteract forces due to
gravity on the displacers. Maximum velocity of the motor displacers for sinusoidal
force inputs is 4.2 m/s.
The force sensors and accelerometers are piezoelectric and are effectively
high-pass filtered with a time constant of 2.5 s due to their design, making control of
low frequencies using these sensors impossible. The force sensors have a
maximum rating of 2670 N, and the accelerometers have a maximum rating of 98
m/s2.
Communication between the control program and the motors and sensors
takes place through a Programmable Multi-Axis Controller (PMAC) board. This
board does encoder interpretation and velocity estimation for the motors, receives
information from the sensors, performs commutation for the three-phase motors,
and sends current commands generated to the motors. Motor force commands are
sent out at 2.3 kHz. The board also performs auto-shutdown of the motors in case
of a position limit fault. The PMAC board has a built in high-level motion control
language, which is interpreted in real time rather than being compiled; this makes
program execution very slow, and unsuitable for running extensive control
programs.
The actual control takes place on a 80486-based PC running at 33 MHz.
The control program is written in C, and compilation is optimized for speed by using
some of the features of the 80486 microprocessor. The control loop runs at 1.1
kHz.
Characterization
Without accurate modeling of motor and composite system behavior, high-
performance control is not possible. In particular, information on the force constant,
mass, friction, maximum force and velocity, and bandwidth of each motor are
needed before any active compensation using the motors can be attempted.
Although the motors have electrical and mechanical characteristics very similar to
148
three-phase rotary motors, the mechanical stops prevent the use of rotary motor
characterization techniques. Instead, techniques similar to those utilized in
robotics were used to prevent motor damage[I]. These methods use small cyclical
forces or motions to obtain data on motor parameters.
During the characterization, it became apparent that there were some
dynamics in the motor and/or the air spring that had not been accounted for.
Further examination revealed the presence of a position-dependent force offset.
This offset requires that, at a certain position, the motors must exert a constant force
to prevent the motor displacers from accelerating. The offset is probably the result
of a "detent force," an attraction of the motor displacers to certain positions along
their tracks, plus position-dependent air spring dynamics. The data taken for one of
the motors, and the function used to model this phenomenon, are shown in Figure
4. The modeling function takes the form of a sinusoid-plus-slope-plus-constant.
Control
The control consists of three discrete parts: the force feed-forward controller,
which directly responds to incoming forces read from the force sensors; the
acceleration feedback controller, which responds to accelerations of the platform
mass; and the motor position controller, which attracts the motors to equilibrium
position, provides software damping for the motors, and also acts as a primary
safety system.
The feedforward force control is a very straight-forward design, similar in
principle to methods used in audio noise reduction. The disturbance forces are
obtained by the force sensors; the signals are then negated (phase inverted) and
reapplied using the actuators. Performance is limited by the design of the force
sensors, motor modeling errors, and the digital delay inherent in all digital systems.
Although only preliminary data has been collected on this control scheme,
simulations have shown that 20 dB attenuation is achievable for frequencies
between 55 rad/s and 150 rad/s.
Control of the platform using feedback of the acceleration data proved to be
a difficult problem. Phase shifts due to the platform itself, the piezoelectric nature of
the sensors, and the time delay inherent in digital systems combined to cause
problems with stability and control bandwidth. Classical control methods would
produce the desired disturbance attenuation at high frequencies only at the
expense of disturbance amplification at low frequencies, and state-space control
seemed encouraging in simulation, but was too sensitive to partly measured or
unmeasured values.
It is necessary to have a motor position controller to attract the motors toward
zero position, so that disturbances caused by the motor triggering the safety system
are kept to a minimum; it is also desirable to have velocity control to provide
damping. The proportional-derivative (PD) control scheme is well documented and
seems suitable for this task, but closer examination reveals limitations in this
scheme. In order to insure that the limits are never overrun, a PD-controller would
149
have to have a resonant frequency of about 36 rad/s, significantly degrading the
lower frequency response of the combined controller.
To alleviate this problem, higher-order functions of position and velocity are
used to achieve a bumper-like effect. These types of functions tend to have small
effect at high frequencies or small amplitude motions, but large effect at low
frequencies or high amplitude motions. This has the effect of allowing high
frequencies, but attenuating low frequencies where the motor cannot exert full force
safely. Careful selection of the gain parameters allows only slight degradation in
frequency response of the force and acceleration controllers, while providing
another level of safety for the motors and attracting motor displacers toward
equilibrium position.
Unfortunately, operation of the nonlinear "bumper" is directly opposed to
operation of the acceleration controller. Any control effort from the bumper shows
up at the platform as an acceleration; if the acceleration controller is working
properly, it will then attempt to cancel this acceleration by applying an opposing
force, defeating the purpose of the bumper controller. This problem can be solved
by including a reference term before the acceleration controller, that is a result of
the bumper control effort filtered through the plant model to give an acceleration.
See Figure 5.
In addition, superimposing the desired forces from all the controllers may
result in a condition where the desired bumper force is defeated, leading to a motor
collision and possible damage. To avoid this, the desired forces from the force
sensor and accelerometer loops are filtered through a nonlinear function that is
dependent on the desired bumper force. The forces are superimposed only if the
sign of the combined force is the same as that of the bumper force; if the signs are
opposite, the combined force is multiplied by a gain of between zero and one,
depending on the magnitude of the bumper force. Lower gain is applied for higher
bumper force, so that the bumper force takes higher precedence. This policy is
summed up in the following equation: Four= Fb+f(Fb)Fc, where Fb is the desired
bumper force, Fc is the desired control force, and f(Fb) is a continuous function
which equals 0 for Fb greater than an upper threshold value, 1 for Fb less than a
lower threshold value, and decreases linearly from 1 to 0 for values of Fb between
the two threshold values.
Conclusions
The force and stroke limits of the motors both serve as actuator
saturation limits. The force limit sets the saturation at high frequencies, while the
stroke limit sets the saturation at low frequencies.
Classical control proved to be ineffective for control in the acceleration
feedback loop. Control using classical methods yielded either small attenuation of
forces or attenuation at high frequencies only at the expense of amplification at low
frequencies. Also, the use of state-space methods in the acceleration controller
proved to be ineffective due to oversensitivity to partly measured or unmeasured
150
quantities, and the inability of state-space controllers to accept reference inputs in
the case of the platform system [2].
The nonlinear "bumper" position and velocity controller proved to be more
desirable than the commonly-used PD controller due to the bumpe:s lower force
commands for high frequency/low amplitude motor motion. This allowed greater
bandwidth of the combined controller.
The anticipated force disturbance rejection for the combined system is at
least -20 dB attenuation for frequencies greater than 55 rad/s, which will extend the
lower bandwidth by 33 rad/s below that of the passive system alone, without an
increase in platform mass or decrease in spring stiffness.
References
1. Velasco, Vergilio B., Jr. "Characterization and Control of the Unique Mobility
Corporation Robot Prototype." Case Western Reserve University thesis,
December, 1990, pp. 53-56
2. Franklin Gene F., J. David Powell and Michael L. Workman. Digital Control of
Dynamic 5/stems. New York: Addison-Wesley, ©1990, pp. 654-720
m molors
forcesensors
accelerometers
Figure 1 Diagram of the platform system.
! i iiiiii = : : = ==;:
:'1....... ;'! ':'! ,:! I i _: :',, - .... _=- '--;--4--;--;-<_- " "-
i i , i i | i J
101 102
Frequency (rad/sec)
_, )o .......... L.....L ..I...L.J..L.LLj .............
I_ 10' *_
Frequency (rsd/sec)
Figure 2 Bode plot of the disturbance force to ground
transfer functzon for the passive platform system.
151
O.tg "i ' '" ........ ' ........ i iilO0
°."J _..............................
=,o4 /_ .......=' ,oo0,0,4 f\
._i / \ ,t°'1 ./\ 1+
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_"+"" 0
1 10 100
tt-e<:p.m_-y(mmV,sl
Figure 3 Maximum position and force versus frequency.
30 detenteforce.dataandcompensatolfunction
N M
X X
20
_ io
-I0
-20 ' '"
-0,15 -0,1 -0.05 0 0,05 0.1 0.iS
motor position (m)
Figure 4 Characterization and m:x:lei' of the
detente force for one of the motors.
Desired _ll _
acceleration occeler,edon
loop b_ controller
ram%,',',,
e¢ciileri_on
reference
.J 0"006605s2 L
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' Bumper I--Yi = !do_r position
t'
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Figure 5 Block diagram showing the. correction_"
for opposing acceleration and posztlon control.
152


Design, characterization, and control of the NASA three degree of freedom reaction compensation platform

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