The MAST Flight System as a test bed for large space structure control algorithms is discussed. An overview is given of the control system architecture. The actuators, the sensors, the control computer, and the baseline damping algorithm are discussed

Davis, L.

Ham, F.

Hyland, D.

Otten, T.

English

NASA Technical Reports Server

N87-16032
MAST FLIGHT SYSTEM DYNAMIC PERFORMANCE
L. Davis
D. Hyland
T. Otten
F. Ham
First NASA/DOD CSl Technology Conference
Norfolk, Virginia
November 18-21, 1986
281
https://ntrs.nasa.gov/search.jsp?R=19870006599 2020-03-20T12:05:45+00:00Z
,, This discussion focuses on the MAST Flight System as a test bed for
large space structure control algorithms. After giving an overview of the whole
system, four main topics will be covered: the actuators, the sensors, the control
computer, and the baseline damping algorithm.
282
MAST PLIGHT SYSTEM DYNAMIC PERFORMANCE
1. Overview of the control system architecture
2. Placement of sensors and actuators
3. LDCM design and performance
4. Sensor performance and models
5. Computer architecture and performance
5. Baseline damping system performance
283
CONTROLSYSTEMARCHITECTURE
The system to be controlled is the 54-bay mast. Sensors and actuators
are placed at the various stations indicated. The control computer system
communicates wlth the sensors and actuators vla a 1553 data bus.
The actuators are Harrls-designed Linear DC Motors (LDCMs). Ten of
these actuators are placed at various stations along the length of the mast. The
four larger, Type I actuators are placed at the tip to provide control over both
the primary bending modes and the torsion modes. The remaining, Type II actuators
provide good control authority over the second and third bending modes.
The control sensors consist of 18 linear accelerometers, 5 angular
accelerometers, and 6 rate gyros. Sensors are collocated with the actuators and
placed at additional locations to measure the motion of the structure accurately.
The slgnal-to-nolse ratio is at least 40 dB for signals to be expected during the
experiment.
The control computer system samples all sensors and commands all
actuators at a rate of 150 Hz. The control computations are performed in the EDS
computer, whlle the MDIS computer handles communications both with the sensors and
actuators and with the Shuttle.
v
BAY 54
X
BAY 44
X
BAY 38
X
BAY 30
X
BAY 24
X
BAY 12
X
BASE
_43 RATE GYROS
LINEAR ACCELEROMETERS
PRIMARY ACTUATORS
_22 ANGULAR ACCELEROMETER
LINEAR ACCELEROMETERS
SECONDARY ACTUATORS
_ LINEAR ACCELEROMETERS
ANGULAR ACCELEROMETER
LINEAR ACCELEROMETERS
ANGULAR ACCELEROMETER
SECONDARY ACTUA ORS
_ LINEAR ACCELEROMETERS
ANGULAR ACCELEROMETERS
_ LINEAR ACCELEROMETERS
ANGULAR ACCELEROMETER
SECONDARY ACTUATO S
_ RATE GYROS
LINEAR ACCELEROMETERS
MDIS
1553 BUS
I E'DS
284
SDSS
TYPE 1 LDCM
The Linear DC Motors (LDCMs) were designed by Harris to act as the
control actuators for the MAST Flight System. Each LDCM consists of two pieces:
the fixed primary, which houses the motor coils, and the moving secondary, which
houses the permanent magnets and provides the reaction mass for the actuator.
The primary coils are rigidly mounted to the motor frame. They are
switched to varying currents based on the commanded force and the position of the
moving secondary as measured via the commutation vanes. The fixed coils rest
between the return irons and the permanent magnets of the secondary. The eight
bearings ensure that the secondary remains centered laterally while keeping
friction to a minimum.
COMMUTATIONVANES
MOTOR FRAME
PRIMARY
COIL CASES
_PERM.MAGNETS
OUTERRETURNPATH
285
POSITIONLOOPOPERATION/ EQUIVALENT USER MODEL
The LDCMs are compensated to provide the user full access to the
capabilities of the devices while ensuring that they operate within their
limitations of stroke and force.
The compensation works, in the absence of user input, to keep the LDCM
secondary fixed in inertial space in spite of transient mast motion. This serves
to decouple the operation of the LDCM from the dynamics of the mast. In this mode,
the stroke of the actuator must be divided between the motion of the mast and the
motion due to control inputs. The total stroke of the device minus the allowed
motion of the mast becomes the effective stroke available for control.
The transfer function as seen by the user through the EDS command is
tailored to give access to the full stroke capability at low frequencies and the
full force capability at high frequencies. The limit on the user command ensures
that the LDCM does not exceed these capabilities.
POSITION LOOP OPERATION
EDS __ LIMITER _-_
COMMAND
°c"-1HPOSITION INERTIALPOSITIONCO LLOCATED ESTIMATOR
ACCELEROMETER
COMMAND
COMPENSATION
POSITION
COMPENSATION
COMMANDED
MOTOR
FORCE
i,=
v
EQUIVALENT USER MODEl
COMMAND
MG 2 S 2
(S+ _ )2
OUTPUT
FORCE
286
COMPENSATED LDCM SPECIFICATIONS
The overall specifications for the compensated LDCMs are shown below.
The mass differential between the Type I and Type II actuators_ combined with their
allowed stroke llmitatlons_ combine to determine the compensated bandwidth sigma.
The user command Is limlted by a single value such that at low frequencies the
stroke is not exceeded. The bandwidth is then adjusted such that at high
frequencies the force limlt is not exceeded.
The user has access to the relative position of the secondary wlth
respect to the primary of each LDCM. This accurate measurement comes from the
commutation vanes of the secondary.
Accurate knowledge of relative secondary position has allowed Harris to
reduce force ripple below i% of nominal output force and additive force errors due
to position feedback below 0.05 N, thus providing users with a quiet actuator for
vibration suppression.
TRANSFER FUNCTION: FORCE OUT
CMD IN
m o 2 S 2
(S+ o) 2
m = 11 Kg, o = 2_ RAD/SEC (TIP)
m = 7 Kg, o = 5.5 RAD/SEC (INTERMEDIATE)
STROKE: _+15 cm (TIP)
_+7cm (INTERMEDIATE)
FORCE OUTPUT: _+30 N (TIP)
_+15 N (INTERMEDIATE)
POSITION MEASUREMENT
RESOLUTION: _+0.48 mm
OUTPUT NOISE: RIPPLE <1% OF OUTPUT
0.05 Nt RANDOM ADDITIVE
287
SENSORSPECIFICATIONS
The sensor complement has been chosen to measure the expected signals
with 40 dB of signal-to-nolse ratio. In some cases, the 12 bits of digital
information available to the computer were not enough to reflect the accuracy of
the sensor. Low ranges are used to expand the dynamic range seen by the computers.
LINEAR ACCELEROMETERS:
RANGE: +/- 1.0 G (HIGH RANGE)
+/- 0.1 G (LOW RANGE)
RESOLUTION: 500 MICRO-G (HIGH RANGE)
50 MICRO-G (LOW RANGE)
BANDWIDTH: 0-50 Hz MIN. (150 Hz MAX.)
ANGULAR ACCELEROMETERS:
2
RANGE: +/- 10 RAD/SEC
RESOLUTION: 0.01 RAD/SEC 2
BANDWIDTH: 0-40 Hz MIN. (200 Hz MAX.)
ANGULAR RATE GYROS:
RANGE: +/- 1 DEG/SEC
RESOLUTION: 0.0005 DEG/SEC
BANDWIDTH: 0-80 Hz
STRAIN GAUGES:
RANGE: +/- 7500 MICROSTRAIN
RESOLUTION: 50 MICROSTRAIN
THERMISTERS:
RANGE: +/- 100 DEG C.
RESOLUTION: 0.5 DEG C.
288
SENSORMODELS
The sensors to be used for control are inertlal. They conform roughly
to the models shown below. The indicated noises are white, zero-mean processes.
Their intensities were chosen to reflect either the resolution of the sensor or the
manufacturers' estimates of rms noise amplitude, whichever was coarser. The
bandwldths are the lowest available for the type of sensor under consideration.
a
(TRUE VALUE)
v%l
i,
v
2
o)o
S 2 +2_ (oo S+(oo 2 _(ME amASURED
' VALUE)
n (NOISE) ,,, (O,v)
(OO = 2_:'50 "_
_= 0.7
V = 220 X 10 -6 g2/Hz
FOR LINEAR ACCELEROMETERS
COo = 2_.40
_= 0.6
V = 60.32 X 10
-3 (rad/sec 2 )2/H z
FOR ANGULAR ACCELEROMETERS
COO = 2_;'80 b
_= 0.5
V = 251.3 X 10 -6 (°/sec) 2/Hz
FOR RATE GYROS
289
CONTROLSYSTEMCOMPONENTS
The control computer system has four major components: the MDIS
computer, the EDS computer, the 1553 bus system, and the remote units.
The MDIS computer controls the overall operation of the system. It
governs the modes of operation, the communication to and from the Shuttle, and the
collection and dissemination of control data and commands.
The EDS computer performs the computations necessary to execute a user's
control algorithm. It receives data directly from the remote units, processes It,
and sends its commands to the remote units, all as coordinated by the MDIS.
The 1553 data bus serves as the communication llnk among the other parts
of the system.
The remote units are located at the various bays along the length of the
mast. Each unlt governs the operation of the instrumentation at Its own particular
bay. On command, it collects data from each of the instruments, formats it, and
sends it on the 1553 bus. In slmllar fashion it receives data from the bus,
decodes it, and distributes it to the actuators.
v
BAY 54
X
BAY 44
X
BAY 38
X
BAY 30
X
BAY 24
X
BAY 12
X
BASE
_43 RATE GYROS
LINEAR ACCELEROMETERS
PRIMARY ACTUATORS
_ ANGULAR ACCELEROMETER
LINEAR ACCELEROMETERS
SECONDARY ACTUATORS
_ LINEAR ACCELEROMETERS
ANGULAR ACCELEROMETER
_ LINEAR ACCELEROMETERS
ANGULAR ACCELEROMETER
SECONDARY ACTUATO S
<_ LINEAR ACCELEROMETERS
ANGULAR ACCELEROMETERS
_ LINEAR ACCELEROMETERS
ANGULAR ACCELEROMETER
SECONDARY ACTUATO S
3 RATE GYROSLINEAR ACCELEROMETERS
MDIS
SDSS
1553 BUS
I EDS
290
1553 BUSTRANSFERANDPROCESSINTERVALALLOCATIONS
The operation of the computer system during one cycle is shown below.
First, data is transmitted from the EDS computer to the remote units, which then
distribute them to the actuators. At the same time, the remote units collect the
sensor data for the next cycle. This data is then sent back down to the EDS
computer for processing. The user's algorithm is executed, and a new cycle is
begun.
The computer system keeps the Jitter under 30 microseconds for one
cycle. This ensures clean, distortion free operation of the control algorithms.
I_/_1 t(k)834MsEXCITATION TRANSFER TO ACTUATORS _
BC; t(k) SAMPLE OF SENSORS; APPLY t(k) EXCITATION24 Ms
t(k) SAMPLE TRANSFER TO EDS AND MDIS776 Ms
t(k)
v
t(k) SAMPLE TRANSFER TO MDIS690 MS
PROCESS t(k) SAMPLES
NEEDING SENSOR DATA
5033 MS MIN AT 150 Hz
6699 MS MIN AT 120 Hz
800 MS MIN
1634 MS MAX
t(k + 1) EXCITATION TRANSFER
TO ACTUATORS
BC
t(k + 1)
291
BASELINE DAMPING ALGORITHM
The MAST Flight System includes a baseline damping control algorithm.
This algorithm Is used as a backup in the case of user algorithm failure or between
testing periods to bring the system to a quiescent state.
Harris has developed a baseline control system that is designed to damp
the first 10 modes of the mast safely and rellably. It achieves the goal of 5%
damping of these modes while operating within the constraints of the hardware. The
decentralized approach uses the concepts of positive real design to assure that the
stated goals wlll be achieved in spite of unexpected mission variations.
The damping system operates by first canceling the two poles of the
LDCM, then introducing two additional poles: one pole cancels a zero of the LDCM
transfer function, the other determines the frequency at which the damping
operation of the loop begins. The parameters of the compensators at the various
stations are tailored to the modes which the stations affect. Of particular
interest is the trade-off between the damping achieved and the actuation level
required. The parameters are adjusted to balance these two factors.
(1)
(10)
EDS
COMPENSATOR
K(S+ _)2 HS(S+_)
LDCM I
MG2S 2
(S+ G)2
VELOCITY v
_1 K(S+ _)2 MG2S2 _1_(S+ _) 2
V
MAST
292
MAST FLIGHT SYSTEM MODAL DATA
ORIGINAL PAGE IS
OF POOR QUALITY
The baseline algorithm was applied to the fully extended mast having the
modal characteristics as shown. The modes apply to a mast connected to a simple
STS model.
The units of interest are MKS. The modal coefficients are amplitude
normalized such that the maximum coefficient of any degree of freedom in the model
Is unity. Mass normalized modes can be obtained by dividing the given coefficients
by the square root of the generalized mass shown for the corresponding mode.
1 2
BAY COMT. ist x-z Ist /1-z
12 X .0344
Y -.0007
_z .0002
24 X .2189
Y -.0006
0z -.0003
30 X .3486
Y -.0004
@z -.0003
38 X .5481
Y -.0001
@z -.0004
44 X .7111
Y .0002
Ez -.0004
54 X .9997
Y .0008
Eg -.0004
f* 0.1813
M** 470.46
MAST F11ght System Modal Data (Includes Orbiter)
Beam Length 54 Bays, Maximum PMD Inertia, LDCMs Locked
3 4 5 6 7 8 9 10
2nd y-z 2nd x-z ist Torsion _Lu_-J_=z _-A x-z 2nd Torslon 4th _-z 4th x-z
.0001 .0044 .4196 .0083 .0059 .9047 .0014 -.0004 .9968
-.1431 .3939 -.0027 -.0152 .8858 -.0052 -.0106 .9892 -.0006
•0000 .0197 -.0124 .2686 .0190 -.0070 .7243 .0155 -.0053
-.0008 .0086 .9158 .0191 .0032 .6197 .0024 -.0006 -.5821
.0125 .8948 -.0063 -.0355 .6240 -.0044 -.0126 -.5645 -.0006
.0000 .0375 -.0231 .5229 .0203 -.0064 1.0000 -.0091 .0043
-.0014 .0090 .9830 .0207 -.0006 -.0095 .0008 .0000 -.8769
.1600 .9682 -.0075 -.0387 .0004 -.0006 -.0072 -.8642 .0000
.0000 .0454 -.0280 .6401 .0153 -.0046 .9090 -.0170 .0074
-.0024 .0070 .7800 .0166 -.0045 -.7236 -.0019 .0003 .2165
•4072 .7745 -.0061 -.0316 -.7157 .0043 .0023 .2060 .0003
.0000 .0545 -.0338 .7810 .0617 -.0015 .5473 -.0120 .0058
-.0033 .0037 .4335 .0094 -.0044 -.7747 -.0025 .0002 .8883
.6133 .4337 -.0035 -.0187 -.7702 .0049 .0050 .8701 .0002
.0000 .0604 -.0376 .8743 .0003 .0008 .1687 -.0045 .0025
-.0047 -.0038 -.3883 .0080 .0016 .2212 .0006 -.0003 -.1605
1.0000 -.3807 .0031 .0120 .2187 -.0015 -.0011 -.1572 .0002
-.0001 .0680 -.0426 1.0000 -.0063 .0036 -.4967 .0004 -.0023
0.2387 1.2276 1.2773 1.3004 3.5079 3.6584 4.3637 6.0100 6.2370
802.76 260.71 264.12 130.76 223.43 227.79 82.97 266.61 273.68
* Natural Frequency, Hz
** Generallzed Mass, kg or kg-m 2 as approprlate
293
54 BAYPOLESHIFTSFORBASELINE
The pole locations shown indicate that the goal of 5% damping has been
achieved. The open-loop damping reflects an assumption of 0.2% damping for the
first bending modes, 0.3% damping for the second bending modes, and 0.5% damping
for all higher modes.
\
\
\
\
\
[] [] \
\
\
\
[]
%
\
[] \
\
\
[]
\
\
m
\
\
\
\
\
\
\
\
40.0
30.0
t_
20.0 <
=E
10.0
-2.0 -1.5 -I .0 -0.5
REAL
0.0
0.0
54 BAY POLE SHIFTS FOR BASELINE
294
DISTURBANCE PSD
Random excitation was applied at bay 44 in the x and y directions to
both the open- and closed-loop systems. The other stations were used for control
in the closed loop. The disturbance was as shown in the diagram. Independent
disturbances were applied in the x and y directions.
I0
A
N -I
= 10
N
O
a.
a
e_
O -210
10"3
10-4
10-1
!
I I I I I I Ill
1
I
I I I I I I III
10
FREQUENCY (Hz)
DISTURBANCE PSD (EACHAXIS)
I I I I I I I
111 2
295
TIP X ACCELERATIONS
The acceleration power spectral densities (PSDs) for the mast tip in the
x direction are shown here for the open- and closed-loop cases. It is easy to see
the marked smoothing achieved by the control system. In both cases, sufficient
signal is detected to be measured by the available sensors.
A
N
A
v
Q.
¢J
¢J
I I I I i I ill
1
II I i i I I I li
10
FREQUENCY (Hz)
TIP X ACCELERATIONS
I l I I ,
10 2
296
TIP Y ACCELERATIONS
Similar results are shown here for the y direction at the mast tip.
Once again, marked smoothinE of the PSD curve is indicative of the improvement in
damping achieved by the control system.
10"5
A
i,,,,i
-,r.'
A
E
eL
,.-'I.
f.p
10-10
10-1
FREQUENCY (Hz)
TIP Y ACCELERATIONS
10 10 2
297
TIP Z ROTATIONALACCELERATIONS
The torsion results shown in this diagram, while smoothed, are not as
clear-cut as the bending results. This is due chiefly to the lack of direct
torsion excitation by the disturbance. In this case, excitation is due to the
couplinE of the bending and torsion modes and to residual spillover of the control
system at the mast tip.
10"5
10-15
10-1
FREQUENCY (Hz)
TIP Z ROTATIONAL ACCEL
10
I I I [ I I I I
10 2
298


MAST flight system dynamic performance

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