This paper presents the design and developmental testing associated with the bearing, motor, and roll ring module (BMRRM) used for the beta rotation axis on International Space Station Alpha (ISSA). The BMRRM with its controllers located in the electronic control unit (ECU), provides for the solar array pointing and tracking functions as well as power and signal transfer across a rotating interface

Obrien, David L.

English

NASA Technical Reports Server

N94- 33295
INTERNATIONAL SPACE STATION ALPHA'S BEARING, MOTOR, AND ROLL
RING MODULE DEVELOPMENTAL TESTING AND RESULTS
David L. O'Brien
Rocketdyne Division
Rockwell International
Canoga Park, California
ABSTRACT
This paper presents the design and developmental testing associated with the
bearing, motor, and roll ring module (BMRRM) used for the beta rotation axis on
International Space Station Alpha (ISSA). The BMRRM with its controllers located in
the electronic control unit (ECU), provides for the solar array pointing and tracking
functions as well as power and signal transfer across a rotating interface.
INTRODUCTION
The BMRRM is part of the beta gimbal assembly (BGA), as shown in Figure 1.
The BMRRM is located between the beta gimbal transition structure (which deploys
the BGA and solar array away from the station) and the BGA platform. The
sequential shunt unit, ECU and solar array are all attached to the BGA platform.
The beta rotation axis is the second of two axes required to allow maximum use
of solar power for the electrical systems aboard the space station. The beta axis
servocontrol compensates for both the seasonal and orbital changes in the station's
orientation to the solar vector (line,of-sight). Under the ISSA program, nominal beta
axis rotational rates vary from zero to 0.096 rad/d (five degrees per day). Shuttle
docking (plume loads) and extravehicular / intravehicluar operations also define
expected beta axis motions. The maximum allowable velocity is 0.076 rad/s
(240 degrees per minute), although the defa_t control parameters limit velocity to
0.025 ra.d/s. The beta gimbal was designed under the Space Station Freedom
requirements, which had an additional requirement of alpha axis rotation in early
flights, which is around 0.078 rad/min, (four degrees per minute). The leading
design drivers of the BMRRM are the beta axis servocontrol, power and signal
transfer through a rotating joint, and structural loading requirements. Small angle
oscillations are also expected due to vibrational modes of the station.
BMRRM DESIGN
The BMRRM consists of two sets of angular contact bearings, a brushless dc
torque motor, resolver, roll ring subassembly, antirotation latches, and a housing to
hold the components together. The electronics to operate the motor, latches, and
resolver are located in the ECU. A cross-sectional view of the BMRRM is shown in
Figure 2. The bearings, motor, and roll ring are all concentric to each other. The
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BMRRM's total mass is 63.5 kg, of which the roll ring is 27.2 kg, the motor 8.2 kg,
and the bearings 5.4 kg.
The angular contact bearings provide structural stiffness about five axes. The
bearing sets are separated by 0.5. m (20.inches), which accommodates bending
loads. The outboard bearing set (toward the solar array) supports axial loading. The
inboard set is free to move axially to accommodate thermal expansion and tolerance
stacking. Each bearing set was consists of two 0.45 m diameter angular contact
bearings mounted face-to-face and preloaded to 0.34 rad (18 degrees) contact
angle.
A brushless dc motor provides the torque about the beta axis. Due to the low
required torque of 1.4 N-m (t2 in-lb) plus friction losses (less than 2 N-m), a direct
drive motor was used. Eliminating a geared system helped pointing accuracy by
reducing frictions losses, thus reducing station vibration disturbances on the inertially
stable array. Eliminating the geared system also helped control stability by
abolishing backlash, reduced power consumption due to lower frictions losses,
reduced mass, and increased life (no gear wear). The motor is capable of providing
45 N-m torque (stall), resulting in about 8 to 1 torque margin. The motor is a 3-
phase, Y-wound, 64-pote device about 0.4 m in diameter. Figure. 3 shows an
outboard view of the BMRRM with the motor and roll ring connector.
The resolver, which is located within the roll ring subassembly, provides arc-
minute pointing accuracy knowledge for the proportional-integral-derivative (PID)
closed servoloop. The PID servoloop is a digital controller located in the ECU. The
BGNBMRRM does not use inertia or solar sensing instruments. The pointing
control comes from the station s guidance, navigation, and control system or the
ground, via the photovottaic controller unit. As a backup, the motor and controller
are designed to allow open-loop stepping. The resolver pointing knowledge is also
used for commutating the motor.
The roll ring subassembly provides bidirectional transfer of source power
(212 A), secondary and dc control power (less than 8 A) and MIL-STD-1553B data
signals. The roll ring, as beino, installed into the BMRRM, is shown on Rgure 4. The
transfer is across a rotating joint through slightly corn.pressed multiple rotating
flexures connecting the. inner and outer conductingrsngs. The rotating flexures ;
greatly reduce the sliding friction, allowing the BMRRM to be rotated with very low
torques. Most of the BMRRM's torsional friction comes from the angular contact
bearings.
There are two antirotation latches in the BMRRM each 1.77 rad
(92.8125 degrees) apart. There are 64 holes in the BMRRM housing flange;
therefore, by oscillating between the latches, 128 latching positions are available
(every 0.05 red or 2.8125 degrees). An antirotation latch is a paraffin actuated pull-
pin device. When 15 Vdc power is applied by the ECU the paraffin solid-to-liquid
phase change results in pulling the pin out of the latch hole and resets a toggle
mechanism. The next time power is applied the paraffin actuator toggles the
mechanism and allows the spring loaded pin to be pushed back into the latching
hole.
The BMRRM can be replaced on-orbit. To facilitate this the roll ring contains a
single input mating connector as shown in Figure 3. This connector includes all
power, motor, latch, and resolver lines.
52
BMRRM TESTS AND RESULTS
Four series of tests were performed: component functional, system functional,
thermal vacuum, and static structural. Both functional tests were performed in a
clean room environment at Rocketydne, Rockwell International, Chatsworth facility.
The thermal vacuum test was performed at Martin Marietta Aerospace, Denver. The
static structural test was performed at Rocketydne, Rockwell International, Canoga
facility. At the time of writing, 60 percent of the component and system functional
tests were completed. The static structural test was fully completed. Results of the
thermal vacuum and remaining functional tests will be presented at the conference.
Component Functional
The purpose of component functional testing was to verify the BMRRM design,
ensure B'MRRM assembly workmanship, verify the control model's component
subroutines, and verify some component performance requirements. Component
functional testing included friction, open-loop servo and position knowledge
accuracy. The BMRRM was installed onto an electrical test set, as shown in
Figure 5. The test set contained a torque cell, an external motor to rotate the
BMRRM, motor voltage sensors, motor current sensors, and a motor controller
(which simulates the ECU). The buildup and test sequence of the BMRRM is shown
on Figure 6.
The friction tests measured the resulting torque of the main bearings, roll ring
bearings, and motor clog_.ing under several conditions. Conditions included
constant velocity tests, in=tial torque tests, small angle dither tests, and open-loop
sine wave voltage inputs. Due to the low rotational rates the BMRRM exhibited little
viscous friction characteristics. Three rates were tested over a complete revolution:
0.076, 0.57, and 6.9 rad/min (4, 30, and 360 degrees per minute). The average
steady-state friction torque for the three rates were 1.2, 1.3, and 1.9 N-m,
respectively. However, over an operating range of zero to 0.078 rad/min the steady-
state friction changes less then 1 percent. The small angle and initial torque tests
show that there was no static friction involved. The friction closely resembles the
Dahl model with a Dahl slope of 565 N-m/radian and a steady state torque betw.een
1.1 and 1.8 N-m. Figure 7 compares the Dahl model and the friction test data Tor a
6.9 rad/min case. The friction "overshoot" shown was probably caused by motor
static torque, which includes cogging as well as hysteresis effects. When the motor
was tested independently a 1 N-m static friction was measured. Test set dynamics
may also play a part in this overshoot, details of which will be presented at the
conference.
Open-loop servo tests included back electromotive force (BEMF) and torque
motor constant. The BEMF test measured the voltage outputs of each phase while
the BMRRM was rotated at a constant 5.74 rad/m rate. The BEMF curves analysis
will be presented at the conference. The data will state the amount of torque ripple
caused by the motor. The torque motor constant test verifies controller motor power
train, that is (1) motor torque, (2) motor to controller alignment, and (3) the controller
current regulator. Prior to performing the torque motor constant test, the motor was
aligned to the resolver by applying current through the +C -B phases. The windings
were then rotated such that torque went to its stable zero (with constant current
through the given phases, the windings have a sinusoidal torque curve with two zero
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torques, one stable and one unstable). As shown in Figure 8, the torque constant
test was within 2 percent of the theoretical maximum value.
Position pointing accuracy and related alignment tests verified the pointing
knowledge requirements and provided the needed accuracy for commutating the
brushless motor. Position accuracy tests to measure resolver accuracy over a _
revolution range in both rotating directions were performed. Figure 9 shows a typical
resolver error DIot The resolver "zero" is adjusted mechanically to the alignment
support equiprnen't zero. The sinusoidal error is typical for resolvers and since the
error is repeatable it can be biased within the controller software.
System Functional
System functional testing included proportional hold, step inputs, rate inputs, and
latching. The latter three required the use of an inertia simulator. This support
equipment simulates the large inertia (8200 kg m2) and dynamic modes of the solar
array, via electrical-mechanical means. At the time of writin_ the inertia simulator
was not complete, thus no rate or latching tests and only limIted step tests were
performed. These tests will be completed prior to the conference and presented
thereupon.
For the proportional hold test the BMRRM was locked down at a specific position
and then commanded to move to various positions. Since only the proportional
constant is used, the torque produced was proportional to the constant and the error
angle: T = Kt P (_cmd "¢_=,ctual).Figure 10 shows results for several command
angles and two proportional constants. As shown the system is very linear, within
2 percent.
The step tests varied from 0.0025 degrees (typical for beta rotation) to as large
as 180 degrees (faulted conditions), although 5 degrees and 30 degrees steps were
the baseline testing conditions. These step tests only used the hardware itself as an
inertia (less than 1/3000 th of the solar array inertia), thus the system reacted
abruptly to the step inputs, often exceeding velocities expected on-orbit (peaked at
1000 degrees per minute). Three control algorithms were tested: proportional (P),
proportional-derivative (PD), and proportional-integral-derivative (PID). A firmware
error was discovered in the integral subroutine, thus the PID reacted similar to a PD
controller. The P controller test data is compared to the simulation model in
Figure 11. Generally the simulation models correlate to the test data within
50 percent. It is uncertain why the model deviates from the test data points,
although two reasons have been proposed: (1) the friction model is invalid at the
higher speeds and (2) the modeled llardware inertia was an assumption. The PD
controller test data is compared to the simulation model in Figure 12. In the PD
controller case, the simulation model correlated to the test data within 30 percent.
The maximum velocity for the PD controller was below the terminal velocity (which is
1.5 A divided by the derivative coefficient for a frictionless system), which validated
the speed control capabilities of this positional controller.
Environmental Testing
Static structural testing was performed to verify the stress and load-deflection
models. The tests represented about 75 percent of the on-orbit bending loads and
400 percent of the on-orbit torsional and shear loads. The bending loads are the
54
main structural design driver. The BMRRM has very large torsional and shear safety
factors, thus the 400 percent loading was required to amplify the deflection. Within
the BMRRM the deflections generated were within 20 percent of expected values.
No structural failures occurred.
Thermal vacuum/thermal balance (TVTB) testing was used primarily to ved_ the
thermal math models. A hot and cold soak as well as transient test (emulating the
60 minute solar, 30 minute eclipse cycle) was performed. Two infrared heat lamp
cages were utilized; one representing the solar flux, and the other, on the anti-solar
side, representing an averaged albedo and earth IR flux. The TVTB testing showed
warm BMRRM internal temperatures during the cold condition, around 5 to -13 C.
Internal BMRRM hardware temperatures are limited to about -65 C. The initial
design concern was that the internal temperatures may become too cold, thus a high
absorptivity black painted surface was chosen. However, this 50 C margin will allow
the design team to proceed with a less costly and more durable clear anodizing
surface, rather than the baseline black painted surface. A 30 degree step test was
planned for the ambient-ambient pressure, ambient-vacuum, cold-vacuum, and hot-
vacuum conditions to measure thermally and vacuum caused differences in the
servoioop. The ambient-vacuum test was successful, showing little difference
between it and the ambient pressure test. However, an open developed in the
B motor phase during the cold-vacuum case, which never closed even after the
hardware was brought back to ambient temperature and pressure conditions. At the
time of writing, the BMRRM has not been disassembled to determine where the
open occurred. A step test using an external power supply and two of the three
motor phases was performed during the cold-vacuum condition, although analysis is
not yet complete.
CONCLUSIONS
All development testing program goals were accomplished, including:
1. The assembly and test sequence of Figure 6 was shown to be an
acceptable hardware flow.
2. All component-level performance requirements were met, with the
exception of the motor line open during cold thermal-vacuum
testing. Once the root cause of the open is found a small design
modification may be needed.
3. The system-level performance test results were within the
tolerances expected, however additional testing with an inertia
simulator is needed.
4. Data from the tests largely verify the control model's component
friction, motor, and controller subroutines. Some additional minor
friction testing is desirable to determine the cause of and model for
small angle movements.
5. Data from both the static structural and thermal testing is
approximate to what was expected.
Overall the BMRRM has proven to be a very tolerant, lightweight, high-accuracy
rotating gimbal with minimal friction torque, and thus high rotating efficiency.
55
Bearing, Motor, Roll Ring
Module end Beta Gimba/
Transition Structure Housing
Beta Glmbel Platform
with Tut Support
Equipment Attached
\
_Kluentlal
Shunt Unit
Electronic
Control Unit
Beta Gimbal Assembly
8C_1-1_1711
Figure 1. Beta Gimabal Assembly .................................
CONTACT
BEARINGS
Figure 2. BMRRM Cross-sectional View
56
Figure3.BMRRMOutboardView
Figure4. RollRingInstallationi toBMRRM
57
Figure 5. Electrical Functional Test Set
BMRRM Assembly
FAIL ICONFIDENCE) & BMRRM/BGA Teit Flow (depends on cause)
.......... Y .--- _ ......
BUILD-UP OF BMRRMt / B ASSF- INSTALl. & I
: INNER/OUTER _-)"." AXIS "_2--:_CONNECT ROLL-RING I-).<.R.ESOL.VE.EI_ ".0;_. )._OLLBiNG/_XIS_:
(HOUSING BEAR NGS _ _ FRICTION AND LATCHES AL;L;UHAI..;y_ \" FR CT ON
•. _N," l 1 T"_,CE'ST-" " 7 I
............ "C.EC;" - --7
-- / ....... _'.4JL(CONFIDENCE)" ;
//\
/ RESOLVE" .x'>_MOTOR C-BPHASEi'(</ H.Or-O-"-3*-," MO_O_,__ I-BRIVEk
• , / PHASING BTEPPING . J
I _ELECT.'O'/ I 10 RESOLVE" I _PHASING/ _. _,,. -/ MOTORI
CONNECT TO , Y FAIL>0._" I I ¥ F_UL_J_V_BzS_)T _A!L ;
CONTROLLER I r- .... .--_. .--'J
Ff dL (wrong oireclion /
, _/_R,VE\ / ...... _ /o.ou_ / "",, r-- ...... 1
i _._./ MOTOR _/ ¢%i_t.%_r_M_ "X_.._ MOTOR "\.___,._d_OPOBTIONA"_L.._,IINSTALL INERTIA
"\ ROTATION " _ '_._'_"_"" / " ".CONSTANT./ " \_n_nT_(: T / "I SIMULATOR
COMPLETE _ // LATCH _ /TESTS USING_E /" S S USING%
'_ TESTS //_-\'elO CONTROl." - \F, PD, & FID..
Figure 6. BMRRM Build-up and Test Sequence
58
_=,
¢...
=-
O
i---
30.0
25.0
20.0
15.0
10.0
5.0
0.0
-5.0
-10.0
-15.0
/ Test data
It= "_=|_l.m_r,#_ io==.
i _ _ ........ Steady state a g
_r uani moaei _m:
_,_ sigma = 5000 in-lb/rad
_ Tss = 15.8 in-lb
I , I I I 1 I
-0.50 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50
Angle, degree
Figure 7. Typical Small Angle Friction Curve. Plot for a 360 degree
per minute test.
.Q
m
!
.D
i...
O
I.--
100
90
8O
70
60
50
40
30 ..,t"
20 ,t-t1"
10 _-_
0 i i
0 0.05 0.1
Figure 8.
.
Predicted maximum motor torque . ._-_"
"_ .._,--,._
-/•.J ._' -._
.,_'_ Least squares fit of data
...y"'_'_ Da_ points
I I I
0.3 0.35 0.40.15 0.2 0.25
Current, amperes
Torque Motor Constant Test
59
0.3
0.2
 o.1
_0.0
O
-'--0.1
LU
-0.2
-0.3
Average
3
Data points
Allowed Range
= r i •
!
t
..... t[--- / lltl
i
0 50 100
.! l _ t l ;
150 200 250 300 350 400
Independent Angle Source, degrees
Figure 9. Resoiver Accuracy Test
o
t--
8O
70
6O
5O
40
3O
20
10
0
-10
-20
T
&
east squares fit from both proportional lines.
P=19.86 ma/deg
Theory at _ ................. C,--._ `_.'"
; ,_ t t ........... "' _,_.- . t .......
Commanded Angle, deg
Figure 10. Proportional Hold Test
6O
50.00
45.00
40.00
35.00
30.00
t,/)
_25.00
_20.00
15.00
10.00
5.00
0.00
Model Prediction
/ _.._...,,
--_/ ,,- \ -',.
/ , "-_, ____
// "_,,,,
• _RBIIIIRg
• Data Points/.
/ ,
/.
/,"
.¢
BNBBBBUB -•#
•g|amgie|no|Dn|Di
Kp = 0.03 amps/deg
I _ I I I- I-
0.00 0.50 1 ._ 1.50 2.00 2.50 3.00
Seconds
Figure 1 1. 30 Degree Step Input with Proportional Only Controller
3.50
40.0
35.0
30.0
25.0
20.0
¢)
c_ 15.0
10.0
5.0
0.0
0.0
Kp + Kd 's/(ts+l)
Kp = 0.00 ampe/deg
Kd, 0.015 aml_/(deg/_e¢ )
t=0.02 se¢
.* ,'1 "---L'--'-- .....
,'ll =/
Data Points_ , / ModalPrediction/
• //'
,/
,,,,,--'/
"//./."_,,_
.. Max theoriticalvelocity(100 deg/sec)
/
1 I I
0.5 1.0 1.5
Seconds
Figure 12. 30 Degree Step Input with Proportional-Derivative
Controller
61



International Space Station Alpha's bearing, motor, and roll ring module developmental testing and results

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