The design and development of a constant-speed solar array drive system for use in high power communications satellites is described. The relationship between continuity of motion in the solar array drive and spacecraft attitude disturbance is investigated. The selection of the system design based on the design requirements including spacecraft disturbance is discussed. The system comprises two main parts: the drive mechanism including small angle stepper motor and reduction gearing and the control electronics including ministepping drive circuits, such that a very small output step size is achieved. Factors contributing to discontinuities in motion are identified and discussed. Test methods for measurement of very small amplitudes of discontinuity at low rotational rates are described to assist in the testing of similar mechanisms

Jones, H. M.

Roger, N.

English

NASA Technical Reports Server

N85-33519
THE DESIGN AND DEVELOPMENT OF A CONSTANT-SPEED SOLAR ARRAY DRIVE
Howard M. Jones and Nell Roger*
ABSTRACT
This paper describes the design and development of a constant-speed solar
array drive system for use in high-power communications satellites. The
relationship between continuity of motion in the solar array drive and
spacecraft attitude disturbance is investigated. The selection of the system
aesign based on the design requirements including spacecraft disturbance is
discussed. The system comprises two main parts: the drive mechanism
including small-angle stepper motor and reduction gearing and the control
electronics including ministepping drive circuits, such that a very small
output step size is achieved. Factors contributing to discontinuities in
motion are identified and discussed. Test methods for measurement of very
small amplitudes of d_scontinuity at low rotational rates are described to
a_sist in the testing of similar mechanisms.
INTRODUCTION
The use of sun-pointing deployable solar arrays of increasing power levels
on spacecraft for various applications has placed increased emphasis on the
design and operational strategies of the solar array drives which maintain the
sun-pointlng attitude of the solar array while the spacecraft typically
remains earth-pointing. As a result of increased solar array size and
: flexibility the potential to disturb spacecraft has increased dramatically.
The use of a constant-speed drive as opposed to the more traditional
intermittent or pulsed drive can improve this situation in two ways:
(a) The elimination of discontinuities in angular momentum between the solar
array and the spacecraft results in reduced disturbance levels. This
permits the use of smaller size or lower power consumption in the
reaction or momentum wheels of the spacecraft pitch axis control loop,
and/or less frequent momentum-dumping operations of the thrusters.
4
(b) The reduced energy levels in the dynamic coupling between the solar array
and drive system leads to improved operating torque margins and
_ eliminates potential problems of stalling and backdriving due to resonant
array dynamics. This phenomenon occurred on-orblt in a large
communications satellite recently, resulting in forcible back-stepping of
the system while a step forward had been commanded.
However, because the required rate of rotation is very low (i.e., l
revolution per day for geosynchronous spacecraft), the maintenance of
constant speed within limits, expressed as a proportion of the nominal
speed, is a difficult proposition. The fundamer_l problem is therefore
one of achieving the necessary continuity of motion in the solar array
drive system without leading to excessive mechanical and electrical
complexity with the resulting penalties in reliability, cost and mass.
°i *Spar Aerospace Limited, Toronto, Canada
103
I
1985025199-113
https://ntrs.nasa.gov/search.jsp?R=19850025206 2020-03-22T19:52:30+00:00Z
REQUIREMENTS
The continuity of motion requirements fo _ the solar array drive system are
derived from analysis of the relationship between speed variations in the
drive system and spacecraft disturbance u3ing computer simulaticns of the
drive system, solar array, spacecraft body with flexible appendages and the
spacecraft pitch axis control loop.
A typical spacecraft configuration is shown in Figure 1 corresponding to a
hlgh-power geosynchronous communloations application. Data for the
spacecraft, its' fle_lble appendages and attitude control system parameters
are summarized In Table I.
Fro_ the computer simulations it was determined that both the amplitude
and frequency of speed variations in the solar array drive were important.
Tb_ approaches used and the continuity of motion achieved are discussed in the
Design and Analysis section below.
The spacecraft disturbance and other relevant drive system requirements
are presented in Table II.
DESIGN AND ANALYSIS
The design and analysis proceeded in three stages:
(a) Analysis of the relatiooship between continuity of motion in the solar
array drive and spacecraft disturbance using computer simulations of the
drive system, solar array, spacecraft body with flexible appendages and
the pitch-axls control loop.
(b) Evaluation and trade-off of various drive system concepts to determine
: which would beat achieve the required continuity of motion requirements
in terms of reliability, cost and mass.
(c) Further design and analysis of the selected concept.
Drive System Concepts and Trade-off
The concepts considered fell into one of the two main categories:
- Open-loop stepper and synchronous systems.
- Closed-loop brushless DC systems using rate or position sensor feedback
(in addition to commutation feedback),
£ variety of gear ratios was also considered for eaoh category.
/,.
i 1-
1985025199-114
It was found that because the nominal rotation rate _s very low (i
revolution per day) the fluctuations in rate, expressed as a proportion nf the
nomipal, are very small and hence difficult to detect and to control for the
closed-loop systems. Therefore the open-loop stepper or synchronous approach
was selected due to its simplicity. The system block diagram for the
open-loop geared stepper concept is illustrated in Figure 2.
The other major trade-off area involved selection of the optimum
combination of techniques for reducing output step size for the stepper or
synchronous concepts. The following methods are available:
(a) Motor with small prime step angle.
(b) Electronic step division (mini-stepping).
(c) Reduction gearing.
Gear reductions are selected for other reasons including motor torque
amplification (reduces system size and weight) and baekdrive characteristics
(e.g., low backdrive efficiency reduces susceptibility to solar array dynamic
torques). The selected concept uses a conventional 1.8° stepper motor with
64-1evel minl-stepplng and 288:1 reduction gearing to achieve a theoretical
output step size of 0.0001 °. This step size is so low and the associated
stepping frequency of 42 Hz so h_gh, that significant interaction with the
solar array and the opacecraft p_tch axis control loop are avoided. The
lamination shapes for a hybrid stepper motor with 1.8" prime step _ngle are
illustrated in Figure 3, with the associated electrfeal schematic and
sinusoidal minlstep drive sequence. The schematic of the ministep electronics
is shown in Figure 4. A cutaway illustration of the 288:1 reduction gearing
is presented in Figure 5. The second stage of the reduction gearing is a
regenerative planetary set which is non-backdrlveable. This set functions in
a slmilar fashion to the harmonic drive except that the nutating flexible
spline of the later has been replaced by the more robust planet gears, which
can withstand much higher backdrive loads. More detailed information on the
gears is presented in Table III.
Factors Affectin 6 Continuity of Motion
As described above the selected drive system provides a theoretical output
step size of 0.0001 e and a stepping frequency of 42 Hz. In practice the drive
mechanism will be part of a solar array drive assembly which includes be&rings
i and slip rings mounted on the main shaft. It has been predicted that _heachievable continuity of motion at the output is limited by stlck-sllp effects
in the system.
Due to the compliance of the drive train and the slow, synchronous speed
of the motor there is a finite rate of drive torque build-up in the main
shaft. Therefore when the system is started from zero rate it takes a finite
time for the drive torque in the shaft to exceed the static friction due to
the slip rings and bearings. Let this time be called T 1. When the breakout
torque is exceeded the shaft accelerates to a speed higher than the nominal
rate under the action of the "wind-up" torque in the drive train. As the
i I05
JL .,
1985025199-115
Jf
"wind-up" torque falls below the friction torque the shaft speed decreases
until it is caught by static friction. Let this time interval (of relative
motlcn at the sllp rings) be called T2. The process is then repeated and is
illustrated in Figure 6A.
The phenomenon could only be completely eliminated by the total
elimination of friction or by the addition of viscous or damping friction into
the system. However, the problems associated with the stick slip effects can
be alleviat_d by controlling the times T1 and T2 and hence the amplitudes
of disconti_ulties in the main shaft rotation. The following parameters were
found to a,f_ct TI and T2:
(a) Drive system stiffness.
(b) Absolute levels of static and dynamic friction.
(c) Relative levels of static and dynamic friction.
(d) Level of viscous friction in the system.
(e) Solar array dynamics.
The ratio of static to dynamic friction in tne slip rings is determined by
the properties of the materials used and is not amenable to manipulation. The
solar array dynamics are not generally subject to alteration for this purpose
while the addition of sufficient viscous friction into the system at these low
rates would not be practical. This generally leaves the drive system
stiffness and the absolute friction levels under the control of the solar
array dri-e designvr. Increasing the drive system stiffness and reducing the
slip ring friction levels will decrease the period (T1 2) and hence the
amplitude of the rate discontinuities. A parametric analysis using the
computer simulations mentioned earlier was conducted to determine acceptable
combinations of stiffness and friction. The predicted effects of various
combinations of sllp ring friction and drive system stiffness on continuity of
motion are illustrated in Figure 6B. This indicates that to maintain an
output step size in the same range as the theoretical value, say 0.0003 °
maximum, the sllp ring dynamic friction should be limited to 1.3 Nm and the
drive system stiffness at the main shaft should be above 150,000 Nm/radian.
While this stiffness is not generally difficult to achieve in a solar array
drive mechani_m, the limit of 1.3 Km sliding friction presents a challenge for
the larger sllp ring assemblies, particularly those of the disc configuration
due to the larger diameter of the rings.
TESTING
An engineering model drive system has been built including the
mlnistepplng electronics. Tne test program in progress includes:
(a) Ambient performance testa (torque-speed, etc.).
(b) Environmental testing (vibration and thermal vacuum).
(c) Continuity of motion (systems dynamics).
106
1985025199-116
!
The ambient performance and environmental tesZlng involve conventional
v
: procedures and techniques and need not be discussed further. However, the
system dynamics test includes a novel approach for measuring continuity of
motion at the output under various operating conditions and is descl oed below.
The purpose of the System Dynamics test is to determine the dynamic
characteristics of the Solar Array Drive Assembly (SADA) under various
operational conditions. The operational conditions to be represented _or
accounted for) are:
(_) Solar array dynamics.
(b) Slip ring friction.
(c) Rotational rates (for sun acquisition and sun-tracking modes).
The system dynamics test rig illustrated in Figure ? was designed and
fabricated to represent t_e appropriate solar array torsional modes and the
sllp ring friction. The ir_rtis and stiffness of the simulated solar array
and the friction of the simulated slip ring can be adjusted to represent
various operational conditions of interest. The drive system can be commanded
to run at a variety of rotational rates including those correspondirg to
typical sun-acquisltion and sun-tracking modes for geosynchronous and
low-earth orbits.
The main indicator of system performance is the continuity of motion
achieved at th_ SADA output. This parameter readily indicates any problems in
the system _uch as temporary stalling, excessive stick-sllp effects or solar
array excitation. The relationship between continuity of motion and levels of
spacecraft disturbance and solar array excitation was discussed above. _ince
the predicted discontinuities in the SADA output motion are low, the question
of measurement techniques is important.
Dynamics analyses discussed above indicated that the e_lected
discontinuities in rotational rate have amplitudes of 0.0001 to O.0OO? degree
and periods of 23 to 170 milliseconds (frequencies of q-60 Hz approximately).
Two alternatives were considared for determining these discontinuities:
(a) Position or rate sensing.
(b) Acceleration sensing.
Position or Rate SensinB
The predicted amplitudes of the discontinuities are so low that extremely
hlSh resolution position sensing is required to measure them to, say, 0.00005
degree. The only feasible techniques identified were either an inducto_yn :
resolver or an optical encoder-reeolver, both with digital interpolation
electronics. The required resolution Is equivalent to a 23-blt encoder. The
J_
_°
107 _"
4
,I J
1985025199-117
sensor generates position versus t_me data from which the velocity profile of
the drive output can be derived.
Several drawbacks were identified with this _pproach:
; o The sensor is a special, long-lead and expensive device•
o The sensor would have to be mounted extremely accurately to the SADA/array
interface or would require its own precision bearing system with flexible
; coupling to the SADa abaft.
o The significant rigid inertia of the sensor rotor would affect the
dynamics.
Acceleration Sensin 5
To overcome the disadvantages associated with the use of the
high-resolution position sensor, an alternative approach involving an
accelerometer to sense the disconblnuitles was developed. In this case a
high-resolution linear accelerometer is mounted on a stiff, light moment arm
attached to the output shaft. The accelerations due to the discontinuities in
motion can then be detected and integrated to obtain the velocity profile as
shown in Figure• 8.
: The limited bandwidth of the accelerumeter is predicted to result in some
truncation of the acceleration spikes at initiation and termination of each
discontinuity when these occur at the higher end of the frequency range
identified above, i.e., 20-60 Hz. This, in turn, will result in errors in the
velocity profiles derived by integration of the accelerometer output and is
illustrated in Figure 8. However, this is not considered serious because:
(a) The errors only become significant at frequencies corresponding to
performance considerably better than the continuity of motion
requirements derived above.
(b) The velcclties can be corrected to some extent by reference to the
average orbital rate. ,
CONCLUSIONS
A constant-speed solar array drive has been developed which offers
significant improvements in the levels of spacecraft disturbance and solar
array excitation compared with more traditional intermittent and stepper
drives. The required continuity of motion has been achieved with an open-loop
system using an optimum combination of small prime step angle in the motor,
; electronic step division, and reduction gearing. This largely preserves the
simplicity associated with open-loop stepper drives. The two-stage
regenerative planetary gearset is an interesting variation on the harmonic
drive conc pt.
q
q
108
1985025199-118
An engineering model drive system including the minlstepping electronics
has been fabricated and is currently undergoing a series of tests. Of
particular interest is the method of measuring the achieved continuity of
motion at the output under various operating conditions. A novel approach was
; adopted involving a sensitive linear acoelerometer mounted on a stiff, light
moment arm attached to the output shaft of the drive system. The
accelerometer signal can be processed in a variety of ways to derive the
angul_." velocity pro'ile, frequency spectrum, etc.
The design and test techniques described in this paper are applicable to
other low-speed mechanisms such as robot arm joints and pointing drives for
iar_e antennae.
ACKNOWLEDGEMENTS
This paper describes work performed under the sponsorship and technical
direction of the Internatlonal Telecommunications Satellite Ocganlzation
(INTELSAT). Any views expressed are not necessarily those of INTELSAT. The
authors would like to thank Mr. W. Alsbach of INTELSAT for his valuable
support and advice.
: The prior support of the Canadian Government Department of
Communications/CRC for developing various aspects of solar array drive
technology is also gratefully acknowledged.
139
1985025199-119
TABLE I SPACECRAFT DATA
A. MASS PROPERTIES
MASS : 6,':50 kg
Mol- ROLL F2,OOO kgm2
- PITCH : 32,000 kgm2 :
- YAW : 46,000 kgm2
B. FREE-FREEMODES OF SPACECRAFT AND ANTENNA
MODE NO. AXIS FREQUENCY
7 ROLL-YAW O.167 Hz
8 PITCH 0.176 Hz
9 ROLL-YAW 0.243 Hz
C. SOLAR ARRAY FIXED-BASE TORSIONAL °
(PITCH-AXIS) MODES
MODE NO. FREQUENCY MODAL INERTIA
1 0.0984 Hz 113 Kgm 2 _"
3 0.1487 Hz 80 Kgm 2
D. ACS PITCH AXIS
POINTING REQUIREMENTS : ± 0.03 °
"; MOMENTUM CAPACITY : ± 10 Nms '
SATURATION TORQUE : 0.1 Nm
: NATURAL,, PERIODS I I 135 AND 125s
7
110
1985025199-120
TABLE II DRIVE SYSTEM DESIGN REQUIREMEI_TS
DRIVE TORQUE
- (3 cx WORST-CASE RESIST=NGTORQUES) 16 Nr_, MINIMUM
SOLARARRAY ORIENTATION ± 2.0=
_- - POINTING ACCURACY
- DEVIATIDN BETWEENNAND S WINGS O'2=
- SUN-TRACKING RATE 1 REVOLUTIONPER DAY
- SUN-ACQUISITION RATE 0.27 ° PER SECOND
INDUCED SPACECRAFTDISTURBANCE!PITCH-AXIS)
- POINTING ERROR 0.03 °, MAXIMUM
- REACTIONWHEELTORQUE 0.1 Nm, MAXIMJM
- REACTIONWHEEL MOMENTUM CHANGE 1.0 Nms, M/'_,IMUM"
' 10% OF TOTAL REACTIONWHEEL CAPACITYFROM TABLE
TABLE III GEAR DATA
: A. FIRST STAGE : EXTERNALS?UR SET
NUMBER OF TEETH ON PINION : 34
NUMBER OFTEETH ON GEAR : 200
RATIO=200 / 34 = 5.88
MODULE(DIAMETI _L PITCH) : 1.O6mm (24)
PRESSUREANGLE : 20_
EFFICIENCY.NOMINAL : 98%
B. SECOND STAGE: REGENERATIVEPLANETARYSET
NUMBER OF TEETH ON PLANET : 27
NUMBER OF TEETH ON FIXED RING : 98
NUMBER OF TEETH ON ROTATING RING : 98
RATIO= 98 x 27 =49
(98 x 27) - (98 x 27)
MODULE (DIAMENTRAL PITCH) : 0.94 mm (28.889)
: PRESSUREANGLE : 22.5°
EFFICIENCY,NOMINAL : 30%
111
1985025199-121
--_
E
' 1 ? \/ "\
_/112
1985025199-122
1985025199-123
oGROTOR
R
MOTOR LAMINATIONS SHAPES
DRIVE AMPLIFIEP (4 PLACES)
RETURN _ _ RETURN
A1 !<- Q _ B1
+Vs e_ • 4V S
A2 B2
RETURN _ _ RETURN
WINDING CURRENT MOTOR AND DRIVER SCHEMATIC
I
A1
1 PRIME STEP
='- TIME (STEPS)
1 2 3 4 1 2 3 4 1
SINUSOIDAL MINISTEP DRIVE SEQUENCE ,
FIGURE 3 MOTOR DETAILS AND DRIVE SEQUENCE
114
1985025199-124
,}
f
- t
-!
z
+
o r _,
_ cr
o
i- _ \ f, V'VV'---_
• ,.u
+ ! m t_
" Z
o, _ I-
_J
w
a.
- _ _7 _ "'
- "-' I-
:  :_0
5 5 r-
_ W
1 °: u,I
©o - r3
_ _ _ 8 -
JU
z
o(J
115
L i
1985025199-125
MOTOR-ENCODER
; (2-PLACES)
1ST STAGE
_; -"_" GEARSET
(5.88:1)
MAIN BEARING
(1 OF 2)
OUTPU]
2ND STAGE
PLANETARY
GEARSET
(49:1)
!
1
Ji
FIGURE 5 CUTAWAY ILLUSTRATION OF REDUCTION GEARING
116
1985025199-126
T
k#
AVERAGEORWTAL.RATE _ ___' _
.--_-- _ OUTPUT STEP SIZE
(A)
SHAFT
STIFFNESS
10-4 (KNnV_Ot
I O0 200 300
(B)
FIGURE 8 8TICK-SLIP MOTION AND EFFECT8 OF FRICTION AND 8TIFFNE$$
1985025199-127
ECONTROL AND t
DRIVE
ELECTRONICS
,RIVE MECHANISM
SIMULATEO SOLAR ARRAY
SIMULATED S COMPRISING TORSION ROD.
INERTIA DISC.
AND WEIGHTS
SUPPORT STRUCTURE ACCICLEROMET[ R
FIGURE 7 SYSTEM DYNAMICS TEST RIG
ACCELERATION
/ / ,
EFFECTS OF ACCELEROMETER.-_
TIME-CONSTANT
VIELOClTY
__ PROFo_ Ol[no_[o BY
,, INTIGRATION OF ACC|LEROMIT|R l f
FIGURE 8 DETERMINATION OF CONTINUITY OF MOTION
118
1985025199-128


The design and development of a constant-speed solar array drive

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server