This paper describes the design, development, and qualification of a new lightweight and compact Antenna Pointing Mechanism (APM). The APM was specially designed to meet the stringent mass, envelope, and environmental requirements of OFFEQ experimental satellite. During the development phase, some problems were encountered with the brushless DC motors, slip ring contact resistance, and bearing drag torque. All of these problems were resolved, and two APM units have been operating successfully in orbit since April, 1995

Halsband, A.

Shmulevitz, M.

English

NASA Technical Reports Server

. Shmulevitz* and A. Halsband* 
This paper describes the design, development, and qualification of a new lightweight 
and compact Antenna Pointing Mechanism (APM). The APM was specially designed 
to meet the stringent mass, envelope, and environmental requirements of OFFEQ 
experimental satellite. During the development phase, some problems were 
encountered with the brushless DC motors, slip ring contact resistance, and bearing 
drag torque. All of these problems were resolved, and two APM units have been 
operating successfully in orbit since April, 1995. 
Introduction 
Data link between OFFEQ experimental satellite and ground Control is provided by two 
redundant APM units. The strict and demanding requirements for minimum mass and 
envelope enhanced a unique design approach featuring a lightweight (1.8 kg) and 
compact (1 50-mm flange diameter x 220-mm length) antenna pointing mechanism. 
Although the APM housings and shafts were manufactured from aluminum alloy, the 
overall system design allows a large temperature range (-30°C to +60°C) without any 
active thermal control. Based on a gimbal (pitch over yaw) arrangement, the 
mechanism has hard-preloaded ball bearings and aluminum gears lubricated with 
sputtered MoS2. The APM is designed for LEO applications and configured to 
minimize effects of monatomic oxygen erosion on the tribological elements. Electrical 
transfer (command & power) to the elevation axis is achieved by a compact slip ring 
unit, and RF feed to the antenna is accomplished via two miniature rotary joints. The 
APM has been tested in thermal vacuum conditions to a 4-year equivalent life in orbit. 
Its overall pointing accuracy is l o ,  and the average power consumption is 2 watts. 
During the development phase, some problems were encountered with the brushless 
DC motors, slip ring unit, and bearing drag torque. Changing the materials in the 
motor resolved the cracks and cuts that formed in the windings. The slip ring unit 
exhibited increased electrical resistance after exposure to laboratory air. Rotation for a 
only few revolutions restored normal conditions. In order to reduce blocking effects, 
the APM bearings contain toroid ball separators and “loose” conformity races. 
However, this approach did not eliminate the increase of drag torque encountered 
during the life test. Also, it has been found that a dependency, which was stabilized at 
an acceptable torque margin, exists between APM mode of operation and drag torque. 
* 
Israel Aircraft Industries / MBT, Yehud, Israel 
291 
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(Figures 1 and 2) provides more than a full hemispherical coverage due to 
lity to rotate continu sly in the azimuth axis and to its 0°-1150 rotation range 
in the elevation axis. The AP azimuth axis consists of a rotating inner housing that is 
simply supported by a pair of bearings: the large bearing is fixed, and the small 
bearing floats with respect to the stationary housing. The bearings used for this 
application were 4406 stainless steel, precision (ABEC 7) angular-contact ball 
bearings (bore diameters 58.73 mm and 19.05 mm, respectively) with double rows of 
balls, preloaded back-to-back, and protected by two metal shields. Only the grooves 
were dry-lubricated with sputtered MoS2, and ball separators were toroid rings made 
of Duroid. 
The rotating assembly is driven by a brushless DC motor via a stainless steel spur 
gear that meshes with a gear made of aluminum alloy and lubricated with sputtered 
MoS2 (same for all other gears in the mechanism). The servo control feedback is 
provided by a resolver and aluminum gear mesh. To enable the continuous azimuth 
rotation, electrical and RF transfers to the elevation axis are achieved by a slip ring unit 
and rotary joint, respectively. 
The elevation axis of the APM consists of two rotating arms: one is axially fixed, and 
the other arm floats axially with respect to the inner housing. A brushless DC motor 
drives the axially fixed arm, whereas the elevation resolver (not seen in the cross- 
section of Figure 2) is driven by the floating arm via an aluminum gear mesh. RF 
transfer to the antenna is accomplished with the elevation rotary joint and coaxial 
cables. The hold down and release mechanism, attached to the antenna backside tip, 
provides the stowage and release capability. The antenna itself consists of a printed 
circuit board made of Duroid and is bonded to a honeycomb sandwich with high- 
modulus, graphite-epoxy face sheets. 
The APM design approach focused on one main objective: to minimize the APM mass, 
yet withstand all other requirements, as defined in Table 1. A compact design and 
selection of proper materials were considered the method to obtain the design goals. 
The selection of materials started with titanium alloy as the first choice for the APM 
shaft and housing material. It has the advantage of a coefficient of thermal expansion 
(CTE) that closely matches the selected 440C stainless steel bearing material and a 
good stiffness-to-weight ratio. However, careful examination of the APM mass budget 
revealed that, with titanium alloy, the allocated overall mass will be exceeded. 
Beryllium and other exotic materials were considered unfavorable due to fabrication 
problems, availability, and price. Aluminum alloy (7075T7351) was the second 
choice to be checked. ith respect to APM mass budget and stiffness-to-weight ratio, 
aluminum alloy is a good choice. But with the big difference in CTE between 
aluminum alloy and 440C stainless steel, this solution seemed unacceptable. 
292 
evertheless, a thorough investigation has been carried out and, with the assistance 
of the European Space Tribology Laboratory (ESTL) computer prediction, bearing 
frictional torque and contact stress were analyzed as a function of different b 
and thermal strains. The results showed that, although there is a large increase in 
bearing frictional torque and contact stress, the obtained values are still acceptable. 
The nominal frictional torque of the whole system was specified to be 10% of motor 
stall torque at ambient conditions, and the servo control was designed to 
accommodate the large change in friction torque. The most significant result of this 
analysis was the understanding of mechanism performance as a function of thermal 
gradients. With a hard preloaded back-to-back bearing arrangement and aluminum 
shaft and housing, bearing torque and contact stress increase rapidly when the outer 
bearing race temperature is lower than the inner race temperature. The opposite will 
occur if the outer bearing race temperature is higher than the inner race temperature. 
In that case, for only a 10°C temperature gradient, the bearings will be off-loaded (Le., 
no preload). System design of OFFEQ satellite required direct installation (Le., no 
booms) of the APM to the OFFEQ plate at the nadir side. Thermal analysis showed 
that, for most cases, the APM outer housing, which is attached to OFFEQ plate, will 
have higher temperatures with respect to the inner housing (shaft), which points to 
space. Therefore, the tendency is that thermal gradients will reduce bearing preload 
and friction torque. The worst condition will be an isothermal case with no gradients. 
In addition, tolerance analysis showed that, with a clearance fit and bearings off- 
loaded, the pointing accuracy and stiffness will not be adversely affected. 
A considerable amount of work has been taken by our subcontractor to develop the 
rotary joint. The critical parameter of the rotary joint is the ability of the bearings to 
conduct the heat dissipated in the rotating section of the co-axial line to the housing. 
The thermal conductance of a ball bearing in vacuum is proportional to the axial and 
radial load, the size of the bearing, and the number and diameter of balls. A face-to- 
face bearing arrangement has been used to ensure that bearing thermal conductance 
increases when the temperature of the shaft rises above that of the housing. Also, a 
compliant spacer was used to compensate for the change in preload due to thermal 
gradients. To improve the thermal conductivity, all metallic parts, except for the 
bearings, were made from aluminum or beryllium copper. 
Testing 
The design and location of the APM antenna impose a large static imbalance with 
respect to the APM elevation and azimuth axes. In order to simulate correctly APM 
performance in orbit and to eliminate all G effects, the antenna has been replaced by a 
dummy payload for all functional and thermal vacuum tests. This dummy payload has 
the same inertial load and stiffness as the antenna, and its center of gravity coincides 
with the APM axes. The APM qualification model has been subjected to the following 
tests: 
293 
unctional tests - These tests have been performed during the various stages of AP 
ualification. They include static and dynamic pointing accuracy check, torque 
threshold at different angular positions, dynamic check, and RF test. Dynamic check 
was executed by operating the APM at a typical tracking mode for seven minutes. 
Current and voltage consumption were monitored, and system tracking error was 
derived by comparison of command versus resolver position. 
Vibration tests - Vibrations were conducted with the APM stowed in the launch 
configuration. Three types of vibration tests were carried out: sine vibration (1 Og 
sinusoidal up to 200 Hz), random vibration (12.5 Grms up to 2000 Hz, 3 min per axis), 
and acoustic test (142 dB overall noise level, 3-min duration). The APM was also 
subjected to three shocks of 50g for 11 ms in each axis direction. 
Thermal vacuum cycling - This test was conducted by exposing the APM to eight 
temperature cycles between -40°C to +60°C at a vacuum level of 10-6 Torr. At each of 
the temperature extremes, functional tests were carried out. 
Thermal vacuum life test - This was an accelerated test divided into two phases. At 
phase 1, temperature condition was +23”Cj whereas temperature condition was -20°C 
in phase 2 (vacuum was 10-6 Torr). In each phase of the test, the APM was operated 
in a typical tracking mode maneuver for seven minutes, then for two minutes. All the 
obtained data was processed and statistically analyzed. Having completed this basic 
cycle (7 minutes operation + 2 minutes processing), repeated cycles, to an equivalent 
life of two years in orbit, were carried out automatically by special software. 
Problems Resolved 
Slip rina contact resistance 
During functional testing of the APM (vacuum, ambient temperature), unexpected 
dither behavior of the elevation axis was observed. Failure investigation revealed an 
excessive increase of contact resistance in the slip ring signal tracks (in the magnitude 
of hundreds and thousands of ohms). A short run-in (even two revolutions) was found 
to be sufficient to restore the slip ring contact resistance to its normal values. Yet, our 
main concern was the cause of slip ring contamination. It was believed that MoS2 
particles from the APM gears contaminated the slip ring, since MoS2 has the property 
of semiconductor resistance. 
A contamination barrier was introduced into the APM to prevent particles from falling 
on the slip ring tracks. However, dither behavior of the elevation axis still occurred 
whenever the APM was operated (in vacuum) after exposure to laboratory air. In 
addition, high contact resistance was also found in the slip ring power tracks, thus 
showing that at least 8 contact points are being contaminated simultaneously, and that 
did not seem reasonable. Further investigation with the slip ring manufacturer 
revealed that oxidation of the slip ring galvanic deposit may occur in air. It was also 
suggested that, for future application and new orders of slip rings, the galvatronics 
shall be coated with a 0.2-pm Au-Co layer, called DUOR, which acts as a dry lubricant 
and corrosion protection. Finally, it was decided that at the beginning of life in orbit, 
the APM will undergo a short run-in as a “cleaning process.” 
294 
-20°C, a serious problem arose 
a large increase in the input volta 
otor resistance reveale 
the brushless DC motor. 
ithout any change in the i 
istance is doubled, 
indicates that there is a cut in the motor windings. It should be stated that this type of 
motor was a qualified item with space heritage. 
The brushless DC motor basically consists of a ring-shaped stator with molded 
windings, a permanent magnet rotor without windings, and an inductive pick-off. A 
review of stator materials and manufacturing procedures revealed that cracks are 
usually formed in the epoxy resin, which is used for the impregnation of the windings. 
In addition, stress analysis showed that a cut will occur with a crack size of 0.02 mm. 
The failed motor was disassembled from the APM and dismantled into parts. Then, by 
thorough inspection of the stator, a cut beneath a crack of 0.04-mm width was found. 
Subsequent efforts were taken by our vendor to qualify stators with various epoxy 
resins and to improve an insulation disc. These stators were subjected to endurance 
thermal tests, including 72 thermal shocks between -55°C to +115”C for a dwell time of 
1 hour and transfer in 7 seconds. It was found that a rigid insulation disc of titanium 
alloy and scotchcast 281 epoxy resin gives the best results. Consequently, all motors 
were repaired to include these modifications. 
Draa toraue 
APM drag torque is composed of its bearing, slip ring, rotary joint, and resolver 
torques. The most sensitive items that contribute to the overall drag torque are the 
bearings. In order to reduce blocking effects, the selected bearings contain toroid ball 
separators and “loose” conformity (56%) inner and outer races. During phase 1 of the 
life test at +23”C, APM voltage consumption, which is related to the drag torque, 
increased from 18 PWM to 28 PWM, where it was stabilized. The stall torque of the 
motor is 200 PWM, and therefore torque margin is high. The notch (Figure 3) is a 
result of a one-day intermission in the test. It indicates that the system has a relaxation 
property when not operated, but this phenomena is not well understood. 
During phase 2 of the life test at -20°C, APM voltage and current consumption were 
considerably higher due to the thermal strain effects. Azimuth axis voltage 
consumption increased from 30 PWM to about 50 PWM, where it was stabilized at an 
acceptable torque margin. Performing a short run-in of the system showed that the 
overall drag torque was reduced. This behavior is attributed to the spread of debris 
piling and the “release” of cage wind up. 
295 
The design, dev ent, and testing approaches h the successful 
operation of two units in orbit. Satellite telem 
between the drag torque obtained during ground testing and the drag torque in orbit. 
Moreover, checking the drag torque with respect to the temperature of the OFFEQ 
plate indicates that the two APM units are exposed to thermal gradients that reduce the 
bearing preload and torque. It is of vital importance to consider the thermal 
environment and temperature during the conceptual design of any mechanism. 
Finally, although developed specifically for the OFFEQ satellite, the APM is a versatile 
design that may be adapted to other gimbal applications. 
The authors wish to thank Dr. Rob Rowntree of ESTL for his consulting concerning the 
analysis of bearing frictional torque, contact stress, and tribology design. We also wish 
to thank the SAGEM team for their collaboration in solving the problem with the 
brushless DC motor and Eli Levy and Dov Varbin of MBT for their contribution to the 
development of the electronics and servo control. 
References 
1. T. A. Harris, Rollina Bearina Analysis, John Wiley and Sons, Inc., 1984. 
2. S. H. Loewenthal, "Two Gimbal Bearing Case Studies: Some lessons learned," 
3. ESTL, Tribology for Spacecraft, Course Notes. 
NASA 22nd Aerospace Mechanisms Symposium, May 1988, pp. 253-269. 
296 
Parameter 
ass (payload excluded) 
imuth rotation [degl 
evation rotation [degl 
Total pointing accuracy [degl 
ax. angular acceleration [deg/sec*] 
ax. angular velocity [deg/sec] 
Eigenfrequency [Hzl 
Torsion stiffness [N-mlrad] 
Torque margin 
Operating temperature ["CI 
Maxim um 
Minimum 
Mode of operation (tracking mode) 
Life [years] 
<2 
+I 80
0-115 
I .8 
10 
10 
>50 
>400 
>3.0 
+50 
-20 
Intermittent 
>2 
Capability 
1.8 
360 cont. 
0-115 
1 .o 
10 
10 
250 
6500 
>5.5 
+60 
-30 
Obtained 
4 
297 
El  ROTARY JOINT 
El DRIVE MOTOR 
INNER HOUSING 
I ROTATES 1 
OUTER HOUSING 
Az DRIVE MOTOR 
A z  RESOLVER 
SLlPRING ASSY. 
A z  ROTARY J O I N T  
igure 2. APM 
P W M  RZ PWPI 
16 
I I I ' d l i  CYCLES u 489 964 1-44? 1929 
est - se 
298 


The design, development and qualification of a lightweight antenna pointing mechanism

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