A recurrent requirement of spaceborne instruments and communications equipment is that of accurate pointing. This need is recognizable in such diverse applications as Star Sensor trimming, Momentum Wheel gimballing, in-orbit adjustment or alignment of equipment, inter-satellite communication and Antenna Pointing. A pointing mechanism of novel design having several advantages over the more conventional gimbal, centre-pivoted, or cross axis pointing concepts is described

Bentall, R. H.

Brunnen, A. J. D.

English

NASA Technical Reports Server

DEVELOPMENT OF A HIGH STABILITY POINTING MECHANISM 
FOR WIDE APPLICATION 
A.J.D. Brunnen * 
R.H. Bentall ** 
ABSTRACT 
A recurrent requirement of spaceborne instruments and communications 
equipment is that of accurate pointing. This need is recognisable in such 
diverse applications as Star Sensor trimming, Momentum Wheel gimballing; 
in-orbit adjustment or alignment of equipment, inter-satellite communication 
and Antenna Pointing. 
As part of the ESA Advanced Supporting Technology Programme, British 
Aerospace is developing a pointing mechanism of novel design having several 
advantages over the more conventional gimbal, centre-pivoted or cross axis 
pointing concepts currently state-of-the-art. 
INTRODUCTION 
The 1977 World Administrative Radio Conference (WARC) stipulated that 
accuracies better that +O.l" should be achieved on communications beamwidths 
of typically 1" to 2", and as a consequence gave rise to an operational 
requirement for Antenna Pointing Mechanisms (APMs) capable of around +O.Ol" 
pointing accuracy, thus relieving the satellite ACCS from the difficult, if 
not impossible, task of achieving this accuracy for multi-antenna systems. 
Experience with thermal vacuum testing has shown that these high 
accuracy requirements are very difficult to achieve, particularly when the 
APM is operating in 'Open Loop' mode(see Ref. 1). Nonlinearities, build errors, 
non-orthogonality effects and, particularly, thermal expansion effects combine 
to reduce the accuracy until, typically, an accuracy of around +O.O25O limits 
the performance of conventional gimbal systems. 
* British Aerospace Dynamics Group, Space and Communications Division, 
Stevenage, England. 
** European Space Agency, ESTEC, Noordwijk, Netherlands. 
159 
https://ntrs.nasa.gov/search.jsp?R=19820015480 2020-03-21T16:27:50+00:00Z
Better 'Closed Loop' performance (i.e. when the APM is responding to an 
external error signal such as that derived from an RF sensor) can normally 
be obtained providing that the movement resolution of the actuators is high 
enough and that mechanism backlash and control response are adequate. 
(Ref. 2, 3). 
The objective of the APM development programme was therefore to achieve 
an APM design which offered, in addition to other attributes, an improved 
stability of pointing. Following trade-offs covering the range of existing 
pointing mechanism concepts, a radically new concept was chosen based on an 
idea developed at ESTEC. This concept, and its derivatives, is the subject 
of an ESA patent application. 
THE SWASH-PLATE PRINCIPLE 
The new mechanism utilises the swash-plate or rotating wedge principle 
in order to achieve and maintain the pointing vector. Figure 1 shows the 
operating principle. The device contains 4 main structural elements, A,B,C 
and D. B and C are wedge-shaped and enabled to rotate about the z-axis 
by means of bearing systems located at 1, 2 and 3. If mutual z-axis 
rotation between A and D is prevented by, for example, a bellows E, and the 
bearings at 1 and 2 are driven, then the device becomes a polar coordinate 
pointing mechanism. 
When B and C are rotated by equal amounts, one clockwise the other 
anti-clockwise, there results a tilting of the pointing vector away from the 
z-axis (nodding). If B and C are now rotated together, the vector sweeps 
around the z-axis at the angle previously achieved. Figure 2 illustrates 
this by showing the circular paths traced by the pointing vector on a target, 
due to the rotation of B and C and the combination B + C. 
It can readily be seen that if either of the two swash-plates is 
prevented from rotating, then the pointing vector can be re-aligned with the 
z-axis by the rotation of the other swash-plate alone. In principle any two 
of the three bearings may be driven, but it is advantageous to drive bearings 
1 and 3 since the actuators may then be mounted on the base and top plates. 
If the actuators prevent back-driving of the two sections by virtue of their 
detent positions, then the pointing vector is maintained without the 
application of power. 
MECHANISM DESCRIPTIONS 
A cross-sectional view of the HAe High Stability AFM is shown in Figure 
3. The tm swash plates, having a swash angle of 2.15 degrees, can be seen 
supported by single bearings and the two motors supported off the top (pay- 
load) interface and the bottom (satellite) interface, respectively. Figure 4 
shows additional views with the installation of the positional encoders. 
160 
Main Bearings 
The three main bearings are.Kaydon four-point contact (Gothic Arch) 
bearings of 7 inches diameter having a minimal preload within the bearings. 
This preload, and the thermal design of the mechanism, necessitated careful 
analysis in order to avoid on the one hand,,! bearing clearance which would, 
in principle, result in degraded pointing accuracy, and on the other hand 
excessive preload and consequent increased torque. 
The four-point contact bearings are well suited to the concept allowing 
a compact design whilst at the same time having a high load-carrying 
capacity (13,600N radial, 34,000N axial, quoted manufacturer's value for 
static loading). Substantial mass and dimensional savings are achieved over 
the use of duplexed pairs of angular contact bearings. ' 
The bearings are lubricated with a low-vapour-pressure oil. The use of 
a liquid lubricant increases the thermal conductivity of the bearing, 
reducing the thermal gradients which would otherwise contribute to preload 
changes. The bellows allow the possibility of hermetic sealing, if required 
by the application. 
Structure 
The material selected for the four main structural components of the 
mechanism is Beryllium. The advantages offered by this material are: 
0 Extremely low mass 
0 Thermal expansion close to that of the bearing steel 
l Good thermal conductivity 
0 High specific heat. 
The three latter features, combined with the cylindrical nature of the 
structure and the externally mounted bellows, provide a protected 
environment within the mechanism, leading to an exceptionally high thermal 
stability. 
The four main structural sections, with the interconnecting bearings, 
constitute the load path through the mechanism. The large diameter of 180mm 
is significant in that high strength and stiffness can be obtained for very 
low mass. The mechanism can typically support unaided a mass of up to 1Okg 
through an Ariane launch, the strength limitations being those of the main 
bearings having a load capacity as given above. In Figure 4 it can be seen 
that the central part of the mechanism is free from obstruction. Thus, 
when desired, an aperture of up to 40mm dia. can be provided through the 
mechanism for the passage of cables, flexible waveguides, or payload 
hold-down mechanisms. 
161 
Actuators 
The actuators, shown mounted off each of the two end sections, are 
comprised of 15“ permanent magnet stepper motors driving through a spur gear 
train onto ring gears mounted on each of the swash-plates. The motors have :. 
an in-line double stacked arrangement such that each motor has a double length 
rotor and a full set of redundant windings. 
The gear ratio is 7Ol:l from motor shaft to ring gear which, in 
conjunction with the swash-plate geometry, gives a typical output step size 
of 0.0008” for each motor step. This high gear ratio also provides a high 
output torque (>35ONm), permitting a slew rate of 0.24O/sec to be applied to 
a load of 50kg rn'. The fact that the mechanism is stiff as well makes it 
suitable for the pointing of complete antennas incorporating relatively 
stiff 'flexible waveguides'. 
Since the actuators prevent back-driving even during launch conditions, 
no launch lock is needed. As a consequence no pyrotechnics are needed. 
Position Sensors 
Two-pin contact encoders mounted off the end-plates allow the position 
of the swash-plates to be determined. The encoders mesh with the ring gears 
on the swash-plates via a two-pass gear and have a resolution equivalent to 
2 motor steps. In addition to the contact encoders, two pairs of redundant 
optical pick-offs identify the '(0.0)' reference position (where the pointing 
vector is aligned with the z-axis). These sensors have an accuracy 
equivalent to +3 motor steps and provide back-up information in the event of 
encoder failure or an alternative means of datum identification for 
applications where encoder information is not required. 
THERMAL DESIGN 
Pointing mechanisms are normally mounted in exposed conditions, and while 
the base of an APM may benefit from the relatively controlled temperature of 
the satellite body, the payload or antenna interface was taken as -170°C to 
+ 120°C and that of the satellite top floor as -2O“C to -5O'C. 
For a satellite-mounted application, the thermal control is completely 
passive. Thermal insulation is placed between the mechanism and the payload 
and the mechanism is surrounded by multi-layer insulation. Thermal straps 
assist the dissipation of heat from the motor so that in the extreme case 
gradients are limited to: 
l 6°C through the main bearings 
0 1O'C between adjacent swash-plates or end-plates 
l 2'C radially across the APM. 
162 
For a boom-mounted application, without power supplied to the motors the 
mechanism can reach very low temperatures. In that case it is necessary to 
employ heaters either to retain acceptable 'start up' temperatures, or to 
provide a warm-up capability prior to a re-pointing activity. The motors may 
act as heaters. In operation the variation in temperatures is expected to 
have very little impact on the mechanism performance. 
OPERATIONAL CHARACTERISTICS 
As already described, the High Stability Pointing Mechanism operates 
in an essentially polar-coordinate fashion such that where the rotation of 
the two swash-plates is defined by the angles 81 and 82 the pointing angle of 
the mechanism in Cartesian coordinates is defined by: 
x = atsin e1 - sin 0,) 
Y = atcos O1 7 cos e21 
where a is the swash angle on each of the rotating sections. 
For simple open loop repointing it is quite straightforward to calculate 1 
the values of 81 and 82 required and drive the swash-plates to the desired 
positions on command from the ground. However, in closed loop operation 
where an RF sensor is employed, it is necessary to respond automatically to 
the error signals generated in the Cartesian axes of the sensor. In this 
case a number of command strategies are possible. 
In a generalised case, where an APM is required to trim about a number 
of alternative RF beacons, use is made of the quasi-orthogonal nature of the 
nod and sweep motions. These motions are inclined to the RF sensor axes and 
an on-board microprocessor is employed in the control electronics to perform 
the necessary conversion. This is greatly simplified by the use of the 
algorithm: 
Ael = Ae + Be 
X Y 
Ae2 = Cex + De 
Y 
where e and e 
X Y 
represent the errors in x and y directions and A, B, C and D 
are four constants inputted to the electronics by telemetry. These depend on 
the location of the trimming position within the pointing range and can either 
be obtained from the encoders or estimated independently. The encoders are 
not necessary for trimming operations. 
163 
This technique allows trimming at any point within the pointing range 
other than at the centre or at the edge. These 'no-go' areas exist because 
of reduced slew rates at these positions due to the harmonic character of the 
pointing vector movement. For a mechanism with a pointing range of 4.3" 
radius and slew rate requirements of O.l"/sec. of the boresight the operating 
range is within an annulus of between 0.7" radius and 4.2" radius. 
In a more realistic situation, the trimming position required is fixed 
and known in advance. Referring to Figure 2, the intersection of the B locus 
and the C locus at point P represents a point where the two motors, acting 
independently, comprise their own quasi-Cartesian coordinate system. These 
axes may be chosen to be coincident with the RF axes and thus a simple control 
system can be achieved. This control philosophy is currently proposed for 
the European Communication Satellite L-SAT. 
While, for trimming operation, the nominal pointing direction is obliged 
to be off-set from the centre of the range, for re-pointing duty it is 
advantageous to make the nominal pointing direction coincident with the z-axis 
or zero reference position. This permits the added feature of a controlled 
'return to zero' capability with a single actuator. Since with power off the 
mechanism will remain fixed, all pointing mechanisms on a spacecraft can be 
controlled by one electronics unit which multiplexes between them. I 
TEST RESULTS 
A prototype model of the High Stability Pointing Mechanism has been 
constructed and tested by British Aerospace. It was constructed using 
commercial standard, off-the-shelf, components for the main bearings and motors. 
The structural elements were made from aluminium. 
Typical results of the test are given in Table 1. Given that the 
clearance present in the commercial standard bearings employed is 
theoretically capable of contributing an error of up to +O.O28O without 
taking into account other error sources such as bearing and housing runout, the 
results show that the design is inherently highly accurate. In these tests the 
preload exerted by the bellows tends to compensate for the effects of bearing 
clearance. Based on these results and supporting analyses, the expected 
accuracies are given in Table 2. The prototype HSAPM is shown in Figure 5. 
PERFORMANCE 
Table 3 summarises the main features of the High Stability Pointing 
Mechanism and its performance. The present design utilises a 2.15O swash 
angle and 7 inch diameter bearings. However both of these parameters may very 
easily be changed, thus allowing the pointing range to be greatly extended and 
the strength and stiffness of the mechanism to be increased dramatically. 
164 
The ability to provide a central aperture is a significant advantage for 
some payloads and, in the case of a steerable antenna dish, is ideally suited 
to the use of a cassegrain-type feed where defocussing due to re-pointing is 
avoided. 
The incorporation of the pin contact encoder is optional. This is 
primarily an instrumentational feature since it is not essential for either 
the trimming or re-pointing duty. A significant reduction in both mass and 
complexity of the mechanisn, compared with other concepts, has been gained by 
obviating the need for a launch-lock device and a separate fail-safe, return- 
to-zero facility. 
CONCLUSIONS 
The High Stability Pointing Mechanism is a new conceptual design which 
provides a simple and rugged mechanical interface between two structures 
(see Figure 6) and which is c.apable of orientating those structures relative 
to one another on demand. With its high strength, high-stiffness torque 
output and accuracy the mechanism has been designed to suit a wide variety of 
applications including antenna pointing for which the mechanism is to be 
employed on L-SAT (see Figure 7). The pointing range will be increased to 
8.6" radius for the L-SAT application. 
REFERENCES 
1. Thermal Vacuum Tests on the Engineering Model 
MSDS APM by J.C. Anderson, ESRO(ESTL)l5. 
2. Systeme d'orientation Fine D’Antenne 
by B. Hubert and P. Brunet, Proc. 15th PerOSpaCe MeChaniSITIS 
Symposium, May 1981. 
3. An Antenna Pointing Mechanism for Large Reflector Antennas 
by H. Heimerdinger, Proc. 15th Aerospace Mechanisms 
Symposium, May 1981. 
165 
TABLE1 - EXPERIMENTAL POINTING ERRORS 
EeX 
0.002 0.007 
0.016 0.001 
0.016 0.023 
0.010 0.041 
0.007 0.017 
0.012 0.000 
0.005 0.007 
0.008 0.004 
0.002 0.005 
0.008 0.012 
0.017 0.000 
0.001 0.015 
0.002 0.005 
0.000 0.000 
0.003 0.001 
0.018 0.022 
0.010 0.020 
0.033 0.029 
0.015 0.005 
0.007 0.013 
0.003 0.015 
0.006 0.008 
0.005 0.009 
0.002 0.010 
E8 
Y 
166 
TABLE 2 - HSAPM POINTING ACCURACIES 
MODE 
Trimming mode with RF sensor 
Repointing using encoder 
Repointing (open loop) with 
encoder set datum 
Repointing using pipper 
Steady state pointing 
Launch configuration (relative 
to interface) 
Failsafe mode using encoder 
Failsafe mode using pipper 
(This assumes use of the encoder 
on the failed section). 
ACCURACY 
+ 0.006" 
-+ 0.0080" 
+ 0.0085“ 
r 0.01" 
+ 0.0080" 
f 0.0080" 
f 0.0080" 
f o.oo94° 
For all modes other than the trimming mode the pointing accuracy 
will be governed by structural component accuracy, bearing run-out, 
thermal distortion, system backlash and motor step size. The 
accuracy for these modes is, therefore, dependent on whether it is 
the encoder or pipper that is used. 
167 
IIIII llllllll 
TAEXE3- HSAEM PERFORMANCE CHARACTERISTICS 
Mass 
Dimensions 
Interface fixation 
Design life 
Position sensors 
Pyrotechnics 
Control aperture 
Load capability 
(individually applied) 
Ground running 
capability 
(individually applied) 
Payload inertia 
Stiffness 
< 4.2kg, depending on configuration 
1OOmm height 
235mm diameter 
: 6, M5 bolts on 19Omm PCD (antenna) 
6, M5 bolts on 215mm PCD (spacecraft) 
10 years continuous trimming duty 
Pin contact encoder (non-redundant) + 0.008° 
Optical datum (redundant) f 0.010 
None 
up to 4omm 
: 
: 
20,000 N axial 
8,000 N radial 
450 Nm cross axis moment 
: 
: 
4,000 N axial 
1,800 N radial 
60 Nm cross axis moment 
Up to 50kgm2 
140 x lo6 N/m longitudinal 
120 x lo6 N/m lateral 
450 x lo3 Nm/rad cross axis rotation 
730 x lo3 Nm/rad torsional 
Output torque 
Swash angle 
Pointing range 
Step size 
Slew rate 
Accuracy 
Power consumption 
>350 Nm 
2.15 deg. (4.3 deg for L-SAT) 
Radius of 4.3“ (8.6 deg for L-SAT) 
0.0008" from each swash-plate 
Up to 0.24O/sec. 
See Table 1 
Trimming 4W/motor 
Repointing 4W/motor 
Steady State pointing OW/motor 
Failsafe return to zero : Available for repointing applications 
168 
. - ORIVEN BEARING 
X- FREE BEARING 
I 
I 
iWASH PLATE POINTING MECHANISM 
PRINCIPLE OF OPERATION 
B LOCUS From(o,o) 
From P 
\ y AL POINTING RANGE.) 
Lo.XS OF POINTING VECTOR DUE TO ROTATION 
OF SWASH PLATE 
FIGURE 1 
FIGURE 2 
169 
Motor Swash Plates 
i 
I - --&----j-p- 
1 
1 I 
I 
G&30X 
I I I 
Satellite Interface 
CROSS SECTIONAL VIEW OF HSAPM 
Encoder Stepper Motor/ 
r. 
Rotating 
Wedges 
FIGURE 3 
ing Gear 
Access IBane 
Therma 
Strap 
End Plates (non-rotating) 
HIGH STABILITY ATTITUDE POINTING MECHANISM FIGURE 4 
170 
PROTOTYPE HASPM FIGURE 5 
MAXIMUM HEAT FLOW 
AT INTERFACE 4 WATTS 
:COND SURFACE MI 
9 
ROR 
-‘-SH, 0.075 METRES 
IUIRED FOR DEPLOYED ,- -._. _ - -... . . 
ANTtNNAs ONLY) 
ANTENNA POINTING ! i 
MEcHAN ISM &PM) 7. T- &~~~-~i&/____lj 1 / 
MULTI-LAYER 
INSULATION 
: I /--BLANKET 
s’/ /---- 
T@fJ 
,I--\\ 
;\ ;! ’ -dJ/ \I ’ 
TYPICAL HSAPM INSTALLATION FIGURE 6 
171 
L-SAT CONFIGURATION (5 APM's SHOWN) FIGURE 7 
172 
A. J. D. Brunnen 
British Aerospace Public Limited Company 
Argyle Way 
Stevenage, Herts. SGl 2AS 
England 
Mr. Brunnen has been employed by British Aerospace Dynamics Group, Space and 
Communications Division, since September 1980. Since joining the company, he 
has been working on the design of the swash plate pointing mechanism and is 
currently project leader for the antenna pointing subsystem on the European 
Communication Satellite, L-SAT. Mr. Brunnen graduated with an Honours degree 
in Engineering Technology from Robert Gordons Institute of Technology in 
Aberdeen, Scotland, in 1980. 
Co-author of this paper is Mr. R. H. Bentall who is with the European Space 
Agency (ESTEC), The Netherlands. 
173' 
la. 


Development of a high stability pointing mechanism for wide application

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