The evolution of an Antenna Deployment Mechanism (ADM) from a Hinge Actuator Mechanism (HAM) is described as it pertains to the deployment of large satellite antennas. Design analysis and mechanical tests are examined in detail

Death, M. D.

English

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EVOLUTION FROM A HINGE ACTUATOR MECHANISM TO 
AN ANTENNA DEPLOYMENT MECHANISM FOR USE ON THE 
EUROPEAN LARGE COMMUNICATIONS SATELLITE (L-SAT/OLYMPUS) 
Martin D. k ' A t h *  
ABSTRACT 
This paper describes the evr>!ution of  an antenna deployment mechanisni (ADM) from a hinge 
actuator mechaniam (HAM). f i e  result is a mechanism capable of deploying large satellite append- 
ages in a controlled manner. 
The development testing of  the HAM identified many improvements t c  thc concept which 
were incorporated into the ADM. Both of  the designs are described in detail and the improvements 
made to the ADM are highlighted. 
INTRODUCTION 
The hinge actuator mechanism (HAM) concept was developed by British Aerospace (BAe) in 
response to an ESTEC development programme.' The experience gained in developing the HAM has 
been applied in the design of  the antenna deployment mechanism (ADM), which originally was for 
the OLYMPUS telecommunications satellite programme but has been h r the r  developed for the 
UNISAT and EUROSTAR sate!lite programmes. 
The HAM was conceived as a device able to deploy large satellite appendages i.n a controlled 
manner with a low speed of deployment such that the disturbance impulse to  the satellite was 
minimal. The energy supply to  power the HAM was self-contai~ed and required no  externzl dctuator 
t o  enable operation. The appendage to be deployed would be held down to the satellite by a pyro- 
technic release assembly, release of the mechanism would only require power to fire thz pyrotechnic 
and would therefore minimize power requkeinents for deploymeat. 
The development of the ADM, which required a significant increase in the aviilable deployment 
torque coupled with a decrease in rotational speed, minimizes any iiicrease in size or  mass. 
Details of the ADM application within the OLYMPUS antenna deployment subsystem has 
been included to show a tyk :a1 app!iwtion of the ADM and to highlight the operational require- 
ments to be satis5ed by the mechanism. 
MECHANISM DESCRIPTION 
Hinge Actuator Mechanism 
Esseniiallv, the HAM is a shaft supported on rolling element bearings that is driven by a large diam- 
eter helical torsion spring. The shaft, which in application, would be coupled to a large inertia, has 
its rate o f  de#loyment controlled by an eddy current damper which is dnven by a shaft at  high 
speed via a high ratio gearbox (F ig~re  1 ). 
*Space 1% Comrnlinizations Division, British Aerospace, Stevenage, Herts, U~uted Kingdom. 
1 2  
I'IIECEDING PAGE BLANK NOT FILMED' n !  
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Shaft and Main Bearings 
The main shaft is supported by a deep groove ball bearing and lineai bearing assembly. The deep 
groove bearing provides the axial location and the in-line linear bsiuing allows for thermal expansion. 
The bearings and shaft are sizcd t o  accommodate anticipated launch loads of 30  g acceleration act- 
ing on the 12  kg mass. Liquid lubricant was used for the main bearing for a low friction torque. The 
linear bearing was not lubricated but ran on a sd id  molybdenum disulphide film. Both bearing 
housings were mounted on a common base to reduce misalignment. In a flight case it was antici- 
pated that any in-line bearings would be separated by about 1 meter and would support an antenna 
backing structure. As previously stated these bearings would be designed to support only radial 
loads, the axial movement being unrestrained to  accommodate thermal expansions. The shaft in the 
HAM represents the backing structure, and was manufactured from steel. The bearing housings and 
base were manufactured from aluminium alloy for lightness. 
Spring 
The drive spring is a large diameter helical torsion type driving the output shaft directly. The spring 
is housed within the gearbox and damper subassemblies and attaches to  the main shaft adjacent to 
the main bearing. The housing allows sufficient clearance for the increase in diameter of the spring 
during deployment and provides adequate support. 
The spring has been sized to generate a torque of 1 Nm, providing a comfortable margin over the 
anticipated resistive torque limit of 0.5 Nm. The steel wire spring was solid lubricated ~ i n g  a 
molybdenum disulphide film to reduce friction between the coils and housing. 
Gears 
The output shaft of the HAM drives the damper assembly via a high ratio gearbox. The overall gear- 
box ratio of 225:l is obtained using four passes, this is to minimize the size of the gearbox while 
maintaining adequate tooth strength. During deployment the damper will balance the excess spring 
torque, the gearbcx must therefore be capable of transmitting 1 Nm applied at  the drive shaft. 
The gears are isolated from the deployable structure launch Imds by mounting them u t  board of 
the main bearing. Tht  geals and their associated bearings need only be cpab le  of trmsmjtting the 
torque and containing thc gear separation forces during deployment. Because of the high overall 
ratio thi friction torc,ue must be minimized. Therefore, the gears are dry film lubricated using 
tno!ybdc.-,um dkulphitle a i d  the bearing are oil lubricated with Fombiin Z25.  
An escapement device was incorporated into the design to eliminate any excessive torque being 
applied to tile gearbox whm back-drivefi, thus the design allows simple integration and stowage 
of the appendage to be deployed. 
The escapement device is a one way rolling element clutch that is oil lubricated using Fomblin 223. 
The gears are standard spur gears with the full tooth form and 20" pressure angle. Because of the 
method of lubrication thc g e m  were manufactured from stainless steel and surface hardened tc  6G 
Rockwell 'C'. 
ORIGINAL PAGE la 
OF POOH QUALITY 
An eddy current damper IS uscd wfich xtilizes the damping effect of eddy currents induced within 
a conducting disc rotating in a magnetic field. The induced current, and the energy dissipated, is re- 
lated to  the speed of rotation and the magnetic field strength. The speed necessary to  provide damp- 
ing can be minimized by optimizing the field with respect ts the magnetic geometry. 
The magnetic circuit was created by 1 ' magget pairs (Figure 2), set radially on the damper housing. 
Simple bar mrignets with shaped pole pieces to  emcentrate the fiux between the magnets were use: 
to optimize the magnetic field. Commercially available samarium c o h l t  Y;pets and low carbon 
steel pole pieces werc used. C 
Mass 
The total mass of 1.45 kg could be significanily reduced by the aaoption of lightwei@it materials, 
for example an aluminium shaft and Delrin, an acetal copolymer resiq, could be cotl?r;..,'tered for the 
housing and gear materials. 
Performance of khe HAM 
A test programme designed to fully test the HAM against its design requirements resulted in the 
following evaluation of the HAM. 
Figwe 3. HAM Magnet Layout 
The HAM satisfied all the functional tests and operating reliability with the exception of depl ~ r -  
ment speed. Tile damper design use,' , pure aluminium disc and the actual manufactured unit used 
an aluminium alloy with a higher rr,. iivity. Under test a deployment speed of 1.3 deg.sec'' was 
obtained. This speed compared with a calculated figure of I .:. aeg.iec-I (using the higher resisitiv- 
ity). The accuracy of deployment was well within 0.05', being O.O1° with a repeatability of d e r b y -  
ment better than t0.004'. 
Conclusions 
The HAM programme proved that it was feasible to ~ r o d u c z  a mechanism capable of accurately 
deploying an inprtia, reliably and s t  a controlle? speed. The error in the deployment speed czused 
by a change in disc material highlighted the sensitivity of the eddy current damper and magnetic 
field theory, and the difficulty of rzliably predicting the magnetic field strength and the speeds a t  
which the desired ddmping torques could be achieved. 
DEVELOPMENT OF THE ANTENNA DEPLOYMENT ME "HANISM 
The OLYMPUS telecommunications satellite programme required r. multipurpose mechanism 
capable of a variation in t o y u e  output and deployment angle but able to control the ratc of deploy- 
ment for c '  expected output torques. During development of the ADM, experience that wis gai:li:d 
from the HAM programme was extensively applie:' t o  the ADM'q design. The areas where the most 
significant improvemellts were made have been highlighted wiihin this paper. 
The ADM concept is fundamentally the same as that of the HAM (i.e., a spring driven shaft 
mounted in rolling element bearings and controlled by an eddy current dalnper). The design require- 
ments for the ADM, which have been modified to satisfy the lllultiourpose roles, are summarized 
for comparison with 'hose of the HAM as follcws: 
Hinge capsci;y-Up to 27 Nm damping rc jdirement 
Deployment angle- Up to 90' 
Deployment accuracy and repeatatility-k0.0 1 " 
6 Resisiive torque--0.5 Nm 
@ Lifetime-5 years storagc, 50 'ground' operations, 10 year operational life 
n Drive torqae-At least 5 times resistive torque 
. . . '-e shock to  spacecraft-,' ! Nms 
: ~ds-27C0 N in any direction 
' : design impruvements in spring motor and damper design are highiighted in the fol- 
!owilia , ..graphs. 

Eddy Current Dam~ine: Theorv 
Equation 1 has been derived from the theory covered in "Permanent  magnet^."^ This equation pre- 
dicts the damping torque that may be expected from a disc rotating within a magnetic field. 
Td = 2 B2 r2 abt >d N SP 
A magnet pole is assumed to  generate a rectangular magnetic field. See Figure 4. 
Where, 
B = 
a = 
b = 
r = 
N = 
t =  
P = 
Wd = 
Td = 
Mpgnetic field strength (Tesla) 
Field or magnetic width (m) 
Field or magnetic length (m) 
Mean radius of magnets around disc of rotation (m) 
Total number of magnets around disc housing (m) 
Thickness of the conducting disc (m) 
Resistivity of conducting disc ( a m )  
Angular velocity of conducting disc (radlsec) 
Damping torque generated by the disc (Nm) 
The magnetic field strength in the gap between the magnets is estimated from equation 2. 
N magnets 
Figure 4. Dam pcr Theory Figure 
Where, 
B = 
Br = 
- K, - 
- 
K2 - 
Ag = 
Am = 
Lg = 
Lm = 
Flux density in gap 
Remanance of magnet material 
Leakage factor 
Magneto motive force factor 
Cross-sectional area of gap 
Cross-sectional area of magnet 
Length of gap 
Effective length of the magnet 
(Tesla) 
(Tesla) 
In equation 1, where a d  and Td represent the speed and torque in the damper, the output speed 
and torque may be represented by the expressions, 
T 
o d  = o. G and Td = 
where G = gear ratio between the damper disc and output shaft. 
By substitution into equation 1 and by rearranging equation 3 is obtained. 
In calculating the flux density in thegap, (equation 2), two factors need to  be quantified, K, and K,. 
Factor K, , the leakage factor, is calculated by relating the useful flux existing between two oppos- 
ing poles through the disc and that lost between poles parallel lo the pane of rotation of the disc. 
The values of K, were quantified during the design stage and are unique to the design of the HAM 
and ADM. 
Factor K, , the magneto motive force factor, determines the ability of the circuit to produce a cur- 
rent. This factor, however, is difficult to quantify accurately and has been taken in both cases to  
be 1.4. 
Summary of Damper Calculation for the HAM 
The following values have been used in the determination of the flux density from equation 2. 
These result in s calculated flux density of 0.250 Tesla. 
During deployment, once a constant speed is obtained, the system is in equilibrium and the damping 
torque is equal to  the excess spring torque. The HAM torsion spring sizing was based on the average 
available torque. 
T = driving torque to be damped 
Ts = average supply torque = 0.9015Nm 
Tr = resistive torque = 0.50Nm 
Td = bearing frictional torque = 0.0005 Nm 
Driving torque to be damped = 0.40 Nm 
The radial distribution of the magnets led to two magnetic circuits with a mean effective radii of 
r, and r, 
Also, 
p = 5.7x10-8i2n1 
G = 225 
t = 0.002 m 
a = 0.005 m 
b = 0.005 m 
N = 12 
Substitution into equation 3 for both circuits and summation of the results reveals the expected 
deployment speed. 
or, 1.19 ur.g.sec-' . 
This indic~tes that the mechanism should take 75 seconds to d e p l ~ y  90'. From testing it was 
discovered that the mechmism took 68 seconds to deploy. 
In comparison the following developments were carried out for the ADM and the results summarized 
in the following section. 
Summary of the ADM Developments 
ORlOlNAL PAGE IS 
OF POOR QUALrrV 
The magnet arrangement was changed from the radial arrangement as shown in Figure 2 to a cir- 
cumferential layout in Figure 5. This resulted in more magnetic pole pairs at an increased mean 
effective radius. Additional benefits of this change are that the reduction in the number of magnets 
has reduced the mass and that the leakage factor has been reduced, resulting in an increase in the 
flux in the gap and consequently the overall efficiency of the damper. 
The gear ratio between the damper and output shaft has been increased from 225 to 1378:l. This 
increase was achieved by incorporating an additional two pass gearbox between the spring motor 
drive shaft and the damper shaft with a ratio of 56.25:l. Therefore, the damper may be expected 
to produce the results that follow. 
Figure 5. ADM Magnet Confipuration 
ORIGINAL PAGE 13 
OF POOR QUALITY 
Summary of ADM Damper Calculations 
-- -- - 
Flux in gap B 
For application in equation 2 .  
. B = 0.5592 Tesla. 
This niay now be used :o predict the deployment speed w. 
Driving torque T. 
Ts = supply torque = 14.85 Nni 
Tr = resistive torque = 0.50Nm 
Tf = frictional torque = 0.01 1356 Nm 
. T = 14.33 Nni 
Also. 
P = 
G = 
B = 
r = 
t z 
a = 
b = 
N = 
Resulting on substitution into equation 3 a deployment speed. 
This indicates a deployment time for a rotation of 90" of 750 seconds. 
At present, test results are not available for the mechanism deploying a resistive torque of 0.5 Nm, 
so the conclusions made are based on the theoretical calculations and the experience geined in eddy 
current damping. 
Conclusions of the ADM Damper Development 
The damping capacity of the ADM has been significantly improved by increasing the overall rhmper 
gear ratio from 225 to 1378 and by increasing the mean radius while at the same time reducing the 
number of magnet pairs. The rate of deployment is down by a factor of four and the damping capa- 
bility has been increased by about 14 times. 
The most gains have been achieved theoretically in this area of development and it is expected that 
these gains will be supported experimentally. 
MATERIAL SELECTION 
During the design of the ADM, extensive use of lightweight materials for lowstressed com- 
ponents and housings has been incorporated. Delrin, an Acetal copolymer, was used for housings 
and the damper gears where considerable mass savings were achieved. 
Mass 
A mass estimate for the ADM includes the additional in-line bearing assembly and results in an over- 
all mass of 2.3 kg. With this slight increase in mass, performance is improved substantially over the 
HAM. 
Conclusion 
The ADM is a versatile mechanism capable of deploying large deployable structures on satellites 
accurately and in a controlled manner such that the disturbance impulse to the satellite is kept to a 
minimum. A significant increass in drive torque and damping capacity was achieved ir. the develop- 
ment of the ADM without a significant increase in mass compared to the HAM. 
APPLICATION OF THE ADM 
The ADM has been used in the OLYMPUS antenna deployment subsystem (ADS). This sub- 
system supports during launch and deploys after launch the 1.2 m reflectors and antenna pointing 
mechanisms (APM) that are to be used on the east and west sidewalls of the spacecraft. 
The ADS is comprised of three main equipments: the ADM, the deployable arm (ARM), and 
the pyrotechnic release assembly (PRA) as shown in Figure 6.3 The PRA is used to hold the ADS 
to the satellite sidewall during launch, and via the pyrotechnics release the ADS after launch. The 
ARM forms the backing structure onto which the APM and reflector are mounted. The ADS is re- 
quired during operation to  form a thermally stable platform from which the APM/ and reflector 
may operate without degradation for the satellite lifetime of 10 years. 
Figure 6. ADS Assembly 
REFERENCES 
1. Gallagher, K., "Hinge Actuator Mechanism Programme - Final Technical Report," TP7951, 
British Aerospace, 1 982. 
2. Hadfie!d, Iliffc, "Permanent Magnets." 
3. Dace, R., "Antenna Deployment Subsystem (ADS) Design Description Report," (RPT/LCS/ 
53636/BAe), British Aerospace, 1983. 


Evolution from a hinge actuator mechanism to an antenna deployment mechanism for use on the European large communications satellite (L-SAT/OLYMPUS)

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