Pointing Control of the Helios Pencil Beam Antenna

H.C.Benohr W.Kleinau R.Alexander POINTING CONTROL OF THE HELlOS PENCIL

BEAM ANTENNA

Hans-Christian Benohr,Helios Engineering Mgr.,MBB, Munich, FRG. Wolfgang Kleinau, System Engineer "" Richard H.L.Alexander,Helios Attitude Subs.Mgr." SUMMARY This paper describes the Despin Drive Assembly and associated control electronics employed for despinning and positioninq the reflector of the HELlOS High Gain Antenna so that the pencil beam is pointed in the required direction, that means towards the earth. Further the nevelopment of the system, with particular emphasis of the critical problems that arose and their solution, and the post-launch experience with the HELIOS-A/B $pac~ craft, launcned in December 1974 and January 1976 respectively, are discussed. INTRODUCTION The Helios spacecraft are solar observatories operating at sun-spacecraft distances between 1 and 0.3 AU and spacecraftearth distances up to 2 AU. An over-all view of the spacecraft is shown in fig. 1. In order to maintain a telemetry down-link with adequate signal-to-noise ratio to enable operation at high bit-rates up to maximum spacecraft-earth separation, a high gain pencil beam antenna providing a gain of 23 dB is used. The spacecraft orbit is shown in fig. 2. The reflector for the antenna has to be despun with respect to the spacecraft (which is spin-stabilised at 60 ± 1 r.p.m.) and set so that the beam pOints toward earth with changing sun-spacecraft-earth angles. Control of the beam is required in the meridian plane only, as the azimuth beam-width of the antenna is sufficiently broad to ensure maximum signal for all expected earth declination angles. Sontrol accuracy required in the meridian plane is 0.7 .

±

SYSTEM OUTLINE The system developed consists of two units: 1) A Despin Drive Assembly (DDA) mounted outside the spacecraft body as in fig. 1. The DDA (fig. 3) consists of 2 main parts, a central portion which forms part of the antenna boom and must be designed mechanically to withstand the antenna flight loads, and an outer portion, supported by bearings top and bottom, which carries the reflector and thermal shielding. The outer part is driven in relation to the inner part by an a-pole brushless D.C.motor, the drive electronics module for which is included in the DDA housing.

227

H.C.Benohr W.Kleinau R.Alexander The speed of the DDA is adjusted by applying a control voltage from an external source, and the DDA generates two signals, known as position and rate signals. The signals are generated electro-magnetically, the position signal (PPU) being delivered when specified points on the inner and outer parts of the DDA coincide and the rate signal (RPU) consisting of 32 pulses per revo~utio~( mechanically equally spaced. (PPU and RPU accuracy ~s ~6 ) 2) A Despin Control Electronics (DCE) mounted inside the main body of the body of the spacecraft. The DCE processes the position and rate signals received from the DDA, signals derived from the sun-crossing sensor, and commands received from ground to establish the control voltage which must be delivered to the DDA. SYSTEM FUNCTIONAL DESCRIPTION Before considering the operation of the control loop, it may be helpful to take a quick look at the sun sensor and its associated electronics. The positioning of the sun senSor is shown in fig. 4. Each time the spacecraft rotates a pulse is generated by the sensor electronics when the sun sensor crosses sun centre. This pulse provides a reference for positioning the DDA. Additionally the time between consecutive sun sensor generated pulses is measured in the electronics, and based on this measurement, pulse trains are generated representing fixed numbers of pulses per revolution. For the operation of the DeE 2048 pulses' per revolution (ppr) and 32768 ppr signals are required. From the diagram can also be seen that a position pick-up pulse is generated when the axis on which the sun sensor lies coincides with the pointing axis of the antenna. A block diagram showing the basic elements of the control loop and motor electronics is shown in fig. 6. In the alpha register the required angle between generation of sun crossing pulse and position pick-up pulse (i.e. the sun-spacecraft-earth angle) is stored in an 11 bit register, which gives an effective setting accuracy of ~ revolution (~.1760). The register is set by ground control according to computed orbit predictions, but to avoid the requirement to send frequent commands,an automatic up-date system is incorporated which can increase 05 decrease the 0 value in the register at rates between 0.23 and 14.7 per day. The rate of change and the requirement to increase or decrease are set by ground command. At switch-on of the spacecraft the register is automatically set to the value wich it is expected will be required after launch to avoid having send a large number of commands to reach the required value required, which could well prove to be necessary if the register could assume a random value when power is first applied.

228

H.C.Benohr W.Kleinau R.Alexander In operation each time a sun pulse is received the value in the alpha register is transferred to an 11-bit down counter. Subsequently the counter, driven by the 2048 pulses per revolution signal, counts down until it stopped by the receipt of a position pick-up pulse. If at this time the value in the counter is zero, no pointing error exists, if zero has not been reached the angle is too small, and if zero has been passed the angle is too great. In addition to the measurement each revelution of the pointing error, the rate at which the reflector is turning relative to the spacecraft is measured 16 times per revolution using the rate pick-up pulses. The measurement is carried out in the omega counter in which the 32768 ppr signal is counted between every second rate pick-up pulse (of which there are 32 every revolution, alternate pulses being ignored). The value in the omega counter, which has 11 bits, should be zero when an effective rate pick-up pulse is received. If the counter has not had time to fill completely the motor is running too fast and if zero has already been passed the motor is too slow. To generate the required control voltage the error values in the alpha and omega counters are transferred to registers, the outputs of which are connected to digitalanalog connectors. To avoid excessively large converters only the 7 least significant bits, together with a sign bit are quantised. In the event of larger errors (greater than 6.25 %) where proportional control is not required, a "full-house"signal together with the necessary sign are applied to the convertor inputs. The control function which has to be realised is Vc = Va o + 0.017<4 (1 +-i-)+ 0.8 Wc (2 + _t_)

(1)

T

where c(e and ?-k are the errors in alpha and omega registers (expressed in decimal with sign) respectively, Vc is control voltage and Va o control VOltage for t = o. T is the time constant = 10 secs. The logic circuitry is designed to avoid "aliasing" effects e.g.motor running at half-speed. In operation errors can arise from systematic errors (mechanical misalignments etc.) and short-term variations of pointing around the desired angle due to variations of bearing torque. In practice the systematic errors can be cancelled out by correcting the alpha required angle. This search, carried out in the first few days of the mission is referred to as an "antenna pointing manoeuvre". Telemetry and Telecommand Interface The following information of despin drive status is available via telemetry. (The circuitry for transferring this information to the data-handling subsyste~ has been omitted from fig. 6 in order to emphasize the essential control loop) o

Required alpha angle

o

Automatic up-date rate

o

Automatic up-date status (increase/decrease/off)

o

Alpha error

( o(e. 229

H.C. Benohr W.Kleinau R.Alexander

( We )

0

Omega error

0

Control voltage

0

Motor current

0

Power supply voltage monitor

0

DDA temperature measurements

Commands (from ground station) are available for the following functions o

Increase alpha register 1 bit (Redundant)

o

Decrease alpha register 1 bit (Redundant)

o

Increase alpha automatically

o o

Decrease alpha automatically Alpha automatic off

o

Increase rate of alpha automatic change

o

Decrease rate of alpha automatic change

(Redundant)

DEVELOPMENT AND DEVELOPMENT PROBLEMS Bearing Design In considering candidate proposals for the DDA, the choice of bearing lubricant suitable for the particular space application was an important criterian. On the one hand wet lubrication was well-proved but would have required the maintenance of the DDA temperature within narrow limits, which would have required a complex active thermal control system in view of the exposed position of the DDA. On the other hand data was lacking on the use of dry lubricated bearings of the type required for use in the DDA, but the less stringent thermal requirements were attractive. Ball Bros.,who manufactured the DDA, had however, experience of operating smaller, higher speed (360 rpm),bearings ~nder vacuum conditions. The bearings were dry lubricated using a Ball Bros. process known as "VacKote" and it appeared this process could also be applied to bearings suitable for the DDA. The decision was taken to build the DDA using dry lubricated bearings and to set up a life test having a duration of 18 months to qualify the bearings and lubricating process. The test specimens used for the life test were a complete despin drive assembly and six sets of bearings driven by a single motor. The test was carried out under thermal vacuum conditions, temperatures being changed at monthly intervals. During the test all the bearings, including those in the DDA, performed well and when they were inspected after completion of the test all were in good condition. The very satisfactory test results indicated that a high level of confidence could be placed in the bearings, and the in-flight performance, described in a later paragraph, has confirmed the suitability of the bearings for the application. 230

H.C.Benohr W.Kleinau R.Alexander control LOOP Redesign The original control loop foreseen for the DDA differed considerably from that finally used and previously described in this paper. In the original concept the rate pulses were only employed during the original run-up of the motor, and when the speed was close to that required, the position pulse together with a lead/lag unit was used to correct the control voltage once per revolution. Tests of the ~ubsystem indicated however that pointing errors up to 10 could be built up. This was found to be due to low frequency variations in bearing torque, the period of one cycle torque variation occupying around 20 revolutions of the DDA. Fig. 5 is typical of the effects Observed, the variation in spin period with a constant drive signal to the motor indicating the effects of the friction variations. Efforts were made to reduce the amount of friction variation by preloading the bearings and by varying the number of balls used, but no significant improvement in performance was achieved, although some variation in frequency and amplitude was observed. Analysis of the test data and a study of the bearing construction showed that the effect was inherent to bearings, and results from relative motions of inner and outer races and ball-bearings, where a particular alignment of these three components repeats itself approximately every 20 revolutions. As a result the Despin Control Electronics was redesigned so that speed changes could be compensated at more frequent intervals, using the rate pick-up pulses as previously described. Fig. 7 shows the result of analysis of pointing errors resulting from a 1 oz.in. sinusoidal ripple torque with the original circuity and fig. 8 the improvement resulting from the circuit design ';hange. DDA Thermal Household All the temperature sensitive experiments and subsystems of HELlOS were accommodated inside the thermally controlled central compartment. However, the DDA had to be mounted outside on a tripod of the antenna mast to carry the reflector to be despun. The distance from the DDA to the central compartment was considerable, because the reflector for the high gain antenna has to be outside of the "shadow" of the solar array cone. Therefore separate thermal control for the DDA itself was necessary. To avoid system complexity, to improve the reliability, to keep performance independent as much as possible from the space interfaces (solar probe 1 to 0.3AU), the following selections were made: - dry lubricated ball bearings - brushless DC motor and magnetic position and rate pick-ups operating without mechanical contact. 231

H.C.Benohr W.Kleinau R.Alexander - passive thermal control provisions. Although dry lubricated ball bearings are less sensitive to changes in temperature environment than wet lubricated ball bearing systems, a considerable effort for thermal control has to be considered. The requirements for the envisaged thermal control of the DDA were: 1. The internal temperatures had to be inside specified limits during the entire mission. The time spent in the so called cold case (near earth) was 70-75% of the mission time, medium case (ambient temperature) ~ 20%, hot case (sun approach) 5- 10%). The temperature range and in particular the upper temperature for the hot case were constraints for electronic components of the integrated motor commutations and drive electronics, whereas low temperatures in the cold case were more critical for the motor torque. 2. The temperature gradients across the bearings and along the DDA axis have to be within specified limits during the entire mission. The temperature range within which the DDA has to be held is also restricted by the differential expansion of shaft and housing material, as the radial clearance of the ball bearings and the axial gap of the upper spring loaded bearing have to be assured, to keep the bearing friction torque below the maximum allowable value. The thermal interfaces for the DDA are: Central Body:

Antenna Mast:

0

Conductance by the antenna mast and the supporting tripod

o

Radiation from top plate and the louver/ radiator plate system (closed in cold case) DDA shaft has been part of the masc

0

o

232

Antenna cables (heat source) radiation

Antenna Reflec-o tor o

Conductance by the supporting truss

Solar' Array:

o

Radiation from the rearside fins of the solar array

o

Reflection of the sun from array sun (worst case orientation)

Reflected sun radiations from the reflector

H.C.Ben5hr w.Kleinau R.Alexander - Space/Sun:

o

Radiation into space

o

Direct sun irradiation for anomolous orientation of spin axis during acquisition manoeuvre

The main objectives for the thermal control of the DDA were: - Decoupling the DDA from the antenna mast and reflector by conductance barriers Insulating the DDA from space using thermal blankets with appropriate absorption/reflection characteristics - Protecting the DDA from direct sun irradiation by location inside solar arraY , cone shadow - Coupling the DDA with the "warm" central spacecraft body - Provide passive thermal control using the above mentioned insulations, the internal heat sources (electronics, drive motor) with radiation interchanges and a small DDA-radiator plate to space. Provisions and improvements for the thermal control during the DDA and spacecraft system development. a) Lowering of the spacecraft body irradiation and the spacecraft, space

DDA positions with respect to the central to prevent the drive from direct sun to have closer irradiation contact to also decreasing the DDA's view factor to

b) Increasing thermal conductivity of the antenna boom/tripod mounting to improve the thermal coupling with the spacecraft. Decreasing the conductive heat flow (heat barriers) and the reflected heat (smaller £ ) of the reflector supporting truss looking towards the DDA. c) Change of thermal blanket material from Mylar to Kepton, outer layer alurninized,with Nylon mesh in order to withstand even direct solar irradiation for a limited time. d) Adding of thermal cover shield for the reflector hub to avoid radiation to space through the gap between the reflector and the antenna mast. e) Adding a heater mat close to the reflector and bearing for cold case operation, further black painting of internal surfaces to increase the lower temperature level without increasing the thermal gradient across the bearings. f)

Increasing the axial gap at the upper bearing between housing and shaft to avoi~ too high friction torques during hot case operation.

233

H.C.Benohr W.Kleinau R.Alexander Low Temperature-High Torque Problem During low temperature thermal vacuum testing a considerable rise in torque together with a marked increase in ripple with a repitition rate of I revolution was observed on some units at around ~20 C. Tests were performed to determine the cause of the torque increase, and it was found to be due to thermal d£stort£ons of the DDA causing a loading of the bearings. The problem was solved by rotating the lower bearing end plate to the most favourable position and by a change in mounting surfaces for the inner race of this bearing. These changes resulted in satisfactory performance down to -50 o C which gave a good margin for the mission. FLIGHT EXPERIENCE Since the launch of HELIOS-l the temperatures measured in 0 the DDA have ranged between approximately -20 and +30 0 for the bearings, motor and commutator, the temperature of the electronics being generally around 100 higher than the other measurements. This is inside the limits to which the DDA was tested during ground testing and neither in the hot nor the cold case has the limit of 50z.in. torque for the bearings been exceeded. At the time of writing HELIOS-2 has not yet reached first perihelion so the DDA has not yet seen the full range of temperatures to be expected, but up to the present time, performance is as good as HELlOS-I. In general the friction of the HELIOS-l unit falls as the spacecraft approaches perihelion and rises again approaching aphelion as the DDA gets colder. Both units have provided the required pointing stability and the pointing manouevers referred to ~reviously have confirmed the nominal sun-spacecraft-earth angles as being the best settings. ACKNOWLEDGEMENTS Acknowledgement is due to the many members of MBB and Ball Bros. who contributed to the development and manufacture of the Despin Drive Assembly and the Despin Control Electronics. In particular thanks are due to Mr. Schindewolf of MBB who established the required control function and to Messrs. Dorl and Mehltreter, also of MBB, who then developed the necessary hardware to realise this and to Messrs. Phinney and Dummigan of Ball Bros. for continuing support throughout the program.

234

H.C.Be nohr W.Klei nau R.Alex ander

HGA. - FEEDE~

HGA.

REFLEC rOR

HELlOS DRIVE

FIG. 1 :

OVERA LL VIEW OF THE CONFIG URATIO N

HELlO S

DE:SPI N

ASSEM BLY

..sPACE CRAFT

Fig. 2 : HELlOS and earth orbits around the sun

235

H.C.Benohr W.Kleinau R.Alexander

HEAT6f1IELDS

M"l6NETlC PICKUP

FIG . 3

236

SCHEMATIC

OF HELlOS DE5PIN DRIVE A55EMBLY

H.C.Benohr W.Kleinau R. Alexander

SVN DIRECTION

SVNSENSOR

ALPHA ANGLE (SVN -SPACECRAFTEARTl-1 ANGLE MEASURED IN SPIN DIRECTION)

SVN SENSOR GENERATES PVLSE WHEN CROSSING .suN LINE.

EARTH DIRECTION

/=1(3.

'+ ;

f--

DDA GENERATES PVLSE WHEN POINT A I WHICH IS ON SUN SENSOR LINE AND ROTATES WITH SPACECRAFT COINCIDES WITH POINT B ON OVTER ODA HOUSING.

SVN SENSOR AND DDA

2'

,.v

GEOMETRICAL ARRANGEMENT

------l

FIG.S ; EFFECT OF TORQUE

VARIATIONS

ON SPIN

PERiOD

237

t:l co

.::c

&uN~UL5E

Z~.8,~[P~R~------lr--------------------------~-:~;7::~~~~~~-:-----I •POSITION PICI(-VP PUL61/! __________~

:>:l~() ~:>:tJj

I-' 1-'(1) (1)

(1)

::s

>< ,... 0:

1lI::S;:T

::s

S IGN,

81r

AL.P'IA

(01.)

11

DOWN

R~GN;TER

D/A

1----""le""",,".

AurOI"1AnC

OM~GA

COUNT£II

RATE

PICK-/JP PULSi!

S2.76E1pPR

Des PIN CONrROl- ELEcrRONICS curLINE

FIG .

6:

III 11

g.~

ALPHA.

BLOCKDIAGRAM OF HELlOS

REFLECTOR

DeSPIN DRIVE

DESPIN

OLlrLINE

CONTROL

LOOP

H.C.Benohr W.Kleinau R.Alexander POINTING ERROR

20 0

0.0001 Hz.

0,001 Hz

O,O-1 H z

"Hz

,:'16.7 POINTING

DURATION

VARIATION . ABOVE CONTROL

F"OR

1oz. in.

$INUSIDAL

TORQUE

ORIGINAL CONrROL -LOOP - BeLOW MOOII=IEO

LOOP.

POINTING ERROR

0,5 0

0,4- 0

0,3 0

0,2 0

O,0001Hz.

0,OO1H%

0, 0-1 Hz.

o,1HZ.

1Hz

o,SHz. I=IG . 8 : RE.$;PONSE 01= CONTROL LOOP FOR NORMALIZED DISTURBANCE ( -1 oz. .-inch) VERSUS FREGUE/VCY

239

H.C.Benohr W.Kle1nau R. Alexander

'"u

.

~ ;;

0

u

·c0

... t

~

c

.~

.~

D..

o :.-:

0

'""

-[

0

6

3

"

.1

1

+

g lS

D..

'"

V

11-··1 cC

0

..

E

:;;

,

'i

~

11·· .. 1

I~ ~

u

~r~'l 11·, .. ·[

.... 2

c

'"{c

240

~ .. 0

U

~

~

et:

«

j

c

.~

....

.0.

f) It·'! 0 0

u

c


a.

c

(;

~

0

Is -;; >j:

" ~

§

0

:>:

..,

t

a:

"5

"l;

~

.~

~

§

.c:

01

<>

.

"5

::.

'" ·c 01

]