DUAL-AXIS SINGLE-MIRROR MECHANISM FOR BEAM STEERING AND STABILISATION IN OPTICALINTER SATELLITE LINKS

W. Coppoolse*, M. Kreienbuehl*, J. Moerschell°, A. Dommann°° and D. Bertsch°°* Contraves Space AG (Schaffhauserstrasse 580, CH-8052 Zürich, Phone +4113062848, Fax +4113062823, E-mailwillem.coppoolse@unaxis.com)° Haute Ecole Valaisanne (CH-1950 Sion),°° Interstate University of Applied Science (CH-9471 Buchs SG)

ABSTRACT

The Contraves Space AG (CSAG) Fine PointingAssembly (FPA) is presented. The FPA is a dual-axis,single-mirror mechanism that can be used for beamsteering and stabilisation in applications such as theContraves Space AG Optical Inter Satellite LinkTerminal series. In these terminals the FPA stabilisesthe line of sight of an optical free space data communi-cation link between satellites. This is the subject ofdifferent papers which have been published in thepast1,2,3.The FPA offers a 1 kHz (+1/-3 dB, -45�) bandwidth,which is sufficiently high to compensate for vibrationalperturbations of the platform. Other main specificationsof the FPA include a � 7 mrad (� 0.4°) mechanical tip-tilt range and less than 1 µrad closed-loop peak-to-peaknoise. A second FPA, with a somewhat differentdynamic characteristic, mounted in the optical terminalequipment, allows realising the point-ahead functionthat is required in bi-directional long distance opticalintersatellite links. The use of custom-built componentsfor all principal functions has reduced size, mass andpower consumption. The CSAG FPA is shown infigure 1.Functional and environmental tests were conducted.One FPA was also successfully used in a system testbed for a free space optical communications link. Thetests have shown the conformity of the FPA to itsspecification. (Table 1)

Fig. 1: The CSAG Fine Pointing Assembly

1 FPA DESIGN OVERVIEW

It is a particular property of the design of an FPA, inthat the basic design parameters are very dependent onthe deflection range and the dynamical propertiesrequired. This is reflected in several papers describingFPAs4,5,6,7,8,9. For the envisaged range of +/- 7 mrad

and a small signal bandwidth of 1 kHz (+1/-3 dB, -45�)elaborate trade-offs for mirror support, sensor type aswell as for the drive actuators, resulted in the basicworking principle that is presented in figure 2.

Table 1: FPA Specification

With this configuration, a commanded mirror position isreached by applying a current to the actuator coils. Theactuators then produce a moment, which accelerates themirror structure and compensates for the back drivingmoment of the flexible support. The differentialcapacitive sensors are incorporated in a control loop,which allows accurate positioning and sub-microradresolution.An exploded view of the FPA Design is presented infigure 3. The FPA incorporates a 28-mm berylliummirror that is suspended on a flexure so that it is free torotate in tip-tilt axes. The mirror also serves as thestructure to which actuators and sensors are directlyattached. The design of the mirror and flexure makes theFPA applicable over a wide temperature range andunder thermal gradients, primarily resulting from thesun impact on the mirror surface, without losing opticaland dynamical performance.Because of the special suspension system design, theFPA is also able to withstand a high level of vibrationload without the necessity of a locking device.

Mirror & Structure for Sensors and Actuators

Actuators in push-pull configuration

Flexible Support

ACT ACT

ACT

ACT

Sens Sens

Sens

Sens

S

S A SA

Differential Sensors

Fig. 2: Basic working principle selected for the FPA

The actuators are of the Lorentz type. Four of theseactuators are used in a push-pull configuration. Themagnetic system is optimised for a moving magnet typearrangement to optimise thermal behaviour regardingdissipated heat by the actuators. The actuators havebeen optimised with the help of extensive finite elementmodelling.The position is sensed using four differential capacitivesensors resulting in superior 0-stability, which is notdependent of the scaling factor. The sensors have anintegrated pre-amplifier. The miniaturised sensors arehighly stable and robust through the use of solderedceramic plates and special bonding techniques.The sensor driver plates that are attached to the mirrorstructure are made of photosensitive glass-ceramics.This material allows a structure and a thermal matchthat avoids distortion of the mirror under thermalgradients over a wide temperature range.The position sensors and sensor driver plates weredeveloped in collaboration with the Interstate Universityof Applied Science in Buchs (CH).For the control system, the position of the poles of themechanical deflection transfer function between theactuators and sensors is basically ruling the maximumfrequency at which the control system can be designedin closed loop. For a closed loop bandwidth target of 1kHz (+1/-3 dB, -45�) the mechanical resonancefrequencies must be above 10 kHz to ease a reasonablysimple and straightforward controller design. For thisreason beryllium has been selected for the mirrorsubstrate because of its E-modulus/specific mass ratioas well as for the thermal properties.The control electronics linearises the sensor charac-teristic, and implements a cascaded feedback scheme,with state space motion controllers and output current

amplifiers to drive the actuators of the FPA mechanism.The configuration with four actuators and four sensorsallows for a degraded mode of operation if one of thesecomponents fails. A pole placement technique adaptedto compensation problems was applied.The control electronics were developed in collaborationwith Haute Ecole Valaisanne in Sion (CH).The FPA design is optimised for the highest bandwidthat the required signal-to-noise ratio, and for very lowdrift in time. The control electronics holds on one euro-size board.

Fig. 3: Exploded view of the FPA

2 DESIGN EVOLUTION

To verify the basic concepts two breadboards have beendesigned and built. They consist of the mechanical unitand a control unit. Both units have been characterisedand their performance showed that the key designspecifications can be met with the selected concept.Figure 4 shows a picture of the Breadboard. Theactuators are performance optimised and are near theirshape required for a flight unit. The sensors have beenput onto larger substrates using standard hybrid designsas per standard PCB layouts. This avoids costly bondingprocesses to contact chip amplifiers. The Breadboardincorporates a small mirror for measurement anddemonstration purposes.

Fig. 4: FPA Breadboard

The design targets for the qualification model differedmainly in the following matters:� Smaller volume for the sensor unit.� Use of space compatible materials and processes� Adapting the design for the thermal impact at sun

exposure.The result of this work is shown in figure 5.

Fig. 5: FPA qualification model (without cover)

3 KEY COMPONENTS

Based on the outcome of the Breadboards the followingkey components have been extensively traded off andtheir performance analysed:� Mirror� Flexible Mirror Suspension� Actuators� Sensors� Controller SystemThese components will be discussed in more detailhereafter.

3.1 MIRROR

The beryllium mirror integrates the mirror with thecarrying structure for sensors, actuators and an interfaceto the suspension system.

For the dimensioning, the following design drivers andrequirements were considered:� 28 mm diameter polished mirror surface area with

27 mm diameter specified high flatness (�/50 rms,wavefront error for 1064 nm) and low roughness.

� High stiffness, avoiding mirror deformation underoperational loads and a first order bending fre-quency of above 10 kHz.

� Low inertia to comply with the limited excitationdrive capability.

� Employment over a large temperature range andunder thermal gradients.

The roughness of the optical polish can be achievedonly by applying a catalytic nickel layer to the substrate.A finite element analysis was performed to define thethickness of the layer on the back and front to minimisethe resulting bimetallic bending. Figure 6 shows themirror design.

Fig. 6: Design presentation for the Mirror assembly

The mirror structure has several pins to integrate it intothe mirror assembly. The centre pin is clamped to thesuspension system using a collet chuck. Another fourpins are used to attach the actuators. The pins haveshown to be very effective in assembling without dis-torting the mirror surface. Around the side, eight armsare located. These are used to attach the four sensordriver plates. Beryllium has been removed wherepossible to lightweight the structure, forming fourcavities in the back.

3.2 FLEXIBLE MIRROR SUSPENSION

The suspension type is considering the temperaturegradients, which are primarily caused by heat impact atdirect sun viewing. A provision for the collet chuck thatis used for integration with the mirror is located in thecentre. The design prevents multiple stable angularpositions by a unique pre-tensioning device. A mechani-cally flexible interface between mirror and base-platewith low stiffness in Tz, Rx, Ry and high stiffness inTx, Ty and Rz can be designed with this concept. Figure7 shows the orientation (T = translation, R = rotation).

Fig. 7: Interface axis Orientation

The concept is based on the interplay of several flexiblemembers offering inherent advantages such as friction-less, stictionless and clean operation.One or more advantages are achieved with this conceptover other concepts:� The Inertia of the interface can be made uniform in

all tip tilt directions (Rx, Ry)� It can be made uncritical to strain (e.g. thermal

strain). The interface can be made to retain positionand stiffness characteristics.

� The stiffness coefficient for Rx, Ry and Tz willchange little over the working range.

The concept is based on two oppositely in planeorientated flexible members. One of these members issoft-preloaded in the direction of orientation with amember that allows one degree of freedom (In directionof the pre-load). With two or more of such configu-rations orientated at an angle towards each other aconstruction can be made which offers the mentionedcharacteristics.The range is limited and the transfer of high loads(primarily during launch) is accommodated by stopsthat limit travel in Rx, Ry and Tz.Stiffness in Tx, Ty plane is maintained, up to approxi-mately the level of the pre-load in the pre-load elements.The transfer of high loads in Tx, Ty plane can beaccommodated by limiting the deflection range of thepre-load elements by stops. This will make the interfacerobust against loads in Tx, Ty.In figure 8 the FPA suspension system design is presen-ted.

Fig. 8: FPA suspension system design

A patent for the suspension system is applied for.

3.3 ACTUATORS

The FPA is equipped with four Lorentz actuators thatprovide the FPA Mirror Assembly with sufficient torqueto achieve the required acceleration and compensate theback-driving moment of the suspension system.The coil and permanent magnet with its pole pieceshave been designed for minimum required drivingpower for a specified deflection service mode. This is asinusoidal excitation on top of static deflection.Optimising parameters were:Permanent magnet material, volume and shape� Material and shape of the pole pieces� Radial position of the actuators within the con-

straints of the overall design of the unit.� Coil winding shape and coil former shape and

material. Here the constraints are the resultingwinding temperature and the electrical parametersof the electronic drivers as well as the availablespace compatible wire dimensions.

An actuator was selected with initially a rotationallysymmetric geometry. A better integration in the FPApackage is possible through cut-outs. A hole through thecentre with 2-mm diameter has to be incorporated forassembly purposes. The geometry was optimised usinga magnetic FE tool. The actuator design is presented infigure 9.

Fig. 9: Design of the actuator magnet and circuit

The material selected for the pole pieces is Fe-Co,because of its high magnetic induction saturation level.After evaluation, Nd-Fe-B magnet material was selec-ted. The conclusion of this evaluation was that Nd-Fe-Bhas the advantage over Sm-Co, mainly because of itshigher energy product. The main disadvantage of usingNd-Fe-B is the higher temperature sensitivity.Several Nd-Fe-B magnet materials were investigated indetail. The magnet material properties were used asinput in the optimised FE model for the breadboardactuators. The output of the FE analysis allows toaccurately predict the performance of the magnet mate-rial. The magnet material was selected on the basis ofsusceptibility to demagnetisation at elevated temperatu-

res (100°C) in this particular magnetic circuit. Anotherimportant parameter is the mean flux density in the coil.

3.4 SENSORS

To achieve the target sensor dimensions for the qualifi-cation model, the active sensor area was reduced and afully integrated thin-film package was designed whichresulted in high dimensional stability.Based on the overall dimension the maximum mirror tiltrange of �7 mrad corresponds to approximately �140µm at the sensor location. The specified noise level ofthe measurement is 1 µrad p-p, which corresponds to 20nm p-p. It is this signal to noise ratio which rules thesensor selection and the signal evaluation principle.Based on the results of a feasibility study, a differentialcapacitive sensor was selected. This Sensor consists offour capacities that are used similar to a Wheatstonebridge.The Sensor can be divided in 2 blocks that are connec-ted via flexprints:� The Sensor Package containing the sensor base,

the sensor plates and the preamplifier� The glass ceramic for the driver plates that are

mounted on the mirrorThe Sensor Base is made of a Ni-Au coated Kovar,which is used as a support for the sensor. Kovar wasselected because of the CTE match with alumina cera-mic. The part was manufactured using common millingtechniques. A tempering process was used to improvethe adhesion of the Ni-Au coating. The coating impro-ves the corrosion resistance of the part.The Sensor Plates are manufactured out of 2.4 mm thickalumina ceramic. The Ceramic had to be lasered, lapped(thickness 1.7mm surface roughness 0.1 µm parallelism10 µm) and gold coated for the required performance.These substrates were subsequently coated with a goldtin alloy to function as a thin film solder material. Thissolder material is only five microns thick and themeasured tensile strength is up to 65 N/mm2. Figure 10shows a test sample used in a tensile strength test. Thefigure shows that a crack runs through the bulk materialand not through the interface of the solder joint.

Fig. 10: Soldered parts after tensile strength test

Before soldering, the alumina parts are diced into thecorrect shape. (fig. 11)

Fig 11: Diced and coated sensor plates

For soldering, two processes were used. The firstprocess (the soldering of the alumina ceramic parts) iscritical due to the required Parallelism of the condenserplates (10 µm). Before soldering the parts are assembledand clamped under clean room conditions. Because ofthe sensitivity to particles between the parts the solde-ring process was performed in a vacuum oven undernitrogen atmosphere. The second soldering process (thesoldering of the Sensor base to the sensor plates) is doneusing a solder with a melting point of 240°C, which isapplied using a syringe dispenser.A small preamplifier in a tight Kovar housing is placeddirectly under the sensor plates. This preamplifier isbonded to the sensor plates using a two componentadhesive. The amplifier circuit amplifies the signal to alevel where it can be sent via a flex print cable to thecontroller system. An electrically conductive adhesive isused, to realise the electrical connections from the sen-sor plates to the preamplifier (hybrid board). A goldwire is used as improvement of the via connections.(fig. 12)

lower sensor plate conductive adhesion wire adhesion

hybrid board

Fig. 12: Connections to the Preamplifier

The same adhesive with an additional underfill is usedto connect the flexprints to the driver plates and to thepreamplifier. The electrically conductive adhesive isused only for the electrical contact to the flexprint whilethe underfill is used for the mechanical stability.(fig. 13)

U n d e rfill C o n d u c tive a d h e s ive

Au fla sh

C u C o n d u cto r

P o ly im id e

F o tu ra n b ro w nC o n d u ctive la ye r

S u b s tra te

F lexp rin t

Fig. 13: Connection of a flexprint

Figure 14 shows an assembled sensor block ready foruse in an FPA.

Fig. 14: FPA Sensor

For the driver plates a material had to be found thatmatches the CTE of Beryllium, acts as an electricalinsulator and allows a gold coating. The material ofchoice was Foturan, a glass ceramic that can be photostructured and etched. After etching, the material can beceramified which gives it a CTE that matches the CTEof Beryllium very well. In addition, the material offersgood adhesion to the gold coating.Two designs were realised. One design has flexiblebeams (fig. 15) to avoid thermo-mechanical loads due tothermal gradients.

Fig. 15: Driver plate design with flexible beams

3.5 CONTROLLER SYSTEM

The mechanical system can be analytically modelled intwo uncoupled rotations and a piston axes as threesecond-order systems (harmonic mass-spring oscillator).For the purpose of control loop design, the mechanicalplant model is completed with a small time constantfirst order system, approximating all small dynamicelements of the loop, i.e. the sensor demodulator lowpass filter, the actuator current driver loop, and control-ler bandwidth limitation. The interest of this way to takeinto account fast dynamics in the model is seen duringthe controller design process. In fact, the small dyna-mics will limit the achievable closed loop performance.

The motion controllers for the FPA are designed for thefollowing objectives:� Damp the open-loop basic resonance frequency in

each axis, so that no amplitude overshoot of theharmonic response larger than 1dB occurs anymore.

� Provide the largest possible bandwidth capabilityfor rejection of mechanical vibrations. This is theprincipal task of the FPA within the optical termi-nal pointing mechanism.

The first objective is reached using a state spacecontroller, comparable in its structure to a PD controller,together with an additional reference feed-forward. Theadditional feed-forward term serves to compensate therepelling forces of the elastic suspension in out-of-zeropositions. The difference between a PD controller andthe state space controller employed, resides in theabsence of a reference derivation. Thus, no zero appearsin the closed-loop transfer function, and by pole-place-ment, the system can be designed to have virtually noamplitude overshoot in its passing band. A suitable polelocation is the one of a Bessel type low pass filter. Ablock diagram of one axis controller is shown in figure16.

Fig. 16: Block diagram of one axis controller

The second design objective is related to the smalldynamics present in the control loop. It is shown in theanalysis of the control loop, that the equivalent firstorder system mentioned in the modelisation of the FPA,limits the achievable closed-loop bandwidth. The modelof the system is of third order, while the controller onlyhas two free parameters. Hence, when imposing a poleconfiguration, i.e. fixing relative positions of the polesto each other, a maximum bandwidth cannot be excee-ded. This bandwidth is that of the small dynamics in theloop, divided by the order of the closed-loop system,which is 3 in our case.It is not useful to compensate for the small dynamicsusing a higher order controller, because this would notimprove the phase response of the closed-loop system,which is critical for a tracking application. Also, caremust be taken to avoid voltage saturation at the outputof the current drivers. The most important electricalperturbation in the loop is located at twice excitationfrequency of the sensor, i.e. the demodulator outputripple voltage. This ripple is amplified by the motioncontrollers and again by the current drivers. It reachesthe highest amplitude on the actuator inputs. Because ofits high frequency, it does not lead to noticeablemechanical motion, as long as the whole loop stayslinear, i.e. no saturation occurs. It must be noted that the

voltage reserve of the current drivers available toaccommodate the sensor demodulator ripple, is thelowest at the limits of the positioning range, becausehere the highest static currents must be driven throughthe actuators. The demodulator output ripple voltage canbe reduced by lowering the cut-off frequency of thedemodulator low-pass filter and by increasing the exci-tation frequency. The first solution is not admissiblebecause it adds to the small dynamics of the motionloop, making it slower. The second solution has an inci-dence on the demodulator noise, and thus on the closed-loop SNR.A block diagram of the FPA electronics is shown infigure 17.

Fig. 17: Block diagram of FPA electronics

Particular attention has been paid to the thermaldissipation of this electronics, which will be operated invacuum. Heat dissipation, which occurs mainly in theoutput stages of the actuator drivers, and in linearvoltage regulators, takes place essentially by conduc-tion. In order not to overheat the PCB itself, thetransistors of the output stages of the actuator drivers,are placed in a cooling bracket conducting the heattowards the frame that holds the FPA electronics. Smallthermal resistance is ensured by mechanically clampingthe PCB using a wedge. All components have beenselected considering the availability of flight qualifiedparts. The print was designed using layout rules forflight applications. At present, the prints are equippedwith commercial components. Where necessary, alter-native pin-outs are foreseen to accommodate flightqualified packages, usually larger in volume. Allcomponents are mounted to withstand vibrational andshock loads equivalent to launch. The FPA electronicsand housing are presented in figure 18.

Fig. 18: FPA Electronics

4 VERIFICATION TEST RESULTS

An experimental measurement of the rotation transferfunction of the open-loop mechanical system togetherwith the current amplifiers and position sensor interface,is shown in figure 19.

Fig. 19: Measured open loop plant response in one axisof rotation

To comply with overall FPA requirements, there arethree design objectives for the sensor linearisation:� The absolute position linearity error should be

reduced to below ±2%.� In order to limit the variation of closed-loop stabi-

lity margins, the slope variation of the sensor char-acteristic should be limited to ±20%.

� The compensation of the sensor non-linearityshould be insensitive to the temperature variations(-20°C to +50°C) to which the electronic circuitryis exposed.

An optimisation of the component values of the linea-risation network has allowed to fulfil these requirementswithin the nominal range of displacement of ±7 mrad, asis demonstrated by the measurements shown infigure 20.

Fig. 20: Measured performance of sensor linearisationcircuit (slope error), influence of temperature variationfrom –20°C to +50°C

The measured closed loop performance of the FPA isshown in figure 21, for one axis of rotation.

102 103 104-50

-40

-30

-20

-10

0

Frequency in Hz

Gai

n in

dB

Closed-loop x-Axis Frequency Response

102 103 104

-150

-100

-50

0

50

100

150

Frequency in Hz

Pha

se in

°

Fig. 21: Closed loop frequency response of FPA, onerotation axis

Noise measurements of the FPA have been done in twoconfigurations: electrically on the sensed and linearisedposition output signals, as well as optically with the helpof a custom built high-bandwidth small angular rangeautocollimator. These measurements have shown goodmatching of results in the interesting frequency rangebelow to 1kHz. The electrical measurements are donewith the help of a band-pass filter having cross-overfrequencies of 10Hz and 2.5kHz. The low-pass sectionis first order, while the high-pass part has a second orderBessel-type response. Fig. 22 shows an x-axis result at amirror position rotated by 4.5mrad in the x-axis. Thenoise observed is minimal in the centre position of themirror, below 0.2�rad, and increases with the deflectionangle.

Fig. 22: x-axis noise measurement of FPA within 1kHzbandwidth, x-axis offset by 4.5mrad w.r.t. centreposition.

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