Pointing Mechanism for Optical Communication

Aage Skullestad

Kongsberg Defence & AerospaceKirkegardsveien 45, 3601 Kongsberg, NorwayTelephone: 47 32738141 / Fax: 47 32739313

E-mail: aage.skullestad@kongsberg.com

ABSTRACT

Optical communication has been advancing at a rapidrate. Optical inter-satellite link systems (OISL) must becapable of high-accuracy pointing for reliablecommunication. The laser beam is normally controlledusing a fine and a coarse pointing mechanism. Thispaper describes the coarse pointing mechanism for lowearth orbit (LEO) applications under development atKongsberg Defence & Aerospace.

INTRODUCTION

Compared to radio frequency (RF) communication,optical communication requires less power, has lowerweight and has a higher bitrate capability. SILEX wasthe first commercial OISL experiment and the firstSILEX terminal was launched on SPOT-4 in 1998.TELEDESIC is the first proposed Gbps OISL networkand will be operational in 2005. Reliablecommunication requires a high accuracy pointingmechanism. The optical pointing mechanism may besplit into a fine pointing and a coarse pointingmechanism. The fine pointing mechanism normallyconsists of a small mirror with a diameter of a few cmand has a bandwidth of 1 kHz or more. The LEO coarsepointing mechanism described in this paper consists of atwo-axes gimballed mirror. Both gimbals, called theazimuth gimbal and the pitch gimbal, are directly drivenby a brushless DC motor and a voice coil, respectively.An Inductosyn transducer provides the necessaryfeedback information in azimuth, and an optical encoderprovides the necessary feedback information in pitch.The pointing mechanism is controlled using a digitalsignal processor (DSP).

1 COARSE POINTING SYSTEM DESCRIPTION

The systems presented in this paper include a two axismirror gimbal and a DSP based servo-controllercontrolling the two gimbals. The systems were designedto meet the following performance requirements.

Mechanical Rotation Range azimuth: 170 pitch: 14

Angular Acceleration: 2.5 /s2Angular Velocity: 5 /sPointing Accuracy: 100 radTotal Mass: 7 kgNatural Frequencies: 200 HzLifetime: > 10 years

The mirror is 100 mm by 141 mm. The mirror issupported by a two-axis gimbal, the inner gimbalproviding pitch motion and the outer gimbal rotatingabout the azimuth axis. A detail of the mirror gimbal isshown in fig. 1.

Figure 1. Coarse pointing mechanism.

Both gimbals are directly driven. A brushless DC motorand a rotary voice coil provide the movement inazimuth and pitch, repectively. The pitch gimbal uses aC-flex bearing. In the azimuth gimbal design, apreloaded bearing is applied. An Inductosyn position

transducer and an optical encoder are used for feedbackin azimuth and pitch, respectively. Digital control isapplied in both azimuth and pitch. It was important tominimise the prototyping time, and dSPACE waschosen as our real time system. dSPACE applies theMATLAB RT workshop and allows rapid controlprototyping within the MATLAB enviroment.Algorithms can quickly be generated and tested withoutwriting a line of native DSP code. dSPACE applies aTexas TMS320C40 DSP. Fig. 2 shows the completesystem.

MATLAB/Simulink

RT Workshop

DSP program

Optical encoder

Voice Coil

Inductosyn

Transducer

Azimuth

motor

Azimuth gimbal

Pitch gimbal

Figure 2. Complete coarse pointing system

2 GIMBAL DESIGN

The mechanical design is a conventional two-axis innerouter gimbal arrangement with intersecting axes ofrotation. Great attention has been given to bearings.They provide sufficient structural stiffness, but theymay also generate friction torques affecting theperformance of the servo.2.1 Material Choice

Before deciding the material type to be used in theprototype, several materials were evaluated. Anoverview over two alternative materials and theirproperties for use in the pointing mechanism is shownin table 1.

Material AluminiumAISI 7075-T651

Titanium

Density g/cm3 2,80 4,43Thermal coeff. ofexpansion[10-6/ oK]

22,9 8,8

Heat conductivity[W/m oK] 134 7,2E-module [GPa] 71 110Microyield tension[MPa] 124 482Specific heat, Cp[J/Kg oK] 838 565

Advantages

- Low cost- High thermal conductivity Fast thermal equilibrium E-module approx. the same as alum.

- Light- Stable- Thermal coeff. of exp. almost like glass

Disadvantages- High coefficient of expansion,

- Expensive- Low thermal conductivity

Table 1. Material properties

7075 T651 aluminium was chosen in the prototype. Thisis an aluminium alloy with good properties, but isdifficult to machine. Manufacturing the mechanism intitanium was evaluated. However, because of the priceand the fact that this only was a prototype model, it wasdecided to use aluminium. In order to get the requiredaccuracy, the parts had to be produced with a finesurface finish of 1.6 m.

2.2 Azimuth GimbalAll parts in the azimuth assembly are designed usingCAD tools. Minimising the complexity was animportant design goal and the final construction consistsof fourteen parts. The main parts can be seen in Fig. 2.The azimuth gimbal is directly driven using a brushlessDC motor. The directly driven gimbal completelyremoves the gear backlash, but, due to the lack of gear,friction torque problem increases.

2.2.1 BearingPoor mechanical stiffness causes static deflections andwill cause undesirable vibrations. Backlash in thebearing should also be avoided. These facts lead to the

selection of a preloaded bearing. A heavily preloadedbearing has good stiffness, but also exhibits a largefriction torque. A large friction torque leads to poorcontrol accuracy, unless the available motor torque ismuch greater than the friction torque. The chosen motorhandles large friction torques, but at the cost of largeweight penalty. It become necessary to choose a bearingthat compromises the preloaded force and the frictiontorque. The selected bearing has a run torque of 0.105Nm and a break-out torque of 0.16 - 0.21 Nm. Thisleads to a motor size reduction.

2.2.2 SensorAn absolute Inductosyn transducer was selected as thefeedback sensor in azimuth. This sensor has an accuracyof about 5 rad. The Inductosyn transducer appliesinductive coupling between two moving parts tomeasure the angle. Because of the non-contactingelements there is zero wear and tear, normally nomechanical adjustment, and no lubrication is required.This sensor is ideal for space applications wherecomponents have to function the entire lifetime withoutmaintenance. The Inductosyn transducer is alsodesigned to resist the effects of light radiation, vacuum,high vibration and shock and a wide temperature range.

2.2.3 MotorThe chosen azimuth motor is a 3-phase brushless DCmotor with peak torque of 3.5 Nm. The torque higherthan the expected requirement was chosen based onexperiments with lightly and heavily preloadedbearings. The motor has redundant windings. Ideally,the brushless DC motor shall generate a torque directlyproportional to the current in the windings. The torqueshall also be independent of the angular position of therotor. In reality there will be small torque disturbancescaused by geometric and magnetic imperfections. Theseeffects are referred to as cogging and ripple and willinfluence the pointing accuracy.

2.3 Pitch GimbalThe construction consists of 6 parts. The main part ofthe pitch gimbal can be seen in Fig. 2. The pitch gimbalis, as the azimuth gimbal, directly driven. However, thepitch angle is limited to 14 , and instead of abrushless DC motor a rotary voice coil is chosen. Thelimited pitch angle also makes it possible to apply a C-flex bearing in pitch. The feedback sensor in pitch islocated on the outside of the gimbal and an opticalencoder was chosen.

2.3.1 BearingA C-flex bearing was chosen. The C-flex bearing isflexible and has extremely long life-time expectancy forlimited ranges of rotation, even when used for constantcycling. They have crossed springs made of high-strength corrosion resistance steel, capsulated in acylindrical housing. The spring element consists of an

inner and a two-part outer spring, which interlock andare crossed at an angle of 90. The bearing requires nolubrication, is maintenance free and has extremely lowhysteresis. Starting and running torque variations, due tothermal and lubrication variations, do not exist. The C-Flex bearing has no friction, no wear and tear, nobacklash, exhibit a constant linear torsion spring rate,and is self centring when deflecting forces are removed.

2.3.2 SensorAn optical encoder was chosen as the feedback sensorin pitch. There are two types of optical encoders, theincremental and the absolute encoder. An incrementalencoder generates a pulse for a given increment of theshaft rotation. The angular shaft rotation is determinedby counting the output pulses of the encoder.An absolute encoder has a number of output channels,such that every shaft position may be described by itsown unique code. Optical encoders, with an accuracy ofless than 1 rad, are available. The mechanicalmounting may influence the accuracy of the encoders.

2.3.3 MotorThe pitch movement is limited to a small angle and avoice coil is chosen. The voice coil actuator has hightorque-to-mass ratio, no backlash, is cogging-free andhas a low-hysteresis motion.

2.4 AnalysisThe natural frequencies of the mechanism reveal thestiffness of the construction. FE analysis was performedon the mechanism. One important goal with thisanalysis is to see if the natural frequencies of theconstruction are above 200 Hz, and also to see the effectof changing the stiffness of the bearings in azimuth andpitch.

2.4.1 Static AnalysisCAD tools were used to find the natural frequencies ofevery mechanical part in the mechanism. In order toobtain the required stiffness of the total construction, itis important that each part has a sufficient stiffness. Allparts have higher frequencies than the required naturalfrequency of 200 Hz.

2.4.2 Dynamic AnalysisThe dynamic FE analysis was performed using amechanism program called FEDEM developed byFEDEM AS. The program is intended for design andoptimisation of the performance of mechanical systems.The program can perform mechanism simulation tooptimise the motion behaviour, run FE analysis ofindividual parts, run FE analysis of the completemechanism and design the control system in order toobtain required damping and optimal systemperformance. FEDEM is a non-linear FEM program.The program also includes full stress analysis andeigenvalue solution for selected time steps. It is possibleto analyse the overall performance of flexible body

systems including control systems in a single runwithout time consuming data transfer operations.

FEDEM analysis shows that the construction issufficiently stiff, as the lowest natural frequency of thecomplete mechanism was higher than 260 Hz. Thisindicates that it is possible to slim the structure and/orchoose a less preloaded bearing. Less preloading resultsin lower friction torque and a slimmer constructionsaves weight. Modelling different stiffnesses of thebearing in azimuth indicates that it is possible to use abearing with 40% lower preloading and still havenatural frequencies above 250 Hz. The weightoptimisation has not yet been performed, but weightreductions are expected. The FEM simulations will becompared to vibration analysis performed on theprotype.

3 CONTROL

3.1 AzimuthThe chosen preloaded bearing in azimuth removes thebacklash and stiffens the construction, but at the cost ofan unwanted larger friction. It is difficult to obtain ahigh precision servo unless the available motor torque ismuch larger than the friction of the bearing. Even if thisrequirement is fulfilled the movement of the gimbalmay be rough and, in order to reduce this problem, thefriction is estimated using the Dahl friction model. Themotor is driven from a pulse width modulated amplifier

Figure 3. Azimuth control loop.

and the motor is current controlled. The dynamicequations of current controlled DC motor reveal that thearmature resistance, the inductance and the back EMFconstant do not influence the current being delivered tothe motor. Since the torque is proportional to thearmature current, the current actually produces torquecontrol. The servo applies digital control and thesampling rate will be chosen according to the band-

width of the servo. Fig. 3 shows a mathematical modelof the azimuth servo, where KT is the motor torqueconstant and the motor amplifier, LIM is the motortorque limit, J is the total moment of inertia, H2(s) is thecontroller of the velocity loop, H1(s) is the controller ofthe position loop. Lead , lag or PID controllers may allbe applied in order to obtain an accurate position andvelocity loop. Other control strategies will also beconsidered. The brushless DC motor applied in azimuthexhibits two unpleasent effects, namely cogging andripple. Both effects are strongly reduced by measuringthe back EMF, and using these measurements as a partof the control signal.

3.2 PitchThe voice coil behaves like an ordinary DC motor. Thepitch gimbal has no friction, no backlash and exhibits aconstant linear torsion spring rate. The maximum motortorque is 0.3 Nm and an ordinary analogue poweramplifier drives the voice coil. The controller is, as inazimuth, digital.

Hardware and software from dSPACE controls both theazimuth and pitch servo. dSPACE applies theMATLAB RT workshop and allows rapid controlprototyping within the MATLAB enviroment.

H1(s) H2(s) D/A LIM 1/J 1/s 1/sKT outin

A/D

A/D