The m echani sm has a three-cha nnel,  144 counts-pe r-revolution  optical encoder that  is nomi nally alig ned  to the detent s of the motor. Th e encoder is robu st in design and p rovide s a simple  signal to  a Field   Programma ble Gate Array (FPGA) b ased control syst em that com mutates the motor an d increments the   positio n count on the edge s of the sig nal wavefo rm. The encoder is de signed such that all t hree signals  are driven by the sa me track on  the mechani sm code  disk, that produce a unique signal eve ry six co unts.  This arra ngement p rovide s constant feedba ck to th e control FPG A about the rotor’s p osition with  respe ct  to the motor’ s six ele ctrical cycle s to fa cilitate co rrect commutation of the motor without a ne ed to find the  mech anism h ome, o r zero, po sition. A small sl ot in th e code disk provide s a once-around unique signal  for this  zero pos ition. 

In normal o peration (shown sch ematically in Figu re 3),  where the HCM rotate s the pola rizing optic to a  specific  positio n fro m many  steps away, the m otor is  comm utated on the  edges of the encoder signal with no  speed control. Roto r braking i s passive a nd is  accompli shed by sho rting t ogethe r the t hree phases of  the motor winding. Th e resultin g dyna mic b raking plus  beari ng frictio n and detent torqu e allow the roto r to co ast  to a stop, typically in a d etent. Becau se the encode r has  a resolution only s ufficient to c ommutate the motor,   there is neith er clo sed-loop control  on stoppi ng po sition  nor repo rting of positio n to great er than 2.5 ° of  accuracy. Th us, the finite  stoppin g distance mu st be  taken into account when driving the mec hanism. Table 1.  HCM Design & Performan ce  Requir ements 

Minimum Aperture 48 mm  Oper ating Temp eratures 0 – 40°C  Survival Temper atures -20 – 55 °C  Total Tor que Ma rgin 100%  Position Repeatability  30 arcseconds 1- σ  Time for 180 ° Move < 1 second  Required Life 600,000 

Table 2.  Hollo w-Core Mo tor De sign 

Phases Three -Phase W ye Winding  Stator Te eth 72  Total Detents 144   Step Size 2.5°  Coil Resistance ~ 128 ohm  Nominal Drive Voltage 15 V  Motor Constant 110 mN•m / sqrt( W)  Static Detent Tor que ~ 64 mN•m     The solution develop ed for mech anisms at LMSAL, and   used in th e HCM, is to con tinue commut ating the motor  for a short ti me after the  target encoder t ransition  is  reached. Thi s extra commu tation time, o r “delay” pu shes  the me chanism reli ably int o the detent past th e defined  encode r tran sition an d allows the HCM  to  be tuned so that it occurs reliably for th e  uniqu e detent  position s. The delay setting   is an 8 -bit, on-orbit settable pa rameter  with 64µ s units. Th e total ra nge of  setting s fro m 0 to 2 55 provides a tim e  range of 16. 32 m s. At th e HCM  nomi nal  spin speed of about 5.5 rad/s (315 d eg/s),  this range allows for adj ustability over  about 5 degrees, or t wo detents. In  a  norm al move , the und riven po rtion of a  movement is a fraction of a step, typica lly  betwe en 1° and 2°. The desi gn not only  provide s a level of ro bustness for on -orbit  operation s, but also provides a flexibl e  option fo r testing the mechani sm. Fig ure  3 illustrates t he operational concept of t he  HCM when moving to  the “1 ” target  detent.  Figure 3. HCM Opera tional Conce pt  

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