There was a condition where a brake di d not relea se in one of the 12 unit s manufactured. T his inte resting   developm ent and the resolution of the  condition  are de scrib ed in detail  in a nother paper presented by  JPL. 

Test Appa ratus 

Due to th e extremely hig h torqu es produ ced by the L PA, it was n ecessary to d esign a spe cial test be nch.  This test bench holds the LPA within  an environme ntal test chamb er while allowing to rque transmission to  the torqu e transdu cer and torqu e generating elem ents m ounte d in-lin e with  one a nothe r outsid e the  cham ber. The LPA unit u nder te st (UUT ) was m ounte d on a  custo m fabri cated cold plate h eld on the  end  of four 5-cm (2.0-i n) dia meter ro ds which passed through the wa lls of a modif ied therm al chambe r and  were an chored to a 4.4 -cm (1.75 -in) thick alumi num plate which comprised the ba se pl ate of the te st  fixture. This configuration i solate d the chambe r from all of reaction  force s and al lowed the torsion al load s  of operatin g the LPA to be resi sted by th e robust co nstruction of the  test bench. 

The goal of the testing p rogram was to measure the torque a nd spee d of the LPA at different ap plied   voltages (24 to 32.8 volts) as well a s different a pplie d torque s (0, 6 80, 1355, 2 033, 2260 Nm) up to 330 0  Nm und er va rious the rmal conditions (-60 to + 60º C). Some sp ecial brake testin g was to be performe d at  –120º C.  

Becau se the LPA moves at su ch a slow rate of about 0.01 8 RPM (on ave rage), conve ntional brakes  would not work. P roblems such a s “stiction” fo r example could cause e rratic torque s leading to  test  anomali es. In the final configuratio n, in o rder to mai ntain smooth, p recise control  over the resisting to rque,  the LPA was allowe d to drive a large  step up ge ar system wi th an overall  ratio of 1:17400 with a  Himmel stein torque  tran sducer mounted between th e LPA and  the loa d. The  input to thi s gear system   was a small control m otor. With the small control  moto r stalled, th e LPA could wind up the system to th e  maximum to rque required. Running the small control motor i n the same  direction as the LPA  would allow  any torqu e loading requi red. Runni ng th e cont rol mot or to match the sp eed of t he LPA woul d pro duce a  no load condition to the LPA. 

During te sting a num ber of param eters,  such as torque, velocity, motor voltag e, brake voltag e as well as  a num ber of t herm o-couples placed at v arious point s on the LPA a nd the te st chambe r were continuo usly  logge d and reco rded. The  actuato r was requi red to  rotate 1 complete revolu tion in both CW and CCW  direction s under ea ch of the spe cified environ ment al condition s at variou s voltages an d loads. A s an  example, at a loa d of 2260 N-m with  a 32.8  volt supply, the  LPA req uired 55 mi nutes to compl ete a  revolution in one directio n. 

To a chieve t he – 120ºC tempe rature i n a “stand ard” chambe r which is only capable of a –7 5ºC lo wer  temperature, a controlled flow of Liquid Nitrog en (L N2) was introd uced into a col d plate inco rporated into  the mou nting base for th e LPA. This was augmented, unde r closed loop control, by dire ct inje ction of LN2  into the ch amber itself. This metho d is not recomm ended by the chambe r manufacturer a s it freeze s the  refrig erant and trips limit re lays. It takes several hou rs for the cham ber to re cove r and op erate  normally.    

The lo w temp eratu re testing wa s to e nsure th at the b rake would  hold du ring th e Mars nig ht and not all ow  the petal to slip a nd u pset the rove r. The te st actu ally included electri cally re leasing the b rake with a   torque a pplie d to the LPA output shaft simulating the rover loa d and then re -enga ging the  brake whe n  electri cally de activated. 

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