ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
On the oth er end of the  actuato r, a p roblem was encounte red in fitting bea rings to th e output stag e  planet s. The se planet s carried a load of 1450 kg at th e pea k outp ut torque of 3 300 N-m. The nearest size  beari ng(s) ca pable of the l oad were larger tha n the planet s. The probl em was resolved by selectio n of a  roller bearing that in multi ples of 3 would be able to carry th e load with sufficient m argin. The se same  roller bearings were al so fitted to the se cond from last stage of pla nets with an attendant mass re duction   of approxim ately 100 g co mpared to the ball bea rings origi nally se lected.  

SLA Modelin g as a Sanit y Check 

Due to the e xtremely tight sch edule a nd the long le ad times for fabri cation of the gear compone nts and  the spa ce rated bea rings, the possibility of a pre-en gineeri ng-release prototype was out of the questio n.  However, a n SLA (Stereo Lithographi c Apparatus) m odel was a possibility.  The fabrication of  the model,  from extra ction of the el ectro nic file s for the SLA process, until re ceipt of the comp onent s took  approximatel y 2 weeks. The pa rts were cl eaned, some min or machini ng was perfo rmed and a ful l  prototype LPA wa s assembled u sing actual steel  bearings and shafts. A prototype, functio nal motor was  fitted and h ence a top to  bottom de sign verification  was accom plished. This unit was actually operated on   the ben ch. It was estimate d that the pl astic unit, ba sed on  the strength of th e SLA material s, wa s capable  of produ cing approximatel y 150 N-m of torque. 

Figure 3.  SLA Model of L PA 

A Scar y Moment 

After su ccessful inte gratio n of the  actuator firs t article, full load t esting began. Being th e first time this  large a lo ad had been atte mpted usi ng the relatively new  Aeroflex techni que, ap prehension was in the air.   Needless to say, desi gn margi ns need to be ca refully honed when the go al is as lig ht a weight as  possibl e. 

The a ctuator achieved the  rated torque  of 3400 N-m  again an d again. At som etime that evening in the   early mornin g hours, a great deal of elation wa s in the air and some p ersonnel had even  gone hom e  when a lou d bang was heard from the t est ap paratus . Of course, t he couplin gs had snapped  or pe rhaps  even the torq ue tran sducer had failed. Fear that it  was the actu ator was p ushed from ou r thinking. 

214
Figure 4. Mo ment Of Tr uth, Actu ator Outpu t at 3390 N-m 

Indeed, after careful in spections, it was determin ed that a structu ral failure h ad occurred i n the a ctuato r  output shaft. The output carri er to which the output  pinion was affixed had sna pped about the base of the  pinion. Unde rstand able since this i s the regio n of most stress. 

Furthe r investigation reve aled that the failure had occurre d at a point whe re the wall thickne ss of the  hollo w output splined shaft wa s only 2.3  mm (0.09 0 in). The original de sign maintained  a constant section   thickne ss of 5 mm (0.20 0 in) th roughout this shaft se ction. Th e section had been  thinne d due  to the  addition  of a  counter-bore to ho use th e pilot bearing from  the planetary  stage behind the final o utput  stage section . The size o f the counte r-bore was chang ed at the last minut e whe n the origin al pilot  beari ng, which mea sured 12.7 mm  (0.5 in) smaller in  diamete r, became unte nable due to exce ssive  lead   time. 

After cal culations and FE A modeling had re-co nfirmed that the  origin al de sign would h ave sufficie nt  margi n, the race again st the clo ck wa s on to resolve the failure mode i n time to delive r the flight  hardware.  

An additional  compli cating  factor wa s that the failure  had occu rred in a regi on of the shaft immediately   adjacent to an EB (Elect ron Bea m) welded area.  The co ndition of the welded regio n was carefully  investigate d to dete rmine if  any an nealin g of the m aterial h ad occurred and had contri buted  to the failure.   A careful mi crosco pic an d metallurgic examination of the failed  part wa s perfo rmed at JPL t o rule out this  possibility. 

215
Figure 5.  Failed Shaft se ction 

A Path For ward 

The challen ge to resolve the pro blem q uickly and safely  was compound ed by the fact  that virtually all the  affected parts were at th e en d of th eir lon g (typic al 16 to  20 week) fab rication cycle s. There were a   minimum of 5 vendors in the fabri cation chai n of the shaft. After a fe w sleepl ess night s, a solution was  devise d and proposed for impleme ntation; rathe r than repla ce the failed part wi th a revise d part with the   intende d section thickne ss (whi ch wo uld dictate the e limination of  the pilot bearing or the substitution of   the ori ginal unavailable  bearing  through  the u se of an off-the-sh elf beari ng), the part would be augm ented   with a seri es of pins inse rted into the load zo ne to  relieve the thin se ction o f any load bearin g. This  solutio n was carefully exa mined by ha nd calculatio ns and FEA mo deling a nd a single part was fab ricated   and in stalled on the o riginal EM test un it. After a fe w tense hours on the dyna mometer, the  solutio n was  deeme d a success. Additi onal pins were procured and in stalled  on the remainder of the parts to support  the delivery sche dule. The net impact on  proje ct sche dule was a pproximately 3 wee ks.  

Motor Desig n 

The moto r is a conventio nal 6-p ole, 1 8-slot Bru shless DC de sign with a ce nter tap whi ch is d riven  unipol arly. The drive ele ctronics (LP AE’s) we re develop ed a nd manufa ctured by JP L. Because the  actuator is capabl e of torques greater than 3300 N-m, current  limiting in the LPAE controllers was  necessa ry to prevent da mage to downlin e mech anisms and was set  using the first actuator.  

The moto rs have a Kt of approxim ately 35 mN-m/am p with a windi ng re sistance of approxima tely 5 ohms.   The m otors were capable of delive ring approximatel y 12 mN-m (1.75 in -oz) of torque at the  current limit.  Motor spee d varied from a pproxim ately 5000 to 70 00 RPM dep ending upo n supply voltage. 

216
Figure 6.  Pinned Shaft s ection 

The motors were com mutated using 3 latchin g ceramic Hall  devices placed with a 120 -degree electrical  separation. Beca use the Hall devi ces contai n ele ctronic comp onents, extrem e cautio n concerning ESD  and voltag e excitation co ndition s had to be ob served at all tim es. When not con nected to the d rive  electroni cs, a ll commutatio n lead s we re twisted tog ether. The m otor used a separate m agnetic targ et to  excite the Ha ll device s. This allo wed a 180-deg ree sensor  pole arc targ et to be  used to en sure CW and  CCW pe rform ance wa s symmetric to within a small p ercentag e of one an other.      Each m otor/brake a ssembl y requi red p assing a separate test procedure befo re integratio n with the gea r  assembly.     Brak e De sign 

The bra ke is a conventio nal electromag netic type us ed in other space mechan ism motors d evelope d by  Aeroflex. The brake has 6 spring s which forces two discs togethe r. The discs have lapped chromi um  oxide su rface s which prod uces the fricti on. One disc is  fixed to a nd rotates  with  the motor’s roto r. The  other disc i s allowed to move axially a nd is rotati onally re strain ed with 3 polished pins. The e ngagi ng  motion is  too short and the pin c learance too large to allow the pins  to cock and hang up. To dis engage  the bra ke, an  electro magnet is energized and attra cts the disc th at is restrain ed from rotati on by the 3  pins. Thi s action se parates the two discs allowin g the dis c affixed to the rotor to rot ate freely.   

Only a  few watts of power i s necessary to release the  brake. A no n- mag netic sp acer wa s inserted  betwe en the electroma gnet and the movable bra ke disc to preve nt resid ual m agneti sm fro m holding th e  brake dis-en gaged once power is removed. The b rake friction to rque al so exceed s the m otor’s capability   of rotating th e syste m (approximately 3 5 mN-m), i. e. if the motor was energized with the brake engaged,   the motor wo uld not turn.  

217
Figure 7.  D ynamomete r Portion of T est Appar atus 

A total of 12 LPA’s we re assemble d, tested an d deli vered to the MER proj ect. 

The results of  the testing  were a validati on of the many desi gn assumptio ns employed in  the construction  of the LPA’s. In pa rticul ar, the pe rforma nce of th e actuators ove r the required temperature range  with out  the use of he aters o r blankets wa s indi cative of excellent gear effici ency an d lubrication p erformance. 

219
LPA ACTUATOR T/S CURVES 00.0050.010.0150.020.025 0 1000 2000 3000 4000 NmRPM28 VO LTS -60 C+60 C ~ 20 C

Figure 8.  Ac tuator Torqu e/Speed Cu rve vs. Temperatu re 

Lesso ns Lea rned 

The LPA’s were  designed, built, tested and delivered  under an  extremely tig ht sche dule. The key to th e  success of the proj ect was exte nsive and early testing. SLA modelin g wa s a gre at reso urce i n  determi ning d esign viability at an early st age. 

An ea rly and  com prehen sive overvie w of the de sign may hel p illustrate  the relevant lon g poles of the   effort at an earlie r stag e. 

The u se of planetary gear reduction  for high to rque application s is a viable a pproach so lon g as stiffness  requi rements ca n be satisfied with th e value s attainabl e with  these syste ms. Careful sele ction  of  material s, gear size and lubrication i s necessary  to balance surface pitting (cont act stre sses), gear fatigue  life (bendi ng stresses) requirements vs. size a nd stiffness.      220
knowledge is provided by an incremental encoder signal  constructed in the LPA electronics from the  motor commutation sensor signals. The LPA electronics are brushless motor drive electronics, physically   Lander Petal Actuator  Rover Egress Ramp  (1 of 3) Lander Petal Actuator  (1 of 3)  Side Petal  (1 of 3) Stow ed Rov er  on Base Petal  

Figure 1.  Lander Petal Actuators open each MER Lander side petal to the iron cross position,  where all petals are coplanar. Petal motion past this condition is called ov er-deploy ed. 

Figure 2.  The MER Lander w ith tw o side petals stow ed and the third side petal closing   Lander Petal Actuator Lander Petal Actuator  under the action of its Lander Petal Actuator  

222
LPA Cable  System Cable  Connects Here LPA  Electronics Brake Brushless  DC Motor 2-Stage Gearbox  44.568:1  2-Stage Gearbox  36.36:1   3-Stage Gearbox  200:1  Drive Output  

Figure 3.  MER Lander Petal Actuator w ith its electronics  

LPA Cable System Cable LPA Brak e LPA Motor System  Electronics LPA  Electronics 

Figure 4.  The LPA motor and brake are separately  pow ered and commanded by  the sy stem  electronics through the LPA electronics,  a brushless motor driv e electronics.  

separate from the LPA and connected by a cable (Figur e 3). Potentiometers at each of the petal hinge  lines provide additional coarse petal position info rmation, an absolute reference for rover egress. 

Lander Petal Actuator   The MER Lander Petal Actuator (Figure 3) is a high torque actuator (3300 N•m output torque) produced  by Aeroflex Laboratories, Inc., whic h consists of a brushless DC motor, a power-off brake, 7 stages of  planetary gearing with an overall ratio of 324,099:1,  and a crowned spline on the output shaft. The LPA  required a brake to meet its backdrive torque r equirements with power removed because of the high  efficiency of the planetary gearing. The brake is mechanically engaged when non-powered to lock the  motor rotor and ensure that the Lander petals cannot move due to external loads. The LPA motor and  LPA brake are separately powered and commanded by  the system electronics through the LPA  electronics (Figure 4). During operation, power is first applied to the LPA brake to release it, followed by  223
power to the LPA motor to initiate petal motion. At the conclusion of motion, power is first removed from  the LPA motor and then the LPA brake to avoid clampi ng the brake rotor at high speed. All LPAs were  tested over temperature at the actuator level to restrain 3300 N •m externally applied to the output s haft.  The brake design is a standard spring-applied, powe r-to-release configuration as shown in the  motor/brake assembly cross-section (Figure 5). The br ake rotor is attached to the motor rotor. With the  brake unpowered, 6 compression springs push the fricti on plate against the brake rotor, preventing motor  rotation. When the brake is energized to release t he motor, the friction plate is guided on 3 pins and  pulled against the springs by the solenoid. The total stroke of the friction plate is 0.13 mm. There is a  0.051 mm annular non-magnetic shim between the friction plate and the solenoid to break the magnetic  flux path and prevent residual magnetism from permanent ly retaining the friction plate on the solenoid  (the stuck-open position). 

Non-Magnetic  Shim  Solenoid Coil Friction Plate   Guide Pins (3)Motor Rotor Brake Rotor Springs (6)  

Figure 5.  Cross-section of the Lander Petal Actuator motor/brake assembly  

Failure and Failure Inv estigation  The LPA failure occurred about nine weeks before the scheduled launch of the first MER spacecraft. In  preparation for closing the Lander for the last ti me, the spacecraft commanded the Lander petals through  their range of motion using the LPAs so that cabling and other hardware near the hinge line could be  observed for proper installation and clearance. Although the petals had been moved with the LPAs many  times before, this was the first time that all the har dware including the flight airbags was installed during  the motion. During a pause in the final flight stow sequence of the Lander petals using the LPAs, one  petal drifted downward under gravity with the spacec raft unpowered. The weight of the assembled petal  applied about 418 N •m of torque to the LPA output shaft or 13%  of the tested backdrive resistance. A  load of this magnitude clearly shoul d not have caused the actuator to  backdrive with the brake engaged.  The failure was initially observed visually as an offset between the commanded position and the actual  position. One petal seemed lower t han it should be. A check of spacec raft telemetry from the hinge line  potentiometers indicated that t here was continued motion after completion of the commanded motion, on  one petal only, although the current draw from the brake was as expected during the motion and went to  zero upon completion of the motion. Once a problem  was suspected, the continued motion was also  visually observed. The anomaly was repeatable. In a separate check, the suspect petal moved when the  LPA was commanded without energizing the brake, a fu rther sign that the brake was not performing  properly.  224
Even though all evidence pointed toward a problem with  the LPA brake, a brake failure did not seem  credible. Swift, decisive action was required to prevent a launch delay, however the closeness to launch  made it even more critical to conclusively isolate the problem prior to removing the hardware from the  spacecraft. Uncertainty in determining the cause of the problem would jeopardize the launch. The  problem was conclusively isolated to  the actuator using the following ra tionale. With the petal backdriving,  the spacecraft was powered off and the connectors bet ween the spacecraft, the LPA drive electronics,  and the LPA were demated sequentially until the LPA wa s completely isolated from the rest of the  system. The petal was still backdriving,  which conclusively placed the failu re in the actuator, eliminating  the possibility that a stray current  in the system or the drive electronics was keeping the brake powered  and in the open position. The failed LPA (SN 007) was removed from the Lander and replaced by a flight  spare and failure analysis began on the removed LPA.  The failure investigation was conducted at  Kennedy Space Center to eliminate the possibility that the failure w ould be lost during transportation of  the LPA to either the Jet Propulsion Labor atory or Aeroflex Laboratories, Inc. 

 Ends of 3 Springs on Pins  Hooking Friction Plate and  Balancing Other 3 Springs  Broken  Springs  Shaft  BrokenRotor Slides Forward Due to Failure of Front  Bearing Retention  Method Square Drive  Rounded Off  + Loss of  Fastener  Preload  Obst ruction  Holding Plates ApartSticky  SurfacesResidual Magnetic Field Holding Brake Open  No Spring  Force Mechanically Stuck  or Jammed on Pins  Coatin g Worn Off Contamination Rotor Not Trans mitting  Torque to Brake Assembly  Brake in Open Condition  (Mechanically Disengaged)Loss of Friction  at Brake Interface  Rotor Turns Under Backdrive Tor que at an Unex pected Value  Brake Not Resistin g Motion

Figure 6.  Fault tree for LPA brake failure w ith the actual failure highlighted in the dotted box  

The evidence at this point only indicated that the brake was not resist ing motion. Many different failure  possibilities were considered which fell into the fo llowing general categories: “Brake in Open Condition”;  “Loss of Friction at Brake Interface”; or “Rotor Not Transmitting Torque to Brake Assembly” (Figure 6).  Many of the failures could only be observed through di sassembly of the brake and some of these had the  additional unfortunate characteristic that disassembly could cause the loss of the failure. After visual  examination of the LPA indicated nothing unusual, t he motor was operated with no power to the brake.  Motor current indicated no-load operation, which meant  the failure was still intact. Real time X-ray  examination revealed the brake in a fully dis engaged position even though no power was applied to the  225
012345678910 -1 01 23 Drop Out Voltage, voltsSpring Force, N0.051-mm shim  0.102-mm shim thickness  (2x 0.051-mm shim)  4 

Figure 8.  Test data from a spare brake assembly  characterizes the v ariation of drop-out v oltage  with spring force for tw o different shim thicknesses.  

05101520253035 01 2 3 4 56 Drop Out Voltage, voltsMagnetic Force, Ny = 0.5758x2 + 2.3089x + 3.8126 R2 = 0.9772, 0.051 mm shim y = 0.1194x2 + 1.3262x + 3.7245 R2 = 0.9897, 0.102 mm shim 0.051 mm shim  0.102 mm shim thickness  (2x 0.051 mm shim) 3.711.95 

Figure 9.  Regression curv es from test data are appropriately  shifted and used to determine the  drop-out v oltage required for a minimum 2.0 factor of safety .  

229
Rew ork 

Two flight units, SN 003 and 008, had drop-out volt ages of 0.79 VDC and 1.25 VDC respectively, well  short of the minimum drop-out voltage of 2.05 VDC r equired for one shim per the margin analysis. These  two units were removed from the second flight lander  and transported to Aeroflex Laboratories, Inc. for  rework. Rework consisted of adding a second, non-magnetic 0.051-mm shim between the solenoid and  the friction plate resulting in a total shim thick ness of 0.102 mm. A shim was removed between the brake  rotor and the motor rotor to maintain the same brake actuation stroke. The rework was relatively  straightforward, however the schedul e intensity of the activity was increased substantially due to the  proximity of the second launch and the criticality of  the LPAs to mission success. The brake housing was  carefully removed from the motor housing (Figure 10) after heating the brake housing up at the bond line  and removing three screws. Brake housing removal wa s performed with the brake leads powered so the  friction plate was secure against the solenoid. Th is prevented the friction plate and the compression  springs from falling out of the housing during disassemb ly. The two brake plates are visible in Figure 11a.  The brake rotor was removed so that its shimming could be adjusted. The six compression springs are  visible after power was removed from the brake assemb ly to enable removal of the friction plate from the  brake housing (Figure 11b). This is the location where the second shim was added after the assembly  was cleaned up and inspected. The brake was reasse mbled. After rework, SN 003 and 008 had drop-out  values of 4.09 and 3.85 VDC respectively, which met the minimum requirement of 3.81 VDC for the 0.102  mm shim thickness. Table 4 indicates the perform ance of these two LPAs, before and after rework.  

Dynamics testing was not performed on the reworked L PAs, however, the actuators were exposed to the  protoflight temperature range of –120 °C to +60 °C. Each actuator was operated at the voltage extremes  of 24 and 32.8 VDC and over the ent ire output torque range at each a cceptance temperature (–60 °C,  ambient, +60 °C). There was no change in the tor que/speed/current performance or backdrive torque  capability from the original acceptance testing. A te st to positively confirm brake drop-out after LPA  operation in each direction, voltage, and temperatur e was added. This test consisted of operating the  LPA motor with the brake unpowered and verifying that  stall current was present. After successfully  completing testing, the LPAs we re returned to the spacecraft.  

The entire process from the initial observ ation of the failure to the return of the LPAs to the  spacecraft took only  three w eeks and caused no slip in the launch schedule. 

Figure 10.  The brake assembly  is remov ed from the Lander Petal Actuator during rew ork to add  an additional shim.  

231
Second Shim  Added Here  Brake roto r Friction Plate  Figure 11a and 11b.  Close-up v iews of the brake assembly  during rew ork 

Table 4.  Drop-out performance for the rew orked brakes 

Before Rew ork  (with 0.051-mm shim) After Rew ork  (with 2x 0.051-mm shim)  SN 

 Drop-Out (VDC) Force Factor of  Safety  Drop-Out (VDC) Force Factor of  Safety   003 0.79 1.08 4.09 2.12  008 1.25 1.38 3.85 2.02 

Analy sis Using Magnetic Circuit Equations 

The results of the failure investi gation, the force/drop-out voltage dat a from the spare brake assembly,  and the corresponding margin analysis provi ded sufficient evidence and understanding to both  conclusively attribute the brake failure to residual  magnetism and to proceed confidently with the defined  rework on two LPAs with marginally acceptable brake assemblies. Some analysis was performed to  further explore the source of the unit-to-unit variat ion in drop-out voltage. The magnetic circuit model of  the brake is shown in Figure 12, wh ich leads to the following equation. 

()()() ()GAP PLATE SHIM CORE R R R R Ni Φ+Φ+Φ+Φ=  (4) 

where  is the value of the magnetomotive force source of the brake coil,  is the number of turns in  the coil, i is the current in the coil, is the flux flowing in the magnetic circuit, and Ni N Φ R is the reluctance  of each component around the circuit. Relu ctance of a material with length  along the flux path and  cross-sectional area  is defined as l A AlRµ=  (5) 

where µis the permeability of the material. Higher permeab ility materials, like the magnetic materials of  the brake core and plate, have a lower reluctance and therefore flux flows more easily through them.  Since flux is conserved around the magnet ic circuit, equation 4 is rewritten as  232
(GAP PLATE SHIM CORE R R R R Ni )+++Φ=  (6) 

The shim is nonmagnetic so its relu ctance is that of free space ( ) just like the air gap. The permeability  of both the core and the plate is much higher than the permeability of free space (0R 0µ), therefore their  reluctance is much less than the reluctance of free space resulting in a simplification of equation 6. 

,0R R RGAP SHIM ==    ,0 CORER R>>PLATER R>>0

AlR Ni 00 022µΦ=Φ=  (7) 

Making the common assumption that the flux density Bis uniform over the cross-sectional  area exposed to the flux results in A

BA=Φ  (8) 

The energy stored in the magnetic field in the gap is 

002 2µAlBU=  (9) 

The magnetic force on the brake plate is equal to the rate of change of energy with the gap length. 

02 02µAB dldUFM==  (10) 

Gap  Shim  Flux path   Φ Brake Rotor  Core i V Friction Plate  N i ΦRCORE RSHIM  RPLAT E  RGAP _ +  

Figure 12.  Magnetic circuit model  

Substituting equation 8 into equation 7, and combini ng with equation 10 gives an expression relating the  magnetic force on the brake plate to the shim thickness, . 0l 2 0022 8lAiNFMµ=  (11)  233