Motors

Motors selected by flown and qualified planetary and orbital robotics programs, with the test campaign or flight that established each one.

Two failure modes recur in the qualification records below, and neither is electrical. On IPEx the accelerated life test of the wheel actuator was ended by the harmonic drive wave generator migrating axially into the flex cup, which broke the springs preloading the motor rotor bearings [1]. On the Apollo Lunar Roving Vehicle the sealed motor was selected over an open brushless alternate, and the contractor was obliged to carry the alternate design until the sealed baseline had been shown to work in simulated lunar conditions [3].

Motors selected and flown

Part	Manufacturer	Used by	Source
LSI 75-12	ThinGap	ipex (wheel actuators)	[1]
LSI 75-30	ThinGap	ipex (bucket drum and arm actuators)	[1]
Series-wound four-pole brush motor, 1/4 hp	Garrett Corporation	apollo-lrv	[3]
Six-pole brushless motor, 80:1 spur	General Electric	apollo-lrv, alternate not flown	[3]
2214X024BXTH with 22GPT gearhead at 862:1	Faulhaber	ipex (radiator cover actuator)	[1]

Ratings and qualification results

• LSI 75-12, ThinGap: a frameless slotless brushless DC motor kit [1]. Ratings: Paired with Honeywell SS511AT Hall sensors and a Harmonic Drive CSF 14-80LW 80:1 gearset. Sizing load cases run to 7.45 N m at the gearbox and 0.24 N m at the motor at 150 rpm, with the long-duration driving cases at 1530 rpm [1]. High pole count gives usable low-speed commutation from Hall sensors alone. Qualification: Characterized under vacuum from 0 to 15 N m in 3 N m steps and from 10 rpm to 1700 rpm, at 35 and 40 C surface temperature and bus voltages of 47.6, 53.2 and 58.8 V; varying bus voltage across that range had negligible effect on motor response [1]. Accelerated life test at 3-sigma load ran 6,547,036 input revolutions against a mission requirement of 5.7 million before failing. Root cause was the harmonic drive wave generator moving axially into the flex cup, which failed the motor rotor bearing springs, not the motor itself [1]. The published output is a characterization curve and fitted equation, Fig. 6, rather than a single torque constant.
• LSI 75-30, ThinGap: a frameless brushless DC motor kit [1]. Ratings: Same outside diameter as the wheel actuator motor, chosen for commonality; drives a Harmonic Drive CSF 20-160LW gearset [1]. Qualification: Covered by the same four-stage KSC actuator qualification campaign as the wheel actuator, with the arm and bucket drum units run at 2-sigma load rather than 3-sigma. Only the wheel actuator result is published [1].
• Series-wound four-pole brush motor, 1/4 hp, Garrett Corporation: a hermetically sealed brush DC traction motor [5]. Ratings: One per wheel at 36 Vdc nominal with pulse-width-modulated speed control from the drive controller, coupled to a United Shoe Machinery 80:1 harmonic drive and instrumented for thermal monitoring on the crew display [5]. Motor and wave generator run together in a sealed 5.17 N/cm2 dry nitrogen atmosphere, transmission lubricated with Krytox 143AZ oil [3]. Qualification: Selected as the baseline and flown on Apollo 15, 16 and 17. The sealed traction drive holds 7.5 psia internal pressure [5]. The contract required the alternate open-to-vacuum design to be carried in parallel until the sealed baseline was demonstrated under simulated lunar conditions [3].
• Six-pole brushless motor, 80:1 spur, General Electric: a permanent magnet brushless DC traction motor with 80:1 planetary spur gearing, open to vacuum, carried as the Apollo LRV alternate design and not flown [3]. Ratings: Same 80:1 overall reduction as the baseline, entire drive open to vacuum rather than sealed. Qualification: Carried as the contractual alternate and not selected. Its stall torque against current was characterized alongside the series-wound baseline, Fig. on p. 4 [3].
• 2214X024BXTH with 22GPT gearhead at 862:1, Faulhaber: a brushed DC motor with planetary gearhead [1]. Ratings: Gearhead rated 1.8 N m continuous and 2.5 N m peak against a computed 1.84 N m peak requirement; motor runs at 431 rpm to open the cover in 30 s. Gearhead is orderable vacuum-rated with vacuum grease preinstalled [1]. Qualification: Not yet qualified. The Hall effect sensor part number inside the motor could not be identified, which the paper records as an open qualification item, and the actuator has still to run the four-stage campaign applied to the drive actuators [1].

Sizing against the seals

The IPEx Gen 1 actuator was instrumented specifically to separate motor losses from gearbox, bearing, seal and resolver losses, because manufacturer motor curves and gear efficiencies assume a particular grease, temperature and ambient pressure [2]. The measured maximum seal drag across that campaign was 0.03 N m, under one percent of the overall load, which is why the layered labyrinth, felt and PTFE seal stack was kept. For a motor sized against a lunar duty cycle the seal is not the constraint; the harmonic drive interface is [1][2].

Two further selections are published without a test result and are therefore not tabled. RASSOR 2.0 uses a Parker Bayside K089050 frameless kit motor direct-driving a Harmonic Drive SHG 32-160 at 161:1 in its shoulder actuator, with factory-installed Hall sensors, chosen because the kit format removes the separate housing and coupling [4].

Why a brush material is qualified against its ambient

A sliding electrical contact is lubricated by a transfer film, and the film is a function of the ambient rather than of the brush. A graphite-carbon body sliding at 216 cm/s under a 9.8 N load at 20 C holds a carbon transfer film on the chromium and iron oxides of AISI 440C down to 1e-9 torr, which caps its friction coefficient at about 0.2; on electrolytic silver, silver oxide is thermodynamically unstable below about 1 torr, no carbon film forms, the carbon strips metallic silver and the coefficient rises to about 0.5 with silver sliding on silver [6]. Wear of graphite sliding bodies becomes very high below 1e-5 torr as the adsorbed moisture the film depends on is pumped away, which is the mechanism behind high-altitude generator brush dusting. The Apollo traction motor above never sees that regime: it runs sealed in dry nitrogen at 5.17 N/cm2 rather than open to the lunar surface [3], so its brush heritage is heritage at an ambient pressure the vehicle’s exterior mechanisms did not have.

Motors qualified by Mars surface programs

Three flown programs published motor qualification campaigns against a part number. The failures they record are at the brush, the commutator and the lubricant, and none is an electrical failure of the winding [8][9].

Part	Manufacturer	Used by	Source
M32 flat motor	maxon	perseverance	[7]
M32 detent brake	maxon	perseverance	[7]
M20 gearmotor	maxon	perseverance	[7]
RE016	maxon	sojourner (wheel drives)	[8]
Brush DC motor lot	maxon	spirit, opportunity (arm)	[9]
USR30E3 ultrasonic motor	Shinsei	MarsArmII wrist, MicroArm	[10]
USR60S1 ultrasonic motor	Shinsei	MicroArm shoulder and elbow	[10]
Iron-core motor, SG54-27 brushes	ATC	Mars Polar Lander study	[8]
BMG planetary gearmotor	Not named	KSC dynamometer article	[11]
DRV8412 motor driver	Texas Instruments	JPL avionics screen	[12]

Ratings and qualification results

• M32 flat motor, maxon. Brushless flat motor at ten flight actuator locations on Perseverance. Ratings: torque constant 50 mNm/A to an acceptance criterion of plus or minus 10 percent, tested at 12, 20 and 28 V in both directions at ambient [7]. Qualification: 99 units were delivered across three lot builds, 10 engineering model, 35 in the first flight lot and 54 in the second [7].
• M32 detent brake, maxon. Passive magnetic detent brake on the same motor. Ratings: 10 and 20 mNm holding torque options over 24 detents per revolution, matching the motor commutation state changes, accepted at plus or minus 10 percent measured at 2 rpm over two revolutions [7]. Qualification: the axially charged detent magnet produced stray flux at the Hall sensor, causing commutation switching errors and reduced detent strength until it was compensated [7]. A passive brake changed the behavior of the sensor that commutates the motor it brakes.
• M20 gearmotor, maxon. Smaller gearmotor from the same qualification campaign. Ratings: gearmotor torque constant 1620 Nm/A to plus or minus 10 percent, verified at plus 70, plus 22 and minus 70 C, with static holding torque required above 50 mNm after one thermal cycle from plus 113 to minus 135 C and two from plus 70 to minus 135 C [7]. Qualification: rotary life testing ran 40400 revolutions at 0.2 N m, 12400 at 0.3, 2100 at 0.4 and 1500 at 0.5, split between directions and alternating between plus 70, plus 22, minus 55 and minus 70 C, completing with minimal wear. Thermal life testing accumulated 3015 cycles representing 4.5 Mars years over 1.5 years of running, on seasonal profiles of minus 80 to plus 85 C for summer and minus 115 to plus 50 C for winter, and was still running when the source was written [7]. Bake-out was 288 hours at 113 C and 1.3e-3 Pa, followed by 40 hours of 80 percent relative humidity at plus 80 C as an accelerated corrosion exposure.
• RE016, maxon. Brush DC motor in the Sojourner wheel drives. Ratings: the mission requirement was about one million revolutions for 100 m of driving [8]. Qualification: two complete actuators ran 30 to 40 million revolutions to failure under load and start-stop, part of it at -70 C in low pressure carbon dioxide, and two motors ran 40 million revolutions unloaded at plus 20 C in low pressure air without failing. Retested for the Mars Polar Lander robotic arm at 30 V rather than Sojourner’s 15.5 V, 29 motors averaged under 10 million revolutions, failing by rotor shorting from conductive commutator debris, by brush galling and micro-welding to the commutator, and in one or two cases by an open winding [8]. Doubling the bus voltage cost the design more than a factor of three in life.
• Brush DC motor lot, maxon, for the Mars Exploration Rover and Phoenix arm actuators. Ratings: lot of 211 motors [9]. Qualification: 168 were fully tested and flight qualified with no indication of degraded or broken brushes. Brushes broke only in component-level simulated pyro shock, at a maximum tolerated 2600 G peak shock response spectrum against a 4000 G requirement, and never in assembly-level pyro shock, which the review attributes to component-level testing being a routine overtest [9].
• USR30E3 ultrasonic motor, Shinsei. Traveling wave piezoelectric motor at the MarsArmII wrist and the MicroArm shoulder and elbow. Ratings: 1 in-lb of stall torque and 300 rpm unloaded, both manufacturer figures, with a minimum rated temperature of -10 C [10]. Qualification: repeatable operation down to -50 C in a 20 mTorr inert atmosphere, with seizure at about -50 C attributed to differential thermal expansion at the stator-rotor interface [10]. One unit was tested, in an inert atmosphere rather than in Martian carbon dioxide, and no lifetime or cycle count is reported.
• USR60S1 ultrasonic motor, Shinsei. Larger traveling wave motor for the same joints. Ratings: 100 rpm unloaded maximum speed [10]. Qualification: stall torque measured on the bench at 4.5 in-lb at room temperature and one atmosphere, against 6 in-lb stated for the same joints in the same paper’s abstract, which the source does not reconcile [10].
• Iron-core motor, SG54-27 brushes, ATC, with silver-graphite brushes. Ratings: complete mechanisms cycled under high load, one test per condition. Qualification: 102 million revolutions to brush wear-out at -70 C in 1.0 kPa carbon dioxide, 36 million at -70 C in 1.0 kPa nitrogen, and 62 million in ambient air, with no-load life exceeding 100 million revolutions and 300 to 500 million extrapolated from brush wear [8]. The authors draw no conclusion from the carbon dioxide against nitrogen difference, and with one article per condition the ordering carries no spread.
• BMG planetary gearmotor, vendor not named, with Cu43Zr43Al7Be7 bulk metallic glass gearing and an iKote dry film. Ratings: run at 1 N m of output torque and 2000 rpm under vacuum, direction reversed every 2000 revolutions [11]. Qualification: 10 million motor revolutions to stall at ambient temperature against 1.5 million below 100 K on the Kennedy Space Center Space Environment Dynamometer, one article per condition [11]. Cold costs this gearmotor a factor of about seven in life, measured rather than derated.
• DRV8412 motor driver, Texas Instruments. Commercial dual full-bridge motor driver screened for a JPL avionics build. Ratings: biased at 13 V on the power supply pin against a 52.5 V rating [12]. Qualification: no latchup above 42.8 MeV-cm2/mg at 25 C and 90 C with silver-109 over 1e7 ions/cm2, with a recoverable functional interrupt observed [12]. A cross section of the die found 12.76 micrometers of copper redistribution above the active silicon, and 287 MeV bromine-81 has only about 17 micrometers of range in that stackup, so an ion that reaches the die in a bare part does not reach it here.

Ion range, not linear energy transfer alone, decides whether a motor driver has been tested at all [12]. The same campaign measured silver at 2954 MeV reaching 113 micrometers in silicon and bromine at 287 MeV reaching 36, against a redistribution layer thick enough to stop the lighter of the two.