Encoders and Resolvers

Position and velocity feedback devices selected by flown and qualified planetary and orbital robotics programs.

Both failures recorded below are contamination or noise at the sensor, not resolution. The IPEx resolver was removed from the design because it injected enough electrical noise to make the motor controller behave erratically [1][2]. The Apollo 15 attitude set control panel thumbwheel resolvers went out of tolerance because contamination built up between the slip rings and the resolvers [4]. A feedback device is a moving electrical contact in a dirty vacuum, and that is what determines whether it survives.

Feedback devices selected and flown

Part	Manufacturer	Used by	Source
SS511AT	Honeywell	ipex (all drive actuators)	[1][2]
Resolver	Not named	ipex (Generation 1 prototype)	[1][2]
DS-70	Netzer	rassor (bucket drum and shoulder actuators)	[3]
EM1 optical quadrature encoder module	US Digital	rassor (all ten actuators)	[3]
Odometer, one per traction drive	Not named	apollo-lrv	[4]
Thumbwheel resolvers	Not named	Apollo command module	[4]
Joint resolver with position sensor	Not named	era	[5]
Custom incremental encoder	Not named	curiosity (SAM)	[6]

Ratings and qualification results

SS511AT, Honeywell. Hall effect sensor for commutation and velocity. Ratings: Three per motor. Combined with the high pole count of the ThinGap motor they meet the low-velocity control requirement without a separate position sensor[2]. Qualification: Ran through the whole KSC actuator qualification campaign including 6,547,036 input revolutions of accelerated life testing on the wheel actuator [1]. A second Hall device elsewhere on the vehicle, inside the Faulhaber radiator cover motor, could not be identified by part number, which the paper records as an open qualification item [1].
Resolver, vendor and part not named: a brushless resolver for speed and absolute position, carried on the Generation 1 prototype and removed before the Alpha build [1][2]. Ratings: Selected to give speed feedback at low motor speeds and absolute arm position across power cycles [1][2]. Qualification: Removed. During setup and tuning it produced enough electrical noise to make the motor controller react erratically, and showed noise and drift across the test range. Hall-only testing showed the resolver was unnecessary for speed control. Removing it lost absolute positioning after a power cycle, which was recovered with a homing routine plus continuous saving of arm position in software [1][2].
DS-70, Netzer. Single-turn absolute capacitive encoder, SSI output. Ratings: 19 bit angular resolution per 360 degrees, accuracy better than 10 millidegrees [3]. Qualification: Selected because absolute arm position had to be known without a homing move. The paper reports the sizing and the dual-loop position-over-velocity control it enables, not a life or environmental test [3].
EM1 optical quadrature encoder module, US Digital. Incremental optical encoder module. Ratings: 10,000 counts per revolution; read directly by Elmo Motion Control G-Sol WHI20/100 motor controllers closing PI position and velocity loops [3]. Qualification: Ground prototype. No environmental test result is published [3].
Odometer, one per traction drive, vendor not named. Pulse generator on the sealed wheel drive. Ratings: Nine pulses per wheel revolution into the navigation signal processing unit, behind an 80:1 harmonic drive and inside a traction drive hermetically sealed to 7.5 psia [4]. Qualification: Flew on Apollo 15, 16 and 17 as the odometry input to lunar surface dead-reckoning navigation [4].
Thumbwheel resolvers, vendor not named. Wire-wound resolvers on slip rings, in the command module attitude set control panel. Ratings: Nominal resistance varies by no more than 1 ohm in service [4]. Qualification: Flight anomaly. Postflight measurement found resistance increased from nominal by as much as 1000 ohms on all three axes, caused by contamination between the slip rings and the thumbwheel resolvers [4]. The measured change was still an order of magnitude too small to explain the roll-axis anomaly on its own, so two candidate relay failure modes, both also caused by contamination, remained open. Corrective action was to wipe the resolvers by rotating them several hundred revolutions, applied to the Apollo 16 and 17 panels [4].
Joint resolver with position sensor, vendor not named, built for the European Robotic Arm joint. Brushless resolver plus an independent optical joint angle sensor. Ratings: The resolver supplies instantaneous rotor position to the current-torque controller and enables ripple-free torque from the two-phase redundant winding. The Joint Position Sensor measures rotation at the joint exit axis and contains a redundant set of read stations, LEDs and signal processing [5]. Qualification: Flown on the ISS since 2021. The architecture separates motor commutation from joint angle knowledge, so a resolver fault does not by itself corrupt the joint angle used by the control electronics [5].
Custom incremental encoder, vendor not named, built for the SAM Sample Manipulation System carousel drive: an incremental encoder behind a 2500:1 reduction [6]. Ratings: 30,000 counts per revolution at the output of a 25:1 planetary and 100:1 harmonic drive combination. Qualification: The Sample Manipulation System recovers from an unexpected power loss through its feedback devices and control architecture, which is what the incremental device plus the mechanical latch scheme buys [6].

Deleting a resolver, and when that trade closes

A resolver was deleted from a flight actuator design and replaced by Hall commutation plus a software homing routine. The IPEx Generation 1 prototype carried a resolver for speed feedback at low motor speeds and for absolute arm position across power cycles [1][2]. During setup and tuning it produced enough electrical noise to make the motor controller react erratically, and showed noise and drift across the test range. Testing in a Hall-sensor-only configuration then established that the resolver was unnecessary for speed control, because the high pole count of the ThinGap motors combined with the Hall sensors already met the low-velocity requirement [1]. The resolver was removed from every actuator on the grounds of the added complexity, mass, volume and wiring it entailed. The one capability lost, absolute positioning after a power cycle, was recovered by a homing routine plus continually saving the current arm position in software.

The trade only closes where a homing move is safe. RASSOR 2.0 states the opposite requirement, that absolute arm position had to be known without a homing move, and therefore keeps a Netzer DS-70 single-turn absolute encoder at 19 bit resolution per 360 degrees and better than 10 millidegree accuracy on its bucket drum and shoulder actuators [3]. The deciding parameter is not sensor performance but whether the mechanism can be moved blind at power-on.

Hall sensors and resolvers with a screening or flight result

Three further feedback devices carry a measured result. Two are Hall sensors screened against temperature and dose, and one is a flown resolver whose accuracy was calibrated on the ground [7][9].

Part	Manufacturer	Used by	Source
SS41 bipolar Hall sensor	Honeywell	perseverance (maxon actuators)	[7]
OMH3075S Hall effect sensor	Optek	Goddard gamma screen	[8]
Suspension resolvers	Not named	curiosity	[9]

Ratings and qualification results

SS41 bipolar Hall sensor, Honeywell. Commutation sensor in the Mars 2020 maxon actuators. Ratings: usable from -100 to +200 C, well outside its published range, with performance deteriorating at both extremes [7]. Qualification: the flight lot was additionally screened with pre-conditioning, lead tinning, thermal cycling and destructive physical analysis, and 64 hours of gamma exposure to an absorbed 300 J/kg, equivalent to 30 krad, produced no failures [7]. The detent brake fitted to the same motor produced stray flux at this sensor and caused commutation switching errors until it was compensated, which is recorded on Motors.
OMH3075S Hall effect sensor, Optek. Commercial Hall effect sensor screened for the NASA Electronic Parts and Packaging program. Ratings: all parameters measured against specification [8]. Qualification: 40 krad(Si) at low dose rate with all parameters in specification, at the Goddard Space Flight Center gamma irradiator [8]. The campaign took no single-event data on this part.
Suspension resolvers, vendor not named, on the Curiosity rocker-bogie suspension. Ratings: sampled at 64 Hz [9]. Qualification: accuracy measured experimentally against an absolute encoder at 0.51 to 0.65 degrees [9]. The figure bounds the suspension angles used in slip and tilt estimation, not the drive actuator feedback.

A Hall sensor qualified against temperature and dose can still be defeated by a magnet fitted next to it, which is what the Mars 2020 detent brake did [7]. The screening envelope and the installed environment are separate qualification questions.

References

Schuler, J. M., Smith, J. D., Nick, A. J., Buckles, B. C., Dyas, J. E., Ortega, V. V., Cloud, J. M., Dokos, A. G., Zhang, E. L., Wang, J. J., Baron, M. A., Muller, T. J., Clark, C. J. and Howe, M. W. (2024). ISRU Pilot Excavator (IPEx) Technology Readiness Level 5 Design Overview. NASA, 20240008162. Source

BibTeX

@inproceedings{schuler2024isru,
  title = {ISRU Pilot Excavator (IPEx) Technology Readiness Level 5 Design Overview},
  author = {Schuler, Jason M. and Smith, Jonathan D. and Nick, Andrew J. and Buckles, Bradley C. and Dyas, Jeffrey E. and Ortega, Victoria V. and Cloud, Joseph M. and Dokos, Adam G. and Zhang, Elizabeth L. and Wang, Jerry J. and Baron, Michael A. and Muller, Thomas J. and Clark, Casey J. and Howe, Musashi W.},
  year = {2024},
  institution = {NASA},
  number = {20240008162},
  url = {https://ntrs.nasa.gov/citations/20240008162},
  booktitle = {AIAA AVIATION FORUM AND ASCEND 2024},
  doi = {10.2514/6.2024-4890}
}

Clark, C. J., Smith, J. D., Nick, A. J., Ortega, V. V., Schuler, J. M., Dyas, J. E. and Lahl, J. (2025). Design and Testing of TRL5 IPEx Actuators. NASA, 20250000003. Source

BibTeX

@inproceedings{clark2025design,
  title = {Design and Testing of TRL5 IPEx Actuators},
  author = {Clark, Casey J. and Smith, Jonathan Drew and Nick, Andrew J. and Ortega, Victoria V. and Schuler, Jason M. and Dyas, Jeffrey E. and Lahl, John},
  year = {2025},
  institution = {NASA},
  number = {20250000003},
  url = {https://ntrs.nasa.gov/citations/20250000003},
  booktitle = {IEEE Aerospace Conference},
  address = {Big Sky, MT}
}

Mueller, R. P., Smith, J. D., Schuler, J. M., Nick, A. J., Gelino, N. J., Leucht, K. W., Townsend, I. I. and Dokos, A. G. (2021). Design of an Excavation Robot: Regolith Advanced Surface Systems Operations Robot (RASSOR) 2.0. NASA, 20210011366. Source

BibTeX

@inproceedings{mueller2021design,
  title = {Design of an Excavation Robot: Regolith Advanced Surface Systems Operations Robot (RASSOR) 2.0},
  author = {Mueller, Robert P. and Smith, Jonathan D. and Schuler, Jason M. and Nick, Andrew J. and Gelino, Nathan J. and Leucht, Kurt W. and Townsend, Ivan I. and Dokos, Adam G.},
  year = {2021},
  institution = {NASA},
  number = {20210011366},
  url = {https://ntrs.nasa.gov/citations/20210011366},
  booktitle = {Earth and Space 2016},
  doi = {10.1061/9780784479971.018},
  pages = {163-174}
}

NASA Manned Spacecraft Center. (1971). Apollo 15 Mission Report. NASA Manned Spacecraft Center, MSC-05161. Source

BibTeX

@techreport{anon1971apollo,
  title = {Apollo 15 Mission Report},
  author = {{NASA Manned Spacecraft Center}},
  year = {1971},
  institution = {NASA Manned Spacecraft Center},
  number = {MSC-05161},
  url = {https://ntrs.nasa.gov/citations/19720021182}
}

Cruijssen, H. J., Ellenbroek, M., Henderson, M., Petersen, H., Verzijden, P. and Visser, M. (2014). The European Robotic Arm: A High-Performance Mechanism Finally on its way to Space. NASA, 20150004070. Source

BibTeX

@inproceedings{cruijssen2014european,
  title = {The European Robotic Arm: A High-Performance Mechanism Finally on its way to Space},
  author = {Cruijssen, H. J. and Ellenbroek, M. and Henderson, M. and Petersen, H. and Verzijden, P. and Visser, M.},
  year = {2014},
  booktitle = {42nd Aerospace Mechanisms Symposium, NASA Goddard Space Flight Center},
  institution = {NASA},
  number = {20150004070},
  url = {https://ntrs.nasa.gov/citations/20150004070}
}

Mahaffy, P. R., Webster, C. R., Cabane, M., Conrad, P. G., Coll, P., Atreya, S. K., Arvey, R., Barciniak, M., Benna, M., Bleacher, L., Brinckerhoff, W. B., Eigenbrode, J. L., Carignan, D., Cascia, M., Chalmers, R. A., Dworkin, J. P., Errigo, T., Everson, P., Franz, H., Farley, R., Feng, S., Frazier, G., Freissinet, C., Glavin, D. P., Harpold, D. N., Hawk, D., Holmes, V., Johnson, C. S., Jones, A., Jordan, P., Kellogg, J., Lewis, J., Lyness, E., Malespin, C. A., Martin, D. K., Maurer, J., McAdam, A. C., McLennan, D., Nolan, T. J., Noriega, M., Pavlov, A. A., Prats, B., Raaen, E., Sheinman, O., Sheppard, D., Smith, J., Stern, J. C., Tan, F., Trainer, M., Ming, D. W., Morris, R. V., Jones, J., Gundersen, C., Steele, A., Wray, J., Botta, O., Leshin, L. A., Owen, T., Battel, S., Jakosky, B. M., Manning, H., Squyres, S., Navarro-González, R., McKay, C. P., Raulin, F., Sternberg, R., Buch, A., Sorensen, P., Kline-Schoder, R., Coscia, D., Szopa, C., Teinturier, S., Baffes, C., Feldman, J., Flesch, G., Forouhar, S., Garcia, R., Keymeulen, D., Woodward, S., Block, B. P., Arnett, K., Miller, R., Edmonson, C., Gorevan, S. and Mumm, E. (2012). The Sample Analysis at Mars Investigation and Instrument Suite. Space Science Reviews. Source

BibTeX

@article{mahaffy2012sample,
  title = {The Sample Analysis at Mars Investigation and Instrument Suite},
  author = {Mahaffy, Paul R. and Webster, Christopher R. and Cabane, Michel and Conrad, Pamela G. and Coll, Patrice and Atreya, Sushil K. and Arvey, Robert and Barciniak, Michael and Benna, Mehdi and Bleacher, Lora and Brinckerhoff, William B. and Eigenbrode, Jennifer L. and Carignan, Daniel and Cascia, Mark and Chalmers, Robert A. and Dworkin, Jason P. and Errigo, Therese and Everson, Paula and Franz, Heather and Farley, Rodger and Feng, Steven and Frazier, Gregory and Freissinet, Caroline and Glavin, Daniel P. and Harpold, Daniel N. and Hawk, Douglas and Holmes, Vincent and Johnson, Christopher S. and Jones, Andrea and Jordan, Patrick and Kellogg, James and Lewis, Jesse and Lyness, Eric and Malespin, Charles A. and Martin, David K. and Maurer, John and McAdam, Amy C. and McLennan, Douglas and Nolan, Thomas J. and Noriega, Marvin and Pavlov, Alexander A. and Prats, Benito and Raaen, Eric and Sheinman, Oren and Sheppard, David and Smith, James and Stern, Jennifer C. and Tan, Florence and Trainer, Melissa and Ming, Douglas W. and Morris, Richard V. and Jones, John and Gundersen, Cindy and Steele, Andrew and Wray, James and Botta, Oliver and Leshin, Laurie A. and Owen, Tobias and Battel, Steve and Jakosky, Bruce M. and Manning, Heidi and Squyres, Steven and Navarro-González, Rafael and McKay, Christopher P. and Raulin, Francois and Sternberg, Robert and Buch, Arnaud and Sorensen, Paul and Kline-Schoder, Robert and Coscia, David and Szopa, Cyril and Teinturier, Samuel and Baffes, Curt and Feldman, Jason and Flesch, Greg and Forouhar, Siamak and Garcia, Ray and Keymeulen, Didier and Woodward, Steve and Block, Bruce P. and Arnett, Ken and Miller, Ryan and Edmonson, Charles and Gorevan, Stephen and Mumm, Erik},
  journal = {Space Science Reviews},
  volume = {170},
  pages = {401--478},
  year = {2012},
  doi = {10.1007/s11214-012-9879-z}
}

Loschiavo, M., Phillips, R., Mikhaylov, R. and Braunschweig, L. (2020). Mars 2020 Maxon Commercial Motor Development from COTS to Flight Qualified Motors, Gearbox, and Detent Brake: Issues Overcome and Lessons Learned. JPL Open Repository. Source

BibTeX

@inproceedings{loschiavo2020mars,
  title = {Mars 2020 Maxon Commercial Motor Development from COTS to Flight Qualified Motors, Gearbox, and Detent Brake: Issues Overcome and Lessons Learned},
  author = {Loschiavo, Michael and Phillips, Robin and Mikhaylov, Rebecca and Braunschweig, Lynn},
  year = {2020},
  booktitle = {45th Aerospace Mechanisms Symposium, Houston, Texas, May 13-15, 2020},
  url = {https://hdl.handle.net/2014/52357},
  publisher = {JPL Open Repository}
}

Topper, A. D., Lauenstein, J.-M., Wilcox, E. P., Berg, M. D., Campola, M. J., Casey, M. C., Wyrwas, E. J., O'Bryan, M. V., Carstens, T. A., Fedele, C. M., Forney, J. D., Kim, H. S., Osheroff, J. M., Phan, A. M., Chaiken, M. F., Cochran, D. J., Pellish, J. A. and Majewicz, P. J. (2020). NASA Goddard Space Flight Center's Compendium of Radiation Effects Test Results. Source

BibTeX

@inproceedings{topper2020nasa,
  title = {NASA Goddard Space Flight Center's Compendium of Radiation Effects Test Results},
  author = {Topper, Alyson D. and Lauenstein, Jean-Marie and Wilcox, Edward P. and Berg, Melanie D. and Campola, Michael J. and Casey, Megan C. and Wyrwas, Edward J. and O'Bryan, Martha V. and Carstens, Thomas A. and Fedele, Caroline M. and Forney, James D. and Kim, Hak S. and Osheroff, Jason M. and Phan, Anthony M. and Chaiken, Max F. and Cochran, Donna J. and Pellish, Jonathan A. and Majewicz, Peter J.},
  year = {2020},
  booktitle = {2020 IEEE Radiation Effects Data Workshop (REDW)},
  pages = {1--12},
  doi = {10.1109/REDW51883.2020.9325841},
  url = {https://ntrs.nasa.gov/citations/20205007136}
}

Rankin, A., Maimone, M., Biesiadecki, J., Patel, N., Levine, D. and Toupet, O. (2021). Mars Curiosity Rover Mobility Trends During the First Seven Years. Journal of Field Robotics, 5. Source

BibTeX

@article{rankin2021mars,
  title = {Mars Curiosity Rover Mobility Trends During the First Seven Years},
  author = {Rankin, Arturo and Maimone, Mark and Biesiadecki, Jeffrey and Patel, Nikunj and Levine, Dan and Toupet, Olivier},
  year = {2021},
  journal = {Journal of Field Robotics},
  volume = {38},
  number = {5},
  pages = {759--800},
  doi = {10.1002/rob.22011},
  url = {https://www-robotics.jpl.nasa.gov/media/documents/ROB-20-0040_R3.pdf}
}