415 vibration/shock environment , and vacuum exposure.  After deter mining that the Ø20 mm and Ø32 mm  models provided the optimal power v ersus mass compromise, the commercial design was analyzed by a  team incorporating both JPL and maxon personnel with the intention of applying the previous experience  of both organizations  to optimiz e the design for ultimate reliability in the harsh Martian environment . Table 2  lists the features in the COTS design that needed modification.  

Table 2.  COTS design and modifications for Martian environment   COTS Design  Design Risk for Mars  2020 Application  Solution   Flange bearing carrier out of several  parts with dissimilar materials  Connection under shock/vibration; c orrosion ;  coefficent of thermal expa nsion mismatch  Create one- piece flange/bearing carrier   Standard Hall sensors  Temperature range limitations, failure due to  radiation exposure  Use Hall sensor that can be qualified for  environmental conditions   Rotor shaft glued to bearing races  Shock and temperature environment breaks  adhesive bond and causes rotor to move Add securing/spacer rings to design and  mount all parts on shaft to hard stop. Weld  rotor to shaft   Main magnet only partially covered  in adhesive  Bond to rotor is not as strong as it could be and  might fail under repeated temperature cycling  Develop new bonding application process  to ensure at least 80% coverage   Flex-print PCB  Can flex under vibration environment , which  leads to mounted components breaking  Replace wit h FR4  and flying lead wire  connections   Winding taps run directly to PCB  ~1 cm lengths of unsupported thin wire  susceptible to vibration- induced resonance and  breakage  Stake winding taps to stator and strain  relief with “expansion loops”  next to solder  joint 

In order for the maxon catalog flat motor to survive the requirements of the Mars 2020 mission, several  changes were made. The PCB material was changed from a flex print to FR4, a material commonly used  for space missions as it does not significantly outgas, is robust enough to support the Hall sensors in a  vibration environment , and fulfill s IPC-A-600 Class 3 requirements and testing required by JPL. The bearing  preload architecture was modified so that the entire system was pushed to a hard stop meaning t he  individual parts could not shift during impact or vibrations. A more robust spring was selected with shim  ends to better distribute the forces and prevent the shim from breaching the bearing. High- quality industrial  bearings were selected from one of maxon’s standard suppliers. Braycote 600EF was chosen as the  lubrication with a fill-factor (15– 20%) of the free volume of each bearing measured (by mass)  and  documented.  

As has been standard practice for decades in space applications, the PCBs in both the M20 and M32  designs have conformal coating to:  1) protect the PCB from corrosion whil e in storage o n the ground  awaiting launch;  2) protect against short circuits from any debris in weightless conditions ; and 3) suppress  any tin whisker growth. maxon h ad a good experience working with Speciality Coating Systems in the  Czech Republic for Parylene HT application for ExoMars and chose to work  with them again for the Mars   2020 motors. The fully populated PCBs, including attached stators with windings were s hipped to Speciality  Coating Systems , coated , and returned to maxon for integration into the rest of the motor.  

Undoubtedly the most difficult component choice to be made when designing a space- rated BLDC motor  is the rotor angle detector required for th e control electronics to correctly commutate the motor. All of  maxon’s BLDC motors use various types of Hall sensor s for this purpose. As described in Ref. [12], the  solution for ExoMars was a radiation and temperature test program with many different types of compact  Hall sensor s. Unfortunately, the Infinion TLE4945 sensor that was selected for ExoMars has been  discontinued, but maxon had already tested and selected a variant of Honeywell’s bipolar SS41 H all sensor  range for other space applications. JPL organized the purchase of a flight batch of sensors that were  subjected to additional screening (pre- conditioning, lead tinning, and thermal cycling as well as lot  qualification including Destructive Physical Analysis) to qualify the sensor lot prior to del ivery to maxon for  population onto the PCBs. These sensors have been shown to operate well outside their published range,  although performance deteriorates at the temperature extremes. Satisfactory operation can be achieved