163 Accommodations for these overages were made in the chuck mechanism by designing components for the  worst case torques and mini mizing actuator output speed.  

Lessons learned   Many key lessons were learned in the design and development of the Mars 2020 chuck mec hanism. Chief  among these are that accommodations should be made for compliance in mechanisms whenever possible  to increase robustness and to decrease the need for overly tight part and assembly tolerances. Initial  prototypes for the chuck mechanism utilized a one- piece  cam, similar to what was used on MSL. Although  ambient testing demonstrated the functionality of using a one-piece  cam, no operations were attempted at  temperature using this prototype chuck. Furthermore, no testing at worst case pullout or rover slip loads  was performed. Analysis indicated that large internal loads could be generated for a chuck with a one-piece  cam due to thermally induced dimensional changes. Such a design would also be very sensitive to  dimensional deviations and would likely require very tight tolerances and assembly controls to ensure  equitable load sharing amongst parts. The use of rollers  and flexures in the chuck eliminates the need for  such tight controls while also resulting in a more robust design.  

Another important lesson is to design for large uncertainties in the torque output of the driving actuator,  especially if the actuator mak es use of a magnetic detent brake. Such a brake requires that the actuator  overcome the magnetic detent in addition to providing output torque, resulting in substantial uncertainties  in applied torque.  Early in the design, it was assumed that actuator torque output could be fairly well  controlled (±25%), and therefore the preload torque applied to the chuck could be known within a  reasonable certainty. However, later analysis and test data for the actuator indicated that much larger   torque overages (±150%) were possible. This required designing parts of the chuck to be able to react  these higher torques even if a much lower torque is commanded. However, the much increased reliability  of a passive magnetic detent brake likely outweighs the increased uncertainty and overages in torque  output.  

Conclusion s 

A novel preloaded ball lock mechanism has been designed and implemented for use as the chuck in the  Mars 2020 coring drill. Through the use of rollers and a flexured cam, the Mars 2020 coring drill chuck is  capable of securing bit assemblies with a preload to provide a stiff connection to the corer and ensure high  quality samples while also preventing potential j amming. This chuck mechanism is capable of operating  across the required proto- flight temperatures and in the dirty environment expected on Mars.  Testing has  demonstrated that the chuck mechanism is capable of reacting and surviving worst case design loads  and  provides the functionality needed for sampling and bit exchange operations.  Future missions requiring a  chuck, end effector, release mechanism, or any other assembly that needs to readily, reliably, and  repeatedly secure and release components can mak e use of the features in this chuck mechanism design.  

References  

1. K. Chrystal, “Percussion Mechanism for the Mars 2020 Coring Drill” Aerospace Mechanisms  Symposium, Houston, TX, 2020.   2. T. Szwarc, J. Parker, and J. Kreuser, “STIG: A Two- Speed Transmission A board the Mars 2020  Coring Drill” Aerospace Mechanisms Symposium, Houston, TX, 2020.   3. M. Loschiavo, R. Phillips, R. Mikhaylov and L. Braunschweig, “ Mars 2020 maxon Commercial Motor  Development from Commercial -Off-the-Shelf to Flight -Qualified Motors, Gearboxes, and Detent  Brakes: Overcoming Issues and Lessons Learned” Aerospace Mechanisms Symposium, Houston,  TX, 2020.   4. L. E. Chu, K. M. Brown and K. Kriechbaum, " Mars 2020  sampling and caching subsystem  environmental development testing and preliminary results, " 2017 IEEE Aerospace Conference, Big  Sky, MT, 2017, pp. 1- 10. 

© 2020. California Institute of Technology. Government sponsorship acknowledged.