179 STIG: A Two -Speed Transmission Aboard the Mars  2020  Coring Drill  

Timothy Szwarc*, Jonathan Parker ** and Johannes Kreuser+ 

The coring drill, part of the Sampling & Caching Subsystem (SCS) aboard the Mars  2020 rover, demands  a wide range of drill bit torque and speed capabilities during sample acquisition operations. The two driving  operating points are high speed , low torque for rotary -percussive coring, as well as low speed , high torque  for separating the rock core sample from its parent rock . The spindle twin- input transmission (STIG) allows  these and other  operating points  to be reached with an actuator of subst antially less peak power and  maximum current draw than that of a single -speed actuator . Rather than containing gearing of its  own, the   transmission interfaces to an actuator  with two outputs of different gear ratios , allowing the transmission to  select one of the two of the outputs  to be coupled to the drill bit . This paper describes the design,  capabilities, and challenges associated with the transmission and the dual-output actuator.  

Introduction  

The Mars  2020 sampling tool, commonly referred to as the “ corer ” and shown in Figure 1, possesses three  methods of surface interaction. T he corer takes its name from its  ability to core and retain  acquired  rock  core samples 13 mm in diameter and 76 mm in length.  An abrading bit allows  for the removal of the several  mm of weathered exterior surface from rock, in patches 50 mm in diameter. Operation of the abrading bit  requires similar torque to that of coring, although the bit is rotated at a much lower speed. Finally, a regolith  bit al lows for collecting and caching surface dirt. This bit operates at  low torque and low speed to  allow  powder to flow into the bit from an opening on the bit’s side.  

Generating  cores typically requires speeds a nd torques at the bit of 200 rpm and 2.5 Nm, respectively.  Following bit penetration into the rock, core breakoff and retention are performed by mis aligning the  eccentric sample tube and the drill bit in which the tube is contained [ 1]. This operation, which fractures  rock in shear, requ ires as much as 28 Nm in the test rock suite used by the mission, although the speed of  core break can be arbitrarily low. Providing margined capability at  the core breakoff and coring operating  points in the absence of a transmission would require an actu ator with a 400 W peak operating point.  Assuming a gearhead with a single  ratio capable of providing coring speed, the driving electronics would  need to provide about 500% more electrical current than is currently possible on the rover  in order to provide  sufficient torque for core breakoff . An actuator coupled to a transmission with suitable ratios would require  a peak power of 200 W and no alterations to electronics.  Therefore, a transmission was determined to be  the preferred solution. The resulting spindle twin- input gearing (STIG) transmission and associated dual  output actuator allow for two modes of operation that differ by a gear ratio of 16.  

Mechanism Architecture  

The dual-output spindle actuator provides motion at its low - and high- gearing output simultaneously . These  outputs, which are offset in different planes along the actuator output axis, are shown in Figure 2. More  detail on this actuator is provided i n a later section of this paper. STIG interfaces with one actuator output  at a time and couples that motion to the coring, abrading, or regolith bit. The STIG interface to the actuator,  including a specific STIG part named the selector which translates axi ally to access one of the two actuator  outputs, is shown in F igure 3. The completed ST IG assembly is shown in Figure 4.  

* Jet Propulsion Laboratory, California I nst. of Technology, Pasadena, CA; timothy.j.szwarc@jpl.nasa.gov   ** Sierra Nevada Corp., Durham, NC; jonathan.parker@sncorp.com   + CEROBEAR GmbH, Herzogenrath, Germany; johannes.kreuser@cerobear.com  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA J ohnson Space Center, 2020