Google Lunar XPRIZE – 4-kg Lunar Rover

One of the coolest classes at CMU is the 16-861 Mobile Robot Development/16-865 Advanced Mobile Robot Development (of which I am now the TA). Even though CMU had formally dropped out of the competition, when I took the class, the goal was still to design a rover for the Google Lunar XPRIZE.

Overview

One of the goals for the Google Lunar XPRIZE was to design a rover under 4-kg. Our ride to the moon, Astrobotic, Inc.’s Lunar Lander, specified that any payloads on their lander needed to be within a 40x33x38-cm envelope. This means designing things as simple and compact as possible. Because of my affinity for mechanisms, I decided to tackle the suspension/body averaging system. The previous semester of the class came up with a drivetrain design where the driven gears have the potential to also act as wheels and adding a gear reduction requiring a smaller motor, potentially saving on mass, shown below:

OldGeartrain.png

This design also allows for mobility using only two motors as part of a skid-steer system. A suitable suspension system for this design is a body averaging system, or “rocker bogie” system that is similar to what Curiosity uses. Applied to this robot, as one side tilts up, the other side of the robot tilts down, “averaging” the position of the body.

Rocker_bogie
Rocker-bogie system on Curiosity. Source: https://en.wikipedia.org/wiki/Rocker-bogie

Unlike on Curiosity which has individually driven wheels, this rover only has two motors, driving two wheels each. This created an interesting design problem, how do you design a suspension where the motor is fixed, but allows for rotation around the same axis of the shaft of the motor?

Design

This was a puzzle that took a while to solve, but a fun puzzle nonetheless. The body averaging system worked like this:

BodyAveraging

RockerCrossSection
Cross-section of mechanism shown above.
Screen Shot 2018-02-17 at 11.35.01 PM.png
Labeled cross section of mechanism. Slightly different than what is shown above because it integrated the final CAD from other members of the team.

Build and Test

IMG_4509
Machined prototype mechanism
IMG_4508
Prototyped mechanism with 3D printed mount and laser-cut rocker arm
IMG_4601
CNC’d rocker arm for the final prototype. I developed the G-Code in Mastercam and set-up the CNC (with some help from the machinists) myself.

 

IMG_4602
Only broke one bit, and on the last pass, so close!
GearTrain_v2
It came alive!

Summary

This class taught me a little more about designing for space and allowed me to design and machine (and use the CNC for the first time) to build my mechanism. It was definitely cool to see the whole thing turning with the gears that someone else in the class designed and machined. It would have been very nice to continue and refine this, but another project came along, this time shooting for a 2-kg lunar rover. While this mechanism didn’t make it into the new design, I was able to use my knowledge about the entire system as the lead systems engineer, which you can read about here.

IMG00362.JPG
The mechanical squad at the final presentation, ft. my crazy mountain man beard
26910207_10155628637353725_1522947846635103535_o.jpg
Assembled final prototype in the Mars dirt simulant testbed at CMU.
2017.10.14 ZZZZDSC07827.JPG.jpg
Generations of CMU’s rovers, from Andy, down to the midterm 2-kg design described here.

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