Sunday, February 10, 2019

QMRD3: The Rover Testbed

To support the Mars Drilling Project (discussed in the previous post), I have started building a rover testbed that I'm calling QMRD3 (Qualification Model Rover Demonstrator for Deep Drilling). This is a 4x scale copy of the Sawppy rover by Github user Roger-Random. Due to the size difference, several changes (mainly relating to motors and power transmission) have been made and an entirely different motor strategy is required. You can find more about Roger's original Sawppy rover at https://hackaday.io/project/158208-sawppy-the-rover and https://github.com/Roger-random/Sawppy_Rover

QMRD3 Screenshot 1.PNG



This rover was born out of the necessity for a mobile testbed with flexible mounting options, but is turning into quite a project, both in the amount of work involved as well in the many ways that it represents the bleeding edge of home-workshop additive manufacturing capability. For example, each wheel weighs 5kg and takes 7 days to print, with a material cost of ~$120. A screenshot with the details of the wheel print is available in the repository. If you are inclined to build a rover like this, the wheel should be your deciding factor. It is by far the most expensive and largest printed component. A Github repository for QMRD3, containing STLs that I have been printing as well as the work-in-progress Fusion360 archive is available here: https://github.com/Shootquinn/QMRD3


Also of note is the 22mm skateboard bearings used extensively on Roger's Sawppy rover have been replaced with 88mm 3D printed gear bearings. The bearings are licensed separately from the rest of the rover and the files can be found here: https://www.thingiverse.com/thing:3116838



The first full rocker-bogie half should be done within the next month. It will likely take until June to print the components for the other side. I haven't decided how best to deal with the rocker-bogie differential yet. It may be worth it to design a printable version and have it made on Shapeways. Also a CNC Router cut plywood assembly may be an option for the differential.

I'll try to get a Bill of Materials put together by the summer. The drive motors are rotisserie servo motors and are around $125 apiece, the steering motors are stepper motors with 50:1 gearboxes. None of this is cheap. The Sawppy was designed with a $500 budget in mind, it appears that the budget moves linearly with the scale at this point, but the scrap costs from prototyping also do. If you want to build one of these I would plan on it costing around $5,000. Eventually the cost could be reduced to $2,000 from lessons learned, etc.


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