Monday, 11 March 2019

First Fully Drive-able Build of my Mars Rover

My first prototype body to mount my rocker bogie arms onto made me realise I needed to better balance the weight of my robot over the main pivot axis. I also needed better clearance between the body and the steering legs. Then there was the challenge of where to put the rest of the electronics and the battery. I decided the battery was the thing to provide the weight to counter balance the body, and being a heavier item should also be located closer to the ground to make the robot more stable. So I drafted a design where the rear of the body was tapered giving less width. Enough to accommodate the battery but narrow enough to give sufficient clearance for the rear steering legs.

Early redesign of main body with narrower rear end

In attempting to wire up all the motors, and with the steering servo cables also needing routing into the chassis body, my robot was suddenly looking rather messy. I had deliberately made my prototypes from 3mm ply so they could be modified easily to iterate on the design. I explored ways to route the motor wires by getting out the drill.

Iterative design: wire routing!

Somebody commented on one of my Twitter posts asking where I planned to mount the Pi Noon attachment required for two of the challenges. This was not something I had considered up to this point. The challenge rules stated the attachment point had to be on the front of the robot, in the middle. My robot design at this point had nothing but thin air in that position! I wondered about making some sort of brace which attached to the two front rocker arms to hold it. This would lock the rocker (but the event would have a flat surface so not a big issue). Then I realised it was also needed for the Spirit of Curiosity challenge and this was expected to have terrain of varying difficulty. This was the challenge my robot was built for, and my robot was based on the namesake Mars rover of this challenge. I needed another plan. Looking at pictures of the real Mars Curiosity rover I realised that the rocker arm linking bar was positioned forwards of the main pivot axis, rather than behind as on my rover. I swapped mine to the same (you can see this difference between the original animated CAD model and the actual plywood first prototype in my earlier post).

Redesigned chassis shape with battery compartment and room for breakout electronics

Looking at my remodelled chassis it suddenly struck me that the back looked awfully like a front now, and what I had always imagined as the front was looking much more like the back (complete with USB and ethernet 'exhaust' ports). This now gave me a front panel to attach the Pi Noon attachment to as well. I extended the new front end to bring it level with the end of the rocker arms, and so my final rover shape was decided.

The new completed chassis built and ready for some road testing

Checking the A4 footprint of my new design

Finally I had something I could road test. The closest thing I had to some martian terrain was a load of rocks and fossils collected on family trips to the Jurassic coast.

Overall I was very pleased with how well my robot handled. It was very easy to control. The grip on steeper slopes was not enough, but I think for the Pi Wars challenges it will be sufficient. It was a little top heavy and prone to toppling over on more challenging obstacles. Here are some of the out-takes.

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