Pennybot

Pennybot is my attempt at a second mini sumobot. A sumobot with brains (or at least a microcontroller), IR sensors to detect opponents and possibly other sensors (a front touch sensor looks like a good idea). Again I will be constructing the chassis mainly from scratch. For motors I had two GM2 motors from Solarbotics. I really liked the offset design of the motors and skinny wheels available. Getting Nudgebot to fit in 10x10cm box was quite a challenge with the standard Tamiya gearbox and wheels. The GM2 motors will make life much easier.

For the chassis I tried many ideas before getting it right. All were around the same concept with the motors mounted on pieces of 90 degree aluminium which in turn are connected to a base plate. Just the positioning and battery location kept changing as I kept finding flaws in the designs. Part of this was due to looking at the design over a few months rather than spending a whole day just to work things out.

In the end I mounted both engines on 80mm lengths of 90 degree aluminium angle. Due to the position of the holes in relation to each other I used a paper template rather than dead reckoning to position the holes. The end result was quite a nice job (compared to other jobs I've done).

For the base I used the Tamiya plastic base plate like I had used on Trackbot and Nudgebot. Pre spaced 3mm holes are wonderful.

I mounted the motors on the bottom of the aluminium angle in order to fit the batteries underneath the base plate. This put the batteries at the lowest point and ensured the centre of gravity was as low as possible. To stop the batteries being damaged or accidently falling out I made a cover plate from some thin metel sheet (The EMF covering from inside an old Sun unipack disk enclosure). Instead of bolting the cover plate to the base plate I used a 3mm tap to put a thread in the cover plate. This meant I didn't waste space by needing room for nuts. Another case of "why didn't I always do this". At least that tap and die set purchased was justified.

For batteries I am using 5 x AA batteries. The motors are rated at 6V so I'm still working out if I either over supply the motors, use a diode to drop the voltage or just make a separate 6V tap. I intend to use a LM2940 low voltage regulator to provide a 5V supply off the 7.5V from all the batteries. How this regulator handles the noise from the motors is to be determined.

I did a quick temporary solder to add a power switch and did some push tests with 4 batteries (6V). No issues pushing the necessary weight (500gm). However I am slightly concerned by the amount of wobble in the axle on the motors. Straight line driving is really needed but there was definitely some travel. Hopefully when I screw the wheels to the axles (rather than just push them on) this will tighten up the slop.