One of the most surprising things about our experience at the European Rover Challenge last year was how incredibly close to total failure we came. Two days before the competition began, while we were in Poland, our control board failed. In addition to having to port all of our embedded codebase to Arduino in two days, we had to fix our overcurrent protection mechanism on the rover’s claw. This was a critical system since it prevents the claw servo from overheating when try to pick up objects. Before we developed our original software solution, a large number of servos had been destroyed due to overheating. Due to errors we’d made in calibration during the port to Arduino, our original software solution didn’t work and we had to think of something else.
Seb Holzapfel and I realised that a hardware solution would also solve this problem. We designed the circuit shown below. It consists of an Op-Amp, a diode, a mosfet and a few resistors. It was designed such that when a large amount of current flows through the 100m Ohm resistor, the PWM signals will be cut off from the claw. This causes the servo motor in the claw to stop drawing current, and therefore prevents overheating.
But why was this worth writing about? Well, we had to build this in a very short period of time and we didn’t really have the correct spare parts on hand. We only had a few op-amps, some jumper cables, some veroboard and a few resistors. This wasn’t enough to build the circuit shown above. We had to improvise. I realised that since our control boards had all failed, we could, in fact, harvest them for the parts we needed. Fortunately, after doing a quick stocktake of the parts on the old control boards, I determined that all the parts we would need were present. We just had to salvage them.
While everyone else was out testing the rover and after we ported the code to Arduino successfully, Seb and I found a bit of spare time on our hands. About 2 hours. We got to work, I desoldered parts from the dead control boards with the hot air gun, while Seb put those parts together into the monstrosity you see below.
We then tested it using a coil of wire as a load and verified that it worked. It was then deployed onto the rover. Despite being built in just an afternoon, it actually worked better than the previous software solution when we tested it with the rover. And with this “solution”, we came 9th.
And that’s how we built a critical system in just 2 hours from parts we salvaged from dead control boards.