The equipment bay for my little rover Micro Sawppy Beta 2 (MSB2) had enough space for the temporary setup of Raspberry Pi 3 and Adafruit PWM/Servo HAT along with all wires and a battery. However, I’m trying to think ahead to more elaborate setups and how I might accommodate their needs. One obvious concern is that bigger computers would need more space and bigger batteries, both competing for room inside the equipment bay.
Mars rover engineers at JPL had already encountered this problem and we can see their solution: Curiosity and Perseverance draw their electrical power from a radioisotope thermoelectric generator (RTG). Unlike their predecessors, who drew electrical power from solar panels that imposed a lot of operational limitations. Mars-bound RTG are large and bulky devices bolted prominently to the rear of those two rovers, giving them a very distinctive rump. Since MSB2 tries to look like the big rovers, it will also have a similar-looking attachment to the back, so we might as well make it serve the same purpose! (And to be clear I meant power supply, not a radioactive generator.)
Since MSB2 could fit its battery inside the main body, I didn’t need to make this faux RTG functional just yet. For MSB2 it is only a test run mockup without any paths for power wiring. I found that if I made it according to proper scale, the mock RTG would be (just barely) large enough to accommodate a single 18650 lithium-ion cell. It also looks pretty easy to make the chamber easily accessible, so the battery could be swapped out.
But with that success, my ambition grew: what if I could find a little more room? I could accommodate a USB power bank built around a single 18650 cell. If micro Sawppy could be built to run off a commodity USB power bank, that would make it a lot more beginner friendly. First, the construction would be simpler and second, beginners would be spared the learning curve of working directly with infamously temperamental lithium ion battery cells.
So that’s another research project: what are the power requirements for a micro rover, and would it fit within the power delivery capabilities of a USB power bank? I know this specific USB power bank in my picture doesn’t have the grunt to keep a Pi 3 running, so it is another motivation to scale down from the current electronics configuration to make room in the power budget for other component like a robot arm.