By Daniel Brown
After joining CNM Hackerspace to work on the Hal 4K for Pi-Wars I was disappointed in two things about the Hal 3000: it didn’t handle very well (like an RC car would) and the battery life was miserable. After testing the motors at 100% capacity, the battery life was about 11 minutes. The handling was poor due to a high center of gravity, and a majority of the weight being biased towards the front of the body (and above the front wheels). Based on my experience in building RC cars and powering high powered flashlights, I immediately knew that I could be of help in the development of Hal 4K for Pi-Wars 2020.
Hal 3000 wasn’t an easy bot to control, mainly because the Raspberry Pi 3B was mounted high in the body, and the battery pack was above the front wheels. This biased the weight towards the front, making the rear of the robot very light and therefore hard to control, especially at speed. Just as with a real car, the lower the center of gravity and the closer to a 50/50 front/rear weight bias, the better handling the vehicle will be. With Hal 4K we have mounted the batteries centrally and much lower inside the body. The Raspberry Pi 4 1GB (also a new upgrade for 2020!) is also mounted above the batteries, in a somewhat central location - in an attempt to achieve a nearly perfect 50/50 front/rear weight bias and much lower center of gravity than
I discovered that the 2019 CNM Hackerspace Pi-Wars team utilized rechargeable Ni-Mh (Nickel-Metal Hydride) AA batteries for power. These were used in a configuration of 8 AA batteries, providing 1.2V and 2000mAh per battery. These batteries were also worn down, after being recharged well over 250 times! After a few minutes of research on alternative power sources, as a team we decided to go with 18650 lithium-ion rechargeable batteries because they pack such a heavy punch in a rather little package. Each 18650 battery provides us with 3.7V (up from 1.2V), 3000mAh (up from 2000mAh), and 35 amps of power. In terms of available power and battery life, this a HUGE upgrade.
While this was not a cheap upgrade, we believe this was money well spent. After testing with our new battery setup, we can run the Raspberry Pi 4 and the motors on Hal 4K at 100% for well over an hour with no noticeable drop in voltage and/or robot performance. This also allows us to expand and experiment in the future using new sensors that previously would have affected performance or battery life further.
After we decided on batteries, we had to develop some sort of quick disconnect system to allow us to swap out battery packs on the fly. Instead of incorporating an on/off switch to a hardwired battery pack (swap individual batteries instead of a pack), we wired an inline quick dis-connector, similar to an RC car battery pack. Not only does this allow us to swap battery packs out on the fly in under a minute, this also acts as the on/off switch for the robot.
Being my first year participating in Hackerspace and Pi-Wars, I’ve realized that there is a place for everyone on a robotics team (and in robotics in general!) and wish I would’ve done this years ago. I don’t know much about computers, coding, and robots, but I still found a spot on the team and was able to help out. With a few people that can code, one or two that know 3D design, my knowledge in power and RC cars…. Together we can come together and create something bigger than any one of us is capable of alone.