It’s been almost two years since I did (or at least started) this project but I never sat down to document it. That’s what I want to do today. As the title says it’s about a little robot based on a RaspberryPi. Like many of its kind it is driven by a pair of stepper motors each driving a wheel directly attached to the respective motor axis. At the back there is another smaller, pivotable wheel to keep the robot in balance.
Here’s a video of the finished robot in action, running a simple demo program demonstrating the various functions. By the way, I’ve started a youtube cannel to share these kind of videos. I’m not really a video guy so this text and photos blog will stay my main medium but some projects like this robot, videos are a welcome addition.
Yes, I’m well aware that many similar designs already exist out there I could just go out and buy a kit like this. But making my own sounded more interesting so when I was looking for a Christmas present for my godson of sorts I did just that.
Above is a close-up of the main PCB that I’ve designed and built for this project. The idea is simple: There is a PIC16F1936 microcontroller that communicates with a RaspberryPi over I2C. The PIC then handles all the low-level details of controlling a pair of Allegro A4982 stepper drivers. These work at up to 35 volts, handle up to 2 amps of current and can hence drive much more powerful motors than the relatively small NEMA 17 size motors I’ve used here. They are easy to use and feature microstepping up to 16th steps.
Besides the two stepper drivers there is a ULN2803 providing 4 power outputs capable of driving up to 1 amp each. The ULN2803 includes free-wheeling diodes so these outputs could be used to directly drive somethingn like a relay or a DC motor. But at least for now these outputs drive some RGB LEDs at the front as well as a buzzer.
The original idea was to power the RaspberryPi from the 5V linear regulator on the board and then draw the power for the PIC from the RaspberryPi’s 3.3 volt rail. Since the PIC uses only a few milliamps that’s entirely possible.
Unfortunately I haven’t given that setup a lot of thought before building the board. Of course, when powered from something like 12V, the LM2931 regulator gets way too hot when powering a RaspberryPi that pulls a few hundred milliamps. So I’ve sacrificed one of my solar charger RevC boards that includes two very powerful USB charging outputs.
During testing and debuggin I’ve used a small 12V AC/DC converter screwed to the bottom side of the robot. Once more or less completed I’ve changed the power supply to an old 3-call (11.1V) LiPo battery from a RC helicopter. It’s no longer fit for flying but still adequate to power this thing for a few hours.
The entire structure is laser-cut from 5mm medium-density fiberboard and held together with M2.5 torx screws with square nuts. M2.5 square nuts measure precisely 5x5mm so that goes together rally nicely. I’ve added and changed a few things as I went along, drilling extra holes to mount the blue PCB for the power supply, the LEDs, to hold the battery in place and some other things. But the structure as such works very nicely. It’s relatively simple if you have some place to do laser cutting (try your local fab lab…) and is inexpensive and sturdy.
The weels are laser-cut from the same material and are sized to measure precisely 200mm in circumference. That’s handy since the steppers feature 200 steps per rotation.
That’s about it, most of the relevant files are on github. The OpenSCAD files for the laser cutting are not so just let me know if you’re interested in them. I’m happy to share them, too. Here are the links for the software and hardware, respectively.
As always, I welcome any thoughts or comments.