Every year, I start a new Gardenbot project, and it rarely gets any further than a wish list. This year is different. I have a fully-functional robot that is battery-powered and can be controlled remotely via WiFi.
Getting this far has not been easy, so I’ll write up what I can remember so you can do the same (or so I can do the same again next year after I forget!)
The biggest problem was the computer itself. I’m using a Raspberry Pi, but powering it was tricky.
The Pi takes a 5V input, but I couldn’t find any ready-made 5V batteries, and didn’t want to use battery packs as I wanted to easily recharge individual batteries.
In the past, my experience with using batteries in series with each other was that one battery would discharge fully way before any others, leaving an apparently dead pack. To solve that, I’m using li-ion batteries scavenged from phones; each with at least 2800mAh in them. I link them in parallel, and “boost” the voltage using some regulators.
There are currently two voltage boosters in the system. The first one powers the Pi, and the second powers the USB hub. You can’t power the USB hub directly off the Pi as the Pi uses 700mA, and there’s not enough left over to power anything useful. So, for anything external, such as the WiFi and the camera on the robot, you need to use a powered hub.
To save space, I stuck the voltage boosters for the USB hub and the Pi inside the Pi case, as you can see in this photo. They’re the rectangular circuits with the large capacitors on them.
The capacitors are there to help stop fluctuations in power supply as various bits and pieces are turned on. There’s nothing quite as annoying as turning on a motor only to find that you have lost WiFi because of it and now have no way to turn off the motor.
The robot chassis is a hand-built case made from two perspex sides, a wooden base, and a wooden front. I didn’t measure anything – it was all done by trial/error.
The tracks are from Tamiya (example store). The box comes with enough for a larger base, but I didn’t need it all.
The claw at the front doesn’t work perfectly yet. The one I currently have is one I bought a few years back. It never seems to work properly for me. I think I need either a stronger servo, or just replace the claw completely.
The servo cable has three wires – ground, power in, and signal. The ground and power in can be plugged directly into the batteries. The signal, I hooked to GPIO 1 on the Pi (using this wiring guide for the main GPIO connector), which is then controlled using pulse width modulation (PWM) through the pin.
The motors for the treads are scavenged from the legs of a Robosapien bot I got for Christmas a few years back. These are standard DC motors, probably for up to 5V, but I’m running them off 3V and happy with them.
To control the motors, I was initially planning to create my own motor controller using some PNP and NPN transistors, but found a motor controller circuit from an old Cybot that handily does exactly what I need.
The camera is a standard web-cam, with the cables shortened.
Turning the machine on is done by simply connecting the little red cable between the battery-side and the “other stuff” side of the breadboard as you can see in the image above.
To charge the battery, I simply hook in a Li-ion charger directly into the left of the board (below). The charging circuit will happily charge multiple Li-ion batteries.
Hardware-wise, I’m almost happy. I want to replace the claw soon, but apart from that, I’m ready to work on software.
I already have code written for controlling the motors, which I’ll upload into Github over the next few days. I’m looking into SLAM now for creating maps via the camera system. I might have to write the solution myself, though, as the code I’ve found so far is written in academicese and I don’t understand it.
Funny, that, as I’m certain I can write the bloody code, but can’t understand the words that the academics use!