Lawnbot v.2 Electrical
February 1, 2012 Leave a comment
I’m happy to report that this project is going to be significantly cheaper than I expected. This is due to the fact that I was originally planning on driving everything off of cheap cordless drills from Harbor Freight. Yeah, that would have worked, but even the cheap ones are expensive enough. He hero to all of this: Ax Man, the local surplus store.
On a recent trip to Ax Man, I found a pair of worm-gear motors. They are 12 volt motors geared down to about 180 RPM. They were initially designed to be motors inside of motorized car seats in Daewoos. That doesn’t really matter. The important part is that they are cheap and they run at a much slower RPM, which means the final gearbox is going to be much less complicated. Yay, Ax Man!
I’ve begun laying out the electrical work on the new lawnbot. The following is a schematic of how the whole thing will work:
The system centers around a simple sealed lead acid battery (SLA). The one I’ve picked out is for a garden tractor, available from Menards for about $24. I will keep the battery charged using a trickle/float charger. By float-charging the battery, I can leave it plugged in without fear of overcharging. An automotive switch is connected to the battery and in turn, powers a bus. Everything else connects to the bus.
An arduino will be used to control the unit. The arduino is powered by the bus via the arduino’s built-in barrel jack. When the power switch at the battery is turned on, the arduino will boot. The arduino will use pins 5, 6, 10, and 11 to control two H-Bridge circuits. I promise that I’ll have a post on what an H bridge is soon enough. The reason these pins are used is that these are the output pins of the arduino that can produce PWM. PWM is a way of turning a circuit on and off fast enough that it actually looks like a varying voltage. While the arduino can’t output an analog voltage, it can vary the PWM in such a way that other systems will see the output like a 0-5 volt analog range. This analog-like signal is important to the H-bridges because they control the speed and direction. The higher the simulated analog voltage coming from the PWM pins, the faster the motor will turn. Each H-bridge gets two PWM signals: one for forward, one for reverse.
Lastly, a digital output pin on the arduino is used to switch a relay for power output. In this case, the pin is #2. The output will be used to power things like the cutting head of the mower attachment. It may also be used to power an inverter if an AC device is required.