Saturday, April 19, 2008

Trackbot part 1

Trackbot and trailer in action

Trackbot was an evolution on Squarebot but with a microcontroller added to the design (a PIC16F84). The engine controller circuit was based on the sumo bot design from "Junkbots, Bugbots and bots on wheels". However the real inspiration came from a visit to the local Dick Smith store.

One day while browsing in the local Dick Smith store I came across the Tamiya track set and Base plate set in the discount bin. Instantly ideas of a tank like bot came to mind and a purchase was made. However I then had a few months wait while I tried to work out a way to attach motors to the track set. Little did I know that Tamiya also had a range of engine kits designed to go with the track set (both single motor kits driving both tracks and dual motor kits). Once I had one of the dual motor kits (from Jaycar) the chassis was done. Since the track set boggies were bright orange I decided to spray paint them black. Big mistake as the paint just peeled straight off, even after I washed the plastic in detergent. So now they are orange with black highlights. Finally I used the cover from an old floppy disk drive as a plate to sit on top of the chassis for all the PC boards to be mounted on. This also had the advantage of providing some shielding from the motors.

The motors are 3V rated. The engine block has four gear ratios that can be setup. I choose the second slowest as I wanted trackbot time to react to objects and I wanted torque. No point having a tank that can't climb over things. Some of the robot sites suggested replacing the 3V motors with 4.5V motors (the motors are the standard "toy" size engines) but speed wasn't a concern on this design.

The next design issue I had was how to control the 3V motors. My existing motor controllers (SN754410, 4424) didn't support such low power motors. I could make dual H bridges out of transistors but that would make the engine controller quite big and space was an issue. In the end I went with the relay based motor controller described in the "Junkbots etc al" book as used in the sumobot design. A DPDT relay is used with the NC points putting the motor in forward. When the relay is triggered the motors go into reverse due to the cross wiring on the DPDT relay. This means that there is no idle. However not having an idle wasn't really an issue on an obstacle avoiding bot. Also as the normal (no power) position of the relay is for the motors to go forward the power consumption by the motor controller circuit would be zero. I even had some nice small 5V PCB telecom relays to use. The other issue was that I was going to have to have two power supplies. 5V regulated for all the ICs etc and 3V for the motors. I considered just using the 5V supply for the motors (and using some diodes to reduce the voltage) but decided against it.

One ideal I tried to stick to was reducing weight and size across the entire design. I wanted Trackbot to be as light as possible as my next bot would be a sumo bot. Given that there are weight restrictions for sumo bots I wanted to see how hard it was to keep weight down.

Next time more about the power board...