Servo - August 2016

THE FUTURE IS BRUSHLESS

Using Open Source Drone Firmware for Robot Drivetrains ● by Charles Guan

It's an indisputable fact that the most critical system in a combat robot is the drivetrain. After all, if you can't move, you're just waiting to be counted out by the referee. Historically, the trend for robot drive motors has been lighter, stronger, and more poweful.

We've come a long way in robot drive technology since the days of cutting windshield wiper motors out of junked cars and rewinding industrial fan motors. Nowadays, we sit at the apex of what good ol' DC brush motors can do.

I found out the hard way at BattleBots™Season 1 in 2015, driving our team's robot, Overhaul that I wanted more. We burnt out several of our Ampflow F30-400 drive motors from drawing too much power. The carbon brushes disintegrated, taking out the motor armature as they did. I was dissatisfied that $300+ motors were limited in their performance by a quarter square inch of graphite.

The best consistent drivers of combat robots know the dynamics of their own machines and use it to their advantage each match. I like drifting around and doing J-turns, and generally being swoopy and unpredictable — a preference gained from driving my 30 lb class bots. Therefore, when Overhaul hit the limits of what we could get from brushed DC motors — even geared for 20 mph — I turned my attention to an emerging corner of the combat robot drive motor market: the brushless motor.

Brushless motors from the RC model industry have been in use for several years as weapon motors, and they have revolutionized the smaller weight classes (1 lb through 60 lb). They offer immensely improved power to weight ratios compared to DC brush motors — even high performance thoroughbred ones like Ampflows. The missing link was control. Generally, RC motor controllers rely on voltage sensing to start and run the motor; so called "sensorless" control. This means the motor has a minimum speed under which it will not behave, since motors generate voltage only while in motion. Yeah, your motor has to be moving before it can move. (I know, right?!) Industrial control systems tended to be very large and expensive since they are designed for precision motion control of, dare I say, real robots. I was looking to strike the middle ground: light weight, good low speed performance, and low cost. During late 2014 and 2015, some small bot builders had begun experimenting with brushless drive using RC controllers modified with open source firmware. These custom firmwares such as SimonK and BLHeli were developed because the builders of high performance multi-rotor drones were dissatisfied with the existing airplane controller’s smooth and gentle behavior.