The HomeBrew Robotics Club (HBRC) invited Chris Tacklind and Cameron Tacklind to demonstrate their first simplified implementation of a low cost brushless motor and control system. This enthusiastic, self-funded father and son team came up with the idea of using cheap but powerful hobby airplane motors.
Model airplane brushless motors are powerful with an excellent weight to power ratio. In model airplanes, they come with simple controllers that lack precise motion control. The challenge is knowing when to apply power to which motor coil in order to produce maximum spin of the rotor. Traditionally this is solved by adding hall effect sensors. The hall effect sensor responds to the magnetic field in the rotor and controls the drive motor timing. Another way is to have a rotational encoder. HBRC has posted the entire presentation on their YouTube channel:
The potential here is to have powerful, cheap model airplane motors that cost $50 for both the motor and controller combined, that have enough precision to be useful for robotics projects. A MELEXIS MLX90316 for about $3 (@3k units) will give the direction of a motor if it has a weak magnet on the axis. The phase should be at 90 degrees for maximum power just like a bicycle crank. Using the bicycle example, when the pedal is pushed down while the crank is at 90 degrees (horizontal) the maximum amount of force is applied. This is why it is important to know the precise direction of the rotor and to have a control system with adjustable timing.
So with a square wave and low speed the brushless motor is able to provide a constant torque. Without any gears or transmission, there is constant, very low speed rotation. With a simple exoskeleton that turns off the motor so the user readjusts their starting position this can be extremely useful! There is no re-zeroing needed because the chip will always give the direction of the axis. This also simplifies the electronics compared to a 4000 position encoder.
This project can be used in wearable robotics and exoskeletons. It demonstrates how much interesting work can be done at the home-grown, DIY level. It is very possible that in the future there will be large collaborative homemade exoskeleton projects where exoskeleton enthusiasts from all over the world work together on a single device.
Limitations of the current project iteration:
- RPM limit of about 500 RPS (revolutions per second)
- FET optimization needed for drive resistance and dead time
- Optimized phase advance
- Open source distribution plan
Additional files on the open source 3 phase sine wave brushless motor controller (WIP) can be found at GitHub:
For more information on the HomeBrew Robotics Club please visit their website at: