A stack of 4 power-stages with a compute module on top. Each power-stage in this design could do about 14 A continuous and 26 A peak. Although I didn’t do a lot of testing we didn’t see any degradation in performance when stacking boards. This 4 stack could do ~56A continuously and ~104 A peak at 48 V.

Stack

c. 2013 ~ 2014

While working on custom motor controllers for the various prototype actuators and robots I started splitting the microcontroller and interfaces from the high-power components (power mosfets, gate-driver and phase-current sensors). This did two things for us:

  1. Make it easier to iterate the design faster. Splitting the board meant we could change microcontrollers, interfaces and sensors (IMUs for example) without wasting money on the power-stage components. Conversely this meant we could do the same with the power-stage. It also made it a lot cheaper to repair - if a power mosfet burnt out we’d just replace the power-stage board.

  2. Help make the board as compact as possible by stacking the two PCBs. Stacking the two halves of the system meant we could nestle the connectors between the boards which meant that the overall design barely grew in height but was significantly shorter.

These two features really helped us hone the design performance and made it a lot easier to integrate quickly into new actuators and robots.

One day while playing around I got the idea to just keep stacking the power-stages. Since the design split the circuit at the PWM and current signals of the power-stage (see diagram) it was easy to hack in a connector to the bottom of a power-stage to send the PWM signals to another power-stage board. I could then combine the motor phase outputs of each power-stage so that each would share the load to drive the motor. While I didn’t have a lot of time to really put it through its paces, we found that you could stack power-stages without any performance degradation (at least up to 12 boards - the most I tried).

Several of our partners utilized the motor controllers for their projects. Here it is in a biped from Georgia Tech and SRI.

Yamaha Motor used it for their MOTOBOT testing platform. You can see two of the stacks along the side of the robot in the upper right of the image.

Another view showing how things stack. We also designed the heatsinks to stack.

The most I tried was 12 power-stages stacked. To help spread the current I tapped the phase outputs in 3 spots (the thick black wires on the top, middle and bottom). You can see this in action driving the sooter project.