A permanent magnet motor controller like moteus has to, at the end of the day, apply voltages to the phase wires of a motor in order to induce currents. Those currents generate magnetic fields that push against permanent magnets in the rotor to make the motor move. I’ve looked at parts of this process before, see “Compensating for FET turn-on time”, but in this post we’ll look at an additional technique that can extend the effective modulation depth, thus increasing the maximum speed that a motor can be driven.

Here’s a bit more in-depth discussion of a yet another new feature from moteus firmware release 2024-10-29: pulse width modulated output on auxiliary ports.

A pulse width modulated signal is a logic level signal of a fixed frequency, where the duty cycle, or width of the pulse, is changed or modulated to communicate a scalar value. Obligatory Wikipedia diagram below:

The new feature does what it claims to do, in that a subset of auxiliary pins can now be configured to output a PWM signal. If so configured, the duty cycle can be controlled using either the diagnostic or register protocol.

When operating a moteus controller with a brushless motor there are two main things that can get hot: the FETs (field effect transistors) on moteus and the motor itself. By default, moteus has built in thermal throttling and fault detection if the FET temperature exceeds rated limits. In many configurations, the motor can be thermally connected to the moteus controller, so that the same FET temperature sensing can be used to prevent damage to the motor, but that isn’t always the case.