Automatic selection of voltage mode control

Features

For some time now, moteus has supported operating in what is called “voltage mode control”. In that mode of operation, the current control loop of the Field Oriented Control process is short circuited. Instead of modulating the output voltage to achieve a desired current, instead it is assumed that there is no inductance and the desired voltage can be selected purely using the phase resistance and back EMF. This is a useful mode of operation anytime the output current range is small relative to moteus’s ability to sense it, often but not always with gimbal motors. The drawback of the mode was that:

  1. there is no torque control
  2. you had to know it existed
  3. you had to manually select it

However, as of release 2025-03-27 / pypi 0.3.77, moteus_tool will now attempt to automatically select voltage mode control if it looks like the motor is high resistance relative to the controller being used.

Implementation

To make this work, during the calibration process moteus_tool measures the amount of random current sense noise captured by moteus. Then it compares the magnitude of the eventual calibration current to this noise to create a “current quality” metric that is just the intended calibration current divided by the standard deviation of the measured noise. A threshold is used, below which, voltage mode control is automatically enabled. This will result in comparable results across all moteus boards, including the moteus-c1 which has a reduced current sense range (re higher accuracy), or in cases where custom current sense resistors have been installed.

This is actually the same metric used to determine if current mode commutation calibration can be used, as documented in this post.

How to use it

For this, all you need is the most recent moteus_tool. To get it, merely update your host side tools:

python -m pip install --upgrade moteus

You can still change servo.voltage_mode_control manually either way after calibration has completed if your application needs it the other way than the auto-detected version.