The moteus brushless controller can drive many motors out of the box, but until now it has been challenging to use with gimbal style brushless motors. They are wound with thin wire so that they have a very high winding resistance, and thus can be driven by inexpensive low current controllers. Using something like moteus with a gimbal motor isn’t absolutely necessary, but does give benefits in terms of high performance trajectory tracking and torque control.

The initial implementation of auxiliary encoders for moteus supported exactly one encoder, the AS5048B. The hardware can support any I2C based encoder, so supporting additional encoders has always been on the TODO list.

I’m excited to announce, that as of firmware release 2021-12-03, AS5600 encoders are now supported as well. They are a lot cheaper than the AS5048 as they have a much lower update rate and resolution, but that isn’t necessarily a problem if it is only used to disambiguate a modest gear reduction.

To use the moteus brushless controller with a motor, you first have to calibrate it with moteus_tool (for history, see “Encoder autocalibration” and “Auto-tuning current control loops”). This calibration process is primarily used to measure the mapping between electrical phases and the encoder, but as a secondary parameters also measures the winding resistance and Kv of the motor and determines the parameters necessary to set the current control bandwidth.

Motivation

To date, this process can be used with any motor, but making it work can involve fiddling with a number of inscrutably named command line parameters to moteus_tool. --cal-power, --cal-voltage, and --cal-speed are all there, however they don’t really do what you think based on their name, but it is necessary to adjust them to make many motors work.

People over in the mjbots discord frequently ask about how to build reliable XT30 cables, so I made a short video describing how I go about it!

This new release makes minor improvements to support for r4.8 moteus boards, notably it makes the Kv and winding resistance calculation more closely match that measured by r4.5 and decreases audible noise when controlling to 0 current or torque.

Get it from github: moteus 2021-09-19

Note, this does require a new version of moteus_tool to be able to flash over CAN, version 0.3.29. You can get it from pypi using any of the normal pip3 methods: https://pypi.org/project/moteus/

A motor driver like moteus switches power to the phases of a brushless motor using a set of 6 (or possibly more), MOSFETs. The typical topology involves 3 high side N channel MOSFETs and 3 low side N channel MOSFETs arranged in 3 half bridges like this:

Since the gates of these FETs need to be driven with potentially high voltages, and you never want the high side and low side to be on at the same time, typically a gate driver is used. For the moteus r4.5 and earlier controllers, the DRV8323 driver from Texas Instruments is what performs this function. This driver lets you configure the drive current for each of the gates for both operations, charging up the gate and discharging it. For high power drive systems, charging up or discharging the gate too fast can result in undesired transients like accidentally switching the other FET on due to capacitive coupling, or inductive ringing as the current starts moving through the FET instead of the body diode. If the gate charges too slowly, then the FET spends much of its time not fully on, which increases power dissipation in the FETs.

I’m excited to announce the release of moteus r4.8!

Due to the ongoing semiconductor apocalypse, this minor release uses some alternate components which were easier to source. It remains compatible with the r4.5 and r4.3 both electrically, mechanically, and with firmware.

For now, the biggest win is that the board and the devkit are actually in stock!

A secondary win is that external primary encoders are now supported, via an unpopulated connector pad on the backside of the board. I’ll write up more about that in a later post.

The moteus controller, communicates exclusively over CAN-FD for command, telemetry, and diagnostics. It will accept either standard or extended frames, and until now, the ID format in terms of bits looked like the following:

33333222222221111111100000000
43210765432107654321076543210
XXXXXXXXXXXXXQSSSSSSSDDDDDDDD

Where:

• X: Don’t care
• Q: 1 for query, 0 for no query
• S: source ID
• D: destination ID

If the lower 8 bits matched the configured ID, all the X bits would be completely ignored and moteus would accept the CAN message as if it were destined for itself. This may not be super desirable, as it consumes nearly all of the available CAN-FD addressing space.

The qdd100 servo uses a planetary geartrain as the transmission reducer. This consists of an outer ring gear, an inner sun gear connected to the rotor as the input, and 3 planets connected to the output. The tolerances of these gears directly impacts the performance of the servo, namely the backlash and noise.

To date, I’ve been hand-binning these and testing each servo for noise at the end of production. To make that process a bit more deterministic, and with less fallout, I’ve built up a series of manual and semi-automated gear metrology fixtures to measure various properties of the gears.

TLDR: moteus can now filter the encoder, resulting in less audible noise. Use firmware version 2021-04-20 and ‘pip3 install moteus’ version 0.3.19, then re-calibrate to get the benefits.

Background

The moteus controller uses an absolute magnetic encoder to measure the position of the rotor. It uses this knowledge to accurately control the current through the three phases of a brushless motor so that the desired torque is produced, i.e. “field oriented control”. This works well, but has some downsides. One, is that magnetic encoders work by sensing the magnetic field produced by a “sensing magnet” that is somehow affixed to the rotor. This sensing process always introduces some noise, so that the sensed rotor position is never perfect.