The most recent moteus firmware release, 2021-12-03, added not one, but two new control modes for less common applications. Previously, mentioned was the “voltage_control_mode” for using gimbal style high resistance motors without changing the sense resistors. In this post, I’ll describe a similarly named, but very different mode “fixed voltage mode” for operating brushless motors as if they were a stepper motor.

For some applications, you don’t care about torque control, or about power consumption at all. Traditionally you would use a stepper motor in those applications, with a correspondingly less expensive stepper motor driver. However, in some cases you may still want to have high rate trajectory control, CAN based telemetry, or have already standardized on moteus controllers for other moving parts of your solution. There are two new options that can be used in such situations:

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.

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 pi3hat, among other things has 5 CAN-FD ports. You can use them to drive a lot of moteus servos, but they are perfectly fine CAN-FD ports generally. The C++ library has always been able to send and receive arbitrary frames (and recently at arbitrary bitrates), but the python interface was lacking, only exposing a portion of this functionality.

As of version 0.3.11, the python library (pip3 install moteus-pi3hat) now exposes everything you need to be able to send and receive arbitrary CAN frames from any of the ports, as well as configure all the timing options for waiting for responses from slave devices.

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.

Since the first public release, moteus has always calibrated motors so that a positive command is equivalent to a fixed sequencing of the phase wires. That means that depending upon which order you solder the phase wires, a positive commanded velocity will result in the motor spinning either clockwise or counterclockwise.

As it turns out, since moteus has an absolute encoder that is immune to such vagarities, it is much more convenient to normalize the direction of rotation around the encoder rather than the phase wires. As of release 2021-04-26, and moteus_tool 0.3.22, that is exactly what moteus does.

The moteus controller uses an absolute magnetic encoder to sense the position of the rotor in order to conduct field oriented control of the motor. In many applications, this sensing is also sufficient to measure the output as well, particularly in direct drive applications. However, if the controller is driving the output through a gear reduction, multiple turns of the input are necessary to make one turn on the output. At power on, this results in an ambiguity, where the controller doesn’t know where the output is.

Some of the new features in the moteus 2021-04-09 firmware release are new limits that can make the overall system more robust to faults or environmental conditions.

Velocity limiting

config: servo.max_velocity

Like the position limits, there now is a configurable velocity limit. If the motor is moving faster than this limit in either direction, then the applied torque will be limited, eventually to a value of 0. This can be used to reduce the likelihood of runaway behavior in systems where high speeds are not expected.

Since the moteus controller was first released, it has implemented a two-stage controller. The outer loop is a combined position/velocity/torque PID controller, which takes as an input a position trajectory, and outputs a desired torque. The inner loops accepts this torque, and uses a PI controller to generate the Q phase voltage necessary to achieve that torque.

Until now, the constants for that PI controller were left as an exercise for the reader. i.e. there were some semi-sensible defaults, but the end-user ultimately had to manually select those constants to achieve a given torque bandwidth. That isn’t too much of a problem for a sophisticated user, but for the rest of us, it is hard to know how to go from a desired torque bandwidth to reasonable PI gains.