The moteus controller, when it implements its control algorithms, uses the internal RC oscillator of the onboard STM32G4 microcontroller to calculate things like velocity and to advance position over time. Typically, this is accurate to within 0.5% which is more than sufficient for most applications. However, there are cases where it does matter.

One common case is when multiple moteus controllers are operated together, and either the relative velocities of the controllers must match closely, or the time required to complete long trajectories must match closely. For example, if a trajectory would take 100s to complete, then a 0.5% difference in clock rate between two controllers would result in one completing 0.5s before the other.

One of the oldest requested features for the moteus brushless controller has been a form of trajectory control beyond constant velocity trajectories. For most applications this is not an actual deal-breaker, because arbitrary trajectories can be approximated by piecewise linear constant velocity trajectories in the application layer. However, for many people, that is big hurdle to jump over to start with, and for some, it can actually limit application effectiveness because a fair amount of CAN bandwidth is required to achieve the high rate control necessary for smooth motion.

Being a switch mode 3 phase motor driver, the moteus controller changes the current flowing through the phases of a motor by rapidly switching the phase terminals between the positive input voltage and the input ground. The control of this switching is denoted “pulse width modulation”, or PWM for short. To date, the rate at which it has switched has been fixed in firmware at 40kHz. As of release 2022-03-12, this can now be altered anywhere between 15kHz and 60kHz to better optimize peak power capability, control bandwidth, maximum speed, and heat generation.

With the r4.8 release of moteus, a not-yet-announced feature was included – the ability to have an off-board primary encoder! It didn’t get announced at the time, because the connectors necessary to populate the board were not obtainable. In fact, that is still the case, but I’ve located a substitute part which works well enough, so here we go!

Theory

The moteus controller uses an absolute magnetic encoder to determine the relationship between the rotor and stator of the motor at each given instant. That allows it to produce torque in the motor at any speed, from standstill to the maximum possible speed. Until now, the only magnetic encoder that was supported is the one mounted to the backside of the board. This is largely acceptable, as moteus is intended to be used in integrated applications.

Now that 64 bit Raspberry Pi OS is a real thing, I finally got around to updating the pi3hat libraries. Now, as of pypi 0.3.20, you can use the pi3hat library to run console only or graphical tview on the most recent Raspberry Pi OS!

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.