This will be the final post describing the fundamentals of configuring the new flexible I/O system. There have been a number of previous posts (part 1, part 2, part 3). In this iteration, we’ll cover how to configure the sinks that consume the “source” encoder data. As a reminder, the block diagram of the I/O system looks like:

Commutation

To perform commutation with field oriented control, moteus needs to know the relationship between the rotor and stator in the magnetic domain. With the addition of the new flexible I/O system, some of the configurable values associated with this remain as they were, where there are some new ones.

In the last two posts (part 1, part 2), I started talking about the new, more flexible I/O subsystem for the open source moteus brushless motor controller. In this post, I’ll continue by describing what a “source” is, and how it is configured.

For reference, the block diagram showing how auxiliary ports, sources, and sinks are related is below:

Each “source” in the above diagram represents a single encoder. To the sinks it provides a position and velocity, along with various validity indications for that data. Each has three basic configuration components: where to get the raw data, how to transform that raw data, and the low-pass filter configuration. We’ll cover each in turn.

In the last post, I covered the goals behind more flexible I/O support in the moteus brushless controller. This time, I’ll start to cover the configuration model that I implemented to make that support work. It is broken up into 3 distinct phases, auxiliary ports, sources, and sinks.

Auxiliary port pin configuration

To begin with, the available connectors and external pins on moteus are organized into “auxiliary ports”. For the moteus r4.3/4.5/4.8/4.11, the correspondence is that the external primary encoder connector, if present (r4.8 and newer), is “auxiliary port 1”. The ABS port and some on-board debug pads are “auxiliary port 2”. For each port, there are two levels of configuration, at the pin level and the function level.

The moteus controller, being a brushless servo drive, needs to use encoders to measure things like how the rotor is positioned relative the stator, and possibly output shafts that have passed through a reducing stage. The support for this has gradually expanded over time, but is still relatively limited as far as those things go. The available options are:

• Primary encoder (used for commutation)
  • The onboard AS5047P
  • An external AS5047P
• Auxiliary encoder (optional, for measuring the output shaft)
  • I2C AS5048A
  • I2C AS5600

However, the moteus hardware has always been capable of more, both because the processor is a very capable one, and the exposed IO pins are relatively flexible. While looking at some future designs that incorporate even more IO options, I decided it was time to update the firmware to finally start taking advantage of that flexibility.

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.

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.

The moteus controller, uses an absolute magnetic encoder to sense the position of the rotor and thus be capable of field oriented control FOC of brushless motors. To date, all the iterations of the controller have used the AS5047P encoder from ams. This is relatively common, works fine over SPI and hasn’t caused any problems. While investigating some other issues, I decided to take a stab at trying some alternate encoders. First, I tried the AS5047U, which is the same basic encoder, but incorporates a digital filter. I also tried the MA732, from Monolithic Power, which uses a different operating principle and also includes a digital filter. The plus side of the MA732 is that it reports full 16 bit values, even if not all of them provide a lot of value.