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.

Here’s yet another update to the moteus line, moteus r4.11!

r4.11 is electrically, mechanically, and software compatible with r4.3, r4.5, and r4.8.

This revision supports two alternate footprints for the CAN-FD transceiver to better support component availability and refines the power stage for the DRV8353 gate driver. moteus r4.8 was the first version to use the DRV8353 because of, once again, component availability issues. However, it was developed on a very abbreviated schedule. With r4.11 the EMI is much improved over r4.8 and r4.5, and the efficiency is much better than r4.8 at all input voltages and PWM frequencies.

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.

Some time ago I wrote about using the wcubed vise for the Pocket NC. While I don’t end up using it very often any more, mostly because I rarely work with rectangular stock, it can be useful from time to time. Unfortunately, it is no longer manufactured. In case anyone is interested in replicating it, I’ve taken at least a minimal stab at modeling it up based on measurements of my unit along with necessary hardware as picked from McMaster. I suspect the model should be good enough to get something that works.

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!

With the Artisan’s Asylum closed for a relocation, I’ve been without access to a manual lathe for a while. Fortunately, import mini-lathes aren’t that hard to come by!

This is a Sieg C4 derivative from Little Machine Shop, which was about the largest machine I could reasonably move into my basement.

It isn’t as rigid as the Colchester at AA was, but it does have power feed and power cross feed which both work just fine. I’ve run into a few minor quality issues, and the spindle runout isn’t great, but it should do for my needs.