If you look around online, there are lots of examples of PCB test fixtures used to perform end of line testing. In the low to medium volume scale, nearly all of these are either clamshell or 2 side affairs, where probe pogo pins or interfaces are connected to the bottom and top of the board.

When developing the moteus-n1, one of the challenges was the number of right angle board edge connectors it has. Those right angle connectors are what allow it to maintain a very low overall stack height when installed in applications, but are also much harder to perform testing on, since by definition the access points are not vertical. On the base n1, there are 6 total right angle connectors, 2 on each of 3 sides, and future variants may have additional bottom side CAN and power connectors populated to make 8 total right angle connectors.

moteus r4.11 has two external connectors, the ABS connector (AUX2) and the ENC/AUX1 connector. The ABS connector was designed initially just to have 2 I2C pins. The ENC connector just has the random pins that were used for the onboard encoder SPI plus one more. Thus the range of external accessories that can be connected is somewhat haphazard and not necessarily all that useful.

When working on a more ground up revision of the controller, I wanted to improve that situation to expose more connectivity options on still a relatively limited connector set. The idea was to use 2 connectors, one which has 5 I/O pins and the other with 4 I/O pins. The onboard encoder SPI would still be accessible on the larger connector to use for at least one external SPI encoder, but how much other functionality could be crammed into the remaining pins? To start, lets see what possible options there are in the current firmware and supported by the STM32G4 microcontroller that moteus uses:

While testing moteus controllers, it is often necessary to experiment with high power conditions. For short durations, any decent sized brushless motor can work, as the windings have a non-zero thermal mass and take a little bit to warm up. However, when testing at high power for extended duration, it can be hard to find a way to get rid of all output energy. Even blowing a fan directly onto a motor only gets you so far when you are trying to get rid of 1kW.

Here’s a not-so-brief story about troubleshooting a problem that was at times vexing, impossible, incredibly challenging, frustrating, and all around just a terrible time with the bare-metal STM32G4 firmware for the moteus brushless motor controller.

Background

First, some things for context:

moteus has a variety of testing done on every firmware release. There are unit tests that run with pieces of the firmware compiled to run in a host environment. There is a hardware-in-the-loop dynamometer test fixture that is used to run a separate battery of tests. There is also an end-of-line test fixture that is used to run tests on every board and some other firmware level performance tests.

I documented the first test fixture I built for moteus some time ago. As the shipment volumes have gone up, the fixture became something of a limitation, and also was a little problematic in a few ways.

The old “state of the art”

First, it relied on attaching 3 connectors by hand for each test, which was a decent fraction of the cycle time. Second, the pogo pins it used were non-replaceable, and also connected only to the debug phase wire test vias, which were tiny. They wore out relatively quickly, and replacing them required building a whole new board. Finally, since the pogo pins were PCB mounted, a PCB needed to be printed for any change in the pin locations or which pins to probe.

First, if you haven’t already, check out these introductory posts:

• Part 1: Motivation
• Part 2: Auxiliary Port Configuration
• Part 3: Source Configuration
• Part 4: Sink Configuration

Second, this article is available in video form:

Third, the official reference documentation can be read here. This describes in detail all of the possible configuration values.

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.