The perils of technical videos is that they can become out of date pretty quickly. For the moteus getting started guide, it has amazingly held up pretty well, but enough has changed since 2021 that it is time for a new one. Now we can use acceleration and velocity limited commands instead of the deprecated stop position and get nicer 4k b-roll for all the intermissions. Everything in the old video, and in this new one, is applicable to both the moteus-r4.11 and the moteus-n1.

Back last year, I walked through bisecting and debugging an annoying problem that caused the STM32G4 ADC on the moteus controller to exhibit higher than expected noise in result largely to either the exact placement in flash of the initialization code, or to the exact timing of the initialization. While the immediate glaring sharp edge was removed, the resulting performance still was confusing to me, and looked like it was not yet optimal. Further, a moderate percentage (2% or so), of production boards failed end-of-line tests related to the current sense noise in ways that were hard to fix by swapping components. Because of this, I wanted to dive in and investigate further. This is that process.

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.

I don’t normally post about moteus firmware releases, but there have been many in the last few months that fix some long standing bugs and regressions. First, the current release as of this post is:

• github: 2023-06-22

With that out of the way, here some of the fixes:

Position drift with non-unity gear reduction

It is embarrassing how long this issue has been outstanding, but ever since the flexible I/O system was introduced, there has been an issue where if the gear reduction was configured to a value other than 1, then the output position would drift from the source encoder over time. If control was not active, you would see the position drift, and if control was active, then moteus would report the correct position, but the actual position would drift.

This is a short and sweet post. The moteus-n1 as previously announced in beta form is now officially available at mjbots.com!

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:

With the release of more flexible I/O support, the moteus controller auxiliary port can be used to monitor encoders using an onboard UART. Now, with firmware release 2023-02-01, those UART pins can be used as an arbitrary logic level serial port controlled by the application! Let’s see how to use it below.

First, you will need to look at the pin configuration table to find pins that support UART functionality, and configure them as UART in the “aux?.pins” configuration tree. Next, “aux?.uart.mode” should be set to “kTunnel”, along with the desired baud rate. That’s it on the configuration front.

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.

Partly to celebrate moteus controllers being back in stock and partly because a lot of important work has backed up, we’ve just released a new firmware version for moteus (2023-02-01) that has a little bit of something for everyone. Future posts will examine some of these features in more detail, but for now you just get the bullet list:

• Support sending and receiving arbitrary data from a UART configured on either of the auxiliary ports

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.