It has been quite some time since moteus gained the ability to be controlled or monitored over a different communication channel. In fact, the last time was with the permanent switch from RS-485 to CAN-FD way back in 2019 with the first moteus-r4. So, I’m happy to share that as of firmware 1.1.2 moteus now has a second supported communication transport out of the box, logic level UART!

Why UART?

Often it is useful to deploy moteus in a system where it is controlled and monitored by a microcontroller, there is only a single moteus controller, and the total distance between the microcontroller and the moteus is short and without significant electromagnetic interference (EMI). In those cases, using a logic level UART on the microcontroller can save the additional complexity and cost of integrating a CAN-FD controller and transceiver into the host system.

Sometimes a bug report comes in that is simple. Sometimes a bug report comes in that is complex. This is the story of a bug report that looked very simple, but in the end was actually much more complex than I had initially expected. Further, it likely contributed to electrical damage on many deployed moteus boards, which is definitely not something I aim to do! Read on to learn more.

I’ve got another very short post today, a new product announcement, the XT90H-M from AMass:

This is an XT90H-M, which mates to the XT90F-S that was already carried. It does not mate to any mjbots products directly, but can be handy if you are building harnessing in a system.

Here’s a small new addition to the mjbots product portfolio, the mjuart - a FT231X breakout board, now available at mjbots.com.

This USB to 3.3V level UART adapter includes appropriate cables to connect to all of the possible auxiliary ports on each moteus controller:

• moteus-r4 ABS: A ZH4 to ZH4 cable with RX/TX swapped
• aux1: A ZH4 to GH8 cable
• aux2: A ZH4 to GH7 cable

The FT231X can communicate without problems at up to 3 Mbps, which lets you capture high rate debug information from a moteus controller. It also works just fine with the yet-to-be-announced UART based control at its default baud rate of 921600 (or up to 3Mbps if you like living on the edge).

When developing features for the moteus controller, testing on actual hardware has long been the primary way that new motor control features were validated. That includes “hero” testing on the lab bench, and the hardware-in-the-loop (HIL) dynamometer fixture used to validate each new firmware release.

While those are valuable techniques, dynamometer testing is not something that you can do rapidly, is limited to validating a small number of things at a time, and has a non-zero fraction of false positive failures due to unmodeled effects. To enable more rapid feature development, I wanted to test a lot of what was validated on the dynamometer fixture in a host side simulation. This would provide a lot of benefits:

• It could run much faster than real time
• 100% deterministic
• Multiple instances can run in parallel
• Agents can use it as success criteria for validating development

Read on to see how it was done:

When using a servo controller, any time the motor needs to either decelerate or resist an external torque, the controller may “regenerate” energy that has to go somewhere. By default, moteus will apply that energy back to the input bus, potentially charging a battery if that is used as the power source. When the input voltage rises too high, either because the battery impedance is high or because a PSU is as the source, then moteus will apply “flux braking” to dissipate the energy in the windings of the motor instead of allowing the voltage to grow without bounds. This often works great, but in some common scenarios flux braking by itself can be inadequate. In order to operate stably, flux braking need to use a filtered version of the bus voltage. This means that if the bus voltage rises fast enough, the maximum configured voltage can be exceeded before flux braking has had a chance to actually begin dissipating significant energy in the motor.

Hot on the heels of BiSS-C encoder support intended for high end applications, I am excited to announce that moteus now supports the RLS Orbis SPI protocol for lower-end systems as of firmware version 1.0.0.

The RLS Orbis is an absolute magnetic encoder suited for use with diametric ring magnets and can be configured to use a number of different communication protocols. If you have a moteus-n1 or moteus-x1, then the newly released BiSS-C support already lets you communicate with them. However that left out the moteus-c1, which does provide the 5V necessary for an Orbis to function, but does not have a built-in RS422 transceiver.

The first public version of moteus-r4 was announced way back in 2019. Since that time, there have been many new board variants and innumerable additional features and capabilities. Amazingly, through all of that, the CAN-FD wire protocol has actually remained backwards compatible the entire time! It is still possible to monitor that very first moteus with tview, and to command it using the register protocol with appropriate configuration. Many projects hem and haw about releasing a version 1.0 of their software, justifying breaking things from a users perspective over and over again. Here, we’ve actually maintained nearly complete backwards compatibility despite having a pure time based release versioning scheme.

I’ve previously written about the moteus performance analysis tool and how it can be used to evaluate different motor control systems for performance purposes. The tool described there is entirely a web based application that requires clicking around in the HTML interface in order to configure an analysis.

Now, there is newly added feature in the main moteus git repository that provides a command line interface (CLI) to the analysis kernel! This capability lets you run sweeps through various parameters or motor configurations much more easily than before, or even wrap the analysis in an optimization loop without resorting to browser based automation techniques.

Since the fdcanusb, and subsequently the mjcanfd-usb-1x, have been released, the only interface they presented to a PC host was a USB CDC (communication class device). That meant they appeared as a modem, on linux as /dev/ttyACMx and on Windows as COMx. Sending and receiving frames was pretty simple using a basic line based protocol.

This works fine as long as you are either using the mjbots tools, or designing a complete custom application. However, if you want to use the adapter with other CAN or CAN-FD based applications on Linux, there wasn’t a whole lot you could do. There was a simple daemon which, with a fair amount of effort, would make the hardware appear as a slcan device, but that was finicky and hard to get working and not terribly reliable.