Probably one of the most frequently asked questions in the mjbots Discord is “how fast can I send new commands to moteus”, or “how fast can I read the status from moteus”. That may be because you want to perform torque based control in your application and require high bandwidth, or just because you have a high torque to inertia ratio system that reacts on very short time-scales. No matter the reason, the principles that control the maximum rate you can send updates are the same.

The pi3hat repository, in addition to hosting a library for sending raw CAN frames using the pi3hat, has long had example source code demonstrating how to use that to communicate with a moteus controller. Recently however, C++ bindings were officially added to the primary moteus repository, that provide a more consistent, flexible and complete interface to moteus controllers. I’ve gone ahead and updated the pi3hat library to take advantage of that new interface, so now the pi3hat can be treated as just another transport like the fdcanusb.

In the initial announcement of the new moteus C++ library, the only transport that was supported out of the box was for the mjbots fdcanusb. That has now been upgraded so that Linux socketcan devices are supported out of the box as well!

You need version mjbots/moteus e33f188b035c7 or newer and for simple cases, no changes to source code are required at all.

For a long time, moteus has had a python library, that lets most people quickly develop motion control applications. However, python is not a suitable solution in every domain. Embedded systems often either don’t have access to python, or cannot tolerate the resource or timing penalties it imposes. In some cases, managing interoperability between python asyncio and systems using legacy python IO like ROS can be challenging. For others, python is just not familiar. The new C++ client library included in the moteus repository is designed to handle all of these scenarios with a flexible, yet ergonomic approach that makes simple things simple, and complex things possible.

To date, all of the development tools for the moteus brushless controller have been available exclusively for Linux based operating systems. I’ve been doing some behind the scenes work, and have gotten to the point where moteus_tool now runs natively on windows and can communicate with moteus controllers using a fdcanusb.

Check out the Windows installer for the latest release:

• moteus_host_tools_0.1.2020.1106.msi

To make this work, I started from the excellent grailbio/bazel-toolchain, which provides LLVM toolchains for Linux based systems based on the official LLVM pre-compiled binaries. I forked that into mjbots/bazel-toolchain and added Windows support. It isn’t perfect, because the LLVM project only distributes Windows binaries in installer form, and it isn’t possible to extract binaries from them without specialized tooling. So, this version relies on a manually re-packed compressed archive of all the executables.

To stay on top of bazel development, and to prepare for some future improvements, I’ve gone ahead and upgraded the moteus firmware build system, rules_mbed to use the new bazel “platforms” toolchain resolution mechanism.

Previously, rules_mbed used the “crosstool_top” bazel mechanism for toolchain configuration. This allowed a single package to contribute a set of C++ toolchains which would be selected based on CPU and compiler. One of the downsides from the rules_mbed perspective, is that it made it difficult to make a build that included both mbed targets and host targets (or anything else non-mbed). rules_mbed worked around this by including a functioning clang host toolchain within it.

Last time I covered the new software library that I wrote to help use all the features of the pi3hat, in an efficient manner. This time, I’ll cover how I measured the performance of the result, and talk about how it can be integrated into a robotic control system.

Test Setup

To check out the timing, I wired up a pi3hat into the quad A1 and used the oscilloscope to probe one of the SPI clocks and CAN bus 1 and 3.

The pi3hat r4.2, now in the mjbots store, has only minor hardware changes from the r4 and r4.1 versions. What has changed in a bigger way is the firmware, and the software that is available to interface with it. The interface software for the previous versions was tightly coupled to the quad A1s overall codebase, that made it basically impossible to use with without significant rework. So, that rework is what I’ve done with the new libpi3hat library:

In the previous issue in this series, I described the schema and data elements of the mjlib serialization format.  Here, I’ll describe the API used to convert between C++ structures and the corresponding schema and data serializations.

First, I’ll start by saying this API is far from perfect.  It hits a certain tradeoff in the design space that may not be appropriate for every system.  I have developed and used similar APIs professionally both at Jaybridge and TRI, so it has seen use in millions of lines of code, but not billions by any stretch.  It is also mostly orthogonal to the rest of the design, and alternate serialization APIs could be built while still maintaining the performance and schema evolution properties described in parts 1 and 2.  Now with that out of the way, the library API:

I used Dear Imgui for the simple Mech Warfare control application I built earlier and was relatively impressed with the conciseness with which one could develop effective (although not necessarily the prettiest), interactive and response user interfaces in C++.  For some time I had been planning on developing a new diagnostic application for the mjbots quad that would allow plotting like the original tplot.py, but would also integrate recorded video and 3D rendering and diagnostics.  I had assumed I would use HTML/JS because it is the cool new thing, but I never got up the energy to make it happen, because every technical step along the way had big hurdles.  I figured I would give Dear Imgui a try, but the big thing it was missing was plotting support.