TLDR: moteus can now filter the encoder, resulting in less audible noise. Use firmware version 2021-04-20 and ‘pip3 install moteus’ version 0.3.19, then re-calibrate to get the benefits.

Background

The moteus controller uses an absolute magnetic encoder to measure the position of the rotor. It uses this knowledge to accurately control the current through the three phases of a brushless motor so that the desired torque is produced, i.e. “field oriented control”. This works well, but has some downsides. One, is that magnetic encoders work by sensing the magnetic field produced by a “sensing magnet” that is somehow affixed to the rotor. This sensing process always introduces some noise, so that the sensed rotor position is never perfect.

In what probably won’t be the last time, mjbots.com now has a new version of the pi3hat due solely to component shortages!

There are no top-level feature changes versus the previous r4.4. What is different is that the connector for the Raspberry Pi has reverted to the fixed height version that the r4.3 and earlier used, and the IMU is slightly better.

Happy building!

Since the first public release, moteus has always calibrated motors so that a positive command is equivalent to a fixed sequencing of the phase wires. That means that depending upon which order you solder the phase wires, a positive commanded velocity will result in the motor spinning either clockwise or counterclockwise.

As it turns out, since moteus has an absolute encoder that is immune to such vagarities, it is much more convenient to normalize the direction of rotation around the encoder rather than the phase wires. As of release 2021-04-26, and moteus_tool 0.3.22, that is exactly what moteus does.

The moteus controller uses an absolute magnetic encoder to sense the position of the rotor in order to conduct field oriented control of the motor. In many applications, this sensing is also sufficient to measure the output as well, particularly in direct drive applications. However, if the controller is driving the output through a gear reduction, multiple turns of the input are necessary to make one turn on the output. At power on, this results in an ambiguity, where the controller doesn’t know where the output is.

Some of the new features in the moteus 2021-04-09 firmware release are new limits that can make the overall system more robust to faults or environmental conditions.

Velocity limiting

config: servo.max_velocity

Like the position limits, there now is a configurable velocity limit. If the motor is moving faster than this limit in either direction, then the applied torque will be limited, eventually to a value of 0. This can be used to reduce the likelihood of runaway behavior in systems where high speeds are not expected.

There are a lot of steps necessary to get a product to market, not just a fancy render. I admit to being far from covering all the bases yet, but we’re getting there. In that spirit, I recently upped the packaging game of the qdd100 with some custom boxes and foam inserts. Pick one up at mjbots.com!

Since the moteus controller has been seeing a lot of updates lately, I decided to make a separate hackaday.io page to track its progress in one place. Check it out!

https://hackaday.io/project/179234-moteus-brushless-controller

Weekly Robotics is a great robotics newsletter I follow authored by Mateusz Sadowski. Every week he posts up to date research notes, industry happenings, and great robot videos. Next week, on Thursday 2021-04-23 I’ll be presenting at the sibling “WR Community Meeting” and hosting a live Q&A session. Sign up with the eventbrite link below!

• Weekly Robotics Community Meeting #7 - mjbots with Josh Pieper

Since the moteus controller was first released, it has implemented a two-stage controller. The outer loop is a combined position/velocity/torque PID controller, which takes as an input a position trajectory, and outputs a desired torque. The inner loops accepts this torque, and uses a PI controller to generate the Q phase voltage necessary to achieve that torque.

Until now, the constants for that PI controller were left as an exercise for the reader. i.e. there were some semi-sensible defaults, but the end-user ultimately had to manually select those constants to achieve a given torque bandwidth. That isn’t too much of a problem for a sophisticated user, but for the rest of us, it is hard to know how to go from a desired torque bandwidth to reasonable PI gains.

On the mjbots discord, people are often looking for the cheapest possible way to command and monitor a moteus controller. One possible solution that comes up over and over again is the CANBed-FD board, as sold by Seeed Studio (and others). I decided to get one of these in house to see if I could make it work:

The first thing I figured out was that the DB-9 connector used a non-standard pinout for CAN_L and CAN_H. I just switched to the terminal block connections instead of the DB-9 to get around that. For the software, I decided to use the acan2517FD Arduino library, as it was quite a bit more robust and featureful than the one provided by Longan Labs.