The most recent moteus firmware release, 2021-12-03, added not one, but two new control modes for less common applications. Previously, mentioned was the “voltage_control_mode” for using gimbal style high resistance motors without changing the sense resistors. In this post, I’ll describe a similarly named, but very different mode “fixed voltage mode” for operating brushless motors as if they were a stepper motor.

For some applications, you don’t care about torque control, or about power consumption at all. Traditionally you would use a stepper motor in those applications, with a correspondingly less expensive stepper motor driver. However, in some cases you may still want to have high rate trajectory control, CAN based telemetry, or have already standardized on moteus controllers for other moving parts of your solution. There are two new options that can be used in such situations:

There’s an updated release out for the mjbots pi3hat which features a useful bug fix and a minor feature.

First, the configuration of automatic retransmission was broken in several ways. The symptoms for previous versions would be that any attempt to change the CAN configuration for a bus would result in all channels attached to that controller having auto-retransmit turned off. Since it was by default on, that would typically result in a decrease in transmission reliability, and in some particular cases with arbitration conflicts, could result in frequent packet loss. Fixing this requires updating both the firmware on the device and the C++ and python libraries. Fortunately, this would only come up if you were communicating with non-moteus devices using the pi3hat, which likely isn’t that common.

The moteus brushless controller can drive many motors out of the box, but until now it has been challenging to use with gimbal style brushless motors. They are wound with thin wire so that they have a very high winding resistance, and thus can be driven by inexpensive low current controllers. Using something like moteus with a gimbal motor isn’t absolutely necessary, but does give benefits in terms of high performance trajectory tracking and torque control.

The initial implementation of auxiliary encoders for moteus supported exactly one encoder, the AS5048B. The hardware can support any I2C based encoder, so supporting additional encoders has always been on the TODO list.

I’m excited to announce, that as of firmware release 2021-12-03, AS5600 encoders are now supported as well. They are a lot cheaper than the AS5048 as they have a much lower update rate and resolution, but that isn’t necessarily a problem if it is only used to disambiguate a modest gear reduction.

To use the moteus brushless controller with a motor, you first have to calibrate it with moteus_tool (for history, see “Encoder autocalibration” and “Auto-tuning current control loops”). This calibration process is primarily used to measure the mapping between electrical phases and the encoder, but as a secondary parameters also measures the winding resistance and Kv of the motor and determines the parameters necessary to set the current control bandwidth.

Motivation

To date, this process can be used with any motor, but making it work can involve fiddling with a number of inscrutably named command line parameters to moteus_tool. --cal-power, --cal-voltage, and --cal-speed are all there, however they don’t really do what you think based on their name, but it is necessary to adjust them to make many motors work.

I made an overview video for my Pocket NC touch probe integration!

The original write ups can be found at:

• Part 1 - Motivation
• Part 2 - Mechanical Hardware
• Part 3 - Electrical
• Part 4 - Software

And the designs and source code can be found at:

This is a series, check out the previous posts at part 1, part 2, or part 3. This time, I’m going to make this probe hopefully do something.

I started out just verifying that the Pocket NC would treat the vers.by probe the same as the built in tool setter probe. So I ran a tool measure cycle, and then just tweaked the probe by hand. Woohoo! It stopped the cycle just like normal. Actually measuring a tool worked too if the probe wasn’t activated, or if it wasn’t plugged in. Success.

In part 1 and part 2, I covered my motivation and the mechanical hardware behind a touch probe add-on for my Pocket NC V2-50. In this post, I’ll cover my prototype electrical hardware.

My intention with the probe was to connect it logically “in parallel” with the existing tool setter probe that the Pocket NC has. I figured that would be likely easiest to integrate with the Linux CNC scripts when I got to the software point. The existing tool setter probe is located in the rotating B axis. That is connected to the Y axis via a single CAT5-ish cable, so my hope was that I could devise something which would pass through the necessary signals on that cable while also paralleling in the new touch probe.

Last time in part 1, I talked about why I wanted to add a touch probe to my Pocket NC. This time, I’ll cover the basic hardware necessary to make it happen.

I decided to start with an inexpensive probe so that as I was figuring things out, I wouldn’t be too sad if I smashed it a few times. I’ve seen a number of other hobby machinists use the “vers.by” probes, so I decided to give them a try too.

When machining, you need to accurately position the cutting tool with respect to the workpiece. With the stock Pocket NC, there are two methods for doing so. The first is to rigidly locate the workpiece with respect to the B axis reference point using a fixture. The second, is to do manual touch offs. Nearly all of my work so far has relied on the former method, as using a manually touch off on a machine without manual controls isn’t all that precise or pleasant. And while possible, it is tedious to touch off against features more complicated than a single edge.