Tag Archives: moteus

Development of next-gen power_dist (part 1)

robots,

The current iteration of the mjbots power_dist board released back in the summer of 2020 is pretty useful. It pre-charges the input, provides a soft switch, and gives you a bunch of output connectors to make wiring easier.

r3.1 Limitations

However, this version did have some limitations and potential problems. The first is that the pre-charge method it uses, a simple on/off pre-charge resistor, is unable to support a wide range of supply voltages. Either the resistor has a low value, in which case large input voltages will cause thermal failure, or for larger values, it isn’t able to actually pre-charge the bus sufficiently before engaging the primary MOSFET.

Secondly, it switches the negative rail. As pointed out in the documentation and by numerous YouTube commenters, if you are not careful, this can result in magic smoke being released if ground on the output and input is connected in any way.

Third, the protection afforded by the board is relatively limited. It merely performs the pre-charge function. It is still possible for short circuits or over-voltage events to result in damage to either downstream circuitry, or the upstream battery.

Fourth, the quiescent current is larger than I would like. At around 2-3 mA, it isn’t that large, but it means you can’t leave a battery connected for more than a day or so. Even worse, some BMS see that quiescent load as something they need to remain active for, which reduces standby battery life more substantially.

Looking forward

Given those shortcomings, I wanted to see if I could do better for the next revision. In the next several posts I’ll walk through my design process.

moteus position anti-windup

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The moteus controller uses a somewhat unique integrated position / velocity / torque controller with per-command configurable proportional and derivative gains. Through various combinations of these settings, it can emulate many different types of controllers, but one that it has struggled with until now was a pure velocity controller.

It has been minimally possible to use moteus as a purely velocity controlled since wraparound support was implemented, but that came with a caveat. Either the proportional term needed to be set to 0, in which case velocity tracking performance was poor, or if the proportional term was non-zero, an external torque would cause the position to drift arbitrarily far from the target position. Then if the external torque were released, the controller would “catch up” for all the lost ground, moving very rapidly.

Now, however, as of release 2021-03-05, an optional configurable parameter has been added to the moteus firmware which enables the “servo.max_position_slip” option. When this is finite, it acts as an anti-windup term on the position tracker, keeping it from getting far out of line. Tuning this lets you control how hard the controller tries to track velocity in the face of external disturbance, and how much catching up it will do when that external disturbance is relaxed.

This wasn’t conceptually hard to implement, but needed careful construction to interact properly with the existing stop position and position bounds that the firmware implements.

Here’s a video demonstrating the problem, and how the new configurable lets you resolve it:

new product day: mj5208 brushless motor

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Welcome to the newest mjbots.com product, the mj5208:

This is a high quality 5208 sized 330Kv wound brushless motor with short pigtails intended to connect to moteus controllers. All the moteus devkits as of last week are shipping using this motor instead of the previous “semi-random” motor.

Specifications:

  • Peak torque: 1.7 Nm
  • Mass (with wires): 193g
  • Peak power: 600W
  • Kv: 330
  • Dimensions: 63x25mm

There are two bolt patterns on the output, a 3x M3 17mm diameter one, and a 2x M3 pattern spaced at 12mm. The stator side has a 4x M3 pattern spaced at 25mm radially and a 3x M2.5 spaced at 32mm. The axle protrudes a few mm from the stator, making it easy to adhere the diametric magnets needed for moteus.

New cross-platform moteus tools!

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After receiving many requests via youtube, discord, and email, I’ve finally gone ahead, bitten the bullet, and updated all of the moteus tools to be pure python and work in a cross platform manner. Now, the only thing you need to do to install pre-compiled versions of tview and moteus tool on most* platforms is:

pip3 install moteus_gui
python3 -m moteus_gui.tview    # (or maybe just tview)
python3 -m moteus.moteus_tool  # (or maybe just moteus_tool)

I’ve personally tested these on Linux, Windows, and Raspberry Pi, and others have at least verified basic operation on Macs. Python 3.7 or greater is required.

….

But wait, there’s more!

Now, both moteus_tool, tview, and the python bindings more generally can use python-can as a transport. That means tview can now be used with socketcan, pcan, and a bunch of other options. To one up that, most users won’t have to even specify any command line options, as tview and moteus tool will automatically select a fdcanusb or python-can depending upon what is available.

I’ll be updating the devkit introduction video soon, although the commands in there will largely continue working for the time being.

Errata

  • Neither pypi or piwheels has pyside2 for the Raspberry Pi, but it is packaged in Raspberry Pi OS. You can follow the instructions in git to find a recipe that works.
  • To use the pi3hat, you need to also do pip3 install moteus_pi3hat

New product day: moteus heat spreader

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The moteus controller is capable of a lot of instantaneous power. However, to fully make use of that power, you’ll need to keep the mosfets cool on the board. moteus has two mechanisms for that:

  1. A heatsink can be mounted to the bottom side of the PCB between the board and the motor. This is most useful when integrated into a servo motor, and the servo housing can be used as a heatsink.
  2. Mounted to the top of the board, attaching to the MOSFETS directly.

In addition to the MOSFETs, the gate driver chip, the DRV8323 can produce large amounts of heat, especially when the controller is run at a higher voltage, like the 44V that the moteus r4.5 supports.

Getting the heat out of all those irregularly spaced components on the top can be tricky, thus mjbots.com now has the moteus heat spreader:

This precision machined and stylish black anodized aluminum piece fits over the top of the PCB and mounts flush against both the MOSFETs and the DRV8323 to ensure optimal heat dissipation from all components. It can be used as-is, or with an additional heat sink fixed to the flat upper surface.

Don’t hesitate to ask any questions in the mjbots discord!

moteus and socketcan

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Various users have been trying to use lower-cost Raspberry Pi CAN-FD adapters for the moteus controller for some time (like this one from Seeed), but have had problems getting communication to work. I buckled down and went to debug the problem, discovering that the root of the issue was that the linux kernel socketcan subsystem calculates very sub-optimal CAN timings for the 5Mbps bitrate that moteus uses. This results in the adapters being unable to receive frames sent at the actual 5Mbps rate, but instead only slightly slower.

The solution is to manually specify the bus timings when configuring the socketcan link. This makes the MCP2518FD boards work, and also PEAK-CAN-FD USB adapters (and probably every other socketcan CAN-FD adapter) work as well. You can find the timings linked in the moteus reference documentation: https://github.com/mjbots/moteus/blob/main/docs/reference.md#bit-timings

General socketcan improvements

As a result of all this debugging, I made some general improvements to socketcan support in all the client side moteus tools.

  1. There is now a documented commandline for invoking moteus_tool from socketcan: https://github.com/mjbots/moteus/blob/main/docs/reference.md#moteus_tool-configuration
  2. I released moteus and moteus_pi3hat 0.2.0 to pypi. These provide socketcan interfaces for python, transparently using them if no fdcanusb or pi3hat peripherals are found.

Thanks for everyone on discord’s patience as we worked through these compatibility issues!

Automated wire stripper and cutter

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Over the Thanksgiving day holiday, I knew I had a bunch of harnesses to build. Rather than being a good corporate steward and actually building them, I instead built a machine to automate the first of the 3 time consuming parts of the harness construction: wire cutting and stripping.

This was just thrown together from two cosmetically damaged moteus devkits, a Raspberry Pi 3 an old development version of a pi3hat, a hand wire stripper, two synthetic rubber bands, an off the shelf 24V supply, and a bunch of 3d printed parts.

Why?

Simple automated wire management at the DIY level is not new. It’s been done many, many, many times before. YouTube has decided that every day I need to see someone else’s take on the problem. Look down in the resources at the bottom for my collection of alternate solutions.

What differentiates this version is (1) I built it most from junk parts I had around, (2) since it uses brushless motors it can be both very fast and very precise. Here’s a clip of it executing a few cycles where it strips 3mm from the front end, pre-cuts 3mm from the other end, then cuts the wire to a total length of 5cm. The overall cycle time for all operations is around 1s per wire for the 30cm wires I needed right now.

By replacing the guides and doing some tuning, it should be capable of managing wire between 30 AWG and 18 AWG, although to date I’ve only tested it on 26 AWG.

It did take a bit longer than the weekend — I printed a second revision of everything early the following week, then waited for a panel mount switch to make the power supply look more professional.

Video

Here’s the overview video, with some more shots of it in operation.

Resources

The BOM, .3mf’s and source code are in github at https://github.com/jpieper/bstrip. There is a hackaday page here for discussion: https://hackaday.io/project/176211-bstrip-wire-cutstrip

Maybe someone else will find it useful?

Other DIY-style solutions

pip3 install moteus

robots, ,

I’m excited to announce new python bindings for communicating with moteus controllers! A simple example from the README:

import asyncio
import math
import moteus

async def main():
  c = moteus.Controller()
  print(await c.set_position(position=math.nan, query=True))
  await asyncio.sleep(1.0)

asyncio.run(main())

This code will try to locate an fdcanusb on your host and use it to communicate with controller with ID 1. All of those details can be customized through code depending upon how you construct things. The library is pure python, although it doesn’t work on Windows currently because it relies on an asyncio aware pyserial wrapper that doesn’t work there.

At the same time, there is a parallel python library “moteus-pi3hat” which only has an armv7l package. This provides an identical API for working with the pi3hat on a Raspberry Pi. It lets you configure which controllers are attached to which bus (by default it assumes everything is on bus #1). After setting that up you can use an identical API to command and monitor the controllers.

Thanks to everyone in discord who helped test!