Here’s the approximately annual giant video update:
If you’re interested in any of the topics in more detail, I’ve collected links to individual posts for each of the referenced items below.
Thanks for all your support in the last year!
Announcement of moteus r4.3: Production moteus controllers are here!
Meet the newest revision of the moteus controller!
Yes, it does look mostly the same as the r4.3 that has been getting a lot of use lately. This revision exists mostly to improve manufacturability, but I snuck in a minor design improvement while at it. Now, the maximum voltage input is rated up to 44V from the 34V of the r4.3! (Note though, that the pi3hat and power_dist still are limited to 34V). Otherwise the new controller is fully electrically, mechanically, and software compatible with the r4.3.
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.
As part of some experimentation in native Windows tools for moteus, I’ve created a dirt simple rules_wix repository in github. It provides a minimal wrapper around the WiX Toolset for creating Window’s installers from within bazel. There’s nothing fancy there yet, but it can at least make an installer with a single executable in it!
The moteus controller has always supported multiple turns when counting positions. It has a one-revolution magnetic encoder built in, but after turn on, it keeps track of how many turns have occurred. However, if you’ve followed previous moteus tutorials, you have probably noticed a persistent caveat that for accurate control, the position of the output shaft needs to stay within a hundred revolutions of 0.0 or so. Now, I’ll describe why that was, and what I’ve done to remove the limitation, allowing unlimited rotations!
At the request of @nichols in discord, I’ve recently implemented a new control mode in the moteus controller, “stay within”. In this mode, as long as the controller is inside the currently commanded bounds, only a feedforward torque is commanded. When either of the optional lower or upper bound is violated, the normal PID controller is used to force the position back to the bound.
Here’s a quick video demo:
Note that this could have been roughly accomplished in a couple of ways by a higher level controller – either by monitoring the position and commanding zero kp/kd scales when inside the boundary, or just solely commanding feedforward torques based on position sensing. However, this approach lets the control run at the full 40kHz of the moteus controller, which results in much smoother operation at the boundary condition.
The moteus controller uses a DRV8323 smart driver IC to drive the power MOSFETs as well as provide various safety functions. One of the capabilities it has which has so far been unexplored in moteus is its ability to control the drive strength and dead time through software configuration.
In a switching power supply or switching motor inverter, MOSFETs are arranged in a half bridge configuration. Depending upon the type of converter, one or more half bridges are used (3 phase inverters like moteus use 3 of them). Each “half bridge” has two MOSFETs, one connected between positive power and the output terminal, and the other connected between the output terminal and ground.
I’ve posted a new release of the firmware for the moteus brushless controller to github!
This release has a number of minor improvements in the host tools (for which there continue to be no distributed binaries, you get to build from source). The biggest improvement in the firmware is the improved low-torque operation as documented here and here. If you have any questions or want help upgrading, hop into discord at #moteus and ask!
Recently I described some changes I made to improve the low speed torque ripple of the moteus controlle. I also built a dynamometer. I decided to use the dynamometer to quantify how much things had improved with the torque ripple, and to see how much room for improvement was left with any anti-cogging implementation.
Here, the test script is relatively simple. I have the “fixture” controller sweep at a very low velocity (0.01Hz) through a bit more than one full revolution using a relatively high I term in the PID controller to ensure that it really holds that position no matter what external torque is applied. Then, the “device under test” controller is just commanded either to be powered off, or in position mode with a pd gain of 0 and a feedforward torque. Then I can just measure the result from the torque transducer while this sweeps through a full revolution, and correlate the measured torque with the encoder position.
Recently dlickindorf pointed out in the mjbots discord that he was having problems with very low torques on his large PMSM hobby-grade motor. While moteus doesn’t have any anti-cogging support yet, it should still be capable of driving motors such that the unexpected torque isn’t much worse than the baseline cogging torque of the motor. However, he was seeing much worse behavior with controlling to 0 current, as much as a full percent of the maximum torque of the motor.
