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!

• https://github.com/mjbots/rules_wix

In the last post, I described the newer gait engine which takes a desired command and produces a set of gait parameters. At that point, the gait engine needs to implement those gait parameters in a way that is stable with respect to disturbances and keeps the two legs properly out of phase with one another.

The gait variables that the gait selection procedure emits are as follows, each “leg” is actually a pair of legs.

As hinted in my earlier video I’ve been working towards some higher speed gaits with the quad A1. To accomplish that, I had to restructure the gait sequencing logic to permit changing cycle times and allow flight phases.

For now, I’ve tentatively broken down the trot gait into 5 regimes, based on how fast the machine is moving:

1. At the slowest speeds, the flight legs swing through a step in the configured maximum flight time. The interval between flight times is fixed at a configured maximum. Here the speed is determined by how far the flight legs move.
2. Once the flight legs are moving through their maximum allowed distance, then the amount of time spent with both legs on the ground is reduced in order to increase speed.
3. At the point when both legs are not on the ground at the same time, then there begins to be a flight phase. Increasing the length of the flight phase increases the speed.
4. When the flight phase reaches a configured maximum, then the swing time is decreased until it reaches a configured minimum.
5. When the swing time is at a configured minimum, the flight time is at a configured maximum, and the legs are moving through their maximum range, then the machine is moving at its maximum speed.

Depending upon the current commanded rotation rate and translation velocity, the distance available for the legs to travel through may change. This uses the same mechanism from the step selection technique to determine the maximum distance at each update cycle, then selects which of the above regimes is active based on the commanded speed.

Here’s another short video only update, I’ve been experimenting with flight phases on the quad A1. With the gait formulation as I have it now, it isn’t terribly stable, but with some coaxing videos are possible:

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’m continuing to make progress with getting the quad A1 to move at higher speeds, furthering my earlier video update. I’ll write up more later, but here’s a quick snapshot showing a 2 m/s trot:

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.

Dynamometer script

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.

The moteus controller, uses an absolute magnetic encoder to sense the position of the rotor and thus be capable of field oriented control FOC of brushless motors. To date, all the iterations of the controller have used the AS5047P encoder from ams. This is relatively common, works fine over SPI and hasn’t caused any problems. While investigating some other issues, I decided to take a stab at trying some alternate encoders. First, I tried the AS5047U, which is the same basic encoder, but incorporates a digital filter. I also tried the MA732, from Monolithic Power, which uses a different operating principle and also includes a digital filter. The plus side of the MA732 is that it reports full 16 bit values, even if not all of them provide a lot of value.

Earlier I showed off a torque transducer and the calibration fixture I used for it. I’ve now got enough assembled to make an entire dynamometer:

This has the torque transducer on one end, coupled to the “fixture” moteus controller through a bearing support. Then that is connected via a 3d printed coupler to the “device under test” moteus controller, which is hard mounted to the base plate. Any net torque between the two controllers will be coupled back to the transducer resulting in a measured torque.