Back when I was getting Super Mega Microbot “Junior” ready for Maker Fair Bay Area 2019, I made it minimally self sufficient through a quick hack of adding some PVC pipe that allowed it to manipulate its feet into a known good position while the robot was safely up in the air.

This worked, but had a number of obvious disadvantages.  For one, it looked ugly!  Second, the machine couldn’t squat down very far before getting stranded on the “resting legs”.  I’ve finally gotten around to doing at least a first attempt at something better!

In part 1, part 2, and part 3, I looked at what was limiting the update rate of the moteus controller when built into a quadruped configuration and how to improve that.  Now, it is time for the final demonstration!

That video was shot with a 150Hz overall update rate.  The plot shows the commanded and actual position of the three joints in the front right leg, although not all to the same vertical scale.  Updating the servos themselves only used about 3.5ms per cycle, but the gait logic used another 1-1.5ms, which made hitting 200Hz not super reliable, thus running at 150Hz.

Back in part 1 and part 2, I looked at problems that limited the rate at which the host computer could command the full quadruped and some of the solutions.  Now, in part 3, I’ll cover more of the solution.

More solution steps

Previously, I switched to using PREEMPT_RT, switched bridging strategies, and optimized the turnaround of the individual servo.  Now, I’ll move on to optimizing the host software.

4. Host C++ software micro-optimizations

The primary contributor in the host software to the overall update rate is the time it takes to turn around from receiving a reply from one servo, to sending the next command.  I first did some easy micro-optimizations which came up in profiling.

Back in part 1, I looked at the driving factors that limited the update rate of the full quadruped.  Now in part 2, I’ll cover the first half of the solution.

Background

To begin with, there were two major paths that I could take based around the network topology.  In one path, I would remove the active bridging capability from the junction board, and rely on the Raspberry Pi to drive all the servos directly, and in the other the active bridge would stick around.  There were a number of key disadvantages to both approaches:

The moteus brushless controller I’ve developed for the force controlled quadruped uses an RS485 based command-response communication protocol.  To complete a full control cycle, the controlling computer needs to send new commands to each servo and read the current state back from each of them.  While I designed the system to be capable of high rate all-system updates, my initial implementation took a lot of shortcuts.  The result being that for all my testing so far, the outgoing update rate has been 100Hz, but state read back from the servos has been more at like 10Hz.  Here I’ll cover my work to get that rate both symmetric, and higher.

My very first sun gear holder I machined myself was something of an artistic feat.  Each operation was re-run many times, and as a result the part was largely a one-off.  The final part properties were not really indicative of the final program.  My next step in my learning adventure was to up my Pocket NC game and get to a single reproducible program that would emit a part that I could use, then be able to run it over and over again.

To demonstrate the dynamic capability of the full rotation quadruped, I figured I would start by doing some full machine jump tests to a relatively low height, just to show that it was capable.

Thus, I rigged up an open loop script which squatted a small fraction of the available distance, and then powered up at a relatively small fraction of the available maximum speed.  I don’t have the telemetry yet to extrapolate how high this will be able to go at maximum, but I think it should be a fair amount higher.  For now, I want to do some more instrumentation and walking testing (and have more spares) before I manage to break things by jumping really high.

Last time, I had finished physically assembling all the motors for the updated quadruped with legs that can rotate freely 360 degrees.  After the long summer break, I powered up and configured all the servos.  Then, after setting up the gait engine for the new configuration (for which there are still a TODOs when the lateral shoulder offset is non-trivial as in this configuration), I was able to achieve some amount of walking.  Here is one of the first videos I took, without much in the way of tuning or work.  The control is a little wobbly still, but so far there are no signs of any mechanical failures as with the older design.

The next step in (re-)building the quadruped with a full rotation leg was getting all the motors ready.  I had to first install reinforcing rings on 6 of the motors:

Then, I needed to lengthen the power leads on 3 of the motors to serve as the lower leg joint.

After doing my first cuts in aluminum, I wanted to actually try and make a real part, to prove that the Pocket NC v2-50 was capable of making things that I can actually use.  My first attempt, was the same part I did my first aluminum cuts in, the sun gear holder from the BE8108 planetary gearbox.

This part attaches to the rotor, the sun gear, the position sense magnet, and has bearing interfaces to the planet input and the back housing.  While not terribly big, the number of features and mating surfaces is relatively large.