The first revision of the brushless servo control board for SMMB was successful in getting a leg to jump.  I ended up doing a small-run second revision that addressed a few minor problems and added a couple more capabilities.

• RS422 Debug/Link Port: I had a 3.3V serial port exposed previously for debugging, however it caused my USB-serial converter to dislike itself due to common mode ground shifts and it wasn’t reliable at high baud rates (>3Mbps).  I also wanted to support “linked” modes, where two servos would perform control in the actuator space at full rate.
• Debug through holes: r1 had a number of debug connections, all of which were unpopulated SMD pads.  I decided that through holes were easier to connect debug wires to.
• Vertical SWD connector: I had initially thought I would hide the SWD connector within an enclosure.  However, the initial enclosure prototypes made that seem less desirable, so I switched it to vertical.
• More debugging points: When bringing up the first board, I ended up doing a lot of carefully balancing scope probes on various pins, when there was plenty of board room to just have through hole debug points.  Lesson learned.
• FET temperature sensing: r1 just had an external temperature sensor port, r2 additionally has a thermistor next to the FETS.

Macrofab’s current pricing scheme provides a great incentive to keep your BOM below 20 parts, as that is the only way to get quick turn service.  Otherwise you pay an extra 2 or 3 weeks of calendar time.  In r1, I went to some lengths to stay under 20, however, it just wasn’t going to work with r2, so I left a few easy-ish or non-critical parts unpopulated to do them myself: the connectors, LEDs, and one really big diode.

One of my intermediate goals for building new actuators for SMMB is to get them robust enough to jump continuously for some duration of time.  Progress is slow, as things break, new parts are ordered, repairs are made, and jumping resumes.  The most recent failure is at least interesting enough to me that it is worth writing up.

To recap, I’m building a brushless servo based around a Turnigy Elite 3508 brushless motor and a custom 5x planetary gearbox.  The 3508 is intended for quadcopter applications, so to install a spur gear I first extracted the original shaft, then pressed in a new shaft with two flats on it.  One flat for the set screw attaching the rotor to the shaft (which had a press fit), and a second for the set screw attaching the spur gear to the output of the shaft.

After OpenCV, the other major dependency the mjmech software has, which is necessary to complete the raspberry pi 3b+ bazel build setup, was gstreamer. Unlike the previous dependencies, this one is a doozy – gstreamer has an enormous transitive dependency set. Additionally, we needed to use its ffmpeg wrappers, which brings in more dependencies.

In this post, I’ll just try to map out the dependencies that we ended up actually needing, so that they can be tackled one by one.

The next level of difficulty in bazel-ifying packages for mjmech was opencv.

First, for the impatient, Apache 2.0 licensed sources are available on github: https://github.com/mjbots/bazel_deps/tree/master/tools/workspace/opencv

OpenCV’s native build system consists of nearly 200 cmake files with over 20,000 total lines of code, plus assorted helper scripts and prototype files which are substituted into.  Fortunately, I didn’t need to support the full complexity of the opencv build system.  Things I didn’t bother to touch:

After the initial leg jumping with the prototype brushless actuator for SMMB, I spent some time actually tuning the control loops and making the firmware not incredibly convoluted to get started.  I also acquired a high speed camera for analysis.

So, here is a brief update of the final jump before I seem to have toasted one of my DRV8323 motor drivers.  It jumped for about 400ms of hang time, running at about half of the maximum current the system should be capable of pulling.

In my first foray into 80/20, I built a slightly more robust jumping fixture for the SMMB leg jumping test:

Overall it is much more rigid than the old one, and looks a little nicer.  To top it off, I laid down a neoprene sheet for surface protection and friction enhancement, which is a step up from the old cardboard surface both in aesthetics and function.

While working on the improved actuators for SMMB, I wanted to be able to perform some quantitative experiments to design the thermal transfer of the controller board and enclosure.  I figured that feeling with my fingers probably wasn’t scientific enough to make consistent progress.

Enter an inexpensive Chinese thermal imager, which you can find for under $300 from time to time.  A non-affiliate Amazon link: https://www.amazon.com/gp/product/B07BDJZ845

It has a resolution 220x160, reads up to 300C and being intended for construction inspection has at least a little software support for reading out actual temperatures and capturing images for reports.  The only downside is the focal length.  It really can’t focus on anything less than about a meter away.  That isn’t too great for PCB inspection.

This is part N in a series describing how I created the bazel infrastructure to build all the third party packages for mjmech.  Previously we have:

• Building mjmech software with bazel
• Configuring bazel to cross compile for the rpi3
• Building mjmech dependencies with bazel
• First bazel-ified packages

We left off with the first, very simple packages configured to build with bazel.  In this installment we will tackle those that require at least minimal configuration, i.e. those that have some files which are normally generated as part of the build process.

I continue to make progress on the improved actuators for SMMB.  To briefly recap, these are based on a home-built brushless servo consisting of off the shelf gears, bearings, 3d printed assemblies, and a custom control board.

Moving on from closed loop vector (FOC) control, I’ve now built up a second motor, set both of them communicating over the same RS485 bus, and wired up a minimal makeshift jumping fixture.  The leg didn’t jump as well as I had expected: I was only able to achieve about 300ms of air time and there are a lot of other minor problems/deficiencies as well.  But on the other hand, I don’t appear to have permanently broken anything yet, so improvement will hopefully be mostly continuous!

I’ve reached a minor milestone in developing improved actuators for Super Mega Microbot.  Previously I demonstrated basic closed loop control using a VESC.  Now I have a custom control board running closed loop vector-based current and position control on a single brushless servo!  I’ll hopefully write up pieces in more depth later, but this post can serve as a proof of existence.

First, boards as received from MacroFab:

Mounted onto the planetary gearbox: