When I first designed the full rotation leg, I didn’t fully appreciate the importance of torque in the knee joint.  Despite the fact that my first force based IK showed that when the legs are immediately under the body, the knee joint carries the entire load of the robot, I still managed to not add any reduction there.

The initial design used a 1:1 ratio, because that allowed me to use the same single piece 3d printed gear design I had used before.  A 28 tooth gear with 5mm pitch resulted in a gear that was larger than the output plate on the qdd100 servo, so it could just be bolted directly on.  To work with a smaller number of teeth, I had to split the gear into two parts, connected by pins, as the gear is now smaller than the qdd100 output plate.

While not its primary purpose, I still plan on entering my walking robots in Mech Warfare events when I can.  In that competition, pilots operate the robots remotely, using FPV video feeds.  I eventually aim to get my inertially stabilized turret working again, and when it is working I would like to be able to overlay the telemetry and targeting information on top of the video.

In our previous incarnation of Super Mega Microbot, we had a simple UI which accomplished that purpose, although it had some limitations.  Being based on gstreamer, it was difficult to integrate with other software.  Rendering things in a performant manner on top was certainly possible, although it was challenging enough that in the end we did nothing but render text as that didn’t require quite the extremes of hoop jumping.  Unfortunately, that meant things like the targeting reticule and other features were just ASCII art carefully positioned on the screen.

Developing the moteus brushless servo controller has been a very long journey, and while it isn’t over yet I have a reached a significant milestone.  The first batch of production moteus controllers are now available for general purchase at mjbots.com and shipment worldwide for $119 USD each!

I’ll repeat some of the specifications here:

• 3 phase brushless FOC control
• 170 MHz 32bit STM32G4 microprocessor
• Voltage: 12-34V
• Peak phase current: 60A
• Dimensions: 46x53mm - CAD drawing in github
• Mass: 14.2g
• Communications: 5Mbps CAN-FD
• Control rate: 40kHz
• Open source firmware: https://github.com/mjbots/moteus

Simultaneously, I’ve got development kits available that give you everything you need to start developing software for the moteus controller out of the box: moteus r4.3 developer kit

I’ve been developing a new bi-directional spread spectrum radio to command and control the mjbots quad robot.  Here I’ll describe my first integration of the protocol into the robot.

To complete that integration, I took the library I had designed for the nrfusb, and ported it to run on the auxiliary controller of the pi3 hat.  This controller also controls the IMU and an auxiliary CAN-FD bus.  It is connected to one of the SPI buses on the raspberry pi.  Here, it was just a matter of exposing an appropriate SPI protocol that would allow the raspberry pi to receive and transmit packets.

With a protocol design in hand, the next step was to go and implement it.  My goal was to produce a library which would work on the nrfusb, and also on the auxiliary stm32g4 on the mjbots pi3 hat.  In this first implementation pass however, I only worked with the nrfusb as both transmitter and receiver.

While developing this, I had more than my share of “huh” moments working from the datasheet and with the components.  To begin with, the initial nrf24l01+ modules I got were all Chinese clone ones.  While I was having problems getting auto acknowledgement to work, I discovered that the clones at a minimum were not compatible with genuine Nordic devices.  Thus I reworked genuine parts into the modules I had:

Last time I discussed the rationale for building a custom control and telemetry solution.  Here I’ll describe the protocol design a little bit, before discussing the implementation in a future post.

Frame design and frequency hopping

The basic idea is that the transmitter sends a frame to the receiver every 20ms, and each frame is sent on a different radio frequency.  A set of frequencies and their order is generated pseudo-randomly based on a “key” that the transmitter and receiver each share ahead of time.  The receiver replies on the same frequency with its telemetry.  Then the transmitter and receiver each switch to the next frequency in the list to get ready for the next frame.

Now that I have both sides of the nrf24l01+ link covered, it was time to design a protocol to take advantage of them.

Design space

To recap, what I needed was a reliable means of commanding the robot and receiving telemetry, even in congested radio environments.  At competitions or events like Maker Faire, Robogames and such, the wireless environment is often totally trashed.  Hundreds of devices are operating in close proximity, across all spectrum bands, including plenty of things that probably aren’t licensed to be transmitting in the first place.  When we first built Super Mega Microbot, we used a custom protocol with a 5 GHz wifi transmitter as the physical layer and selected USB based dongles which allowed control over the physical layer.  USB proved problematic, and with national RF regulations, it is extremely challenging to find wifi devices which provide that level of control at the RF layer.  Also, even with full physical layer control, wifi is difficult to make work in a reliable manner as there is so much congestion in both the 2.4GHz and 5GHz bands and the channels are so wide.

Thankfully, I’m now at the point where I’m fixing actual dynamics problems on the robot.  Doubly thankfully I have a robot which is pretty robust and keeps working!  That said, it is still, shall we say, “non-ideal”, to be testing code for the first time ever on a real robot.

Back with my HerkuleX based Super Mega MicroBot, I had a working DART based simulation which was decently accurate.  However, the actuators for that machine were so limited that it didn’t really make sense to do any work in simulation.  The only way to be effective with that machine was to tweak and tweak on the real platform and rely on exactly the right amount of bouncing and wiggling that would get it moving smoothly.

While I was able to make the r2 power distribution board work, it did require quite a bit more than my usual number of blue wires and careful trace cutting.

Thus I spun a new revision r3, basically just to fix all the blue wires so that I could have some spares without having to worry about the robustness of my hot glue.  While I was at it, I updated the logo:

First, a limited number of qdd100 servos are available for sale to beta testers!  Check them out at mjbots.com.

After building up the first set of qdd100 servos, I wanted to empirically measure their performance parameters.  Some astute commenters uncovered in my terrible juggling video, that I didn’t actually have any ground truth measure of torque with these actuators.  Given that the ultimate torque is a pretty useful performance metric, it’s a good thing to have a solid understanding of.