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.

In order to bring up the final piece of the raspberry pi 3 hat, the nrf24l01+, I wanted a desktop development platform that would allow for system bringup and also be useful as a PC side transmitter.  Thus, the nrfusb:

Similar to the fdcanusb, it is just an STM32G474 on the USB bus, although this has a pin header for a common nrf24l01+ form factor daughterboard.

The next steps here are to get this working at all, then implement a spread spectrum bidirectional protocol for control and telemetry.

After I restructured my control laws to take advantage of high rate force feedback for the pronking experiments, I haven’t actually managed to port the walking gait yet.  Now that I have a brand new robot, it seemed like a good time!

This gait is basically the same thing as I ran on the quad A0 in principle.  The opposing feet are picked up according to a rigid schedule, and moved to a point opposite their “idle” position based on the current movement speed.  Any feet that are completely placed on the ground just move with the inverse of the robot’s velocity.

After getting all the legs swapped out, I ran my existing software to validate that all the pieces worked together.  Here’s a quick video showing basically what I’ve shown before, but with all new hardware: