This post will be short, because it is just re-implementing the functionality I had in my turrets version 1 and 2, but this time using the raspberry pi as the master controller and two moteus controllers on each gimbal axis.

I have the raspberry pi running the primary control loop at 400Hz.  At each time step it reads the IMU from the pi3 hat, and reads the current state of each servo (although it doesn’t actually use the servo state at the moment).  It then runs a simple PID control loop on each axis, aiming to achieve a desired position and rate, which results in a torque command that is sent to each servo.  Here’s the video proof!

To date, I’ve used the moteus controllers exclusively for joints in dynamic quadrupedal robots.  However, they are a relatively general purpose controller when you need something that is compact with an integrated magnetic encoder.  For the v3 of my Mech Warfare turret I’m using the moteus controllers in a slightly new configuration, with a gimbal motor, one for each of the pitch and yaw axes.

Gimbal motor theory and current sensing

From an electrical perspective, gimbal motors are not that all that different from regularly wound brushless outrunners.  The primary difference being that they are wound with a much higher winding resistance.  That enables them to be driven with a much lower current, at the expense of a lower maximum angular velocity.  In this case, I’m using the GM3506 from iFlight which has a winding resistance of 6 ohms, that results in working currents being on the order of 2A maximum.

Another of the tasks I’ve set for myself with regards to future Mech Warfare competitions is redesigning the turret.  The previous turret I built had some novel technical features, such as active inertial gimbal stabilization and automatic optical target tracking, however it had some problems too.  The biggest one for my purposes now, was that it still used the old RS485 based protocol and not the new CAN-FD based one.  Second, the turret had some dynamic stability and rigidity issues.  The magazine consisted of an aluminum tube sticking out of the top which made the entire thing very top heavy.  The 3d printed fork is the same I one I had made at Shapeways 5 years ago.  It is amazingly flexible in the lateral direction, which results in a lot of undesired oscillation if the base platform isn’t perfectly stable.  I’ve learned a lot about 3d printing and mechanical design in the meantime (but of course still have a seemingly infinite amount more to learn!) and think I can do better.  Finally, cable management between the top and bottom was always challenging.  You want to have a large range of motion, but keeping power and data flowing between the two rotating sections was never easy.

I made a number of tweaks to the quad A1’s raspberry pi hat to get it ready for production, resulting in r4.1 of the board:

None of the changes were particularly big, but each has some value:

• The correct switch mode regulator is installed.
• The auxiliary CAN transceiver was switched to one that supports a larger common mode voltage.  This will allow it to be connected to the power distribution board without smoking.
• Each of the STM32s now has some GPIO pins connected directly to GPIOs on the raspberry PI primarily to be used for interrupts.
• Pin headers expose a few gpio pins from each STM32 for interfacing with random external things.
• The NRF radio module changed orientation and has improved power filtering.
• I added a microphone to the auxiliary STM32.  The goal is to eventually be able to use that to synchronize external video with onboard data collected during operation more easily.

I’ll bring this up in a future post!

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.

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.