I finally got around to fixing a number of minor glitches in the quad A1’s chassis recently.
1. The raspberry pi is now far enough away from the left panel that you can connect the HDMI if you choose.
2. I no longer have vestigal studs for the pre quad A0 junction board on the other side.
3. The switch got moved down to between the legs.
As part of provisioning a quad A1, or anytime the mechanical configuration has been changed, I need to go and record where the zero position of all the joints is. The “0” position for the software now is with the shoulders perfectly horizontal, and the upper and lower leg sticking straight down.
Up until now, every time I’ve done this it has just been by eyeballing and with lots of foam and bubble wrap to shim things into place long enough to record the level. Sometimes I had to go back and try a few times, as even determining when something is straight is not, well, straightforward.
Like with the fdcanusb, I built a programming and test fixture for the moteus controllers. The basic setup is similar to the fdcanusb. I have a raspberry pi with a touchscreen connected via USB to a number of peripherals. In this case, there is a STM32 programmer, a fdcanusb, and a label printer. Here though, unlike with the fdcanusb fixture, I wanted to be able to test the drive stage of the controllers and the encoders too.
The Mech Warfare turret concept I’m developing involves having basically two independent robots, the actual robot and the turret. To make that be controllable in a sane way, the control station will command and receive telemetry from both simultaneously, allowing control actions to be given in the camera frame of reference. Otherwise, remote piloting is… very challenging.
This could be done just by having two separate transmitters. Since the nrfusb that I’m using is spread spectrum, many transmitters can easily co-exist at the same time. However, the protocol is designed such that a single transmitter can simultaneously communicate with multiple slaves at the same time, simply by switching channels more frequently.
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.
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:
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.
