Continuing in my series of developments with the Mech Warfare turret, I’ve now managed to replicate the primitive target tracking functionality I had in the v2 version of the turret. This works using a pretty simple principle:
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.
I’ve made enough progress with the turret, that I got around to actually implementing this on the nrfusb side. The result is a few new serial commands, “slot tx2” and “slot pri2”, which let you select *which* remote you are talking to, and the ID configuration now supports multiple ids. Unfortunately, this doesn’t lend itself to a good video demo, but the changeset is up:
This post will be short, because it is just re-implementing the functionality I had in my turrets version 1and 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.
The moteus controllers are designed to drive motors with phase currents in the 20-60A range. To operate in current control mode, they use a current shunt resistor connected to a dedicated amplifier for each phase. The current sensing resistor determines the range of currents that can be accurately sensed. The resistor that the controllers have by default measures 0.5 milliohms, which gives a reasonable tradeoff between accuracy and power dissipation for 60A operation. However, for 2A operation, it results in pretty low resolution current feedback. Thus for this application, I substituted in 5 milliohm current shunts:
Control modes and noisy velocity
The other potential challenge, is that the velocity signal that can be derived from the AS5047 absolute magnetic encoder in the moteus controller is relatively noisy when operated with no gearbox reduction. That limits how much derivative gain can be used in a position control loop. You could get around that by incorporating more plant knowledge in a filter or state observer, but that shouldn’t be necessary here.
Fortunately for this application, I won’t be controlling position based on the absolute magnetic encoder, but instead based on the inertial measurement unit in the primary controller. The absolute magnetic encoder will be used solely for performing the DQ transform to implement torque control, for which the noise in velocity is irrelevant.
Next up is making these controllers actuate the turret.
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.
My concept with this redesign is twofold, first make the turret be basically an entirely separate robot with no wires connecting it to the main robot and second, try to use as many of the components from the quad A1 as I could to demonstrate their, well, flexibility. Thus, this turret will have a separate battery, power distribution board, raspberry pi, pi3 hat, and a moteus controller for each axis of motion. These are certainly overkill, but hey, the quad A1 can carry a lot of weight.
The unique bits will be a standalone FPV camera, another camera attached to the raspberry PI for target tracking, a targeting laser, and the AEG mechanism, including a new board to manage the firing and loading functions.