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:

• raspicam is used to read the raspberry pi camera
• ArUco is used to find any fiducials in view
• The target closest to the center is deemed, the “active target”
• The pitch and yaw rate are set based on a simple P controller to bring that target to a known point

This works passably, as shown in the video below:

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.

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.