I've mentioned previously that I think it's beyond me to visually judge the performance of the PID tuning values in use in my servo controller.

I had created a simple program to test the response, but wasn't happy with the point to point style test it was carrying out. It's more important for a CNC machine to follow curves closely, than have outright movement speed.

To fix this, I've added a feature to the microcontroller, a sine wave test. The test was added to the controller, because for a good test, a position update needs to be present every time the PID loop recalculates. By placing the sine wave path generation in the controller, a new movement value is present every single PID update. I couldn't achieve this sort of high performance timing by sending updates down a serial line.

Also, the standard point to point test produces massive errors when the set-point jumps, masking the fine detail of an "error" plot. I've added it now (on it's own axis), and it's very helpful in seeing exact levels of error (the large scale of the movement would otherwise hide this relatively small error). 

In the above images you can see the reduction in error during a slower test (from +/-10 to +/-3). On the slower test, that's within 0.9 degrees.

I've also included a "steady state" option. This doesn't instruct the controller to make any movement, but it does record and display it. This lets you see the response to an external disturbance on the servo (the handy shifter!).

Armed with this new information... I still haven't gone back and re-tuned the PID values.