We are having a problem with a Lurch or Glitch while changing the speed during a turn.

For example, we are turning a tank-style robot with a command like

motor[motorD] = 50;
motor[motorE] = -50;

Then when we get close to the target heading, we are slowing down:

motor[motorD] = 10;
motor[motorE] = -10;


At the transition between the speeds, the robot will sometimes move straight forward or maybe straight backward for an inch or 2.

This seems to not happen if PID control if Off, so I am thinking maybe it has to do with accumulated error in the PID control or something like that. But that is total guess on my part.
My gut feel is that the error accumulated on each side should be about the same and in opposite directions, so it surprises me that the result is a lurch forward, not just a bump in the rotation speed.

Anyone have any thoughts on this? Similar experiences?

And is there a way to peek at what is going on inside the PID algorithm?

For example:
What is the current error?
What is the current commanded output to the motor?
If in the Ramp Profile, what is the current target speed?
What are the gain values?

Thanks in advance,
David Fort
Rockwell Automation
Team 1001
Charles F. Brush High School

[Edit update] I notice I can get this to happen just using the "NXT Device Control Display" dialog with the robot up on blocks.
If I check the Reg checkbox, then set Speed on motorD to 50, then change to 10 then 50, etc., it will sometimes reverse direction. There might be a correlation with resisting force, not sure.

Do you have shaft encoders installed on your motors and plugged into the motor controller? PID control doesn't work if you do not have the shaft encoders installed.

yes.

We seem to be getting good results with the PID except during these speed changes.