Omron Rotary encoders - stepper motors.
My Allen-Bradley 7320 had this feature, but it did not have a smooth on/off logic. When the feedrate caused increasing error, the machine jerked to 1/2 the feedrate, the motors caught up, and the cycle repeated. I had to disable it to prevent tipping the mill over.
It _might_ be possible to connect the pid error value to the adaptive feed pin so that step-rate reduces while still making up the lost steps, but I don't know if anyone has ever tried it.
Moronicsmurf wrote: @andypugh i wonder how those "closed loop stepper motors" drivers deal with this then
They run like a very high pole-count brushless motor. I think that they also have a torque reserve, the driver is allowed to temporarily over-current as it "knows" what is going on.
I've wondered if a high resolution encoder couldn't help with improving the accuracy of a high microsteoped stepper (32x microstepping or so) and limit max following error to something less than 1 full step.