motion control / servos / robotics

I need to become an overnight expert in driving (smallish, DC) motors, with particular emphasis on smoothness and control of the acceleration/deceleration profile, for a mechatronics/robotics application. I have a background in PID control, and analogue/power electronics, so I'm not a total novice - but the sample rates I now need are orders of magnitude higher than I have experience of. I also need very high resolution, probably using absolute encoders (e.g. Stegmann/Hiperface et al).

I've tried, and failed, to find one or more killer reference books ("Dummies Guide to Motion Control" or something ;)). I've googled and found *some* material, but so far without the detail I need. Could any kind soul point me at a source of detailed data on the subject?

One particular question - why do certain motor controllers need both an encoder (for position) and a tacho (for speed) input to the control system? Or is this only common when the final axis of rotation is > 360 degrees?

TIA,

Steve

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It depends on the encoder resolution, if you have a fairly coarse encoder a tachometer is needed for a high performance system,as you do not get velocity feedback except when the encoder count changes (in other words between encoder counts you have no idea what the motor is doing). The end result of this is that the damping (What you need velocity feedback for) gets "crunchy" at low speeds. There are tricks to improve this (count-count period measurement at low speeds, digital filtering of velocity info etc) but they all have their problems

The tachometer feedback gives you real velocity info down to very low speeds.

If you need a very high resolution encoder for other reasons, a tachometer is most likely not needed.

PCW