motion control / servos / robotics

You will also get good answers to this question by posting it to comp.robotics.misc

Noted - thanks.

Steve

Most (or all) the controller I know of use only an encoder as feedback. If you are planning to work at low speeds, you might have some problems with speed measurement from an encoder due to the very low sample rate then (remember, an encoder read position, not speed). If you're dealing with robotics, sometimes you can go on without speed measurement at all, since the positiong is what's in your objective. As you mentioned using an high resolution encoder, you should have no problem to take the speed from position measurement also.

Ricardo

Logical - thanks.

Steve

Hi Steve,

I don't know the *generic* answer to this, but the *specific* situation where I have needed both is where there is a big gear ratio involved (500:1 in my case). The position encoder in my system went on the output of the geartrain, and the tach was on the motor shaft directly.

The tach had to go on the motor shaft because of the annoyance/error in accurate measurement of very slow time intervals.

The position encoder was best placed on the final output for a couple of reasons:

1. this drastically reduces interrupt loading on the micro.
2. so it accurately indicates actual position of the final shaft in the geartrain, taking into account "slack" in the gears.

system?

slack or backlash in the geartrain , together with flexing , have the undesirable effect of making the PID motor control loop unstable , *if* the encoder is situated after the geartrain . Placing the encoder on the motor shaft is the cheapo solution, but then it doesn't read (and the loop doesn't compensate for ) the geartrain inaccuracies . In order to stabilize the loop with the encoder after the geartrain, a second loop is closed based on motor velocity . This can be a tacho if the motor turns fast enough , another encoder , or other less orthodox methods like detecting brush spikes or back EMF . There are some variations that are possible, some drivers provide a current mirror output , and together with the PWM value , the actual EMF can be calculated rather than measured .

best regards, matt tudor

Besides the microchip datasheets, you could have a look at a Silicon Labs application note. Look for motor control on their website.

To get some hands-on experience, the (expensive) national LM628 or excellent and lower cost JR-Kerr

devices are great. Jrkerr has some great boards, chips, software and examples to drive small motors. It will be hard work to implement a simple motion controller that has as much functionality as his devices.

The only good introduction i know of is 'Designing With Motion Handbook' by Chuck Raskin. (Available at amazon). He gives a good introduction on servo loops, tuning , feedback etc. Also some code and examples for the 8052.

Stijn

Thanks for all the responses, guys. Most helpful. I'm not an expert yet, but I'm no longer quite so ignorant ;).

Steve