Brian wrote: > EMC adds the screw compensation to the measured value from the > encoder before it is interpreted as actual joint position. Therefore, > if I want to use screw comp and scale feedback at the same time, it > wont work. > The whole POINT of the screw comp table is to correct for measurement errors from the encoder. It doesn't care whether the encoder data is from a shaft encoder or a linear encoder. (Now, the question is why you'd need a screw compensation table IF you have a linear encoder, it should be free from short-term variations, so a single value of counts/unit of measure should be fine. And, of course, a linear encoder should not have backlash, either.)
I don't understand why we keep going around on this. If you have linear encoder info flowing into EMC, you have accurate position info, and should need NO correction table. Once you have accurate position measurement, steps to the stepper driver are NO LONGER positioning commands, they become relative movement commands. We know where we are NOW, we need to move 3 steps right, so issue 3 steps, then check position again. That's why everybody said to set stepgen to velocity mode. Therefore, you don't need the screw comp table, unless your encoder is out of whack. The only fly in this ointment is backlash, between the motor and the linear scale. I believe that might be compensated for with the deadband setting, but that may be unacceptable. Also, proper PID tuning may make this something you can live with. I think the whole problem you are having is you think you MUST use stepgen as a positioning system. Obviously, with no position measuring device, you have to. But, you HAVE an encoder. The encoder takes care of short and long-term errors, so you don't have to deal with them, so screw comp is not needed. The motor only needs to supply velocity to get from here to there, the scale measures position 1000 times a second and makes sure the error is arbitrarily small at any time. Jon ------------------------------------------------------------------------------ This SF.net email is sponsored by Make an app they can't live without Enter the BlackBerry Developer Challenge http://p.sf.net/sfu/RIM-dev2dev _______________________________________________ Emc-developers mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-developers
