kf2qd:
If you are not supplying power to the motors at all times, and if there is any torque on the motor when you remove power then the motor can move and you are out of position. Proper use of steppers keeps them powered at all times. You can reduce power when not moving, but you must supply enough power to hold position. Sorry, but with no other feedback you have to keep the motor powered to be anywhere near accurate with positioning. A common rue of thumb is that you need steppers with 2 times the required torque.
hi kf2qd,
The steppers each moves a sled on a greased rail. Once the step has been accomplished, there is no more torque on the axel. For now, I'll go with the precision this setup gives.
If it really is an issue, both steppers should be exposed to it equally .. it's still only one of them that misses a little. I havn't had time to look at the "starting at wrong phase" possibility. I'll post back on the outcome.
You may say that detecting missing steps is a sort of precautionary measure .. if one step is not pulled through, I can add an extra, but if stepping is gone all together, something serious is wrong. I would like to be able to turn my back on it and feel safe that any eventuality is dealt with properly.
Setups of my photo-interrupters often involves a small capacitor on the receiver voltage, but that's not where the 'jumpiness' is .. and the capacitor would act as a high-pass filter, right? The analog low is well-behaved, but the high is very serrate. I wonder if I could put a low-pass filter on it.