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General Mach Discussion / Newbie question- using PM-DC motor as servo

« on: September 20, 2010, 07:15:32 PM »

I may be asking a ridiculous question, but could a pair of 90 vdc PM-Brush motors with quad encoders installed be operated from MACH. I know that a stepper motor is supposed to synchronize a specific turn angle per pulse, but have not figured out how a servo is kept synchronised. I thought that it was simply turned on till the encoder count matched the proper travel, but in reading the MACH manual I see all this talk of step pulses and such and am totally confused. Since a straight dc motor will turn a given distance entirely dependent on current, drag, pulse length and so many other variables, how is it actually being synchronized to a digital count. Do the step pulses continue until the encoder reaches a predetermined count based on the exact coordinates programmed into the tool track, or would changes in speed and drag potentially cause the motor to stop travel in an unsynchronized position. Also since these particular motors are high voltage/current and controllers like the gecko can't operate at those limits I suppose standard scr motor drives would have to be employed. In this case how would they be affected by a pulse based drive signal? If anyone has attempted this and found a solution a block diagram and material list would be of great assistance in assessing how to proceed. Thanks, Les