On 19 January 2016 at 07:04, Chris Albertson <[email protected]> wrote: > I've read that it is hard to use steppers for closed loop CNC. If this is > still true could some one explain the theory.
This may be a misunderstanding. It is hard to do anything useful with an encoder attached to a stepper motor. You can tell that the motor is out of position, but unlike with a DC or AC servo there is no way to "step harder" to recover the error. You can step _faster_ but that will reduce rather than increase torque. (Recently a family of stepper motors with integrated encoders and drivers to suit have appeared, but they can me more clearly understood as very high pole-count 2-phase brushless motors and are not immediately relevant to the question) > I have a small mill with DRO scales that are good to about .001 inch and > I'd like to put them to use with my planned CNC conversion. In this case you definitely can do closed-loop CNC with steppers. What you need to do is to run the step-generators in velocity mode. Then, just like any other servo system, you compare the linear scale position with the commanded position, and the further you are from commanded position the faster you drive the motor. You do this with the normal LinuxCNC PID controller. The controller will need to be tuned just like any other PID loop, but the stepper motors behave like almost perfect velocity-mode servos in this configuration. -- atp If you can't fix it, you don't own it. http://www.ifixit.com/Manifesto ------------------------------------------------------------------------------ Site24x7 APM Insight: Get Deep Visibility into Application Performance APM + Mobile APM + RUM: Monitor 3 App instances at just $35/Month Monitor end-to-end web transactions and take corrective actions now Troubleshoot faster and improve end-user experience. Signup Now! http://pubads.g.doubleclick.net/gampad/clk?id=267308311&iu=/4140 _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
