2010/5/15 Stephen Wille Padnos <[email protected]>: > Viesturs Lācis wrote: >> I had a thought about that and there is one BUT... I am going to use >> stepper motor to control the rotary axis and as far as i know there is >> only one way to create a closed loop system - EMC outputs PWM signal >> and receives encoder data and then there has to be PWM-to-step signal >> converter. >> > EMC can output whatever you want, it doesn't have to be PWM. The problem > with closed loop and steppers is that the available torque of a stepper > decreases with speed, so when the motor falls behind enough for PID to > try to correct, it's already lost. The stepper has no extra torque to > give in response to the "faster" command it gets, unlike a servo. > > Closed loop monitoring of a stepper position, using that for following > error detection, is perfectly reasonable and can be done with EMC2, > without a hardware PWM-to-step converter. > >
Motors will be moving relatively slowly, so loss of torque in high speeds is not a problem. I expect that rotary axis will have to hold their existing position more than do any movement at all and steppers are better in holding their position than servos without a brake. 2010/5/15 Jon Elson <[email protected]>: > Tom Easterday wrote: >> On May 15, 2010, at 9:24 AM, Andy Pugh wrote: >> >>> There are people running closed-loop stepper systems. >>> >> >> Really? I keep getting told you can't do it for various reasons. >> >> >> I would love to see someone's configuration files who has made a closed loop >> with steppers work. >> > The Pico Systems Universal Stepper Controller is a sort of closed loop > system all the time. The default method is to count the step pulses > sent out. By flipping a switch for each axis, you can set it to read > the encoder pulses, instead. > > So, in fact, EMC doesn't actually even KNOW whether it is physical > closed-loop or electronic closed-loop! The only changes in the .ini > file would be to change the INPUT_SCALE parameter to reflect the encoder > resolution vs. the step resolution per user unit. > > See the configs file directory included in your EMC2 distro under univstep. > > Now, the reason you've been told you "can't do that" is because it > doesn't PREVENT stalls. It WILL detect a stalled motor and cause a > following error stop. It will also silently fix a single lost step here > and there caused by setup time violations during direction reversals. > Some machines suffer from this kind of problem. > > Jon > I (and seems like other EMC users as well) did not know that stepper motors are used in closed-loop systems. I will try to search for more information about that Pico Systems Universal Stepper controller. Thank You for sharing the information? Then I have one more question - what are the options to use servo motors and stepper motors together in one machine? I have 3 servos (2 on X axis and 1 on Y axis) and currently I am trying to control servo drives in step/dir mode, which does not seem very clever idea, besides movement of axis with old, DOS based control system is smoother (analog +10V/-10V signal is used) than EMC with step/dir mode performs. I want to add 3 stepper motors on Z, A and B axis respectively. I already have stepper drivers and motors, so i could save some money and as I mentioned - i believe that steppers on rotary axis are better. Viesturs ------------------------------------------------------------------------------ _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
