On 8/7/2013 12:23 PM, Przemek Klosowski wrote: > On Wed, Aug 7, 2013 at 12:16 AM, Don Stanley<[email protected]> wrote: > >> Most stepper controllers have idle torque. >> Some will have full torque at idle (stopped). >> Most modern stepper controllers will have a selectable >> idle torque and some will wait a few seconds >> before switching from run torque to idle torque. >> > You missed Stephen's point---yes, there's a holding current going > through the stepper coils, but in the middle of the step the torque is > zero. In other words, the stepper motor holds the position by having > local maxima of holding torque at half step ahead and after the > desired position. > There's no closed loop like in a servo, where you can increase the > precision by increasing the resolver resolution and/or the gain. > > Don't get me wrong---I actually like steppers, because they are simple > and reliable, and accurate enough if engineered properly by matching > their inherent accuracy to the desired movement precision. It's just > that we have to understand their limitations. > > ------------------------------------------------------------------------------ > Get 100% visibility into Java/.NET code with AppDynamics Lite! > It's a free troubleshooting tool designed for production. > Get down to code-level detail for bottlenecks, with<2% overhead. > Download for free and get started troubleshooting in minutes. > http://pubads.g.doubleclick.net/gampad/clk?id=48897031&iu=/4140/ostg.clktrk > _______________________________________________ > Emc-users mailing list > [email protected] > https://lists.sourceforge.net/lists/listinfo/emc-users > >
Theoretically that may be true, but you can't feel slop in the middle of the step at all on these new stepper motors. They feel tight when they are being held in position.. If you grab a knob on the shaft when they are energized stationary there appears to be no slop. If you want to use steppers you need to make one step of the motor equal to a machine movement of less than the desired precision of the machine.. That dictates your motor gearing ratio to the load. Unfortunately that also dictates the max speed of the machine since steppers run out of usable torque at relatively low speeds compared to servos. But for many applications that is not a problem. I have an old Bridgeport mill with steppers and I think it was geared such that one step of the motors was equal to .001 of an inch which apparently was good enough for thousands of machines shops. On the other hand the mill I have is no speed demon during rapids with the old heavy finned motors..50-60 ipm is quite reliable. which at this point is fast enough. Dave ------------------------------------------------------------------------------ Get 100% visibility into Java/.NET code with AppDynamics Lite! It's a free troubleshooting tool designed for production. Get down to code-level detail for bottlenecks, with <2% overhead. Download for free and get started troubleshooting in minutes. http://pubads.g.doubleclick.net/gampad/clk?id=48897031&iu=/4140/ostg.clktrk _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
