On 01/19/2016 01:04 AM, Chris Albertson 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. Stepper drivers have fixed current settings for the windings. When the load increases, the voltage applied by the driver may increase to maintain the current. But, this gives a maximum torque, above which the stepper will lose steps or stall. A stepper with encoders will allow you to detect loss of position or stall, but it WON'T prevent it from happening.
A servo drive gives the minimum current to the motor to keep at the commanded position. When load increases, the current is increased to maintain position and velocity. Since the minimum current required is used, the motors stay cooler, and therefore are available to deliver greater than continuous torque when the load demands it. Generally, servos can deliver a peak torque about 4 X the continuous rating. Steppers are good up to a few hundred Watts of mechanical power. Remarkably small servos can deliver this much power, and modest ones can deliver a KW or more. Of course, it is still possible to stall a beefy servo, but it is a LOT less likely. Another advantage of the servo is that you can go to E-stop and back on-line without losing the position alignment. (Yes, steppers with encoders can do this, too.) > 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. I understand > software. I've worked in the field for 30 years and have experience with > Linux (and other OS) device drivers, embedded real time code and so on. > If the problem is simply that no one has bothered to write the code, I can > fix that. But if there is something of a mechanical nature I'm not > thinking of maybe some one can point me at what I need to read up on. > > At least I think I might be able too automatically measure backlash using > the DRO scale data. But my goal is to go closed loop if I can > Many low-cost DRO scales are NOT quadrature encoders reporting position in real time, but some kind of interpolated sensor that gives several position reports a second. These cannot be used for closed-loop servo operation. So, check what sort of scale you have. See http://pico-systems.com/minimill.html for some pics and discussion of my minimill servo conversion. This is not a machine I use for general machining, (I have a CNC Bridgeport that is a lot more capable) but is portable for demos, and I also use if to experiment with motion control. I (Pico Systems) have all the hardware you need for such a conversion, using either brush or brushless motors. Jon ------------------------------------------------------------------------------ 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
