Marc van Doornik wrote: > * Optical encoders with 100 nm (~0.000004 inch) resolution and > differential output
> One problem that springs to mind is the spring-like behaviour of the > motor. This manifests itself as a hysteresis in the output position due > to the friction of the roller guides (about 15 N). Obviously, this > hysteresis should be compensated for as fast as possible, preferably > with the maximum velocity of the stage (i.e. 2 m/s). However, the > maximum velocity the servo card can keep up with (because of encoder > resolution) is 200 mm/s. I think you have painted yourself into a corner, and there may be no good solution. Is the drive a programmable one? Can you set it to limit the max velocity? I think there is great danger in having a system that can exceed the velocity that can be sensed. Let me repeat that word, DANGER! If this system goes into runaway, the encoder counter is likely to not detect any counts, while the stage is moving at 2+ m/sec! This is a catastrophe waiting to happen, unless you have some other method of sensing velocity. Maybe you could add a little circuit with a couple one-shots that would detect a very high frequency on the encoder signals and comannd an E-stop. If the limitation is that the stage can move faster than the encoder ITSELF can count, then the problem is even worse, the one-shots won't have a signal to look at. It might be possible to look at the commutation encoder signals to do the same trick. Possibly limiting the power supply voltage can limit the peak velocity, depending on the motor's resistance. If it has a lot of resistance, and you need high acceleration, then the peak speed at light load would be much higher. Still, you have a 10:1 discrepancy here. You can't compensate for the "hysteresis" at 2 m/sec due to the time it takes to accelerate. You may want to use a lot of acceleration, but never a lot of velocity, to overcome this. What kind of velocity and acceleration do you actually need to provide with this system? What kind of positional accuracy do you need? Only at the end points of the move, or all along the path? Do you really NEED 100 nm resolution on the encoders? (My G*D, how much did you pay for those?) Is there a way to divide down the encoder resolution? Do these encoders use interpolator boxes to get higher resolution? These usually have settings to select from some different resolutions. But, beware that some interpolators cause a phase shift in the output. Anyway, 100 nm resolution is less than a wavelength of visible light, presumably your cutter is using IR, so it seems like useless resolution. Jon ------------------------------------------------------------------------- This SF.net email is sponsored by: Microsoft Defy all challenges. Microsoft(R) Visual Studio 2005. http://clk.atdmt.com/MRT/go/vse0120000070mrt/direct/01/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
