e ----- Original Message ----- From: "Jack Ensor" <[EMAIL PROTECTED]> To: "Enhanced Machine Controller (EMC)" <emc-users@lists.sourceforge.net> Sent: Monday, June 04, 2007 1:30 PM Subject: Re: [Emc-users] How much is too much jitter?
> Jon Elson wrote: > >>Jack Ensor wrote: >> >> >>>I noticed when jogging at my maximum rate of 90 ipm the 2 quadrature >>>signals coming at a rate of 1 Khz have approximately 50 micro seconds of >>>jitter. Is this excessive? How much would it contribute to tracking >>>error? My tracking error is insignifcant when homing but but huge when >>>running Axis.ngc. >>> >>> >>Would you please define this term "tracking error"? I do not >>know what it means. From context, I believe you mean to say the >>machine's position differs from the displayed position. Is that >>correct? >> >> > Yes, position displayed on the axis screen differs from what my dro says > except when I home they always agree. > >>How are you seeing the quadrature signals? On a "box" >>oscilloscope, or somehow with halscope? Unless halscope AND the >>software or hardware encoder input facility is sampling at a >>fast enough rate, you would miss some of the edges. >> > Yes, I understand the hal (storage scope) better now. When I used the > faster sample rate, I then got more resonable results > >>For >>instance, on my minimill, at 60 IPM, with 16 TPI leadscrews and >>4:1 motor reduction, and with 500 CPR encoders producing 2000 >>counts/revolution, you get 128,000 counts per second. >>Therefore, counts are coming at a rate of one every 7.8 us. >>Obviously, my jitter must be less than yours. But, a scope >>would need to be sampling it at a rate of once a microsecond or >>better before you could even begin to discern jitter on the >>signal. If you are using an analog oscilloscope, then there is >>no sampling. But, without specifying the rate of encoder pulses >>when you see the 50 us jitter, it is hard to know what it means. >>If you had 50 us jitter when the count rate was one millisecond, >>it is not a big deal. If it was when the count rate was 50 us, >>it would be reducing the quadrature angle to zero, and would >>clearly cause errors. So, you have to compare the jitter to the >>count rate. >> > The rate as I originally stated was 1 Khz which translates to a pulse > period of .5 milliseconds low and .5 milliseconds high. So I suppose 50 > micreoseconds jitter isn't too bad then. (About 5%). > >>Ideally, there should be 90 degrees between the 4 >>states of the encoder's A and B signals. They never are, due to >>tiny errors in the manufacturing of the encoder's optics. The >>greater the error, the narrower some of the count states become, >>until they become so small the encoder counter's logic misses >>them. Then, the position will be off by multiples of 4 counts. >> >>When you say homing is OK, but axis is bad, is that all due to >>speed? >> >> > Slowing things down by a factor of ten makes no difference in position > error. It still jumps all over the place. > > Could you explain why the following speed calculation is in error? > I have a unipolar motor, driven in quadrature phase A, phase A not, > Phase B, and phase B not., where phase B lags phase A by 90 degrees. > Motor plate specifies 200 steps/rev > step down from motor to screw: 2.5 to 1 > Screw pitch: .2 in/rev > .2 in/rev x 1/2.5 rev/rev x1/200 rev/step = .0004 in/step. This is > correct because this is what I see the system do. > > For speed: > The max jog speed is set in emc to 90 ipm (1.5 in/sec). When jogging at > the max rate I measured a step frequency of 813 Hz on phase A. > Calculating the table speed: > 800 pulses/sec x 60 sec/min x .0004 in/step = 19.2 ipm > However just by looking at the table move, it is moving much faster than > that. > Is this because due to the nature of quadrature drive, the table > actually moves 4 times faster than the step rate? > This would put it more in the ball park of what I am seeing. > > Jack ensor > > ------------------------------------------------------------------------- > This SF.net email is sponsored by DB2 Express > Download DB2 Express C - the FREE version of DB2 express and take > control of your XML. No limits. Just data. Click to get it now. > http://sourceforge.net/powerbar/db2/ > _______________________________________________ > Emc-users mailing list > Emc-users@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-users ------------------------------------------------------------------------- This SF.net email is sponsored by DB2 Express Download DB2 Express C - the FREE version of DB2 express and take control of your XML. No limits. Just data. Click to get it now. http://sourceforge.net/powerbar/db2/ _______________________________________________ Emc-users mailing list Emc-users@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-users
