Encoder feedback velocity error

Hi there, newbee to CNCLinux coming over from Mach.

I am setting up both a mill and lathe, and have a spindle speed reading error of ~4% on both machines which is bugging me (possibly for no reason), in that speed measured by LinuxCNC is ~104% of what I measure actual spindle speed at with a handheld optical tachometer (ie, setting speed M3 S500 gets spindle speed of 500rpm in LinuxCNC velocity reading, but actual measured is only 480, and the percent error stays same as I increase or decrease speed). In Mach3, measured and reported are spot on with single pulse per rev input.

I'm running LinuxCNC 2.5, through paraport. Same computer on both machines.

My mill is setup with a single index pulse per rev, encoder in counter mode, scale of 60 to convert encoder-velocity from Rev/sec to Rev/min. Using speed out in PID loop, LinuxCNC goes to commanded speed and holds it there, it is just off from 'actual' speed measured with tachometer. Error probably doesn't matter on the mill, as I have no plan for putting on real encoder and doing rigid tapping.

My lathe is similar setup, except spindle has a 2048 line encoder run through a frequency divider circuit (a couple of 74LS74 flip-flops to divide by 8) to result in 256 counts per rev. Encoder is in count mode (channel A and index only), 256 count/rev. I've tried both using just the one/rev and the 256/rev counts, and just like the mill, the speed reported in LinuxCNC is ~4% higher than actual. The lathe encoder setup and PID loop appear correct to me, and threading cycles work correctly. However the speed error does worry me in that it could affect threading operations.

Is this anything anyone has seen before, and does reported velocity error actaully matter, if threading operations actually sync the axis moves to encoder counts, does error in velocity reported matter? I'd hate to change the scale factor of 60 (going from Rev/sec to Rev/min) to something like 57.7 to get correct actual speed... doesn't seem right.

Thanks for any insight
Eric

Hi John,

Yes, I did tune PID and it appears to give good performance in controlling during step changes, etc.

It's just that if I put Halmeter on the encoder-velocity pin while it is running, and I just multiply that value by 60, the value is off by 4% from actual measured speed. Since it is the encoder-velocity value (x 60) I'm feeding back into the PID for feedback, it controls it to match the encoder-velocity to the commanded value.

I've attached the ini, and hal, perhaps I've made an obvious error I'm not seeing?

thanks again for the input.
Eric