servo motor indexing distance adjusting

Controllogix 5k, Kinetix 6000, RSLogix 5K, Kinetix servo motor.


We are using a servo motor to index-drop parts into a container slots. The drop position error was gradually increasing as the indexing progresses away from the home position. So we repeatedly adjusted the Conversion Constant value under the Conversion tab of the axis property menu. It worked and now the parts are dropped exactly into the center of the slots all the way to the last drop. But it took us a long time to accomplish this. The conversion constant was drive counts per 1 inch. I have two questions:

1) What are these drive counts referring to? Incremental pulses from the servo motor's encoder?

2) was there a better way to adjust the indexing distance than the way I did?

Please let me know if I omitted info about my system.

Thanks.

unsaint32 said:

1) What are these drive counts referring to? Incremental pulses from the servo motor's encoder?

Click to expand...

Yes.

2) was there a better way to adjust the indexing distance than the way I did?

Click to expand...

You should have been able to specify the slot positions in inches. The counts per inch scale factor is used to convert the position in inches to counts internally. You shouldn't need to mess with the counts.

The problem that often occurs is that
1. the counts per inch is not an integer number. It usually isn't when PI is involved. Usually a lead or ball screw system doesn't have this problem but a rotary gear or belt system will.
2. The indexing unit 'slips'.

If neither of the two problems above are true then you simply made it harder than it needed to be.

which version of RS5000?
I think Version 18 or 19 has a calculate scaling button on the servo properties.

In SERCOS Rockwell allows you to set the counts per revolution of the motor before scaling to user units - this allows you to use irrational factors (eg 1/3)

in a positioning system you can pre-calculate the scaling exactly as you know the screw pitch (or tooth belt pitch) Gearbox ratio and motor counts per rev

IF you are using a compressed wheel (eg a nip wheel and no direct drive) then you will get errors due to the mechanics of the system ie PI - In that case I prefer to use wheel circumference in the scaling to bypass PI and I usually use a multiplier eg *10 or 1000 of the circumference to improve scaling

Note: Any servo system will perform better if you can use higher counts/rev so in RS5000 do not use 1024 counts per motor rev better values are greater than 200000