[gavztheouch]

Hi I am building a laser cutter, bed size is about 1.2 by 0.9m.

I have two 400w panasonic series S servo motors one on each ballscrew driving the y axis

They have a resolution of 5000 pulses per revolution.

Motors are coupled directly onto 20mm pitch ballscrew

I am struggling to keep the postion error low. Depending on the speed it can be as high as 140 pulses. I would like it if possible below 10.

What I would like to know is is this sort of accuracy possible in servo motors considering my application.

Here is a photograph of one of the ballscrew as you can see the gantry is lightweight aluminium.



Many thanks

Gav

[Al_The_Man]

Is the 5000p before x4?
If so that is a resolution of .001mm so 140p error would result in .0055" error?
140 pulses is usually considered high following error for normal cutting position operation?
Al.

[gavztheouch]

Hi Al 5000p is after x4. I think I can have 10000p after encoding but its set at 5000p for now.

I have a large cnc router with a gantry weighing nearing 1ton and I have had it running with an error of below 10 all day. So im confused that these servos are struggeling with such a lightweight gantry on the laser cutter.

Cheers

Gavin

[mike_Kilroy]

I am struggling to keep the postion error low. Depending on the speed it can be as high as 140 pulses. I would like it if possible below 10.

Assuming you have vel loop and pos loop gains adjusted as well as possible,
so since your PE goes up with speed (as it must with a feedback loop w/o feedforward) sounds like you do not have feedforward turned on or high enough?

got a term like that you can increase or enable?

[gavztheouch]

Hi Mike, I re-read the manual again this morning and spotted the velocity feedforward setting and I set it at around 60%.

Now at the same speed I have an error of +-20 happy days.

I would still like it lower and im sure they could be tuned better but its a good start.

Thanks for your help

Gav

[mike_Kilroy]

FF does not 'tune' anything or effect 'tuning'

if set too hi PE goes the other direction like you have - that is a bad thing - ur FF is too high. set it to not bounce ahead of the command - that is bad. if the result is still too hi then you need to address it in 'tuning' the proportion, integral, derivitive terms.

[gavztheouch]

What is a normal ff value? Panasonic recommend 50 as a starting point.

Is it wise to completely remove ff when tuning?

Cheers gav

[mike_Kilroy]

nothing like normal for across the board answer; if P says 50, then I would use that.

yes, ff should be 0 during normal tuning to be sure it does not cause erroneous results. after tuning is done, THEn put in FF until you get very small PE that does NOT bounce thru 0 - that means the motor is IN FRONT of the command, a bad thing that is called instability.