Servo & Load Inertia Ratios

[azsigns]

Has anyone experienced unstable servo operation due to high load/motor inertia ratios?

I've found several articles on the subject, the best one being:
http://www.motionvillage.com/welcome...es/Inertia.PDF

Just wondering if anyone has had some practical issues with this in their past designs?



Chris

[Al_The_Man]

Chris, What symtoms are you getting, usually wrong inertia match shows up when you want to move an axis in rapid and a servo goes into overload, or a stepper will stall, there are quite a few programs where you can plug in your numbers and find out, the magic number is usually to keep the motor to load inertia ratio lower than 10:1 . the answer of course is to apply a reduction of some sort, fortunately the ratio is reduced by a squared of the reduction.
Al

[azsigns]

The machine is not complete. I was just deciding if I should use a belt drive reduction or not to avoid a potential problem.

The servo's I have are of sufficent torque without reduction.

I was using Danaher's Motioneering software and if the input's are correct I'm running at about 7.5:1 with the servo's I have.

The table is very very heavy ~1500lbs but quite rigid.

Before using the software I was not even aware of matching inertias.
I was just wondering if this was really significant or more of a theoretical rambling

Thanks
Chris

[Al_The_Man]

Chris, If you want to achieve a certain rapid feed acceleration rate, it is well worth designing for it, I made the mistake a long time ago of winging it and regretted it, I usually shoot for 5:1 now to be on the safe side, and that gives a better margin in case of oversight or the desire to push the acceleration a bit more.
Al

[azsigns]

Thanks!

I've put alot of time, money, & sweat into this machine so I should not skimp on the belt/pulley now.

I just don't have alot of room for a pulley.

BTW, do you use Danaher's Motioneering software?
If so do you know what the difference between "slide weight" & "carried axis weight"?
Also what do you think the GIB force of a 30mm THK is? Or does it even have one?

Chris

[Al_The_Man]

Chris, I have used Motioneering, its not bad. as far as I know the slide weight is the weight of a particular axis and the carried axis weight is the the weight of an axis when carried by another axis, this would be like a gantry table where the y axis is also carrying the x and z axis.
Its a bit like a CNC mill where they ask for the table weight and the max weight carried by the table.
it seems a bit convoluted.
Al