Servo system tuning problem??

[REVCAM_Bob]

O.K. this one is for the servo Guru's.
I've got a servo system that moves a load up and down on a rail system.
The total load is about 400 lbs.
I am driving it with a 1.5" dia. ball screw with 1/2" pitch. I am useing a 1Kw
servo motor coupled to the ball screw with a 5 to 1 gear reduction.
I do not have a counter balance for the system.

The problem I am having is when moveing the load down, or lowering it, when I tell it to stop, the system jerks, sort of uncontrollable jerking upwards.
It will also do this same jerking intermitantly when moveing down at constant
speed. It seems to do it more so above 125" per minute, and motion above
200" per minute is not feasable. I have tried every tuning parameter (that I can figure out how to use) and nothing seems to help. It seems the motor
(1Kw and max of 50 in*lb torque) should be strong enough according to calculations.

I had in the system prior to this configuration a 3kw motor with a direct drive to the same ball screw, and it did the same thing. The only way I could get it to work was to leave the brake on when moveing down. This seemed to stop the jerking motion. Someone suggested it was to high a load to inertia ratio (what ever that means!) and I thought the gear reduction would be a sure fix. But to no avail!!

Will a counter balance fix it? Is a counter balance the typical scenario for
a hanging load like this? Is there a tuning problem?

I am using a galil card, and amc drives if that makes any difference.

Thanks for any thoughts you may have!!

[Al_The_Man]

I would most definately put this down to the Load inertia, you have a fairly high lead ball screw coupled with the 400lb load.
The jerking you are getting at constant speed is probabally due to the following error correction created by the overhauling load.
The vast majority of Mills and slant-bed lathes with this kind of vertical load use a counter balance of some kind.
Al.

[REVCAM_Bob]

Thanks Al, I am going to attempt installing about 330lb of counterbalance
today. Any one else have any thoughts?