How to fine tune motion?

[Lasershop]

V2XT w/FMDC. New EMI motion cards (all 3) installed a year ago. The paperwork has the adjustment numbers like "Adjust the balance pot for 800 at rest". Problem is.... I dont know what the hell that means. Is this something I can do on screen or is there some kind of external monitoring something? Runs fine but seems to be out about .015". Screws are in great shape but I suspect the motion needs adjustment. Our local Bridgeport guy is an idiot and it seems to be $400.00 a visit so I have been hesitant to call.

[machintek]

I am not familiar with what the EMI software will allow. I will tell you and alternative way of doing this.
The encoders go to the FMDC. take the covers off the motor pulleys. Move axis to the center of travel. With the drives ON, pull the plug on an axis encoder at the FMDC. If the balance is off, the pulley for that axis will start to move. Adjust the balance pot so the "drift" stops.
I am assuming that the 800 refers to a DAC command. On the machines with the FMDC, it was shipped with a plug on the RS232 port. This plug had some jumpers that allowed viewing the "terminal" on the machine CRT. This allowed viewing the fist event log and the status of each axis. See maintenance manual.

George

[Lasershop]

Software is standard V2XT. There is a screen "SysTerminal" that shows XYZ status with just a bunch of scrolling characters and numbers. I have read the test section of the manual but for the life of me cannot interpret what it is talking about. The EMI manual says to adjust their numbers according to the steps of the V2XT manual.

[Paraprop]

George,

could yo please tell me what FMDC stands for in your thread here above ?

Thanks

Daniel.

[machintek]

The FMDC is the board that runs the axis. It has a Motorolla 68000 processor.
It has 8 LED across the top of assorted colors.

George

[HuFlungDung]

George,

Doesn't "pulling the plug on an axis encoder" cause a runaway? Or does this also disconnect the feedback loop so that the controller does not attempt to move the motor?

Just wondering, that's all, as you are the expert on these things

[Paraprop]

There are much issues about ball screw acuracy and backlash with the Bridgeport and clone retrofits.

I noticed that Industrial Hobbies (IH) mounts the servo encoder onto the end of the ballscrew and not onto the servo motor which is on the other end of the reduction. This allows them to make sure the reduction does not introduce any further acuracy/backlash problems.

Now, my question to the more experienced people in this forum. Why is it not feasible to use a linear encoder similar to the ones used for DRO's to provide a feedback signal to the servo drive as this would also take care of all the backlash issues remaining even with a ballscrew as well as the inacuracies of the ballscrew lead. For sure there is agood reason for not doing so but I just can't figure it out myself.

Thanks,

Daniel

[gus]

It's all about the money.

New linears are upwards of 300 bucks a pop.

If you have a tight ballscrew, it can be extremely close. Linear encoders have other issues[ IE distance between center of ballscrew and edge of table causing errors when table is cocked under load

[Paraprop]

Do you know of anybody who did this kind of control loop ?

[gus]

Most VMCs have linear scales as an option. Most closed loop controls don't 'care' what the encoder is, just counts per mm or whatever