VM-3 Linear Scales?

[100]

I am looking at the VM-3 machine and they have an option for linear scales. I am wondering how the linear scales work with the cnc control? Does the control only look at the information coming from the scales for positioning and is this a true closed loop system? I have had problems with ball screws that are worn in one area and it makes it hard to do precision work. Will the linear scales solve the problem of a ball screw worn in one area. Thanks.

[Al_The_Man]

One, motor encoder or linear scale is still a closed loop system.
But you may increase your problems, to implement scales, especially if you have backlash requires a dual feedback system, an encoder on the motor as well as the scale input, and for a system with backlash, the PID LOOP is divided between the scale and the motor encoder, otherwise you will most likely get 'hunting' problems.
So unless you are sure the machine can be set up this way, you may need a rethink.
Al.

[AMCTony]

Linear Scales are a direct from Haas Option so I do not see any problems that could occur because there is no DIY involved.

[100]

Al, I don't think the encoder is a true closed loop system because the encoder does not know how far the table has moved. If a belt brakes the motor keeps turning. Thanks for the info about how the PID LOOP is divided between the scale and the motor encoder, that seems like a good system. I wonder if the control has a memory about how the machine acts or if it just uses the data as it enters. If you had backlash in one spot along the ball screw would the control remember for future, faster, more accurate movements?

[Gandalf]

They come only as factory installed. You can not add them after.

[jimgarner]

My impression from discussing this with Haas engineers at IMTS is that the linear scales are used to detect expansion in the ball screws due to heat so that a correction factor can be applied to the positioning of the screws and that output from the scales are not used in a classical "feedback loop" to control how far the screw is turned to position the table. The system still remains an open loop system.

Jim

[100]

Thanks for the info Jim... I think the true closed loop is a hard thing to integrate into the machines or it would be more common...

[jimgarner]

While the loop is closed on the rotational position of the ball screw, I do think it must be far more difficult to close the loop on the output of a linear scale attached to the table.

Probably related to the fact that a small amount of "hunting back and forth" is acceptable with the rotational position of the screw, but would not be with the linear scale because a good deal more actual table movement would be occurring with the scale in the loop if hunting occured.