timer control

Im currently using RSlogix 5000 but the programming language really matter much.

I have a time preset that I need to change depending on the speed of a belt. As the speed goes up, i need to reduce a time which activates a reject system. This is to allow for belt slippage and or speed changes.

Basically I have a value that goes up I need another value to go down in reverse proportion to that first value and vise versa.

I have tried using a PID controller but the problem with that is that I have no setpoint as I only have a speed reference to work with that changes the time value. I have also looked at using the Scaling FBD (SCL) but this does not allow inverse scaling.

Any info or points in the right direction would be great because chances are I have been looking at it to long and I am missing something simple.

I think all you have to do is a little math:

If you have a point that you can detect the object that needs to be rejected, then you need to know the speed of the belt and how far the reject is away from the point of detection. You can then calculate the time that it will take the object to get from the point of detection to the reject.

A simple compute (CPT) will do what I described. (inches for my example)

D = Distance from Detection point to Reject
S = Belt Speed in FPM

D/(S/60*12)*1000

This will give you a timer preset in milliseconds. "D" will never change, but "S" will vary when the belt speed changes. "/60*12" converts to inches per second and *1000 will give you milliseconds.

The more accurate you are on the distance and speed (decimal points) the more accurate your timer preset will be. If you use whole numbers, it will round.

Make sure you put limits to NEVER let the timer preset get negative!

Number of Decimal Points != accuracy.