Help with Calculation

I have a machine that I am trying to modify to run like it's older twin.

It's a sheet material coming into a center winder and we need the line speed of the material for the winder to run correctly and the nip that feeds into the winder is ran by a power flex 755 VFD and the motor goes into a gearbox and drives a chain to drive the roll.

It's in a very bad place and will take a lot of work to get a look at the details for the mechanical such as gearbox ratio drive and idle sprocket teeth for the chain and roll circumference / surface area which is everything I think I would need to determine the line and material speed.

The PF 755 sends a 4-20 analog output to the winder PLC based to the motor RPM feedback.

in the existing unit that works like this the compactlogix has a SCPC instruction that has input min 6200 and input max 29476 and scaled min 0 and scaled max 34.15 with the output tag being Line speed feet per minute
but I am trying to understand where they got ht e numbers for input min and max and scaled min and max.

Also on the new machine the line speed drive analog out to the winder plc comes from a PF 40 4-20 ma analog not a 755 analog output but I don't think that makes a difference does it?

Any tips on how to figure this out?

David,
I am not familiar with the SCPC Instruction, based on your description it sounds like a Scaling Add On Instruction. I use the SCLP (Scale with Parameters) AOI from Rockwell in my programs. Sounds like yours is similar so I will take a stab at it.

The instruction will take a input value and will scale a output based on the value of the input min\max and output min\max. This type of instruction is used very often for getting a "real world" number from a analog input.

To answer your questions about where the input min\max number come from, most likely they come from the Analog Input Cards A to D counts (found in the manual). So 4mA = 6200 counts and 20mA = 29476 counts. The output min\max numbers come from what the input actually represents. In your case 4mA = 6200 counts = 0 fpm speed and 20mA = 29476 counts = 34.15 fpm speed.

Now to find your speed. Do you know if the new machine is geared the same as your old one? Can you get any info from the OEM?

If you can't get any info from your OEM then here are a couple of ideas you can try:
1. If you have a roller that is easy to measure and get to you can use a portable device to determine the RPM of the roller. If you know the Diameter of the roller you can get the Circumference of the roller x RPM will tell you the inches per minute.
2. Mark a piece of material every x Inches and time how many inches go into the machine after x Seconds. Then you can convert to Feet per Minute.

Hope this helps.