Analog scaling

dajohn6

Member
Join Date
Sep 2006
Location
Georgia
Posts
21
I'm need some help writing a program to control 4 A-B 160 drives with a SLC 5/03, using rslogix 500, using a panelview 550(touchscreen only), using panelbuilder 32 software . We are presently controling the drives using 5k ohms pots. We want to take out the pots and use presets according which recipe is chosen. We want to use 0-60 Hz(that is what the operators are use to) displayed on the PV. We also want to be able to manually inc/dec the output if needed. We are using 4-20mA output to the drives. I'm not knew to plc's, but I use them for troubleshooting and can write simple programs. I don't really understand how to use the scaling parameters (rate and offset), or is there a better way to write it not using the scl instruction? I have read the help section on rslogix 500, but I think that if had a good example I could understand it better.
 
What kind of example would you prefer - the RSLogix ladder for a PID instruction to control a drive, OR a Panelbuilder 32 example of the operator controls for a variable speed drive?
 
Here is an RSLogix 500 program that has many examples of scaling for analog inputs and analog outputs. I used the CPT (Compute) instruction instead of SCL as it more closely duplicates the manual calcualtions (see comments above rungs) that I do to check my equations. Look at Lad 4 for PID loops and examples of Variable Speed Drive rung logic for some fans.

This project used RSVIEW32 as the graphic operator interface, instead of a Panelview, but the procedures are similar.
 
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there are a couple of PDF files on "scaling" available for download in the "Sample Lessons" area of my website ... they should answer all of your questions ...

is there a better way to write it not using the scl instruction

if your processor supports it, the SCP instruction is usually the easiest way to do scaling ... the CPT method that Lancie mentioned usually takes up less processor scan time ...

good luck with your project ...
 
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I have three letters for you: SCP

This is my all time favorite RSLogix instruction.

Learn it, live it love it.

Really simple. You put 0 and 60 into the input min and input max of the function and scaling numbers for you 4-20 output in the output min and max. Add the addresses of your registers and you're good to go.
 

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