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Old February 9th, 2018, 09:26 AM   #1
richleva
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Canada

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Join Date: Sep 2012
Location: ontario
Posts: 64
Question Alalog Output Scaling, Please help !

Hello,

I need to control an Acid pump at a water treatment plant, Using an analogue output of a micrologix 1400. This PLC only has 0-10V analogue output card. So I installed a 500Ohm resister across the pair of my instrument cables, to get my 4-20Ma. So at 2Votls will be my 4mA and 10 Volts will be 20mA.

PLC Resolution (0-4095)

Acid pump 1
0-60 L/H

Acid pump 2
0-12000 strokes per hour

In The PLC. I'm using RXLogix micro developer, and I'm using the SCP instruction to scale

I tried using one SCP block, but the input number that is entered is much lower than what is display in the pump

Example:

F8:1

input: F8:1 --->> entered 7 l/h in the HMI

input min: 0
input MAX: 60

scale min: 819
Scale max: 4095

Output O:0/0 --->> Pump said about 12 l/h


I tried using two SCP block per pump, to scale input number from the HMI to 4-20 and the second would scale the 4-20 to the PLC 12 bit analog card (4095)

but I got similar results...

SCP 1

Input: F8:1 ---->> entered 15l/h in the HMI
input min: 0
input MAX: 60

scale min: 4
scale max 20
Output F8:2 ---->> 8 mA

SCP2

Input: F8:2 ---->> 8 mA

Input min: 4
input max: 20

scale min: 819
scale max: 4095

Output: O:0/0 ----->> 1638


The results for example

today I entered 30 L/H in the HMI the pump said 41.8

for the other pump I entered 6000 strokes per hour, pump would say 12000 strokes per hour.

PLEASE HELP, if anyone has ideas....


Thanks everyone

Last edited by richleva; February 9th, 2018 at 09:29 AM.
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Old February 9th, 2018, 09:36 AM   #2
RDS
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Posts: 169
The 1400 analog output can only source into 1000 ohms or greater. The 500 ohm resistor is loading down the output so you will never reach full scale.
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Old February 9th, 2018, 09:38 AM   #3
lfe
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France

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Join Date: Jun 2007
Location: Barcelona
Posts: 228
You can't convert 0-10V to 4-20 mA output by simple adding a resistor, you need a converter like this:

http://new.abb.com/products/1SVR0117...v-4-20ma-24vdc

and probably it will solve the scaling too

In addition a the voltage output need to be connected to high impedance input, if not the voltage drop across the cable generate a significant error.

Last edited by lfe; February 9th, 2018 at 09:47 AM.
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