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

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.

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/1SVR011712R1700/cc-e-v-i-signal-converter-singlef-0-10v-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.