Every instrument has at least one input and at least one output. For instruments responding linearly, the correspondence between input and output is proportional:
A practical example of this is a pressure transmitter, in this case one with an input range of 0 to 1023 PSI and an output of 4-20 mA
If you happened to measure an output current of 14.7 mA from this pressure transmitter
x = { ( 14.7 – 4 ) / 16 } * 1023 = 684.13 PSI
It would be a simple matter for you to calculate the corresponding input pressure to be 684.13 PSI.
Now consider a pressure transmitter is connected to the PLC system. The pressure transmitter is connected to the Analog input card of the PLC system. The AI card have inbuilt Analog to Digital Converter (ADC). The ADC converts the input 4-20mA value coming from pressure transmitter into equivalent Raw Counts.
The AI card finally sends this Raw counts value to CPU card. The CPU have the pre-defined Pressure transmitter values like its Range (LRV & URV), units etc. The CPU calculates the Process variable (pressure value from pressure transmitter) using these Raw Count from AI card & pre-defined LRV, URV, units etc… and displays the process variable on operator workstations/HMI.
Also Read : Formula to Convert 4-20mA into Process Variable
Take for example this analog-to-digital converter, with a 10-bit output (a “count” range of 0 to 1023) and a 4-20 mA input:
Calculate the corresponding “count” output of this ADC circuit given a 6.82 mA input signal.
Here we are calculating the ADC output in terms of count. So Input to the AI card is “x” and output is “y” as shown in below fig.
y = { ( 6.82 – 4 ) / 16 } * 1023 = 180 count
In a PLC system, CPU converts this count into equivalent Process variable and displays on the operator workstations.
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View Comments
Dear Mr. S.Bharadwaj Reddy
I like to say 'Thank You' for your wonderful contribution to the people who are in search of knowledge like me for instrumentation and especially about PLC as I m from a Mechanical background; as well inspires me to contribute back to the community.
Regards
Rajan.
Hello sir,
very useful article for me ,i got clear idea from this article .thank you very much.
But one thing, better to check the formula mentioned in the field pressure transmitter to PLC ADC count convertion. in this formula x,y interchangely mentioned.
Hi Lukkuman hakkim,
Thank you for the update. Corrected.
thank you so much.
Thank you sir for your kindness! you are a great man!!
please how can i scale parameters in logix 500 and logix 5000 controllers
I need your help on these
Thanks very much
Hi Mr.bharadwa reddy first of all thanks for your updation ..
Can you update what is different between Instrument earthing and electrical earthing ,why it is need separate for earth pits.
Electrical system handles high voltage applications . Instrumentation systems handles low voltages system.
Instrument earthing is used to ground any noise ( low level interfering signal) in the signals .
Also instrument earth pit resistance is maintained at low ( nearly 1 ohm )
.
If electrical earth is connected with Instruments earth , there is a potential of current flow in between these earthing systems. Hence these two are kept different.