How to scale the analog inputs and outputs - PLC scaling examples

How to scale the analog inputs and outputs - PLC scaling examples

 

Analog I / O whether it is 4-20mA or 0-10 V are connected to pressure transducers, level probes and other analog devices. As the PLC programmer or engineer control Will you need to get some knowledge about scaling plc analog signals from the I / O modules. Scaling helps to convert the analog value to measure units (pressure, level etc..).
Most industrial applications usually require measurements in engineering units, the which Provide more meaningful data. We can achive this by using the conversion formula shown: Example by AutomationEngineering.

Time for example
If We Want to measure levels in tanks from 0 to 65 meters then our L = 0, H = 65 max value from our analog cards 32 000 (in engineering units). A current value from the analog inputs is 13 454. Now if We calculate it using equation We Will get our current level of 23.7 meters.

Looks slightly different situation now with Analog Output.
Your PLC program has to calculate the digital value to send to the analog I / O module. There are many Airways to do this, but most industrial applications, are understood more easily if you use measurements in engineering units. We can do this by using the conversion formula shown.


Consider the following example with the level of measurement

We know there is 38 meters of liquid in tank. Our H = 65m L = 0m. Max analog output value is 32000. We use our calculation if We Will get 18 707 the which We Are going to send to our Analog Output.

Programmable Logic Controllers have many ready to use the instruction for scaling.
Siemens S7-200 PLC scaling of the analog input to the output data format REAL (S_ITR)
The S_ITR function permits you to convert the analog input signal into a normalized value of the between 0.0 and 1.0 (type REAL)
The following table explains the abbreviations used:

Parameter	Description
Ov	Scaled output value (output value)
Iv	Analog input value (Input value)
Osh	Upper scale limit for the scaled output value (Output scale high)
OSL	Lower scale limit for the scaled output value (Output scale low)
Ish	Upper scale limit for the scaled input value (Input scale high)
Isl	Lower scale limit for the scaled input value (Input scale low)

our instruction