Explain analog value scaling in the PLC using an example.

Explain analog value scaling in the PLC using an example.

Application:-Make PLC program to scale analog value. Temperature controller value in the display so it can show the actual temperature.

Explanation:-

Assume one temperature controller system in the plant. The temperature controller is used to controlling the temperature. Here temperature controller is measuring temperature 200 degrees and we want to display this temperature in the HMI. So we need to scale the voltage value.

Generally in the Siemens PLC count value is 0 to 27648 according to voltage value in the range between 0 to 10 V DC. So here we need to scale 0-10 VDC voltage into count to show the actual temperature.

The temperature controller is generating voltage in the temperature range between 0 to 400 degrees maximum. Hence if the temperature is 0 then generated voltage by temperature controller I 0 and if the temperature is 400 degree then voltage generated by controller is 10 VDC.

 

So the calculation for the controller,

If the measured temperature is 200 degree then generated voltage?

Measured temperature X Maximum voltage/ maximum temperature

So,

Output voltage from the controller=200 x 10/400=5 V DC

 

 

 

 

Voltage to PLC count scaling, equation,

Measured PLC count=Measured voltage x maximum PLC count/ Maximum voltage

                                  =5 x 27648/ 10

                                  =13824

Calculation for HMI

Here the measured voltage is 5 V dc from the temperature controller

So

Actual temperature scaling equation for HMI

              =Measured count X max temperature / maximum PLC count value

              =13824 x 400 /27648

              =200 degree

So HMI will show 200-degree temperature.

PLC program:-

Write the ladder program for the above application using ladder diagram language. Here for ladder logic, we can use any supported PLC systems like S7-1200, S7-300, S7-1500 or any other PLC which can support this instruction. For PLC logic we need digital inputs and outputs.

 M memory:-

Start switch S1=I0.0

Digital Output:-

Measured count=MW10

Calculated value=MW12

Actual Temperature=MW14

PLC program Explanation:-

In network 1, we have done the multiplication of measured value and maximum temperature value and stored the result in MW12.

In network 2, after multiplication, we have done the division of calculated value and the maximum count value of PLC and stored result in the MW14.

The above calculation method is the general method for scaling, it can be used for any PLC. Some PLCs are providing special functions or special instructions for scaling.