Problem Description
Heating of the liquid in the tank is to be performed. To heat this liquid, steam flow is controlled. If the
temperature is detected less than the set point, increase the steam flow and vice-versa. Implement
automation of this process in PLC using Ladder Diagram programming language
Problem Solution
To detect temperature, RTD PT100 is used. This RTD has a range from -200°C to 650°C.
Let’s say that the temperature of liquid in the tank is to be maintained at 100°C. Calibrate RTD to
operate in this range.
RTD PT100 has a resistance of 100Ω when it is at 0°C and 138.4Ω when at 100°C.
Calibrate 4-20mA output such that when input is 0°C, it gives 4mA output and when it is 100°C, it
gives output 20mA.
Feed this data to Analog Input Module of PLC. Input module converts 4-20mA into equivalent 0000h
to FFFFh hex number. This means when temperature is 0°C, data stored in the register is 0000 0000 0000
0000, and when output is 100°C, data stored in register is 1111 1111 1111 1111.
Analog Output module converts this data into equivalent 4-20mA which is then converted into 3-
15psi to operate control valve.
One more method to solve this problem is by calculating Output in terms of 4-20mA for -200°C-650°C
that when -200°C, it gives 4mA and when 650°C, it gives 20mA.
But then this would not be precise control action because data conversion gives minor errors while
performing conversion.
PLC Program
Here is PLC program to Heat the Liquid in Tank by Steam Flow, along with program explanation and
run time test cases.
List of Inputs and Outputs
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