How scaling works in PLC?

Converting the input ( analog voltage 0 to 10V / current 4 - 20 mA) into the understandable engineering values as per industry standard ( analog voltage 0 to 10V / current 4 - 20 mA) is called “ SCALING ” .

The Input instrument which is producing analog values( for eg. 0–10V) is give to Analog Input Module. This module converts that analog values into counts/raw value (i.e) 0 to 32768.

Whenever the input instrument produces 0V our PLC reads 0.

Similarly , Input Instrument 5V = PLC reads 16384.

Input Instrument 10V = PLC reads 32768.

So using this raw value , we can scale our PLC to convert the Input to Output as engineering values as per industry standard so that we can control the output device.

Author - Monish Kumar M


Scaling in a Programmable Logic Controller (PLC) refers to the process of converting raw input values from sensors or other devices into meaningful engineering units. It allows PLC programmers and users to work with values that are easier to interpret and use in control logic or HMI (Human-Machine Interface) displays.

Scaling is commonly used when dealing with analog signals, such as temperature, pressure, level, or flow.

Here’s a step-by-step explanation of how scaling works in a PLC:

Understand the Input Range

Determine the minimum and maximum values that can be measured by the input device. For example, if you are scaling a temperature sensor with a range of 0 to 100 degrees Celsius, those values will be used as the input range.

Define the Desired Output Range

Determine the desired range of values in engineering units that you want to work with. This could be a specific temperature range, such as -40 to 150 degrees Celsius.

Calculate the Scaling Factors

Calculate the scaling factors that will map the input range to the desired output range. This involves determining the slope and intercept of a linear equation that relates the input and output values.

Slope (M)

The slope represents the rate at which the input values change with respect to the output values. It is calculated as the difference between the desired output range divided by the input range. In the example above, the slope would be (150 - (-40)) / (100 - 0) = 1.9.

Intercept (B)

The intercept represents the offset or bias between the input and output values. It is calculated as the desired output range start point minus the product of the slope and the input range start point. In the example above, the intercept would be -40 - (1.9 * 0) = -40.

Apply the Scaling Formula

Once the scaling factors (slope and intercept) are determined, apply the scaling formula to convert the raw input value (X) into the scaled output value (Y). The formula is: Y = (M * X) + B.

For example, if the raw input value is 50, the scaled output value would be: Y = (1.9 * 50) + (-40) = 55 degrees Celsius.

Implement Scaling in PLC Programming

In PLC programming, you would typically use the scaling formula within the program logic to convert the raw input values into scaled engineering units. This can be done using mathematical instructions or function blocks provided by the PLC programming software.

By applying scaling in a PLC, you can work with meaningful engineering units that are easier to interpret and manipulate within the control system. It allows for better accuracy, control, and visualization of process variables.