Adjusting Analog Scaling for Reversed Output Range

Need a Reversed Analog Output?

Struggling to scale an analog signal to a reversed output range in your PLC? This reverse analog scaling equation modifies the standard linear scaling formula to flip the output (e.g., 100% input = 0% output). This is common in automation for applications like tank level sensors. Here’s how to adjust the standard scaling equation, with an example.

Why Reverse the Scaling Equation

The standard scaling equation, y = mx + b, maps an input range (e.g., 4-20 mA) to an output range (e.g., 0-100%). For a reversed output (e.g., 20 mA = 0%, 4 mA = 100%), you adjust the slope (m) and offset (b). This ensures accurate device measurements using Allen-Bradley PLCs and the associated programming software: Studio 5000.

Here is how to Adjust for Reversed Output

• Understand the Standard Equation: Use y = mx + b, where x is the raw input (e.g., 4-20 mA), y is the scaled output, m = (y_max – y_min) / (x_max – x_min), and b = y_min – m * x_min. For reversed output, y_max and y_min swap roles.

• Define Input/Output Ranges: Identify raw input (e.g., 4-20 mA = 4000-20000 counts) and reversed output (e.g., 100-0%). For example, 4 mA maps to 100%, 20 mA to 0%.

• Calculate Reversed Slope (m): For reversed output, m = (0 – 100) / (20000 – 4000) = -100 / 16000 = -0.00625. The negative slope flips the output direction.  BUT, if you already have a standard analog scaling logic block written that you are using, there is a way to keep using the same equation.

• Change Analog Scaling Variables: Using the normal equation, change the input raw range max form  20000 to -20000, and change min from 4000 to -4000.  Then make sure EU Max = 0% and EU Miin = 100%.  Finally, negate the raw input value (i.e. 8mA) going into the equation.

• Implement and Test in PLC: Now, if possible, allow the new sensor to read at different levels.  You should get readings such as: 8mA = 75%, 12mA = 50%, 16mA = 25%.

Master Reversed Scaling

Adjusting the reverse analog scaling equation ensures precise control for unique applications. Apply these steps in your PLC setup today!  Visit Rockwell Automation for more details.  Let Automation Ready Panels design and program your next PLC control system today!  Contact Us

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