Converting Between Percentage of Full Scale and Engineering Units

The default setpoint, process variable, and output ranges for the PID VIs correspond to percentage of full scale. In other words, proportional gain (K_c) relates percentage of full scale output to percentage of full scale input. This is the default behavior of many PID controllers used for process control applications. To implement PID in this way, you must scale all inputs to percentage of full scale and all controller outputs to actual engineering units, for example, volts for analog output.

You can use the PID EGU to Percentage VI to convert any input from real engineering units to percentage of full scale, and you can use the PID Percentage to EGU VI to convert the controller output from percentage to real engineering units.

Note��The PID VIs do not use the setpoint range and output range information to convert values to percentages in the PID algorithm. The controller gain relates the output in engineering units to the input in engineering units. For example, a gain value of 1 produces an output of 10 for a difference between setpoint and process variable of 10, regardless of the output range and setpoint range.