The proportional band is the amount of change in the process variable that produces a full change in the PID output.
The proportional band is related to the gain as shown in Figure 1. The Metasys PID controller uses proportional band as the input to the controller, rather than the term gain. For controllers that use PRAC+ automatic tuning, these controllers also report the effective (eff) proportional band. See PID Inputs, Outputs, and Properties for descriptions of eff proportional band and these additional inputs involved in tuning:
- integral time: The time required by integral action to contribute the same amount to the controller output as the proportional term for a constant error.
- process dead time: The amount of time until the process variable sees results from a step change.
- time constant: The amount of time that elapses from when the process variable starts to change until it reaches 63% of its total change in value.
- process range: The change in process variable as the output changes from 0 to 100%.
- saturation time: The amount of time the PID output is at the low or high limits for the saturation status to be set to low or high.
- interval (period): The amount of time between PID executions.
Note: Prior to Release 5.1, or in certain upgrades where the PID pre-processor is not connected to the PID inputs of interval, proportional band, integral time, and saturation time, the initial values of these PID inputs are calculated by dead time, time constant, and process range.
For information on how proportional band is calculated, see Process IDs.
Figure 1. Proportional Band and Gain
