Pulse is Okay, but Doublet is Better

    Picking the best Bump Test is Essential when Tuning PID Controllers

    In order to tune a PID controller, a change to the process must be observed. The bump test is how practitioners initiate such a change and reveal the cause-and-effect relationship that exists between a loop’s Process Variable (PV) and its Controller Output (CO). In the absence of a bump test, tuning a plant’s PID controllers is little more than guesswork.

    There are various types of bump tests that can be performed. The Step, Pulse, Doublet and PBRS are among the most commonly used. Some are more practical than others. Here we focus on the Pulse and Doublet tests since both allow practitioners to observe the process moving in opposite directions.

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    Setting the Standard

    Pulse Test

    The Pulse test involves the execution of two steps both in succession and in opposite directions. It begins by moving a controller’s Set Point (SP) in one direction. Best-practice is to move SP by a value that is 3-4 times the noise evident in the process. When the measured Process Variable (PV) shows a response to the change the SP is then stepped in the opposite direction and returned to its original value. While the Pulse test shows the process moving in both directions, it only reveals one-half of a process’ dynamic behavior. As such it is not the best test. The Doublet is the superior option.

    Doublet Test

    The Doublet test is essentially two Pulse tests that are conducted in rapid succession and that span both sides of a controller’s normal range of operation. Like the Pulse test the Doublet begins by moving the controller’s SP in one direction so that a clear response to the PV results. A key difference comes into play once the process initially responds to the change. Instead of returning the SP to its original value the SP is moved in the opposite direction and to the other side of the process’ operating range. Note that it’s not necessary for the process to settle at either of these two new SP values. The test is completed by returning the controller’s SP to its original value.

    Why is the Doublet Test the best?

    Whenever practical Control Station highly recommends the Doublet Test when tuning PID control loops. The Doublet provides data that is representative of a controller’s full range of operation. In contrast with a Pulse that only examines one side of a controller’s operating range, the model and tuning parameters resulting from a Doublet test allow for safer and more effective control.