Which statement describes the process reaction curve in the Cohen-Coon tuning method?

• A. The process output response after a step input is used to derive model parameters.
• B. The process reaction curve is the graph of output vs input at steady state only.
• C. It is unrelated to step tests.
• D. It is used to measure energy consumption.

Answer: (A)

The main idea is that Cohen-Coon tuning relies on the open-loop step response of the process to build a simple model. You apply a step change in the manipulated variable and watch how the process output changes over time. This dynamic response, the process reaction curve, lets you extract key parameters like steady-state gain, dead time, and time constant. Those parameters are then fed into the Cohen-Coon formulas to compute controller settings. So the best description is that the process output response after a step input is used to derive model parameters. The other statements miss the dynamic, step-tested nature of the reaction curve and its role in tuning.