Question

Process reaction curve method of tuning is done for a system in

• A. open loop
• B. closed loop
• C. interacting loop
• D. non interacting loop

Hint:

Ziegler-Nichols has two methods:
1. Process Reaction Curve Method: Done in open loop using step response.
2. Continuous Oscillation Method (Ultimate Gain): Done in closed loop by bringing the system to its stability limit.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks to identify the system loop configuration (open loop or closed loop) required to perform PID controller tuning using the Process Reaction Curve (Cohen-Coon) method.

Step 2: Detailed Explanation:

• The Process Reaction Curve method (originally developed by Ziegler and Nichols, and refined by Cohen and Coon) is an experimental tuning technique.

• To perform this test, the control loop is set to manual mode, which breaks the feedback loop, creating an open-loop system.

• A step change of magnitude $M$ is applied directly to the controller output (manipulated variable), and the resulting transient response of the process variable (controlled variable) is recorded over time.

• This dynamic step-response curve is called the Process Reaction Curve.

• From this curve, key parameters such as the process gain ($K_p$), dead time ($t_d$), and time constant ($\tau$) are calculated to determine the optimal PID controller settings.

• Since the feedback loop must be disconnected during the step test, the method is strictly performed in open loop.

Step 3: Final Answer:
The process reaction curve method of tuning is conducted on an open-loop system.