Practical Guide to Process Control

Practical Loop Tuning is an important aspect of control system design, as it ensures that the system behaves as desired and meets the required performance specifications. There are several techniques and methods that can be used for loop tuning, including Zieghler-Nicholes, Cohen-Coon, and Lopez. In this blog, we will discuss these methods in detail, along with some key loop performance measurement techniques such as quarter-wave decay and disturbance rejection. Zieghler-Nicholes Method: The Zieghler-Nicholes method is one of the most widely used methods for loop tuning. It is based on the concept of the ultimate gain and ultimate period, which are defined as the gain and period at which the system becomes unstable. The method involves adjusting the proportional, integral, and derivative (PID) gains until the system reaches the ultimate gain and ultimate period. This is done by first adjusting the proportional gain until the system reaches the ultimate gain, and then adjusting the integral and derivative gains to achieve the desired period. Cohen-Coon Method: The Cohen-Coon method is another popular method for loop tuning. It is based on the concept of the process reaction curve, which is a plot of the process response to a step change in the input. The method involves adjusting the PID gains to achieve the desired shape of the process reaction curve. This is done by first adjusting the proportional gain to achieve the desired level of responsiveness, and then adjusting the integral and derivative gains to achieve the desired level of overshoot and settling time. Lopez Method: The Lopez method is a more recent method for loop tuning that is based on the concept of the process reaction curve. It is similar to the Cohen-Coon method in that it involves adjusting the PID gains to achieve the desired shape of the process reaction curve. However, the Lopez method also takes into account the effect of the control system's time delay on the process reaction curve. This allows for a more accurate tuning of the system, particularly for systems with long time delays. Loop Performance Measurement Techniques: Measuring the performance of a control loop is crucial for ensuring that the system meets the desired specifications. Two commonly used techniques for measuring loop performance are quarter-wave decay and disturbance rejection. Quarter-wave decay: Quarter-wave decay is a technique for measuring the damping ratio of a control loop. It involves applying a step change in the input and measuring the decay of the process response over time. The damping ratio is calculated by dividing the logarithm of the amplitude of the process response by the logarithm of the time. Disturbance Rejection: Disturbance rejection is a technique for measuring the ability of a control loop to reject disturbances. It involves applying a disturbance to the system and measuring the deviation of the process response from the desired value. The disturbance rejection ratio is calculated by dividing the deviation of the process response by the amplitude of the disturbance. In conclusion, loop tuning is an important aspect of control system design, and there are several methods and techniques that can be used to achieve the desired performance. The Zieghler-Nicholes, Cohen-Coon, and Lopez methods are popular methods for loop tuning, and measuring the loop performance using techniques like quarter-wave decay and disturbance rejection can help ensure that the system meets the desired specifications.

You are correct that the Zieghler-Nicholes method includes both open-loop and closed-loop tuning methods. In open-loop tuning, the system is not connected to a controller, and the process is operated in a manual mode. In this case, the method involves adjusting the proportional, integral, and derivative (PID) gains based on the process reaction curve obtained from a step change in the input, without any feedback from the controller. The goal is to achieve the desired level of responsiveness and overshoot, while avoiding instability. In closed-loop tuning, the system is connected to a controller, and the process is operated in an automatic mode. In this case, the method involves adjusting the PID gains based on the process reaction curve obtained from a step change in the setpoint, while taking into account the feedback from the controller. The goal is to achieve the desired level of responsiveness, overshoot, and stability, while minimizing the effect of disturbances on the process response. The Zieghler-Nicholes method is widely used in closed-loop tuning, as it provides a simple and effective way to adjust the PID gains based on the process reaction curve. The method involves first adjusting the proportional gain until the system reaches the ultimate gain, and then adjusting the integral and derivative gains to achieve the desired period. This is done by adjusting the PID gains until the process reaction curve reaches the ultimate gain and ultimate period, which are defined as the gain and period at which the system becomes unstable. In summary, the Zieghler-Nicholes method provides a simple and effective way to adjust the PID gains for both open-loop and closed-loop tuning. The open-loop method is useful for manual operation and the closed-loop method is useful for automatic operation. The goal of the method is to achieve the desired level of responsiveness, overshoot, and stability while minimizing the effect of disturbances on the process response.