How can you use the Cohen-Coon method for PID tuning?

2 How to apply the Cohen-Coon method?

In order to apply the Cohen-Coon method for PID tuning, you need to begin by performing an open-loop step test on the process. This involves applying a step change in the input and recording the output over time. Subsequently, graphical methods or software tools can be used to identify the process parameters K, t_d, and T from the step response data. These values are calculated using equations such as K = (y_final - y_initial)/u_change , t_d = t_1 - t_0 , and T = (t_2 - t_1)/(ln(3) - ln(2)) , where y_final and y_initial are the final and initial values of the output, u_change is the magnitude of the step change in the input, t_0 is the time when the input changes, t_1 is the time when the output reaches 63.2% of its final value, and t_2 is the time when the output reaches 86.5% of its final value. After that, you can calculate the PID parameters K_p, T_i, and T_d using the Cohen-Coon formulas. Then, you can implement the PID controller with these parameters and test its closed-loop response through simulation software or hardware. Performance metrics such as rise time, settling time, peak overshoot, and steady-state error can be used to evaluate its quality. If necessary, you can adjust the PID parameters to improve its performance or meet specific requirements of the system through trial-and-error methods or optimization algorithms.

3 What are the advantages of the Cohen-Coon method?

The Cohen-Coon method is a popular and useful technique for PID tuning due to its many advantages. It is easy to apply and does not require complex calculations or models, and it is based on open-loop data, meaning it does not disturb the system's normal operation or require a feedback loop. Additionally, this method can handle processes with time delays, which are common in many industrial applications, and it can achieve a good balance between stability and responsiveness by aiming for a quarter-decay ratio.

4 What are the limitations of the Cohen-Coon method?

The Cohen-Coon method has some limitations that should be taken into consideration, such as the assumption that the process can be approximated by a FOPTD model which may not be accurate for nonlinear or complex processes. Additionally, this method does not account for noise, disturbances, or changes in operating conditions which could affect control performance. It may also not be optimal for certain control objectives like minimizing overshoot, settling time, or error. Lastly, manual adjustments or fine-tuning of the PID parameters may be required in order to achieve the desired control performance.

5 How to use other methods for PID tuning?

The Cohen-Coon method is not the only method for PID tuning. There are many other methods you can use, such as the Ziegler-Nichols method, the Tyreus-Luyben method, the IMC method, and the Lambda method. Each of these has its own advantages and disadvantages, and you should compare and test them to find the best one for your system. You can also use software tools or online calculators to help you with the PID tuning process.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?