 Welcome to the Ziegler-Nickels Reaction Curve Process Identification Procedure. The primary function of a closed loop system is to make the controlled variable a desired value established by the set point. Some of the variables are listed here. They include temperature, level, physical position, and the speed of movement. Whenever the controlled variable becomes different than the set point, the objective of the closed loop system is to make them the same as quickly as possible. The controlled variable becomes different than the set point under three conditions. These are the set point change, the disturbance, and the load demand change. The closed loop system will respond in a desirable way only if its controller is properly tuned. This means that its proportional, integral, and derivative, or PID settings, are properly made. A popular procedure for tuning a controller is the Ziegler-Nickels Reaction Curve Tuning Method, which was developed in the 1940s. The procedure requires a step change of the controller's output that alters the controlled variable, which is observed on the computer monitor. Based on the shape and magnitude of the controlled variable's reaction curve in reference to the step changes, values are obtained and used in mathematical formulas. Values are found for the process reaction rate, the unit reaction rate, and the effective delay. These values are then used to determine the PID settings to program into the controller. The method used to make the step change and measure the controlled variable on the monitor is called the process identification procedure. The Ziegler-Nickels Reaction Curve Method begins by switching the controller to the manual mode. This controller setting puts the system into an open loop condition. Next, a step change is made to the controller that causes a 5-10% change in the controller output. The signal from the sensor that measures the controlled variable is displayed on a computer monitor to show the rate at which the process responds. Called the reaction curve, three different values are obtained from the graph on the monitor. These are the process reaction rate, the unit reaction rate, and the effective delay. These values are used in mathematical calculations to determine the proper controller's settings. The following steps, called the process identification procedure, are performed to determine the values for the calculations. Step 1. Put the controller in the manual mode. Step 2. Change the controller's output to make a 5-10% change to the controlled variable. The reaction curve is then displayed. Step 3. Draw a tangent line along the slope on the reaction curve. And step 4. Calculate the slope of the tangent by drawing two lines. Horizontal line A begins at the starting point of the tangent. Vertical line B is drawn vertically in an upward direction from line A to the end of the tangent. Step 5. Determine the process reaction rate from the slope of the tangent using the following formula. R equals B divided by A, where R equals the process reaction rate, A equals the time in minutes, and B equals the percentage of the controlled variable change. This graph shows the value of B is 10%, and the value of A is 1 minute. Step 6. Calculate the unit reaction rate, R1, by dividing the process reaction rate, R, found in step 5, by the percentage of the set point change, or X. The graph shows the actuator step change is 5%. The process reaction rate of 10% was determined by step 5. In some applications, there is a delay from the time when the set point change is made to when the reaction starts. This delay is called an effective delay. In this situation, it is necessary to make one more calculation. Step 7. Determine the effective delay, or D, which is the time that elapses from the point when the step change is made and when the tangent line crosses the line of the initial process variable status. The graph shows D equals 0.9 minutes. Once the process identification procedure information is obtained, the next step is to make calculations to determine the controller setting. Three control modes of operation are commonly performed by the controller. These include proportional, or P, proportional integral, or PI, and proportional integral derivative, or PID. Using the process identification procedure values, different calculations are performed to determine the proper setting for each mode. This is the chart for the Ziegler-Nickels reaction curve formulas. The Ziegler-Nickels reaction curve tuning method is an alternative to the Ziegler-Nickels continuous tuning procedure in which the controlled variable is made to oscillate. Oscillations can be undesirable in some situations because the process can go outside of an acceptable tolerance range from the set point. If it does, a food product may be ruined or a dangerous condition in a nuclear power plant may develop. Now, let's check your understanding. Please pause the video if you need more time to answer the questions. Question 1. The Ziegler-Nickels reaction curve tuning method is the most appropriate procedure for a food processing or nuclear power plant application. Question 2. The step change is made when the controller is in the manual mode for the Ziegler-Nickels reaction curve tuning process identification procedure. Question 3. Which of the following types of values is obtained from the graph during the process identification procedure for the Ziegler-Nickels reaction curve tuning method? The correct answer is all of the above, the process reaction rate, the unit reaction rate, and the effective delay. Congratulations! 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