MAY-JUN 2017

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INTECH MAY/JUNE 2017 39 SPECIAL SECTION: PROCESS INDUSTRY TRENDS sample six due to the large error rate of change, but lessened it at sample seven and eight as the PV quickly approached the SP. This dampened the control out- put to avoid overshoot of the PV. P, PI, then PID manual tuning A process does not always require a three- mode PID control loop. Many applica- tions run fine on only PI terms and some on just the P term, so it is common to dis- able parts of the PID equation. Selecting appropriate values for the gain (Kc), reset (Ti), and rate (Td) makes a P, PI, PD, I, ID, and D loop possible. This is also what is done to tune the PID loop manually. It is difficult to tune all values of a PID control loop at once. To start, it is bet- ter to cancel out the integral and deriva- tive, and tune the proportional term. Find a proportional value that provides a quick response of the process variable. Raise the proportional term until the PV becomes unstable and oscillates, and then reduce it until a stable response is achieved with slight oscillation or error. With a stable PV, add a small value for the integral term to help the error reach zero. The error will get smaller when this term is approaching the optimal value. At this point, the PI control should have the desired response with a stable PV and minimal error. With many applications, especially temperature control, a PI loop is all that is needed. The derivative term is used in other applications, but it can slow the control response. It reduces gain when large rates of change in the PV are detect- ed to reduce possible overshoot. This of- ten causes undershoot—an over-damp- ened and slower response. Although each PID term can be manually entered to tune the control loop, the PID process- ing engine in some PLCs and advanced controllers can autotune the loop by au- tomatically calculating the values. Autotuning Autotuning, if available in the controller, often reduces or eliminates the trial and error of manual PID tuning. In most cas- es, autotuning a control loop will provide terms close to optimal values. However, it is often necessary to perform some manual tuning to attain optimal values. Many temperature controllers and PLCs provide an automatic tuning func- tion (figure 3). During the autotune cycle, a controller controls the output value while measuring the rate of change, over- shoot, and response time of the process. The Ziegler-Nichols method is then often used to calculate the controller PID term values. Typically, the Ziegler-Nichols method creates a square wave on the control output to create a step response that is measured and analyzed. Based on the response of the PV, the autotune func- tion calculates the terms and the sample time. Several full-span step cycles are used to compute the terms/gains. When manually tuning a loop, take the time to understand the equation and start with the proportional term. With a responsive and stable PV, add the inte - gral to the equation, and then add de- rivative if necessary. When an autotune function is used, a little adjustment to the terms may be all that is needed to optimize the control loop. n ABOUT THE AUTHOR Bill Dehner (bdehner@automationdirect. com) has spent most of his 11-year en gi- neering career designing and installing industrial control systems for the oil and gas, power, and package-handling in - dustries. He holds a bachelor's degree in electrical engineering with an associate's in avionics from the U.S. Air Force. He is currently a technical marketing engineer at AutomationDirect. View the online version at Figure 3. Many control devices, such as these AutomationDirect SOLO process controllers, have PID control autotuning functions.

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