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MAY-JUN 2017

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36 INTECH MAY/JUNE 2017 WWW.ISA.ORG A better understanding of closed-loop control, and how to tune it, can be found by looking at the individual parts of the PID equation and the effect of each on the controlled output By Bill Dehner M achines and processes are controlled using many strategies, from simple ladder logic to custom algorithms for specialized process control, but proportional- integral-derivative (PID) is the most common control method. Different programmable logic controllers (PLCs) handle PID control loops in different ways. Some loops need to be set man- ually, while others can use an autotune process embedded in the PLC's software. Even before loop tuning starts, the design may have created a slow-to-respond control loop with built-in lag. For example, a temperature sensor positioned a long distance from a heater can slow response to dynamic changes. Changes in machines and processes due to disturbances and set point changes are why PID control is often needed. The amount, length of time, and rate of change of the process error are all part of the PID equation, as is correcting the error to bring the process variable closer to the set point. This article will look at the PID equa- tion and some tuning tips, along with a brief re view of autotuning and applications benefiting from PID control. What is PID control? The application almost always determines whether open- or closed-loop analog control is used. Many applications will work with on-off, open-loop control using an analog sensor mea- suring temperature, pressure, level, or flow as an input to control a discrete output. The same analog sensors are also used for PID closed-loop control, but in a more complex strategy. In on-off, open-loop control of room tempera- ture, for example, a heat or cooling cycle is trig- gered by hysteresis in the thermostat. When the room temperature is approximately 1°F or 2°F above the set point, a cooling cycle is turned on. Once the set point—or possibly 1°F or 2°F be- low the set point—is reached, the cooling cycle is turned off. This can result in a 2°–4°F swing in room temperature. The temperature swing can be even worse with a slight overshoot at the turn- off point and undershoot at the turn-on point. This temperature swing (error) around the set point is not accurate enough in many industrial control processes. To reduce the process variable error, the closed-loop control function in PID controllers is commonly used. A PID controller reads a process variable (PV ), compares it to a desired set point (SP) value, and uses a continuous feedback loop to adjust the control output. The equation behind PID loops For many control system programmers, PID loops can be difficult to set and tune. Many have forgotten the calculus involved or never learned Optimizing to the tune of a PID equation

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