Proportional-integral-derivative (PID) control is one of the most common types of automatic control used in the power industry. A PID controller continuously calculates an error value as the difference between a measured process variable and a desired setpoint.
By tuning three parameters, a PID controller can deal with specific process requirements. The proportional term refers to the amount added to the output based on the current error, the integral term refers to the amount added to the output based on the sum of the error, and the derivative term refers to the amount subtracted from the output based on the rate of change of the error.
Even though it has a relatively simple algorithm, there are many subtle variations in how the structure is applied in the industry. This has probably resulted in confusion for plant engineers and operators, who are often happy to leave control loops in a suboptimal configuration rather than spend time improving the process. The mentality can be, “Let’s leave well-enough alone.”
According to ControlSoft Inc.—a provider of advanced process control, performance monitoring, and process diagnostic tools—about 80% of control loops in plants today are not optimally tuned. The company says it finds around 25% of control loops have factory-default settings in place and some 65% have less variation in manual mode than they do in automatic.
However, there are some relatively easy ways to improve the situation. ControlSoft recently released an updated PID loop tuning pocket guide. The twelve-page pamphlet includes a short, easy-to-use guide for tuning PID loops, along with updated controller references for many of the more common controllers on the market today. The pocket guide is available for free at: bit.ly/PID-Guide. Visitors are required to enter contact information prior to viewing or downloading the guide.
ControlSoft also markets INTUNE tuning software to assist with PID control loop tuning. The tool is said to reduce the time required to tune PID loops by up to 70%. It connects to most input/output devices and their associated controllers using OPC technology. A full year of technical support is provided with the software purchase. ■
—Edited by Aaron Larson, POWER’s executive editor.