Category: Education

The final element of PID control is the “D” term, which stands for derivative. This is a calculus concept like integral, except most people consider it easier to understand. Simply put, derivative is the expression of a variable’s rate-of-change with respect to another variable. Finding the derivative of a function (differentiation) is the inverse operation of integration. With…

Imagine a liquid-level control system for a vessel, where the position of a level-sensing float sets the position of a potentiometer, which then sets the speed of a motor-actuated control valve. If the liquid level is above setpoint, the valve continually opens up; if below setpoint, the valve continually closes off: Unlike the proportional control system where valve position…

A fundamental limitation of proportional control has to do with its response to changes in setpoint and changes in process load. A “load” in a controlled process is any variable not controlled by the loop controller which nevertheless affects the process variable the controller is trying to regulate. In other words, a “load” is any factor…

Imagine a liquid-level control system for a vessel, where the position of a level-sensing float directly sets the stem position of a control valve. As the liquid level rises, the valve opens up proportionally: Despite its crude mechanical nature, this proportional control system would in fact help regulate the level of liquid inside the process vessel. If…

Once while working as an instrument technician in an aluminum foundry, a mechanic asked me what it was that I did. I began to explain my job, which was essentially to calibrate, maintain, troubleshoot, document, and modify (as needed) all automatic control systems in the facility. The mechanic seemed puzzled as I explained the task…

Negative feedback systems, in general, tend to cause much confusion for those first learning their fundamental principles and behaviors. The closed-cycle “loop” formed by the interaction of sensing element, controller, final control element, and process means essentially that everything affects everything else. This is especially problematic when the feedback control system in question contains a fault and…

Instrumentation is the science of automated measurement and control. Applications of this science abound in modern research, industry, and everyday living. From automobile engine control systems to home thermostats to aircraft autopilots to the manufacture of pharmaceutical drugs, automation surrounds us. This chapter explains some of the fundamental principles of automatic process control. 29.1 Basic feedback…