Posts Tagged ‘Derivative’
The “D” in PID Stands for: Do Not Use (Sometimes)!
The Derivative Term is not only the last letter in PID (i.e. Proportional-Integral-Derivative) it’s also the most maligned of the three. With its big kick, the Proportional Term provides an immediate correction for changes in control and it is clearly the star of the PID controller. So too, the Integral Term is credited with its tenacious correction of Offset and for steadily pushing a loop back to Set Point. And then there’s Door #3: The lowly Derivative Term. Derivative works to counteract the rate of change of the process variable.Read More
Common Industrial Applications of PID Control
A previous post about the Derivative Term focused on its weaknesses. As noted, the primary challenge associated with the use of Derivative and PID Control is the volatility of the controller’s response when in the presence of noise. Noise is a major stumbling block for Derivative and PID Control as production data is routinely replete with process noise and other sources of variability. The use of PID Control in such an environment can drive frenetic changes in a loop’s Controller Output (CO) and unnecessarily wear out the associated Final Control Element (FCE). In summary: Little to gain; lots to lose.Read More
Common Industrial Applications of PI Control
Studies show that when individuals are given a set of three options they are instinctively biased to prefer the middle one. When this finding is applied to purchasing behavior a common outcome is that consumers pick the middle priced option with little-to-no rationale other than a desire to avoid being viewed as either too cheap or too lavish. It’s known as the Compromise Effect. While the PID controller offers three options – P-Only, PI and PID – the rationale for selecting the middle option is generally clear. But PI Control is not only the instinctive choice, on many occasions it is also the superior and simpler one.Read More
How Does the Derivative Term Affect PID Controller Performance?
Derivative is the third term within the PID. In mathematical terms the word derivative is defined as the slope of a curve. Seen in the context of strip chart data derivative represents the rate of change in error – the difference between the Process Variable (PV) and Set Point (SP). Like the proportional and integral terms within a PID controller, the derivative term seeks to correct for error. Valuable as the third term can be in maintaining effective control, experience suggests that appropriate uses of derivative are not entirely clear.Read More