Question #5 - Interacting and Non Interacting Reset and
Derivative
December 12, 2003
Question:
What is the difference between interacting and non-interacting PID relating to the
reset and derivative. In the typical PID equation:
it appears that the reset and derivative are not interacting.
Answer:
The terms “Interacting” and “non-interacting” are used many ways and can lead to confusion. I will use the terms “Series” and “parallel”, respectively.
The form of the PID equation shown in Figure 1, which is the way the PID is often represented in text books, differs from most industrial implementations in the basic structure. Most implementations place the derivative section in series with the integral or reset section.
Figure 1 - Parallel PID block diagram
We can modify the diagram illustrate the series algorithm more commonly used in industrial controls, shown in Figure 2.
Figure 2 - Series PID block diagram
The difference between this implementation and the parallel equation shown at the top is that the derivative has an effect on the integration. The equation becomes:
Out = (RD+1)G(e + R+D )
R = the reset rate in repeats per minute,D = the derivative in minutes, and
G = the gain.
The effect is to increase the gain by a factor of RD + 1, while reducing the reset rate and derivative time by the same factor. Based on common tuning methods, the derivative time is usually no more than about 1/4 the reset time (1/R), therefore the factor RD+1 is usually 1.25 or less.
Almost all analog controllers and most commercial digital control systems use the series form. Such tuning methods as the Ziegler-Nichols methods were developed using series form controllers.
Unless derivative is used there is no difference between the parallel (non-interactive) and series (interactive) forms.