Adaptive cascade control algorithm

Details Adaptive cascade control algorithm Adaptive cascade control algorithm

1997-07-31

2001-05-29

Gordon, Paul P. (Department: 2121)

Data processing: generic control systems or specific application

Generic control system, apparatus or process

Cascade control

C700S033000, C700S034000

Reexamination Certificate

active

06240324

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the field of controllers, and more specifically to cascaded controllers.
The purpose of cascade control has been to provide superior control of a process through continuous controlled-medium set point adjustments. As shown in Prior Art
FIG. 1
, “Classic” cascade control was usually defined as a control method where the output from one control module
10
was fed into the set point input of another control module
30
, passing through some scaling module
20
on the way. These functions were typically implemented using a Direct Digital Control (DDC) controller. In a DDC controller, the control modules were usually PID operators, or functions and the scaling module was typically a ratio operator.
The purpose of the scaling module was to transform the output from the first PID operator (typically ranging from 0 to 100%) into a usable set point appropriate for the controlled medium (i.e., air temperature, air pressure, water temperature, etc.). A direct linear relationship was commonly used, such as that provided by a standard ratio operator.
The problem with classic cascade control was that the system installer or end-user rarely knew what values to enter into the minimum and maximum set point limit parameters (inputs
21
and
22
into the scaling module
20
) to achieve optimal control of the process. In fact, installers or end users often picked values which led to poor system operation. Also, as system (process) gains change, due to different load levels, seasonal loads, etc., the optimum values of limit parameters may change.
Accordingly, it is an object of the invention to provide installers and end users of cascaded controls with systems which have fewer parameters to be entered by them leading to reduced installation time and consequently, better performance.
SUMMARY OF THE INVENTION
The present invention is an apparatus and a process for adaptive cascade control. An adaptive process module intelligently sets parameters during system operation to attain the best possible control performance, while providing an easier-to-use system for the user.
The process includes the steps of determining whether the measured variable has a first predetermined relationship with a MINLIMIT and if so, lowering the value of the MINLIMIT, determining whether the measured variable cannot reach the MINLIMIT with the controller output ON full and if so, raising the value of the MINLIMIT, determine whether the measured variable cannot reach a MAXLIMIT with the controller output OFF and if so, lowering the value of the MAXLIMIT and determining whether the measured variable has second predetermined relationship the MAXLIMIT and if so, raising the value of the MAXLIMIT.
Implementing this new control results in a control system with fewer parameters which must be configured by the installer or operator. Further, the parameters which are removed from the operator's access are ones which are difficult to set to correct values, and the correct values may change depending on seasonal, or other load differences which affect process dynamics. Therefore, the new algorithm saves installation and set-up time, as well as providing on-going ease-of-use for the operator.


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