A rule based design methodology for the control of non-self regulating processes

Date of Completion

January 2004


Engineering, Chemical




Non-self regulating (integrating) processes move in an unbounded manner when perturbed in open loop by a bounded manipulated or disturbance variable. It is not uncommon for some temperature, level, and pressure control loops to display this type of behavior. Integrating processes are surprisingly challenging to control and can move to extreme and even dangerous levels if left unregulated. An additional challenge is that the controllers and tuning methods proven for self regulating processes can yield poor and often unstable performance when applied to integrating processes. ^ A rule based methodology for controller selection and design for non-self regulating processes is developed and documented. This work fills the gaps of previous research by providing a completely characterized set of controller design strategies encompassing a wide range of non-self regulating processes and control objectives. The rule structure developed guides the decision making pathways through the various design options. ^ The fundamental approach taken is built upon model based design methods. For a model based control approach to be beneficial, its design must take into account an accurate representation of the process dynamics. In this work existing model based control strategies for self regulating processes, including IMC based PID Control, DMC/MPC, Smith Predictors, Feed Forward and Cascade control structures, are modified to work with non-self regulating processes. This modification can take the form of an enhanced tuning parameter correlation, or a complete re-design of the control structure. ^ The novel contributions of this rule based methodology include a complete package of automated process analysis tools that include both data capture and model fitting capabilities, a set of simulation tools that can be used to quickly evaluate controller performance, an extension of the IMC PID controller tuning method to encompass the entire family of PID (4-mode) controllers, a design procedure that is valid for a broad range of control strategies, a series of decision blocks that provide a pathway for controller selection by posing precise questions concerning the process dynamics, system limitations and the specific control objectives, and a demonstration of the proposed control strategies performed on a bench scale experimental process. ^