Systematic Approach for Control Structure Design

Abstract

<p> Control structure design is an essential step in control system synthesis and has big impact on achievable closed-loop performance. This thesis develops a systematic approach of selecting optimal control structures based on closed-loop dynamic performance and other criteria, such as integrity.</p> <p> The main contribution of this thesis is a rigorous mathematical formulation for control structure design problem that includes full closed-loop transient analysis with additional integrity requirement. The multi-objective framework is extendable so that different control performance objectives can be easily added. Unique process requirements and engineer inputs can be taken into account as additional constraints. The proposed formulation is a Mixed Integer Nonlinear Programming (MINLP) with complementarity constraints. The research scope is limited to linear process models and linear controller algorithms.</p> <p> The tailored solving strategy that makes this challenging problem computationally tractable is introduced in this thesis. The modified Branch and Bound algorithm takes advantage of the special problem structure by using control knowledge to generate valid lower bound efficiently. Prior knowledge can be cooperated as heuristic tuning parameters to guide the solving process so that a reasonably good solution can be found early in the solving process. The complexity study shows the solving strategy can attack design problem size up to 8x8. Considering the percentage of good structures needing evaluation will decrease with problem size even larger problems will be tractable.</p> <p> The common control structures in process industries, such as square and nonsquare Single-Input-Single-Output (SISO) loop pairing using PID controller and block-centralized structure using Model Predictive Controller (MPC), are addressed in this thesis. The usefulness of this research has been demonstrated by several case studies, include Tennessee Eastman problem. The proposed methodology finds a physically sound pairing with good performance for Tennessee Eastman problem in less than one hour, while several off-the-shelf NLP, MINLP and global solvers cannot find a solution in five days.</p>