Jietae Lee

Multiloop PI controller tuning for interacting multivariable processes

Abstract A trial-and-error method that is a variation of Ziegler–Nichols tuning has been employed in the past for field tuning of PID controllers. This method is now extended for tuning of multiloop PI controllers. The Nyquist array method is refined to determine ultimate gains for multiloop tuning. The proposed method is simple to use and simulation results show that it avoids sluggish responses and unbalanced responses between loops that occur with other tuning methods such as BLT and sequential autotuning.

Conditions for decentralized integral controllability

Abstract The term decentralized integral controllability (DIC) pertains to the existence of stable decentralized controllers with integral action that have closed-loop properties such as stable independent detuning. It is especially useful to select control structures systematically at the early stage of control system design because the only information needed for DIC is the steady-state process gain matrix. Here, a necessary and sufficient condition conjectured in the literature is proved. The real structured singular value which can exploit realness of the controller gain is used to describe computable conditions for DIC. The primary usage of DIC is to eliminate unworkable pairings. For this, two other simple necessary conditions are proposed. Examples are given to illustrate the effectiveness of the proposed conditions for DIC.

Closed-loop identification of wafer temperature dynamics in a rapid thermal process

Single wafer rapid thermal processing (RTP) can be used for various wafer fabrication steps such as annealing, oxidation and chemical vapor deposition. A key issue in RTP is accurate temperature control, i.e., the wafer temperatures should be rapidly increased while maintaining uniformity of the temperature profile. A closed-loop identification method that suppresses RTP drift effects and maintains a linear operating region during identification tests is proposed. A simple graphical identification method that can be implemented on a field controller for autotuning and a nonlinear least squares method have been investigated. Both methods are tested with RTP equipment based on a design developed by Texas Instruments.

An Analytic Expression for Closed-Loop Output Behavior under Multiloop PID Control

An analytic expression is derived for closed-loop output behavior under a multiloop PID control. Based on the analytic expression obtained, optimization problems are formulated to assess 1) best achievable quadratic performance using multiloop PID control, 2) best achievable quadratic performance on key process variables while maintaining reasonable performance on other less critical process variables, 3) achievable performance improvement with decouplers, and 4) effects of loop pairing on achievable performance. It is shown through a simulated example that individual loop performance as well as the overall multiloop PID control performance can be assessed by using the proposed method.

Upper bounds of structured singular values for mixed uncertainties

Upper bounds of structured singular values for mixed uncertainties whose computations are efficient and reliable have been available. For uncertainties without real blocks and repeated complex scalar blocks, existing upper bounds are known to be quite tight. On the other hand, when the block structures of uncertainties include purely real or repeated complex block, tightness can be arbitrarily degraded. Here, improved upper bounds for mixed uncertainties with purely real uncertainties are proposed.

A Tuning of the Nonlinear PI Controller and Its Experimental Application

Many industrial chemical process control systems consist of conventional PID and nonlinear controllers, even though many advanced control strategies have been proposed. In addition, nonlinear control methods are widely used even for linear processes to achieve better control performance compared with linear PID controllers. However, there are few tuning methods for these nonlinear controllers. In this work, we suggest new controller tuning methods for the error square type of nonlinear PI controller. These control methods can be applied to a large number of linear and nonlinear processes without changing control structures. We also propose new tuning rules for integrating processes. In addition, we suggest application guidelines for performing the proposed tuning rules at the pilot scale multistage level control system. Finally, in this work we confirmed good control performances of the proposed tuning methods through both simulation studies and experimental studies.

PID Controller Tuning Rules Using an Inner P Controller

Using an inner P controller, a tuning rule useful for processes with wide ratios of time delay over time constant is proposed. Internal model control method and pole assignment method are utilized. It can be used for processes with wide range of the ratio of time delay to time constant without incovenience to choose different tuning rules.