Wonhui Cho

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.

Control system design based on a nonlinear first-order plus time delay model

Abstract Most chemical processes are nonlinear in nature. When large set point changes or load disturbances occur frequently, nonlinear control systems are required. Instead of using the differential geometric method or nonlinear model predictive method, simple gain scheduling may be sufficient for many nonlinear single input-single output (SISO) processes. To design such simple control systems systematically, a nonlinear first-order plus time delay model is proposed for model-based control. A logarithmic transformation which is very useful for control of high purity distillation columns is shown to be effective in general. Several chemical process examples are also given.

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.

Comparison of controller tuning methods for temperature uniformity control in a rapid thermal processor

Multiloop PI control and quadratic dynamic matrix control (QDMC) algorithms are applied for the temperature uniformity control of rapid thermal processing (RTP). Input and output variable transformations are done based on the singular value decomposition (SVD) of the process gain matrix and the (mu) -interaction measure analysis of compensated transfer function matrix. Multiloop PI control with transformation of variables shows improvement in transient responses to set point changes compared with the multiloop PI control without variable transformations and QDMC. Further studies on systematic tuning method for this multiloop PI control is needed to reduce the offsets and to obtain the uniform transient responses.

Iterative identification of temperature dynamics in single wafer rapid thermal processing

In the rapid thermal processing (RTP), a control system to improve the processing time, uniformity and repeatability of processing is required. As the standard size of silicon wafer grows and integration of integrated circuits increases, this requirement is increasing. Identification and control are complicated because of highly nonlinearity, drift and time varying nature of the wafer dynamics. Various physical dynamic models for RTP are available and they show diagonal nonlinear first order dynamics with multivariable static gains. Accurate identification of the multivariable static gains is very important for better control. However, these model structures of RTP are not well utilized yet for identification and control. Here, an identification method which refines the multivariable static gains iteratively is proposed. It will simplify the identification procedure and improve the accuracy of identified model.