Computation of multiloop controllers having desired closed-loop responses

For n-by-n multivariable processes, multiloop controllers have n degrees of freedom and hence the n diagonal elements of closed-loop transfer functions can be designed to have desired closed-loop responses. Multiloop controllers having desired closed-loop responses can be considered as an extension of the single-input single-output internal model control and they can be used as reference controllers. However, computations of such multiloop controllers have not been well developed. The Newton-Raphson method and the iterative sequential loop closing method can be used, but they can suffer from a divergence problem for some processes. Here, the continuation method is applied to obtain multiloop control systems with desired closed-loop responses for a robust computation. The multiloop controllers with desired closed-loop responses can be used to obtain dynamic interaction measures and design multiloop PID controllers.     

MP criterion based multiloop PID controllers tuning for desired closed loop responses

A tuning method of multiloop PID controllers is developed based on the generalized IMC PID tuning rule by Lee et al. (1998a). To extend the SISO PID tuning method to MIMO systems, a new tuning criterion is proposed. The criterion is based on the closed loop frequency response method to meet desired performance and robustness as close as possible. Examples for 2x2, 3x3 and 4x4 systems are used to illustrate the proposed method. The results show that the proposed method is superior to conventional methods such as the BLT tuning method.     

Estimating performance enhancement with alternate control strategies for multiloop control systems

A filter based methodology, studied earlier for SISO systems, is extended to MIMO systems. The presented approach facilitates the calculation of best achievable performance for proportional-integral (PI) controller and the optimal multiloop (ML) PI settings for stochastic disturbance rejection in ML control systems. The filter based approach is further extended to answer some of the key questions for ML control systems such as: (a) performance enhancement possible with the alternate pairing scheme, (b) benefits that will accrue through the employment of decouplers and (c) the performance achievable with the use of multivariable controller (as opposed to an ML controller). Further, the trade-off curve between output variance and control effort is generated for the various control configurations within PI controller domain.     

An energy effective PID tuning method for the control of polybutadiene latex reactor based on closed-loop identification

The PBL (Polybutadiene Latex) production process is a typical batch process. Changes of the reactor characteristics due to the accumulated scaling with the increase of batch cycles require adaptive tuning of the PID controller being used. In this work we propose a tuning method for PID controllers based on the closed-loop identification and the genetic algorithm (GA) and apply it to control the PBL process. An approximated process transfer function for the PBL reactor is obtained from the closed-loop data by using a suitable closed-loop identification method. Tuning is performed by GA optimization in which the objective function is given by ITAE for the setpoint change. The proposed tuning method showed good control performance in actual operations.     

Controlled output variance based diagnosis tree for feedforward/cascade control systems

This paper aims at providing a framework for detection and diagnosis of the performance of a combinational feedforward (FF) and cascade (CC) control system. It is the extension of our previous work [1,2]. The main idea is to extract the only CC effect and the combination of FF with CC effects, respectively. In the only CC effect, the output variances of the primary and the secondary loops can be turned into the cascade-invariant and cascade-dependent terms, respectively. The combination of FF with CC effect can also be decomposed into the cascade/feedforward invariant term, the cascade-invariant/feedforward-dependent term and the cascade/feedforward dependent term. The diagnosis tree based on these decomposition terms is proposed to assess the performance of the FF/CC control system. The sequence of the statistical inference system is developed to diagnose fault causes. The capability of the proposed method is demonstrated via a cascade control system with the feedforward loops and multiple faults.