ROBUST ANALYSIS AND PID TUNING OF CASCADE CONTROL SYSTEMS

The robustness and performance of the cascade control system are analyzed in this article. A robustness measure is defined, and it is shown that it can reflect the interaction between the inner loop and the outer loop and has a clear indication of the robustness of each loop. The measure can serve as a graphic aid for tuning the cascade controller. Moreover, it is observed that the cascade controller can be tuned individually for each loop instead of sequentially.     

ROBUST ANALYSIS AND PID TUNING OF CASCADE CONTROL SYSTEMS

The robustness and performance of the cascade control system are analyzed in this article. A robustness measure is defined, and it is shown that it can reflect the interaction between the inner loop and the outer loop and has a clear indication of the robustness of each loop. The measure can serve as a graphic aid for tuning the cascade controller. Moreover, it is observed that the cascade controller can be tuned individually for each loop instead of sequentially.     

OPTIMAL TUNING OF PID CONTROLLERS FOR SINGLE AND CASCADE CONTROL LOOPS

Design of one parameter tuning of three-mode PID controller was developed in this present study. The integral time and the derivative time of the controller were expressed in terms of the time constant and dead time of the process. Only the proportional gain was observed to be dependent on the implemented tunable parameter in which the stable region could be predetermined by the Routh test. Extension of the concept towards designing cascade PID controllers was straightforward such that only two parameters for the inner and outer PID controllers required to be tuned, respectively. The optimal tuning correlative formulas of the proportional gain for single and cascade control systems were obtained by the least square regression method.     

A SIMPLE TITO PI TUNING METHOD SUITABLE FOR INDUSTRIAL APPLICATIONS

A simple tuning method for TITO (Two-Input, Two-Output) PI controllers will be presented in this article. The method is an extension of the SISO PID tuning method based on Internal Model Control (IMC). An approximate model for tuning purpose will be developed using the information from the interaction measure of the TITO system. The procedure of the tuning method is very simple and straightforward utilizing only two bias-relay feedback tests. The tuning method can easily be applied to various industrial situations with almost no need for a priori process knowledge. Various TITO examples will be used to demonstrate the performance of this tuning method. Both setpoint and load disturbance responses will be examined     

A CLOSED-LOOP REACTION-CURVE METHOD FOR CONTROLLER TUNING

This article proposes a closed-loop, reaction-curve method to provide a procedure for establishing controller settings on the classical three-mode (P1D) controllers. It uses the partial extreme data available during the early period of step set-point change under porportional control mode to determine the ultimate period and the ultimate gain of the open-loop process. The resulting ultimate data are critical for subsequent controller settings according to Ziegler-Nichols rules. Simulation examples and test on a pilot-scale stirred-tank reactor demonstrate that the method can provide adequate critical process data, and that it is applicable to processes that are underdamped, characterized by large dead-times, open-loop unstable or with inverse-response.     

LINEAR PI TEMPERATURE-CONCENTRATION CASCADE CONTROL FOR TUBULAR REACTORS

In this article we explore the application of linear PI cascade control schemes to improve the performance of industrial PI/PID controllers for controlling outlet reactor concentration. By departing from simple I/O first-order dynamical models obtained from step responses, it is shown that the incorporation of a secondary loop for regulating the reactor temperature at a given interior position significantly improves the control performance in the face of feed composition and temperature disturbances. The effects of the temperature sensor location and the usage of multiple temperature measurements are also evaluated.     

LINEAR PI TEMPERATURE-CONCENTRATION CASCADE CONTROL FOR TUBULAR REACTORS

In this article we explore the application of linear PI cascade control schemes to improve the performance of industrial PI/PID controllers for controlling outlet reactor concentration. By departing from simple I/O first-order dynamical models obtained from step responses, it is shown that the incorporation of a secondary loop for regulating the reactor temperature at a given interior position significantly improves the control performance in the face of feed composition and temperature disturbances. The effects of the temperature sensor location and the usage of multiple temperature measurements are also evaluated.     

A SIMPLE AUTOTUNING METHOD USING PROPORTIONAL CONTROLLER

In this study, we propose a simple autotuning method using proportional controller. A second order plus time delay model is used in the on-line closed-loop identification and tuning of P1D controllers. This model can approximate processes over a wider frequency region than a first order plus time delay model, which is often employed in model-based autotuning methods. Moreover, since the identification method requires only a simple least squares method to estimate the model parameters, no complex numerical techniques such as root-finding or optimization methods are employed, thus making it practical, unlike many other previous identification methods. Simulation studies demonstrate that the proposed method shows satisfactory identification and better control performances than other previous methods for various processes. In the identification step, it shows insensitivity to measurement noise and input disturbances. The results of the experimental study confirm its validity and capability and show its promise in industrial situations.     

SIMPLIFIED MODEL PREDICTIVE CONTROL FOR SINGLE AND CASCADE LOOPS

An analysis of the simplified model predictive control (SMPC) has been carried out so that the digital algorithm involved could be conveniently applied to single loop control of complex higher order processes with noise and dead time. Simulation results are presented to show the excellent capability of the control technique particularly in the presence of noise. Certain aspects related to SMPC tuning, such as development of a simple closed loop tuning procedure, have been discussed. Finally, the control law is extended to cascade control structure involving a primary and a secondary loop.     

TUNING METHOD FOR INTERACTIVE MULTILOOP IMC, PI AND PID CONTROLLERS

An efficient method to adjust a multiloop control system based on IMC, PI or PID controllers is presented. The first step of the new method consists in designing and adjusting the individual IMC controllers assuming a noninteractive condition and using information related to model uncertainties. In a second step, a single tuning parameter is used to adjust the multiloop interactive system in order to improve both stability and performance characteristics. The extension to multiloop PI and PID controllers is made through alternative parameterizations of IMC (Rivera el al., 1986).

Several simulation results obtained using transfer function representations of distillation systems from the literature show the convenience of the proposed technique by yielding good settings with a reasonable amount of engineering and computational effort.     

PID CONTROLLER TUNING FOR UNSTABLE SYSTEMS BY OPTIMIZATION METHOD

Simple Tuning formulae are provided for optimal PID controller settings for unstable first order plus time delay systems. The method is based on minimization of integral squared errors (ISE). A method of calculating the set point weighting parameter is proposed to reduce the overshoot for servo problems. The performances of the proposed PID settings are compared with the settings recently proposed by Huang and Chen (1997, 1999) for both the servo and regulatory problems. The performance of the controller is also evaluated under parameter uncertainty in time delay and separately in process gain. Tuning formulae are given for PI controller along with the set point weighting. The performance of the PI controller is compared with that of Poulin and Pomerleau (1996). Two simulation studies, one on control of an unstable nonlinear bioreactor and a second on an unstable chemical reactor using the proposed PID controller settings, show improved performances both for servo and regulatory problems.     

CONSISTENCY PRINCIPLES FOR STABILITY IN DECENTRALIZED CONTROL SYSTEMS

The Niederlinski Index (NI) is generalized to systematically address closed loop stability from steady state information of both the process and the controller. A series of stability conditions, referred to as 'consistency principles for stability', which allows for joint consideration of controller design and variable pairing, are established. It is shown that systems paired on negative NI can still be stabilized by properly setting directions of individual controllers, and that positive NI is not only desirable to facilitate controller tuning and failure tolerance, but is also always achievable for any decentralized control system.     

基于LONWORKS现场总线的自整定PID控制系统

将ＰＩＤ自整定技术和现场总线技术有机地结合起来,实现了基于ＬＯＮＷＯＲＫＳ现场总线的ＰＩＤ自整定系统。自整定系统由两层组成,上层为ＰＣ计算机,下层为一个智能节点。它们之间通过ＬＯＮＷＯＲＫＳ现场总线进行通信。ＰＣ计算机给操作人员提供一个良好的人机界面,同地控制着整个ＰＩＤ参数自整定的过程。实验证明了该自整定系统的适用性和有效性,必将在工业过程控制中得到广泛的应用。     

一种新的氨冷器出口温度－液位自动选择性串级控制系统

本文提出一种采用可编程调节器实现的氨冷器出口温度－液位自动选择性串级控制系统，实现了氨冷器的安全运行和自动调节。     

TUNING PID CONTROLLER FOR OPEN-LOOP UNSTABLE PROCESSES WITH TIME DELAY

A calculation method of PID controller tuning for the first- and the second-order open-loop unstable process models with time delay is presented in this study. Optimum PID controller tuning data based on the models and minimum IAE criterion were obtained via Powell searching technique, and these data were then empirically correlated into several multiple-regression equations by a least-squares method. Thus PID controller tuning based on the models can easily be obtained by the calculation of these correlated equations. Simulation with a reset-feedback PID control algorithm has demonstrated that the proposed tuning method based on the first-order model can provide better results than the latest studies. In addition, simulation has also unveiled that tuning results based on the second-order models are superior to the first-order model for a higher-order process.     

净水厂混凝投药模糊控制方法

本文结合净水厂工程实例介绍水厂混凝投药模糊控制方法。该方法能够解决水厂混凝剂投加控制系统中存在的非线性、时滞性和模糊性问题,从而实现现场混凝投药自动控制。以一个单点模糊控制器的工作原理为例说明了混凝投药模糊控制系统中的核心技术的设计原理。本文提出的方法经工程实践取得了预期成效。     

AUTOMATIC TUNING OF SYSTEMS WITH ONE OR TWO UNSTABLE POLES

Stabilizability and identification of systems with one or two unstable poles is addressed. Two limit points of conditional stability are characterized using closed-loop dynamics with a relay and a novel nonlinear element. An algorithm is developed to analyze information from the resultant stable limit cycles in the process input and output. An easily tuned cascade PID control structure is then proposed to stabilize the system and achieve desirable performance. The proposed autotuning technique is tested with good results on a wide variety of unstable systems.     

A DIRECT NONLINEAR ADAPTIVE CONTROL OF STATE FEEDBACK LINEARIZABLE SYSTEMS

A direct nonlinear adaptive control of state feedback linearizable single-input single-output systems is proposed in the case when parametric uncertainties are represented linearly in the unknown parameters. The main feature of the proposed nonlinear adaptive control system is that the linearizing coordinate transformation and the state feedback are updated by parametric adaptive law, derived using the second method of Lyapunov. The proposed adaptive control scheme is relatively straightforward and simple in the sense that it does not use the concept of augmented error. This adaptive control scheme is numerically applied to an exothermic chemical reactor system and is compared with the nonadaptive stale feedback linearization which has an integral action. The simulation shows that the proposed adaptive control scheme can be applied effectively to highly nonlinear, uncertain chemical systems.     

TUNING RULES FOR A NON-INTERACTING MULTI-DIMENSIONAL PI CONTROL ALGORITHM

By expanding the method of Dahlin (1968) to n-dimensions, tuning rules are derived for a non-interacting discrete proportional-integral control algorithm and their usefulness is demonstrated with an example.