Implementation of on-line control in chemical process plants

A range of identification, estimation and control algorithms has been implemented and tested on a chemical process plant/process control computer system which is typical of installations in the process industries. The plants studied are a gas-separating unit consisting of a pair of 9 m high absorption/distillation columns and a two-stage fractional crystallization plant. All on-line estimation and control was performed by a Honeywell 516 computer system. The topics studied on the plants included continuous on-line estimation of states and chemical process parameters using Kalman filtering techniques, the use of these estimates in various control algorithms, the application of optimal control theory to a variety of problems (minimum variance control, adaptive control, time optimal control), and the identification of process models for subsequent use in the design and implementation of multivariable control algorithms. In all cases, the implementation of these aspects of modern control theory on a real process plant was successful, but pointed to several non-trivial complications which must be resolved before these algorithms can be adopted for general use in the process industries.     

TCP/IP networking in process control plants

This paper refers to the utilization of Transmission Control Protocol/Internet Protocol (TCP/IP) in process control plants. Pertinent characteristics of classical control systems are presented, as a base to comparison with TCP/IP control systems. The advantages and shortcomings of adopting the latter one are pointed out. Integration with Internet-based technologies and systems management is explained next. Some products that provide these features are also presented as well as common components used in corporate networks, which can fully exploit and enhance the versatility of these control networks. Finally, economical aspects of this design are briefly commented.     

Pioneering work in the field of computer process control

Dreams of using digital computers in industrial control systems surfaced almost as soon as such a computer was invented in the mid to late 1940s. By the early fifties, the concepts of such use were fairly well established. However, actual applications had to wait until relatively small, reliable, and also relatively inexpensive machines were available, along with vendor companies with the will and the initiative to pursue this field vigorously Such a company was the Ramo-Wooldridge Company, which entered this field in the mid-fifties. The company found ready acceptance of its products among the companies in the process industries. By the mid-sixties, there were installations in almost every process industry and many other vendors had entered the field. Such installations became the norm for computer applications to industrial control until the microprocessor and its associated distributed computer control systems superseded them beginning in the mid-seventies. The article chronicles the development of this early field by describing several of the early installations and their successes and difficulties     

A hierarchical expert system for computer process control

This paper reports on the design and implementation of an expert system for computer process control (HESCPC). The complexity of the expertise necessary for computer process control applications requires that the expert system architecture be structured into a hierarchy of classes of specialized experts. The architecture of HESCPC integrates four classes of expert systems: operator/manager companion expert class, control system algorithm design expert class, hardware expert class, and software expert class. The paper is concerned with the design and implementation of the general system architecture, an operator adviser expert for a nuclear power plant and a control system designer expert using a state space feedback approach. Although the design and implementation aspects of all experts are discussed, the emphasis is on the latter.

At this stage of the HESCPC development, the declarative knowledge represented by 423 metarules and 1261 rules is distributed on a hierarchical structure among 20 experts on different levels of the hierarchy which are able to communicate among themselves to solve difficult control problems.

Examples of control system design sessions of linear mono and multivariable systems using feedback state space approach are given. A run time of an operator-adviser data-driven expert system for a nuclear plant is also presented.     

Multilevel optimization algorithms in computer aided production control in process industry

The use of optimization techniques in production control is discussed. Two optimization problems in relation to typical process industry complexes are formulated. A modified version of Tamura's algorithm is reviewed. The promising performance of the computerised algorithms is illustrated by numerical results. DISPATCHER, a practical operative decision support system, is described.     

Advanced methods of process computer control for industrial processes

In recent years, the requirements for the performance of multilevel process control, including feedforward and feedback control, monitoring and optimization have increased. Applying process computers and micro computers, the functions of analog equipment and hardwired logic devices cannot only be replaced. Extended or quite new methods can be realized improving the performance of multilevel process control. These advanced methods for process control are characterized by: more sophisticated, better adjusted control algorithms, forecasting of process variables, estimation of not directly measurable variables, computer aided design of algorithms and adaptive or selftuning algorithms. The basis of these advanced methods are mathematical models of the processes and their signals, often gained by the process computer itself during on-line operation.The present paper discusses first how process models in open and closed loop can be obtained by on-line identification methods. Then, based on these models, the computer aided design of control algorithms, adaptive control algorithms and adaptive steady-state on-line optimization will be regarded. Monitoring of not direct measurable variables will be mentioned. For some methods, practical results with real and simulated processes are shown. Interactive process computer software packages are used which can easily be transferred to other process computers.     

Improved quality of continuous flow manufacturing through the combination of statistical process control and conventional computer process control

This paper represents a portion of the evaluation of the use of Statistical Process Control within the realm of Conventional Computer Process Control. Various control charts commonly utilized in continuous processes are evaluated when subjected to process disturbances typical of continuous processes. The control charts considered are the Moving Average and Moving Range Chart combination, the Individual and Moving Range Chart combination, and the Exponentially Weighted moving Average Chart. The types of process disturbances considered are unit step and linear trend disturbances to the process average. The evaluation is based on the determination of the Average Run Lengths (ARLs) resulting from computer simulations.     

A process and control simulator for large scale cryogenic plants

This paper presents a process and control simulator for industrial helium cryogenic plants controlled by Programmable Logic Controllers (PLC). This simulator can be used for different purposes such as operator training, test of the PLC programs or the optimization of the plant. The different component models used in the simulator are detailed and explained. Various large scale cryogenic plants used for the particle accelerator LHC (Large Hadron Collider) at CERN have been modeled and simulated. The good agreement between the simulation results and the dynamic behavior of real plants is demonstrated with experimental results. Various discussions complete the presentation.     

Retrofitting machine tools with computer control

Many numerically controlled machine tools used in industry predate the era of microelectronics, and rely on controllers based on discrete logic. There are clear advantages in converting these machine tools to computer control. Part programmes can be loaded from a central library over a local area network, and status information can be returned to a central point to help with overall planning. A screen near the machine tool can display helpful information for the operator. Finally, computer control opens the way to integrating the machine tool into a flexible manufacturing system where human operators are partly or completely replaced by robots. This paper describes a simple and automatic way of converting a machine tool to computer control. The method relies on a commercially available microcomputer, and it can be applied to most existing numerically controlled machine tools with a minimum of disturbance.     

Hydrothermal scheduling problems with pumped-storage hydro plants

In this paper, we developed an algorithm in the calculation of hydrothermal generation schedules with pumped-storage plants. In our algorithm, the system is decomposed into a hydro subproblem and a thermal subproblem by using Lagrange relaxation. The hydro subproblem is solved using Mixed Coordination of [TAN91a], and the thermal subproblem is solved analytically. A method is developed based on the Lagrange multipliers (incremental cost rates) in deciding the operation of the pumped-storage plants. Numerical tests show that by properly operating the pumped-storage hydro plants, savings on fuel-cost can be achieved.     

Optimization problems in computer network design

This paper examines optimization problems associated with computer network design and routing. The emphasis is focused on modifying conventional heuristic search procedures to enhance their solution optimization capability.The techniques discussed include fuzzy heuristics, adaptive compositions of heuristics and suboptimum algorithms based on dynamic programming concepts. The discussion is illustrated with examples and experimental computer implementation results.The results obtained demonstrate that the application of the above methods improves solutions derived by means of conventional heuristic procedures and brings such solutions closer to optimum ones.     

A predictive control strategy for nonlinear NOx decomposition process in thermal power plants

For the load-dependent nonlinear properties of the nitrogen oxide (NOx) decomposition process in thermal power plants, a local-linearization modeling approach based on a kind of global Nonlinear AutoRegressive Moving Average with eXogeneous input (NARMAX) model, named the exponential ARMAX (ExpARMAX) model, is presented. The ExpARMAX model has exponential-type coefficients that depend on the load of power plants and are estimated offline. In order to take advantage of existing conventional controllers and to reduce the cost of the industrial identification experiment, we propose a model structure that makes it possible for the ExpARMAX model to be identified using commercial operation data. On the basis of the model, a long-range predictive control strategy, without resorting to parameter estimation online, is investigated. The influence of some intermediate variables treated as process disturbances is studied, and the scheme using a set of multi-step-ahead predictors of the intermediate variables to improve control performance is also presented. A simulation study shows that the ExpARMAX model can give satisfactory modeling accuracy for the NOx decomposition (de-NOx) process in a large operating range, and the control algorithm proposed significantly improves the control performance.     

Stability of digital control with computer interruptions

This note analyzes centralized digital control systems in which feedback loops are closed through a digital computer which generates the control law. Intermittent computer interruptions result in the deterioration of control quality and may even render the system unstable. A criterion that guarantees asymptotic stability under any admissible interruption pattern is presented in this note.