Real-time process control in the 80s

Guest editor Julian Morris presents an overview of real-time process control which is the subject of this special issue Microelectronics and microprocessors have without doubt aroused industry and engendered some strong feelings both for and against new technology. There are many plants, unit processes, machines, etc. where the application of microsystems could be envisaged as offering increased efficiency and profitability. However, it is necessary to employ good instrumentation as well as theoretically sound and robust control; the microprocessor can then be regarded as simply a small, relatively inexpensive, flexible and reliable data manipulation device, offering gains in both instrumentation and control.

There are, however, considerable problems to the almost 'open door' philosophy that the microprocessor has fostered. These relate to equip- ment reliability, maintainability and servicing, necessary staff training and staff grading levels, backup by manu- facturers, contractors and servicing agencies, methods of implementing large scale applications and union involvement.

It is not the remit of this paper to detail the above areas; all that should be added here is that all parties involved in industrial microprocessor applications need to sort out these inherent problem areas before this new technology kills, or is killed by, industry. This is especially important in view of the expectations we have for real time process control in the eighties.

With the high cost and probable shortage of energy, together with market competition, it is no surprise that energy saving and maximum demand control are arousing much interest and this is one important area that micro-based systems could play a major role. It is most appro- priate that LSI technology has reached its present stage of develop- ment in time to play an influential role in reducing energy consumption.

It is expected that microprocessor, and associated technology, develop- ments during the next decade will see advances and increasing applica-

tions in all areas of instrumentation and control. Some of the most important of these are shown in Table 1.

Arguably, the principal control engineering trend in the 80s will be towards distributed control systems, . offering considerable advantages in reliability, operator acceptability and the ability to install systems in a series of economic and separate operational stages.

The development of LSI techno- logy and similar developments in the fields of telecommunications, display

Department of Chemical Engineering, University of Newcastle-upon-Tyne, Merz Court, Claremont Road, Newcastle- Upon-Tyne NE1 7RU, UK

techniques and data processing has led to a major revision of the design of process control systems. It is now possible to distribute intelligent devices around the control system in a systematic and economic manner, with intercommunication between these devices to provide facilities for coordinated control and super- vision by the plant operator. A high degree of system integrity can be achieved by ensuring that each distri- buted control module handles only a small part of the overall control task thus minimizing the consequence of failure of any one device.

A typical distributed system should feature the following elements:

• local autonomous digital controllers, each of which handles a limited control task function, with com- munication to sensor devices for

Table 1. Projected advances in instrumentation and control

• digital process variable transmitters both independent of, and incorporated into, sensor transducer housings

• digital transmission of measurement and control signals over distances greater than just a few metres

• fibreoptic data and control signal transmission bringing advantages of reduced costs, freedom from electrical interference and increased safety through electrical isolation

• digital processors in actuator housings for data highway access to, and control of, actuators

• computer based sensors operating on several process measurements to output inferential relationships of direct meaning for optimization and control

• basic sensing techniques used to yield direct digital outputs and eliminate the need for signal conditioning of primary process variables

• pumps driven by AC variable frequency drives for flow control and AC variable frequency drives for speed control

• central supervisory control rooms replacing central control rooms as distri- buted control systems move process controllers closer to plant units

• flat-screen colour display technology with solid state reliability for control system operator-machine interfacing

• touch-screen methods of operator display and function selection based on menue and prompt programming

• diagnostic techniques that slowly extend self-monitoring methods to self-repair • onchip computer-based programming to solve user control problems rather

than programmable general purpose computers in specific applications • computer-based solutions in problem-oriented control firmware/hardware

easily applied by users without special computer programming knowledge - especially useful for the newer advanced control techniques (adaptive control, kalman filtering, optimizing control)

• exclusive use of high level, flexible programming languages (PASCAL, C) for target use in small computer systems rather than machine or assembly language, except in specialized time-critical cases

• high level languages offering concurrent programming or multitasking capabilities in small comPuter systems and portability, especially with real time control hardware features very large scale memories to remember the results of past control actions to control inputs so that future control actions can be based upon an online learning experience or pattern recognition speech synthesis techniques to system diagnostics and maintenance, annun- ciator applications for audible fault detailing and voice recognition techniques for operator-machine interfacing

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direct and inferential processing of measured process variables

• an operator information centre based on one or more VDUs with keyboard input for commands

• a digital data communications highway connected to all the system modules and communicat- ing data and controller settings between them. Three data-highway structures are usually encountered; the radial highway (providing efficient communication between local modules and the central station but not for communication between local modules), the ring highway (requires nodes in the communication ring to connect modules into the network which can cause problems if a node fails), the direct-drop highway (here modules are plugged into T-junctions on the highway, easy to implement but a communica- tions failure can affect all modules)

• a multiplexer unit for additional process data input from noncon- trolled process variables

• links from the data highway to general purpose mini- or micro- computer-based outstations (for laboratories, process investigations, etc.)

SyStems application

The application of distributed systems is in its infancy and several design aspects have yet to be resolved:

• Geographical as well as functional distribution should become com- mon but here environmental and safety considerations will play an inhibitive role

• Further distribution of control to the sensor and actuator level is

possible with digital data trans- mission but here the ability to programme more complex cascade, feedforward, multivariable and modern control schemes may inhibit progress

• System integrity could be improved by having arrays of single-loop controllers giving rise to single-loop- controller integrity to minimize the consequence of failure

• System specification is more

complex than in conventional systems and there is need to reduce the engineering effort required

Current approaches to distributed control are rather specialized, not particularly user-oriented and restric- ted to a limited range of applications. In the next decade the need to handle more sophisticated applications is likely to require more generalized networks of single- and multiloop controllers, sequence controllers, display stations and process com- puters. These architectures may carry the disadvantage that the simplicity of concept of distributed instru- mentation and control systems will be lost and with it the acceptance and understanding of control engineers.

Although the ability to mix differ- ent manufacturers' equipment in a distributed system is desirable from a users' point of view, much still remains to be done in the standardization of hardware structures, high level portable programming languages and communi- cations protocols.

Users' interests

The engineering of industrial control systems always includes, as ultimate objectives, demonstrative economic gains for the users. These gains can take many forms: higher product quality, increased produc- tivity of plant or personnel, reduced raw materials usage and reduced energy usage. Instrumentation and control systems require economic justification just as any other invest- ment in plant equipment.

Trends to distribute control designs have developed from the need to increase system integrity and reduce the installation costs associated with wiring of hundreds of sensors and actuators to controllers in centralized control rooms.

It would appear however that in general some equipment is designed with little thought for the end users' economic objectives, but rather the manufacturers' economic objectives resulting in maximum flexibil ity of application and minimum cost of manufacture. It is not too surprising

therefore that the instrumentation and control world is suddenly full of microprocessor-based devices.

The end users' economic concerns include a lot more than low first cost. Reliability, usually in the form of process availability, is probably much more important than flexibility. Self-diagnostics can certainly reduce maintenance time and increase plant availability and such capabilities are now available to board level. However,. the very presence of diagnostics would tend to suggest that a high level of complexity still exists and that can run counter to inherent high reliability.

LSI permits a tremendous complex- ity of circuitry on any single chip without endangering system reliabi- lity but the reliability of monolithic ICs does not necessarily permit a proliferation of chips. (There are more chips and connections per chip in a digital controller than in an IC op-amp-based analogue controller.)

Microprocessor solution

The microprocessor solution, the general purpose small computer, can demand a large increase in chip count whilst the provision of special solutions for real-time process control on large scale integrated chips could signifi- cantly reduce chip count and increase system reliability. It is hoped that the next decade will see better things from the LSI technology that has given us the microprocessor aimed at solving some of the real problems faced in process control.

The microprocessor solution from one point of view, however, is ideal. The parts are inexpensive, and applica- tion flexibil ity is maximized. Perfor- mance and special features are con- jured in software which can easily be changed and hardware redesigns and production line changes are almost completely eliminated.

Better solutions still are needed though for increased integrity and reliability of control, which always translates into an economic advan- tage for the control user. This must remain the ultimate objective for real-time process control.

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