distributed process control concept spreads

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Technology Distributed process control concept spreads Need to segregate control functions by process area and managerial level prompts hierarchical distributed control trend The profusion of computing hard- ware and programing languages in process control isn't diminishing. But an organizational concept in indus- trial control being evolved by control engineers and system designers is bringing more order. The concept is based on recognition of two spectra of functions that must be accommo- dated simultaneously by a control system: hierarchical functions and distributed functions. Hierarchical functions are those involved in the instrument and com- puter pecking order—from, say, data acquisition and hardware actuation at the lowest level through control loop regulation and unit optimization to management information systems at the highest level. Distributed functions refer to the proper place- ment of the control functions at any given hierarchical level within the equipment areas, process units, plants, or facilities. In summarizing the general in- dustrial control picture for the recent 1979 Joint Automatic Control Con- ference in Denver, D. Grant Fisher and Sirish L. Shah of the University of Alberta see two separate but re- lated developments occurring now. One is the maturing of microcompu- ters; the other is the designing of distributed networks of computers. These developments, they point out, are revolutionizing information processing and process control in in- dustry. Although any given computer can probably find application some- where, there is a growing class of ap- plications best handled by distributed networks of computers of various sizes. It is important, Fisher and Shah say, to use the right amount of com- puting power when and where it is needed and provide the right amount of redundancy in the system. This very rapid and continuing evolution of control systems has yielded the hierarchical distributed control (HDC) concept. According to P. Ficarro and R. E. Jones of Foxboro Co., Foxboro, Mass., the concept stems from the need to segregate process control functions by both process area and managerial level of the function. The evolution of the concept occurred at the same time that system designers experienced sharply increased demands on the abilities of control systems resulting from a natural growth in process complexity. There also was an in- crease in demands by managements for more process information, often in response to regulatory require- ments. Hardware improvements that permit a degree of control not possi- ble before also have contributed to development of the HDC concept. And, in addition to the need to inte- grate control functions, further in- centive came from the ability of an HDC network to provide the safety inherent in redundant units without requiring nonworking duplicates. Thus, HDC involves considerably more than just connecting units of hardware, Ficarro and Jones say. The provision of redundancy without duplicate units, for example, involves machines making managerial deci- sions over other machines, although these decisions are subordinated to machine control by the manager. Such subtle interactions are the essence of HDC, practical examples of which are growing in number and complexity. One such system, for ex- ample, has been developed by Union Carbide at its South Charleston, W.Va., facilities. Carbide's William K. Greene says that the first hierar- chical system was installed there in Sea urchin is mold for artificial blood vessels Donald V. Hillegass, a scientist at Goodyear, examines a South Pacific sea urchin that is used as a mold for small artificial blood vessels such as the one in his right hand. Replacing small vessels has been a problem because body cells growing inside the vessels can clog them and cause clotting. Specially formulated rubber is injected into the pores of the sea urchin. After the rubber is vulcanized, the skeletal sections are chemically dissolved. The rubber used contains special carbon black compounds that help concentrate the body's natural electrical charge near the artery wall. This electrical charge causes a selective growth pattern for the developing inner lining of the artery and decreases clogging. July 23, 1979 C&EN 27

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Page 1: Distributed process control concept spreads

Technology

Distributed process control concept spreads Need to segregate control

functions by process area

and managerial level

prompts hierarchical

distributed control trend

The profusion of computing hard­ware and programing languages in process control isn't diminishing. But an organizational concept in indus­trial control being evolved by control engineers and system designers is bringing more order. The concept is based on recognition of two spectra of functions that must be accommo­dated simultaneously by a control system: hierarchical functions and distributed functions.

Hierarchical functions are those involved in the instrument and com­puter pecking order—from, say, data acquisition and hardware actuation at the lowest level through control loop regulation and unit optimization to management information systems at the highest level. Distributed functions refer to the proper place­ment of the control functions at any given hierarchical level within the equipment areas, process units, plants, or facilities.

In summarizing the general in­dustrial control picture for the recent 1979 Joint Automatic Control Con­ference in Denver, D. Grant Fisher and Sirish L. Shah of the University of Alberta see two separate but re­lated developments occurring now. One is the maturing of microcompu­ters; the other is the designing of distributed networks of computers.

These developments, they point out, are revolutionizing information processing and process control in in­dustry. Although any given computer can probably find application some­where, there is a growing class of ap­plications best handled by distributed networks of computers of various sizes. It is important, Fisher and Shah say, to use the right amount of com­puting power when and where it is needed and provide the right amount of redundancy in the system.

This very rapid and continuing evolution of control systems has

yielded the hierarchical distributed control (HDC) concept. According to P. Ficarro and R. E. Jones of Foxboro Co., Foxboro, Mass., the concept stems from the need to segregate process control functions by both process area and managerial level of the function. The evolution of the concept occurred at the same time that system designers experienced sharply increased demands on the abilities of control systems resulting from a natural growth in process complexity. There also was an in­crease in demands by managements for more process information, often in response to regulatory require­ments.

Hardware improvements that permit a degree of control not possi­ble before also have contributed to development of the HDC concept. And, in addition to the need to inte­

grate control functions, further in­centive came from the ability of an HDC network to provide the safety inherent in redundant units without requiring nonworking duplicates.

Thus, HDC involves considerably more than just connecting units of hardware, Ficarro and Jones say. The provision of redundancy without duplicate units, for example, involves machines making managerial deci­sions over other machines, although these decisions are subordinated to machine control by the manager.

Such subtle interactions are the essence of HDC, practical examples of which are growing in number and complexity. One such system, for ex­ample, has been developed by Union Carbide at its South Charleston, W.Va., facilities. Carbide's William K. Greene says that the first hierar­chical system was installed there in

Sea urchin is mold for artificial blood vessels

Donald V. Hillegass, a scientist at Goodyear, examines a South Pacific sea urchin that is used as a mold for small artificial blood vessels such as the one in his right hand. Replacing small vessels has been a problem because body cells growing inside the vessels can clog them and cause clotting. Specially formulated rubber is injected into the pores of the sea urchin. After the rubber is vulcanized, the skeletal sections are chemically dissolved. The rubber used contains special carbon black compounds that help concentrate the body's natural electrical charge near the artery wall. This electrical charge causes a selective growth pattern for the developing inner lining of the artery and decreases clogging.

July 23, 1979 C&EN 27

Page 2: Distributed process control concept spreads

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the 1960's to minimize the problems of software maintenance and of ex­pensive redundancy that beset tjie stand-alone systems of the times. The first system designed by Carbide en­gineers consisted of a central com­puter and two satellites and was de­signed to be process independent, with no software peculiar to the chemical process being controlled.

The main problem facing Carbide at the time was one of interunit com­munications. Communications soft­ware simply didn't exist. According to Greene, there was no way, for exam­ple, to allow a FORTRAN programer to communicate across a machine/ machine link. Carbide's staff virtually invented its own local languages and temporarily solved the communica­tions problem until vendors began providing consistent software.

By the mid-1970's, Carbide's sys­tem had assumed its present shape. Greene describes the system as con­taining three levels of process control and involving 20 separate computer systems. A large central computer is available for handling large numbers. Three intermediate-level host sys­tems are connected to satellite sys­tems assigned to specific process units. In addition to providing re­dundancy, the host systems integrate the communications and software for eventual monitoring by the higher-level central computer. One of the host systems is the original central computer of the 1960's network.

The satellites are directly con­nected to the processes and are used for loop control functions, data log­ging, program testing, and conversion of process signals into appropriate engineering units. Some satellites communicate with the distributed control network, some with micro­computers that control process ana­lyzers, and some even with the control computer directly.

A major provision of the Carbide system is for future expansion. For example, Greene says, the company expects to connect the entire system with an IBM 370 data processor at a location remote from South Charleston.

Another example of an HDC sys­tem is at Armco, where all phases of the company's steel mills eventually will be brought into the system. Long-range planning for the conver­sion began in 1973.

According to Armco's Lawrence C. Long and Jerry H. Schunk, the final incentive to develop true distributed control was provided by the ability to connect machines in a network. In­deed, they say, the rapid development of distributed networks is due in large measure to the provisions for ma­chine/machine communication now

available. Before, it was virtually impossible to bridge an electronic interface, or a man/machine inter­face, unless a unique software lan­guage transition were devised for each case. Today, they point out, it is pos­sible to connect machines from a single manufacturer, although it is still difficult to bridge the gaps be­tween machines from different man­ufacturers.

The Armco system, like many others, was strongly influenced by the general systems developed in recent years at the Purdue University com­puter workshops directed by Theo­dore Williams. By combining certain functions, Armco engineers reduced the Purdue, six-hierarchical, level system to four levels, but in essence the Armco and Purdue systems are much the same.

A prime consideration in any HDC system is security. Security may be greater with HDC systems than with the older stand-alone systems. One reason is that no part of the system can be perturbed for very long with­out the disturbance being manifested elsewhere in the system. •

Acoustic flowmeters now under test Although much of the thought in process control circles these days is with the systemic aspects themselves, there also is effort being expended on development of new instruments to be controlled. An example, described at the 1979 Joint Automatic Control Conference in Denver, is a family of acoustic flowmeters being developed for use in coal conversion plants.

In 1976, Argonne National Labo­ratory identified a need for a solids flowmeter that could be used with coal slurries and suspensions. This need led to a development program, now in progress at Argonne, dealing with acoustic flowmeters. Acoustic flowmeters are based operationally on the attenuation of sound with dis­tance in a given medium and with some parametric variations due to frequency. In slurries and suspensions with less than 18% solids content, the variations appear to be linear.

One of the first results of the pro­gram is a density /volume transducer that is being tested at the Hygas coal gasification plant operated in Chicago by the Institute of Gas Technology. A second unit, based on the Doppler effect, also is being tested.

An advantage of acoustic flowme­ters is that they do not require ob­structions in the line. They can be placed within a pipe's wall and be ef­fective without impeding flow. •

28 C&EN July 23, 1979

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