Microcomputer control for industrial plants Mike Murphy describes INSCRIBE, a language developed for a particular series of programmable controllers

The requirements o f a hiqh level programming language for control o f batch plants, widely used in the process industries, are discussed. The main features o f INSCRIBE, a language used in programming a series o f controllers based on LSI- I 1 processors, are described with examples o f typical statement types.

The control of batch plants, widely used in process industries, presents some interesting yet complex problems. It has to achieve balance between the increasing flexibility demanded by production and marketing and the simplicity and standardization required by maintenance staff. All this of course must be within progressively tighter capital budgets with ever shortening delivery requirements.

Is the microprocessor the answer or is big still beautiful in computer control? Can control be done effectively with relay ladders or are large process computer languages the answer? What other solutions are there?

Control of batch plants can be considered as the centre of a triangle (Figure I): it requires a subtle blend of the attributes of the PLC, the plant computer and the analogue controller. What ingredients are required for a successful control system?

B A T C H C O N T R O L R E Q U I R E ~ i E N T S

In common with all industrial control, the ideal system must be based .on reliable, industrialized hardware which is suitable for the typical plant environments in which it must operate. The control language must be high level and capable of performing the required control actions with a maximum of ease and flexibility, since application program- ming is preferably by process of instrument engineers rather than computer programmers. Modifications should be easy to implement and document to reduce system test and commissioning times to a minimum. The control environment must be capable of simultaneously controlling numerous tasks or functions in real time and handling multiple communication links or peripherals. Efficient digital and analogue input and output handling is obviously essential together with the ability to quickly diagnose equipment and plant failures thereby minimizing plant downtime. In addition, it must of course be capable of performing the mixture of functions and facilities required for batch control.

Typical batch processes require the ability to weigh or meter ingredients, route them to some predetermined

Kratos Instem Ltd, Walton Industrial Estate, Stone, Staffs ST15 0LT, UK

Data

Analogue Logic

Figure I. Control o f batch plants

destination, perform control actions dependent on time, temperature, energy or pressure and then produce diagnostic and production information for plant operators and managers. The size of system varies considerably, as also does the sophistication of the operator communication and management reports. Batch systems are required to produce stock level information, material, product usage and shift reports, alarm messages, status displays and breakdown analysis reports.

Where product processing involves temperatures, pressures or flows analogue limit checking, three term control and trend recording are often required. Routing and plant alarm diagnostics require frequent scanning of valve or conveyor states as well as sequencing route changes at the appropriate time. Colour VDUs may be required to present the operator with a technicolour view of life on the plant.

One system that can do all this within a tight budget and provide facilities allowing plant engineers to maintain and modify the system easily is Kratos Instem's program- mable controller, Instematic. This has evolved over the last four years and now fulfils many functions required by both large and small batch control systems. In its latest form - Series 200 - it is available as a range of four controllers based on the DEC LSI-11¢23 and LSI-11/2 microprocessors.

The key is undoubtedly its engineers' control language, INSCRIBE.

H I G H L E V E L L A N G U A G E

INSCRI BE was conceived after many years experience in batch control system design using assembler code computer systems and hardwired diode matrix programmers. Assembler coded computers with fast real-time executives provided the necessary response, flexibility and data handl- ing but at the expense of complexity of program which required skilled programmers to implement batch systems

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successfully. Modifications were laborious and almost impossible if the programmers concerned were no longer available or accurate records of program patches had not been kept.

Hardwired controllers provided the simplicity of programming and operation necessary but were somewhat restrictive in the functions they were able to perform. Printing was almost impossible and special hardware circuits were often necessary to complete a control scheme. INSCRIBE combines the benefits of both the approaches without the previous disadvantages. It provides a flexible batch control language which is easy to use and suitable for engineers to implement their own control systems.

The major decision to be made in any control language is the format it takes. Since INSCRIBE was to perform the many different functions required by batch control systems, it was felt that a relay ladder approach would be restrictive in providing a suitable mechanism for expres- sing printing and data handling requirements. A high level English language approach, similar to BASIC, was there- fore adopted which subsequently has enabled more complex functions to be added without the need for any significant structural or philosophical changes to the language.

The necessity to have an interpretive rather than a compiled program language was dictated by the require- ments of ease of programming and subsequent modifica- tion together with the desire to have a language system which is self contained. The ability to be able to modify programs then immediately run the system, even in the middle of Russia, without the support of a large computer bureau, was obviously a desirable objective.

MULTITASKING Batch control requires the simultaneous execution of a number of sequences, functions or tasks. Any language structure must therefore be capable of multitask operation in real time. INSCRIBE provides up to 128 such tasks, each of which may be independently sequencing through its language statements at different speeds and priorities. The tasks are organized on a 'round robin' basis where each task executes one statement before control is passed to the next task. Priority can be assigned to tasks and interrupting input modules can be given immediate priority.

This structure provides an efficient basis for organizing the application programs in plant-related blocks which are easy to understand while minimizing the response times for any particular control function.

Tasks can only intercommunicate via the database which minimizes the effects that program errors have on the rest of the system and makes fault location easier. Each task has its own set of local memories which are inaccessible to other tasks. The action taken by a task after powerfail restart or emergency stop can be individually specified.

In order to eliminate system declarations, the database has a fixed maximum size and the system automatically determines the location and number of tasks and records present in the system on restart. The database comprises datawords which can be floating point, binary of BCD numbers, ASCII text and up to 4096 bits for digital inputs, outputs and flags. The digital and analogue interface modules to the plant are automatically scanned by hard- ware into the database so that all input and output trans- actions are transparent to the user programmer.

Where larger quantities of data are required for recipes, displays or trend recording, up to 240 memory-held records are available. Each record may contain up to 64 floating point numbers or 256 ASCII characters. This structure may be extended by addition of a 2 Mbyte floppy disc if required. All tasks and records are held either on battery-backed RAM or EPROM memory modules to provide the system with the necessary level of security required for industrial control system programs and data.

HIGH LEVEL STATEMENTS The different functions to be performed by a system are generated by one of 24 types of high level statements. These statements are specifically orientated towards control applications and can be grouped into the following areas:

• sequencing • general data and handling • operator communication • interprocessor communication • specific batch functions • analogue monitoring and control • alarm detection • printing

Statements are entered into tasks under the control of the system generator program. This enables tasks and records to be displayed, printed and modified by single letter commands. This mechanism avoids syntax errors on enter- ing programs but provides a detailed English language printout of the application program. For example,

$10 WHEN B256 AND/B273 = 1 SET B821 = 1 & A D V A N C E BUT I F B 9 2 = I G O T O S 4

where bold characters indicate user-entered data, / is logical NOT and $10 is the statement number.

Sequence statements such as WHEN wait until the appropriate conditions are satisfied before the task advances to the next statement in sequence ($11). Should the BUT IF conditions be satisfied while waiting, then the task executes the GOTO part of the statement ($4) instead. Such sequencing structures enable the current statement number of any task in the system to be readily monitored thereby assisting inprogram testing and fault diagnosis.

Arithmetic statements include MAKE:

$20 MAKE DA1 - DCIO + DR(I) X D150, RESULT = 0 GOTO $10 $20 $30

This enables the database to be manipulated and decisions to be made on the result. Datatype conversions are done automatically and are transparent to the user. For example,

DA1 is an analogue dataword in binary DC10 is a floating point common dataword D150 is a BCD input word DR(I) is an indexed variable record item.

Specific batch control functions include COMPARE:

$23 COMPARE DA1 WITH DR3 AS DATAIF 1st = 2nd SET/B256 = 1 & ADVANCE

BUT IF B352 = 1 GOTO $20

A comparison of the analogue weight input DA1 with a recipe target DR3 results in the feeder output B256 being switched off when the target is reached. If an alarm B352

24 microprocessors and microsystems

is generated during the comparison then the sequence immediately goes to statement $20.

Communication facilities include statements for operator data entry and printing and the TRANSFER statement for interprocessor communication. The system supports up to 12 serial links.

$5 TRANSFER RECORD (D5) FROM LINK L1 TWO WAY TO RECORD D6

THEN ADVANCE AND RETAIN LINK BUT GOTO $3 IF ERROR

This statement transfers a record of up to 64 items of data from one processor to another in binary format with a secure communication protocol, thereby minimizing transmission errors.

An analogue statement is available for monitoring, converting, limit checking and alarming analogue flows,

temperatures and pressures. Another statement allows a three-term control to be executed using record held parameters.

Comprehensive monitoring and diagnostic facilities are built-in for program debugging equipment and plant fault diagnosis. The system also supports a user-oriented colour VDU mimic diagram drawing capability which allows plant data and states to be quickly specified and auto- matically updated.

This approach to solving batch control problems has been successfully employed over the last four years by end users, OEMs and inhouse staff in a variety of applica- tion areas. These include rubber blending and mixing, feedmill control from intake, through blending to out- loading, glass plant furnace monitoring, polyethylene pellet blending and routing, batch chemical reaction control, edible oil blending, engine testing and concrete mixing.

Announcing a new international journal to be published by IPC Science and Technology Press in association with the European Computer Measurement Association

published quarterly from June 1980

Main subject areas to be covered:

• m e a s u r e m e n t app l i ca t i ons • m e a s u r e m e n t t e c h n o l o g y • ana ly t ica l m e t h o d s

• mode l l i ng t e c h n i q u e s • capac i t y p lann ing • pe r fo rmance theory

For further details please contact: Christine Mullins, Computer Performance, IPC Science and Technology Press Ltd., PO Box 63, Westbury House, Bury Street, Guildford, Surrey GU2 5BH, England. Telephone: 0483-31261 Telex: 859556 SCITEC G

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