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One of the basic requirements set to control systems is their high reliability while usage shall be simple and convenient. The requirement of reliability is understood, because those systems control the operation of expensive installations, which downtimes, malfunctions or high operation cost must be minimized. Also safe process execution requires the reliability. A convenient and simple use makes it easier for process operators to control functions of the installation and helps to operate them in optimal conditions. Reliability and effectiveness of control systems made by ASKOM is determined by a few key factors:

• use of reliable PLC's made by worldwide companies,

• implementation of elements for structural system reliability,

• use of distributed configurations,

• process supervising based on a clear set of screens implmented by visualization and control system,

• system opening for the level of production management.
   

Many functional advantages while maintaining user-friendliness is ensured by ASKOM made SCADA system : asix for Windows.

  System Configuration

ASKOM bases its solutions on programmable controllers of worldwide suppliers (Siemens, GE Electric, Allen-Bradley, Schneider-Electric). Standardization of controller programming tools included in IEC 1131 standard makes possible to perform control using almost any controller, by development on the basis of its components a reliable structure of computer control and supervisory system.

To fulfill strict requirements of maintaining control system availability, which is especially important in power industry, ASKOM uses since a couple of years a system configuration proven by success of many installations (see diagram) which improves its reliability structurally.

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Key features of this structure are:

• separate process (control) and information (Ethernet) buses,

• redundancy of process bus,

• function of operator stations within supervisory system in hot-redundancy mode,

• system opening for data access from the level of production management.

Depicted control system structure includes 3 layers:

• process layer comprising control stations,

• operator layer comprising PC operator stations (including engineering station),

• management layer comprising process data server for workstations on plant network.

Measurements

Measurements of technological parameters are entered to the system by usage of various types of analog cards. Digital I/O module cards are also available in wide range of types, depending on input circuit voltage range, number of I/O channels, separation from controller's internal bus, reaction time, requirements of channel separation etc. In case the technical requirements do not force to choose special type of card, one tries to achieve maximum degree of standardization, which reduces cost of spare parts. By standard, the 24 VDC object voltage is used. For typical contactless and hermetic sensors, or contacts made of good contact materials, this is the most rational solution.

For most applications, where economical factors decide, sensors and executive elements are connected directly from object to the card using screw terminals. Disadvantage of this approach is a possibility to transmit overvoltage from object to the module, which may damage single channel or entire module. In control installations, which have high requirements for reliability, inputs and outputs are separated from system's electronic circuits by coupling relays. Higher price of such application is compensated by complete independence of system operation from hazards, which may be transmitted from external circuits.

Drives  

The drives are controlled by typical functional blocks of the PLC program, made for specific type of the drive depending on its electric diagram. Functions of the blocks ensure service of local buttons, remote control, drive operation diagnostics and data exchange with visualization system. Typically, drives may be controlled by local control box or remotely by operator station. The control itself can be fulfilled in one of the following operating modes:
 

• maintenance mode, without technological limitations, eventually using only some internal security features,

•  manual mode (local or remote), maintaining all interlocks and security,

• automatic mode, maintaining all interlocks and security, where turning the drive on and off is performed by algorithm of operation run within the controller.

Operator stations within the control system of ASKOM company have a so-called symmetry of application feature, which means, they have identical software configuration in scope of operator's services, which ensures their immediate interchangeablity. Any operator computer can execute all functions related to the technical supervision. It is the operator's choice, which segment of installation (technological group) shall be presented on the monitor. Such configuration ensures maximum security of operation and does not cause a possibility to loose control over process during malfunction of any station.

Control stations communicate with each other and with operator stations through separate process bus. For purposes of responsible processes and installations, process bus has been duplicated to improve system reliability. Type of the bus depends mostly from requirements for the number of exchangeable process data within the system - typically this is PROFIBUS or Ethernet bus.

Operator servers

Operator servers have got process bus connections for data exchange with control stations on one side, on another side connections to independent Ethernet information bus for data exchange and making data available to operator terminals.

To achieve high system reliability, operator servers operate in pairs, being process data servers within the information network on so-called "hot-redundancy" principle. Servers within a pair run the same system and operator functions, ensuring duplication of access to process variables and duplication of data archives, alarm and event log. Each operator server has got two communication channels with the PLC:

• basic - through process bus, and

• spare - through Ethernet and "neighbour" server.

Servers communicate with the PLC's through independent interfaces and do independent archives of data, alarms and reports. During failure-free operation, both servers are a source of data for other stations within the Ethernet network.

If any server detects malfunction of communication channel to the PLC, it switches automatically to alternative channel, which is Ethernet interface to "twin" server within the network. Malfunction of one server within a pair or its connection to the process bus does not cause system function degradation - operator terminal computers within the Ethernet network automatically switch to the "neighbour" server operating in "hot-redundancy" mode. After defect is fixed, the restarted server automatically does resynchronization with its twin server and rebuilds content of its data archive.

Operation of every system requires reliability of its function. The operator and the user must be sure the process data is correct and required controls and regulations have been executed. Many system elements diagnoses and checks system function continuously to report any defects of its operation.

Reliable operation of ASKOM system bases, among other things, on the following elements:

• diagnostics of proper process bus communication,

• check of proper process data transfer,

• check of proper measurements,

• redundancy operation of operator servers,

• automatic process data server finding by terminal stations during failure state of actual data source,

• automatic synchronization of data archive and alarm log between servers operating in "hot redundancy" mode.

Information bus (Ethernet) is used for:

• second way of process data transfer between operator servers,

• synchronization of data archive on operator servers,

• synchronization of astronomical time between operator and control stations,

• data exchange with operator terminals,

• information exchange about alarm system status,

• building of open system prepared for data exchange with corporate computer network.

• engineering software for PLC's of control stations,

• software of so-called designer of asix SCADA system,

• diagnostic software for process and information bus,

• devices and software for back-up of long-term process data archive to removable drives or CD's,

• data base (a so-called repository) of configuration files and application software of asix SCADA system and control station PLC's,

• modem and software for remote service of application software,

• other special software for testing and development.

In control systems based on programmable controllers and PC based SCADA visualization, the software remains divided onto controller and computer part. In solutions of ASKOM, the PLC and operator station software is linked by common process data base, which is maintained in an Excel spredsheet. It declares a list of process variables and their parameters, both for controller application and visualization software. Standards of control system solutions developed by ASKOM ensure extended library of PLC's functional blocks and corresponding library of visualization objects (windows) and pictures, which build together a coherent system of functionality comparable to DCS systems. 

PLC programming is made by engineering software of their manufacturers. Those programs are compatible with commonly accepted IEC 1131 standard, thus they have same languages and methods quite independent from the type of PLC.

Designer mode of asix visualization system ensures on-line application parameterization and automatic broadcast of changes to the operator stations connected via Ethernet LAN. Operator station software gets a message about changes in standard database on the engineering station (or separate application server) and updates its definitions on-line. Also process variables database modifications (adding of the variable, removal, parameter change) can be broadcast on-line in the system. asix and application software administration functions meet expectations of every system engineer and make the same easy development of the software for single station and multi-station configuration.

Engineering station equipped in a modem allows remote access to the control system and facilitates software service and immediate technical support when a failure occurs.

Within the operator layer there is also network print server, which allows to make text or graphic printouts from any computer station - screen (windows) captures, alarm and event logs for current or selected period, reports, trends etc. A typical printing equipment of control system is a pair of printers - laser and inkjet color one. In smaller configurations, printers can be connected directly to the operator stations. 

Supervisory station 

Within control system of large installations, there is often foreseen separate supervisory PC station for extended reporting and process analysis functions. The station can also take over, from the engineering station, the tasks of data back-up on removable media. Supervisory station (KE) can access process database through Ethernet within the segment of operator control room and execute among other things the following functions:

• view process windows,

• print reports, print trends,

• view and print alarm log,

• calculate process efficiency rates,

• back-up data to removable media.

KE computer is equipped in large hard disk and removable ZIP media drive with capacity of 100/200MB as well as CD recording drive (650MB), Which is applicable for back-up of long-term (many months) archive of process data. A selection of CD as archive media allows to read and process data on any computer station with asix terminal license and a cheap and popular CD drive. Experiences from implemented control systems show that monthly data archive for typical large installation (e.g.. boiler, turbine, desulfurization) has a size of roughly dozen MB. For large-capacity drives, this ensures possibility of direct access to many years of archive.

Process data on KE station can be processed directly in asix  environment (using reporter, dialog panel or trend module), by utility programs of Windows e.g. spreadsheet, database, editor (e.g.. MS OFFICE) or using user developed programs, e.g. in VISUAL BASIC. Thanks to OPC/DDE/OLE2 data server included in asix, MS Windows programs can access on-line, by standard protocols, current and archive process database of asix system within the same station or remotely through Ethernet.

Process data server  

Openess of the operator system to the plant network is made by process data server within the management layer. The server, on the one hand, has connection to Ethernet information bus, on the other hand, has got an independent connection to the corporate network. The process data server gets periodically all process data from operator stations and builds on its own the current database, data archive as well as event and alarm logs, in order to serve asynchronous requests from workstations of the plant network requesting access to current and archive data, alarm and event log, reports etc. without "engaging" operator stations.

Ethernet segment of operator stations is separated from the corporate network, thanks to which a potentially big message traffic within this network does not transfer to control room stations. Such solution ensures fast access to process data and protects operator stations against failures of corporate network, limits data traffic in operator station segment and protects them against overload by access requests.

For a low number of workstations within the corporate network, functions of process data server can be fulfilled by a selected operator server equipped with additional Ethernet. card.

Workstations 

Workstations within the corporate network with asix for Windows installed have in above way full on-line access to current data, data archive with one second resolution, alarm and event log and reports. Workstations have full process information, they can be used as remote operator stations (with locked possibility to control), supervisory stations, process and data analysis stations to support process management, optimization, monitoring and control, maintenance, etc. Functions of those stations depend only on their own application software while process data server makes data available without limitation of their possible processing, analysis and presentation.

Workstations equipped with OPC/DDE/OLE2 server (ASIX Connect program), supporting high Ethernet data traffic, make process database available for Windows program environment basing on standard OPC , DDE or OLE2 Automation protocols. They ensure read and write access for current values, read of archive data and event log. That way a presentation of temporary values and processing of historic data can take place using tools of MS Excel spreadsheet, Access database, or other Windows. applications.

PC Gateway

The system described above is also open to connect to the supervisory systems of other vendors. Process data availability using standard OPC/DDE/OLE2 protocol is the first possibility. Higher data exchange throughput can be achieved by extension of process data server within the management layer with dedicated GATEWAY function according to protocol required by external system, e.g. based on RPC standard.

Solutions of automation systems by ASKOM standard are an example of decreasing gap between PLC/SCADA and DCS control systems.
 

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 © ASKOM  01.2002, MKo