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1. Functions of the system.

The block scheme of OpenSCADA system (74 Κα)
Fig. 1. The block scheme of OpenSCADA system

1.1. Modularity.

In order to achieve flexibility and a high degree of scalability OpenSCADA is constructed in a modular fashion. The process of developing our own modules imposed a great risk; possible errors could introduce an element of instability into the system, however tight integration of the modules with kernel lessened this issue and the ability to create a distributed configuration was seen as a greater benefit. In the end a more flexible and stable system was created.


OpenSCADA modules are stored within dynamic libraries and each shared library can contain modules of various types. The specific functional modules that are contained in a library is determined by the specifics of the modules connections. These dynamic libraries are hot swappable, which allows for the updating of a specific module without affecting the system as a whole. This method of storing code modules in dynamic libraries is essential for OpenSCADA, because it is supported by virtually all modern operating systems (OS). However, this does not exclude the possibility of developing other methods of storing code modules.


OpenSCADA has the following functional parts or modules:


Management of the modules is carried out by the "Modules Management" subsystem, whose functions include connection, switching off, updating, and other operations concerned with the management of the modules and their libraries.

1.2. Subsystems.

Architecturally OpenSCADA is divided into subsystems or two types, regular and modular. Modular subsystem have the ability to expand through the addition of modules, with each modular subsystem containing sets of modular objects. For example the modular Database subsystem contains modular objects of the database type, thus the modular object is the root of the module.


The basic configuration of OpenSCADA consists of nine subsystems with seven being modular. These nine subsystems are present at every configuration. Additional subsystems can be created by adding additional modules. The following lists the basic subsystems of OpenSCADA:

1.3. PLC and other sources of dynamic data. A subsystem "Data acquisition".

The Data Acquisition subsystem supports dynamic data sources whether PLC controllers, USO boards, virtual, or other sources. The functions of this subsystem are to provide data in a structured manner and the management of the data, i.e. data modification.


Since the Data Acquisition subsystem is modular it contains objects of the dynamic data source type. For example in October 2007 OpenSCADA supported the following data sources:


Each data source requires a separate module that can be connected or disconnected, with a module communicating to one or more devices(controllers).


Each controller contains parameters with the types, defined by the module. The parameter provides the list of attributes which contain the data. Parameter's attributes can be one of four basic types, string(text), integer, float and boolean. For example an analog parameter can contain data in either integer or float format.


The structures of a controllers, parameters and their types are contained in the Data Acquisition subsystem so that the module objects can specifically fill in these structures.


A source of dynamic data can be on a remote OpenSCADA system. In this case the data source would be the OpenSCADA data transport. The function of this type of data source is to mirror of the data sources on the local system.

1.4. Databases. A subsystem of "Database"

For a data storage of system databases (DB) are everywhere used. With a view of systematization of access and management of databases in OpenSCADA system the subsystem "Database" is provided. For support of various DB/DBMS the subsystem is modular.


In a role of the modular objects, containing in a subsystem, type DB/DBMS acts, i.e. the module of a subsystem "Database", which practically contains realization of access to the certain type of a DB. For example modules: DBF, MySQL, SQLite.


The object of type DB/DBMS, in its turn, contains the list of objects of separated DB of the given type. And the object of a DB contains the list of objects of tables which are contained by data in the tabulated form.


Practically all the data of OpenSCADA system are stored in this or that DB. The toolkit of system allows to transfer easily the data from one type of a DB on another and as consequence provide an optimum selection of DB type under the concrete area of OpenSCADA system. Transfer of the information from one DB to another can be made by two ways. The first is a change of the address of a working DB and save of all system on it, the second is a direct copying the information between DB. Except for copying the function of direct editing of contents of tables of a DB is supported also.


For the organization of the centralized access of the allocated system to a uniform DB two ways are provided. The first is using of network DBMS, for example MySQL. The second way is using of transport type of a DB on local systems for access to one central DB (It is planned.). Function of a transport DB is transfer of queries to a DB on remote OpenSCADA system.


Data can be stored also in a configuration file of system. The mechanism of full reflection of structure of a DB on structure of a configuration file is realized. I.e. the standard configuration can be placed in a configuration file. An essence of such mechanism that by default for example at start without a DB, it is possible to describe the data of system in a configuration file. In the further, these data can be redefined in a DB. Besides for cases of impossibility of start of any DB generally, it is possible to store all data in a configuration file.


For access to databases the mechanism of registration of a DB is used. Registered DB in system are accessible to all subsystems of OpenSCADA system and can be used in their work. Owing to this mechanism it is possible to provide an allocation of data storage. For example, various libraries can be stored and extend independently, and connection of library will consist in simple registration of the necessary DB.


In the further, realization of duplication of a DB by linkage of the registered DB is planned. This mechanism will allow to increase considerably reliability of OpenSCADA system as a whole by reservation of the mechanism of a data storage. (It is planned.)

1.5. Archives. A subsystem "Archives".

Any SCADA system gives an opportunity of archiving the acquisition data, i.e. formation of history of change (dynamics) of processes. Archives, conditionally, it is possible to divide into two types: archives of messages and archives of values.


Feature of archives of messages is that the subject of archiving are, so-called, events. A characteristic attribute of event is time of occurrence of this event. Archives of messages, usually, are used for archiving messages in system, i.e. conducting logs and reports. Depending on a source, messages can be classified by various criteria. For example, it can be reports of emergencies, reports of actions of operators, reports of failures of connection, etc.


Feature of archives of values is their periodicity defined by the time interval between two adjacent values. Archives of values are applied for archiving of history of continuous processes. As far as process is continuous and it's archiving is possible only by introduction of conception of quantization of interrogation of values as differently we receive archives of the infinite sizes, in view of a continuity of the nature of process. Besides, practically, we can receive values with the period limited by sources of data. For example, qualitative enough sources of data, in the industry, data with frequency more 1kHz seldom allow to obtain. And it without taking into account sensors having even less qualitative characteristics.


For the decision of tasks of archiving data flows in OpenSCADA system the subsystem "Archives" is provided. The subsystem "Archives" allows to conduct both: archives of messages and archives of values. The subsystem "Archives" is modular. The modular object containing in a subsystem "Archives" the type of the archiver acts. The type of the archiver defines the way of a data storage, i.e. storehouse (file system, DBMS, a network, etc.). Each module of a subsystem "Archives" can realize both: archiving of messages, and archiving of values. The subsystem "Archives" can contain set of the archives served by various modules of a subsystem.


The message in OpenSCADA system is characterized: by date, by level of importance, by category and the text of the message. Date of the message specifies for the period of creation of the message. The level of importance specifies a degree of importance of the message. The category determines the address or the conditional identifier of a source of the message. Usually, the category contains a full way to a source of the message in system. The text of the message, actually, also carries meaning content of the message.


During archiving messages are passed through the filter. The filter works on a level of importance and a category of the message. The level of the message in the filter specifies that it is necessary to pass messages with specified or higher level of importance. To filtering on a category templates or regular expressions are used, which define what messages are applied to pass. Each archiver contains own options of the filter. Consequently it is possible to create easily various specialized archivers for archive of messages. For example archivers of messages it is possible to dedicate on:


In view of the similar nature of the messages and the alarms, the subsystem "Archives" contains a buffer of current alarms, which contains active at the time the alarms with using the message category as a key identifier of the alarm. Access to the list-buffer of current alarms specifying by a negative value level messages. Thus, the formation of negative message with level -2 cause place in this message to buffer active alarms with level 2, as well as duplication of directly to messages archive. At the subsequent formation of the message in the same category, but a positive level, say 1, will be carried deletion of the specified alarm from the buffer of alarms and also the message fall into messages archive. This mechanism allows you to simultaneously keep track of active alarms and log their passage into the messages archive. When requesting to archive messages, an set of a positive level makes a request to archive messages, and a negative to buffer-list of current alarms.


The archive of values in system OpenSCADA acts as an independent component which includes the buffer processable by archivers. Key parameter of archive of value is the source of data. In a role of a source of data attributes of parameters of OpenSCADA system and also other external sources of data (a passive mode) can act. Other sources of data can be: network archivers from remote OpenSCADA systems, the environment of programming of OpenSCADA system, etc.


Key component of archiving of values of continuous processes is the buffer of values. The buffer of values is intended for intermediate storage of a file of the values received with certain periodicity (quantum of time). The buffer of values is used as for direct storage of big arrays of values in archives of values, before direct "retire" on physical carriers, and for manipulations with the staff of values, i.e. in functions of frame-accurate query of values and their placement in buffers of archives.


For the organization of the dedicated archivers, in the allocated systems it is possible to use transport type of the archiver (It is planned.). Function of transport type of the archiver is reflection of the remote central archiver on local system. As consequence, archivers of transport type carry out data transmission between local system and the archiver of the remote system, hiding from subsystems of local system the real nature of the archiver.

1.6. Communications. Subsystems "Transports" and "Transport protocols".

As far as the OpenSCADA system is pawned as is high-scaled system that support of communications should be flexible enough. For satisfaction of a high degree of flexibility, communications in OpenSCADA system are realized in subsystems "Transports" and "Transport protocols" which are modular.


The subsystem "Transports" is intended for an exchange of the not structured data between OpenSCADA system and external systems. In a role of external systems can act even remote OpenSCADA systems. Not structured data are understood as a file of symbols of the certain length. The modular object containing in a subsystem "Transports", the type of transport acts. The type of transport defines the mechanism of transfer of not structured data. For example it can be:


The subsystem "Transports" includes support of input and output transports. Input transport is intended for service of external queries and sending of answers. Output transport, on the contrary, is intended for sending messages and expectation of the answer. Consequently, input transport contains a configuration of the given station as server, and output transport contains a configuration of the remote server. The module of a subsystem "Transports" realizes support both: input and output transports.


The subsystem "Transport protocols" is intended for structuring of data received from a subsystem "Transports". As a matter of fact, the subsystem "Transport protocols" is continuation of a subsystem "Transports" and carries out functions of check of structure and integrity of the received data. So, for the indication of the protocol together with which transport should work, the special configuration field is provided. The modular object containing in a subsystem "Protocols" is the protocol. For example, transport protocols can be:


The full chain of connection can be written down as follows:


Protocols for output transports are supported also. The output protocol incurs function of dialogue with transport and realization of features of the protocol. The internal side of access to the protocol is realized by data-flow way with own structure for each protocol module. Such mechanism allows to carry out transparent access to external system, by means of transport, simply specifying a name of the protocol by means of which to serve transfer.


Owing to standard API-access to transports of OpenSCADA system it is possible to change easily a way of data exchange not touching exchanging systems. For example, in the case of a local exchange it is possible to use faster transport on the basis of shared memory, and in the case of an exchange through the Internet and a local network to use TCP or UDP sockets.

1.7. Interfaces of the user. A subsystem "Interfaces of the user".

SCADA-systems as a class, assume presence of user interfaces. In OpenSCADA, for granting the user interfaces, the subsystem "The user interfaces" is provided. The user interface of OpenSCADA system is understood not only as the environment of visualization from which the end user should work, but also as everything, that concerns the user, for example:


The subsystem "The user interfaces" is modular. As modular object of a subsystem the concrete interface of the user actually acts. Modularity of subsystem allows to create various interfaces of users on various GUI/TUI libraries and to use optimal of decisions in particularly taken case, for example, for environments of performance of programmed logic controllers it is possible to use configurators and visualizers on the basis of Web-technologies (WebCfg, WebUI), and in case of stationary workstations to use the same configurators and visualizers, but on the basis of libraries Qt, GTK.

1.8. Security of system. A subsystem "Security".

The OpenSCADA system is the branched out system which consists of ten subsystems and can include set of modules. Consequently, granting of unlimited access by all to these resources is at least unsafe. Therefore, for differentiation of access in OpenSCADA system, the subsystem of "Security" is provided. The basic functions of a subsystem "Security" are:

1.9. Management of libraries of modules and modules. A subsystem "Management of modules".

The OpenSCADA system is constructed by a modular principle that means presence of set of modules with which it is necessary to operate. For performance of function of management by modules of OpenSCADA system the subsystem "Management of modules" is provided. All modules, for the present moment are delivered in system by means of shared libraries (containers). Each container can contain set of modules of various type.


The subsystem "Management of modules" realizes the control over the status of containers and allows to carry out hot addition, removal and updating of containers and modules containing in them.

1.10. Unforeseen opportunities. A subsystem "Special".

Certainly, to provide all probable functions it is impossible, therefore in OpenSCADA system the subsystem "Special" is provided. The subsystem "Special" is modular and is intended for addition in OpenSCADA system unforeseen functions by modular expansion. For example, by means of a subsystem "Special" can be realized:

1.11. The user functions. Objective model and the environment of programming of system.

Any modern SCADA system should contain the mechanisms giving an opportunity to program at the user level, i.e. to contain the environment of programming. The OpenSCADA system contains such environment. By means of the environment of programming of OpenSCADA system it is possible to realize:


The environment of programming of OpenSCADA system represents a complex of assets organizing the computing environment of the user. Into structure of a complex of assets are included:


Modules of libraries of functions give set of functions of the certain orientation expanding objective model of system. Libraries can be realized both: by the set of functions of the fixed type, and functions supposing free updating and addition.


Libraries of functions of the fixed type can be given by standard modules of system, organically supplementing objective model. Functions of such libraries will represent the interface of access to assets of the module at a level of the user. For example, "The environment of visual data presentation" can give functions for delivery of various messages. Using these functions the user can realize interactive algorithms of communication with system.


Libraries of functions of free type give the environment of a writing of the user functions on one of programming languages. Within the limits of the module of libraries of functions mechanisms of creation of libraries of functions can be given. So, it is possible to create libraries of devices of technological processes, and in a consequence to use them by linkage. Various modules of libraries of functions can give realizations of various programming languages.


On the basis of the functions given by objective model, computing controllers are under construction. Computing controllers carry out linkage of functions with parameters of system and the mechanism of calculation.


 
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2013-05-13 20:43:09 | Owner: Maxim Lysenko | Print version | Search: