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1:42 م | Posted by
mostafa sayed |
SCADA
Supervisory Control And Data Acquisition))
. SCADA MEAN1
SCADA stands for Supervisory Control And Data Acquisition. As the name indicates, it is not a full control system, but rather focuses on the supervisory level. As such, it is a purely software package that is positioned on top of hardware to which it is interfaced, in general via Programmable Logic Controllers (PLCs), or other commercial hardware modules.
SCADA systems are used not only in industrial processes: e.g. steel making, power generation (conventional and nuclear) and distribution, chemistry, but also in some experimental facilities such as nuclear fusion. The size of such plants range from a few 1000 to several 10 thousands input/output (I/O) channels. However, SCADA systems evolve rapidly and are now penetrating the market of plants with a number of I/O channels of several 100 K: we know of two cases of near to 1 M I/O channels currently under development.
SCADA systems used to run on DOS, VMS and UNIX; in recent years all SCADA vendors have moved to NT and some also to Linux.
2. Architecture
This section describes the common features of the SCADA products that have been evaluated at CERN in view of their possible application to the control systems of the LHC detectors [1], [2]
2.1 Hardware Architecture
One distinguishes two basic layers in a SCADA system: the "client layer" which caters for the man machine interaction and the "data server layer" which handles most of the process data control activities. The data servers communicate with devices in the field through process controllers. Process controllers, e.g. PLCs, are connected to the data servers either directly or via networks or fieldbuses that are proprietary (e.g. Siemens H1), or non-proprietary (e.g. Profibus). Data servers are connected to each other and to client stations via an Ethernet LAN. The data servers and client stations are NT platforms but for many products the client stations may also be W95 machines.
2.1.1 ELEMENTS OF SCADA
- . The remote terminal unit (RTU).
- . The master terminal unit (MTU).
- . The communications equipment
2.1.1.1 The remote terminal unit (RTU).
· (RTU) gather information from their remote site from various input devices.
- · hold the information gathered in their memory and wait for a request from the MTU to transmit the data.
- · Receive data and control signal from (MTU) and transmit control signal to the site devices.
- · The old (RTU) was “all-in-one” PCB was fixed I/O.
· (RTU) now are based on PLC which able to program, extend I/O, or even stand alone when the communication lost with (MTU).
2.1.1.2 The master terminal unit (MTU).
- · The heart of the system is the master terminal unit (MTU).
- · (MTU) make the communication, gathers data, stores information, sends information to other systems, and interfaces with operators.
- · The primary interface to the operator is the monitor and printers.
2.3. Communications
Internal Communication
Server-client and server-server communication is in general on a publish-subscribe and event-driven basis and uses a TCP/IP protocol, i.e., a client application subscribes to a parameter which is owned by a particular server application and only changes to that parameter are then communicated to the client application.
Access to Devices
The data servers poll the controllers at a user defined polling rate. The polling rate may be different for different parameters. The controllers pass the requested parameters to the data servers. Time stamping of the process parameters is typically performed in the controllers and this time-stamp is taken over by the data server. If the controller and communication protocol used support unsolicited data transfer then the products will support this too.
A single data server can support multiple communications protocols: it can generally support as many such protocols as it has slots for interface cards.
COMMUNICATION OPTION
- . Wired systems.
- .Wireless systems.
- . Public transmission systems.
2.4 Interfacing
1 Indirect Communication
OPC - OLE for Process Control
1.1 Functionality
· Introduction
OPC (OLE for Process Control) refers to a standard, manufacturer-independent
Software interface. The OPC interface is based on Microsoft Windows COM
(Component Object Model) and DCOM (Distributed Component Object Model)
technology. OPC XML, on the other hand, is based on the Internet standards XML,
SOAP and HTTP
· COM
COM is the standard protocol for communication between objects located on the
Same computer but which are part of different programs. The server is the object
Providing services, such as making data available. The client is an application
This uses the services provided by the server.
· DCOM
DCOM represents an expansion of COM functionality to allow access to objects on
Remote computers.
This foundation allows standardized data exchange between applications from
Industry, administrative offices and manufacturing.
Up to that point, applications which accessed process data were tied to the access
Protocols of the communication network. The standard software interface OPC
Allows devices and applications from various manufacturers to be combined with
one another in a uniform manner.
The OPC client is an application which accesses process data, messages and
archives of an OPC server. Access takes place using the OPC software interface.
An OPC server is a program which provides the applications from various
Manufacturers with a standard software interface. The OPC server is the
Intermediate layer between the applications for handling process data, the various
Network protocols and the interfaces for accessing these data.
Only devices with operating systems based on Windows COM and DCOM
Technology can use the OPC software interface for data exchange. At the present
Time, Windows NT, Windows 98, Windows 2000 and Windows XP use this
Software interface.
· XML
Communication via DCOM is restricted to local networks. Data exchange via XML
operates with SOAP (Simple Object Access Protocol). SOAP is a platform
Independent XML-based protocol. SOAP can be used to enable applications to
communicate with each other Internet or heterogenic computer networks via HTTP
(Hypertext Transfer Protocol).
1.1 Direct Communication - Basics
· Introduction
Communication is the exchange of data between two communication partners.
· Communication
A communication partner can be any element of a network that is able to
Communicate and exchange data with others. In the context of the WinCC
environment, these communication partners could be central modules or
Communication modules in automation systems (AS) or communication processors
in PCs.
The data transferred between the communication partners can serve entirely
different purposes. In the case of WinCC, these include:
• Controlling a process
• Requesting data from a process
• Reporting unexpected states in a process
• Archiving process data
The PC products provide support for the Microsoft standards such as Dynamic Data Exchange (DDE) which allows e.g. to visualize data dynamically in an EXCEL spreadsheet, Dynamic Link Library (DLL) and Object Linking and Embedding (OLE).
Database
The configuration data are stored in a database that is logically centralized but physically distributed and that is generally of a proprietary format.
For performance reasons, the RTDB resides in the memory of the servers and is also of proprietary format.
The archive and logging format is usually also proprietary for performance reasons, but some products do support logging to a Relational Data Base Management System (RDBMS) at a slower rate either directly or via an ODBC interface.
2.5 Scalability
2.6 Redundancy
The products often have built in software redundancy at a server level, which is normally transparent to the user. Many of the products also provide more complete redundancy solutions if required.
3. Functionality
3.1 Access Control
Users are allocated to groups, which have defined read/write access privileges to the process parameters in the system and often also to specific product functionality.
3.2 MMI
The products support multiple screens, which can contain combinations of synoptic diagrams and text.
They also support the concept of a "generic" graphical object with links to process variables. These objects can be "dragged and dropped" from a library and included into a synoptic diagram.
Most of the SCADA products that were evaluated decompose the process in "atomic" parameters (e.g. a power supply current, its maximum value, its on/off status, etc.) to which a Tag-name is associated. The Tag-names used to link graphical objects to devices can be edited as required. The products include a library of standard graphical symbols, many of which would however not be applicable to the type of applications encountered in the experimental physics community.
Standard windows editing facilities are provided: zooming, re-sizing, scrolling... On-line configuration and customization of the MMI is possible for users with the appropriate privileges. Links can be created between display pages to navigate from one view to another.
3.3 Trending
The products all provide trending facilities and one can summaries the common capabilities as follows:
- the parameters to be trended in a specific chart can be predefined or defined on-line
- a chart may contain more than 8 trended parameters or pens and an unlimited number of charts can be displayed (restricted only by the readability)
- real-time and historical trending are possible, although generally not in the same chart
- historical trending is possible for any archived parameter
- zooming and scrolling functions are provided
- parameter values at the cursor position can be displayed
The trending feature is either provided as a separate module or as a graphical object (ActiveX), which can then be embedded into a synoptic display. XY and other statistical analysis plots are generally not provided.
3.4 Alarm Handling
Alarm handling is based on limit and status checking and performed in the data servers. More complicated expressions (using arithmetic or logical expressions) can be developed by creating derived parameters on which status or limit checking is then performed. The alarms are logically handled centrally, i.e., the information only exists in one place and all users see the same status (e.g., the acknowledgement), and multiple alarm priority levels (in general many more than 3 such levels) are supported.
It is generally possible to group alarms and to handle these as an entity (typically filtering on group or acknowledgement of all alarms in a group). Furthermore, it is possible to suppress alarms either individually or as a complete group. The filtering of alarms seen on the alarm page or when viewing the alarm log is also possible at least on priority, time and group. However, relationships between alarms cannot generally be defined in a straightforward manner. E-mails can be generated or predefined actions automatically executed in response to alarm conditions.
3.5 Logging/Archiving
The terms logging and archiving are often used to describe the same facility. However, logging can be thought of as medium-term storage of data on disk, whereas archiving is long-term storage of data either on disk or on another permanent storage medium. Logging is typically performed on a cyclic basis, i.e., once a certain file size, time period or number of points is reached the data is overwritten. Logging of data can be performed at a set frequency, or only initiated if the value changes or when a specific predefined event occurs. Logged data can be transferred to an archive once the log is full. The logged data is time-stamped and can be filtered when viewed by a user. The logging of user actions is in general performed together with either a user ID or station ID. There is often also a VCR facility to play back archived data.
• Automation system (AS): saves the process values.
• Data manager (DM): processes the process values and returns them to the
archive system via process tags.
• Archive system: processes the acquired process values (e.g. forming the
average value). The method of processing depends on the way the archive is
configured.
• Runtime database (DB): saves the process values that are to be archived.
3.6 Report Generation
One can produce reports using SQL type queries to the archive, RTDB or logs. Although it is sometimes possible to embed EXCEL charts in the report, a "cut and paste" capability is in general not provided. Facilities exist to be able to automatically generate, print and archive reports.
3.7 Automation
The majority of the products allow actions to be automatically triggered by events. A scripting language provided by the SCADA products allows these actions to be defined. In general, one can load a particular display, send an Email, run a user defined application or script and write to the RTDB.
The concept of recipes is supported, whereby a particular system configuration can be saved to a file and then re-loaded at a later date.
Sequencing is also supported whereby, as the name indicates, it is possible to execute a more complex sequence of actions on one or more devices. Sequences may also react to external events.
Some of the products do support an expert system but none has the concept of a Finite State Machine (FSM).
SCADA SYSTEM BENEFITS
- . Reduced number of man-hours required to troubleshoot a pump or other electrical device that did not operate as designed.
- . Automated report generation is a major labor saving feature and facilitates.
- . Reduced operating costs of a PLC-based SCADA translate to a superior return on investment (ROI) when compared to the typical proprietary system.
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