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Redundancy
Using Redundancy in Plant Control and Monitoring Systems
The sophisticated features provided by the current generation of plant control and monitoring systems have ensured their acceptance in plants and factories throughout the world. All successful systems provide high-level control and monitoring facilities, such as graphic displays, alarm handling, trending, reports, and data exchange. In a welldesigned system, these features improve a plants reliability and efficiency and therefore its protability. However, when implementing a plant control and monitoring system, designers often overlook one important aspect what happens if the system hardware fails?
Info
Figure 1 A Single Node System are eliminated. Citect for Windows incorporates redundancy through a simple yet effective method, unavailable in any other control and monitoring system.
Citect for Windows incorporates redundancy through a simple yet effective method, unavailable in any other control and monitoring system.
Client-Server Processing
In 1992, Citect for Windows pioneered the use of client-server architecture in plant monitoring and control. By distributing the processes in the control
The control systems for both the single node application shown in Figure 1 and the network application shown in Figure 2 have a single point of failure. These systems fail totally if one piece of hardware (the computer connected to the control and monitoring units) fails. Most modern computers are designed for reliability, yet breakdowns can still occur, especially where they are used in harsh environments. If some (or all) of the processes in a plant are critical, or if the cost of non-production due to system failure is high, redundancy must be incorporated in to the system. In a system with inbuilt redundancy, interruptions due to equipment failure Figure 2 A Network System
LAN
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Dual Server Redundancy
Citect Clients Citect Clients
To provide redundancy, a second (standby) server can be added, also dedicated to communication with the plant control devices (see Figure 4). If
LAN
the primary server fails, the clients requests for data are channelled to the standby server.
Citect Server
PLC Network
Citect for Windows offers a capacity to work in an integrated manufacturing environment unequalled in todays market.
Figure 3 A Client-Server System and monitoring application across two or more computers (using a LAN) you can increase the speed and efficiency of the system. In a simple application, the computer connected to the control and monitoring units becomes the server, dedicated to communication with the plant control devices, while the display nodes are clients (see Figure 3). When a client computer requires data for display, it requests the data from the server, and processes that data locally.
The standby server does not duplicate the primary servers functions, or both servers would have to communicate with the PLCs, doubling the load on the PLC network, and reducing performance. In the Citect client-server system, only the primary server communicates with the PLCs. The primary server also communicates with the standby server, continually updating it with the plants status. If communication with the primary server is interrupted, the standby server assumes that the primary server has failed, and takes over as the primary server. When the primary server is repaired and returned to service, it reads the plants status from the standby server and resumes its role as the primary server. The standby server automatically reverts to its former role.
LAN
Citect Clients
Citect Clients
File Server
Split-Task Redundancy
Primary Server Standby Server
With Citects client-server architecture and dual Citect servers, you can do more than simply maintain continuous communication with the plant-oor
PLC Network
devices. You can ensure that all alarm and trend data is also maintained in the event of failure. This is achieved by splitting the servers task into four
Figure 4 Dual Server Redundancy 2
sub-tasks:
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I/O (Input/Output) Alarms Trends Reports
Each of these tasks manages its own database independently of the other tasks, so you can handle redundancy differently for each task. For example, to maintain integrity of trends, you would parallel the trend task in both servers (unlike the I/O task that uses primary/standby processing). When the primary is returned to service after a failure, it can update its lost trend data from the standby server. Both servers then have continuous, uninterrupted trend data.
Citect Clients
File Servers
LAN 2 LAN 1
Primary Server
Standby Server
PLC Network
Figure 5 Dual LANs
Dedicated File Server
If you also add a dedicated le server, you can centralise the databases and display screens. Continuity is then maintained if the primary server fails. (Centralised databases are also easier to manage and maintain.)
LAN 1 LAN 2 Citect Clients File Servers
Dual LANs
The arrangement in Figure 4 secures the system by removing the single point of failure (the I/O server). However, if the LAN fails, control and monitoring by the display nodes is lost. A second LAN and le server would ensure system stability even in the event of network failure (see Figure 5).
Primary Server
Standby Server
PLC Network 2 PLC Network 1
Figure 6 Units Level Redundancy the plants uninterrupted operation by more than simply duplicating hardware
Parallel Units Redundancy
For maximum system stability, you should connect parallel units (PLCs) to the same eld devices as in Figure 6. Any hardware component in this system can fail without disrupting the control and monitoring of the plant. This level of redundancy is aimed directly at the centralised part of the processing. Citect for Windows ensures
all processing power is utilised. In the event of a failure the spare hardware actually works, using plant-oor data that is always up-to-date. Citect for Windows offers a capacity to work in an integrated manufacturing environment unequalled in todays market.
Citect for Windows ensures the plants uninterrupted operation by more than simply duplicating hardware all processing power is utilised.
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The most advanced plant monitoring and control system on the market.
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