This content has been developed as a training module to give the user an overview of the Honeywell TDC Distributed Control System (DCS). The information and specifications in this document are subject to change Honeywell, TotalPlant, and TDC are U.S. registered. The cables can also be switched manually from a universal station. Features of LCN: LCN uses Honeywell proprietary protocol. Structure similar to IEEE
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A distributed control system DCS is a computerised control system for a process or plant usually with a large number of control loopsin which autonomous controllers are distributed throughout the system, tcd there is central operator supervisory control.
This honeywwll in contrast to systems that use centralized controllers; either discrete controllers located at a central control room or within a central computer. The DCS concept increases reliability and reduces installation costs by localising control functions near the process plant, with remote monitoring and supervision. Distributed control systems first emerged in large, high value, safety critical process industries, and were attractive because the DCS manufacturer would supply both the local control level and central supervisory equipment as an integrated package, thus reducing design integration risk.
Today the functionality of SCADA and DCS systems are very similar, but DCS tends to be hhoneywell on large continuous process plants where high reliability and security is important, and the control room is not geographically remote.
The key attribute of a DCS is its reliability due to the distribution of the control processing around nodes in the system. This mitigates a single processor failure. Tdf a processor fails, it will only affect one section of the plant process, as opposed to hneywell failure of a ddcs computer which tdcc affect the whole process. The accompanying diagram is a general model which shows functional manufacturing levels using computerised control.
Levels 1 and 2 are the functional levels of a traditional DCS, in which all equipment are part of an integrated system from a single manufacturer. Levels 3 and 4 are not strictly process control in the traditional sense, but where production control and scheduling takes place. The processor nodes and operator graphical displays are connected over proprietary or industry standard networks, and network reliability is increased by dual redundancy cabling over diverse routes.
The processors receive information from input modules, process the information and decide control actions to be signalled by the output modules. The field inputs and outputs can be analog signals e.
Honeywell TDC LM manual
DCSs are connected to sensors and actuators and use setpoint control to control the flow of material through the plant. A typical application is a PID controller fed by a flow meter and using a control valve as the final control element.
The DCS sends the setpoint required by the process to the controller which instructs a valve to operate so that the process reaches and stays at the desired setpoint. Processes are not limited to fluidic flow through pipes, however, and can also include things like paper machines and their associated quality controls, variable speed drives and motor control centerscement kilnsmining operationsore processing facilities, and many others.
DCSs in very high reliability applications can have dual redundant processors with “hot” switch over on fault, to enhance the reliability of the control system. Modern DCSs also support neural networks and fuzzy logic applications. Recent research focuses on the synthesis of optimal distributed controllers, which optimizes a certain H-infinity or the H 2 control criterion.
Distributed control systems DCS are dedicated systems used in manufacturing processes that are continuous or batch-oriented.
Process control of large industrial plants has evolved through many stages. Initially, control would be from panels local to the process plant. However this required a large manpower resource to attend to these dispersed panels, and there was no overall view of the process.
The next logical development was the transmission of all plant measurements to a permanently-manned central control room. Effectively this was the centralisation of all the localised panels, with the advantages of lower manning levels and easier overview of the process. Often the controllers were behind the control room panels, and all automatic and manual control outputs were transmitted back to plant.
However, whilst providing a central control focus, this arrangement was inflexible as each control loop had its own controller hardware, and continual operator movement within the control room was required to view different parts of the process. These could be distributed around plant, and communicate with the graphic display in the control room or rooms.
The distributed control system was born. The introduction of DCSs allowed easy interconnection and re-configuration of plant controls such as cascaded loops and interlocks, and easy interfacing with other production computer systems. It enabled sophisticated alarm handling, introduced automatic event logging, removed the need for physical records such as chart recorders, allowed the control racks to be networked and thereby located locally to plant to reduce cabling runs, and provided high level overviews of plant status and production levels.
Early minicomputers were used in the control of industrial processes since the beginning of the s. The DCS largely came about due to the increased availability of microcomputers and the proliferation of microprocessors in the world of process control.
Computers had already been applied to process automation for some time in the form of both direct digital control DDC and setpoint control. Sophisticated for the time continuous as well as batch control was implemented in this way. A more conservative approach was setpoint control, where process computers supervised clusters of analog process controllers.
Distributed control system
A workstation provided visibility into the process using text and crude character graphics. Availability of a fully functional graphical user interface was a way away.
Central to the DCS model was the inclusion of control function blocks. One of the first embodiments of object-oriented software, function blocks were self-contained “blocks” of code that emulated analog hardware control components and performed tasks that were essential to process control, such as execution of PID algorithms. Function blocks continue to endure as the predominant method of control for DCS suppliers, and are supported by key technologies such as Foundation Fieldbus  today.
Midac Systems, of Sydney, Australia, developed an objected-oriented distributed direct digital control system in The central system ran 11 microprocessors sharing tasks and common memory and connected to a serial communication network of distributed controllers each running two Z80s.
The system was installed at the University of Melbourne. Digital communication between distributed controllers, workstations and other computing elements peer to peer access was one of the primary advantages of the DCS. Attention was duly focused on the networks, which provided the all-important lines of communication that, for process applications, had to incorporate specific functions such as determinism and redundancy.
As a result, many suppliers embraced the IEEE This decision set the stage for the wave of migrations necessary when information technology moved into process automation and IEEE In the s, users began to look at DCSs as more than just basic process control. The system installed at the University of Melbourne used a serial communications network, connecting campus buildings back to a control room “front end”.
Each remote unit ran two Z80 microprocessors, while the front end ran eleven Z80s in a parallel processing configuration with paged common memory to share tasks and that could run up to 20, concurrent control objects. It was believed that if openness could be achieved and greater amounts of data could be shared throughout the enterprise that even greater things could be achieved.
The first attempts to increase the openness of DCSs resulted in the adoption of the predominant operating system of the day: As a result, suppliers also began to adopt Ethernet-based networks with their own proprietary protocol layers. Plant-wide historians also emerged to capitalize on the extended reach of automation systems.
The drive toward openness in the s gained momentum through the s with the increased adoption of commercial off-the-shelf COTS components and IT standards. Probably the biggest transition undertaken during this time was the move from the UNIX operating system to the Windows environment.
While the realm of the real time operating system RTOS for control applications remains dominated by real time commercial variants of UNIX or proprietary operating systems, everything above real-time control has made the transition to Windows. The introduction of Microsoft at the desktop and server layers resulted in the development of technologies such as OLE for process control OPCwhich is now a de facto industry connectivity standard.
The s were also known for the “Fieldbus Wars”, where rival organizations competed to define what would become the IEC fieldbus standard for digital communication with field instrumentation instead of 4—20 milliamp analog communications. The first fieldbus installations occurred in the s. Fieldbus technics have been used to integrate machine, drives, quality and condition monitoring applications to one DCS with Valmet DNA system.
The impact of COTS, however, was most pronounced at the hardware layer. The initial proliferation of DCSs required the installation of prodigious amounts of this hardware, most of it manufactured from the bottom up by DCS suppliers.
Standard computer components from manufacturers such as Intel and Motorola, however, made it cost prohibitive for DCS suppliers to continue making their own components, workstations, and networking hardware. As the suppliers made the transition to COTS components, they also discovered that the hardware market was shrinking fast. COTS not only resulted in lower manufacturing costs for the supplier, but also steadily decreasing prices for the end users, who were also becoming increasingly vocal over what they perceived to be unduly high hardware costs.
The gaps among the various systems remain at the areas such as: While it is expected the cost ratio is relatively the same the more powerful the systems are, the more expensive they will bethe reality of the automation business is often operating strategically case by case. The current next evolution step is called Collaborative Process Automation Systems. To compound the issue, suppliers were also realizing that the hardware market was becoming saturated. Many of the older systems that were installed in the s and s are still in use today, and there is a considerable installed base of systems in the market that are approaching the end of their useful life.
Developed fcs economies in North America, Europe, and Japan already had many thousands of DCSs installed, and with few if any new plants being built, the market for new hardware was shifting rapidly to smaller, albeit faster growing regions such as China, Latin America, and Eastern Europe. Because of the shrinking hardware business, suppliers began to make the challenging transition from a hardware-based business model to one based on software and value-added services.
It is a transition that is still being made today. The applications portfolio offered by suppliers expanded considerably in the ’90s to include areas such as production management, model-based control, real-time optimization, plant asset management PAMReal-time performance management RPM tools, manuaal managementand many others. To obtain the true value from these applications, however, often requires a considerable service content, which the suppliers also provide. Increasingly, and ironically, DCS are becoming centralised at plant level, with the tcc to log into the remote equipment.
This enables operator to control both at enterprise level macro and at the equipment level micro both within and outside the plant as physical location due to interconnectivity primarily due to wireless and remote access has shrunk. As wireless protocols are developed and refined, DCS increasingly includes wireless communication. DCS controllers are now often equipped with embedded servers and provide on-the-go web access.
With these interfaces, the threat of security breaches and possible damage to plant and process are now very real. From Wikipedia, the free encyclopedia. Archived from the original on Retrieved from ” https: Control engineering Applications of distributed computing Industrial automation. Archived copy as title Use British English from February All articles with unsourced statements Articles with unsourced statements from August Views Read Edit View history.
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