Distributed control system architecture:

Distributed control system is basically a series of microprocessor based multi-loop controllers distributed along the communication network often called data highway.

Or

A collection of small computers connected by a network to form a system specially designed for process control.

Or

At DCS is a form of instrumentation that locates the controlling portion of the control system in the processing area where it measures process values (and discrete inputs) and produce output signal to position the actuators as a function of deviation from the set point, while by means if electrical transmission process information is communicated to a central location where the operator can sit and manipulate on the loop of control in the system.

DCS offers powerful automation capabilities from basic control functions to production and quality management.

The system performs following functions:

1. Control of process parameters.2. Data acquisition and monitoring.3. Alarming4. Logging5. Report generation 6. Recipe control7. Logic operations8. Sequence control9. Corrects history data and display it in the trend display 10. Process management11. Provides interface to other system

The functions of the system are distributed to different stations which are connected to one another through a bus interface. The stations are able to operate independently of the rest of the system.

The DCS should have the following characteristics.

1. The system should be functional and topographically distributed design and should have a hierarchical structure.

2. It should distribute the functions among devices and stations in accordance with the hierarchy and modularity of the automation task and to suit the individual requirements involved.

The devices in this architecture are grouped three categories.

1. Those that interface directly to the process to be controlled or monitored.2. Those that perform high level human interfacing and computing functions.3. Those that provide the means of communication between the other devices.

A brief definition of each device is given below:

1. Local control unit (LCU):The smallest collection of hardware in the system that can do closed loop control. The LCU interfaces directly to the process.

2. Low level human interface (LLHI):A device that allows the operator or instrument engineer to interact with the local control unit (e.g.:- to change the set points, control modes, control configurations or tuning parameters) using a direct connection. LLHI’s can also interface directly to the process operator oriented hardware at this level is called a low level operator interface; instrument engineer oriented hardware is called a low level engineering interface.

3. Data input/ output unit (DI/OU):A device that interfaces to the process only for the purpose of acquiring or outputting data is called Data input/ output unit. In performs no control functions.

4. High level human interface (HLHI):A collection of hardware that performs functions similar to the LLHI but with increased capability and user friendliness is called high level human interface. It interfaces to other devices only for the Shared communication facilities. Operator oriented hardware at this level is called the high level operator interface; instrument engineer oriented hardware is called the high level engineering interface.

5. High level computing device (HLCD):The collection of microprocessor based hardware that performs plant management functions traditionally performed by a plant computer. It interfaces to other devices only over the Shared communication facilities.

6. Computer interface device (CID):

A collection of hardware that allows an external general purpose computer to interact with other devices in the DCS using Shared communication facilities.

7. Shared communication facilities:One of more levels of communication hardware and associated software that allows the sharing of data among all devices in the distributed system is Shared communication facility. Shared communication facilities do not include dedicated communication channels between specific devices or between hardware elements within a device.

Several LCU’S are used to implement the functions required in controlling the process; therefore, the control is functionally distributed. However, the LCU’s are all located in a central equipment room area and so it is not a geographically distributed control system. Both high level and low level human interface devices are located in the control room area for operational purposes. Most of the operator control functions are performed using high level interface; their low level interface is included in the configuration primarily to serve as a backup in case the high level interface fails.

The HLHI is located in the instrument engineer area so that control system monitoring and analysis can be done without disturbing plant operations. This type of installation is typical of early DCS configurations in which equipment location and operator interface design follows convention practices.The fully distributed control system configuration is shown below. In this case, each in LCU is located in the plant area closest to the portion of the process that it controls. Associated LLHI equipment is also located in this area.

The control room and instrument engineering areas contain HLHI units, which are used to perform all of the primary operational and engineering functions. The lower level units are used only as manual backup controls in case the high level equipment fails or needs maintenance. This configuration takes advantage of two areas of the equipment savings that result from a totally distributed system architecture. 1. Reduction in control room size by eliminating panel board equipment.2. Reduction in field wiring costs by placing LCU’s near the process.

Control Bailey micro Z system:

The controller has two microprocessors namely the control microprocessor and the communication microprocessor.

The algorithm library resides in a ROM. The specific settings of the controller to match a problem are featured in a removable personalization card. The personalization card consists of the process control scheme and the control of parameters. The communication processor manages two serially connected asynchronous 2400 baud lines, allowing communication with the operating center through the concentrator or the selector. Both the lines are independent and galvanically isolated.

To achieve a video operating center, a high speed serially connected bus is used to transmit data between the various items. Distance may be as much as 2 km between a cabinet room and the control room.

The data is transmitted along the following two types of supports.

1. Shielded twisted pairs in the areas where due to the presence of several subscribers by passers are necessary on the bus.

2. Optic fiber for long distances without bypass between two groups of subscribers.

On the general purpose control station, the various functions of separate set point station and control station are connected. The station allows sequential control of various process loops. A loop is called the selection keyboard which allocates the selector which is indeed a solid state multiplexer.

Transmission between micro Z cards and the general purpose control station is asynchronous up to 2400 baud. The selector control station assembly is completely independent of the connected bus system. Since each micro Z card is provided with two 2400 baud serially connected communication lines, two communication systems can coexist in the same unit and on the same micro Z card.

The operation can be from one of the three types

1. Centralized video station2. General purpose control (GPC) centre3. Conventional control stations

The general purpose stations can be connected to any control loop and adjustments made to the GPC can act as a plant level control. Either GPC or conventional control station can act as redundancy for the video control center.

Examples of such new Automation Systems are numerous. In the meantime, that is over 50 modern distributed computer control systems in the market today, such as

CP 80 (AEG- telefunken) DCI 4000 (Fisher and Porter) MAX 1 (Leads and Northup) MICON (VDO)

PROCONTROL (Brown Boveri) MOD III (Taylor and ABB) MV 8000 (Beckmann) PCS 8000 (Phillips) PLS 80 (J.C. Echardt) P 4000 (Kent) PMS (Ferranti) PROVOX (Fisher Controls) TDC 3000 (Honey Well) TELEPERM-M (Siemens) TOSDIC (Toshiba) TPS system (Honey Well) Lonwork System (Echelon) and so on

These systems have been introduced only after a few years after first DCS has been installed.