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Distributed control systems.

Introduction:

The requirement of DCS has been evolved due to the advancement of Electronics, which can make the operators of the plant to see and control process with ease and provide reports and data at the stroke of button. The thanking process has to be attributed to Database originators and Graphic Builders.

History of DCS:

In earlier part of decades the plant details are brought to inform of process details and controls shown in panels and the fault conditions are brought as Annunciators mounted at the top of the units. The critical and alarm conditions are shown as yellow and red lamps. The plant layout and piping arrangement were shown as mimic diagrams on top of the panels. The panels are segregated as sections of plants. For example the Boiler plants will have a length of panels consisting of section like boiler feed water pump section. Demineralised water plant. Dearator section, Furnace area, Drum level area, Condensate and hot well section, Coal handling area, Force Draft and Induced draft area. Fuel oil storage area. HP LP bypass section etc. The panel operator sits in front of console moves to the section and changes the operation mode or setpoints in the controller. Also when abnormalities are indicated the operator moves to the necessary area to acknowledge the buttons and take the corrective action. The hourly readings are taken from panel instruments in tabular forms for reporting the plant conditions. The information is collected on shift basis and reported the daily production figures to the higher up. The plant process behaviors are read from strip chart recorders to know the flow or pressure pattern in continuos for two or three hours. In essence operator takes the following actions.

View the process parameter

Acknowledge alarms

Action like pump start stop

Data collection

Isolate equipment

Load changes for production increase decrease

Observe safety conditions

Data collection

Hourly reports

Trend the process parameters

Prepare shift reports

Prepare daily report

Chronologically details the failures of equipment.

Where as in the present day DCS are manufacture by various vendors. DCS are the micro processor based equipment with built electronic boards run by software provides information of plant at one figure tip action. Displays are called by pressing buttons calling different displays and view them on Display stations. Reports are generated automatically at a desired time with the combination of various process points. Process behaviors are available for extended period of time which can be used to analyze the process upsets. The alarms are generated instantly and can be acknowledged at any work station. These are recorded in hard disk and can be printed anytime. Also segregation can be done by time, type, area and all possible combination for analysis. Operator actions are stored for future analysis. Unlike earlier days Process graphics are displayed and operator know exactly which is the location at which the process change takes plays.

Also the operator can have startup displays which can only be observed for easier startup. The plant can be started with one button recipes are available for varies combination of process changes.

Communication devices are installed to transport plant information to the management to decide the process changes to suit the commercial need.

Also facilities are built to analyze the behavior of equipment and can be used for planning the preventive maintenance schedule.

The diagnostic tools exits to know the failure of equipment and Systems.

So the ultimate utility of DCS is for operation, Maintenance and the Management functions.

The following displays are required for operator to view the plant

Overview display

Group displays

Details displays

Graphic display

Alarm display

Trend display

Report display

The parameters which operator looks are

Pressure

Flow

Temperature

Level

Analyzer

Status of equipment

Safety parameters

Start up conditions.

Equipment failures and conditions

The parameters are classified as Continuously variable and discrete variables.

Continuous variables:

Pressure

Flow

Level

Temperature

Analyzer details

Vibration

Speed

 

Discrete variables:

On/off conditions like

Level high

Level high high

Pressure low

Pressure low low

Temperature high

Temperature low low

Valve close

Valve open

Pump run

Pumpo stop

Vibration high

Vibration low

Speed low

Speed high

Signal details:

In DCS, transmitters send the continuous parameters to DCS as 4 to 20 Ma signals for example

Pressure in a vessel of range 0- 500 psi is sent to DCS as 4 to 20 ma

i.e 4 ma = 0 psig and 20 ma = 500 psig

Hence operator can see the intermediate values. The view can be seen to any decimal places.

The same principle goes to all process variables.

The converted signal range can be in 4 to 20 ma or 1 to 5 v or -10Vdc to -10V dc as per the choice of the DCS vendors.

The discrete signals are the state change or on/off signals routed to DCS as I 0r 0 signals.

i.e. level high as high signal =24V DC and level low signal as 0V DC

Motor start = high signal 24V DC and Motor stop = low signal as 0V DC

The logic signal can be chosen as negative or positive also the voltage levels vary as 125 V DC/AC, 24 V DC, 220 V DC/AC etc by the cards chosen at DCS and also choice of the vendor.

DCS Hardware:

The DCS built by major vendors basically have the following hardware structure The hardware consist of six areas which are as follows:

Input output interfacing cards

Power supply cards

Processor cards

Memory storage cards

Display devices and connected electronics

Peripheral devices to interface external devices.

Communication cards

 

Input output cards:

These are the interface electronic cards which are connected to the filed devices such as transmitters, switches and Motor control devices, valves etc.

Analog Input output cards:

The signal levels vary with the vendor. As mentioned earlier the signal will be 4 to 20 ma signal or 1 to 5 V dc. These signals are converted by the Input cards to digital values using Analog to digital converters. Also the Input output cards have the facility to power the transmitters. The power selections are done by the hardware jumpers or by software switches. Normally these cards have either 8 channels or 4 channel input and 4 channel output. The cards are also classified as follows:

  1. Current devices 4 to 20 ma
  2. Voltage input such as mv., volts
  3. Rtd input cards
  4. T/c input cards
  5. Smart input cards
  6. Field bus devices.
  7. Pulse input cards.

The following give some of the devices hooked to Input output cards

  1. Pressure Transmitters
  2. Flow transmitters
  3. Level transmitters
  4. Temperature transmitters
  5. Analyzer Transmitters
  6. Speed transmitters
  7. Thermocouple
  8. Resistance thermometers
  9. Pulse devices like Turbine flow meters
  10. Current to Pneumatic Converters
  11. Position transmitting devices.

 

Digital input cards

The signal levels for digital inputs and outputs are normally 1 or 0 levels and the digital signals conditions are generated by switches installed in the field.

 

The contact input/output cards:

The signal levels are normally 24V DC, 48V DC, 120V DC, 120V AC, 240V AC or the potential free relay contact. The contact can be powered from DC or External. The jumper selections are available. Also the selection includes the normally open or normally closed configuration also exists. Normally the contacts, both in / output can be isolated utilizing Opto couplers to isolate the filed interference with the DCS cards. These signals can be contact sense or voltage sense. These can be achieved by jumper selection or routing the signal to the selected terminals. Normally these cards have 16 or 32 channels. The cards are as follows.

1. Discrete input 24V DC 32 channel.

2. Discrete input 120V AC 16 channel

3. Discrete input 220v AC 16 channel

4. Discrete input 125 V DC 16 channel

5. Relay input/ output card 24V DC 8 in/ 8 out

6. Relay input/output card 120V DC 8 in / 8 out

 

Power Supply Cards:

The system components works at low voltage and mostly Cmos devices and processors. These require a stable power supply as most of them work at 5V DC and +24, -24v. These power supply units are mostly switching mode using troidal coils. Normally the power supply is common to system components and Input output devices. For external power supplies are installed for driving field components. Generally two power supply cards are installed for redundancy to avoid failures due to insufficient power. The incoming power is normally from uninterrupted power supply units and emergency generators. The supply voltage values and frequency varies with the regulation of local areas.

 

Processor Cards:

Processors are the heart of the system which regulates the flow of information from and to the Input output devices and does the calculation. Processors in coordination with the memory cards do the programmed actions. The software switches decides the process of programmed actions. Some of the actions of the processors are as follows.

The process control requires different algorithms to control the process depending on the process lag and equipment configuration. Some of the algorithms are as listed below:

PID, PIDE, PID GAP, Summer Low selector, High selector, Characterizer,

Ramp generator, Switches, Logic, Ratio and Bias. Sequence blocks, Math functions, Calculator, Counter, Valve movements, Motor control devices. Etc.

The above are generic functions generated by mathematical calculation based on input and output values. These functions are stored in ROM and can be utilised whenever it is required. Since process control require all these are buit as defaults and can be connected to the inputs by software switches.

Also the process control require to control the process parameter in a definite time the scan time for the operation of input, calculation and output is very important and the processor section speed of operation is based on this. The Scan time is normally in the range of 1/4 sec to 1 sec.

Also the hardware is manufactured in such way that any input / output can be linked to any other inputs / outputs.

The processors are also programmed to transfer information between various devices and cards inside the system. The details of these functions are elaborated in the later part of this article with examples of selected manufacturer for understanding purpose.

Normally the processor cards are dual and if one fails the other takes over. Also the processor is responsible for fault analysis in the system and report the failure by LEDs installed in the cards.

The processor does the communication functions between various cards and display processors.

The processors also provided with Boot ROM which is responsible for cold starts.

Memory storage cards.

The system has both RAM and ROM cards. These cards are required to store the application programs and user programs. These are static devices and normally backed by lithium batteries. The application programs are manufacture defined informing the architecture and utility requirements for various applications used the same hardware components. The system is defined and manufactured in such a way that the same cards can be used for batch processing and continuous processing plants.

Normally these are CMOS devices operates at low voltages and can be removed on line as backup memory cards are installed to have the mirror image of the information.

Also these memory cards are provided with the facility to program the devices for user application on line without affecting the running process. The buffer memory built in the system takes care of this function along with the processor instructions.

The memory cards are normally in range of 2 to 4 Meg. More the memory the better the utility and processing time.

 

Display devices and connected electronics:

The display devices are the Human interface for viewing the process parameters. The display devices has the following components:

Power supply unit,

RGB generator

Processor

Communication interface

SCSCI board to communicate peripherals

Memory board.

Cathode ray Tube.

Power supply unit is the basic card which supplies power to all the electronic cards. This has +12V., -12V, 5V DC required for the system cards and the power is distributed to the cards through the mother board.

RGB generator has both ramp generator and color cards, which drive the Red Green, Blue coils. Basically these are responsible for color generation in the display units.

Processor is responsible for coordinating the picture generation based on in/out values, operator commands and also communication between the cards in the display units.

Communication Interface is responsible for the data transfer between the various devices connected to the display unit. Basically this direct the operator commands and process input /output values to the controllers which is interfacing with the Input output devices.

SCSCI Board is responsible to control the read write actions of peripheral devices like Floppy drive, Tape drive, hard disk and printer. The system application and user configuration details are saved and restored through these devices for startup and running of the system.

Memory board is buffer device responsible for information between devices and display units. Also the Graphic static display is fetched and stored in the buffer memory before displaying in the CRT. Normally these are of 4 Meg. Higher the memory capacity, better the display update time. Normally the display update time would be 1 to 2 sec depend on the complexity of graphic and links of process information.

CRT: cathode ray tube is the final viewing device, which display all process information to the operator. The number of pixels and and colour segments decide the clarity of information shown in the CRT. Also the capability like viewing multiple screen in the same CRT is based on the color gun and coils provided in the CRT.

 

Peripheral devices to interface external devices.

These are the devices which interface with external system and also the storing Devices. Normally these are attached to the display unit electronics Also the units are also provided with bridges and gateways which talk to external systems which also gather process details. The storing device include the tape and disk drives which can be utilised for loading and saving system and application software during initial boot up or while starting up the systems. The peripheral devices are Floppy drives, Tape drives, Hard disks.

Communication cards:

These are Gateway cards which are attached to the DCS Systems which talks to other systems like PLCs Vibration Monitoring system or other embedded systems for dedicated purposes. These devices are generally talks in different protocols and communication media. Some of the protocols are Modbus, Controlnet, Device net etc and also the communications media such as RS232, Rs485 etc. Some are Fibre optic type. The communication media is decided by the distance and the method each individual supplier implemented in their systems.

 

Software details of DCS Systems:

The software is the language in which the system understand the System components and also the user requirements. The software is classified into two categories and they are

1 System Software

2. Application software

System Software:

System software is details the method of interconnections and the optimum hardware utility for many functions. The software also provides standard templates of process, systems, and maintenance functions. Also the standard templates for operator viewing capabilities, report and History requirements are generated. The system software provides tools for expanding the system online.

The system also defines the method of transfer of information between Input/ output devices and Human interface. The software for graphic generation tools, print functions, remote maintenance of system components and Diagnostics are included.

Application software:

Application software is mainly the utility tool for the people who utilize the system for Operator functions and Engineering function. The software tool is to do the following functions.

Operator Functions.

1. Graphic generation detailing the Plant process lines and vessels

2. History of the Process detailing on chronological basis.

3. Alarm Management based on criticality, Area basis and Unit basis.

4. Daily, Weekly, Hourly report generation based on Process points at various

Units.

5. Defining the control function based on the Process flow diagrams.

6. Scheduling the repeated activity using simple commands.

7. Operator journal for supervising and monitoring operators actions.

Engineering Function:

1. For Inter connecting the Input / outputs.

2. Defining the control scheme and interconnections based on the process requirements.

3. Defining the alarm generation and defining the colour details based on critically and setting preferences and permissions.

4. Defining the Process Point ranges, alarm values, Failure actions, control algorithms,

5. Linking the process points to the graphics.

6. Define Batch process.

7. Define the auto start function.

8. Prepare various reports

9. Backup and load the both application software and System software.

 

The following session will give the details of Sizing the DCS, Configuration and other practical aspects of DCS.

Configuration In the system software the basic definition of the hardware flexibility and User requirements are in built. Configuration is the method to tell the system how we utilize the system based on the requirement of the user. An e.g. It is like the town planning. The total area developed is defined. The numbers of Communities are defined. The communities are named. The basic requirement like sewage, water and communication facilities are built. The street and block and avenue names are given. The limitation and facilities are well defined. Based on the above the user built his house. The house also has various kinds and which has different shades, The interior and exteriors are different and the numbers of rooms are different and the numbers of occupants are different. Exactly the similar principle the DCS Configuration is organized.

The DCS system architecture defines the similar way as Town planning the maximum number of devices are well defined. The communication speed is defined. The number of devices transferring information is defined. The Input output cards types and quantities are defined. Even the definition of filtering characterization, alarm requirements are defined in input cards. The communications between main and sub devices are defined. Templates are defined for entering the details. The path of interconnections is defined. The following will give the detail explanation of the above facts.

In this article the author has taken an example of three systems for the sake of academic explanations. There is no bias against a particular system and this is to explain how Hardware and software configurations are done and the similarity exists in all other systems also.

Some of the definition need to be understood and are as follows.

Node: A device sitting on the main communication Route.

Module: Module is same as Node a device sitting on the main communication Route.

Node Bus: The communication path which transfer signals between Node devices.

Field bus: The secondary network which transfer information between Input out cards and Node devices.

I/O cards: The hardware interface between Field devices and Node bus devices.

Termination assemblies: The connection terminals between Field device and I/O cards. Also the power supply route to individual filed device and the bulk power supply of the system

Hardware Details and Configuration information