UNIT-3DISTRIBUTED CONTROL SYSTEM

INTRODUCTION A distributed control system (DCS) refers to a control system usually of a manufacturing or processor any kind of dynamic system, in which the controller elements are not central in location (like the brain) but are distributed throughout the system with each component sub-system under the control of one or more controllers. The entire system may be networked for communication and monitoring.

APPLICATIONDistributed control systems (DCSs) are used in industrial, electrical, computer and civil engineering applications to monitor and control distributed equipment with or without remote human intervention; the nomenclature for the former 'manual control' and the latter 'automated control'.

EVOLUTION

1934- Direct-connected pneumatic controls dominate market.1938- Transmitter-type pneumatic control systems emerge, making centralized control rooms possible.1958- First computer monitoring in electric utility.1959- First supervisory computer in refinery.1960- First solid-state electronic controllers on market.1963- First direct digital control (DDC) system installed.1970- First programmable logic controllers (PLCs) on market.1970- Sales of electronic controllers surpass pneumatic.1975- First distributed control system on market.

SYSTEM ARCHITECTURE

1. Hybrid system architecture 2. Central computer system architecture 3. Distributed control system architecture

Hybrid system architecture

Hybrid system architecture.

It making use of a combination of discrete control hardware and computer hardware in a central location to implement the required control functions.In this approach, first level or local control of the plant unit operations is implemented by using discrete analog and sequential logic controllers (or PLCs).Panel board instrumentation connected to these controllers is used for operator interfacing and is located in the central control room area.

Hybrid system architecture.

A supervisory computer and associated data acquisition system are used to implement the plant management functions, including operating point optimization, alarming, data logging, and historical data storage and retrieval.The computer also is used to drive its own operator interface, usually consisting of one or more video display units (VDUs).A substantial amount of interfacing hardware is required to tie the analog and sequential control equipment to each other as well as to the supervisory computer.

Central computer system architecture

Central computer system architecture.System functions are implemented in high-performance computer hardware in a central location.Redundant computers are required so that the failure of a single computer does not shut the whole process down.Operator interfacing for plant management functions is provided using computer-driven VDUs, like hybrid control system architecture.

Central computer system architecture.Operator interfacing for first-level continuous and sequential closed-loop control also may be implemented using VDUs.Optionally, the computers can be interfaced to standard panel board instrumentation.

Distributed control system architecture

Distributed control system architecture1.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 LCU using a direct connection. LLHIs can also interface directly to the process Operator oriented hardware called Low Level Operator Interface; instrument engineer- oriented hardware called Low Level Engineering Interface

Distributed control system architecture3. Data Input/ Output Unit (DI/OU): A device that interfaces to the process solely for the purpose of acquiring or outputting data, It performs no control functions.4. High Level Human Interface (HLHI): A collection of hardware that performs functions similar to LLHI but with increased capability and user friendliness. It interfaces to other devices only over the shared communication facilities.5. High-Level Computing Device (HLCD): A collection of microprocessor-based hardware that performs plant management functions traditionally performed by a plant computer.

Distributed control system architecture6. 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 or more levels of communication hardware and associated software that allow the sharing of data among all devices in the distributed system. It do not include dedicated communication channels between specific devices or between hardware elements within a device.

COMPARISON OF ARCHITECTURE

COMPARISON OF ARCHITECTURE

LOCAL CONTROL UNIT

LOCAL CONTROL UNIT.Definition: The smallest collection of hardware in the system that can do closed-loop control. The LCU interfaces directly to the process.Basic Elements: The microprocessor along with the associated clock comprise the CPU of the controller. ROM is used for permanent storage of controller programs. RAM is used for temporary storage of information. Input/ output circuitry can communicate with the external world by reading in, or receiving, analog and digital data as well as sending similar signals out. CPU communicates with other elements in the LCU over an internal shared bus.

LOCAL CONTROL UNIT. The controller structure must be enhanced to include the following: 1. Flexibility of changing the control configuration. 2. Ability to use the controller without being a computer expert. 3. Ability to bypass the controller in case it fails so that the process still an be controlled manually. 4. Ability of the LCU to communicate with other LCUs and other elements in the system.

LCU Architecture..

ARCHITECTURAL PARAMETERS:Size of Controller: It refers to the no. of function blocks and/or language statements that can be executed by the controller, as well as the no. of process I/O channels provided by the controller.2. Functionality of Controller: It refers to the mix of function blocks or language statements provided by the controller. Also the mix of process input and output types provided by the controller.

LCU Architecture..

3. Performance of Controller: It refers to the rate at which the controller scans inputs, processes function blocks or language statements, and generates outputs: it also includes the accuracy with which the controller performs these operations. 4. Communication Channels: It provide other communication channels to operator interface devices in the system . The number, type and speed of these channels are key controller design parameters.

LCU Architecture..

5. Controller output security: Provided to ensure that the control output is maintained despite a controller failure so that a process shutdown can be avoided.

LCU Architecture

1. LCU Architecture- Configuration A 2. LCU Architecture- Configuration B 3. LCU Architecture- Configuration C

LCU Architecture

1. LCU Architecture- Configuration A

LCU Architecture

2. LCU Architecture- Configuration B

LCU Architecture

3. LCU Architecture- Configuration C

COMPARISON

COMPARISON

Control Complexity Ratio: It is defined as the ratio of number of function blocks in control system to the number of control system outputs.

PROCESS INTERFACING ISSUES

PROCESS INTERFACING ISSUES It expands on the basic LCU elements through the addition of interfaces to external communication facilities and to a HLHI device.Functions:It allows 1. several LCUs to implement control strategies. 2. transmission of process data to the higher-level system elements. 3. higher-level elements to transmit information requests and control commands to the LCUs. 4. two or more LCUs to act together as redundant controllers to perform the same control or computational functions.

PROCESS INTERFACING ISSUESSECURITY DESIGN ISSUES FOR THE LCU:-> Security Requirements: 1. Maximize the availability of the automatic control functions of the system. 2. If failure occurs, make sure that there is a mechanism that allows the operation to take over manual control of that portion of the process.

PROCESS INTERFACING ISSUESSECURITY DESIGN ISSUES FOR THE LCU:-> Overview of Security Design Requirements: 1. Provide manual backup only: Each LCU is designed to implement only one or two control loops. The control output is fed back to the manual backup station and to the computation section of the controller so that the inactive element can synchronize its output with the active element.

PROCESS INTERFACING ISSUES2. Provide a standby redundant controller: The LCU is backed up by another LCU that takes over if the primary controller fails. Full automatic control is maintained even under failure conditions.

PROCESS INTERFACING ISSUES3. Provide multiple active controllers: Several LCUs are active at the same time in reading process inputs, calculating control algorithms, and producing control outputs to the process. Since only one output can be used at a time, voting circuitry selects the valid output

PROCESS INTERFACING ISSUES

PROCESS INTERFACING ISSUES-> On-Line Diagnostics: 1. LCU should be able to alarm or report the failure to both the LLHI and HLHI and computing elements. 2. Should be able to switch a contact output to provide an external hardware indication of failure. 3. The internal application logic of the LCU should be able to rive a failure indicator.

PROCESS INTERFACING ISSUES->Secure control output Design: Some of the techniques the designer should follow to improve the security of the control output circuitry include the following: 1. No. of analog outputs per DAC to a minimum. 2. Design both analog and digital output circuitry so that the control outputs go to a safe state when the LCU fails. 3. Power the o/p circuitry from a supply i.e independent of the supply used to power the rest of the LCU.

PROCESS INTERFACING ISSUES 4. Design the o/p circuitry so that the actual value of the o/p can be read back by the rest of the LCU. 5. For maximum reliability of each o/p channel, minimize the no. of components and electrical connections between the control o/p driver hardware and the field termination point for the control actuator.

PROCESS INTERFACING ISSUESMultiplexed Control Output Configuration:

PROCESS INTERFACING ISSUESSingle D/A converter is used to produce several control o/ps by including an analog multiplexer.To generate each o/p P writes the proper value to o/p register, and DAC generates corresponding analog voltage.Processor instructs the MUX to switch o/p of DAC to proper hold circuit, which stores the o/p value and causes the current driver to generate the appropriate o/p current, usually in 4-20 mA.This process occurs on the cyclic basis at least several times per second.

PROCESS INTERFACING ISSUESSecure Control Output Configuration:

PROCESS INTERFACING ISSUESDAC is dedicated to generating a single control output.Provision is made to allow the processor to read back the value of the control o/p, done by means of current-to-voltage converter and ADC.

PROCESS INTERFACING ISSUESPulsed Control Output Configuration:

PROCESS INTERFACING ISSUESLCU processor is involved in o/p generation process by generating raise and lower command to an up/down counter in the o/p channel.This counter responds the commands by incrementing or decrementing a digital value in memory.This value is fed to the DAC, which generates a control o/p through the current driver.The processor keeps track of the o/p through the current-to-voltage converter and ADC.

PROCESS INTERFACING ISSUESBasic Digital Output Configuration: This configuration suffers from many of the same defects as the corresponding analog version of Multiplexed control output configuration.

PROCESS INTERFACING ISSUESSECURE DIGITAL OUTPUT CONFIGURATION:

PROCESS INTERFACING ISSUESThe added security features correct these defects.An o/p read back capability also can be added to this configuration if desired.The failsafe o/p selection section is much simpler in the digital o/p case than in the analog one.In digital, there are only two states(0 or 1), and selecting and generating the safe state is a relatively straightforward process.

PROCESS INTERFACING ISSUES-> Manual Backup Designs:Principles:Should be a minimal amount of h/w between LLOI station and control actuator.The operator should be able to remove and replace the LCU electronics that implement the automatic control function without shutting down the process.Through LLOI, to observe the values of both manual o/p signal and process variables being controlled.Manual o/p command should be available to the automatic controller.Some mechanism for manual backup from the control room must be provided

PROCESS INTERFACING ISSUESManual Backup-Configuration 1:

PROCESS INTERFACING ISSUESManual Backup-Configuration 1:Both the LCU processor and LLOI station communicate with the i/o cards.One operator station can back up several output cards.Station communications port allows transmission of process variable and control o/p information between the station and the LCUs.If both the processor and the station fails, the failsafe o/p select circuitry takes over to generate a pre selected safe o/p signal.

PROCESS INTERFACING ISSUESManual Backup-Configuration 2:

PROCESS INTERFACING ISSUESManual Backup-Configuration 2:It is similar to Configuration 1 in all respects expect for the way in which the manual backup commands are transmitted to the control o/p section.It uses an up/down counter to generate the control o/p signal.LLOI cannot be shared among o/ps, it must be dedicated to a single o/p channel.It increases the cost of backup, but also increases the level pf security.

PROCESS INTERFACING ISSUESManual Backup-Configuration 3:

PROCESS INTERFACING ISSUESManual Backup-Configuration 3:LLOI is completely independent of the LCU and interfaces with it only over the station communication ports.

PROCESS INTERFACING ISSUESManual Backup for Digital output: An o/p FF or latching circuit is included that will accept a manual o/p override signal from the LLOI.

PROCESS INTERFACING ISSUESRemote Manual Backup Unit: It is designed to interface with the control o/p circuitry of the LCU using any of the techniques illustrated before.

PROCESS INTERFACING ISSUES-> Redundant Controller Designs: Control system security require some form of controller redundancy to ensure that automatic control of the process is maintained in spite of an LCU failure. Addition of redundant elements will increase the system cost as well as in additional maintenance to service the extra hardware.

PROCESS INTERFACING ISSUESGuidelines for designing:Redundant architecture should be kept as simple as possible.The architecture must minimize potential single points of failure.Should be transparent to the userProcess should not be bumped or disturbed by the failed or repaired elements.Until repair or replacement, the system should not provide an information to other elements.Must have the capability of hot spare replacement.

PROCESS INTERFACING ISSUESRedundant LCU Architecture: 1. CPU redundancy 2. One-on-One redundancy 3. One-on-Many redundancy 4. Multiple Active redundancy

PROCESS INTERFACING ISSUESCPU redundancy:

PROCESS INTERFACING ISSUESOnly the CPU portion of the LCU is redundant: while the I/O circuitry is not redundant.CPU is redundant because its failure affects all of the control outputs.Arbitrator monitors the operation of the primary CPU.If it detects any failure in the primary, arbitrator transfers priority to the backup.Only the primary is active in transmitting and receiving messages over this link.

PROCESS INTERFACING ISSUESAdvantages: *Easy to understand * Cost-effectiveDisadvantages: * Problems occur if not designed properly * If LCU is located at some distance from the central control room, no means of manual backup exists.

PROCESS INTERFACING ISSUES2. One-on-One redundancy:

PROCESS INTERFACING ISSUESIt provides a total backup LCU to the primary LCU.O/P switching block must be included to transfer the outputs when the controller fails.Arbitrator has the additional responsibility of sending a command to the o/p switching circuitry if the primary LCU fails, causing the backup LCU to generate the control outputs.

PROCESS INTERFACING ISSUESAdvantages: * No manual backup is needed, since all of the LCU hardware is duplicated. * It eliminates any question that may arise with a partial redundancy approach.Disadvantages: * It is an expensive approach to redundancy. * It has potential single-point failure problems with the arbitrator and the output switching circuitry.

PROCESS INTERFACING ISSUES3. One-on-Many redundancy

PROCESS INTERFACING ISSUESSingle LCU is used as a hot standby to backup any one of several primary LCUs.Arbitrator is required to monitor the status of the primaries and switch in the backup when a failure occurs.There is no way of knowing ahead of time which primary controller the backup would have to replace.As a result very general switching matrix is necessary to transfer the I/O from the failed controller to the backup.It is loaded into the backup LCU from the primary LCU only after the primary has failed.

PROCESS INTERFACING ISSUESAdvantages: * Low cost * This approach violates the second and fifth design guidelines.Disadvantages: * Complex design * The approach relies on the failed controller to provide a copy of the control system configuration to the backup LCU.

PROCESS INTERFACING ISSUES4. Multiple Active redundancy:

PROCESS INTERFACING ISSUESOne or more redundant LCUs are used to perform control functions.All of the redundant controllers active at the same time.A mechanism is provides to allow the controllers to synchronize their operations in time and to periodically read and check others internal states.In analog control o/ps- voting device is designed to select the media signal and in digital control o/ps- voting device is designed to select the signal generated by at least two out of the three controllers.

PROCESS INTERFACING ISSUESAdvantages: * As long as the output voting device is designed for high reliability, it significantly increases the reliability of the control system.Disadvantages: * High cost * Implemented for fixed configuration * Added hardware requires increased maintenance and that the system is very complex.

PROCESS INTERFACING ISSUESPROCESS INPUT/OUTPUT DESIGN ISSUES:The monitoring and control of a large industrial process requires that many inputs of various types be brought into the control system.The control system then computes the proper actions to be taken and transmits the control outputs to the process and the indications to the operator.The following are some of the requirements: -> Input/Output requirements -> Input/Output Design Approaches

COMMUNICATION FACILITIESINTRODUCTION: In conventional non-distributed control system, the connections that allow communications between the various system elements are configured on point-to-point wiring.Replacing dedicated point-to-point wiring and cabling with the communication facility provides a considerable no. of benefits to the user: 1. It reduces the cost 2. Flexibility of making changes increases. 3. Less time to implement large systems 4. More reliable

COMMUNICATION FACILITIES.POINT-TO-POINT WIRING:

COMMUNICATION FACILITIES.COMMUNICATION SYSTEM REQUIREMENTS:Transmission of control variables between LCUs in the system.Transmission of process variables, control variables, and alarm status information from the LCUs to the HLHI and to the LLHI.Downloading of control system configuration, tuning parameters, and user programs from HLHI to LCUs.Transmission of information from DI/OU to high-level computing devices.Transfers large blocks of data and programs.Synchronization.

COMMUNICATION FACILITIES.KEY PARAMETERS: Maximum size of the systemMaximum delay time through the systemSensitivity to traffic loadingSystem scalabilitySystem Fault toleranceInterfacing requirementsEase of application and maintenanceEnvironmental specificationsRate of undetected errors occurring in the system

COMMUNICATION FACILITIES.ARCHITECTURAL ISSUES:-> Channel structure-> Levels of sub networks-> Network Topologies

COMMUNICATION FACILITIES.COMMUNICATION SYSTEM PARTITIONING:

COMMUNICATION FACILITIES.Several HLOI and computing elements located in the central control room area must communicate with each other at moderate levels of message traffic.Able to communicate with data acquisition and control elements located near the process unit to be controlled.

COMMUNICATION FACILITIES. The requirements of communication system partitioning has three levels: 1. A local bus or sub network in each cabinet allows the individual controllers to inter communicate without interfering with message traffic in other cabinets. 2. A local sub network in the central control room area allows the high-level devices to intercommunicate. 3. A plant wide communication system interconnects the control room elements with the distributed elements in the process areas.

COMMUNICATION FACILITIES.OTHER ISSUES: - Selecting a Communication Medium - Message security - Efficiency of Bandwidth usage