1. Distributed Control SystemsEmad AliChemical Engineering DepartmentKing SAUD University

2. Distributed Control Systems Collection of hardware and instrumentationnecessary for implementing control systems Providethe infrastructure (platform) forimplementing advanced control algorithms 3. History of Control Hardware Pneumatic Implementation: Transmission: the signals transmittedpneumatically are slow responding andsusceptible to interference. Calculation: Mechanical computation devicesmust be relatively simple and tend to wear outquickly. 4. .(History (cont Electron analog implementation: Transmission: analog signals are susceptible tonoise, and signal quality degrades over longtransmission line. Calculation: the type of computations possiblewith electronic analog devices is still limited. 5. .(History (cont Digital Implementation: Transmission: Digital signals are far less sensitive to noise. Calculation: The computational devices are digital computers. 6. Advantages of Digital System Digital computers are more flexible because theyare programmable and no limitation to thecomplexity of the computations it can carry out. Digital systems are more precise. Digital system cost less to install and maintain Digital data in electronic files can be printed out,displayed on color terminals, stored in highlycompressed form. 7. Computer Control Networks1. PC Control:Display Good for small processes such as laboratory prototypeMain or pilot plants, where Computer the number of control loops isFinalcontrol PROCESSData acquisition relatively small element 8. Computer Control Networks2. Programmable Logic Controllers: specialized for non-continuous systems such as batch processes. It can be used when interlocks are required; e.g., aflow control loop cannot be actuated unless a pumphas been turned on. During startup or shutdown of continuous processes. 9. Computer Control Networks Supervisory (host)Computer3. DCSOperator MainOperatorArchivalControl ControlControlDataMostPanel Computer PanelStoragecomprehensiveData highwayTo other Processes To other Processes Local data acquisition andLocal Local Localcontrol computers ComputerComputerComputerLocal Display Local DisplayPROCESS 10. DCS Elements-1 Local Control Unit: This unit can handle 8 to 16individual PID loops. Data Acquisition Unit: Digital (discrete) and analog I/Ocan be handle. Batch Sequencing Unit: This unit controls a timingcounters, arbitrary function generators, and internallogic. Local Display: This device provides analog displaystations, and video display for readout. Bulk Memory Unit: This unit is used to store and recallprocess data. 11. DCS Elements-2 General Purpose Computer : This unit is programmedby a customer or third party to perform optimization,advance control, expert system, etc Central Operator Display: This unit typically containseveral consoles for operator communication with thesystem, and multiple video color graphics display units Data Highway : A serial digital data transmission linkconnecting all other components in the system. It allowfor redundant data highway to reduce the risk of data loss Local area Network (LAN) 12. Advantages of DCS Access a large amount of current information from the datahighway. Monitoring trends of past process conditions. Readily install new on-line measurements together with localcomputers. Alternate quickly among standard control strategies andreadjust controller parameters in software. A sight full engineer can use the flexibility of the framework toimplement his latest controller design ideas on the hostcomputer. 13. Modes of Computer control1. Manualsignals from digitalcomputerLocal PID2. AutomaticFCcontroller PID with local set pointSupervisory Control mode3. SupervisoryFlow measurement to computer PID with remote set valve setting from computerpoint (supervisory) Direct digital Control mode4. Advanced 14. Additional Advantage Digital DCS systems are more flexible.Control algorithms can be changed andcontrol configuration can be modified withouthaving rewiring the system. 15. Categories of process information Type Example Digital. 1 Relay, SwitchSolenoid valveMotor drive Generalized. 2 Alphanumerical displays digital Pulse. 3 Turbine flow meterStepping motor Analog. 4)Thermocouple or strain gauge (mill voltProcess instrumentation (4-20 am)Other sensors (0-5 Volt) 16. Interface between digital computer andanalog instruments (A/D)Transducers convert analog signals todigital signals. (SensorComputer) (D/A)Transducers convert digital signals toanalog signals. (ComputerValve) 17. Data resolution due to digitization Accuracy depends on resolution. Resolution depends on number of bits:Resolution = signal range 1/(2m -1)m = number of bits used by the digitizer (A/D) to represent the analog data 18. Data Resolution Signal = 0 - 1 Volt, 3 bit digitizer:BinaryDigitalAnalog Analog rangerepresentationEquivalent equivalent covered000 00to 1/14 0001 11/7to 3/14 1/14010 22/7to 5/14 3/14011 33/7to 7/14 5/14100 44/7to 9/14 7/14101 55/7to 11/14 9/14110 66/7to 13/14 11/14111 71to 14/14 13/14 19. Data Resolution 7 6Digital data 5 4 3 2 1 0 0 1/7 2/7 3/7 4/7 5/7 6/7 1 Analog data 20. Utilization of DCS DCS vendor job: installation Control Engineer Job: Configuration Built-in PID control: How to Tune the PID control? 21. Utilization of DCS Implementation of advanced control: Developed software for control algorithms, DMC,Aspen, etc. Control-oriented programming language suppliedby the DCS vendors. Self-developed programs using high-levelprogramming languages (Fortran, C++) 22. Advanced control topology Two-ways Data Bus Process PI DCS systemPI interface Online: Real-time data PI interface: C++ or FortranWorkstation 23. DCS Vendors Honeywell Fisher-Rosemont Baily Foxboro Yokogawa Siemen 24. Thank You