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الموضوع: Terminology and Symblos in Control Engineering

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    Terminology and Symblos in Control Engineering

    Part 1 Fundamentals
    Terminology and Symbols
    in Control Engineering
    Technical Information
    1
    Part 1: Fundamentals
    Part 2: Self-operated Regulators
    Part 3: Control Valves
    Part 4: Communication
    Part 5: Building Automation
    Part 6: Process Automation
    Should you have any further questions or suggestions, please
    do not hesitate to contact us:
    SAMSON AG Phone (+49 69) 4 00 94 67
    V74 / Schulung Telefax (+49 69) 4 00 97 16
    Weismüllerstraße 3 E-Mail: schulung@samson.de
    D-60314 Frankfurt Internet: http://www.samson.de
    Technical Information
    Terminology and Symbols in
    Control Engineering
    Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
    Terminology in Control Engineering . . . . . . . . . . . . . . . . . . 6
    Open loop control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
    Closed loop control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
    Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
    Control loop . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
    Abbreviations of variables relating to closed loop control. . . . . . . . . 10
    Symbols in Control Engineering . . . . . . . . . . . . . . . . . . . 12
    Signal flow diagrams. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
    Blocks and lines of action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
    Device-related representation. . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
    Instrumentation and control tags . . . . . . . . . . . . . . . . . . . . . . . . . . 19
    Control Systems and Structures . . . . . . . . . . . . . . . . . . . . 22
    Fixed set point control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
    Follow-up control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
    Cascade control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
    Ratio control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
    Appendix A1: Additional Literature . . . . . . . . . . . . . . . . . . 26
    3
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    *******S
    Preface
    The technical informations presented in this ******** are based on definitions
    according to DIN, the German organization of standardization (Deutsches
    Institut für Normung). Continuous efforts are being made to determine
    international definitions in order to achieve an increasing similarity in the terminology
    used. Nevertheless, differences in designations and representations
    do exist in international use. Literature presented at the end of this
    ******** includes international standards and publications relating to DIN
    standards, or being derived from them.
    Representations and **** sections referring to DIN are often cited in short
    form, summarizing the *******s. The precise facts must always be read - also
    because of possible extensions or amendments - in the current edition of the
    respective standard.
    4
    Fundamentals × Terminology and Symbols in Control Engineering
    SAMSON AG × V74/ DKE
    Introduction
    Planning, design and start-up of process control systems require clear and
    unambiguous communication between all parts involved. To ensure this, we
    need a clear definition of the terms used and – as far as the ********ation is
    concerned – standardized graphical symbols. These symbols help us
    represent control systems or measurement and control tasks as well as their
    device-related solution in a simple and clear manner.
    5
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    Terminology in Control Engineering
    To maintain a physical quantity, such as pressure, flow or temperature at a
    desired level during a technical process, this quantity can be controlled either
    by means of open loop control or closed loop control.
    Open loop control
    In an open loop control system, one or more input variables of a system act
    on a process variable. The actual value of the process variable is not being
    checked, with the result that possible deviations – e.g. caused by disturbances
    – are not compensated for in the open loop control process. Thus, the characteristic
    feature of open loop control is an open action flow.
    The task of the operator illustrated in Fig. 1 is to adjust the pressure (p2) in a
    pipeline by means of a control valve. For this purpose, he utilizes an assignment
    specification that determines a certain control signal (y) issued by
    the remote adjuster for each set point (w). Since this method of control does
    not consider possible fluctuations in the flow, it is recommended to use open
    loop control only in systems where disturbances do not affect the controlled
    variable in an undesired way.
    6
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    p1
    y
    p2
    Fig. 1: Operator controls the process variable p2 via remote adjuster
    Assignment:
    wa => ya => p2a
    wb => yb => p2b
    etc.
    open action flow
    disturbances are
    not recognized
    Closed loop control
    In a closed loop control system, the variable to be controlled (controlled
    variable x) is continuously measured and then compared with a
    predetermined value (reference variable w). If there is a difference between
    these two variables (error e or system deviation xw), adjustments are being
    made until the measured difference is eliminated and the controlled variable
    equals the reference variable. Hence, the characteristic feature of closed
    loop control is a closed action flow.
    The operator depicted in Fig. 2 monitors the pressure p2 in the pipeline to
    which different consumers are connected. When the consumption increases,
    the pressure in the pipeline decreases. The operator recognizes the pressure
    drop and changes the control pressure of the pneumatic control valve until
    the desired pressure p2 is indicated again. Through continuous monitoring of
    the pressure indicator and immediate reaction, the operator ensures that the
    pressure is maintained at the desired level. The visual feedback of the process
    variable p2 from the pressure indicator to the operator characterizes the
    closed action flow.
    7
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    p1 p2
    Fig. 2: Operator controls the process variable p2 an a closed loop
    closed action flow
    disturbances are
    eliminated
    The German standard DIN 19226 defines closed loop control as follows:
    Closed loop control is a process whereby one variable, namely the variable
    to be controlled (controlled variable) is continuously monitored, compared
    with another variable, namely the reference variable and, depending on the
    outcome of this comparison, influenced in such a manner as to bring about
    adaptation to the reference variable. The characteristic feature of closed
    loop control is the closed action flow in which the controlled variable continuously
    influences itself in the action path of the control loop.
    A control process can also be regarded as ‘continuous’ if it is composed of a
    sufficiently frequent repetition of identical individual processes. The cyclic
    program sequence of digital sampling control would be such a process.
    Note: In English literature we only find one term, that is ‘control’, being used
    for actually two different concepts known as ‘steuern’ and ‘regeln’ in the German
    language. When translating into German, we therefore come across
    the problem whether ‘control’ means ‘steuern’ or ‘regeln’. If both methods
    are involved, ‘control’ often is translated as ‘automatisieren’ or ‘leiten’ (control
    station). An exact distinction can be made if the German term ‘Regelung’
    is made obvious by using the English term ‘closed loop control’.
    Process
    A process comprises the totality of actions effecting each other in a system in
    which matter, energy, or information are converted, transported or stored.
    Adequate setting of boundaries helps determine sub-processes or complex
    processes.
    8
    Fundamentals × Terminology and Symbols in Control Engineering
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    definition of
    closed loop control:
    DIN 19 226
    difficulties with the
    English term ´control´
    • Examples:
    4Generation of electricity in a power plant
    4Distribution of energy in a building
    4Production of pig iron in a blast furnace
    4Transportation of goods
    Control loop
    The components of a control loop each have different tasks and are distinguished
    as follows:
    The components of the final control equipment are part of the controlling sy
    stem as well as part of the controlled system.
    The distinction made above results directly from the distribution of tasks. The
    actuator processes and amplifies the output signal of the controller, whereas
    the final control element – as part of the controlled system – manipulates the
    mass and energy flow.
    9
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    Controlling system Controller and acuator
    + Controlled
    system
    Final control element, pump,
    pipeline, heating system etc.
    + Measuring
    equipment
    Temperature sensor, pressure sensor,
    converter etc.
    = Control loop
    components of the
    control loop
    components of the
    final control equipment
    Actuator (controlling system) Actuating drive
    + Final control element
    (controlled system)
    Closure member
    = Final control equipment Control valve
    Abbreviations of variables relating to closed loop control
    The abbreviation of variables allows the determination of standardized symbols.
    The symbols used in German-speaking countries and specified in DIN
    19221 correspond with the international reserve symbols approved by the
    publication IEC 27-2A. Aside from that, IEC also determines so-called chief
    symbols which considerably differ from those used in DIN in some important
    cases.
    x (IEC chief symbol: y)
    In a control loop, the process variable to be controlled is represented by x. In
    process engineering, usually a physical (e.g. temperature, pressure, flow) or
    a chemical (e.g. pH value, hardness) quantity is controlled.
    w (IEC chief symbol: w)
    This variable determines the value that must be reached (set point) by the
    process variable to be controlled. The physical value of the reference variable
    – this may be a mechanical or electric quantity (force, pressure, current,
    voltage, etc.) – is compared with the controlled variable x in the closed control
    loop.
    r (IEC chief symbol: f)
    This variable results from the measurement of the controlled variable and is
    fed back to the comparator.
    e = w – x (IEC chief symbol: e)
    The input variable e of the controlling element is the difference between reference
    variable and controlled variable, calculated by the comparator. When
    the influence of the measuring equipment is included, the equation e = w – r
    applies.
    xw = x – w
    The equation above shows that the system deviation yields the same result as
    error, however, with an inverse sign. When the influence of the measuring
    equipment is included, xw = r – w applies.
    10
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    DIN or IEC
    controlled variable,
    actual value
    reference variable
    feedback variable
    error
    system deviation
    y (IEC chief symbol: m)
    The manipulated variable is the output variable of the controlling equipment
    and the input variable of the controlled system. It is generated by the controller,
    or in case an actuator is being used, by the actuator. This variable depends
    on the setting of the control parameters as well as on the magnitude of
    error.
    yR
    When dividing the controlling system into the controller and actuator, the variable
    yR stands for the output variable of the controller or the input variable
    of the actuator.
    z (IEC chief symbol: v)
    Disturbances act on the control loop and have an undesired effect on the
    controlled variable. Closed loop control is used to eliminate disturbance variables.
    Yh
    The manipulated variable y can be determined by the controller within Yh,
    the range of the manipulated variable :
    11
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    ymin £ y £ ymax
    manipulated variable
    controller output
    variable
    disturbance variable
    range of the
    manipulated variable
    Symbols in Control Engineering
    Signal flow diagrams
    A signal flow diagram is the symbolic representation of the functional interactions
    in a system. The essential components of control systems are represented
    by means of block diagrams. If required, the task represented by a
    block symbol can be further described by adding a written ****.
    However, block diagrams are not suitable for very detailed representations.
    The symbols described below are better suited to represent functional details
    clearly.
    Blocks and lines of action
    The functional relationship between an output signal and an input signal is
    symbolized by a rectangle (block). Input and output signals are represented
    by lines and their direction of action (input or output) is indicated by arrows.
    • Example: Root-extracting a quantity (Fig. 3)
    (e.g. flow rate measurement via differential pressure sensors)
    12
    Fundamentals × Terminology and Symbols in Control Engineering
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    xe xa
    Fig. 3: Root-extracting a differential pressure signal
    xe = differential pressure xa = root-extracted differential pressure
    • Example: Representing dynamic behavior (Fig. 4)
    (e.g. liquid level in a tank with constant supply)
    • Example: Summing point (Fig. 5)
    The output signal is the algebraic sum of the input signals. This is symbolized
    by the summing point. Any number of inputs can be connected to one summing
    point which is represented by a circle. Depending on their sign, the inputs
    are added or subtracted.
    13
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    xe xa
    Fig. 4: Development of a liquid level over time
    xe = inflow xa = liquid level
    xe1
    xe2 +
    +
    _
    xe3
    xa
    xa = xe1 + xe2 – xe3
    Fig. 5: Summing point
    • Example: Branch point (Fig. 6)
    A branch point is represented by a point. Here, a line of action splits up into
    two or more lines of action. The signal transmitted remains unchanged.
    • Example: Signal flow diagram of open loop and closed loop control
    The block diagram symbols described above help illustrate the difference
    between open loop and closed loop control processes clearly.
    In the open action flow of open loop control (Fig. 7), the operator positions
    the remote adjuster only with regard to the reference variable w. Adjustment
    is carried out according to an assignment specification (e.g. a table: set point
    w1 = remote adjuster position v1; w2 = v2; etc.) determined earlier.
    14
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    x1
    x2
    x1 = x2 = x3
    x3
    Fig. 6: Branch point
    w x
    Fig. 7: Block diagram of manual open loop control
    man
    remote
    adjuster system
    control
    valve
    signal flow diagram
    of open loop control
    In the closed action flow of closed loop control (Fig. 8), the controlled variable
    x is measured and fed back to the controller, in this case man. The controller
    determines whether this variable assumes the desired value of the
    reference variable w. When x and w differ from each other, the remote adjuster
    is being adjusted until both variables are equal.
    Device-related representation
    Using the symbols and terminology defined above, Fig. 9 shows the typical
    action diagram of a closed loop control system (abbreviations see page 10).
    15
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    w + x
    _
    Fig. 8: Block diagram of manual closed loop control
    man
    remote
    adjuster
    control
    valve
    system
    z
    w + e


    yr y x
    r
    Fig. 9: Block diagram of a control loop
    controlling
    element
    measuring
    equipment
    controller
    final
    control
    element
    system
    actuator
    signal flow diagram
    of closed loop control
    elements and signals
    of a control loop
    Whenever the technical solution of a process control system shall be pointed
    out, it is recommended to use graphical symbols in the signal flow diagram
    (Fig. 10). As this representation method concentrates on the devices used to
    perform certain tasks in a process control system, it is referred to as solution-
    related representation. Such graphical representations make up an essential
    part of the ********ation when it comes to planning, assembling,
    testing, start-up and maintenance.
    16
    Fundamentals × Terminology and Symbols in Control Engineering
    SAMSON AG × V74/ DKE
    graphical symbols
    for detailed, solutionrelated
    representations
    Fig. 10: Graphical symbols for describing temperature control
    of a heat exchanger system
    1 Sensor (temp.) 2 Transmitter
    3 Signal converter 4 Controller
    5 Pneumatic linear valve 6 Heat exchanger
    1
    3 2
    4
    5 6
    Each unit has its own graphical symbol that is usually standardized. Equipment
    consisting of various units is often represented by several lined-up symbols.
    17
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    PI
    Fig. 11: Graphical symbols for controllers, control valves and software-based
    functions according to DIN 19227 Part 2
    hand-operated
    actuator
    motor-driven
    actuator
    diaphragm
    actuator
    valve with
    diaphragm
    actuator
    motor-driven
    butterfly valve
    valve
    controller controller
    (former symbol)
    valve with
    diaphragm actuator
    and attached
    positioner
    PI controller
    root-extracting
    element,
    software-based
    software counter
    with limit switch
    functions performed by
    software are marked
    with a flag
    Graphical symbols used for process control are specified in DIN 19227, including
    symbols for sensors, adapters, controllers, control valves, operating
    equipment, generators, conduits and accessories (Figs. 11 and 12). However,
    there are a number of other DIN standards covering graphical symbols,
    such as DIN 1946, DIN 2429, DIN2481, DIN 19239 and DIN 30600 (main
    standard containing approximately 3500 graphical symbols).
    It is recommended to always use standardized graphical symbols. In case a
    standardized symbol does not exist, you may use your own.
    18
    Fundamentals × Terminology and Symbols in Control Engineering
    SAMSON AG × V74/ DKE
    P
    F
    F
    T
    Pt 100 DIN
    P
    P L
    L
    I
    Fig. 12: Graphical symbols for sensors, transmitters, adjusters and
    indicators according to DIN 19227 Part 2
    level
    sensor
    temperature
    sensor
    pressure
    sensor
    analog indicator adjuster
    flow sensor
    pressure transmitter
    with electric
    standardized output
    signal
    current transmitter
    with pneumatic
    standardized output
    signal
    i/p converter,
    electr. into pneum.
    standardized
    signal
    graphical symbols
    for process control
    Instrumentation and control tags
    Apart from the solution-related representation, process control systems can
    also be represented by means of instrumentation and control tags (DIN
    19227 Part 1) which describe the task to be done.
    An instrumentation and control tag is represented by a circle. When the circle
    is divided by an additional line, editing and operating procedures are not
    carried out on site, but in a centralized control station. In the bottom half of
    the circle, you will find the instrumentation and control tag number. The identifying
    letters in the top half specify the measuring or input variable as well as
    the type of signal processing, organizational information and the signal flow
    path. If additional space is needed, the circle is elongated to form an oval
    (Fig. 13).
    The typical use of identifying letters in an instrumentation and control tag is
    shown below:
    19
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    TI

    106
    TI

    106
    FRCA

    302
    Fig. 13: Instrumentation and control tags disignated according to
    DIN 19227 Part 1
    Example: P D I C
    First letter (pressure)
    Supplementary letter (differential)
    1st succeeding letter (indication)
    2nd succeeding letter (control)
    instrumentation and
    control tags
    The meaning and the order of the identifying letters are listed in the following
    table.
    20
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    Group 1: Measuring or input variable Group 2: Processing
    First letter Supplementary
    letter
    Succeeding letter
    (order: I, R, C, ...any)
    A Fault message, alarm
    C Automatic control
    D Density Differential
    E Electric quantities Sensing function
    F Flow rate, troughput Ratio
    G Distance, length, position
    H Hand (manually initiated) High limit
    I Indication
    K Time
    L Level Low limit
    O Visual signal,
    yes/no indication
    P Pressure
    Q Material properties Integral, sum
    R Radiation Record or print
    S Speed, rotational speed,
    frequency
    Circuit arrangement,
    sequence control
    T Temperature Transmitter function
    U Multivariable
    V Viscosity Control valve function
    W Velocity, mass
    Y Calculating function
    Z Emergency interruption,
    safety device
    for further details,
    see DIN 19227
    The two possible methods of graphical representation are compared with
    each other in the Figs. 14 and 15. The device-related representation according
    to DIN19227 Part 2 (Fig. 15) is in general easily understood. Whereas
    instrumentation and control tags according to DIN19227 Part 1 (Fig. 14) are
    more suitable for plotting complex systems.
    21
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    VL
    RL
    TI

    2
    TI

    3 TIC

    8
    KS

    2
    TIC

    7
    TI

    4
    GOS

    6
    SOSA

    1
    5
    Fig. 14: Representation of a control loop according to DIN 19227 Part 1
    instrumentation and
    control tags
    ZLT
    ZLT
    ZLT
    tAU
    %
    T
    T
    T
    PI
    VL
    RL
    0 1
    Fig. 15: Representation of a control loop according to DIN 19227 Part 2
    device-related
    symbols
    Control Systems and Structures
    Depending on the job to be done, many different structures of control can be
    used. The main criterion of difference is the way the reference variable w is
    generated for a certain control loop. In setting the controller, it is also important
    to know whether the reference variable is principally subject to changes
    or disturbance variables need to be compensated for.
    4To attain good disturbance reaction, the controller must restore the original
    equilibrium as soon as possible (Fig. 16).
    4To attain good reference action, the controlled variable must reach a
    newly determined equilibrium fast and accurately (Fig. 17).
    22
    Fundamentals × Terminology and Symbols in Control Engineering
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    x
    z
    t
    t
    Fig. 16: Disturbance reaction
    w
    t
    x
    t
    Fig. 17: Reference action
    designed for good
    disturbance reaction
    or reference action
    Fixed set point control
    In fixed set point control, the reference variable w is set to a fixed value. Fixed
    set point controllers are used to eliminate disturbances and are therefore designed
    to show good disturbance reaction.
    The temperature control system in Fig. 18 will serve as an example for fixed
    set point control. The temperature of the medium flowing out of the tank is to
    be kept at a constant level by controlling the heating circuit. This will provide
    satisfactory results as long as high fluctuations in pressure caused by disturbances
    do not occur in the heating circuit.
    Follow-up control
    In contrast to fixed set point control, the reference variable in follow-up control
    systems does not remain constant but changes over time. Usually, the reference
    variable is predetermined by the plant operator or by external
    equipment. A reference variable that changes fast requires a control loop
    with good reference action. If, additionally, considerable disturbances need
    to be eliminated, the disturbance reaction must also be taken into account
    when designing the controller.
    23
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    Fig. 18: Temperature control by means of fixed set point control
    w
    x
    follow-up controllers
    require good
    reference action
    fixed
    reference variable
    Cascade control
    Cascade control systems require a minimum of two controllers, these are the
    master or primary and the follower or secondary controller. The characteristic
    feature of this control system is that the output variable of the master controller
    is the reference variable for the follower controller.
    Employing cascade control, the temperature control of the heat exchanger
    (Fig. 19) provides good results also when several consumers are connected
    to the heating circuit. The fluctuations in pressure and flow are compensated
    for by the secondary flow controller (w2, x2) which acts as final control element
    to be positioned by the primary temperature controller.
    In our example the outer (primary) control loop (w1, x1) must be designed to
    have good disturbance reaction, whereas the inner –secondary– control
    loop requires good reference action.
    Ratio control
    Ratio control is a special type of follow-up control and is used to maintain a
    fixed ratio between two quantities. This requires an arithmetic element (V). Its
    input variable is the measured value of the process variable 1 and its output
    variable manipulates the process variable 2 in the control loop.
    24
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    Fig. 19: Temperature control by means of cascade control
    w1=wsoll x2
    x1 w2
    q
    master and
    follower controller for
    high-quality control
    Fig. 20 illustrates a mixer in which the flow rate q2 of one material is controlled
    in proportion to the flow rate q1 of another material.
    25
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    Fig. 20: Ratio control
    V
    q2
    q1
    q2 = V q1
    w
    x
    Appendix A1:
    Additional Literature
    [1] Controllers and Control Systems
    Technical Information L102EN; SAMSON AG
    [2] DIN 19226: Control technology
    [3] DIN 19227: Graphical symbols and identifying letters for process
    control engineering
    26
    Fundamentals × Terminology and Symbols in Control Engineering
    SAMSON AG × V74/ DKE
    APPENDIX
    Figures
    Fig. 1: Operator controls the process variable p2 via remote adjuster . . 6
    Fig. 2: Operator controls the process variable p2 an a closed loop . . . 7
    Fig. 3: Root-extracting a differential pressure signal . . . . . . . . . 12
    Fig. 4: Development of a liquid level over time . . . . . . . . . . . . 13
    Fig. 5: Summing point . . . . . . . . . . . . . . . . . . . . . . . 13
    Fig. 6: Branch point . . . . . . . . . . . . . . . . . . . . . . . . 14
    Fig. 7: Block diagram of manual open loop control. . . . . . . . . . 14
    Fig. 8: Block diagram of manual closed loop control . . . . . . . . . 15
    Fig. 9: Block diagram of a control loop . . . . . . . . . . . . . . . 15
    Fig. 10: Graphical symbols for describing temperature control . . . . . 16
    Fig. 11: Graphical symbols according to DIN 19227 Part 2 . . . . . . 17
    Fig. 12: Graphical symbols (2). . . . . . . . . . . . . . . . . . . . 18
    Fig. 13: Instrumentation and control tags disignated. . . . . . . . . . 19
    Fig. 14: Representation of a control loop: DIN 19227 Part 1 . . . . . . 21
    Fig. 15: Representation of a control loop: DIN 19227 Part 2 . . . . . . 21
    Fig. 16: Disturbance reaction . . . . . . . . . . . . . . . . . . . . 22
    Fig. 17: Reference action . . . . . . . . . . . . . . . . . . . . . . 22
    Fig. 18: Temperature control by means of fixed set point control . . . . 23
    Fig. 19: Temperature control by means of cascade control . . . . . . . 24
    Fig. 20: Ratio control . . . . . . . . . . . . . . . . . . . . . . . . 25
    27
    Part 1 × L101 EN
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    FIGURES
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  2. #2
    مشرف قسم الموضوعات العامة الصورة الرمزية محمد صلاح
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    مشاركة: Terminology and Symblos in Control Engineering

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