• 1.2 Block Diagrams

    Dr James E. Pickering

    Learning Outcomes

    By the end of this, learners will be able to:

    1. Identify and describe the fundamental elements of a feedback control system using block diagrams.
    2. Explain the function and interdependence of system elements in determining behaviour.
    3. Differentiate between open-loop and closed-loop systems using block diagrams.
    4. Relate theoretical configurations to real-world engineering examples.

    1.2.1 Introduction to Block Diagrams

    Block diagrams are the standard method for illustrating closed-loop control systems in engineering industry, as they clearly show the system configuration and signal flow.

    Open-loop and closed-loop control systems are often represented using block diagrams (see below), which provide a clear visual structure of how signals and components interact. In these diagrams:

    • Blocks represent the main components of the system.
    • Lines and arrows show the signals and their direction of flow.
    • Summing junctions indicate where signals are added or subtracted.

  • 1.2.2 Open-Loop Systems

    The open-loop system block diagram is given in (a) - refer to this throughtout this Section.

    Based on this representation, an open-loop system consists of:

    • Input: A command signal (electrical, mechanical, or thermal) that drives the system.
    • Actuator: A device that converts the input signal into physical action, such as movement, force, or position (e.g., motor, valve, hydraulic piston).
    • Process (plant): The part of the system that changes due to the actuator’s action, with this producing the output.

    Negatives of Open-Loop Systems

    • No feedback link between input and output, so the desired control is often not achieved, leading to persistent errors.
    • Cannot compensate for disturbances (e.g., a window opening in a heated room when the heater supplies constant power).
    • Lacks robustness to variations such as system changes, ageing, or component wear.

    Examples

    • Examples include toaster, washing machine and sprinkler system.hine.

  • 1.2.3 Closed-Loop Control Systems

    The closed-loop control system block diagram is given in (b) - refer to this throughtout this Section.

    As well as the open-loop system format, to form a closed-loop control system, this consists of:

    • Measurement (use of Sensor/Transducer): A device that is configured to provide a measurement of the system output (e.g., speed sensor, temperature probe)
    • Feedback: The signal from the measurement that is returned for comparison with the reference.
    • Reference: The desired output of the system (e.g., vehicle speed, room temperature).
    • Error: The signal formed by subtracting the feedback (measured output) from the reference.
    • Controller: The 'decision-making' unit that processes the error and generates a control output signal/command.
    • Actuator: A device that executes the controller’s output command by applying physical action to the process (e.g., motor, valve, hydraulic piston).
    • Process (Plant): The physical process being controlled, which responds to the actuator’s action and produces the output.
    • Dotted arror: represents a physical connection between the measurment unit and system, but this is not a signal, i.e., the measurement unit is located within the system

    Benefits of Closed-Loop control Systems

    • When properly configured (although more complex to do so!), the system output closely follows the reference (desired output).
    • More robust to disturbances and uncertainties, e.g., a vehicle maintaining speed on changing gradients.
    • Essential for applications requiring high precision, reliability, and safety.

    Examples

    • Examples include aircraft autopilot, vehicle cruise control, CNC machines and air conditioning.
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