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2. Introduction to the Principles of Feedback

2.10 Summary

  • Control is concerned with finding technically, environmentally, and commercially feasible ways of acting on a technological system to control its outputs to desired values while ensuring a desired level of performance.
  • Fundamental to control engineering is the concept of inversion.
  • Inversion can be achieved by a feedback architecture.
  • Feedback refers to an iterative cycle of
    • quantifying the desired behavior,
    • measuring the actual values of relevant system variables by sensors,
    • inferring the actual system state from the measurements,
    • comparing the inferred state to the desired state,
    • computing a corrective action to bring the actual system to the desired state,
    • applying the corrective action to the system via actuators, and then
    • repeating the above steps.

  • The principal components in a feedback loop are shown in Figure 2.12.

    Figure 2.12: Typical feedback loop
    Typical feedback loop

  • The desired performance is typically quantified on the following bases:
    • accuracy with which the outputs should attain their desired values;
    • required tolerance level to inaccurate assumptions, disturbances and plant changes;
    • specification of transients;
    • constraints on acceleration, overshoot, energy consumption, and so forth.

  • Control system objectives usually include the following:
    • maximization of throughput, speed, yield safety, and more;
    • minimization of energy consumption, waste production, emissions, and more;
    • decreasing the impact of disturbances, noise uncertainty, time variations, and so forth.

  • The chapter gives a first indication that the desired performance objectives are usually in conflict with each other and therefore form a network of trade-offs.
  • By control-system design we mean the process of
    • understanding the trade-off network,
    • making deliberate design decisions consistent with these trade-offs, and
    • being able to systematically translate the chosen goal into a controller.