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QFT Frequency Domain Control Design Toolbox - Background

In the 1960s, as a continuation of the pioneering work o f H.W. Bode, Isaac Horowitz introduced a frequency domain design methodology that was refined in the 1970s to its present form. This methodology is now commonly referred to as Quantitative Feedback Theory.

One of the main objectives of QFT is to design a simple, lower order controller with minimum bandwidth that satisfies performance specifications in the presence of uncertainties (plant changes and/or external disturbances). Minimum bandwidth controllers reduce problems with noise amplification, resonances, and unmodeled high frequency dynamics. QFT offers direct insight into the tradeoffs between controller complexity and specifications.

In general, QFT can be considered a natural extension of classical frequency domain design approaches. It relies on the observation that feedback is needed principally when the plant is uncertain and/or when there are uncertain inputs (disturbances) acting on the plant.

The QFT design methodology is characterized by certain key features:

  • The amount of feedback is tuned to the amount of plant and disturbance uncertainty, and to performance specifications.
  • Design tradeoffs at each frequency are highly transparent between stability, performance, plant uncertainty, disturbance level, controller complexity, and controller bandwidth.
  • The method extends classical frequency domain loop-shaping concepts in order to cope with simultaneous specifications and plant uncertainties.