Disturbance attenuation in single-loop dithered adaptive systems

Dithered non-linear feedback systems form one of the very few adaptive classes for which a quantitative feedback theory (QFT) exists. QFT enables a direct design to achieve given performance tolerances over specified plant uncertainty. This is due to the decoupling of the system response to the dither signals from its response to the slower and smaller (at the proper place in the loop) control signals. The system is basically linear time invariant (LTI) for the latter signals. Also, the adaptive action of the non-linearity is decoupled from the benefits of the LTI component of the feedback loop. This paper extends QFT to disturbance attenuation, for both the single-loop externally excited adaptive system (EEAS) and the self-oscillating adaptive system (SOAS). The former is more flexible than the latter, because its LTI feedback properties are almost totally independent of all other system parameters. However, both the SOAS and EEAS may be inadequate for fast disturbances at the plant output. An example with time delay is given which cannot be handled at all by an LTI design, but can be solved by the SOAS, and more easily by the EEAS. Design simulations are included showing the system's ability to cope with fast plant parameter changes.