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Feedback is used to decrease the sensitivity of a system to plant uncertainty or to disturbances. This thesis is focused on the reduction of sensitivity to additive disturbances applied at the output. Systems are represented by linear multivariable frequency responses whose inputs and outputs belong to / (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI) / Rigorous conditions under which feedback can reduce sensitivity are derived. / It is shown that given a minimum phase single input-single output plant, there exists a feedback compensator which reduces sensitivity below any arbitrarily positive value on any finite frequency interval while not exceeding a specified upper bound in the right half complex plane. / It is also shown that given a multivariable invertible plant which approaches diagonal dominance at high frequencies, it is possible to build a diagonal feedback compensator to reduce the sensitivity below any arbitrarily value on any finite frequency interval while not exceeding a specified upper bound in the right half complex plane. / A relationship between sensitivity reduction and decentralized control is established. It is shown that reducing sensitivity to additive disturbances at the outputs is in essence the same as achieving local control of a multivariable system.

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Display/control gain is the amount of movement that occurs on a display in response to a unit amount of movement on the control. Two studies were conducted to determine the adequacy of identifying the optimum gain for an interface as a method of control-display interface optimization. The first study examined the effects of changes in both the maximum control input and the display width on target acquisition performance with a touch tablet and a trackball. The hypothesis that an interaction between the control input and the display output would determine performance was not supported for either device. There was a main effect of the control input for the touch tablet, and significant effects of the control input and the display width for the trackball. The results also indicate that, at least for the touch tablet, gain is not a sufficient specification for performance. The second study evaluated the effects of changes in the display amplitude, the display target width, and the control amplitude. There were significant interactions among these three factors for both touch tablet and trackball target acquisition performance. These results extend the findings of the first study with respect to the inability of gain to predict performance. In addition, the inadequacy of Fitts' Law as it applies to the given interfaces is discussed. / Ph. D. / incomplete_metadata