An optimal multivariable proportional-integral (PI) controller with pr
eassigned eigen-spectrum is designed so that the system output would t
rack constant reference inputs in spite of constant but unknown plant
disturbances. A sequential strategy for modal servomechanism design is
given for the above problem. In addition, since the proposed PI contr
oller is of state feedback type, an observer capable of estimating the
unmeasurable state variables in spite of the presence of unknown dist
urbances is introduced, thus resulting in a suboptimal overall system.
The separation property for the overall servo/observer system is illu
strated. The design strategy is successfully used for position control
of a brushless DC motor, and in a regulator mode for controlling a he
adbox which is an integral part of the modern paper machines used in t
he pulp and paper industry.