Control systems are used to regulate an enormous variety of machines, products, and
processes. They control quantities such as motion, temperature, heat flow, fluid flow,
fluid pressure, tension, voltage, and current. Most concepts in control theory are based
on having sensors to measure the quantity under control. In fact, control theory is
often taught assuming the availability of near-perfect feedback signals. Unfortunately,
such an assumption is often invalid. Physical sensors have shortcomings that can
degrade a control system.
