Random linear systems: A special case

IN CONTROL SYSTEMS, elements are frequently used whose ¿gain¿ is a function of variables outside of the primary control loop. For example, the gain of an armature controlled servomotor depends on the motor field current. The gain of a hydraulic valve (with respect to flow rate as output and valve displacement as input) depends on the static head on the valve. The effectiveness of a control surface in turning a body moving in a turbulent medium varies with time: The gain from control surface deflection to turning torque may be assumed to vary randomly with time.