In missiles the control function is to ensure stability of the missile and implement the guidance signals received from external sources or generated onboard. The control, after processing the guidance signals, actuates the aerodynamic surfaces or thrust vector to generate turn of the missile speed and direction as required.
The guidance system is to detect whether the missile is flying above or below, to the left or right, of the required path. It obtains these deviations or errors and sends signals to the control system to reduce these errors to zero. The task of the control system therefore is to manoeuvre the missile quickly and efficiently making use of these signals.
In order to appreciate controls we shall briefly describe the motion of the missile as a free body. The missile has a total of six degrees of freedom of movement. Out of this, three degrees are translational or linear about the three axes viz., x, y and z; while the other degrees are rotational movement about three axes termed as pitch, yaw and roll.
Pitch is the turn of missile when it climbs up or down. Yaw is its turn to left or right. The roll is when the missile rotates about its longitudinal axis, which is also called roll axis. The longitudinal axis is the one running from nose to tail. If a missile is resting horizontally then, the pitch axis is the one which is normal to longitudinal axis and parallel to the horizontal axis and pitch axis. Missiles can roll when in motion due to various reasons.
There are missiles in which roll is controlled. Roll can be sensed onboard using a free gyro sensor and eliminated through actuation of controls. Some missiles have roll induced by design to use it for stability. The other axes which are controlled for motion are pitch and yaw axes.