Analysis of the function of an accelerometer as feedback instrument within a lateral autopilot

Autopilot is a minor loop inside the main guidance loop in terms of the guidance and control system for a missile. Lateral autopilot is used for either controlling the motion in the pitch or yaw planes. The paper focuses on a lateral autopilot with an arrangement of a rate gyro and an accelerometer in feedback paths, while the accelerometer mounted away from the center of gravity of the missile. Based on a selected example of a missile from a text book, the paper presents mathematical model and parameters for the missile in pitch plane, gives the design results of a lateral autopilot, and shows the step responses of the designed lateral autopilot at three different velocities in the time domain. By deriving the signal the accelerometer measured and the whole feedback, it is showed that the accelerometer which is mounted at a distance away from the center of gravity not only provides lateral acceleration feedback, but also gives the first and second derivatives of the lateral acceleration feedbacks. Further analysis shows that different mounting location of the accelerometer will not change the open loop gain of the lateral autopilot, but will change the phase angle of the open loop of the lateral autopilot. And the Bode diagram of the feedback path with accelerometer mounting place changed shows that for an lateral autopilot which usually has an open loop frequency between 20rad/s and 100rad/s, the mounting place of the accelerometer needs to be carefully designed.