The theoretical basis is Newton's second law: according to the basic physical principles, within a system, the speed is not measurable, but its acceleration can be measured. If the initial velocity is known, the linear velocity can be calculated by integration, and then the linear displacement can be calculated, so it is actually an acceleration sensor using the principle of inertia.
When the tilt sensor is stationary, that is, there is no acceleration in the side and vertical directions, then only the gravitational acceleration acts on it. The angle between the vertical axis of gravity and the sensitive axis of the acceleration sensor is the angle of inclination. In general, the tilt sensor is a static measurement or a quasi-static measurement. Once there is external acceleration, the acceleration measured by the acceleration chip contains the external acceleration, so the calculated angle is not accurate. Therefore, the common practice now is to increase the mems. Gyro chip, and uses the preferred Kalman filter algorithm. Acceleration 3 axes, gyroscope 3 axes, all products here are also called 6-axis or VG (vertical gyro).