When testing a vehicle, you often must measure its dynamic motions and its angle relative to the road. Accelerometers let you measure the forces caused by turning, accelerating, or braking, but the turning measurements won't be accurate unless the vehicle is level relative to the earth during the turn. By using a combination of sensors, however, you can compensate for the gravity component and accurately measure angle and dynamic motion.
Most tilt sensors sense the direction of gravity as a reference direction.
In a tilt measurement, you want to measure gravitational acceleration only. When you want to measure motion acceleration, you don't want to measure any tilt component. Unfortunately, a tilt sensor will make inaccurate angle measurements when subjected to motion acceleration, and an accelerometer will make inaccurate acceleration measurements when the vehicle tilts. Fortunately, each measurement technology can compensate for weaknesses in the other. You can combine acceleration measurements with angular-rate measurements to produce accurate measurements of the vehicle's dynamic motions.