Low-g Motion Sensing

The principle of acceleration sensing is simple and reliable: mass inertia according to Newton's second law is the phenomenon that gives people a sense of acceleration.

The basic elements of the accelerometer are the body, the spring and the proof mass. When the speed of the sensor body changes, the mass is forced to follow the change via the spring coupling. A force is needed to change the motion of the proof mass. Due to the force the spring is bent and the distance between the body and the proof mass changes in proportion to the body acceleration.

The operating principles of sensors differ in how the movement between the body and the mass is detected. In a capacitive sensor the body and the mass are insulated from each other and their capacitance, or charge storage capacity, is measured. As the distance decreases, the capacitance increases and electric current travels towards the sensor; when the distance increases, the opposite occurs. The sensor converts the acceleration of the body into an electric current, charge or voltage.

The superb performance of VTI´s sensors based on capacitive measurement technology makes them highly suitable for detecting small changes in motion. The acceleration sensing elements is made of single crystal silicon and glass. This gives the sensor exceptional reliability, unprecedented accuracy and outstanding stability in terms of time and temperature.

Typically a 1g element can withstand more than 50,000 g of acceleration (1 g = the acceleration caused by the Earth's gravity). The capacitive sensing element measures acceleration in both positive and negative directions and is sensitive to static acceleration as well as vibration.

The core of the VTI low-g accelerometer and inclinometer is a symmetrical, bulk-micromachined acceleration sensing element, which has two sensing capacitors. Symmetry decreases temperature dependence and cross-axis sensitivity, and improves linearity. Hermeticity has been achieved by using anodic bonding to attach the wafers to each other.

This facilitates the packaging of the element, improves reliability and enables the use of gas damping in the sensor element.