Design of MEMS based capacitive accelerometer

Abstract

MEMS are the manufacturing of a wide variety of items that are electronic and mechanical in nature. In addition to sensors, small motors, pumps, hydraulic systems, warhead fuses, high resolution displays, mass data storage devices are but a few of the devices that can be manufactured using MEMS technology. The characteristics of MEMS fabrication are miniaturization, multiplicity, and microelectronics. Miniaturization not only allows for small, lightweight devices, but these same devices have high resonant frequencies which mean higher operating frequencies and bandwidths for microsensors and microactuators. An accelerometer measures proper acceleration, which is the acceleration it experiences relative to freefall, and is the acceleration that is felt by people and objects. Such accelerations are popularly measured in terms of g-force. At any point in space time the equivalence principle guarantees the existence of a local inertial frame, and an accelerometer measures the acceleration relative to that frame. As a consequence an accelerometer at rest relative to the Earth's surface will indicate approximately 1 g upwards, because any point on the earth's surface is accelerating upwards relative to the local inertial frame, which would be the frame of a freely falling object at the surface. To obtain the pure acceleration due to motion with respect to the Earth, this “gravity offset” must be subtracted. This is generally true of any gravitational field, since gravity does not produce proper acceleration, and an accelerometer is not sensitive to it, and cannot measure it directly. An accelerometer behaves as a damped mass on a spring. When the accelerometer experiences acceleration, the mass is displaced to the point that the spring is able to accelerate the mass at the same rate as the casing. The displacement is then measured to give the acceleration. There are many different ways to make an accelerometer. Some accelerometers use the piezoelectric effect - they contain microscopic crystal structures that get stressed by accelerative forces, which cause a voltage to be generated. Another way to do it is by sensing changes in capacitance. Capacitive interfaces have several attractive features. In most micromachining technologies no or minimal additional processing is needed. Capacitors can operate both as sensors and actuators. They have excellent sensitivity and the transduction mechanism is intrinsically insensitive to temperature.