An accelerometer is a transducer that converts acceleration forces into electronic signals. Accelerometers are applied and used in a variety of devices. For example, accelerometers are often included in automobile systems for air-bag deployment and roll-over detection. Accelerometers are also used in computer devices, such as for motion-based sensing (e.g. for drop detection) and motion-based control (e.g. in gaming).
A MEMS accelerometer typically includes, among other things, a proofmass and one or more sensors for sensing movement or changes in position of the proofmass that are induced by external accelerations. A MEMS accelerometer can be configured to sense acceleration along one or more axes. Typically, the proofmass is configured in a predetermined device plane, and the axes of sensitivity are referenced relative to this device plane. For example, accelerations sensed along an axis or axes parallel to the device plane are usually referred to as X-or Y-axis accelerations, and accelerations sensed along an axis perpendicular to the device plane are usually referred to as Z-axis accelerations. A single-axis accelerometer might be configured to detect only X-or Y-axis accelerations or only Z-axis accelerations. A two-axis accelerometer might be configured to detect X-and Y-axis accelerations or configured to detect X-and Z-axis accelerations. A three-axis accelerometer might be configured to detect all three of X-, Y-, and Z-axis accelerations.
In the MEMS field, using a pressure-or force-sensitive element, such as a cantilever, in conjunction with a strain-sensing element for measuring acceleration, force, or pressure, is known in the art. Accelerations on the strain-sensing element along the relevant axis will put that element into tension or compression, thereby changing the element's cross-sectional area and the resistance to the flow of electrical current in proportion to the acceleration. The change in resistance is measured using techniques such as a Wheatstone bridge to determine the amount of acceleration.
Prior attempts to achieve high sensitivity in acceleration sensors have been plagued by susceptibility to cross-axis, i.e. out-of-plane, accelerations. Thus, there is a need for a MEMS accelerometer that is sensitive to Z-axis accelerations, i.e. accelerations sensed along an axis perpendicular to the device plane, while being insensitive to X-and Y axis accelerations, i.e. accelerations sensed along an axis or axes parallel to the device plane.