High performance MEMS accelerometer (Gemini accelerometer)

UTC Aerospace Systems (UTAS) has developed a family of high performance open loop accelerometers, which, branded under the name "Gemini", aim to meet the needs of aerospace and commercial applications. There are five different acceleration ranges in the family: 0.85g, 2g, 10g, 30g and 96g. The sensor is manufactured and marketed by Silicon Sensing Systems Limited (SSSL). It comprises a silicon MEMS (Micro-Electro-Mechanical System) structure providing two in-plane axes of sensing fabricated using deep reactive ion etching (DRIE). The silicon MEMS is in the form of a lower glass layer, a silicon sensing layer and an upper glass layer. The glass layers are anodically bonded to the silicon layer to form a hermetic assembly. The manufacturing process and equipment used for these accelerometers has been proven over many years by Silicon Sensing Systems Limited in their MEMS gyro production. Differential capacitance sensing, provided by an asymmetric gap between two sets of interdigitated fingers with a fixed set and a moving set, is at the heart of the design. This provides the electrical output. The hermetic sealed structure is backfilled with atmospheric pressure gas giving near critical air squeeze film damping for all variants of the MEMS. This has the linearity characteristic of a differential capacitance and can be modelled to high precision using low order polynomials. The resonant frequency of the MEMS is selected to suit the operational `g' range with three different MEMS designs covering the five `g' ranges. This is packaged with a mixed signal ASIC to provide an analogue output and a digital output via a SPI bus.