and vary between extremes. They also offer good noise performance, low noise performance and a high sensitivity - all achieved within a compact package. In simple terms, the MEMS capacitive sensor
makes use of a single moveable mass, springs and fixed reference silicon substrates or electrodes. Movement of the mass relative to the fixed electrodes causes a change in capacitance. Because acceleration is related to change in capacitance of a moving mass, these capacitance values can be used to derive the mass’ displacement and its direction. These accelerometers may be structured as
either a single-sided or a differential pair. The single-sided is simpler in construction but provides an undesirable nonlinear output. To illustrate, let us consider one mass and a single reference plate. As the mass moves, the capitative value
changes with respect to the single reference plate. This can be an incredibly small value, which can be a challenge for systems to register. Adding a second reference plate on the
other side of the mass - known as a sandwich- type construction - means that when the mass moves, we can obtain two capacitance values insteadofjustone. The additional measurements help to linearise
the data, improving accelerometer precision as well as facilitating system calibration. Using more electrodes in parallel allows for greater capacitance changes, which again helps to boost sensor accuracy and makes this method of measurement more feasible. The analogue mass voltage will then go
through charge amplification, signal conditioning and demodulation before conversion into a digital domain via an analogue-to-digital converter (ADC).
OPTIMISING PERFORMANCE Fault signatures with a small amplitude can be hard for systems to register. It is therefore essential that the sensor can also operate with low noise and with sufficient resolution to isolate the low amplitude signals associated with these faults. Achieving this is possible with bespoke electronics. Standard commercially available ICs maybeabletoperform the required signal conditioning and conversion of analogue voltage signals into digital ones that be communicated with external equipment. But for a more optimised solution, it may be preferable
to opt for an Application Specific IC, or ASIC. An ASIC is a bespoke chip designed specifically to meet its application to offer a superior overall solution. Freedom in design means that the ASIC can be designed to interface directly with the accelerometer sensor to provide sensor-specific conditioning, fine- tuned to meet the demands and requirements of the application. An ASIC also comes with the benefit of obsolescence protection. Many defence and industrial applications require products with extended lifetimes and a guaranteed supply in times of need. Where standard ICs may be removed from production at any time and a manufacturer left stuck with a limited stock of LTB chips, a reputable ASIC designer will in contrast provide a full non-obsolescence plan. This includes choosing a suitable silicon process with the required longevity, as well as giving ample notice for a solution to be obtained depending on the manufacturer’s needs and product lifetime. The applications of accelerometer and gyroscope technology reach far beyond our consumer devices. Yet whatever their final application, it is crucial that they operate with precision every time.With a bespoke IC at their core, it is possible to optimise these micro sensors down to the finest level for a performance superior to off the shelf alternatives.
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