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piezo sensor as high pass filter - The Principles of...

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The Principles of Piezoelectric Accelerometers Read this and you will learn how quartz accelerometers work and how to power and instrument them. Feb 1, 2004 By: Guy Kulwanoski, Kistler Instrument Corp. , Jeff Schnellinger, Kistler Instrument Corp. Sensors The piezoelectric effect, discovered in 1880 by Pierre and Jacques Curie, remained a mere curiosity until the 1940s. The property of certain crystals to exhibit electrical charges under mechanical loading was of no practical use until very high input impedance amplifiers enabled engineers to amplify the signals produced by these crystals. In the 1950s, electrometer tubes of sufficient quality became available and the piezoelectric effect was commercialized. Most piezoelectric accelerometers are made of quartz crystal, piezoelectric ceramics, or, for high-temperature operation, tourmaline or lithium niobate. They obey Newton's second law, F = ma, in that the force acting on the measuring element is directly proportional to the acceleration produced. Available in a wide range of configurations and operating specifications, these devices are used wherever shock or vibration is of interest. The charge amplifier principle was patented by W.P. Kistler in 1950 and gained practical significance in the 1960s. The introduction of MOSFET solid-state circuitry and the development of highly insulating materials such as Teflon and Kapton greatly improved performance and propelled the use of piezoelectric sensors into virtually all areas of modern technology and industry. Piezoelectric measuring systems are active electrical systems. That is, the crystals produce an electrical output only when they experience a change in load?they cannot perform true static measurements. However, it is a misconception that piezoelectric iýstruments are suitable for only dynamic measurements. Quartz transducers, when paired with adequate signal conditioners, offer excellent quasistatic measuring capability. There are countless examples of applications where quartz-based sensors accurately and reliably measure quasistatic phenomena for minutes and even hours. There are two types of piezoelectric sensor: high and low impedance. High-impedance units have a charge output that requires a charge amplifier or external impedance converter for charge-to-voltage conversion. Low-impedance types use the same piezoelectric sensing element as high-impedance units and also incorporate a miniaturized built-in charge-to-voltage converter. They also require an external power supply coupler to energize the electronics and decouple the subsequent DC bias voltage from the output signal.
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Quartz-Based Piezoelectric Sensors Although this article focuses on accelerometers, the response function for force and pressure sensors is nearly identical. In fact, many force applications are closely related to acceleration. On the other hand, pressure sensors are designed to minimize or eliminate (by direct compensation of the charge output) the vibration effect.
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