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A piezoelectric sensor is a device that measures pressure or strain using piezoelectricity, a phenomenon in which certain materials can generate an electric current when deformed. Thus a piezoelectric sensor can convert force exerted on it by compression, impact, acceleration or vibration into an electrical signal whose strength is proportional to the strength of the force. There are a number of naturally occurring materials that demonstrate this effect, most notably quartz and tourmaline. Piezoelectric materials can also be synthesized industrially. Synthetic piezoelectric materials can be single crystal materials similar to natural crystals but with enhanced properties, such as gallium phosphate crystals; piezoceramic materials such as lead zirconate titanate (PZT), which can be manufactured cheaply by sintering, or heating powdered ingredients in a furnace; and polymers such as polyvinylidene fluoride (PVDF) that can be made into thin, flexible films.
The electrical signal generated by piezoelectric sensors falls rapidly after the force is first applied. This means that they are unsuitable for measuring a static force; however, they are excellent at measuring forces of very brief duration, such as impact forces. Single crystal and piezoceramic sensors are very rigid and have a very high natural frequency. The natural frequency of a sensor imposes limitations on its uses, as measurement errors will occur when the frequency of the vibration to be measured is close to that of the sensor. A piezoelectric sensor's high natural frequency allows it to accurately measure very high frequencies, however. This means that it can be used to detect unusual vibration patterns that might be caused by defects in materials and components.
The high rigidity of single crystal and piezoceramic sensors means they often have similar physical properties to the materials, such as steel, from which tools and machines are made. Sensors can therefore be incorporated into tools and machinery as part of their structure, so that they can be constructed with built-in sensors. This type of piezoelectric sensor is also very robust, being able to handle strong impact forces and operate effectively at high temperatures, and is used in many industrial applications for measuring pressure and strain on materials. They can also be used to measure acceleration — when an object accelerates it exerts a force in the opposite direction that can be measured by a sensor.
Piezoelectric sensors have a wide range of other applications. One common application is in microphones, where the force exerted by sound waves is converted to electrical signals and recorded or amplified. Flexible, lightweight piezoelectric materials such as PVDF can be used in touch screens and touch pads, and also have applications in scientific research. For example, tiny sensors can be attached to small animals and even insects to monitor their activity and behavior.