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| | | Basics of Piezoelectric Measuring Technology | | |
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| | | Longitudinal effect The electric charge Q produced by this effect is developed on and can be collected from the surfaces to which the force is applied. Its magnitude depends only on the applied force Fx and not on the dimensions of the crystal washers. The only way to increase this charge is to connect several washers mechanically in series and electrically in parallel. The output charge is then: Qx = du • Fx • n The piezoelectric coefficient du depends on direction and is an indication of the crystal's force sensitivity along the corresponding axis. The direction in which the crystal is sliced therefore determines the properties and hence the application of the quartz force link. Piezo elements sliced to exhibit the longitudinal effect are sensitive to compression forces and are therefore the most suitable for simple, robust sensors to measure forces. | | |
| | | Unloaded crystal Crystal under load | | |
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| | | Principle of longitudinal piezoelectric effect | | |
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| | | Option for increasing charge output | | |
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| | | du : piezoelectric coefficient Fx : force in x-direction (-2,3 pC/N for quartz crystal) n : number of crystal washers | | |
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| | | Shear effect As with the longitudinal effect, the piezoelectric sensitivity involved in the shear effect is independent of the size and shape of the piezo element. The charge also develops on the loaded surfaces of the element in this case. With a load in the x-direction applied to n elements connected mechanically in series and electrically in parallel, the charge is: | | |
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| | | Shear-sensitive piezo elements are used for sensors measuring shear force, torque, strain and acceleration. They are suitable for manufacturing sensors whose excellent performance is unimpaired by temperature fluctuations, as the changes in the stresses in the sensor structure caused by these fluctuations act normal to the sensitive shear axis. | | |
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