Force and Pressure Sensors
There are several techniques to measure forces and pressures. Very often, the force to be measured is converted into a change in length or height of a piece of material, the spring element. The change in dimensions is subsequently measured by a sensor element, e.g. a (piezo-) resistive or resonant strain gauge or a changing capacitance. This is illustrated in Fig. 6.1. Sometimes the sensor element and the spring element can not be distinguished, i.e. the sensor element itself is also the spring element. For example, in piezoelectric force transducers, the deformed crystal both supports the load and supplies the output signal. More sophisticated systems incorporate an electronic feedback to balance the external force or pressure by an equal but oppositely directed counterforce or pressure. The obvious advantage of such a system is that the spring element can be omitted, thus eliminating problems like linearity, creep and hysteresis related to the spring element. However, application of such systems is limited to relatively small forces and pressures because of the limited size of the counterforce or pressure that can be exerted.
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