Sensors with oscillating elements

Variable frequency can be used as an information parameter in signals for measuring techniques if the measuring principle chosen favours its application, if freedom from interference is essential, transmission over long distances is necessary or easy digital conversion is desirable. The variation in frequency is in general obtained from the more or less direct influence of the quantity to be measured on an oscillator parameter. Examples with various input variables and principles of conversion are presented. With forced oscillations the phase and resonant amplitude depend on the ratio of the exciting to the natural frequency and on the damping. As examples of the utilisation of these relations, sensors for small mass increments, force and pressure, density, mass flow and viscosity are chosen.