Figure 1 illustrates the basic components of a vibrating wire sensor. A tensioned steel wire, hermetically sealed, is coupled with an electromagnetic coil. The wire is “plucked, by applying momentary electrical pulses to the coil, inducing vibrations in the wire. The wire will settle to it’s resonant vibrating frequency, and these vibrations in turn induce current back onto the signal wires. This signal can then be measured and used to determine the tension in the wire, and therefore also the strain in the structure to which the vibrating wire gage is attached.
Figure 1. Basic Operation of Vibrating Wire Sensors
More specifically, the resonant frequency of the tensioned wire is given by:
where LW is the length of the wire, T is the tension in the wire, and m is the mass of the wire per unit length. Since the tension T of the vibrating wire is directly proportional to the strain in the wire, and to the strain in the attached structure, the measured strain can be calculated as
where k is the gage factor, f is the measured frequency, and f0 is the initial frequency.
Temperature also affects the frequency of the vibrating wire and can lead to errors if not compensated. Therefore, most vibrating wire sensors include a thermistor embedded within the sensor packaging for this purpose.