Use of velocity transducers in low-frequency modal testing of structures

One challenge in performing modal testing of structures at low frequencies is obtaining deflection sensors that provide reasonable output in this frequency range. Deflection sensors that require a fixed datum are generally not practical on full-scale structures. Sensors that use an inertial reference such as accelerometers and geophones are necessary. Velocity transducers are typically used at frequencies above their resonant frequency where their amplitude and phase response is flat. They can be used at frequencies near and below their resonant frequency. Thought, in this region, their response is frequency dependent. The advantage of using velocity transducers at low frequencies is that velocity is equal to displacement times frequency, the output of a velocity transducers falls off more gradually with decreasing frequency than the output of an accelerometer. Using velocity transducers at low frequencies requires characterization of the low frequency response of the transducer. A velocity transducer's response can be modeled accurately as a single- degree-of-freedom, damped resonator. Laboratory methods of calibrating velocity transducers to determine the coefficients that describe their behavior are presented. Methods of correcting the amplitude and phase measurements to account for the transducers response are demonstrated. These procedures are illustrated on modal testing data from highway bridges.