A sensing and actuating transducer for measuring point impedance to moment

In dynamic testing of structures, rotational frequency response functions have been less utilized than its linear counterparts because no suitable moment generators and sensors for measuring moments and rotational motions are available. In this paper, a method and a new transducer for measuring point rotational impedance of structure is reported. The transducer contains two PZT bimorph cantilevers symmetrically attached to a solid base which is affixed to the measuring point of test structure. When the bimorph cantilevers are excited to vibrate out of phase with ac power, the transducer exerts a sinusoidal moment to the structure. To let the transducer measure rotational impedance simultaneously, a transducer model called “transduction matrix” is exploited. The matrix quantitatively relates the input voltage and current and the output moment and rotational velocity. Once the matrix is identified via tests and/or numerical simulation, the mechanical impedance at the output port can be evaluated from the electrical impedance measured at the input port of the transducer. To validate, the rotational impedance at a point of a beam was detected by the transducer and calculated by a finite element model. Comparisons of the results show that the proposed transducer fulfills the aimed measurement accurately.