Multi-Tonal Based Impedance Measurements for Microsecond State Detection

Abstract

This paper concerns the development of a system capable of microsecond state detection via the electromechanical impedance (EMI) method utilizing a novel multi-tonal excitation approach. Structures that operate in highly dynamic environments, such as aircraft and drilling equipment, can benefit from a system capable of quickly detecting changes in the structure’s dynamic state. These changes of state can occur due to phenomenon, such as high velocity impacts, and necessitate a measurement system capable of working at millisecond to microsecond timescales. Traditionally, the electrical impedance of the PZT utilized in the EMI method is measured across a broad range of frequencies using an impedance analyzer, such as an HP 4194A; however, they are heavy, slow, and limited to a small amount of data points for each measurement. These disadvantages are overcome by using an alternative measurement system using data acquisition hardware, an auxiliary measurement circuit, and a custom coded analysis system. A key part of this measurement system is the use of a customizable excitation signal to drive the PZT. Due to the small amount of time in which a microsecond state detection system has to collect and analyze data, the excitation signal should be carefully designed to minimize measurement time while retaining accuracy. The use of conventional broadband frequency sweep excitations in a short amount of time presents challenges due to the fact that the total energy available to excite the structure becomes limited. This work investigates a novel multi-tonal excitation approach where only targeted frequency bands containing relevant structural information are excited in order to reduce the excitation time. The timing advantage of the multi-tonal signal is shown by matching the frequency dependent voltage of targeted frequency bands to that of a wideband chirp signal, which results in a 36% reduction in excitation time. The accuracy of the multi-tonal signal is also demonstrated; the impedance spectrum shows good agreement with both the wideband chirp signal and the HP 4194A. Damage detection of a structure is also presented using the multi-tonal excitation signals.