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Electro-mechanical impedance (EMI) method utilizing smart piezoelectric sensors has emerged as a promising
technology for structural health monitoring in civil, mechanical and aerospace engineering. However, two major
limiting factors have prevented field deployment of this method in real life. First, smart piezoelectric sensors, such as
Lead Zirconate Titanate (PZT) patches, are highly sensitive to environmental changes such as temperature, humidity,
and vibration. Secondly, bulky and expensive equipment is needed for performing impedance measurement. This paper
proposes a feedback-enhanced electro-mechanical impedance (FEMI) technique for improving robustness against
environmental variations and a design of a low-power EMI sensor with built-in measurement circuitries based on this
new technique. The proposed FEMI technique employs a feedback scheme to amplify the peaking characteristics of the
natural resonance frequencies in the EMI frequency response. The feedback loop includes a phase-locked loop (PLL)
and a transimpedance amplifier (TIA). An analog EMI measurement circuit is developed to replace bulky EMI
measurement instruments. To keep the power consumption low, the proposed system does not require any analog-to-digital
conversion or DSP circuit blocks, but uses a simple analog mixer to multiply input and output waveforms of the
PZT sensor, and then extract the EMI amplitude by passing the mixer output through a low-pass filter (LPF).
The performance of the proposed FEMI sensor is verified by simulations using MATLAB. Simulated natural frequency
peaks in the EMI spectrum are noticeably sharper with the feedback scheme than the one without feedback. As a result,
the natural frequency shift due to any structural change can be more easily detected. To quantify the shift of these
natural frequency peaks, the root mean square deviation (RMSD) of the difference between cases with and without
damage is calculated. The simulation results show that the RMSD with feedback is greater than the RMSD without
feedback by a factor of 3.2, when the damage is emulated by a 30% decrease in stiffness. This result confirms that the
FEMI technique with the proposed EMI measurement circuits can detect structural damage with higher sensitivity
compared to existing methods. Our future goal is to build a prototype for the FEMI sensors and integrate all the
circuitries in a single CMOS chip.
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