1. Piezoelectric Accelerometers Based on an Asymmetrically Gapped
Cantilever for heart sound and lung sound monitoring
Yating Hu, Qinglong Zheng, and Yong Xu
Department of Electrical and Computer Engineering, Wayne State University, Detroit, MI, USA
1. Background 3. Prototypes and test
The cardio-respiratory signal is the most fundamental vital sign to A. Characteristic of the prototype accelerometres
assess a person’s health. The typical methods to acquire these signals
include electrocardiogram (ECG) and stethoscope. Although these
methods represent the current standard, they remain obtrusive, and are
cumbersome for continuous and ambulatory monitoring. In order to
derive both cardiac and respiratory signal in a simple manner with a
non-obstrusive, highly portable device, we developed a miniature chest-
worn accelerometer with high sensitivity. By integrating the wireless
readout system, the signal detected by the millimeter scale sensor could
Aluminum proof mass
be transferred to investigate on the PC or even smart phone as
visualized waveforms or sound.
Charge amplifier
Fig. 5 frequency response of
18 mm
PZT
the prototype accelerometer
Fig. 4 Inside view of the prototype
accelerometer
Fig. 6 Noise spectrum of the
prototype accelerometer
2. Design Principle B. Testing and results
Preliminarily tests for recording both cardiac and respiratory signal are
The accelerometer is based on an air-spaced cantilever which is
carried out on human body. The data from the sensor is transferred to a
composed of a bottom mechanical layer and a top piezoelectric layer
PC through DAQ board and further process in Labview and matlab
separated by a gap. This novel cantilever structure helps to increase
program. The sampling rate is 20kHz. A filter with a bandwidth from 20 to
the sensitivity by orders of magnitude. From the energy point of view, it
500Hz is applied to extract the cardiac signal while a filter with bandwidth
enables the majority of mechanical energy to be effectively used to
from 350 to 1900Hz is applied to exact the respiratory signal.
strain the piezoelectric layer. The overall energy conversion efficiency
is over 90% for air-spaced cantilevers and only below 39% for
conventional cantilevers. Inhale Exhale
First heart
sound
Second
heart sound
Hold breath
Fig. 7 A sample waveform of respiratory Fig. 8 A sample waveform of heart
sound. The inhale and exhale breathing sound. The first and second heart sound
cycle as well as the “no breath” period timing and strength are distinguished.
are distinguished.
Second
heart sound
Majority energy
First heart
sound
Fig. 9. The corresponding frequency analysis of the heart sound in Fig.8 by
short Fourier transform. The majority of heart sound energy is concentrated
under 500Hz.
REFERENCES
(a) (b) (c) [1] Yuefa Li, Qinglong Zheng, Yating Hu, and Yong Xu "Micromachined
Piezoresistive Accelerometers Based on an Asymmetrically Gapped
Fig. 3. (a) Photograph of a fabricated device. (b) SEM image of one fabricated
Cantilever" Journal of microelectromechanical systems, vol. 20, pp83-94
accelerometer based on an asymmetrically gapped cantilever. (c) Magnified
view of one free-standing top piezoelectric beam. Feb 2011
ACKNOWLEDGEMENT
This project is partially supported by MSGC (Michigan Space Grant
Consortium) and NSF awards ECCS-747620.