Design of SAW sensor for longitudinal strain measurement
with improved sensitivity
Received: 19 January 2018 / Accepted: 18 May 2018
Ó Springer-Verlag GmbH Germany, part of Springer Nature 2018
This paper presents the design of a highly sensitive surface acoustic wave (SAW)-based sensor with novel structure for the
longitudinal strain measurement. The sensor utilizes thin lithium niobate (LiNbO
) diaphragm as the sensing element rather
than the bulk substrate. The application of the diaphragm effectively decreases the cross-sectional area of the strain
sensitive element, and meanwhile reduces the resistance between the sensor and the specimen. The newly designed strain
sensor is to operate around a frequency of 50 MHz. The insertion loss of - 12 dB and quality factor of 63 are obtained
analytically from impulse-response model. The sensor performance with tensile testing of the steel beam is predicted by the
ﬁnite element method. The prestressed eigenfrequency analysis is conducted with the COMSOL commercial software. The
simulation shows the resonance frequency of the sensor shifts linearly with the strain induced in the testing beam. For the
SAW sensor with traditional conﬁguration applying 1 mm thick substrate, the strain sensitivity is obtained as 0.41 ppm/le.
For the sensor with the novel design employing thin diaphragm with the thickness of 200 lm, the strain sensitivity is
increased to 0.83 ppm/le. With the availability of the bulk micromachining of LiNbO
, the application of the piezoelectric
diaphragm as sensing element in SAW strain sensor can be an alternative way to enhance the sensor sensitivity.
The efﬁcient management of structure safety relies heavily
on the strain sensors to gather relevant information about
the structure aging and deterioration. Strain sensors with
high sensitivity are therefore increasingly demanded in
giant infrastructures, such as tunnels, bridges, highways
and buildings, to prevent disasters. The technology of SAW
resonator offers a promising sensor option due to its
advantages of low power consumption, moderate low cost
and high performance (Drafts 2001). Since the resonance
frequency of the SAW can be modiﬁed by the changes in
the piezoelectric surface length as a result of strain, the
sensor can be designed with improved sensitivity.
Konno et al. (2007) developed a highly sensitive SAW
strain sensor with a resonant frequency of 40 MHz by
micromachining of AT-quartz crystal. Bao et al. (2015)
reported a strain sensor by employing SAW on layered
structure deposited on silicon substrate.
The sensor resonated at 107.8 MHz with a strain sensitivity
of 0.99 ppm/le. It is noted that LiNbO
substrate has been
widely applied to maximize the device efﬁciency for its
high electro-mechanical coupling coefﬁcient. Konno et al.
(2013) developed a SAW strain sensor resonating at
384 MHz based on 128°YX LiNbO
. The coupling coef-
ﬁcient was obtained as high as 7.63%, and the strain sen-
sitivity is evaluated around 0.184 ppm/le with tensile test.
Moreover, by using a 500 lm thick 128° YX LiNbO
substrate, Fischerauer et al. (2013) applied 99.8 MHz delay
line achieving a sensitivity of 0.35 ppm/le, while Fu et al.
(2014) applied 151 MHz delay line achieving a sensitivity
of 0.85 ppm/le. Similarly, Humphries and Malocha (2015)
presented a passive and wireless SAW strain sensor by
implementing SAW delay line on a 500 lm thick YZ
substrate. By operating at a frequency as high as
915 MHz, an ultra high sensitivity of - 1.80 ppm/le was
The sensitivity to strain for a SAW sensor can further be
improved by the structural modiﬁcation of sensing element.
Previous devices commonly used the full thickness of
& Xueyong Wei
School of Construction Machinery, Chang’an Univeristy,
Middle Nan’er Huan Road, Xi’an 710064, Shaanxi, China
State Key Laboratory for Manufacturing Systems
Engineering, Xi’an Jiaotong University, No. 28 Xianning
West Road, Xi’an 710049, Shaanxi, China