-
Research ArticleA Wireless Pressure Microsensor Fabricated
inHTCC Technology for Dynamic Pressure Monitoring inHarsh
Environments
Ronghui Gao,1,2 Yingping Hong,1,2 Huixin Zhang,1,2 Wenyi Liu,1,2
Ting Liang,1,2
Wendong Zhang,1,2 and Jijun Xiong1,2
1Key Laboratory of Instrumentation Science & Dynamic
Measurement, Ministry of Education, North University of China,Tai
Yuan 030051, China2Science and Technology on Electronic Test &
Measurement Laboratory, North University of China, Tai Yuan 030051,
China
Correspondence should be addressed to Jijun Xiong;
[email protected]
Received 25 August 2014; Revised 7 December 2014; Accepted 17
December 2014
Academic Editor: Gour C. Karmakar
Copyright © 2015 Ronghui Gao et al. This is an open access
article distributed under the Creative Commons Attribution
License,which permits unrestricted use, distribution, and
reproduction in any medium, provided the original work is properly
cited.
The partially stabilized zirconia (PSZ) ceramic has wide
applications due to its excellent mechanical toughness and
chemicallyinert and electrical properties for fabricating various
devices. In this paper, a novel high temperature pressure sensor
with thePSZ was designed and fabricated. The sensor was designed
based on the small deflection theory, which enables its
theoreticpressure-capacitance capability up to 60 bar. HTCC process
technology was used to fabricate the sensor, which would realize
acompletely passive LC resonant circuit integrated on the ceramic
substrate. According to the coupling principle, noncontact
testingis achieved using the designed readout system, with average
sensitivity up to 38 kHz Bar−1 presented. Compared to the
fabricationandmeasurement of traditional sensors, excellent
packaging process is demonstrated, and the sensor can be completely
tested from0 to 60 bar.
1. Introduction
Instantaneous precise measurement pushes the limit of thedynamic
testing technology continuously. Until now, thetest technology
referring to the key parameters under harshenvironment is still
unreachable [1]; the wireless pressuremeasurement especially under
high temperatures has becomeincreasingly critical in automotive,
aerospace, and industrialapplications [2–4]. The conventional
pressure sensors basedon silicon material have been used widely in
different occa-sions. However, they would face great challenges in
hightemperatures for the intrinsic limits of the material.
Othermaterials, such as silicon nitride and silicon carbide,
despitehaving excellent robustness, are rarely used for
standardfabrication because their process is not so mature
comparedto silicon. Seen as a new-typematerial, the partially
stabilizedzirconia ceramic (PSZ) has very high fracture toughness
andhas one of the most highest maximum service
temperatures(0–1850∘C) among all of the ceramics. It would also
keep
mechanical properties when the temperature is close to
itsmelting point (2500∘C), which makes great sense for fabri-cation
of pressure-sensitive devices under high temperature.
Noncontact wireless passive telemetric sensing is one ofthe
methods using frequency for continuous and reliablepressure
measurements. The sensing methodology requiresan external reader to
interrogate environment pressure vari-ations electrically
registered by an implanted sensor througha wireless inductive
coupling link. The concept, shown inFigure 1, was first proved in
1967 using a sensor with resonantcircuitry implanted to the
anterior chamber of the eye [5],which then was further researched
in the development ofvarious pressure microsensors [6–8]. The model
enablesstraightforward pressure sensing by utilizing an
implantedsensor that records pressure variations in
high-temperatureenvironments, so that the pressure can be directly
measuredby using an external reader wirelessly interrogating
theimplant. In the last decades, due to the development
ofmicromachining technology,microelectromechanical system
Hindawi Publishing CorporationInternational Journal of
Distributed Sensor NetworksVolume 2015, Article ID 974742, 11
pageshttp://dx.doi.org/10.1155/2015/974742
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2 International Journal of Distributed Sensor Networks
Impedance analysis
Sensor (inside the engine)
External reader antenna(outside the engine)
Inductive coupling
−−
Zeq
V1LsLr
RsC0
++
Cs
Lm
Figure 1: Conceptual schematic of the passive wireless pressure
sensing.
(MEMS) sensors made of silicon are the major devicesused in
detecting pressure [9]. However, silicon sensorswith PN-junctions
exhibit a restriction; that is, they cannotbe used above 150∘C,
since the leakage current across thejunctions drastically increases
above 150∘C [10]. In addi-tion, the mechanical properties of
silicon will deteriorateas the material becomes easily deformable
when pressure isapplied above 500∘C [11]. Using SOI material can
increasethe operation temperature of sensor, but sensor
becomesinvalid because the silicon material will lose elasticity
at500∘C [12]. Sensors demonstrated in the literature havecertain
deficiencies: the Georgia Institute of Technology hasdesigned a
wireless high temperature pressure sensor usinglow temperature
cofired ceramic (LTCC) material. However,the sensor is only tested
up to 450∘C [13–16]. Anotherteam in Novi Sad (Serbia) demonstrated
a better structure,but worse performance [17–20]. Recently,
Professor Xiong’steam has demonstrated a greater sensitivity and a
betterperformance, but the measuring range was not up to 50 barand
the operating temperature was only up to 500∘C [21,22]. However,
the coupling distance and performance of thesensors designed before
do not operate so efficiently and thepressure tests for these
elements have not been taken undercomposite high-temperature
environment.
These facts seriously hinder its application especiallyunder
extraordinaryhigh-temperature environments (>800∘C).Thus, we
need to further improve the full-scale range and theoperating
temperature of the sensor.
In this paper, we developed a microfabricated wirelesspassive
pressure sensor with HTCC technology [13] to realizea better
applicability and feasibility of the sensor and morestable pressure
data in real time under harsh environmentssuch as high-temperature
environment. The working princi-ple is described as follows: the
external pressure would causevariation of capacitor, so we can
obtain the pressure valuefrom the frequency as the LC resonant
frequently changes. Areadout system is designed to extract the
pressure informa-tion contained in the sensor’s resonant frequency.
Differentfrom the previous sensor, a new material is applied for
theproposed sensor, and an embedded structure design andfabrication
process are studied. Measurement and character-ization analysis in
high-pressure environment (0–60 bar) are
achieved. Featuring zirconia ceramic as the high
temperatureresistant material, the pressure sensor is designed to
be com-pletely implantable in the high-temperature environment
sothat long-term pressure monitoring inside the engine can
befulfilled.
2. Design
2.1. Principle of Measurement. The concept of the
wirelesspressure sensing system in the passive electrical
sensingscheme is shown in Figure 1; the implanted sensor can
recordreliable pressure variations using corresponding
electricalcharacteristic changes, which are measured from the
externalreader through a wireless inductive coupling link. The
asso-ciated pressure monitoring method as shown in Figure 2
isproposed for continuous monitoring in harsh environments.
The sensor implant is designed to have an electrical LCresonant
circuit with a corresponding resonant frequencyrepresented as
𝑓𝑠
=
1
2𝜋
√1
𝐿𝑠
𝐶𝑠
−
𝑅2
𝑠
𝐿2
𝑠
≅
1
2𝜋√𝐿𝑠
𝐶𝑠
(1)
if
𝑅2
≪
𝐿
𝐶
, (2)
where 𝐿𝑠
, 𝐶𝑠
, and 𝑅𝑠
are the inductance, capacitance, andresistance of the sensor,
respectively. By using an externalreader antenna to build an
inductive coupling link with theimplanted sensor, the equivalent
input impedance viewedfrom the reader antenna side can be derived
using circuitanalysis as
𝑍eq = 𝑅𝑟 + 𝑗2𝜋𝑓𝐿𝑟
× (1 − (
𝑓𝑟
𝑓
)
2
+
𝑘2
(𝑓/𝑓𝑠
)2
1 + (𝑗/𝑄𝑠
) (𝑓/𝑓𝑠
) − (𝑓/𝑓𝑠
)2
) ,
(3)
where 𝑅𝑟
and 𝑓𝑟
are the resistance and the self-resonancefrequency of the reader
antenna, respectively, 𝑓 is the excita-tion frequency, 𝑄
𝑠
= 𝑅−1
𝑠
(𝐿𝑠
𝐶𝑠
− 1)1/2 is the quality factor of
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International Journal of Distributed Sensor Networks 3
Insert
ing
PCB test antenna
Sensor
Waterproof device
Signal collecting
Signal output
unit
Computer
3 cm
Readout
Figure 2: Integrated reader antenna and the sensor on the
waterproof device in harsh environments.
the sensor at resonance, 𝑘 represents the coupling coefficientof
the inductive link (totally dependent on the physicalgeometries
planar size of the sensor and the reader antennacoils and the
separation distance between the inductancecoils) [16, 17].
Therefore, from (3), an input impedance-risetechnique can be
applied to wirelessly detect the resonantfrequency of the sensor.
When the sensor is excited atresonance, the real part of the
equivalent input impedance𝑍eq becomes
max𝑓
Re {𝑍eq} ≈ Re {𝑍eq}𝑓=𝑓
𝑠
= 𝑅𝑟
+ 2𝜋𝑓𝑠
𝐿𝑟
𝑘2
𝑄𝑠
. (4)
The real part-rise magnitude determines the signalstrength in
the wireless sensing system and is dependenton 𝑘 and 𝑄
𝑠
. Because of such relation between the inputimpedance’s real
part and the resonance frequency of thesensor, the latter can be
identified if one can detect areal part-rise in the frequency scan
of the equivalent inputimpedance. As long as the real part-rise is
detectable inthe frequency scan, the resonant frequency of the
sensorcan be accurately characterized. As a result, if the
sensorhas pressure-sensitive electrical components, its
resonantfrequency will be changed because of external pressure
vari-ation; thus the environmental pressure would be recorded.This
change can be interrogated using the external readerantenna so that
continuous wireless pressure monitoring canbe accomplished. As a
result, by finding the updated real partof 𝑍eq change, the
frequency shift can be obtained, and thecorresponding pressure
change can be analyzed.The involvedelectrical characteristic change
can be interrogated usingthe external reader antenna coil to
accomplish continuouswireless pressure monitoring.
In case of high-temperature environment, it is demon-strated
that the sensor’s 𝑄 value which related to the changesof
temperature limits the operating temperature at 400∘C[13, 14]. The
value of mutation point of impedance’s real partat peaks changes
less obviously as the𝑄 value varies; even theresonant
characteristics for extracting the resonant frequency
20.8 20.85 20.9 20.95 21 21.05 21.1 21.15 21.2 21.25 21.31
2
3
4
5
6
7
8
Frequency (MHz)
Q = 270.17
Q = 135.08
Q = 90.06
Q = 67.54
Q = 54.03
Q = 45.03
Q = 38.09
Q = 33.77
Q = 30.02Q = 27.02
Q = 24.56Q = 22.51Q = 20.78Q = 19.30
Q = 18.01
|Z𝜃(f
)|(kΩ
)
Figure 3: The real part-rise magnitude of antenna coil from
inputimpedance for the quality factor variation.
disappear. The real part-rise magnitude of antenna coil
frominput impedance for the quality factor variation is shownin
Figure 3. The main reason is that the relative dielectricconstant
and loss of inductance coil from ceramic substrateof sensing
elements become different as temperature changes.The
characteristics disappear when the value of 𝑄 reduces,in other
word, the coupling energy cost totally at the activepower rather
than reactive power which beneficial for detect-ing.
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4 International Journal of Distributed Sensor Networks
0
10
20
30
40
50
60
Frequency (MHz)
|Z𝜃(f
)|(kΩ
)
20.5 20.6 20.7 20.8 20.9 21 21.1 21.2 21.3 21.4 21.5
k1 = 0.1k1 = 0.2k1 = 0.3
k1 = 0.4
k1 = 0.5
k1 = 0.6
k1 = 0.7
k1 = 0.8
k1 = 0.9
Figure 4:The impedancemagnitude of antenna coil for the
couplingcoefficients variation.
Another key parameter which affects the extraction ofresonance
frequency feature is the coupling factor 𝐾, whichis related to
geometry size, coupling distance, and magneticpermeability of
inductance coil of antenna and passive sensorranging from0 to 1.The
impedancemagnitude of antenna coilfor the coupling coefficients
variation is shown in Figure 4.In the actual detection system, the
coupling factor𝐾 reducesunder 0.3 when coupling distance is far
away or the sizeof the inductance coil is very small; the mutual
inductancecoupling characteristics of the remote sensor cannot
bedetected at reading antenna (even including the high preci-sion
impedance analysis instrument).Therefore, the couplingfactor 𝐾 of
readout testing system proposed in this work ischosen at 0.3∼1;
however the ideal case 𝐾 = 1 cannot berealized since there always
exists a certain coupling distancebetween the sensor and readout
antenna inductance coil.
2.2. Sensor Circuit Structure Parameters. Figure 5 shows
thedesign of the microsensor comprising a
pressure-sensitiveparallel-plate variable capacitor embedded in a
deformablediaphragm chamber, a spiral metal wire serving as
planarinductance, and a flexible ceramic sensitive membrane
withelectrodes on as the capacitor.
The design particularly features a larger square substrateto
incorporate a larger planar spiral inductance. From theengineering
aspect, the electrical characteristics of the sensorcan be
determined by using the established models [22],where the
electrical inductance of such a circular spiral coilcan be
calculated as
𝐿𝑠 = 2.34𝑢0
𝑛2
𝑑avg
1 + 2.75𝐹
, (5)
Spiral inductor
Pressure-sensitive variable capacitor
Via
Capacitance plate
Ceramicsubstrate
Sealed cavityd0
Figure 5: Pressure sensor designing schematics with
zirconiaceramic substrate.
where 𝑛 is the number of turns of the inductor coil,
𝑑avgindicates the averaged diameter of the coil windings, (𝑑avg
=((𝑑in + 𝑑out)/2)), and 𝐹 is the fill ratio defined as 𝐹 = (𝑑out
−𝑑in)/(𝑑out + 𝑑in), where 𝑑in and 𝑑out are the inner and
outerdiameters, respectively. 𝑢
0
is permeability of vacuum. Theelectrical resistance is mainly
contributed by the inductorwire which imperatively has a series
resistance calculated,with consideration of the high-frequency skin
effect, as
𝑅𝑠 =
𝜌𝑙
𝑤√(𝜌/𝜋𝑓𝑢) (1 − 𝑒−𝑘⋅(𝜋𝑓𝑢/𝜌)
)
, (6)
where 𝜌 is the electrical resistivity of the metal, 𝑢 is
themagnetic permeability of the metal, 𝑤 is the metal linewidth,
and ℎ indicates the metal line height. The electricalcapacitance of
the sensor can be expressed as
𝐶𝑠 =
𝜀0
𝑎2
𝑡𝑔
+ (𝑡𝑚
/ (2 ∗ 𝜀𝑟
))
⋅
tanh−1 (√𝑑0
/ (𝑡𝑔
+ (𝑡𝑚
/ (2 ∗ 𝜀𝑟
))))
√𝑑0
/ (𝑡𝑔
+ (𝑡𝑚
/ (2 ∗ 𝜀𝑟
)))
;
(7)
𝑑0
is the center defection of the membrane assumes bothbending and
stretching of a uniformly loaded circular plateand is given by
[10]
𝑑0
=
3𝑃𝑎4
(1 − V2)
16𝐸 (𝑡𝑚
)3
⋅
1
1 + 0.448 (𝑑0
/𝑡𝑚
)2
, (8)
where 𝑎 is the length of the square electrode, 𝑃 is
theatmospheric pressure outside the sensor, 𝑡
𝑔
represents thedepth of the cavity and 𝑡
𝑚
is the thickness of the membrane,and 𝜀
0
and 𝜀𝑟
are the free space permittivity and relativedielectric constant,
respectively.
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International Journal of Distributed Sensor Networks 5
Table 1: Parameters of the designed sensor.
Parameters ValueOverall dimension 25mm × 25mm × 0.9mmCapacitance
plate radius 4mmHeight of the embedded cavity 100 umHeight of the
sensitive membrane 400 umDiameter of inner inductor 22.5mmDiameter
of outer inductor 12mmNumber of coil turns 9.5Width of coil turns
0.4mmDistance between neighbor coils 0.4mmVia dimension 0.2mm ×
0.2mmThickness of the printed pattern 20 um
Readoutcircuit
Reader antenna Pressure sensor
Figure 6: Major components of the designed readout system.
For the case when the deflection is small compared to theplate
thickness (𝑑 ≪ 𝑡
𝑚
), (8) above simplifies to
𝑑0
=
3𝑃𝑎4
(1 − V2)
16𝐸 (𝑡𝑚
)3
(9)
which is exactly the maximum center deflection for purebending
of a circular plate with clamped edges.
Deduced from the aforementioned description, parame-ters of the
sensor including capacitance plate radius, height ofthe embedded
cavity, and height of the sensitive membranewere designed and the
specific parameters of the sensordesigned were shown in Table
1.
2.3. Readout System. In this paper, a readout system isdesigned
tomeasure the pressure sensor’s resonant frequency.Figure 6
demonstrates the major components of the read-out system, which
include the reader antenna fabricatedon printed circuit boards, the
pressure sensor inductivelycoupled to the reader antenna, the
readout circuit, and themeasurement cable (regular 50Ω coaxial).
The sweep signalis generated with a direct digital synthesizer
(DDS). Thistype of oscillator has a wide output frequency range
and
Table 2: Parameters of the ESL 5570.
Parameters ValueScreen mesh/emulsion ∼325mesh/25 ± 5
𝜇mResistivity (17.5) ∼30 ± 10Ω/squareFiring range 1500∘C ± 10∘CTime
at peak temperature ∼120minViscosity 70 ± 20 Pa⋅sSodium
concentration ≤50 ppm
Table 3: Parameters of the PSZ tape 42020.
Parameters ValueYoung’s modulus ∼220GpaPoisson’s ratio
∼0.32Stabilizing agent Y2O3Unfired thickness ∼125 um ± 10%𝑋, 𝑌
shrinkage ∼16.5 ± 1.0%𝑍 shrinkage ∼18.0 ± 1.0%
Fired density (1450∘C for 1.5 hours) ≥95% of
theoreticalcalculating value
very fine frequency resolution and the output frequencychanges
almost instantly. The sweep signal and the voltagesignal across a
reference resistance aremultiplied through theGilbert
cell-basedmixer (multiplier), and a low-pass filter cir-cuit is
used to filter the mixer’s output signal into a dc outputvoltage
while the dc output voltage is processed and digitizedin a digital
system by a fast 16-bit ADC (AD7667). Thereadout circuit also
contains a microcontroller unit (MCU)for communicating with the PC
via a USB interface andperforming the requested frequency sweeps at
the specifiedspeed and with the specified starting and ending
frequencies.The PC postprocessing software analyzes the digital
data andcalculates and obtains the sensor’s resonant frequency.
The readout system is able to measure frequenciesbetween 1MHz
and 100MHz. The method used to extractthe sensor’s resonant
frequency is based on the changes in theshape of themeasured dc
output voltage curve.Themeasureddc output voltage is related to the
real part of reader antennaimpedance: when the sensor is excited at
resonance, the realpart of reader antenna impedance changes
greatly; then, thereadout system can extract the sensor’s resonant
frequencyfrom the dc output voltage information.
2.4. Sensor Structure Scheme. Similar to devices designedwith
LTCC (low-temperature cofiring ceramic) material,devices with HTCC
(high-temperature cofiring ceramic)material mainly take three
essential parts into consideration:formation of a flexible
membrane, a sealed cavity, and theintegration of an LC resonant
circuit. One of the majordifferences between the previous sensor
and the sensor wedesigned is the introduction of ESL 5570 and PSZ
tape 42020,whose parameters are shown in Tables 2 and 3,
respectively.
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6 International Journal of Distributed Sensor Networks
Capacitor electrode
Via
Diaphragm
Inductor coils
Sealed cavity
P
P
Figure 7: Schematic cross-sectional view of the sensor.
ESL 42020 high-temperature PSZ (partially stabilized zirco-nia
tape) tape is a flexible cast film of partially stabilizedzirconia
(PSZ) powder dispersed in an organic matrix. Thismaterial is
designed to be sintered in the temperature rangeof 1450∘C–1550∘C to
yield a dense white-colored ceramic.ESL 42020 tape is provided on a
silicone-coated polyesterfilm to protect the tape from mechanical
damage and aidin handling. Pt inks (ESL 5570 Series) are used for
printing,which can be cofired for use applications such as
planarsensors.
ESL 49000, a flexible cast film of fugitive powder dis-persed in
an organic matrix, is firstly introduced to generatethe sealed
cavity.Thismaterial is designed to be burned out inthe temperature
range of 600∘C–800∘C to yield a void wherethe tape was placed. ESL
49000 tape is provided on a silicone-coated polyester film to
protect the tape from mechanicaldamage and aid in handling.
In addition, according to the small deflection theory,increase
of the thickness of the sensitive membrane anddecrease of the
sensitive membrane area would both con-tribute to improvement of
the pressure range. Embeddedcapacitance electrode inside the sensor
is designed to increasethe capacitance as the distance between
capacitor plates isshortened, as shown in Figure 7. Capacitance
increases willlead to the decrease of the frequency, to some
extent; thisis advantageous to signal collection in low frequency
range.By the simulation using the ANSYS software, the
deflection,stress, and strain of the zirconia ceramic membrane
under60 bar and at 800∘C, respectively, are shown in Figure 8.
3. Fabrication
The first step is to cut the green tape using punchingmachine.
The punch file is used to cut accurate cavity, via,and alignment
holes, as illustrated in Figure 9, step 1. Detailedschematic layout
and geometrical values used for cutting thisdesign are described in
Table 1.
To achieve embedded passives series resonance circuitwithin the
substrate, ESL 5570 platinum conductor was
screen-printed while the ceramic tape was in a green
state,illustrated in Figure 9, step 1. The top planar spiral and
elec-trodes are screen-printed on the first layer, shown in Figure
9,step 1. The wet ink is allowed to dry in an oven at 150∘C for
15minutes prior to lamination. The fifth layer is defined as
thecavity. Capacitance electrode plates are printed in the
fourthand the seventh layers, respectively. All sheets, except for
thelast three sheets, have vias on, through which the inductorand
capacitor would be connected to form a series resonantcircuit; the
last two layers are used merely for increase ofthe sensitive
membrane thickness. All layers are assembledto form a ceramic body.
ESL 5575 platinum conductor, usedto fill the via hole, is then
allowed to dry in oven at 170∘C for5 minutes. Assembly of the
device begins with laminating thenine layers separately illustrated
in Figure 9, step 2, in vacuumcondition. The top section is
assembled over the bottom andmiddle sections to form the final
stack and then is laminatedunder pressure of 21MPa at 80∘C, which
ensures that areasover the cavity are well laminated before final
assembly. Thetop, middle, and bottom sections are then assembled
andlaminated, illustrated in Figure 9, step 2. Contact betweenthe
top metal spiral and via during lamination is sufficientto ensure
the metal melts and create a contact during sinter-ing.
As shown in Figure 9, step 3, the laminated stack issintered in
a box furnace in air for 85 minutes from 430∘Cto 600∘C (2∘C⋅min−1
ramp rate) to bake off the organics; slowheating rate from 600∘C to
800∘C (3∘Cmin−1 ramp rate) isused to make the fugitive tape
volatile and then 120 minutesat 1500∘C to form a dense zirconia;
then the structure iscooled at 5∘Cmin−1 ramp rate or slower; the
specific sinteringcurve is shown in Figure 10. The microfabricated
sensorswith sealed cavity and image of inductor coils observedby
scanning electron microscope (SEM) are illustrated inFigure 11.
4. Results and Discussion
The sensor testing was conducted using the PCB readerantenna
connected to the readout circuit to serve as theexternal reader for
electrical measurements. The microfab-ricated devices were tested
on-bench to characterize theirelectrical, physical behaviors.
Electrical parameters of thefabricated microsensor were firstly
obtained by analyzingthe measurement data from both the actual
device withthe external wireless readout method and several test
struc-tures with on-chip probing. Table 4 lists the
experimentalresults which were in good agreement with device
designestimates.
Measurements for the sensor were taken using pressurecylinder to
simulate the high-pressure environments. Thepressure can be
controlled from atmospheric pressure upto 20MPa using air pump as a
source of stress and apressure control instrument. A customized
pressure controlconfiguration, as shown in Figure 12, was utilized
for wireless
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International Journal of Distributed Sensor Networks 7
(a) (b) (c)
(d) (e) (f)
Figure 8: Ansys simulation of the membrane deflection (a),
stress (b), and strain (c) load under 60 bar. ANSYS simulation of
membranedeflection (d), stress (e), and strain (f) at 800∘C.
Table 4: Electrical parameters of the sensors.
Parameters ValueInductance ∼3.6 𝜇HResistance ∼5.3ΩCapacitance
∼3.6 pFResonant frequency ∼43MHZQuality factor at resonance
∼186Sensing distance ∼3 cmPressure sensitivity ∼38 kHz/bar
pressure sensing demonstration. Accurate environmentalpressure
variations could thus be created for the device withthis pressure
control setup. The device was placed insidethe cylinder connected
to the designed readout system tocomplete the high-pressure
testing.
Although the pressure sensitivitywas expected to be smallfrom
the device design, the sensing was compensated by thehigh sensor
resonant frequency to reach reasonable pressureresponsibility for
detection of the readout circuit’s dc outputvoltage shift with
respect to environmental pressure varia-tions. Figure 13 shows the
measured dc output voltage curvesfor the sensor by varying the
pressure from atmosphericpressure to 60 bar.
For the sensors in the variable capacitor design, thenormalized
shifted resonant frequency can be written as
𝑓max (Δ𝑃)
𝑓max (Δ𝑃 = 0)=
1/2𝜋√𝐿𝑠 (𝐶𝑠 + Δ𝐶𝑠)
1/2𝜋√𝐿𝑠𝐶𝑠
≅ (1 − 𝛼Δ𝑃)1/2
,
(10)
where Δ𝐶𝑠 is the changed capacitance due to diaphragmdeflection
and 𝛼 is the fitting parameter incorporating themechanical behavior
of the diaphragm. Accurate environ-mental pressure variations could
thus be created for thedevice (Δ𝑃 = 𝑃outside sensor − 𝑃inside
sensor) with this pressurecontrol setup. The wireless pressure
sensing behavior ofthe device was characterized with the measured
dc outputvoltage curves as shown in Figure 14 with approximately38
kHz/bar.
In the previous literatures of our research team, somestatic
performances of sensor at high temperature are testedin the closed
furnace. In this work, pressure tests are designedunder 650∘C
high-temperature environment on the high-temperature platform as
shown in Figure 17.The experimentsunder high-temperature
environment consist of two parts intotal: one is 50∘C∼650∘C
temperature changes experimentat 100 kpa and the result is shown in
Figure 15; the other ispressure testing experiments with a heat
preservation after 90min under 650∘C and the result is shown in
Figure 16. As theplatform takes water cooling way to reduce the
temperatureof readout antenna, the tests should be taken after the
systemreaches its thermal equilibrium; the pressure range is from70
kpa to 190 kpa and step value is 20 kpa; the test results are
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8 International Journal of Distributed Sensor Networks
Bottom section
Top section
Center section
1
987
654
2 3
ESL Pt inks 5570
ESL fugitive film 4900
ESL PSZ tape 42020
(1) Laser-cut samples of green tape.Then screen-print top and
bottom coils
(2) Laminate all threesections separately
(3) Laminate sections together
(4) Sinter laminated sections togetherFigure 9: Fabrication
process for pressure sensor.
0 200 400 600 800 1000 1200
0
200
400
600
800
1000
1200
1400
1600
Sintering time (min)
Sint
erin
g te
mpe
ratu
re (∘
C)
(610, 1500) (735, 1500)
(552, 800)
(485, 600)
(400, 430) (850, 445)
(1050, 25)
Figure 10: Temperature process control curve.
shown in Figures 13 and 14. The resonant frequency range
ofsensor is 84 kHz with a linearity of 1.17% and repeatabilityof
7.1%, and the hysteresis is 1.94% and the sensitivity is51 kHz/bar
under 650∘C as well.
24.5mm × 24.5mm(planar)
Figure 11: Device images: (left) microsensors; (right)
micrograph ofthe planar spiral inductor in SEM; (underneath)
micrograph of thesealed cavity.
5. Conclusions
In this paper, a design and fabrication method of a
wirelesspassive microsensor fabricated in HTCC technology
waspresented. Featuring zirconia ceramics (PSZ) and Pt inks,the
sensor achieves a satisfactory performance suitable for
-
International Journal of Distributed Sensor Networks 9
Airpump
Pressure controller
Impedance analysis
SensorAntenna
Pressure barrel
Figure 12: Schematic of the on-bench pressure testing setup
forcharacterization of the sensor.
35 35.5 36 36.5 37 37.5 38 38.5 39 39.5 40
1.4
1.6
1.8
2
2.2
2.4
2.6
2.8
3
Frequency (MHz)
4bar8bar12bar16bar20bar24bar28bar32bar
36bar40bar44bar48bar52bar56bar60bar
Vou
t(V
)
Figure 13: Overlay plot of the measured dc output voltage
curvesof the readout circuit by varying the pressure in on-bench
wirelesspressure sensing test.
applications in high-pressure environments. A sealed
cavity,flexible ceramicmembranes, and a fixed inductance𝐿 formedthe
series resonance circuit of the sensor, which were inte-grated on
amonolithic substrate without the need of complexprocess
technologies. A readout system was designed to testthe sensor’s
resonant frequency, and the performance ofthe sensor was
demonstrated in high-pressure environment.From the test result, the
pressure dependence of the sensorcan be tested up to 60 bar. The
average sensitivity andaccuracy of the sensor are up to 38 kHz/bar.
And the resultsprovide substantial evidence that the sensor has
great poten-tial for fulfilling continuous dynamic pressure
monitoring inharsh environments.
Freq
uenc
y ra
tiof
max/f
max
(ΔP=0
)
0.94
0ΔP (bar)Applied pressure difference
10 20 30 40 50 60
0.95
0.96
0.97
0.98
0.99
1.00 R = fmax /fmax|ΔP=0 ≅ (1 − 0.001761102ΔP)1/2
Responsivity ≈ 38kHz/bar
Curve fitMeasured data points
Sensitivity =ΔP=0
≈ 8850 ppm/bar⏐⏐⏐⏐⏐⏐⏐⏐⏐⏐⏐⏐
𝜕R
𝜕(ΔP)
Figure 14: Frequency versus pressure Δ𝑃 (Δ𝑃 = 𝑃outside sensor
−𝑃inside sensor).
0 100 200 300 400 500 600 700
38.80
38.81
38.82
38.83
38.84
38.85
38.86
38.87
38.88Fr
eque
ncy
(MH
z)
Temperature (∘C)
Figure 15:The resonant frequency of the sensor versus
temperature.
0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
38.74
38.75
38.76
38.77
38.78
38.79
38.80
Freq
uenc
y (M
Hz)
Pressure (bar)
Pressure decline Pressure raise Pressure decline
Pressure raise Pressure decline Pressure raise
Figure 16: The resonant frequency of the sensor versus pressure
at650∘C.
-
10 International Journal of Distributed Sensor Networks
Readoutunit
Operating control centre
Pressurecontrol
Nitrogentank
Reading antenna Sensor
Temperaturecontrol
Figure 17: The customized temperature-pressure measurement
system.
Conflict of Interests
The authors declare no conflict of interests.
Acknowledgments
Thiswork is supported by theNational Natural Science Foun-dation
of China (61335008) and the State Key DevelopmentProgram of Basic
Research of China (2010CB334703). Theauthors especially thank Dr.
Xiong and Dr. Liang for theirvaluable comments on experimental
procedures and WenyiLiu for his valuable discussion and
assistance.
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