CMUT Time of Flight Gas Sensor By Phase Shift Measurement
Introduction The most common approach to measure a gas concentration consists in coating the exposed part of a sensor with a functionalized film in order to absorb the targeted gas [1]. However, it is well known that a sensitive coating compromises the long term stability of the sensor mostly due to...
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Zusammenfassung: | Introduction
The most common approach to measure a gas concentration consists in coating the exposed part of a sensor with a functionalized film in order to absorb the targeted gas [1].
However, it is well known that a sensitive coating compromises the long term stability of the sensor mostly due to aging of such film [2]. This has resulted in an increasing interest in uncoated sensors, which rely on the change of the physical properties (mass density for instance) of their surrounding gas despite their lack of selectivity.
One example of uncoated sensors are time of flight sensors. Capacitive micromachined ultrasonic transducers (CMUTs) [3], have been previously used for time of flight gas sensing due to their easy integration capabilities. However, these methods rely on peak detection algorithms which can be very complex and hard to integrate.
In this study, we present a new simple approach using the phase shift of a continuous ultrasonic wave to measure the time of flight between two CMUT arrays. The principle of this method along with the experimental setup and results will be presented in the following sections.
Sensor
Figure 1 shows a picture of the sensor used in this study. It consists of an array of thousands of CMUTs as the one illustrated in Figure 2 fabricated according to [4]. By applying a bias
V
dc
and an alternative voltage
V
dc
between its electrodes, the top electrode vibrates generating an ultrasonic mechanical vibration in the gas (emitter mode). Similarly, when the membrane vibrates due to a mechanical vibration of the gas, the CMUT’s capacitance changes generating an electrical signal (receiver mode).
Principle
The experimental setup schematics are shown in Figure 3. One CMUT array working as emitter (1) generates a continuous ultrasonic wave at a given angular frequency (
ω
) through a network analyzer in gain/phase configuration (2). The wave propagates through the gas at a velocity (
C
) that depends on the gas composition. For this study the gas consists of N
2
(similar to air) and either CO
2
or H
2
. In the case of an ideal gas, its expression is given by Equation 1 where
R
= 8.314 J.K
-1
.mol
-1
,
T
= 20°C,
x
is the molar fraction of a gas (either N
2
or the mixture),
M
is the corresponding molar mass and
γ
is the adiabatic index given by Equation 2 where
c
p
and
c
v
are, respectively, the isobar and isochoric heat capacitance per unit mass. After a delay
τ
, the wave reaches a second CMUT array (3) working in receiver mode. Th |
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ISSN: | 2151-2043 2151-2035 |
DOI: | 10.1149/MA2020-01312323mtgabs |