Electricity: measuring and testing – Fault detecting in electric circuits and of electric components
Reexamination Certificate
2000-07-26
2003-07-01
Karlsen, Ernest (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
C324S658000, C324S1540PB, C073S514320, C073S718000, C326S017000, C702S183000
Reexamination Certificate
active
06586943
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a sensor signal processing apparatus for extracting a signal from a sensor section as a function of physical quantity to be measured and, more particularly, to a sensor signal processing circuit for eliminating errors originating from elements due to temperature changes and the like (these errors will be referred to as element-based errors hereinafter) and errors originating from a circuit due to the offset of an operational amplifier and the like (these errors will be referred to as circuit-based errors hereinafter) by signal processing.
Recently, in the field of pressure measurements, electronic pressure gages have been rapidly replacing mechanical pressure gages. The electronic pressure gages can be roughly classified into a resistance type that converts a pressure change in a pressure-sensitive diaphragm into an electric resistance change and a capacitance type that converts a displacement of a pressure-sensitive diaphragm into a capacitance change. Of these types of sensors, a capacitance type pressure sensor is excellent at fine pressure measurement.
FIG. 16
shows the structure of the above capacitance type pressure sensor. Referring to
FIG. 16
, a first recess portion
101
a
is formed in the central portion of the surface of a pedestral substrate
101
. A second recess portion
101
b
is formed in the shape of a groove to surround the first recess portion
101
a
through a barrier
104
. A thin diaphragm substrate
102
is joined to that surface of the pedestral substrate
101
on which the recess portions
101
a
and
101
b
are formed. The spaces surrounded by the first and second recess portions
101
a
and
101
b
and diaphragm substrate
102
on the pedestral substrate
101
form capacitor chambers
103
a
and
103
b.
As shown in
FIG. 17
, a square fixed electrode
105
a
is formed on the bottom surface of the first recess portion
101
a.
A movable electrode
105
b
is formed on the diaphragm substrate
102
to oppose the fixed electrode
105
a
at a predetermined distance therefrom. The electrode
105
a
is extracted to the outside by a lead
105
c.
The electrode
105
b
is also extracted to the outside by a lead (not shown). A sensor capacitor
114
a
(
FIG. 18
) to be described later is constituted by the pair of electrodes
105
a
and
105
b
and the air existing between the electrodes
105
a
and
105
b
and serving as a dielectric member.
As shown in
FIG. 17
, a belt-like electrode
106
a
is formed in the shape of a square frame on the bottom surface of the second recess portion
101
b.
An electrode
106
b
is formed on the diaphragm substrate
102
to oppose the electrode
106
a
at a predetermined distance therefrom. The electrode
106
a
is extracted to the outside by a lead
106
c.
The electrode
106
b
is also extracted to the outside by a lead (not shown). A reference capacitor
114
b
(
FIG. 18
) to be described later is constituted by the pair of electrodes
106
a
and
106
b
and the air existing between the electrodes
106
a
and
106
b
and serving as a dielectric member.
Note that the barrier
104
between the capacitor chambers
103
a
and
103
b
is partly removed to allow the air in the capacitor chambers
103
a
and
103
b
to easily mix.
A portion of the diaphragm substrate
102
which is part of the capacitor chamber
103
a
serves as a pressure-sensitive diaphragm
102
a.
As shown in
FIG. 16
, therefore, when a positive pressure P is externally applied to the diaphragm substrate
102
, the pressure-sensitive diaphragm
102
a
deflects toward the capacitor chamber
103
a.
Since the electrode
105
b
is displaced as the pressure-sensitive diaphragm
102
a
deflects, the gap between the electrodes
105
a
and
105
b
decreases, and a capacitance C
s
of the sensor capacitor
114
a
increases. At this time, a portion of the diaphragm substrate
102
which is part of the capacitor chamber
103
b
does not deflect upon application of the pressure P, and hence a capacitance C
r
of the reference capacitor
114
b
does not change. That is, the sensor capacitor
114
a
functions as a first sensor element whose capacitance C
s
changes in accordance with a change in the pressure P.
That portion of the diaphragm substrate
102
which is part of the capacitor chamber
103
b
does not deflect upon application of the pressure P because the capacitor chamber
103
b
is narrow. For this reason, the capacitance C
r
of the reference capacitor
114
b
does not change. That is, the reference capacitor
114
b
functions as a second sensor that exhibits the constant capacitance C
r
even with a change in the pressure P.
The reference capacitor
114
b
is formed to eliminate measurement errors (element-based errors) due to temperature changes around a sensor section
114
, humidity changes in the capacitor chamber
103
a,
and the like. More specifically, the pressure P from which the above measurement errors are eliminated can be theoretically obtained by calculating
K
1
=(
C
s
−C
r
)/
C
s
(1)
on the basis of the capacitance C
s
of the sensor capacitor
114
a
and the capacitance C
r
of the reference capacitor
114
b.
Letting ∈ be the dielectric constant of the air in the capacitor chambers
103
a
and
103
b,
d be the gap between the electrodes
105
a
and
105
b
in the sensor capacitor
114
a
(in non-measurement period) and the gap between the electrodes
106
a
and
106
b
in the reference capacitor
114
b,
&Dgr;d be the pressure sensitivity displacement of the pressure-sensitive diaphragm
102
a,
and S be the area of each of the opposing surfaces of the electrodes
105
a
and
105
b
and the areas of the opposing surfaces of the electrodes
106
a
and
106
b
for the sake of simplicity, the capacitances C
s
and C
r
can be given by
C
s
=∈S/
(
d+&Dgr;d
) (2)
C
r
=∈S/d
(3)
Substitutions of equations (2) and (3) into equation (1) yield
K
1
=−&Dgr;d/d
(4)
Obviously, therefore, the pressure P can be obtained from equation (1).
FIG. 18
shows a sensor signal processing circuit for extracting a signal from the sensor section
114
in
FIG. 16
as a function of the pressure P.
Referring to
FIG. 18
, the input side of the sensor capacitor
114
a
of the sensor section
114
is connected to an AC power supply
111
through a buffer
113
a
and switching section
112
. The input side of the reference capacitor
114
b
is connected to the AC power supply
111
through a buffer
113
b
and the switching section
112
. An amplifying section
115
is connected to the output side of the sensor section
114
. A CPU (Central Processing Unit)
117
is connected to the output side of the amplifying section
115
through an A/D (Analog-to-Digital) converter
116
.
The amplifying section
115
is comprised of an operational amplifier
115
a
and capacitor
115
b.
The noninverting input terminal (+), inverting input terminal (−1), and output terminal of the operational amplifier
115
a
are respectively connected to the ground (G), a node
114
c
of the capacitors
114
a
and
114
b,
and the A/D converter
116
. The capacitor
115
b
is connected to the node
114
c
of the capacitors
114
a
and
114
b
and the output terminal of the operational amplifier
115
a.
The CPU
117
outputs control signals
118
for switching operation to the switching section
112
, and performs arithmetic processing upon combining signals output from the A/D converter
116
for every switching operation of the switching section
112
.
Letting V
i
be the output voltage from the AC power supply
111
, and C
f
be the capacitance of the capacitor
115
b,
an output voltage V
101
from the amplifying section
115
when the power supply
111
is connected to the sensor capacitor
114
a
can be given by
V
101
=−C
s
V
i
/C
f
(5)
An output voltage V
102
from the amplifying section
115
when the power supply
111
is connected to the reference capacitor
114
b
can be given by
V
Masuda Takashi
Yoshikawa Yasuhide
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