Signal processing circuit of sensor

Details Signal processing circuit of sensor Signal processing circuit of sensor

1999-04-13

2002-05-14

Bui, Bryan (Department: 2857)

Data processing: measuring, calibrating, or testing

Measurement system

Measured signal processing

C702S085000, C702S098000, C702S104000

Reexamination Certificate

active

06389371

ABSTRACT:

BACKGROUND OF THE INVENTION
1.Field of the Invention
The present invention relates to a signal processing circuit for processing an electric signal of a sensor, the sensor having a sensor element for electrically detecting a change in physical amount of a workpiece and a signal output means for detecting the change of the sensor element and outputting the electric signal.
2.Description of the Related Art
Conventionally, a sensor having a sensor element for electrically detecting a change in a physical amount of a workpiece and a signal outputting means for detecting the change of the sensor chip and outputting an electric signal is used.
For example, an electro-capacitance type sensor having a diaphragm deforming in proportion to a change in pressure of the workpiece, a substrate for supporting the diaphragm, a sensor element for detecting the deformation of the diaphragm as a change in electro-capacitance between the diaphragm and the substrate and an electro-capacitance signal output means for detecting the change in the sensor element and outputting an electro-capacitance signal is known as such sensor.
Such electro-capacitance sensor is used as a pressure sensor and an acceleration sensor. In the pressure sensor, for example, the deformation of the diaphragm in proportion to the change in the pressure of the workpiece can be electrically detected as a change in the electro-capacitance, which is suitable for controlling and measuring pressure utilizing a computer.
As a specific example, a pressure detector
90
using an electro-capacitance pressure sensor is shown in FIG.
10
.
In the figure, the pressure detector
90
has a base member
91
which has a fitting
92
fixedly screwed to a portion to be detected. The fitting
92
is provided with a pressure inlet
93
to which pressure is introduced from an inside of the portion to be detected. The base member
91
has a greatly enlarged diameter remote from the fitting
92
and a pressure sensor
1
is installed thereon to cover the enlarged opening. A sealing member
94
such as an O-ring is inserted between the pressure sensor
1
and the base member
91
in order to ensure sealability therebetween.
The pressure sensor
1
has a diaphragm
11
on a surface facing the pressure inlet
93
, the diaphragm
11
receiving the pressure from the pressure inlet
93
to displace in a direction intersecting the surface. The pressure sensor
1
outputs the displacement of the diaphragm
11
as a change in electro-capacitance.
A processor
5
is mounted on the pressure sensor
1
remote from the fitting
92
. The processor
5
is connected to an electrode of the pressure sensor
1
through a through-hole etc. provided on the pressure sensor
1
to receive a signal showing the change in the electro-capacitance and to output to the outside after amplifying and conducting predetermined arithmetic processing etc.
An output substrate
95
is mounted for outputting the signal from the processor
5
to the outside. The processor
5
and the output substrate
95
are wired by wire-bonding etc. and a durable cable
96
is wired to connect the output substrate
95
and the outside.
The base member
91
is covered by a cover member
97
, in which all of the pressure sensor
1
, the processor
5
, the output substrate
95
etc. are accommodated.
In the electro-capacitance sensor used in the above-described pressure detector and the like, the actual pressure change and the electro-capacitance change do not linearly correspond within all the measurement range. Moreover, when the electro-capacitance sensors are mass-produced, slight deviation is caused in each product.
Accordingly, a calibration work is conducted in order to measure highly accurately irrespective of measurement range and the electro-capacitance sensor employed, in which respective sensors are adjusted by a potentiometer or a thick-film resistor or a thin-film resistor is trimmed by laser trimming.
However, since such calibration work requires an outside adjusting device as well as the sensor and the calibration work has to be conducted for respective sensors, large cost is necessary for the calibration work after manufacturing the sensors.
In view of above problem, a self-calibrating sensor is proposed (Japanese Patent Publication No. 2676959), in which a signal processing circuit having processor including adjusting device is integrally provided on the electro-capacitance sensor, so that the calibration work is conducted by the sensor itself.
Specifically, the self-calibration method by the processor of the above publication is summarized as follows.
In general, there is a relationship represented by following formula (1) between a sensor voltage output V in proportion to the change in the electro-capacitance of the sensor and pressure P.
V
=
a
×
P
b
-
P
2
+
c
(
1
)
Here, a, b and c are calibration values. Calibration of the sensors is equal to finding proper values for the calibration values a, b and c.
When standard pressures for the calibration are set as P
0
, P
1
and P
2,
and sensor outputs corresponding thereto are set as V
0
, V
1
and V
2
, following formulas (2) to (4) can be derived by assigning above values into the formula (1).
V
0
=
a
×
P
0
b
-
P
0
2
+
c
(
2
)
V
1
=
a
×
P
1
b
-
P
1
2
+
c
(
3
)
V
2
=
a
×
P
2
b
-
P
2
2
+
c
(
4
)
The processor solves simultaneous equations of the formulas (2) to (4) to calculate the calibration values a, b and c and the calibration values are stored in a memory such as E2PROM (or EEPROM; Electrically Erasable and Programmable Read Only Memory) provided on the sensor.
In actual measurement, the pressure P is calculated after correcting the detected sensor voltage output V using the calibration values a, b and c calculated by the above formulas (2) to (4). More specifically, in the processor on the sensor, the calibration values a, b and c obtained by the above formulas (2) to (4) are assigned to formula (5) representing solution of the quadratic equation of formula (1), and the detected sensor voltage output V is corrected.
P
=
a
±
a
2
+
4
×
(
V
-
c
)
2
×
 
⁢
b
-
2
×
(
V
-
c
)
(
5
)
According to the self-calibrating sensor, since the adjustment is conducted by the processor of the signal processing circuit integrally provided on the sensor, no calibration work is necessary for respective sensors using separate adjusting device after manufacturing the electro-capacitance sensor, thereby largely reducing the cost required for the calibration work of the sensors.
However, there are following disadvantages in the above-described self-calibrating sensor.
The above-described self-calibrating sensor calculates the calibration values a, b and c using the simultaneous equations of formulas (2) to (4) and corrects the output by the formula (5). Accordingly, the processor of the signal processing circuit needs to include both calculating section of the calibration values a, b and c by the formula (2) to (4) and output correcting section using the formula (5), resulting in complicated structure of the processor. Moreover, the calibration values calculating section according to the formulas (2) to (4) is not used in the actual pressure measurement after calibration, inevitably resulting in unnecessary parts not used for the actual measurement. Accordingly, even though the cost for the calibration work can be reduced, total cost including the cost for respective components is not necessarily reduced on account of complicated structure of the processor in the sensor.
Similar disadvantages also occur to a self-calibrating sensor, in which physical amount of the workpiece is electrically detected and an electric signal is outputted by detecting the change in a sensor element, such as a strain gauge sensor for detecting a deformation of a diaphragm as a change in a strain gauge, not limited to the above-described electro-capacitance sensor.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a signal processing circuit of a sensor for efficiently utilizing the processor wi

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