Measuring and testing – Fluid pressure gauge – With pressure and/or temperature compensation
Reexamination Certificate
2003-03-24
2004-11-02
Lefkowitz, Edward (Department: 2855)
Measuring and testing
Fluid pressure gauge
With pressure and/or temperature compensation
Reexamination Certificate
active
06810745
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field of the Invention
The present invention relates generally to a sensor which works to measure, for example, a physical pressure acting thereon, and more particularly to an improved temperature dependent sensitivity compensation structure of such a sensor designed to compensate for a change in sensitivity of the sensor arising from a change in temperature thereof.
2. Background Art
Pressure sensors are known for use in measuring the pressure of brake fluid in a brake actuator or the pressure of fuel in a fuel injection device of automotive vehicles. Most of this type of pressure sensors includes a thin diaphragm formed on a semiconductor substrate and two pairs of gauge resistors connected on a central and a peripheral portion of the diaphragm in the form of a Wheatstone bridge to form a sensing element. When a physical pressure is exerted on the sensing element, it will cause the resistance of the gauge resistors to be changed by the piezoelectric effect, thereby resulting in a potential difference between middle points of the pairs of gauge resistors installed on the central and peripheral portions of the diaphragm. The pressure sensor amplifies and modifies such a voltage output to produce an electric signal as a function of the pressure applied thereto. In such a type of pressure sensor, the gauge resistors have a temperature coefficient of resistance (TCR) sensitive to a temperature change. Similarly, the sensing element has a temperature coefficient of sensitivity (TCS) sensitive to a temperature change. Therefore, pressure sensors designed to compensate for the TCS of the sensing element have been proposed.
FIG. 5
shows an example of such a pressure sensor which will be described below.
The pressure sensor consists of a constant current circuit
1
, a sensing element
2
, and a temperature compensating resistor
3
.
The constant current circuit
1
is made up of an operational amplifier
100
a
, Darlington-connected transistors
100
b
and
100
c
, current mirror-connected transistors
100
d
,
100
e
, and
100
f
, and a resistor
101
. A voltage VK is inputted from an external to one of input terminals of the operational amplifier
100
a
. The other input terminal of the operational amplifier
100
a
is connected to an emitter of the transistor
100
c
. The operational amplifier
100
a
serves as a voltage follower which drives the current flowing through the transistor
100
d
through the transistors
100
b
and
100
c
. The constant current Is flows through the transistor
100
e
in proportion to the current flowing through the transistor
100
d
. In this way, the constant current Is flows through the transistor
100
e
in proportion to the input voltage VK. Note that the constant current Is is proportional only to the input voltage VK, but does not depend on the power supply voltage VCC1.
The sensing element
2
is made up of gauge resistors
202
to
205
connected in a form of the Wheatstone bridge. The gauge resistors
202
to
205
each have the TCR which is sensitive to the pressure applied to the sensing element
2
and the temperature of the sensing element
2
. The voltage VSI of the transistor
100
e
of the constant current circuit
1
is applied to the sensing element
2
. Voltages VS+ and VS− appear at joints B and C of the sensing element
2
as a function of the voltage VSI applied to the sensing element
2
.
The temperature compensating resistor
3
has the TCR which is sensitive to the temperature thereof and is connected in parallel to the sensing element
2
.
The TCR of the temperature compensating resistor
3
is greater than those of the gauge resistors
202
to
205
. The current Is
1
flowing through the temperature compensating resistor
3
decreases as the temperature rises, while the current Is
2
flowing through the sensing element
2
increases as the temperature rises. The increase in current Is
2
results in an increase in voltage VSI applied to the sensing element
2
. The TCS of the sensing element
2
depends upon the voltage VSI, i.e., the current Is
2
flowing through the sensing element
2
.
Therefore, increasing the current Is
2
serves to compensate for the TCS of the sensing element
2
if the TCS changes at a negative slope. Additionally, use of a resistor as the temperature compensating resistor
3
which has a TCR lower than that of the gauge resistors
202
to
205
enables the TCS of the sensing element
2
to be compensated for if it changes at a positive slope. In this case, the TCR of the temperature compensating resistor
3
may be smaller than those of the gauge resistors
202
to
205
. For example, it may be zero (0). Specifically, the compensation of the TCS of the sensing element
2
is accomplished by providing a temperature characteristic to the current Is
2
flowing through the sensing element
2
using the temperature compensating resistor
3
which has a temperature characteristic different from that of the gauge resistors
202
to
205
of the sensing element
2
.
The TCR of the gauge resistors
202
to
205
of the sensing element
2
, if implemented by diffused resistors, depends upon the concentration of impurities contained therein. A rise in temperature of the sensing element
2
during flow of constant current through the gauge resistors
202
to
205
results in a rise in voltage VSI applied to the sensing element
2
. Specifically, the decrease in TCS is compensated for by the concentration of impurities in the diffused resistors, which is usually called sensitivity self-compensation, however, it is not always achieved. Accordingly, the pressure sensor further uses the temperature compensating resistor
3
having a given TCR for compensating for the TCS of the sensing element
2
.
FIGS.
6
(
a
) and
6
(
b
) show an ideal TCS and an actual TCS of the pressure sensor, respectively. In a case of the ideal TCS of FIG.
6
(
a
), the sensitivity of the sensing element
2
is kept constant by the sensitivity self-compensation free from the temperature thereof. In a case of the actual TCS of FIG.
6
(
b
), the sensitivity changes, as indicated by a solid line, with a change in temperature of the pressure sensor. The pressure sensor of
FIG. 5
uses the temperature compensating resistor
3
for compensating for the sensitivity by an amount &dgr;.
However, the above pressure sensor encounters a difficulty in compensating for a change in the TCS completely if the TCS changes, as indicated by a chain line, at a greater slope.
SUMMARY OF THE INVENTION
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an improved circuit structure of a sensor designed to provide a temperature characteristic to current supplied to a sensing element, thereby compensating for a change in TCS of the sensing element.
According to one aspect of the invention, there is provided a sensor circuit which may be employed to measure a physical pressure acting thereon. The sensor circuit comprises: (a) a first resistor forming a sensing element; (b) a second resistor connected in parallel to the sensing element, the second resistor having a temperature characteristic different from that of the first resistor; (c) a current source supplying given currents to the first and second resistors; and (d) a compensating circuit installed in the current source. The compensating circuit works to provide temperature characteristics to the currents flowing through the first and second resistors, thereby compensating for any changes in TCS of the sensing element.
In the preferred mode of the invention, the current source may be designed to increase the currents flowing through the first and second resistors with a rise in temperature thereof. This enables compensation for a change in TCS when changing at a negative slop with a rise in temperature.
The current source may alternatively be designed to decrease the currents flowing through the first and second resistors with a rise in temperature thereof. This
Jenkins Jermaine
Lefkowitz Edward
Posz & Bethards, PLC
LandOfFree
Temperature dependent sensitivity compensation structure of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Temperature dependent sensitivity compensation structure of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Temperature dependent sensitivity compensation structure of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3350483