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 100a, Darlington-connected transistors 100b and 100c, current mirror-connected transistors 100d, 100e, and 100f, and a resistor 101. A voltage VK is inputted from an external to one of input terminals of the operational amplifier 100a. The other input terminal of the operational amplifier 100a is connected to an emitter of the transistor 100c. The operational amplifier 100a serves as a voltage follower which drives the current flowing through the transistor 100d through the transistors 100b and 100c. The constant current Is flows through the transistor 100e in proportion to the current flowing through the transistor 100d. In this way, the constant current Is flows through the transistor 100e 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 100e of the constant current circuit 1 is applied to the sensing element 2. Voltages VS+ and VSxe2x88x92 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 Is1 flowing through the temperature compensating resistor 3 decreases as the temperature rises, while the current Is2 flowing through the sensing element 2 increases as the temperature rises. The increase in current Is2 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 Is2 flowing through the sensing element 2.
Therefore, increasing the current Is2 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 Is2 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 xcex4.
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.
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 enables compensation for a change in TCS when changing a positive slop with a rise in temperature.
According to the second aspect of the invention, there is provided a sensor circuit which comprises: (a) a first resistor forming a sensor 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 third resistor installed in the current source. The third resistor has a resistance which changes as a function of a change in temperature thereof and works to provide temperature characteristics to the given 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 given 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 given 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 positive slop with a rise in temperature.
The current source has a first and a second transistor connected in the form of a current mirror. The current source produces a flow of a reference current through the first transistor to produce flows of a constant current proportional to the reference current from the second transistor to the first and second resistors as the currents flowing through the first and second resistors. The resistance of the third resistor increases with a rise in temperature thereof to increase the reference current.
The resistance of the third resistor may increase with a rise in temperature thereof to decrease the reference current, thereby enabling compensation for a change in TCS when changing at a positive slope with the rise in temperature.
The voltage inputted to one of input terminals of the operational amplifier may be changed by the resistance of the third resistor.
The current source may have an input voltage applied from an external to the other input terminal of the operational amplifier. The input voltage may be in proportional to the constant current.
The current source may have a first and a second transistor connected in the form of a current mirror, a fourth resistor connected in series with the first transistor, and an operational amplifier having two input terminals. An input voltage is applied to one of the input terminals of the operational amplifier. The voltage appearing at an end of the fourth resistor is applied to the other of the input terminals of the operational amplifier. An output of the operational amplifier serves to produce a flow of a reference current through the first transistor to produce flows of a constant current proportional to the reference current from the second transistor to the first and second resistors as the given currents flowing through the first and second resistors. The input voltage is applied to the other of the input terminals of the operational amplifier through the third resistor. The reference current is provided by a current flowing through the fourth resistor minus a current flowing through the third resistor. This enables compensation for a change in TCS when changing at a negative slop with a raise in temperature.
The current source may alternatively have a first and a second transistor connected in the form of a current mirror, a fourth resistor connected in series with the first transistor, and an operational amplifier having two input terminals. An input voltage is applied from an external to one of the input terminals of the operational amplifier. The voltage appearing at an end of the fourth resistor is applied to the other of the input terminals of the operational amplifier. An output of the operational amplifier serves to produce a flow of a reference current through the first transistor to produce flows of a constant current proportional to the reference current from the second transistor to the first and second resistors as the given currents flowing through the first and second resistors. The input voltage applied to the one of the input terminals of the operational amplifier is provided by a fraction of a power source voltage produced by a voltage divider using the third resistor. This enables compensation for a change in TCS when changing at a negative slop with a raise in temperature.
The current source may alternatively have a first and a second transistor connected in the form of a current mirror, a fourth resistor connected in series with the first transistor, and an operational amplifier having tow input terminals. An input voltage is applied from an external to one of the input terminals of the operational amplifier. An output of the operational amplifier serves to produce a flow of a reference current through the first transistor to produce flows of a constant current proportional to the reference current from the second transistor to the first and second resistors as the given currents flowing through the first and second resistors. A fraction of a voltage appearing at an end of the fourth resistor provided by the third resistor and a fifth resistor is applied to the other of the input terminals of the operational amplifier. This enables compensation for a change in TCS when changing at a negative slop with a raise in temperature.
According to the third aspect of the invention, there is provided a sensor circuit which comprises: (a) a first resistor forming a sensor 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, the current source having a first and a second transistor connected in the form of a current mirror, a third resistor connected in series with the first transistor, and an operational amplifier having two input terminals, an input voltage being applied from an external to one of the input terminals of the operational amplifier, a voltage appearing at an end of the third resistor being applied to the other of the input terminals of the operational amplifier, an output of the operational amplifier serving to produce a flow of a reference current through the first transistor to produce flows of a constant current proportional to the reference current from the second transistor to the first and second resistors as the given currents flowing through the first and second resistors; and (d) a fourth resistor installed in the current source, the fourth resistor having a resistance which changes as a function of a change in temperature thereof and working to provide temperature characteristics to the given currents flowing through the first and second resistors. The input voltage is applied to the other of the input terminals of the operational amplifier through the fourth resistor. The reference current is provided by a current flowing through the third resistor minus a current flowing through the fourth resistor.