Flow rate detector

A flow rate detector includes a detection circuit, which is configured to output as an analog signal a voltage in accordance with a flow rate of air flowing through an intake pipe, and a conversion circuit, which is configured to convert the analog signal input from the detection circuit to a digital signal based on an analog-to-digital conversion characteristic to output the digital signal. The analog signal that corresponds to a forward flow direction and is input to the conversion circuit is set to have a value larger than an input voltage range in which a missing code may occur in the analog-to-digital conversion characteristic.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a flow rate detector configured to detect a flow rate of air sucked into an internal combustion engine.

2. Description of the Related Art

In an electronically controlled fuel injection system applied to an internal combustion engine mounted in, for example, a motor vehicle, a flow rate detector configured to detect a mass flow rate of air sucked into the internal combustion engine is widely used. General detection methods of the flow rate detector include a thermal-type detection method and a Karman vortex-type detection method, for example.

A thermal-type flow rate detector is configured to detect a flow rate of air based on a current that is required to be caused to flow through a thermal line to return the temperature of the thermal line that is changed by air flowing around to an original temperature. A Karman vortex-type flow rate detector is configured to detect a flow rate of air based on the generation frequency of a Karman vortex, which is regularly generated alternately on the right and left sides on a downstream side of an object arranged in fluid.

In addition, there is proposed a flow rate detector configured to detect a temperature of air flowing through a flow passage, load a voltage in accordance with the temperature onto an arithmetic processing circuit, and perform signal processing, for example, digital conversion, by an A/D converter (see, for example, Japanese Patent No. 5304766).

The related art described in Japanese Patent No. 5304766 has a configuration in which linear correction of A/D conversion is performed in a circuit in a subsequent stage of the A/D converter in order to prevent detection accuracy from being deteriorated by a nonlinear error of the A/D converter. As a result, the circuit configuration of the flow rate detector is complex.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above-mentioned problem, and it is an object of the present invention to provide a flow rate detector that is capable of enhancing detection accuracy of a flow rate while preventing complication of its circuit configuration.

According to one embodiment of the present invention, there is provided a flow rate detector, which is configured to detect a flow rate of air flowing through an intake pipe for introducing the air into an internal combustion engine, the flow rate detector including: a bypass passage, which is configured to allow part of the air to flow therethough; a detection circuit, which is configured to output as an analog signal a voltage in accordance with a flow rate of the air flowing through the intake pipe by dividing a reference voltage based on a magnitude of the flow rate and whether a flow direction of the air is a forward flow direction or a backward flow direction, the forward flow direction being a direction from the flow rate detector toward the internal combustion engine, the backward flow direction being a direction opposite from the forward flow direction; and a conversion circuit, which is configured to receive as input the analog signal output by the detection circuit, and to convert the input analog signal input into a digital code based on an analog-to-digital conversion characteristic to output the digital code obtained by conversion as a digital signal, the analog-to-digital conversion characteristic being a characteristic obtained by associating an input voltage input to the conversion circuit with the digital code output by the conversion circuit, in which a forward flow analog signal, which is the analog signal that corresponds to the forward flow direction and is input to the conversion circuit, is set to have a value larger than a missing input voltage range, which is an input voltage range in which a missing code is capable of occurring in the analog-to-digital conversion characteristic.

According to the present invention, it is possible to provide the flow rate detector that is capable of enhancing detection accuracy of the flow rate while preventing complication of its circuit configuration.

DESCRIPTION OF THE EMBODIMENTS

Now, a flow rate detector according to exemplary embodiments of the present invention is described with reference to the accompanying drawings. In the illustration of the drawings, the same or corresponding components are denoted by the same reference symbols, and the overlapping description thereof is herein omitted.

First Embodiment

FIG. 1is a configuration diagram for illustrating a flow rate detector1according to a first embodiment of the present invention. InFIG. 1, a side cross section obtained when the flow rate detector1mounted to an intake pipe2for introducing air into an internal combustion engine is cut on a plane parallel to a flow direction X of the air is illustrated. Further, a direction of the air flowing in the intake pipe2from the flow rate detector1side to the internal combustion engine side is defined as a forward flow direction X1, and a direction of the air flowing in the intake pipe2from the internal combustion engine side to the flow rate detector1side, that is, the direction opposite from the forward flow direction X1, is defined as a backward flow direction X2.

InFIG. 1, the flow rate detector1includes: a body portion3, which is inserted into the intake pipe2through an insertion hole4formed in the intake pipe2; a flange portion5, which is fixed to the intake pipe2; and a connector portion10, which is formed in the flange portion5.

In the body portion3, a bypass passage6, inside which a flow rate detection element7is arranged, a circuit board8, and a circuit accommodation part9, which accommodates the circuit board8, are provided.

The bypass passage6is located in the intake pipe2, and part of the air flowing through the intake pipe2flows through the bypass passage6. That is, the bypass passage6takes in part of the air flowing through the intake pipe2through an inflow port61. The air taken in by the bypass passage6flows through the bypass passage6, and then is returned to the intake pipe2through an outflow port62.

The circuit board8is accommodated in the circuit accommodation part9. On the circuit board8, a detection circuit15and a conversion circuit16, which are to be described later, are mounted. The detection circuit15and the conversion circuit16, which are mounted on the circuit board8, are electrically connected to an external power source and a control device (not shown) via the connector portion10. With this configuration, power required for the detection circuit15and the conversion circuit16to be driven is supplied from the external power source, and the output by the conversion circuit16is input to the control device.

Next, description is given of a configuration of the flow rate detection element7with reference toFIG. 2andFIG. 3.FIG. 2is a plan view of the flow rate detection element7in the first embodiment of the present invention.FIG. 3is a sectional view as viewed from the direction indicated by the arrows A-A ofFIG. 2.

InFIG. 2andFIG. 3, the flow rate detection element7includes a detection portion11, a silicon substrate13, and an insulating film14, which is formed on a surface of the silicon substrate13to cover the detection portion11. The detection portion11includes a first resistor for flow rate detection (hereinafter simply abbreviated as “resistor”)12aand a second resistor for flow rate detection (hereinafter simply abbreviated as “resistor”)12b. The resistor12aand the resistor12bare arranged in the insulating film14.

As illustrated inFIG. 3, the back surface side of the silicon substrate13is removed through etching, for example, and thus the layer in which the resistor12aand the resistor12bare formed has a thin film structure.

Next, description is given of configurations of the detection circuit15and the conversion circuit16with reference toFIG. 4andFIG. 5.FIG. 4is a circuit diagram for illustrating a configuration of the detection circuit15in the first embodiment of the present invention.FIG. 5is a block diagram for illustrating a configuration of the detection circuit15and the conversion circuit16in the first embodiment of the present invention.

InFIG. 4, the detection circuit15includes a constant voltage source E and a series circuit including the resistor12aand the resistor12b. The constant voltage source E outputs a reference voltage Vref. The series circuit has a configuration in which the resistor12aand the resistor12bare connected in series to each other. In the series circuit, one terminal is grounded and another terminal is connected to the constant voltage source E.

One terminal of the resistor12aand one terminal of the resistor12bare connected in series to each other. Another terminal of the resistor12ais grounded, and another terminal of the resistor12bis connected to the constant voltage source E.

Between the resistor12a, which is located on the upstream side in the bypass passage6, and the resistor12b, which is located on the downstream side in the bypass passage6, a temperature difference in accordance with a flow rate Qm of the air flowing through the intake pipe2is generated. With such a temperature difference being generated, a ratio between resistance values of the resistor12aand the resistor12bis changed.

Accordingly, the detection circuit15is capable of outputting a voltage in accordance with the flow rate Qm as an analog signal Vm by dividing the reference voltage Vref based on a magnitude of the flow rate Qm and whether the flow direction of the air is the forward flow direction X1or the backward flow direction X2. Specifically, as illustrated inFIG. 4, the detection circuit15is configured to output a voltage based on the resistor12aas the analog signal Vm.

InFIG. 5, the detection circuit15outputs the analog signal Vm in accordance with the flow rate Qm. The analog signal Vm output by the detection circuit15is input to the conversion circuit16.

The conversion circuit16receives as input the analog signal Vm output by the detection circuit15, and converts the input analog signal Vm into a digital code based on an analog-to-digital conversion characteristic to output the digital code obtained by conversion as a digital signal Dm. The analog-to-digital conversion characteristic of the conversion circuit16is a characteristic obtained by associating an input voltage input to the conversion circuit16with the digital code output by the conversion circuit16.

Now, description is given of the analog-to-digital conversion characteristic of the conversion circuit16with reference toFIG. 6.FIG. 6is a schematic graph for showing an example of the analog-to-digital conversion characteristic of the conversion circuit16in the first embodiment of the present invention.

InFIG. 6, the horizontal axis indicates the analog signal Vm input to the conversion circuit16, that is, the input voltage, and the vertical axis indicates the digital code output as the digital signal Dm by the conversion circuit16correspondingly to the input voltage. Further, inFIG. 6, an example of the analog-to-digital conversion characteristic in a case where a general conversion error is present in the conversion circuit16is shown.

As shown inFIG. 6, with regard to the relationship between the input voltage and the digital code, an input voltage width per 1 LSB, which is the least significant bit, is ideally constant. However, depending on the type of the conversion circuit16, an input voltage width corresponding to a specific digital code is larger than an input voltage width corresponding to another digital code.

Depending on the input voltage width per 1 LSB, a phenomenon in which a digital code that is to originally be output correspondingly to an input voltage is not output, that is, a missing code, may occur.

In a case where such a conversion error is present in the conversion circuit16, it is concerned that, even when the analog signal Vm in accordance with the flow rate Qm is input from the detection circuit15to the conversion circuit16, the analog signal Vm is not correctly converted into the digital signal Dm, with the result that detection accuracy of the flow rate Qm is deteriorated.

Next, description is given of a relationship between the flow rate Qm and the digital code in a case where a general allocation of the flow rate Qm and the digital code is applied to the conversion circuit16with reference toFIG. 7.FIG. 7is a schematic graph for showing an example of the relationship between the flow rate Qm and the digital code in the case where a general allocation of the flow rate Qm and the digital code is applied to the conversion circuit16in the first embodiment of the present invention.

InFIG. 7, the vertical axis indicates both the input voltage input to the conversion circuit16correspondingly to the flow rate Qm and the digital code output by the conversion circuit16correspondingly to the input voltage, and the horizontal axis indicates the flow rate Qm corresponding to the digital code.

Further, inFIG. 7, in a range of values that can be taken by the digital code, an input voltage corresponding to the maximum is set as the reference voltage Vref, an input voltage corresponding to the minimum is set as zero, and an input voltage corresponding to the boundary at the time of carrying from “0” to “1” at the most significant bit of the digital code is set as a half of the reference voltage Vref.

Further, inFIG. 7, the flow rate Qm indicates the magnitude of the flow rate of the air by the magnitude of the value and the flow direction of the air by the sign of the value. Specifically, the flow rate Qm takes a positive value or a negative value with zero serving as the boundary. When the sign of the flow rate Qm is positive, it indicates that the flow direction of the air is the forward flow direction X1, and when the sign of the flow rate Qm is negative, it indicates that the flow direction of the air is the backward flow direction X2.

As shown inFIG. 7, with the most significant bit serving as the sign of the flow rate, the same gradients of digital codes are allocated in each of the forward flow direction and the backward flow direction. In this case, the digital codes around the carrying from “0” to “1” at the most significant bit are particularly liable to be affected by the nonlinear error. In the example shown inFIG. 7, such a flow rate range liable to be affected by the nonlinear error is a flow rate range in which air hardly flows, that is, a flow rate range in which the flow rate is nearly zero.

Next, with reference toFIG. 8, description is given of a relationship between the analog signal Vm and the digital signal Dm in a case where a missing code occurs in the analog-to-digital conversion characteristic of the conversion circuit16.FIG. 8is a schematic graph for showing an example of the relationship between the analog signal Vm output by the detection circuit15and the digital signal Dm output by the conversion circuit16in the first embodiment of the present invention.

As shown inFIG. 8, when the analog signal Vm in accordance with a continuous temporal change of the flow rate is input from the detection circuit15to the conversion circuit16, the conversion circuit16outputs the digital code corresponding to the analog signal Vm as the digital signal Dm.

Further, as shown inFIG. 8, when the value of the analog signal Vm is included in a particular input voltage range, a digital code corresponding to the analog signal Vm that is originally to be output is not output by the conversion circuit16. That is, when the value of the analog signal Vm input from the detection circuit15to the conversion circuit16is included in such a particular input voltage range, a missing code occurs.

When a missing code occurs, the digital code corresponding to the analog signal Vm that is originally to be output is not output, and hence the detection accuracy of the flow rate Qm is deteriorated. In addition, when the flow rate detector1is configured such that advance correction is performed by a signal processing circuit (not shown) in a subsequent stage thereof, the detection accuracy of the flow rate Qm may be further deteriorated.

In view of the consideration described above, the flow rate detector1is devised such that the detected flow rate Qm is not affected by a conversion error due to the conversion circuit16in an actual-use flow rate range, that is, a range in which the flow direction of the air is the forward flow direction, so as to suppress deterioration of the detection accuracy of the flow rate Qm. Specifically, in the flow rate detector1, the analog signal Vm (hereinafter referred to as “forward flow analog signal”) that corresponds to the forward flow direction and is input to the conversion circuit16is set to have a value larger than an input voltage range (hereinafter referred to as “missing input voltage range”) in which a missing code may occur in the analog-to-digital conversion characteristic.

For example, when the allocation shown inFIG. 7is applied to the conversion circuit16, the missing input voltage range is a voltage range obtained by taking a margin upward and downward with respect to a value that is a half of the reference voltage Vref.

Examples of methods of setting the forward flow analog signal to have a value larger than the missing input voltage range include, for example, a method of adjusting a ratio (hereinafter referred to as “first ratio”) between resistance values of the resistor12aand the resistor12bincluded in the detection circuit15.

FIG. 9is a schematic graph for showing an example of a relationship between the flow rate Qm and the analog signal Vm in a case where the ratio between the resistance values of the resistor12aand the resistor12bof the detection circuit15in the first embodiment of the present invention is changed.

InFIG. 9, the relationship between the flow rate Qm and the analog signal Vm is shown for each of a case in which the first ratio is a ratio A and a case in which the first ratio is a ratio B.

When the resistance value of the resistor12ais represented by Rhu, the resistance value of the resistor12bis represented by Rhd, and the reference voltage output by the constant voltage source E is represented by Vref, the analog signal Vm output by the detection circuit15is represented by the following expression (1).
Vm=Rhu/(Rhu+Rhd)×Vref  (1)

From the expression (1), it can be understood that the analog signal Vm is a signal depending on a ratio between Rhu and Rhd, that is, the first ratio.

As shown inFIG. 9, when the first ratio is the ratio A, the analog signal Vm whose value is included in the missing input voltage range corresponds to the forward flow direction. When the first ratio is the ratio B, the analog signal Vm whose value is included in the missing input voltage range corresponds to the backward flow direction.

Accordingly, when the first ratio is the ratio A, the value of the forward flow analog signal may be included in the missing input voltage range depending on the magnitude of the flow rate Qm. That is, when the first ratio is the ratio A, the flow rate Qm detected in the flow rate range in which the flow direction of the air is the forward flow direction may be affected by a conversion error due to the conversion circuit16.

Meanwhile, when the first ratio is the ratio B, the value of the analog signal Vm corresponding to the backward flow direction may be included in the missing input voltage range depending on the magnitude of the flow rate Qm. However, in this case, the value of the forward flow analog signal is not included in the missing input voltage range irrespective of the magnitude of the flow rate Qm. That is, when the first ratio is the ratio B, the flow rate Qm detected in the flow rate range in which the flow direction of the air is the forward flow direction is not affected by a conversion error due to the conversion circuit16.

In view of this, the detection circuit15is set so that the forward flow analog signal has a value larger than the missing input voltage range by adjusting the first ratio in advance.

With this configuration, it is possible to allocate the digital codes around the carrying from “0” to “1” at the most significant bit only to a flow rate range that does not correspond to the actual-use flow rate range without allocating those digital codes to the actual-use flow rate range. That is, in the actual-use flow rate range, the analog signal Vm whose value is not included in the missing input voltage range is input from the detection circuit15to the conversion circuit16, and thus occurrence of a missing code in the actual-use flow rate range can be suppressed.

Consequently, the detection accuracy of the flow rate Qm can be enhanced only by changing the first ratio without changing the configuration of the conversion circuit16.

As described above, the flow rate detector according to the first embodiment includes the detection circuit, which is configured to output as an analog signal a voltage in accordance with the flow rate of the air flowing through the intake pipe, and the conversion circuit, which is configured to convert the analog signal input from the detection circuit to a digital signal based on the analog-to-digital conversion characteristic to output the digital signal. Further, the forward flow analog signal, which is an analog signal that corresponds to the forward flow direction and is input to the conversion circuit, is set to have a value larger than the missing input voltage range, which is an input voltage range in which a missing code may occur in the analog-to-digital conversion characteristic.

In the first embodiment, there is exemplified a case in which the ratio between the resistance values of the first resistor for flow rate detection and the second resistor for flow rate detection of the detection circuit is adjusted so that the forward flow analog signal is set to have a value larger than the missing input voltage range.

With this configuration, it is possible to enhance the detection accuracy of the flow rate by eliminating influence of the conversion characteristic of the conversion circuit. Further, in order to enhance the detection accuracy of the flow rate, it is not required to complicate the circuit configuration of the flow rate detector, and hence it is possible to achieve downsizing and cost reduction of the flow rate detector. As described above, it is possible to obtain the flow rate detector capable of enhancing the detection accuracy of the flow rate while preventing complication of its circuit configuration.

Second Embodiment

In a second embodiment of the present invention, description is given of the flow rate detector1further including an adjustment circuit17in addition to the configuration of the above-mentioned first embodiment. In the second embodiment, description of the same points as those of the above-mentioned first embodiment is omitted, and differences from the above-mentioned first embodiment are mainly described.

As a method of setting the forward flow analog signal to have a value larger than the missing input voltage range, the first ratio is adjusted in the above-mentioned first embodiment, whereas the adjustment circuit17is included in the flow rate detector1in the second embodiment.

FIG. 10is a block diagram for illustrating a configuration of the detection circuit15, the conversion circuit16, and the adjustment circuit17in the second embodiment of the present invention.

InFIG. 10, the detection circuit15outputs the analog signal Vm to the adjustment circuit17. The adjustment circuit17adjusts the analog signal Vm output by the detection circuit15so that the forward flow analog signal input to the conversion circuit16has a value larger than the missing input voltage range, and outputs the adjusted analog signal Vm to the conversion circuit16as an analog signal Vm′. The conversion circuit16converts the analog signal Vm′ input from the adjustment circuit17to the digital signal Dm.

Next, description is given of a configuration example of the adjustment circuit17. The adjustment circuit17is formed of an adder circuit, for example.

The adder circuit offsets the analog signal Vm input from the detection circuit15to the positive side to output the offset analog signal Vm as the analog signal Vm′. In other words, the adder circuit outputs the analog signal Vm′ obtained by adding a certain offset amount to the analog signal Vm.

Thus, it is possible to set the forward flow analog signal to have a value larger than the missing input voltage range by setting an offset amount of the adder circuit in advance so that the forward flow analog signal input to the conversion circuit16has a value larger than the missing input voltage range.

There has been exemplified a case in which the adjustment circuit17is formed of the adder circuit, but the present invention is not limited thereto. The adjustment circuit17may be formed of an inverting amplifier circuit, for example.

As described above, the flow rate detector according to the second embodiment further includes the adjustment circuit in addition to the configuration of the above-mentioned first embodiment. The conversion circuit is configured to receive as input an analog signal output by the adjustment circuit instead of an analog signal output by the detection circuit to output a digital signal.

With this configuration, it is possible to set the forward flow analog signal to have a value larger than the missing input voltage range with use of the adjustment circuit without adjusting the first ratio by the detection circuit in advance. Further, the analog signal is to be adjusted in a subsequent stage of the detection circuit, and hence, as compared to the above-mentioned first embodiment, the analog signal input to the conversion circuit as well as a variation of the detection circuit can be adjusted with higher accuracy.

Third Embodiment

In a third embodiment of the present invention, description is given of the flow rate detector1in which the configuration of the detection circuit15is different from that in the above-mentioned first embodiment. In the third embodiment, description of the same points as those of the above-mentioned first embodiment is omitted, and differences from the above-mentioned first embodiment are mainly described.

FIG. 11is a plan view of the flow rate detection element7in the third embodiment of the present invention.FIG. 12is a sectional view as viewed from the direction indicated by the arrows A-A ofFIG. 11.

InFIG. 11andFIG. 12, the flow rate detection element7includes the detection portion11, a resistor for air temperature detection (hereinafter simply abbreviated as “resistor”)18, the silicon substrate13, and the insulating film14, which is formed on the surface of the silicon substrate13to cover the detection portion11and the resistor18. The detection portion11includes the resistor12aand the resistor12b.

Similarly to the resistor12aand the resistor12b, the resistor18is a heat-sensitive resistor, and is arranged on a portion of the surface of the flow rate detection element7other than a portion of the detection portion11. The resistor18is used for detecting the temperature of the air.

Similarly to the above-mentioned first embodiment, the back surface side of the silicon substrate13is removed through etching, for example, and thus the layer in which the resistor12aand the resistor12bare formed has a thin film structure.

Next, description is given of the configuration of the detection circuit15with reference toFIG. 13.FIG. 13is a circuit diagram for illustrating the configuration of the detection circuit15in the third embodiment of the present invention.

InFIG. 13, the detection circuit15includes: the constant voltage source E; a first series circuit, which includes a fixed resistor19and a fixed resistor20; a second series circuit, which includes the resistor12aand the resistor12b; an operational amplifier23; and the resistor18. The first series circuit has a configuration in which the fixed resistor19and the fixed resistor20are connected in series to each other. The second series circuit has a configuration in which the resistor12aand the resistor12bare connected in series to each other.

In each of the first series circuit and the second series circuit, one terminal is grounded and another terminal is connected to the constant voltage source E. That is, the resistor12aand the fixed resistor19are grounded, and a node of the resistor12band the fixed resistor20is connected to the constant voltage source E.

A non-inverting input terminal of the operational amplifier23is connected to a node of the fixed resistor19and the fixed resistor20, and an inverting input terminal of the operational amplifier23is connected to a node of the resistor12aand the resistor12b. One terminal of the resistor18is connected to the inverting input terminal side of the operational amplifier23, and another terminal of the resistor18is connected to an output side of the operational amplifier23. The detection circuit15is configured to output, as the analog signal Vm, output of the operational amplifier23.

With this configuration of the detection circuit15, an intermediate potential of the first series circuit and an intermediate potential of the second series circuit are input to the non-inverting input terminal and the inverting input terminal of the operational amplifier23, respectively. Further, the output of the operational amplifier23is fed back to the inverting input terminal of the operational amplifier23via the resistor18.

A heating current Ihu and a heating current Ihd flow through the resistor12aand the resistor12b, respectively, with the result that Joule heat is generated. When air flows over the detection portion11, the resistor12a, which is located on the upstream side, is cooled more as compared to the resistor12b, which is located on the downstream side. For that reason, in the detection circuit15, the output of the operational amplifier23, that is, the analog signal Vm output by the detection circuit15, changes so that a potential V−at the node of the resistor12aand the resistor12bchanges to become equal to a potential V+at the node of the fixed resistor19and the fixed resistor20.

Thus, the flow rate Qm can be detected through confirmation of the analog signal Vm. The analog signal Vm output by the detection circuit15is represented by the following expression (2). In the expression (2), an amplification degree of the operational amplifier23is represented by A, a voltage at the non-inverting input terminal of the operational amplifier23is represented by V+, and a voltage at the inverting input terminal of the operational amplifier23is represented by V−.
Vm=A×(V−−V−)  (2)

As described above, V−represents the potential at the node of the resistor12aand the resistor12b, and thus can be changed by adjusting the ratio of the resistance values of the resistor12aand the resistor12b, that is, the first ratio. As can be understood from the expression (2), the analog signal Vm can be changed by changing V−.

That is, also when the detection circuit15has the configuration illustrated inFIG. 13, similarly to the above-mentioned first embodiment, the forward flow analog signal can be set to have a value larger than the missing input voltage range by adjusting the first ratio in advance.

Consequently, even without changing the configuration of the conversion circuit16, similarly to the above-mentioned first embodiment, the detection accuracy of the flow rate Qm can be enhanced only by changing the first ratio. In addition, in the detection circuit15illustrated inFIG. 13, a configuration of a differential amplifier circuit using the operational amplifier23is employed, and hence it is possible to obtain the analog signal Vm that is more sensitive to changes of the flow rate Qm as compared to that in the above-mentioned first embodiment.

As described above, according to the flow rate detector of the third embodiment, in the detection circuit illustrated inFIG. 13, the forward flow analog signal is set to have a value larger than the missing input voltage range by adjusting the ratio between the resistance values of the first resistor for flow rate detection and the second resistor for flow rate detection.

Also with this configuration, the same effects as those of the above-mentioned first embodiment can be obtained. Moreover, it is possible to achieve the flow rate detector that is more sensitive to changes of the flow rate of the air as compared to the above-mentioned first embodiment.

Fourth Embodiment

In a fourth embodiment of the present invention, description is given of the flow rate detector1further including the adjustment circuit17in addition to the configuration of the above-mentioned third embodiment. In the fourth embodiment, description of the same points as those of the above-mentioned third embodiment is omitted, and differences from the above-mentioned third embodiment are mainly described.

As a method of setting the forward flow analog signal to have a value larger than the missing input voltage range, the first ratio is adjusted in the above-mentioned third embodiment, whereas the adjustment circuit17is included in the flow rate detector1in the fourth embodiment.

FIG. 14is a block diagram for illustrating a configuration of the detection circuit15, the conversion circuit16, and the adjustment circuit17in the fourth embodiment of the present invention.

InFIG. 14, the detection circuit15outputs the analog signal Vm to the adjustment circuit17. The adjustment circuit17outputs the analog signal Vm′ to the conversion circuit16. The conversion circuit16converts the analog signal Vm′ input from the adjustment circuit17to the digital signal Dm.

The configuration of the adjustment circuit17is the same as that of the above-mentioned second embodiment. Consequently, as illustrated inFIG. 14, similarly to the above-mentioned second embodiment, the forward flow analog signal can be set to have a value larger than the missing input voltage range by providing the adjustment circuit17to the configuration illustrated inFIG. 13.

As described above, the flow rate detector according to the fourth embodiment further includes the adjustment circuit in addition to the configuration of the above-mentioned third embodiment. The conversion circuit is configured to receive as input an analog signal output by the adjustment circuit instead of an analog signal output by the detection circuit to output a digital signal.

With this configuration, it is possible to set the forward flow analog signal to have a value larger than the missing input voltage range with use of the adjustment circuit without adjusting the first ratio by the detection circuit in advance. Further, the analog signal is to be adjusted in a subsequent stage of the detection circuit, and hence, as compared to the above-mentioned third embodiment, the analog signal input to the conversion circuit as well as a variation of the detection circuit can be adjusted with higher accuracy.

Fifth Embodiment

In a fifth embodiment of the present invention, description is given of the flow rate detector1in which the configuration of the detection circuit15is different from those in the above-mentioned first and third embodiments. In the fifth embodiment, description of the same points as those of the above-mentioned first and third embodiments is omitted, and differences from the above-mentioned first and third embodiments are mainly described.

FIG. 15is a plan view of the flow rate detection element7in the fifth embodiment of the present invention.FIG. 16is a sectional view as viewed from the direction indicated by the arrows A-A ofFIG. 15.

InFIG. 15andFIG. 16, the flow rate detection element7includes the detection portion11, a first resistor for air temperature detection (hereinafter simply abbreviated as “resistor”)18a, a second resistor for air temperature detection (hereinafter simply abbreviated as “resistor”)18b, the silicon substrate13, and the insulating film14, which is formed on the surface of the silicon substrate13to cover the detection portion11, the first resistor18a, and the second resistor18b. The detection portion11includes a resistor for flow rate detection (hereinafter simply abbreviated as “resistor”)12, a first resistor for temperature detection (hereinafter simply abbreviated as “resistor”)24a, and a second resistor for temperature detection (hereinafter simply abbreviated as “resistor”)24b.

The resistor18aand the resistor18bare heat-sensitive resistors, and are arranged on a portion of the surface of the flow rate detection element7other than a portion of the detection portion11. The resistor18aand the resistor18bare used for detecting the temperature of the air. The resistor24aand the resistor24bare used for detecting the temperature of the resistor12.

In the insulating film14, the resistor24a, the resistor12, and the resistor24bare formed along the forward flow direction. Similarly to the above-mentioned first embodiment, the back surface side of the silicon substrate13is removed through etching, for example, and thus the layer in which the resistor24a, the resistor12, and the resistor24bare formed has a thin film structure.

Next, description is given of the configuration of the detection circuit15with reference toFIG. 17.FIG. 17is a circuit diagram for illustrating the configuration of the detection circuit15in the fifth embodiment of the present invention.

InFIG. 17, the detection circuit15includes: the constant voltage source E; a first series circuit, which includes the fixed resistor19, the resistor18a, the resistor18b, and the fixed resistor20; a second series circuit, which includes the fixed resistor21, the resistor24a, and the resistor24b; a third series circuit, which includes the fixed resistor22and the resistor12; the operational amplifier23; and a transistor Tr.

The first series circuit has a configuration in which the fixed resistor19, the resistor18a, the resistor18b, and the fixed resistor20are connected in series to each other. The second series circuit has a configuration in which the fixed resistor21, the resistor24a, and the resistor24bare connected in series to each other. The third series circuit has a configuration in which the fixed resistor22and the resistor12are connected in series to each other. In each of the first series circuit, the second series circuit, and the third series circuit, one terminal is grounded and another terminal is connected to an emitter side of the transistor Tr.

The non-inverting input terminal of the operational amplifier23is connected to a node of the resistor24aand the resistor24b, and the inverting input terminal of the operational amplifier23is connected to a node of the resistor18aand the resistor18b. A base side of the transistor Tr is connected to the output side of the operational amplifier23, and a collector side of the transistor Tr is connected to the constant voltage source E. The detection circuit15is configured to output a voltage based on the fixed resistor22as the analog signal Vm.

With this configuration of the detection circuit15, an intermediate potential of the first series circuit and an intermediate potential of the second series circuit are input to the non-inverting input terminal and the inverting input terminal of the operational amplifier23, respectively. Further, a voltage based on a difference between the intermediate potential input to the inverting input terminal and the intermediate potential input to the non-inverting input terminal is output from the operational amplifier23, and is fed back to upper ends of the first series circuit and the second series circuit.

At this time, a heating current Ih flows through the resistor12, with the result that Joule heat is generated. As illustrated inFIG. 15andFIG. 16, the resistor12is arranged in the proximity of the resistor24aand the resistor24b, and hence the temperature of the resistor24aand the resistor24bbecomes equal to the temperature of the resistor12. In addition, the detection circuit15has a configuration in which the temperature of the resistor24aand the resistor24bis kept higher than the temperature of the resistor18aand the resistor18bby a fixed temperature.

As the flow rate of the air flowing over the detection portion11increases, a heat transfer amount from the resistor12to the air increases. For that reason, in order for the temperature of the resistor12, the resistor24a, and the resistor24bto be kept higher than the temperature of the resistor18aand the resistor18bby a fixed temperature, the heating current Ih depending on the flow rate Qm is required.

Thus, the flow rate Qm can be detected by causing the voltage based on the fixed resistor22corresponding to the heating current Ih to be output as the analog signal Vm. In this case, the analog signal Vm output by the detection circuit15can be represented by the above-mentioned expression (2).

It is possible to change V−shown in the expression (2) by adjusting a ratio (hereinafter referred to as “second ratio”) of resistance values of the resistor18a, the resistor18b, the fixed resistor19, and the fixed resistor20, which are included in the first series circuit, which is connected to the inverting input terminal of the operational amplifier. As can be understood from the expression (2), the analog signal Vm can be changed by changing V−.

That is, also when the detection circuit15has the configuration illustrated inFIG. 17, similarly to the above-mentioned first embodiment, the forward flow analog signal can be set to have a value larger than the missing input voltage range by adjusting the second ratio in advance. Consequently, even without changing the configuration of the conversion circuit16, similarly to the above-mentioned first embodiment, the detection accuracy of the flow rate Qm can be enhanced only by changing the second ratio.

As described above, according to the flow rate detector of the fifth embodiment, in the detection circuit illustrated inFIG. 17, the forward flow analog signal is set to have a value larger than the missing input voltage range by adjusting the ratio of the resistance values of the first resistor for air temperature detection, the second resistor for air temperature detection, the first fixed resistor, and the second fixed resistor.

Also with this configuration, the same effects as those of the above-mentioned first embodiment can be obtained. Moreover, it is possible to achieve the flow rate detector that is more sensitive to changes of the flow rate of the air as compared to the above-mentioned first embodiment.

Sixth Embodiment

In a sixth embodiment of the present invention, description is given of the flow rate detector1further including the adjustment circuit17in addition to the configuration of the above-mentioned fifth embodiment. In the sixth embodiment, description of the same points as those of the above-mentioned first embodiment is omitted, and differences from the above-mentioned fifth embodiment are mainly described.

As a method of setting the forward flow analog signal to have a value larger than the missing input voltage range, the second ratio is adjusted in the above-mentioned fifth embodiment, whereas the adjustment circuit17is included in the flow rate detector1in the sixth embodiment.

FIG. 18is a block diagram for illustrating a configuration of the detection circuit15, the conversion circuit16, and the adjustment circuit17in the sixth embodiment of the present invention.

InFIG. 18, the detection circuit15outputs the analog signal Vm to the adjustment circuit17. The adjustment circuit17outputs the analog signal Vm′ to the conversion circuit16. The conversion circuit16converts the analog signal Vm′ input from the adjustment circuit17to the digital signal Dm.

The configuration of the adjustment circuit17is the same as that of the above-mentioned second embodiment. Consequently, as illustrated inFIG. 18, similarly to the above-mentioned second embodiment, the forward flow analog signal can be set to have a value larger than the missing input voltage range by providing the adjustment circuit17to the configuration illustrated inFIG. 17.

As described above, the flow rate detector according to the sixth embodiment further includes the adjustment circuit in addition to the configuration of the above-mentioned fifth embodiment. The conversion circuit is configured to receive as input an analog signal output by the adjustment circuit instead of an analog signal output by the detection circuit to output a digital signal.

With this configuration, it is possible to set the forward flow analog signal to have a value larger than the missing input voltage range with use of the adjustment circuit without adjusting the second ratio by the detection circuit in advance. Further, the analog signal is to be adjusted in a subsequent stage of the detection circuit, and hence, as compared to the above-mentioned fifth embodiment, the analog signal input to the conversion circuit as well as a variation of the detection circuit can be adjusted with higher accuracy.

Seventh Embodiment

In a seventh embodiment of the present invention, description is given of the flow rate detector1in which the configuration of the detection circuit15is different from those in the above-mentioned first, third, and fifth embodiments. In the seventh embodiment, description of the same points as those of the above-mentioned first, third, and fifth embodiments is omitted, and differences from the above-mentioned first, third, and fifth embodiments are mainly described.

FIG. 19is a plan view of the flow rate detection element7in the seventh embodiment of the present invention.FIG. 20is a sectional view as viewed from the direction indicated by the arrows A-A ofFIG. 19.

InFIG. 19andFIG. 20, the flow rate detection element7includes the detection portion11, the resistor18, the silicon substrate13, and the insulating film14, which is formed on the surface of the silicon substrate13to cover the detection portion11and the resistor18. The detection portion11includes the resistor12a, the resistor12b, a third resistor for flow rate detection (hereinafter simply abbreviated as “resistor”)12c, and a fourth resistor for flow rate detection (hereinafter simply abbreviated as “resistor”)12d.

In the detection portion11, four heat-sensitive resistors, namely, the resistor12a, the resistor12b, the resistor12c, and the resistor12d, are formed. With respect to the flow of the air, the resistor12aand the resistor12care arranged on the upstream side, and the resistor12band the resistor12dare arranged on the downstream side.

Further, the resistor12aand the resistor12bare arranged so as to be opposed to each other, and the resistor12cand the resistor12dare arranged so as to be opposed to each other. The two resistors arranged so as to be opposed to each other are arranged so as to be as close to each other as possible so that, as described later, when air flows, the air heated through exposure to one resistor is exposed to another resistor before the air is cooled down. The resistor18is arranged on a portion of the surface of the flow rate detection element7other than the portion of the detection portion11.

Similarly to the above-mentioned first embodiment, the back surface side of the silicon substrate13is removed through etching, for example, and thus the layer in which the resistor12a, the resistor12b, the resistor12c, and the resistor12dare formed has a thin film structure.

Next, description is given of the configuration of the detection circuit15with reference toFIG. 21.FIG. 21is a circuit diagram for illustrating the configuration of the detection circuit15in the seventh embodiment of the present invention.

InFIG. 21, the detection circuit15includes: the constant voltage source E; a first series circuit, which includes the fixed resistor19and a parallel circuit including the resistor12a, the resistor12b, the resistor12c, and the resistor12d; a second series circuit, which includes the resistor18and the fixed resistor20; the operational amplifier23; an operational amplifier27; and the transistor Tr. The parallel circuit has a configuration in which the resistor12aand the resistor12b, which are connected in series to each other, and the resistor12cand the resistor12d, which are connected in series to each other, are connected in parallel to each other. The first series circuit has a configuration in which the parallel circuit and the fixed resistor19are connected in series to each other. The second series circuit has a configuration in which the resistor18and the fixed resistor20are connected in series to each other.

The non-inverting input terminal of the operational amplifier23is connected to a node of the parallel circuit and the fixed resistor19, and the inverting input terminal of the operational amplifier23is connected to a node of the resistor18and the fixed resistor20. A non-inverting input terminal of the operational amplifier27is connected to a node of the resistor12cand the resistor12d, and an inverting input terminal of the operational amplifier27is connected to a node of the resistor12aand the resistor12b.

The base side of the transistor Tr is connected to the output side of the operational amplifier23, and the collector side of the transistor Tr is connected to the constant voltage source E. In each of the first series circuit and the second series circuit, one terminal is grounded and another terminal is connected to the emitter side of the transistor Tr.

A bridge circuit25corresponds to the above-mentioned parallel circuit, and includes the four resistors12ato12d. A bridge circuit26includes the bridge circuit25, the resistor18, the fixed resistor19, and the fixed resistor20.

As described later, a current flowing through the bridge circuit26is controlled so that the bridge circuit26is in a balanced state, that is, a potential difference between an intermediate point ca and an intermediate point cb becomes zero. The resistance values of the fixed resistor19and the fixed resistor20are set so that the temperature of the four resistors12ato12dbecomes a predetermined temperature when the bridge circuit26is balanced.

In this configuration of the detection circuit15, when air flowing in the forward flow direction flows over the detection portion11, first, the resistor12aand the resistor12c, which are arranged on the upstream side, of the four resistors12ato12dhave heat removed therefrom by the air, with the result that the resistance values thereof are reduced. Further, as the flow rate of the air increases, heat that is removed from the resistor12aand the resistor12cby the air increases. As a result, the decrease amount of the resistance values increases in proportion to the flow rate of the air.

Meanwhile, the resistor12band the resistor12dare arranged on the downstream side, and hence are exposed to the air that is heated by the resistor12aand the resistor12c. Thus, the resistance values of the resistor12band the resistor12dare hardly changed before and after the air flows. As a result, differences between voltages across both ends of each of the resistors12ato12dare generated. That is, when voltages across both ends of each of the resistor12aand the resistor12b, which are connected in series, are compared to each other, the voltage across both ends of the resistor12ais smaller. Similarly, when voltages across both ends of each of the resistor12cand the resistor12dare compared to each other, the voltage across both ends of the resistor12cis smaller.

Thus, the flow rate Qm can be detected by causing a potential difference generated between the intermediate point ca and the intermediate point cb of the bridge circuit25to be output as the analog signal Vm via the operational amplifier27. In this case, the analog signal Vm output by the detection circuit15can be represented by the above-mentioned expression (2).

It is possible to change Vm shown in the expression (2) by adjusting a ratio (hereinafter referred to as “third ratio”) of resistance values of the resistor12a, the resistor12b, the resistor12c, and the resistor12d, which are included in the bridge circuit25.

That is, also when the detection circuit15has the configuration illustrated inFIG. 21, similarly to the above-mentioned first embodiment, the forward flow analog signal can be set to have a value larger than the missing input voltage range by adjusting the third ratio in advance. Consequently, even without changing the configuration of the conversion circuit16, similarly to the above-mentioned first embodiment, the detection accuracy of the flow rate Qm can be enhanced only by changing the third ratio.

As described above, according to the flow rate detector of the seventh embodiment, in the detection circuit illustrated inFIG. 21, the forward flow analog signal is set to have a value larger than the missing input voltage range by adjusting the ratio of the resistance values of the first resistor for flow rate detection, the second resistor for flow rate detection, the third resistor for flow rate detection, and the fourth resistor for flow rate detection.

Also with this configuration, the same effects as those of the above-mentioned first embodiment can be obtained. Moreover, it is possible to achieve the flow rate detector that is more sensitive to changes of the flow rate of the air as compared to the above-mentioned first embodiment.

Eighth Embodiment

In an eighth embodiment of the present invention, description is given of the flow rate detector1further including the adjustment circuit17in addition to the configuration of the above-mentioned seventh embodiment. In the eighth embodiment, description of the same points as those of the above-mentioned seventh embodiment is omitted, and differences from the above-mentioned seventh embodiment are mainly described.

As a method of setting the forward flow analog signal to have a value larger than the missing input voltage range, the adjustment circuit17is included in the flow rate detector1in the eighth embodiment, while the third ratio is adjusted in the above-mentioned seventh embodiment.

FIG. 22is a block diagram for illustrating a configuration of the detection circuit15, the conversion circuit16, and the adjustment circuit17in the eighth embodiment of the present invention.

InFIG. 22, the detection circuit15outputs the analog signal Vm to the adjustment circuit17. The adjustment circuit17outputs the analog signal Vm′ to the conversion circuit16. The conversion circuit16converts the analog signal Vm′ input from the adjustment circuit17to the digital signal Dm.

The configuration of the adjustment circuit17is the same as that of the above-mentioned second embodiment. Consequently, as illustrated inFIG. 22, similarly to the above-mentioned second embodiment, the forward flow analog signal can be set to have a value larger than the missing input voltage range by providing the adjustment circuit17to the configuration illustrated inFIG. 21.

As described above, the flow rate detector according to the eighth embodiment further includes the adjustment circuit in addition to the configuration of the above-mentioned seventh embodiment. The conversion circuit is configured to receive as input an analog signal output by the adjustment circuit instead of an analog signal output by the detection circuit to output a digital signal.

With this configuration, it is possible to set the forward flow analog signal to have a value larger than the missing input voltage range with use of the adjustment circuit without adjusting the third ratio by the detection circuit in advance. Further, the analog signal is to be adjusted in a subsequent stage of the detection circuit, and hence, as compared to the above-mentioned seventh embodiment, the analog signal input to the conversion circuit as well as a variation of the detection circuit can be adjusted with higher accuracy.