JP2000018991A - Heater drive circuit for flow-rate measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a heater drive circuit by which a temperature can be corrected with good accuracy, irrespective of an irregularity in a resistance value in the heater of a flow-rate sensor and irrespective of an irregularity in a characteristic regarding an element inside the circuit. SOLUTION: In a heater drive circuit 102 for a flow-rate measuring apparatus, a diode D1 as an ambinent-temperature monitoring element is provided, and a variable resistor VR1 which generates an offset voltage V2 is provided. An ambient-temperature monitoring voltage V1, at one end of the diode D1, which is changed according to an ambient temperature is added to the offset voltage V2. An added voltage V3 is amplified by an amplifier circuit in an operational amplifier AMP1 and by an amplifier circuit in an operational amplifier AMP2, so as to be adjusted, and a heater application voltage V4 is generated. The heater application voltage V4 is current-amplified via a transistor Tr1 so as to be supplied to a heater 111.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heater drive circuit of a flow rate measuring device for determining a flow rate of a fluid such as gas using a heater and a temperature sensor.

[0002]

2. Description of the Related Art As one type of flow rate measuring device, a so-called flow rate sensor having temperature sensors disposed upstream and downstream of a heater is used. Heat is generated from the heater and the temperature on the upstream side and downstream side is measured. 2. Description of the Related Art A thermal diffusion detection type flow rate measuring device that detects a degree of diffusion of heat through a fluid to be measured by measuring with a sensor and obtains a flow rate of a fluid in the vicinity of the flow rate sensor based on the degree has been conventionally proposed.

As a heater drive circuit used in such a flow rate measuring device, there is one having a temperature correction function as disclosed in, for example, Japanese Patent Application Laid-Open No. 4-34315. By performing the temperature correction, the flow rate can be measured accurately regardless of the ambient temperature of the flow rate sensor. FIG. 5 shows a configuration example of a conventional heater drive circuit.
The heater drive circuit includes an operational amplifier AMP3, a resistor R1
2, an amplifier circuit formed by R13, R14, and a transistor Tr11. The heater Rh is heated by supplying power to the heater Rh connected to the output terminal of the amplifier circuit. ing. A reference input Rr for ambient temperature monitoring, which is pulled up to a DC power supply + V by a resistor R11, is connected to a negative input terminal of the operational amplifier AMP3, and a voltage obtained by dividing the power supply voltage + V by the resistor R11 and the reference resistor Rr. Va is supplied.

In the configuration of FIG. 5, assuming that the resistance value of the resistor R11 and the resistance value of the reference resistor Rr are R11ｒRr, the current flowing through the reference resistor Rr is constant even if the resistance value changes. . The reference resistor Rr has a predetermined temperature coefficient, and the resistance value changes depending on the ambient temperature. As a result, the voltage Va supplied to the operational amplifier AMP3 changes, and accordingly, the voltage Vb of the collector output of the transistor Tr11 changes. As described above, the heater driving circuit uses the temperature coefficient of the reference resistor Rr to change the heater Rh when the ambient temperature changes.
Is corrected by changing the voltage Vb supplied to the power supply.

[0005]

However, the above-described conventional heater drive circuit utilizes a temperature coefficient of the heater Rh and the reference resistor Rr to change the voltage supplied to the heater in accordance with a change in the ambient temperature. Therefore, there is a problem that accurate temperature correction cannot be performed unless the temperature coefficients of the heater Rh and the reference resistor Rr match. In addition, due to variations in the characteristics of each element such as the operational amplifier AMP3 in the amplifier circuit, variations in the resistance value of the reference resistor Rr, variations in the resistance value of the heater Rh, and the like,
There is also a problem that the manner in which the temperature correction is applied differs depending on the individual difference of the flow rate measuring device, and the temperature correction may not be performed well.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and includes variations in resistance values of heaters of a flow sensor, variations in resistance values and temperature characteristics of an ambient temperature monitoring element, and variations in characteristics of elements in a circuit. It is an object of the present invention to provide a heater driving circuit of a flow rate measuring device capable of accurately performing temperature correction regardless of individual differences.

[0007]

According to a first aspect of the present invention, a heater driving circuit for a flow rate measuring device includes a heater for generating heat and a temperature sensor disposed in the vicinity of the heater. A heater driving circuit of a flow measuring device provided with a flow sensor comprising: an ambient temperature detecting unit that outputs an ambient temperature monitor voltage that changes according to the ambient temperature of the heater; and an offset voltage for adjustment. An offset voltage generating means for generating, by adding and amplifying the ambient temperature monitor voltage and the offset voltage,
Heater application voltage generation means for generating a heater application voltage such that the output value of the flow sensor becomes constant with respect to the ambient temperature, power supply means for supplying power to the heater according to the heater application voltage, It is characterized by having.

In the above configuration, the heater application voltage is generated from the sum of the ambient temperature monitor voltage and the offset voltage and is supplied to the heater, so that the resistance value of the ambient temperature monitor element and the temperature characteristics vary, and In this case, accurate temperature correction can be performed regardless of individual differences of the flow rate sensors.

According to a second aspect of the present invention, in the heater driving circuit of the flow rate measuring device, at least one of the offset voltage and the amplification factor of the heater applied voltage generating means can be changed according to the resistance value of the heater. It is characterized by.

In the above configuration, by changing at least one of the offset voltage and the amplification factor of the heater applied voltage generating means in accordance with the resistance value of the heater, it is possible to cope with variations in the resistance value of the heater of the flow rate sensor. Accurate temperature correction is performed irrespective of individual differences between sensors.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram illustrating a configuration of a heater driving circuit according to an embodiment of the present invention, and FIG. 2 is a block diagram illustrating a schematic configuration of a flow rate measuring device according to the embodiment.

As shown in FIG. 2, the flow rate measuring device according to the present embodiment comprises an upstream temperature sensor 112 and a downstream temperature sensor 113 on both sides of a heater 111 composed of a micro heater or the like.
And a flow sensor 101 formed by arranging them. As the flow sensor 101, for example, a small sensor element in which a micro heater and two thermopiles are formed on a silicon substrate can be used. The thermopile constituting the upstream temperature sensor 112 and the downstream temperature sensor 113 senses the temperature of the fluid around each of them as a voltage from the difference between the thermoelectromotive force and the difference in heat capacity between the cold junction and the hot junction. The flow sensor 101 includes a heater 1
A heater drive circuit 102 that drives the heat generation 11 and a processing circuit 103 that obtains a flow rate based on the outputs of the upstream temperature sensor 112 and the downstream temperature sensor 113 are connected.

When the flow rate is measured, the heater drive circuit 1
02 to the heater 111 to supply electric power to the heater 111.
And the upstream temperature sensor 112 and the downstream temperature sensor 113 measure the temperatures on the upstream side and the downstream side.
Then, the outputs of the upstream temperature sensor 112 and the downstream temperature sensor 113 are supplied to the processing circuit 103, and the processing circuit 10
In step 3, a predetermined calculation process is performed to determine the flow rate value Q.
In the processing circuit 103, for example, a calculation process of calculating the difference between the peak value of the temperature on the upstream side and the peak value of the temperature on the downstream side after the heating of the heater and obtaining the flow rate value Q by a predetermined calculation formula based on the difference value is performed. Done. As the arithmetic expression, a regression function (expressed by a polynomial) representing the relationship between the difference value and the flow value is obtained and held in advance, and by substituting the difference value into this arithmetic expression, the flow value Q is calculated. Is calculated. As described above, by detecting the degree of diffusion of heat from the heater 111 via the fluid to be measured, the flow rate of the fluid near the flow rate sensor 101 can be obtained.

Next, the configuration of the heater drive circuit 102 according to the present embodiment will be described in detail with reference to FIG. The heater driving circuit 102 is provided with an ambient temperature monitoring diode D1 whose forward voltage changes according to the ambient temperature. One end of the diode D1 is grounded, and the other end is pulled to a DC power supply + V by a resistor R1. Has been up. The diode D1 and the resistor R1 constitute an ambient temperature detecting means. A connection point between the diode D1 and the resistor R1 is connected to the positive input terminal of the operational amplifier AMP1 via the resistor R2.

In parallel with the diode D1 and the resistor R1, there is provided a variable resistor VR1 as an offset voltage generating means having one end connected to the DC power supply + V and the other end grounded. The tap terminal is connected to the positive input terminal of the operational amplifier AMP1 via the resistor R3. That is, the temperature monitor voltage V1 that changes according to the ambient temperature generated by the diode D1 and the offset voltage V2 generated by the variable resistor VR1 are added, and the voltage V3 after the addition is added to the operational amplifier AMP.
1 is supplied.

The diode D1 can be replaced by another ambient temperature monitoring element provided inside or outside the flow sensor, such as a thermistor or a resistance temperature detector.

A resistor R is provided around the operational amplifier AMP1.
4 and R5 are provided to constitute a negative feedback type amplifier circuit. The output terminal of the operational amplifier AMP1 is connected to an amplifier circuit including the operational amplifier AMP2 and the resistors R6, R7, and R8. Amplifying circuits of the operational amplifier AMP1 and the operational amplifier AMP2 constitute a heater applied voltage generation unit. A transistor T as a power supply unit for performing current amplification is provided at a stage subsequent to the operational amplifier AMP2.
r1 is connected. Further, a heater 111 is connected to the collector terminal of the transistor Tr1.
11 is supplied with drive power P.

Next, the heater driving circuit 1 according to the present embodiment
02 will be described. The forward voltage of the diode D1 changes according to the ambient temperature of the flow sensor 101, and the voltage V3 obtained by adding the offset voltage V2 by the variable resistor VR1 to the temperature monitor voltage V1 is used as an operational amplifier AMP1 as a temperature-corrected voltage corresponding to the ambient temperature. Supplied to This voltage V3 is first amplified by the amplifier circuit of the operational amplifier AMP1, and then adjusted by the amplifier circuit of the operational amplifier AMP2 to become the heater applied voltage V4.
The current is amplified by 1 and applied to the heater 111. As a result, the driving power P is supplied to the heater 111, and the heater 111 generates heat.

The voltage V4 is a voltage applied to the heater such that the output of the flow sensor becomes constant at each ambient temperature.
FIG. 3 is a characteristic diagram showing an example of a temperature characteristic of a heater applied voltage at which a constant flow sensor output is obtained with respect to an ambient temperature. The heater according to the present embodiment has such a characteristic that the heater applied voltage at which the output of the flow sensor becomes constant is linearly increased in the range of about 1.2 V to 1.8 V in accordance with an increase in the ambient temperature.

As described above, by applying a voltage corresponding to the ambient temperature as shown in FIG. 3, the temperature can be corrected, and the fluid to be measured can be heated at a predetermined heating value regardless of the ambient temperature of the flow rate sensor 101. Is heated, a constant output value is obtained from the flow rate sensor 101 with respect to the ambient temperature, and the flow rate can be measured accurately.

In the present embodiment, a temperature monitor voltage V1 that changes according to the ambient temperature due to the action of an ambient temperature monitor element such as a diode, and an offset voltage V2 that changes by adjusting the resistance value of the variable resistor VR1 are added. Voltage V
3 and amplified by an amplifier circuit and applied to a heater applied voltage V4.
, The temperature can be easily and accurately corrected. Since the heater applied voltage is directly adjusted at the time of temperature correction as described above, even if the characteristics of the ambient temperature monitoring element such as the diode D1 vary, the resistance values of the resistors R4 and R5 are changed. This can be handled by adjusting the amplification factor of the amplifier circuit or adjusting the offset voltage V2 by changing the resistance value of the variable resistor VR1. In addition, it is possible to cope with variations in the characteristics of each element in the heater drive circuit 102 in a similar manner.

In addition, the characteristics of the ambient temperature and the applied voltage of the heater are measured in advance for each of the plurality of heaters, and those having similar resistances are grouped, and the offset voltage and the amplification factor are determined for each resistance. By changing the setting, it is possible to cope with the variation of the heater provided in the flow sensor 101. That is, the variation is absorbed by changing the offset voltage and the amplification factor of the heater drive circuit 102 according to the resistance value of the heater, so that the optimum heater application voltage is always output. Accordingly, the temperature can be corrected in accordance with the resistance value of each heater without measuring the temperature characteristics of the flow rate sensor one by one for setting the circuit constant, thereby improving the productivity.

FIG. 4 shows an example of a temperature characteristic of a heater applied voltage with respect to an ambient temperature obtained for each group of heater resistance values. As shown in FIG. 4, with respect to groups 1 to 3 of heater resistance values grouped for each resistance value within a predetermined range, the slope of the temperature characteristic according to the heater resistance value so that an appropriate heater applied voltage can be supplied. The setting of the offset voltage and the amplification factor of the heater drive circuit 102 is changed in each group so that the temperature changes, and the temperature is corrected.

As described above, in the present embodiment, the temperature monitor corresponding to the ambient temperature obtained by utilizing the temperature characteristics of the ambient temperature monitor element so that the output value of the flow sensor becomes constant with respect to the ambient temperature. An appropriate power is supplied to the heater by adding and amplifying the voltage and the offset voltage set by the variable resistor. This allows
Even if there is variation in the resistance value of the heater of the flow rate sensor or variation in the characteristics of the elements in the circuit, the temperature can be accurately corrected regardless of individual differences.

At this time, it is not necessary to make the temperature coefficients of the resistance values of the ambient temperature monitor element and the heater equal. That is, no matter what kind of ambient temperature monitoring element is used, the ambient temperature can be detected using the temperature characteristics of the element and the temperature can be corrected. Therefore, according to the heater drive circuit of the present embodiment, the optimum temperature correction can always be performed, so that accurate flow rate measurement can be performed regardless of the ambient temperature.

[0026]

As described above, according to the present invention, an ambient temperature detecting means for outputting an ambient temperature monitor voltage which changes according to the ambient temperature of a heater, and an offset voltage generating means for generating an offset voltage for adjustment. And a heater application voltage generation unit that generates a heater application voltage such that the output value of the flow sensor becomes constant with respect to the ambient temperature by adding and amplifying the ambient temperature monitor voltage and the offset voltage. Since the power supply means for supplying power corresponding to the heater applied voltage to the heater is provided, even if there are variations in the resistance value and temperature characteristics of the ambient temperature monitoring element, variations in the characteristics related to elements in the circuit, etc. There is an effect that the temperature can be accurately corrected regardless of individual differences. Further, by making it possible to change at least one of the offset voltage and the amplification factor of the heater applied voltage generation means in accordance with the resistance value of the heater, it is possible to cope with variations in the resistance value of the heater of the flow rate sensor, and The temperature can be accurately corrected regardless of the temperature.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of a heater drive circuit according to an embodiment of the present invention.

FIG. 2 is a block diagram illustrating a schematic configuration of a flow measurement device according to the present embodiment.

FIG. 3 is a characteristic diagram showing an example of a temperature characteristic of a heater application voltage with respect to an ambient temperature.

FIG. 4 is a characteristic diagram showing an example of a temperature characteristic of a heater applied voltage with respect to an ambient temperature obtained for each group of heater resistance values.

FIG. 5 is a circuit diagram showing a configuration example of a conventional heater drive circuit.

[Explanation of symbols]

101 Flow rate sensor 102 Heater drive circuit 103 Processing circuit 111 Heater 112 Upstream temperature sensor 113 Downstream temperature sensor AMP1, AMP2 Operational amplifier D1 Diode R1, R2, R3, R4, R5, R6, R7, R8 Resistor Tr1 Transistor VR1 Variable resistor vessel

Claims (2)

[Claims] 1. A heater driving circuit of a flow measuring device provided with a flow sensor having a heater for heat generation and a temperature sensor disposed near the heater, the heater driving circuit corresponding to an ambient temperature of the heater. An ambient temperature detecting unit that outputs a changing ambient temperature monitor voltage; an offset voltage generating unit that generates an offset voltage for adjustment; and adding and amplifying the ambient temperature monitor voltage and the offset voltage, thereby obtaining Heater application voltage generation means for generating a heater application voltage such that the output value of the flow sensor becomes constant with respect to temperature; and power supply means for supplying power to the heater in accordance with the heater application voltage. A heater drive circuit for a flow measurement device. 2. The apparatus according to claim 1, wherein at least one of the offset voltage and the amplification factor of the heater applied voltage generating means can be changed according to a resistance value of the heater.
4. A heater drive circuit of the flow rate measuring device according to 1.