JP5178261B2 - Thermal flow meter - Google Patents

Description

  The present invention is configured using a sensor chip in which a pair of heat sensitive elements provided on the same chip with a heat generating element sandwiched in the fluid flow direction and a temperature detecting element for detecting ambient temperature are formed. The present invention relates to a thermal flow meter with high measurement accuracy that suppresses variations in flow rate detection characteristics caused by variations in temperature characteristics.

  For example, as shown in FIG. 3, the thermal flow sensor has a pair of heat sensitive elements in a flow direction F of a fluid (gas) on a thin diaphragm D formed on a silicon substrate (sensor chip) B with a heating element Rh therebetween. The elements Ru and Rd are provided, and a temperature detection element Rr for detecting the ambient temperature is integrally provided on the periphery of the silicon substrate B. A thermal type flow meter configured using such a thermal type flow sensor has the above-mentioned fluid (from the change in temperature distribution in the vicinity of the sensor surface due to the fluid (gas) flowing along the sensor surface formed by the diaphragm D). Gas) is configured to detect a flow rate (flow velocity) [see, for example, Patent Document 1].

  However, the heating element Rh, the pair of thermal elements Ru and Rd, and the temperature detection element Rr are made of a resistor such as platinum (Pt), and generally the resistance value thereof varies depending on the temperature as shown in FIG. It has temperature change characteristics. Moreover, it cannot be denied that the temperature change characteristic varies from production lot to production lot due to the influence of various factors in the production process. Therefore, the thermal flow sensor having the above-described configuration usually has a certain temperature characteristic in which the sensor output changes depending on the temperature.

Therefore, conventionally, for example, as disclosed in Patent Document 1 described above, the temperature characteristics of a thermal flow sensor are measured in advance, and the sensor output at the time of flow measurement is subjected to temperature correction (zero point correction). It should be noted that a temperature correction resistor is incorporated in the sensor circuit for measuring the flow rate from the change in resistance value of the pair of thermosensitive elements Ru and Rd, and the sensor output at the reference temperature is equal to the sensor output when the temperature changes. It has also been proposed to correct so that it becomes [see, for example, Patent Document 2].
JP 2004-93174 A JP 2006-329638 A

  However, as disclosed in Patent Document 1, in order to perform temperature correction (zero point correction) for the sensor output during flow rate measurement, the temperature change characteristic of the thermal flow rate sensor is measured in advance and a temperature correction table or the like is prepared. It is necessary and very annoying. Moreover, since it is necessary to incorporate a temperature correction function into the thermal flow sensor, it cannot be denied that the price is increased. Further, as disclosed in Patent Document 2, in order to incorporate a temperature correction resistor into the sensor circuit, for example, it is necessary to individually adjust the correction resistor before shipment from the factory, and the adjustment cost increases. Absent.

  The present invention has been made in consideration of such circumstances, and its purpose is to suppress variations in flow rate detection characteristics caused by variations in temperature characteristics of thermal elements and the like, and to easily perform temperature correction on the flow rate detection characteristics. An object of the present invention is to provide a thermal flow meter with high measurement accuracy.

In order to achieve the above-mentioned object, the thermal flow meter according to the present invention is
<a> A heat generating element Rh, a sensor chip in which a pair of heat sensitive elements Ru and Rd provided with the heat generating element Rh sandwiched in the fluid flow direction and a temperature detecting element Rr for detecting the ambient temperature are formed on the same chip; ,
<b> For a heater formed by connecting a first fixed resistor R1 in series to the heating element Rh and a second fixed resistor R2 in series to the temperature detecting element Rr, and connecting these series circuits in parallel. A heater circuit comprising a bridge circuit and a feedback circuit for controlling the drive voltage of the heater bridge circuit to keep the output of the heater bridge circuit constant;
<c> The pair of thermosensitive elements Ru and Rd are connected in series and the third and fourth fixed resistors Rx and Ry are connected in series. These series circuits are connected in parallel, and a constant voltage is applied. A sensor bridge circuit to be driven, and a sensor circuit including an amplifier that amplifies and outputs the output of the sensor bridge circuit;
<d> The temperature change characteristic of the flow rate detection with respect to the change in the ambient temperature is canceled out by the temperature change characteristic of the sensor circuit that is in direct proportion to the change in the ambient temperature. The resistance values of the third and fourth fixed resistors and the pair of heat sensitive elements are set so that is flat .

  Incidentally, the heating element Rh, the pair of thermal elements Ru, Rd, and the temperature detection element Rr are made of a resistor of the same material, for example, platinum (Pt). Preferably, the heating temperature Th of the heating element Rh is set high, and the driving voltage of the sensor bridge circuit is set low, whereby the heating element Rh, the pair of thermal elements Ru, Rd, and the temperature detection element. It is preferable to suppress variation in each temperature characteristic itself in Rr.

  According to the above-described configuration, the temperature detection characteristic of the thermal flow meter is kept constant overall (overall) by offsetting the positive temperature change characteristic of the heater circuit with the negative temperature change characteristic of the sensor circuit. Can be. As a result, it is possible to realize a thermal flow meter having a flow detection characteristic that is thermally stable and highly accurate by facilitating temperature correction of the temperature detection characteristic. In particular, the temperature change characteristics of the sensor circuit are offset by the temperature change characteristics of the heater circuit by actively utilizing the temperature change characteristics of the heating element Rh, the pair of heat sensitive elements Ru and Rd, and the temperature detection element Rr. Therefore, the variation in the sensor output depending on the temperature is suppressed, so that the temperature correction for the sensor output can be performed easily, and the temperature correction itself can be performed easily and with high accuracy. Therefore, practically great effects such as being able to maintain the flow rate measurement accuracy sufficiently high are exhibited.

  The heating element Rh, the pair of thermal elements Ru, Rd, and the temperature detection element Rr are formed of a resistor of the same material, and the heating temperature Th of the heating element Rh in the heater circuit is set high, and the sensor bridge circuit If the drive voltage is set low, this can suppress variations in the temperature characteristics of the heating element Rh, the pair of thermal elements Ru and Rd, and the temperature detection element Rr, thereby further improving the flow rate detection accuracy. In addition, effects such as stabilization of flow rate detection characteristics can be achieved.

Hereinafter, a thermal type flow meter according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram of a thermal type flow meter according to the present invention. Reference numeral 1 denotes a pair of thermal elements Ru, Rd, a heating element (heater element) Rh, and temperature detection on a semiconductor substrate (sensor chip) such as silicon. For example, a thermal flow sensor having an element structure as shown in FIG. 3 in which an element Rr is formed. Incidentally, the pair of heat sensitive elements Ru, Rd, the heating element (heater element) Rh, and the temperature detecting element Rr are formed of a thin film resistor formed by depositing platinum (Pt) on the sensor chip 1 or the like. In particular, these elements Ru, Rd, heat generating element (heater element) Rh, and temperature detecting element Rr are collectively formed on the sensor chip 1 by using a resistor of the same material, so that these elements Ru, Rd , Rh, and Rr, the variation in the temperature change characteristic of the resistance value is suppressed.

  Now, the drive circuit of the thermal flow sensor 1 basically drives the heat generating element Rh in accordance with the ambient temperature detected by the temperature detecting element Rr, and in the vicinity of the pair of heat sensitive elements Ru and Rd. The heater circuit 3 that raises the temperature by a constant temperature T and the temperature sensitive elements Ru and Rd detect the temperatures Tu and Td in the vicinity thereof, and the temperature difference ΔT (= Tu−Ud) is detected by the thermal flow sensor 1. The sensor circuit 4 is obtained as a flow rate (flow velocity) Q of the fluid flowing along

  Specifically, the heater circuit 3 includes the heating element Rh and a first fixed resistor R1 connected in series to the heating element Rh, and the temperature detection element Rr and a second fixed resistance connected in series to the temperature detection element Rr. Each of the resistors R2 is a half-bridge circuit, and the first bridge circuit 3a for temperature control formed by connecting these two half-bridge circuits (series circuit) in parallel is mainly configured. Then, the drive voltage Vdriv of the bridge circuit 3a is generated through the transistor 3b that operates by receiving the power supply voltage Vcc, and the bridge output voltage (potential difference between bridges; Vh−Vr) of the bridge circuit 3a is generated by the differential amplifier 3c. The operation of the transistor 3b is feedback controlled so that the bridge output voltage becomes zero (0), and the drive voltage Vdriv of the bridge circuit 3a is varied. By the feedback control of the transistor 3b by the differential amplifier 3c, the heat generation temperature Th of the heat generation element Rh is always set higher than the ambient temperature (atmosphere temperature) detected by the temperature detection element Rr by a constant temperature T. .

  The sensor circuit 4 includes a pair of thermal elements Ru and Rd provided in the fluid flow direction with the heating element Rh therebetween, and a pair of fixed resistors Rx, which are third and fourth fixed resistors. The flow rate measuring second bridge circuit 4a configured using Ry is mainly used. Specifically, the second bridge circuit 4a forms a half bridge circuit by connecting the pair of thermal elements Ru, Rd in series, and forms a half bridge circuit by connecting the fixed resistors Rx, Ry in series. These two half-bridge circuits (series circuits) are connected in parallel.

  The second bridge circuit 4a is driven by being applied with a constant voltage VR from a constant voltage source 5, and more specifically, a bridge in which a thermal resistor Ru on the upstream side of the heating element Rh is arranged. A constant voltage VR is applied to the upper side, and a constant voltage drive is performed by grounding the lower side of the bridge where the thermal resistor Rd on the downstream side of the heating element Rh is disposed. The bridge output voltage (potential difference between bridges; Vs−Vf) corresponding to the change in the resistance value of the thermal elements Ru and Rd in the second bridge circuit 4a is detected by the differential amplifier 4b and has a predetermined amplification gain (gain). ) It is amplified by G and obtained as a sensor output Vout having a predetermined voltage level. The amplification gain (gain) G is determined by the feedback resistor Rf of the differential amplifier 4b.

  In the thermal flow meter configured as described above, the behavior when the flow rate is zero [0] will be described. As described above, the first bridge circuit 3a constituting the heater circuit 3 basically has an ambient temperature. The voltage Vr obtained by dividing the drive voltage Vdriv by the temperature detection element Rr and the fixed resistor R2 whose resistance value changes according to the voltage Vh, and the voltage Vh obtained by dividing the drive voltage Vdriv by the heating element Rh and the fixed resistor R1. And the drive voltage Vdriv is controlled so that the heat generation temperature of the heat generating element Rh is higher than the ambient temperature by a constant temperature T.

  However, when the drive voltage Vdriv is feedback-controlled to control the heat generation temperature of the heat generating element Rh, the current flowing through the temperature detecting element Rr changes with the change of the driving voltage Vdriv, and as a result, the temperature detecting element Since the resistance value of Rr slightly changes, the heater circuit 3 having the above-described configuration has a positive temperature change characteristic as shown in FIG.

  That is, for example, as the ambient temperature rises, the resistance value of the temperature detection element Rr increases, and the divided voltage Vr of the drive voltage Vdriv by the fixed resistance R2 increases, so that the fixed resistance R1 and the heating element Rh As the drive voltage Vdriv is increased to increase the divided voltage Vh of the drive voltage Vdriv, the current flowing through the temperature detection element Rr increases accordingly. Then, as the current increases, the resistance value of the temperature detection element Rr is slightly increased, but eventually, the heat generation temperature Th of the heat generation element Rh is set to a constant temperature T from the ambient temperature. It becomes higher by the addition of the minute temperature t resulting from the resistance value change of Rr. This means that the heater circuit 3 formed by forming the bridge circuit 3a as described above has a positive temperature change characteristic with respect to the temperature change.

  On the other hand, the above-described bridge circuit 4a constituting the sensor circuit 4 is driven at a constant voltage VR at a constant voltage VR, and the pair of thermal elements Ru and Rd are respectively incorporated on the bridge lower side of the bridge circuit 4a. It is. When the resistance values of the pair of thermal elements Ru and Rd change due to changes in the ambient temperature, the currents flowing through the thermal elements Ru and Rd change accordingly, and as a result, the thermal element Ru , Rd also change, so that the heater circuit 3 having the above-described configuration has a negative temperature change characteristic as shown in FIG.

  That is, for example, when the temperature of the gas (fluid) is increased as the ambient temperature increases, the resistance values of the pair of thermal elements Ru and Rd are also increased accordingly. Then, as described above, since the bridge circuit 4a is driven by the constant voltage VR, the currents flowing through the thermal elements Ru and Rd respectively decrease as the resistance values of the thermal elements Ru and Rd increase. As a result, the respective resistance values of the thermal elements Ru and Rd are slightly reduced. As a result, the bridge output of the bridge circuit 4a slightly decreases as the resistance values of the thermal elements Ru and Rd decrease as the ambient temperature increases. In other words, the bridge output of the bridge circuit 4a is a voltage slightly lower than the voltage corresponding to the temperature of the gas (fluid) described above. Therefore, as described above, the sensor circuit 4 that forms the bridge circuit 4a and is driven at a constant voltage has a negative temperature change characteristic with respect to the temperature change.

  As described above, the heater circuit 3 causes the heat generation temperature Th of the heat generation element Rh to change with respect to the temperature change due to the temperature change characteristics of the resistance values of the heat generation element Rh and the temperature detection element Rr. It has a positive temperature change characteristic that changes as shown in FIG. The sensor circuit 4 has a negative temperature at which the sensor output Vout changes as shown in FIG. 2B with respect to the temperature change due to the temperature change characteristics of the resistance values of the thermal elements Ru and Rd. Has change characteristics.

  Accordingly, in the thermal type flow meter configured to include the heater circuit 3 and the sensor circuit 4 having the above-described temperature change characteristics, the sensor circuit 4 is comprehensively (overall) according to the temperature change in the heater circuit 3. The temperature change at is canceled out. Therefore, if the positive temperature change characteristic of the heater circuit 3 is set so as to be offset by the negative temperature change characteristic of the sensor circuit 4, ideally a flat temperature change as shown in FIG. It becomes possible to realize a thermal flow meter having characteristics. In other words, according to the thermal flow meter configured as described above, a constant flow rate that is not affected by variations in resistance value change characteristics with respect to the temperatures of the heating element Rh, the thermal elements Ru and Rd, and the temperature detection element Rr. It is possible to provide detection characteristics.

  Therefore, according to the thermal type flow meter of the present invention, the fixed resistors Rx, Ry are previously set so that the positive temperature characteristic of the heater circuit 3 and the negative temperature characteristic of the sensor circuit 4 have a complementary relationship. By setting circuit constants defined by the respective resistance values of Ru, Rd, Rd, it is possible to make the temperature change characteristics of the sensor output Vout constant, so that the temperature correction can be easily performed. It becomes possible. In addition, the temperature change characteristics of the heating element Rh, the pair of heat sensitive elements Ru and Rd, and the temperature detection element Rr are positively utilized, and the temperature change characteristics vary between the heater circuit 3 and the sensor circuit 4. Since the temperature change characteristics can be made uniform, the flow measurement accuracy can be sufficiently increased under simple temperature correction. A great effect is produced.

  In particular, the heating element Rh, the pair of thermal elements Ru, Rd, and the temperature detection element Rr are formed of a resistor made of the same material, and the heating temperature (heater temperature) Th of the heating element Rh in the heater circuit 3 is set high. If the drive voltage VR of the bridge circuit 4a in the sensor circuit 4 is set low, it is possible to suppress variations in the temperature characteristics of the heating element Rh, the pair of thermal elements Ru and Rd, and the temperature detection element Rr. Therefore, the above-described positive temperature change characteristic and negative temperature change characteristic can be easily combined, so that the flow rate detection accuracy can be further improved and the flow rate detection characteristic can be stabilized. An effect is produced.

  Furthermore, according to the configuration described above, even if the temperature change characteristics of the resistance values of the thermal elements Ru and Rd, the heating element (heater element) Rh, and the temperature detection element Rr described above vary depending on the manufacturing lot, The temperature characteristic can be made constant by canceling the variation in the temperature change characteristic. Therefore, it is possible to easily realize a thermal flow meter with uniform characteristics regardless of the difference in production lots. In addition, since the temperature characteristics of the thermal type flow meter can be made uniform in this way, the zero point correction of the sensor output with respect to the temperature can be easily carried out without examining the individual sensor output characteristics in advance. The effect of.

  The present invention is not limited to the embodiment described above. For example, the heat generation temperature (heater temperature) Th of the heat generating element Rh, the drive voltage VR of the bridge circuit 4a constituting the sensor circuit 4 and the like are the above-described heat sensitive elements Ru and Rd, the heat generating elements (heater elements) Rh, and the temperature detecting element. According to the circuit specifications determined by the resistance values of Rr and the resistance values of the fixed resistors R1, R2, Rx, and Ry, the conditions of the positive temperature change characteristic and the negative temperature change characteristic described above may be satisfied. It ’s good. In addition, the present invention can be variously modified and implemented without departing from the scope of the invention.

The schematic block diagram of the thermal type flow meter which concerns on one Embodiment of this invention. The figure which shows the temperature change characteristic of the thermal type flow meter shown in FIG. The figure which shows the schematic element structure of a thermal type flow sensor. The figure which shows the temperature change characteristic of the resistance value in the thermosensitive elements Ru and Rd, the heat generating element Rh, and the temperature detection element Rr.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Sensor chip 3 Heater circuit 3a Bridge circuit 4 Sensor circuit 4a Bridge circuit 5 Constant voltage source Ru, Rd Thermal element Rh Heating element Rr Temperature detection element R1, R2, Rx, Ry Fixed resistance

Claims (2)

A sensor chip in which a heating element, a pair of thermal elements provided with the heating element sandwiched in the fluid flow direction, and a temperature detection element for detecting the ambient temperature are formed on the same chip;
A bridge circuit for a heater formed by connecting a first fixed resistor in series with the heating element and a second fixed resistor in series with the temperature detecting element, and connecting these series circuits in parallel, and for the heater A heater circuit comprising a feedback circuit for controlling the driving voltage of the bridge circuit to keep the output of the heater bridge circuit constant;
A sensor bridge circuit formed by connecting the pair of thermosensitive elements in series and connecting the third and fourth fixed resistors in series, and connecting these series circuits in parallel, and driven by applying a constant voltage; and A sensor circuit including an amplifier that amplifies and outputs the output of the bridge circuit for the sensor;
The temperature change characteristic of the heater circuit that is directly proportional to the change in the ambient temperature is offset by the temperature change characteristic of the sensor circuit that is inversely proportional to the change in the ambient temperature, and the temperature change characteristic of the flow rate detection with respect to the change in the ambient temperature is flat. The thermal flow meter is characterized in that the third and fourth fixed resistors and the resistance values of the pair of heat sensitive elements are set .   The thermal flow meter according to claim 1, wherein the heat generating element, the pair of heat sensitive elements, and the temperature detecting element are made of a resistor made of the same material.

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