JP2006258434A - Thermal flowmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thermal flow meter capable of constituting the detection circuit simply and capable of detecting precisely. <P>SOLUTION: The 1st series circuit 1 connected serially with a pair of upstream and down stream temperature sensitive electric resistances (Ru, Rd) and the fixed resistance (Ra), and the 2nd series circuit 2 connected serially with the temperature measurement resistance (Rt), and the fixed resistances (Rb, Rc) constitute an electric bridge circuit (BC). The 1st operational amplifier circuit (A1) for detecting the balance of the bridge circuit (BC) controls the potential Vc of the point c of the bridge circuit BC, and the 2nd operation amplifier (A2) for detecting difference of the voltage Ve of the connection point e of the upstream and the downstream pair sensors and the specific voltage V2 controls the voltage Vd of the point d of the bridge circuit BC. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a thermal type flow meter, and more particularly to a thermal type flow meter in which temperature-dependent electrical resistance sensor elements are arranged upstream and downstream of a flow path, respectively.

In general, as a thermal flow meter, a pair of temperature-dependent electrical resistance sensor elements are arranged upstream and downstream along the flow path, and power is supplied to both sensor elements to generate heat, and the fluid is thermally It is known to measure the mass flow rate by utilizing the relationship between the amount of heat exchanged between the upstream and downstream sensors. In this type of flow meter, the method of supplying electric power is as follows: 1. a method of supplying the same constant current or voltage to the upstream and downstream sensors; 2. a method for maintaining the resistance value of each of the upstream and downstream sensor elements and hence the temperature at the same temperature; A method of keeping the total average temperature of the upstream and downstream sensors constant is known (see, for example, Patent Document 1). Among these, the methods 2 and 3 are provided with a temperature sensor, and control the difference between the temperature sensor detected temperature and the flow rate sensor temperature with respect to the detected temperature of the temperature sensor element to cope with the temperature change of the flow rate detection. FIG. 5 shows a specific implementation circuit of the system for keeping the total average temperature of the above three upstream and downstream sensors constant. In FIG. 5, a series circuit 1 in which a pair of temperature-dependent electric resistance sensor elements Ru, Rd and a fixed resistor Ra provided upstream and downstream are connected in series, and a resistance temperature detector for detecting a fluid or ambient temperature. An electric bridge (bridge) circuit BC is constituted by the series circuit 2 in which the element Rt and the fixed resistors Rc and Rb are connected in series.

  The connection point a between the temperature-dependent electrical resistor sensor element Rd and the fixed resistor Ra of the bridge circuit BC is connected to the input terminal of the operational amplifier A1, and the connection point b between the fixed resistor Rc and the fixed resistor Rb is calculated. The amplifier A1 is connected to the + input terminal, and the output terminal of the operational amplifier A1 is connected to one connection point c of the series circuit 1 and the series circuit 2 of the bridge circuit BC. The other connection point d of the series circuit 1 and the series circuit 2 of the bridge circuit BC and the connection point e of the electric resistance sensor elements Ru and Rd are connected to the input terminal of the operational amplifier A2, and the connection of the bridge circuit BC. The point e and the connection point a are connected to the input terminal of the differential amplifier A4, and the output terminal of the operational amplifier A2 and the output terminal of the differential amplifier A4 are connected to the input terminal of the differential amplifier A3.

  The bridge circuit BC is balanced by (Ru + Rd) / Ra = (Rt + Rc) / Rb. In this circuit, when the resistance temperature detector Rt for detecting the fluid or the ambient temperature is constant, and therefore the fluid or ambient temperature is constant, the temperature-dependent electrical resistance sensor element Ru, The applied voltage Es = Eu + Ed is controlled so that the sum Rs of Rd, that is, the total average temperature of the upstream and downstream electric resistance sensor elements Ru, Rd is kept constant.

In this case, the flow rate output signal is obtained from the difference dEs = Eu−Ed between the sensor voltage Eu of the upstream sensor element Ru and the sensor voltage Ed of the downstream sensor element Rd. The influence of the temperature measuring resistance element temperature Ta on the flow rate signal is that when the fluid or the ambient temperature Ta changes, the change in the temperature Ta of the temperature measuring resistance element Rt is Rs = Ru + Rd = Rso (1 + αs * Ts) = { Rto (1 + αt * Ta) + Rc} (Ra / Rb)
From Ts = {(1 + αt * Ta + Rc / Rt0) (Ra / Rb) (Rto / Rso) −1} / αs, the average temperature of the upstream and downstream temperature-dependent electric resistance sensor elements with respect to changes in fluid or ambient temperature Ta By changing Ts, the output characteristic temperature dependency on the measured flow rate is compensated.

The flow rate of the fluid to be measured is determined by the amount of heat conduction of the upstream temperature-dependent electrical resistance sensor element Ru with respect to the fluid, which is detected from the difference in conduction heat amount between the upstream and downstream temperature-dependent electrical resistance sensor elements Ru and Rd. It is larger than the heat conduction amount of the downstream temperature-dependent electrical resistance sensor element Rd, and the sum Rs = Ru + Rd of the upstream and downstream temperature-dependent electrical resistance sensor elements Ru and Rd is constant. The resistance of the resistive electrical sensor element is Ru = (Rs−dRs) / 2, Rd = (Rs + dRs) / 2, where dRs is a value accompanying an increase / decrease phase with respect to the flow rate. On the other hand, the currents Is flowing through the upstream and downstream temperature-dependent electrical resistance sensor elements are common to the upper and lower sides (the larger the flow rate, the higher the current). Therefore, the upstream and downstream temperature-dependent electrical resistance sensor element voltages are Esu = IsRsu, Assuming that Esd = IsRsd, the flow signal output is obtained as dEs = Esd−Esu = Is (Rsd−Rsu) = Is * dRs.
JP 2002-122454 A

  In the conventional flow meter circuit described above, one sensor voltage Eu can be easily converted by a circuit using one general-purpose operational amplification device, while the other sensor voltage Ed is obtained by subtracting the voltage Eu from the sum Eu + Ed of both voltages. The required differential amplifier circuit is required and the circuit becomes complicated, and there are many problems in detecting an extremely small voltage difference of 1/100 to 1/1000 compared to the sensor voltage.

  In addition, the common-mode input voltage of a commercially available operational amplifier needs to be within 1 to 1.5 V from the potential at both ends of the power supply of the operational amplifier. In the conventional circuit, the potential at the balanced detection end of the bridge circuit is It is necessary to enter this range, and the voltage range applied to each temperature sensor is narrowed by about 2V to 3V, and the required flow rate detection voltage sensitivity cannot be obtained. In particular, the final use voltage of commercially available dry batteries is 0.9V per battery. If two or three batteries are used, the minimum power supply voltage is required from 1.8V to 2,7V. The end voltage (common potential) becomes a problem.

  The present invention has been made paying attention to the above-mentioned problems, and the detection circuit can be simply configured and can be detected with high accuracy, and can be reduced with a lower power supply voltage corresponding to battery driving or the like. An object of the present invention is to provide a thermal flow meter that can operate with power supply power.

  In the thermal flow meter of the present invention, a pair of temperature-dependent electric resistance sensor elements (Ru, Rd) are arranged upstream and downstream along a fluid flow path, and a voltage is applied to both of the sensor elements to provide fluid. A thermal flowmeter that measures the mass flow rate by utilizing the relationship between the amount of heat exchanged between the upstream and downstream sensor elements, and the upstream and downstream sides of the flow rate change. The average temperature of the sensor elements (Ru, Rd) is not changed, and the temperature dependence between the sensor element average temperature and the fluid is controlled with respect to the ambient temperature or the fluid temperature. A first series circuit (1) in which an electric resistance sensor element (Ru, Rd) and a fixed resistor (Ra) are connected in series, a temperature sensor element (Rt) and a fixed resistor (Rb, Rc) in series. The second series circuit (2) connected to the two sides A first operational amplifier circuit (A1) that constitutes a bridge circuit (BC) and detects the balance of the bridge circuit controls one potential (Vc) of the bridge circuit, and the upstream and downstream pair sensor elements The other potential (Vd) of the bridge circuit is controlled by a second operational amplifier (A2) that detects a difference between the connection point of the voltage and a predetermined potential.

  In the present invention, a first series circuit in which a pair of temperature-dependent electrical resistance sensor elements and a fixed resistor are connected in series in the upstream and downstream, and a second in which the temperature sensor element and the fixed resistor are connected in series. Means for adjusting the same voltage at the balance detection end of the bridge circuit so that the input potential of the first operational amplifier circuit for detecting the balance of the bridge circuit with both sides of the series circuit satisfies the potential condition of the operational amplifier. Can be provided.

  In the present invention, the voltage adjusting means includes, for example, a first addition circuit provided between a connection point between the temperature-dependent electrical resistance sensor element of the first series circuit and the fixed resistor and a predetermined potential, A second adder circuit provided between a connection point of the temperature sensor element and the fixed resistor of the second series circuit and a predetermined potential, and outputs the first and second adder circuits to the first calculation It is good to receive at the input of the amplifier circuit.

  According to the first aspect of the present invention, the potential difference between the connection point of the upstream and downstream sensors and the potential difference between the connection point of the upstream and downstream sensors are detected by the addition circuit using a simple resistor, and the flow rate output can be easily obtained. I can do it.

  Further, according to the inventions according to claims 2 and 3, the effective voltage range of the bridge circuit can be greatly expanded with respect to the operational amplifier power supply for limiting the supply power supply voltage of this type of device. It is possible to reduce the power accompanying the reactive voltage due to the difference in the effective range of the bridge circuit, and it is particularly effective when it is desired to use the battery until the power supply becomes low, such as battery driving.

Hereinafter, the present invention will be described in more detail with reference to embodiments. FIG. 1 is a circuit diagram showing a detection circuit of a thermal flow meter according to an embodiment of the present invention. In FIG. 1, a series circuit 1 in which a pair of temperature-dependent electrical resistance sensor elements Ru, Rd and a fixed resistor Ra provided upstream and downstream are connected in series, and a resistance temperature detector for detecting fluid or ambient temperature A bridge circuit BC having the series circuits 1 and 2 as both sides is constituted by the series circuit 2 in which the element (temperature sensor element) Rt and the fixed resistors Rc and Rb are connected in series. The temperature-dependent electric resistance sensor element Ru among the elements constituting the bridge circuit BC is arranged in the flow path 11 through which a fluid flows as shown in FIG. 2, and the temperature-dependent electric resistance sensor element. Rd is disposed downstream of the electric resistance sensor element Ru in the flow path 11. When the temperature measuring resistance element Rt detects the temperature of the fluid, the temperature-dependent electric resistance sensor element Ru is not affected by the heat generation of the temperature-dependent electric resistance sensor elements Ru and Rd. Further, when detecting the ambient temperature upstream, it is arranged outside the flow path 11. Further, the fixed resistors Ra, Rb, and Rc are provided outside the flow path 11.

  The + V1 power source is connected to one connection point c between the series circuit 1 and the series circuit 2 of the bridge circuit BC via the collector and emitter of the transistor Q1, and the series circuit 1 and the series circuit 2 of the bridge circuit BC are connected. Is connected to the power source V3 via the collector and emitter of the transistor Q2.

  The operational amplifier A1 has a + input terminal connected to the connection point a between the electric resistor sensor element Rd and the fixed resistor Ra of the bridge circuit BC, and a negative input terminal connected to the fixed resistors Rc and Rb of the bridge circuit BC. And the output terminal of the operational amplifier A1 is connected to the base of the transistor Q1. The operational amplifier A1 is an operational amplifier that detects the balance of the bridge circuit BC. The output of the transistor Q1 is such that Va = Vb and (Ru + Rd) = (Ra / Rb) * (Rc + Rt). Input to the base and control the potential Vc of the supply voltage to the bridge circuit BC.

  The operational amplifier A2 has a negative input terminal connected to the reference comparison voltage V2, a positive input terminal connected to a connection point e between the electric resistance sensor elements Ru and Rd of the bridge circuit BC, and an output terminal of the operational amplifier A2. Is connected to the base of transistor Q2. In addition, the operational amplifier A2 is configured so that the potential Ve at the connection point e between the upstream electrical resistance sensor element Ru and the downstream electrical resistance element Rd is equal to a predetermined potential V2. The electric potential Vd of the power supply for the bridge circuit is controlled. The connection point a of the bridge circuit BC is connected to the positive input terminal of the operational amplifier A3 via the resistor Rpd, and the connection point d of the bridge circuit BC is connected to the positive input terminal of the operational amplifier A3.

  By controlling the operational amplifiers A1 and A2, in the operation of the operational amplifier A1, the average temperature Tsu + Tsd of the upstream and downstream electrical resistor sensor elements becomes a function of the temperature sensor Rt regardless of the fluid flow rate to be measured, and the required temperature compensation is performed. Can be done. Further, in the operation of the operational amplifier A2, the potential Ve at the connection point e of the upstream and downstream temperature-dependent electric resistance sensor elements Ru and Rd is always held at the reference potential V2, and the potential Va at the point a in FIG. The value Va = V2 + Ed obtained by adding the voltage Ed of the downstream side electric resistor sensor element Rd, and the potential Vd at the point d in FIG. 1 becomes the reference potential V2, and the voltage Eu of the upstream temperature dependent electric resistor sensor element Ru is set to the reference potential V2. The subtracted value Vd = V2−Eu. As a result, since the sensor output signal dEs = Ed−Eu = Va + Vd with respect to the flow rates of the upstream and downstream temperature-dependent electric resistance sensor elements Ru and Rd, it can be obtained by a simple addition circuit of points a and b in FIG. it can. In the adding circuit of the resistors Rpu and Rpd and the operational amplifier A3 in FIG. 1, dVs = (Ed−Eu) / 2.

  FIG. 3 is a circuit diagram showing a circuit according to another embodiment of the present invention. In the circuit of this embodiment, in addition to the circuit shown in FIG. 1, an adder circuit (voltage dividing circuit) 3 including resistors R1 and R2 is further provided between the connection point a of the bridge circuit BC and the potential V4. An adder circuit (voltage dividing circuit) 4 including resistors r1 and r2 is provided between the connection point b of the bridge circuit BC and the potential V4, and the connection point of the resistors R1 and R2 (output of the adder circuit 3) is connected to the operational amplifier A1. Are connected to the positive input terminal of the operational amplifier A1, and the connection point of the resistors r1 and r2 (the output of the adder circuit 4) is connected to the negative input terminal of the operational amplifier A1. The output of the operational amplifier A1 is connected to the base of the transistor Q1.

  In the circuit of this embodiment, the resistors R1, R2, r1, and r2 of the adder circuits 3 and 4 are R1 / R2 = r1 / r2, and ideally R1 / r1 = R2 / r2 = Ra / Rb. If each resistance value is larger than each resistance of the bridge circuit BC, R1 / R2 = r1 / r2.

  Since the outputs va and vb of the adder circuits 3 and 4 are va = (Va + V4 * R1 / R2) / (R1 / R2 + 1) and vb = (Vb + V4 * r1 / r2) / (r1 / r2 + 1), R1 / R2 When r = r1 / r2 and va = vb, Va = Vb.

  In the circuit of this embodiment, the outputs of the adder circuits 3 and 4 are compared by the operational amplifier A1, the transistor Q1 is controlled by the comparison output, and the bridge circuit BC is balanced. A predetermined potential between the potential Vb and the potential V4 can be input to the operational amplifier A1.

  FIG. 4 is a circuit diagram showing a circuit according to still another embodiment of the present invention. This embodiment circuit further comprises an adder circuit comprising resistors R1 and R2 between a connection point a of the bridge circuit BC and a connection point d (potential Vd) of the bridge circuit BC in the circuit shown in FIG. Voltage divider circuit) 3 and an adder circuit (voltage divider circuit) 4 comprising resistors r1 and r2 between a connection point b of the bridge circuit BC and a connection point d (potential Vd) of the bridge circuit BC. The connection point between the resistors R1 and R2 (output of the adder circuit 3) is connected to the + input terminal of the operational amplifier A1, and the connection point between the resistors r1 and r2 (output of the adder circuit 4) is Connected to the input end.

  In the circuit of this embodiment as well, the resistance values of the resistors R1, R2, r1, and r2 of the adder circuits 3 and 4 are R1 / R2 = r1 / r2, and ideally R1 / r1 = R2 as in the circuit of FIG. / R2 = Ra / Rb, but R1 / R2 = r1 / r2 may be used as long as each resistance value is larger than the resistance of each bridge circuit.

  Since the outputs va and vb of the adder circuits 3 and 4 are va = (Va + Vd * R1 / R2) / (R1 / R2 + 1) and vb = (Vb + Vd * r1 / r2) / (r1 / r2 + 1), R1 / R2 = If r1 / r2 and va = vb, then Va = Vb.

  Also in the circuit of this embodiment, the output of the adder circuit is compared by the operational amplifier A1, the transistor Q1 is controlled by the comparison output, and the bridge circuit BC is balanced.

It is a circuit diagram which shows the circuit structure of the detection part of the thermal type flow meter which concerns on one Embodiment of this invention. It is a figure explaining arrangement | positioning inside and outside the flow path of each element which comprises the electric bridge circuit of the thermal flow meter of the embodiment. It is a circuit diagram which shows the circuit structure of the detection part of the thermal type flow meter which concerns on the other same embodiment of this invention. It is a circuit diagram which shows the circuit structure of the detection part of the thermal type flow meter which concerns on further another embodiment of this invention. It is a circuit diagram which shows an example of the circuit structure of the detection part of the conventional thermal type flow meter.

Explanation of symbols

Ru Temperature-dependent electrical resistor sensor element Rd on the upstream side Temperature-dependent electrical resistor sensor element Rt on the downstream side Temperature measuring resistance element (temperature sensor element)
Ra, Rb, Rc Fixed resistor R1, R2 for the bridge circuit Fixed resistor r1, r2 for the adder circuit Fixed resistor A1, A2, A3 for the adder circuit Operational amplifier Q1, Q2 For transistor / bridge circuit Series circuit / adder circuit 11

Claims (3)

A pair of temperature-dependent electrical resistance sensor elements are arranged upstream and downstream along the fluid flow path, and a voltage is applied to both sensor elements to thermally act on the fluid so that the upstream and downstream sensor elements are connected to each other. A thermal flow meter that measures the mass flow rate using the relationship between the amount of heat exchanged between them, and the average temperature of the upstream and downstream sensor elements remains unchanged with respect to the flow rate change, and the ambient temperature or fluid temperature In the thermal flow meter for controlling the temperature difference between the sensor element average temperature and the fluid,
A first series circuit in which a pair of upstream and downstream temperature-dependent electrical resistance sensor elements and a fixed resistor are connected in series, and a second series circuit in which the temperature sensor element and the fixed resistor are connected in series Configure a bridge circuit on both sides,
The first operational amplifier circuit that detects the balance of the bridge circuit controls one potential of the bridge circuit,
A thermal flow meter, wherein the other potential of the bridge circuit is controlled by a second operational amplifier circuit that detects a difference between a connection point of the upstream / downstream pair sensor element and a predetermined potential.   A first series circuit in which a pair of temperature-dependent electrical resistance sensor elements and a fixed resistor are connected in series; and a second series circuit in which the temperature sensor element and a fixed resistor are connected in series. Means for adjusting the same voltage at the balance detection end of the bridge circuit is provided so that the input potential of the first operational amplifier circuit for detecting the balance of the bridge circuit on both sides satisfies the potential condition of the operational amplifier. The thermal flow meter according to claim 1.   The voltage adjusting means includes a first adder circuit provided between a connection point of the temperature-dependent electrical resistor sensor element and the fixed resistor and a predetermined potential, and the temperature sensor element and the fixed resistor of the second series circuit. And a second adder circuit provided between a connection point of the capacitor and a predetermined potential, and the outputs of the first and second adder circuits are received at the input of the first operational amplifier circuit. 2. The thermal flow meter according to 2.

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