JPS5828621A - Thermal flow meter - Google Patents

Abstract

PURPOSE:To eliminate temperature dependent elements and to enhance temperature characteristics, by performing both zero-point adjustment and span adjustment within a closed loop. CONSTITUTION:Control current is supplied to a heat-sensitive resistor 2 set in a flow path 1 through a branching point A, and the output signal is extracted through a point B. The output side bridge branching is formed between the points A and C, and the resistor 2 and a fixed resistance R1 are connected in series within the branching. Then the control current is supplied from a power supply 4. The voltage dividing ratio is controlled between the voltage dividing resistances r1 and r2, and the temperature of the resistor 2 is changed. Then the degree of inclination is increased for the output signal to the flow rate to perform the adjustment of span. At the same time, the resistance value of a current detecting resistance R3 connected to the branching at the side of the temperature compensation is varied. Thus the bridge balance level is controlled to perform the adjustment of zero point. The span adjustment and the zero-point adjustment are performed repetitively. Thus the charactristic curve of the output voltage is controlled within a desired range to the flow rate.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thermal flowmeter, and more particularly to a structure of a thermal flowmeter that can accurately and easily adjust a characteristic curve of change in an output signal with respect to flow rate, regardless of fluid temperature or the like.

Thermal flowmeters have a temperature-sensitive resistor, usually called a hot wire, and a temperature-compensating resistor, usually called a cold wire. While supplying the Lm antibody, a control current is maintained so that the temperature difference between the temperature-sensitive resistor and the temperature-compensating resistor is always approximately constant, and the flow rate is detected based on the value of the output voltage of the temperature-sensitive resistor at that time. It is something to do.

This type of thermal flow meter is widely used, for example, as an intake air amount detector for automobile engines.

In this thermal flowmeter, each flowmeter has a zero point (for example, when an automobile engine is idling) and a fully open point (
By inspecting and adjusting the output voltage with respect to the flow rate at the time of wide oven throttle (wide oven throttle), it is possible to keep the change characteristic curve of the output voltage with respect to the flow rate within a predetermined range. That is, zero point adjustment and span adjustment are performed to keep the characteristics between desired upper and lower limit values.

Q: When used as an intake air amount detector for an automobile engine, it is necessary to maintain relatively high accuracy in the low flow range due to the characteristics of a thermal flow meter.
For example, it ensures accuracy of ±3% in the range up to 0% flow rate, and ±3% accuracy in the high-speed range from 40% flow rate to full opening.
Stone requires an accuracy of 4 to 5%. In order to achieve the required performance specific to automobile engines, for example, the zero point adjustment is set at idle rotation, and the span adjustment is performed at a fully open 40% flow point to compare the adjustment of output characteristics. A temperature-sensitive resistor 2 and a temperature-compensating resistor 3 are installed in the target path 1. A fluid flows in the flow path 1 in the direction of arrow F, and a temperature-sensitive resistor 2 is installed on the upstream side. The supply current from the power supply 4 is passed through the collector and emitter of the transistor 5 to the branch point A.
supplied to From this branch point A, it branches into an output side bridge branch and a temperature compensation side bridge branch and reaches a collection point C. A temperature sensitive resistor 2 and a fixed resistor R1 are connected in series to one branch, that is, the output bridge branch. The other branch, that is, the bridge branch on the temperature compensation side, has a temperature compensation resistor 3.
and a fixed resistor R3 are connected in series.

Therefore, each point B, D of the diagonal branch of the bridge circuit
are respectively connected to input terminals of an amplifier 6 for supplying control current. In the illustrated example, the connection point B of the output side bridge branch is connected to ten phases of the amplifier 6, and the connection point B of the hot gate compensation side branch is connected to one phase of the amplifier 6. The output side of the amplifier 6 is connected to the base of the transistor 5.

In the circuit configuration shown in FIG. 1, a fixed resistor 7 is connected in parallel between one phase of the amplifier 6 and the output side of the amplifier 6.

The drive circuit (control circuit) as shown in FIG. An output signal (flow rate detection signal) can be extracted from the point B while maintaining the control current of .

The drive circuit shown in FIG. 1 cannot perform the above-mentioned zero point adjustment and span adjustment, and it is extremely difficult to keep the output characteristics within a desired accuracy.

Therefore, in order to add the functions of zero point adjustment and span adjustment to the drive circuit shown in FIG. 1, it has been proposed to add an adjustment circuit as shown in FIG. 2.

In the drive circuit shown in FIG. 2, an additional circuit 8 for zero point and span adjustment is connected to the output terminal of the drive circuit shown in FIG. This adjustment circuit 8a Zener diode 9
A reference voltage (for example, 5-8 ports) is supplied through the resistor 10, and this reference voltage is supplied to the ten phases of the amplifier 11 through the resistor 10. A variable resistor 13 is connected to the branch point 12 of the reference voltage supply circuit. On the other hand, an output point B° of the drive circuit is connected to one phase of the amplifier 11, and an output signal V is taken out from the output side of the amplifier 11. Further, a variable resistor 14 is connected in parallel between the input side to one phase of the amplifier 11 and the output side of the amplifier.

In the conventional zero point and span adjustment circuit described above with reference to FIG. 2, by adjusting the variable resistor 13, the value of the output voltage V relative to the flow rate can be moved vertically in parallel, thereby making it possible to perform zero point adjustment. On the other hand, by adjusting the gain using the variable resistor 14, the slope of the characteristic curve of the output signal V with respect to the flow rate is adjusted, thereby adjusting the span.

In the conventional adjustment method shown in FIG. 2, the characteristics of the Zener diode 9 change depending on the temperature, so the Zener voltage, which is the reference voltage, also changes depending on the temperature. is extremely difficult to ensure within the desired accuracy. That is, in automobile engines, etc., the intake air temperature ranges from 12 oc to 12 oc.
It is extremely difficult to ensure the desired detection accuracy in any of such a wide temperature range. However, Zener diode 9
Although it is technically possible to construct a circuit with excellent temperature compensation function instead, this would require a complicated circuit and be expensive, and in the end, an open circuit as shown in Figure 2 would be required. It is virtually impossible for loop regulation circuits to ensure output signal accuracy over wide temperature variations.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art as described above and to provide a thermal flowmeter that can accurately and easily perform zero point and span adjustment regardless of fluid temperature.

A feature of the present invention is that span adjustment is performed by adjusting the gradient of the output signal with respect to the flow rate by adjusting the voltage dividing ratio of the voltage dividing resistor connected in parallel with the temperature-sensitive resistor. Zero point adjustment is performed by adjusting the resistance value of the connected resistor and adjusting the bridge balance level.

That is, according to the present invention, there is an output bridge branch having a temperature-dependent temperature-sensitive resistor placed in the flow path and heated by a control current, and a first resistor in series with the resistor, and a resistance value that changes depending on the fluid temperature. a bridge circuit configured with a temperature compensation side bridge branch having a temperature compensation resistor whose value changes and a second resistor connected in series thereto; a diagonal branch of the bridge circuit connected to an input terminal of an amplifier for supplying a control current; In a thermal flowmeter that controls the current of the temperature-sensitive resistor in a closed loop so that the temperature difference between the temperature-sensitive resistor and the temperature-compensating resistor is approximately constant, the temperature-sensitive resistor and the temperature-compensating resistor are adjustable in parallel with each other. In addition to connecting a voltage dividing resistor, the resistance value of the second resistor of the temperature compensation side bridge branch can be adjusted, and by adjusting the dividing ratio of the voltage dividing resistor, the slope of the output signal with respect to the flow rate is adjusted. A thermal flow meter is provided that is characterized in that a bridge balance level is adjusted by adjusting the resistance value of a resistor.

The present invention will be described in detail below with reference to FIG.

In FIG. 3 as well, parts that are equivalent to or correspond to the conventional circuit elements in FIGS. 1 and 2 are designated by the same reference numerals.

In FIG. 3, a control current to a temperature-sensitive resistor 2 installed in a flow path 1 is supplied from a branch point A, and an output signal V is taken out from a point B.

A fixed resistor R1 is provided on the ground side of point B and is grounded via point C. An output bridge branch is formed between the branch point A and the point C, and the temperature sensitive resistor 2 and the fixed resistor R1 are connected in series within the branch. A control current is supplied to the output bridge branch from the power supply 4 via the collector and emitter of the transistor 5.

Branch resistors r, , r2 are connected in parallel with the temperature-sensitive resistor 2. One of the voltage dividing resistors made up of these two resistors, that is, the resistor r2 in the illustrated example, has a resistance value that can be adjusted. That is, the partial pressure ratio can be adjusted. The midpoint Z of these voltage dividing resistors rl+'2 is input to the half phase of the amplifier 6.

On the other hand, the temperature-compensating bridge branch from point C is connected via an adjustable resistor R3 to one diagonal branch of the bridge circuit, which is further connected to the further layer of the amplifier 15. . A connection in parallel with the amplifier is provided between the single-phase input side of the amplifier 15 (point D) and a point A1 on the output side of the amplifier 15, and a temperature compensation resistor 3 and a fixed A resistor 16 is connected in series. or,
A diagonal branch point B from which the output signal V is taken out is connected to a half phase of the amplifier 15. The output side of this amplifier 15 is connected to one phase of the amplifier 6.

According to the configuration of the drive circuit of the thermal flowmeter explained above with reference to FIG. 3, at point A on the output side of the amplifier 15,
It is possible to change the voltage at a constant rate corresponding to the bridge supply voltage (the excitation voltage at point A) and take out the voltage at point A and point A.

According to the embodiment of the present invention shown in FIG. 3, the zero point adjustment and span adjustment of the thermal flowmeter can be performed as follows.

First, the temperature of the temperature sensitive resistor 2 can be changed by adjusting the voltage dividing ratio of the voltage dividing resistors r, , r, and changing the voltage at point Z. For example, by adjusting the voltage division ratio and increasing the voltage at point Z, the temperature of the temperature sensitive resistor 2 can be raised, and while maintaining the voltage at one diagonal branch point B, the temperature at the other diagonal branch point B can be increased. The voltage at or the output voltage can be increased, thereby increasing the slope of the output signal versus flow rate to provide span adjustment. Naturally, during this span adjustment, the flow rate at the time of adjustment is maintained at a certain constant value.

The zero point adjustment of the thermal flowmeter can be performed by adjusting the resistance value of the resistor R8 of the bridge branch on the temperature compensation side. That is, when the resistance value of the resistor R3 is decreased, the potential at the point decreases, and the potential at the output side point A of the amplifier 15 can be increased. When the potential at point A increases, the excitation voltage at bridge branch point A decreases, allowing the signal voltage at point B to decrease. In this way, by lowering the output voltage at point B, the level of the output signal relative to the flow rate is lowered in parallel overall, thereby making it possible to perform zero point adjustment. That is, by lowering the level of the control current supplied to the temperature sensitive resistor 2,
When controlling the temperature difference between the temperature-sensitive resistor 2 and the temperature-compensating resistor 3 to be constant, the level of the output signal V can be generally shifted downward in parallel to perform zero point adjustment. If you want to adjust the zero point upward, use a variable resistor, contrary to the above.

This can be done by increasing the resistance of .

In the drive circuit of the thermal flowmeter shown in Fig. 3, the temperature difference between the temperature-sensitive resistor 2 and the temperature-compensating resistor 3 is also controlled to be almost constant. By increasing the current and controlling it in a direction that widens the temperature difference, it is possible to increase the gradient of the output voltage with respect to the flow rate.

That is, the slope of the characteristic curve can be increased so as to increase the output voltage in the high speed region. In this way,
A voltage dividing resistor r, connected in parallel with the temperature sensitive resistor 2.

The slope of the output characteristic can be changed by adjusting the heat transfer coefficient of the temperature-sensitive resistor 2 by changing the voltage division ratio of r, and by changing the voltage at point B (output voltage V) relative to the point.

On the other hand, by changing the resistance value of the current detection resistor R5 connected to the branch on the temperature compensation side, the voltage at both point B and the switch, that is, the bridge balance level, can be adjusted, thereby performing zero point adjustment. be able to. By repeating such span adjustment and zero point adjustment several times, the characteristic curve of output voltage versus flow rate can be adjusted within a desired range.

According to the embodiment of the present invention described above with reference to FIG.
By performing zero point adjustment and span adjustment in a closed loop, it is possible to eliminate the need for temperature-dependent elements and significantly improve temperature characteristics. In other words, as explained in Fig. 2, conventional zero point adjustment and span adjustment use the reference voltage of a Zener diode and adjust the reference voltage in an oven loop. It was not possible to perform temperature correction during zero point adjustment and span adjustment. That is, in conventional open-loop control, a reference voltage is required and the characteristics of the Zener diode used to create the reference voltage change depending on the temperature, but according to the embodiment of the present invention shown in FIG. Zero point adjustment and span adjustment can be performed with a simple circuit configuration within the device, and an element for generating a reference voltage is not required, so that temperature dependence during zero point adjustment and span adjustment can be eliminated. Follow?
Therefore, according to the embodiment shown in FIG. 3, a thermal flowmeter is obtained which can accurately and easily perform zero point adjustment and span adjustment without being affected by fluid temperature.

Furthermore, since the conventional adjustment method shown in FIG. 2 uses the amplifier 11, the output signal is primarily amplified (linear amplification), but between the flow rate Q and the signal V there is a , V', which includes four power terms, has the disadvantage that the linear amplification function of the amplifier 11 cannot amplify accurately. According to embodiments of the present invention, these drawbacks are overcome and other advantages are also obtained.

Furthermore, in a conventional thermal flowmeter, the temperature-sensitive resistor 2 and the temperature-compensating resistor 3 have different resistance values (for example, 1
However, according to the embodiment shown in FIG. 3, parts with the same resistance value and the same specifications can be used as the temperature-sensitive resistor 2 and the temperature-compensating resistor 3. .

That is, by setting the ratio of the voltage at point A1 in FIG. 3 to the voltage at control current supply point A to an appropriate value, the resistance value of each resistor 2.3 can be freely selected. Therefore, by selecting the voltage ratio at point A, it is possible to use the same resistors with the same resistance value as each resistor.

As described above, according to the embodiment of the present invention shown in FIG. The scope of application of the flowmeter can be greatly expanded. Furthermore, since it is not necessary to use an amplifier for span adjustment, zero point adjustment and span adjustment can be performed with a compact circuit configuration.

As is clear from the above description, according to the present invention, a thermal flowmeter capable of performing zero point adjustment and span adjustment with excellent temperature characteristics can be obtained.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram illustrating the circuit configuration of a conventional thermal flowmeter, and FIG. 2 is an explanatory diagram illustrating a state in which a conventional adjustment circuit for zero point adjustment and span adjustment is connected to the circuit in FIG. Third
The figure is an explanatory diagram showing an example of the circuit configuration of a thermal flowmeter according to the present invention. DESCRIPTION OF SYMBOLS 1... Flow path, 2... Temperature sensitive resistor, 3... Temperature compensation resistor, 4... Power source, 6... Amplifier, 15... Amplifier, F... Flow direction, ■...Output signal, A...
Bridge junction, B. D... Diagonal branch point of the bridge, R1... Fixed resistance, R3... Resistance of the bridge branch on the temperature compensation side, r,, r
.

Claims (1)

[Claims] 1. Output side bridge branch that has a temperature-dependent temperature-sensitive resistor installed in the flow path and heated by a control current and a first resistor in series with it, and temperature compensation whose resistance value changes depending on the fluid temperature. A bridge circuit is provided with a bridge branch on the temperature compensation side having a resistor and a second resistor in series with the bridge circuit, and a diagonal branch of the bridge circuit is connected to an input terminal of an amplifier for supplying control current, and the temperature-sensitive resistor In a thermal flowmeter that controls the current of the temperature-sensitive resistor in a closed loop so that the temperature difference between the body and the temperature-compensating resistor becomes approximately constant, an adjustable voltage dividing resistor is provided in parallel with the temperature-sensitive resistor. At the same time, the resistance value of the second resistor of the temperature compensation side bridge branch can be adjusted, and the slope of the output signal with respect to the flow rate is adjusted by adjusting the voltage dividing ratio of the voltage dividing resistor, and the slope of the output signal with respect to the flow rate is adjusted. A thermal flow meter characterized by adjusting the bridge balance level by adjusting the resistance value.