JPS62250312A - Flow speed sensor driving circuit - Google Patents

Abstract

PURPOSE:To measure a flow speed by a single power source, by a method wherein one terminal of a bridge constituted of first and second resistors having the same temp. coefficient is held to constant voltage higher than earth potential and the output of the bridge is differentially amplified to heat the second resistor, and differentially amplified output and constant voltage are differentially amplified. CONSTITUTION:When a power source is turned OFF, resistances are R1=R2 and Rf=RS, and, when said power source is turned ON, voltage Va-Vb is applied to both terminals of a bridge and, because RS is not heated by Rf, a value of voltage V1-V2 is determined by (r). The voltage V1-V2 is differentially amplified by a differential amplifier 9 to heat RS by RH. Therefore, voltage v2 increases and voltage v1-v2 is reduced to become v1-v2 0 at last by a feedback system. The voltage Va of a power source 1 is applied to differential amplifiers 9, 10 and, in order to adjust amplified output V by a volume, voltage Va is converted to Vb<Va by a constant voltage circuit 2 to set voltage Vb to apparent earth potential and inherent earth potential is regarded as a negative power source to be applied to the -VEE terminal of the differential amplifiers 9, 10 and input offset is subjected to zero adjustment to make voltage V equal to voltage Vb. Voltage V0 showing a true flow speed is obtained from the output V0=V-Vb of the second differential amplifier 10.

Description

[Detailed Description of the Invention] <Industrial Application Field> The present invention provides a flow velocity sensor that uses a heated resistance temperature sensor to measure the flow velocity of a fluid with extremely low power consumption and with a single power source. The present invention relates to a drive circuit.

<Prior art> A method of determining the flow velocity of a fluid by heating a heat generating resistor and utilizing changes in the amount of heat transferred from the heat generating resistor to the fluid is based on a method in which the temperature of the fluid and the temperature difference between the heat generating resistor are constant. There is a method of amplifying the output voltage from the bridge circuit by connecting a temperature-measuring resistor in the bridge circuit, and feeding it back to the heating resistor, thereby compensating for the influence of fluid temperature and measuring the flow velocity of the fluid. It had been taken. However, in this method, the differential voltage entering the differential amplifier circuit from the bridge circuit has a small level difference, and due to variations in the input offset voltage of the elements used as the differential amplifier (for example, an operational amplifier), When an amplifier is used with a single power supply, it often operates outside the active region, and to solve this problem, it has been necessary to use positive and negative side power supplies.

Therefore, as a current meter that requires portability, there is an inconvenience in that the number of dry cells used as a power source increases.

<Objective of the Invention> An object of the present invention is to provide a flow rate sensor drive circuit that can measure the flow rate of a fluid using a hot wire resistance with low power consumption without requiring two bipolar DC power supplies. .

<Embodiment> FIG. 1 is a flow velocity sensor drive circuit diagram for flow velocity measurement showing one embodiment of the present invention. Reference numeral 1 in the figure is a dry battery that corresponds to the power source of this circuit. 2 is a constant voltage circuit for obtaining a stable voltage from the power source 1, and specifically uses a Zener diode, an IC constant voltage element, or the like. Hereinafter, for convenience, the voltage of the power supply is assumed to be Va, and the output voltage of the constant voltage circuit is assumed to be vb. Components 3 to 7 are resistors constituting bridges, respectively. Of these, R of resistance 3
1 and R2 of the resistor 6 are fixed resistors having the same resistance value. Rf of the resistor 5 and Rs of the resistor 7 are fixed resistors each having the same temperature coefficient and resistance value.

The resistor r of the resistor 4 is a fixed resistor for setting the temperature difference between the temperature of the fluid and Rs of the resistance temperature detector 7. If this value is increased, the above-mentioned temperature difference will increase. Reference numeral 8 denotes a heater having a resistance 7 and a resistance value RH. 9 is a differential amplifier, and 11 is an input offset zero adjustment volume. The heater 8 generates heat due to the output voltage of the differential amplifier 9, and the amount of heat generated per unit time is determined by the output voltage of the differential amplifier 9 being V and the resistance value of the heater 8 being RH.
Then, it is 5 (W).The resistor 7 and the heater 8 are arranged very close to each other, and the heat generated by the resistor 8 causes the resistor 7 to
It is assumed that the temperature is K so that it is heated efficiently. For example, the specific arrangement of the resistor 7 and the heater 8 is as follows.
It will look like the figure. In FIG. 2, 21 is a suitable substrate made of glass, ceramic, resin, etc., and 22 and 23 are resistors 7 and 23, respectively. The heater 8 has resistance values Rs and RH. These are made by depositing metal on the substrate 2I. The above objective is achieved by depositing the resistor 7 and heater 8 in close proximity as shown.

Next, the circuit operation of the drive circuit shown in FIG. 1 will be explained assuming that there is no wind.

When power supply 1 is not connected, R1=R2゜Rf=
However, at the moment when power supply 1 is connected, a voltage of Va-Vb is applied across the resistor bridge, and Rs becomes RH.
Since Vl and v2 in the figure are not heated by the resistance r
The value of oVI-V2, where vl>v2 due to the existence of
v2 becomes larger. This voltage v)-v2 is amplified by the differential amplifier 9, and the output voltage V causes the heater 8 to generate heat. Since this temperature is efficiently transmitted to the temperature measuring resistor 7, the resistance value Rs of the resistor 7 increases according to its temperature coefficient. Therefore, the voltage v2 increases and the voltage vl - v2 decreases. Such a feedback system ultimately results in Vl-V220. At this time, the output voltage V of the differential amplifier 9 becomes a value necessary to adjust the resistance value Rs of the resistor 7 in order to heat the heater 8 and satisfy the condition of Vl y v2.

The above explanation requires that the input offset of the differential amplifier 9 be zero-adjusted. Normally, the current flowing through Rf and RsK is the same as the current flowing through these resistors 5.
, 7 must be set to a sufficiently small value so that self-heating due to Joule heat does not occur, and if the value r of the resistor 4 is increased, the increase in the resistance value Rs due to heating of the resistance temperature detector 7 will be increased. Therefore, the heater 8
This means that the amount of heat generated will also have to be increased.

Therefore, the overall power consumption increases and the burden on the power supply increases.

For these reasons, the current flowing through Rf and Rs is reduced,
Also, it is better for r to be as small as possible. Therefore, since the voltage drop caused by the resistor 4 cannot be set large, the value of vl-v2 inevitably becomes very small.

On the other hand, the input offset voltage of the integrated general-purpose operational amplifier used in the differential amplifier 9 is about several mV, which is equivalent to the value of vI-v2 that should become a signal. Therefore, the input offset voltage is made extremely small. For this purpose, the input offset must be zero-adjusted. If the entire circuit is powered by a single power supply, the input offset cannot be adjusted to zero. (The voltage vl==v2 is applied to both sides of the differential amplifier, and the offset adjustment volume 11 is adjusted so that the output V becomes 0. However, with a single power supply, near OV it is out of the active region and normal operation is not possible. For zero adjustment, the differential amplifier 9
-VEE terminal of must be connected to the negative power supply.

) So to make this adjustment possible even with a single power supply!
The voltage Va of the dry battery, which is , is converted by the constant voltage circuit 2 to a voltage vb smaller than the voltage Va, and this V is regarded as the upper ground potential, and the original ground potential is regarded as a negative power source,
By connecting the -VER terminal of the differential amplifier 9 to this, the input offset can be adjusted to zero. However, in this case, the output voltage V when the above adjustment is performed accurately is v=
It becomes vb.

As a result, the differential amplifier 9 operates after accurately adjusting the input offset to zero, so that the differential amplifier 9
The output voltage V is 0, which is a value related to the flow velocity of the fluid (gas or liquid) hitting the resistance temperature detector 7. In other words, as the flow velocity of the fluid increases, it is heated by the heater 8, which is a heat generating resistor. As the temperature of the resistance temperature detector 7 decreases, its resistance value Rs decreases, and eventually vI-v2 increases. Therefore, the output voltage V of the differential amplifier 9 also increases, the amount of heat generated by the heater 8 increases, and the resistance value Rs of the temperature measuring resistor 7 increases. Finally, V is a larger value than before, vl-v2::0
The system becomes stable.

By the way, as explained above, the output voltage V of the differential amplifier 9 represents the flow velocity of the fluid, but a voltage KVb is usually added thereto. Therefore, the voltage V representing the true fluid flow velocity
Q must be subjected to the calculation vO=v−vb.

A second differential amplifier 10 is provided for this purpose. The output vo of the differential amplifier 10 is a voltage representing the true flow velocity.

<Effects of the Invention> As detailed above, by making it possible to measure the fluid flow velocity even with a single power source, dry cell battery operation becomes easier, and this is expected to be useful for flowmeters, anemometers, etc. that require portability. be done.

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[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a flow rate sensor drive circuit showing one embodiment of the present invention. Figure 2 shows the layout of the main parts of the heater and resistance temperature sensor. Resistance temperature detector 6...Fixed resistance 7...Resistance temperature detector 8...Heating resistor 9...Differential amplifier 10...Differential amplifier
11...Volume for input offset adjustment 21...
・Substrate 22... Heater 23... Resistance temperature sensor Patent attorney Takeshi Sugiyama (and 1 other person) Figure I2 Figure 2

Claims (1)

[Claims] 1. A resistance bridge circuit consisting of a first resistance temperature detector and a second resistance temperature detector having the same temperature coefficient as the first resistance temperature detector, and one end of the bridge circuit set to a temperature higher than the ground potential. a differential amplifier that amplifies the output voltage from the bridge circuit; a heating resistor that heats the second temperature measuring resistor of the resistance bridge circuit by the output voltage of the differential amplifier; A flow rate sensor drive circuit comprising: a second differential amplifier circuit that differentially amplifies the output voltage of the differential amplifier and the voltage of the constant voltage circuit.