JPS62182670A - Flow velocity measuring instrument - Google Patents

Abstract

PURPOSE:To accurately measure a flow velocity without being affected by the change in temperature by making the resistance value of a flow velocity measuring thermistor equivalent to a heating temperature obtained by an arithmetic unit and rendering the difference between a fluid temperature and the heating temperature constant independent of the fluid temperature. CONSTITUTION:The voltage across a resistor R1 obtained by applying a voltage to a flow velocity measuring thermistor TH is inputted to a divider D1 via a voltage dividing circuit DVC and the output of the divider D1 is fed back to a wideband amplifier AP. On the other hand, a fluid temperature detected by a temperature measuring element PT is inputted to an arithmetic unit X. The unit X inputs the resistance value of the thermistor TH at a heating temperature wherein a temperature difference determined in advance is added to the fluid temperature to the other terminal of the amplifier API as voltage signals. As a result, the difference between the fluid temperature and the heating temperature becomes constant independent of the fluid temperature. Measuring signals divided by the voltage divider DVC and passed through a multiplier MLT are inputted to the arithmetic unit X and converted to an actual flow velocity value to be outputted outside.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention automatically and highly accurately compensates for temperature changes in a fluid that causes large temperature changes from -10°C to +70°C, and also converts flow velocity values so that they can be directly read. It is connected to the output flow rate measuring device.

A thermistor is used as a current sensor because its resistance change with respect to temperature changes is significantly larger than that of other resistors. That is, when a current is passed through a thermistor, it will self-heat due to Joule heat below a certain current value, the temperature of the thermistor itself will rise in proportion to the increase in current, and the resistance value will decrease according to the temperature resistance characteristics of the thermistor.

Then, when the heated thermistor is placed in the measuring fluid,
It is cooled by the flow, and the resistance value increases in proportion to the flow velocity.

Here, the following relationship is known between the power W of the flow rate measuring thermistor, the temperature difference T-Tθ between its heating temperature T and the fluid temperature TII, and the flow rate V of the measuring fluid.

W=V・I
...No. (1) W=K (v) ・ (T
Ts ) ”...(2),',K (
v) =P/ (T T 11 ) ””(
3) Here, ■: Voltage applied to the thermistor I: Current flowing through the thermistor K (v): Heat radiation coefficient determined by the shape of the thermistor, etc. A function of the flow velocity V. Therefore, the heating temperature of the thermistor for flow velocity measurement is constant with the fluid temperature. By controlling the current flowing through the thermistor so as to have a temperature difference of Desired.

Conventionally, the following methods are known as such current meters.

(1) In a bridge circuit in which two resistors with equal resistance values are connected to the upper side and a thermistor for measuring the flow velocity and a field effect transistor are connected to the lower side, the output of the thermistor for measuring the fluid temperature is obtained by adjusting the amplification degree and offset. This method changes the resistance value between the source and drain by connecting a reference voltage to the gate of the field effect transistor, controls the heating temperature of the thermistor for measuring flow velocity, and at the same time determines the flow velocity from the power consumption of the thermistor for measuring flow velocity.

(2) Connect the thermistor for flow velocity measurement and a resistor in series, convert the voltage across the resistor to a voltage proportional to the power consumption and resistance value of the thermistor for flow velocity measurement using a multiplier and a divider, and The output is compared with the reference voltage obtained by adjusting the amplification and offset of the output of the thermistor for measuring the temperature of the fluid, and in order to equalize them, a feedback circuit is connected to control the current flowing to the thermistor for measuring the flow velocity. , a method for determining the flow velocity from the output of a multiplier (Japanese Patent Publication No. 59-4) However, these methods have the following drawbacks.

In both cases (2) and (2) above, a reference voltage is generated from the output of the temperature measuring thermistor in order to keep the temperature difference between the heating temperature of the flow rate measuring thermistor and the fluid temperature constant.Here, The reference voltage is not a voltage proportional to the resistance value of the thermistor for temperature measurement, but must be a voltage proportional to the resistance value of the thermistor for flow rate measurement when a certain temperature is added to the fluid temperature. In the method described above, an attempt was made to match the output proportional to the resistance value when the flow velocity measuring thermistor is heated to a constant temperature by simply adjusting the amplification degree and offset of the output proportional to the resistance value of the temperature measuring thermistor.

At this time, the fluid temperature T and the resistance of the thermistor for measuring the flow velocity at that time [RH1 resistance 11 of the thermistor for temperature measurement]
The relationship between Rc is expressed by the following formula.

Rc=RIIexp [:B (1/T-1/TL1)
]・・・・・・(4) Rs=RIIexp [B (1/(T+DT)
1/Ts) ]・・・・・・(5) Here, Re: (t-resistance value of mister B at temperature TL1)
:Thermistor's B constant DT:Heating temperature As can be seen from the above equation, Rc has no proportional relationship with RH, so it is difficult to make Rc and RH equal at all temperatures just by adjusting the amplification degree and offset. , Rc and RH can only be made close to each other within a fixed and narrow temperature range. Moreover, since the amplification degree and offset are adjusted so that Rc and RH match at two or three temperature points, an error occurs between the two or three points, and this error extends over the temperature range. The more you spread it out, the bigger A
It becomes. In addition, when the temperature range is widened, the range of change in the resistance value of the thermistor for flow rate measurement becomes larger, and it may exceed the operating input range of the multiplier or 1-subtracter. The error may become large. Therefore, with the conventional method, it is possible to measure the flow velocity with high accuracy when the fluid temperature is in a constant steady state and the temperature change is small, but when the fluid temperature is in a large change, it is possible to measure the flow velocity with high accuracy. Adjustments must be made again, and it is impossible to automatically measure flow velocity with high accuracy over a wide temperature range.

In addition, in the conventional method, the voltage proportional to the power consumption of the thermistor for flow velocity measurement is directly used as the output.
As can be seen from equation (3) above, the output voltage that is proportional to the thermistor's power consumption is proportional to the heat radiation coefficient, which is a function of the flow rate, and the relationship between the actual flow rate and output voltage is nonlinear. becomes. Therefore, in order to directly read the flow velocity value, you can use an output voltage-flow velocity conversion table and read the flow velocity from the table, use the output as an analog pointer-type voltmeter with uneven scales, or use an external device to measure the output voltage. It is necessary to use a method such as providing an arithmetic circuit for nearization.

The present invention achieves -1, which was difficult with the conventional method described above.
The purpose of this project is to provide a flow rate measurement device that automatically measures flow velocity with high accuracy over a wide temperature range from 0°C to 70°C, and that directly displays the flow velocity value or outputs it as a voltage to the outside. In the feedback circuit that changes the current flowing through the thermistor for flow rate measurement, which is constructed by comparing the voltage proportional to the resistance value of the thermistor with the output of the temperature measurement device, Using a calculation device with characteristics memorized,
It converts into an output proportional to the resistance value of the thermistor when heating temperature is added to the fluid temperature and uses it in the feedback circuit, and it also adjusts the input terminals of the divider in the feedback circuit and the multiplier in the output circuit. By using a pressure circuit in combination with a constant voltage source, it is possible to control the flow velocity measurement thermistor so that it always has a constant temperature difference from the fluid temperature at any temperature, allowing highly accurate flow velocity measurement over a wide temperature range. At the same time, by linearizing the voltage proportional to the power consumption of the flow velocity measuring thermistor using the same arithmetic unit, the flow velocity measurement value can be directly read.

Hereinafter, one embodiment of the present invention will be described in detail based on the drawings.

FIG. 1 is a block diagram of the entire flow rate measuring device. The thermistor TH for flow velocity measurement is connected in series with a resistor R1 with a small temperature coefficient and a constant voltage source OFS, and the voltage VIV2 across R1 is
is the voltage dividing circuit D via buffer amplifiers AP3 and AP4.
Connected to VC.

By the functions of the voltage dividing circuit DVC and the constant voltage source OFS, Vl and V2 are divided so as not to exceed the input voltage ranges of the divider DIV and multiplier MLT at any temperature within the operating temperature range. The output VD of the divider DIV becomes an input to a wideband amplifier API, and together with the amplifier AP2, forms a feedback circuit that controls the current flowing through the thermistor TH for measuring the flow rate.

The other input of the broadband amplification WAPI is connected to the output vS obtained by processing the outputs of the platinum thin film resistor PT and the resistance voltage converter RV by the arithmetic unit X. The output VM of the multiplier MLT is input to the arithmetic unit X, and after being processed, the output v
Displayed as O or output as voltage. Also, arithmetic unit #L
X is the multiplexer MP, A-D, as shown in Figure 2.
Converter AD, microcomputer CPU, storage device R
It consists of an OM, a display device DSP, and a DA converter DA.

Next, the operation of the flow rate measuring device will be explained.

When measuring the flow rate, when the flow rate measurement thermistor TH and the platinum thin film resistor PT are placed in the measurement fluid and a voltage is applied to the resistance R and the flow rate measurement thermistor TH, a current flows through the resistance R1 and the flow rate measurement thermistor TH. , the thermistor TH for flow rate measurement self-heats, and V is applied to both ends of the resistor R1.
1, V2 voltage is generated. Vl and V2 are connected to the voltage dividing circuit DVC via buffer amplifiers AP3 and AP4, respectively.
In order to meet the input conditions of the divider DIV and multiplier MLT, that is, even if the resistance value of the thermistor TH for flow velocity measurement changes greatly due to changes in fluid temperature, the divider DIV and multiplier MLT The voltage is divided so as not to exceed the operating input voltage range of 2. Here, constant voltage ROFS
acts to apply a constant voltage, and when dividing vl, ■2 with the voltage divider circuit DVC, the output voltage of the constant voltage source OFS is used as a reference instead of the ground of the circuit as a reference. , the input conditions of the divider DTV and the multiplier MLT can be matched over a wide temperature range that cannot be adjusted by the voltage divider circuit DVC alone.

The voltage value of the constant voltage ff0Fs changes depending on the temperature range of the fluid and the operating input voltage range of the divider DIV and the multiplier MLT, so it is necessary to adjust it to suit each of them. The output voltage VD of the divider DIV is the wideband amplifier API
, and is compared with the output voltage vS of the arithmetic unit X, forming a feedback circuit that controls the current flowing through the thermistor TH for flow velocity measurement so that both voltages become equal. Here, the output voltage Vs of the arithmetic device X is 7H of the fluid.
The resistance of the platinum thin film resistor PT for measuring the temperature is converted into a voltage signal by the resistance voltage converter RV, and then the multiplexer M
After passing through P, the fluid is converted into a digital signal by the A-D converter AD, and a preset temperature difference is added to the fluid temperature T@ according to the temperature-resistance characteristics of the thermistor TH for measuring the flow velocity, which is stored in the storage device ROM. The resistance value of the flow velocity measuring thermistor TH at the heating temperature T is calculated by the microcomputer CPU, converted into a voltage signal by the DA converter DA, and output. Therefore, to compare the output voltage vS of the arithmetic unit aX and the output voltage VD of the divider DrV and control the current flowing through the thermistor TH for flow velocity measurement so that both voltages are equal, the thermistor TH for flow velocity measurement
This is nothing more than making the resistance value of 2 coincide with the heating temperature T calculated inside the arithmetic device X. Therefore, the temperature difference T −T @ between the fluid temperature Tg and the heating temperature T is always constant regardless of the fluid temperature. As is clear from equation (3) above, when the temperature difference T-Tθ between the heating temperature T of the thermistor for flow velocity measurement and the fluid temperature T@ is constant, the power consumption W of the thermistor for flow velocity measurement is proportional to the flow velocity. It is proportional to the heat dissipation coefficient, which is a function of . Therefore, in order to be able to directly read the actual flow velocity, the output voltage VM of the multiplier MLT is input to the arithmetic unit X, and the platinum thin film resistor P is input to the multiplexer MP.
After being distinguished from the input from T and the resistive voltage converter RV,
The A-D converter AD converts it into a digital signal, and the microcomputer CPU linearizes it. The linearized result is directly displayed on the display device DSP, and at the same time is converted into a voltage signal by the DA converter DA, and is output as a voltage o proportional to the flow velocity separately from vS.

As described above, according to the flow rate measurement device of the present invention, temperature compensation is automatically performed in a wide temperature range even when the fluid temperature changes, so it can be used in a very wide temperature range from -10°C to 70°C. Even in this case, highly accurate flow velocity measurements that are unaffected by temperature changes can be performed. Also,
The heating temperature of the thermistor for flow rate measurement can be set using software, so it can be done directly and easily. Furthermore, even if a thermistor with significantly different resistance or temperature coefficient is used as a flow velocity measurement thermistor, it can be easily used by simply changing the software. It is also possible to use detection elements. In addition, since the output voltage linearization required to directly read the flow velocity can be performed inside the device, there is no need for an output voltage-flow velocity conversion table or an external linearization circuit as in conventional methods. The flow velocity value can be displayed or output as a voltage. In addition, by replacing the constant voltage source connected in series with the thermistor for flow rate measurement with a variable voltage source, the voltage value can be changed according to the ambient temperature, allowing use in a wider temperature range and increasing the operating input terminal range. It is also possible to use multipliers and dividers with a narrow width.

[Brief explanation of drawings]

FIG. 1 is a circuit configuration diagram showing an embodiment of the present invention. R1...Resistor, TH...Thermistor for flow rate measurement, DVC...Voltage divider circuit DIV...Divider MLT...Multiplier PT...Platinum thin film resistor RV ...Resistance voltage converter X...Arithmetic unit API...Broadband amplifier AP2...Amplifier AP3. AP4...buffer amplifier FIG. 2 is a circuit diagram of the portion at the calculation position g indicated by X in FIG. 1. MP...Multiplexer AD...A-D converter DSP...Display device CPU...Φ/Microcomputer ROM...Note t! Device DA...D-A converter.

Claims (2)

[Claims] (1) A voltage dividing circuit is added to the thermistor heating circuit, which is formed by connecting a resistor with a small temperature coefficient, a thermistor for flow rate measurement, and a constant voltage source in series, and the voltage across the resistor and the output voltage of the constant voltage source. After passing through the voltage, input it to a divider that outputs a voltage proportional to the resistance value of the thermistor, compare the output voltage of the divider with a reference voltage, and control the current flowing through the thermistor to make them equal. In the flow velocity measuring device connected to the feedback circuit, the fluid temperature is measured with another temperature measuring device, and the voltage proportional to the fluid temperature is converted into digital data by an A-D converter, inputted to the calculation device, and stored in advance. The resistance value of the thermistor is calculated to keep the difference between the heating temperature of the thermistor and the temperature of the measured fluid constant from the temperature resistance characteristics of the thermistor, and a voltage proportional to the resistance value is fed back to the above using a DA converter. A temperature compensation circuit that is applied in place of the reference voltage of the circuit and configured to control the current flowing through the thermistor so that the voltage matches the output voltage of the divider, and the voltage across the resistor and the constant voltage. The output voltage of the multiplier, which is a voltage proportional to the power consumption of the thermistor obtained by passing the output voltage of the source through a voltage dividing circuit and inputting it to a multiplier, is converted into an actual flow velocity value. A flow velocity measuring device characterized by having an output circuit for outputting to the outside. (2) The output circuit digitally converts the voltage proportional to the power consumption of the thermistor output from the multiplier using the A-D converter and inputs it to the arithmetic unit, using a conversion formula stored in advance. 2. The flow velocity measurement device according to claim 1, wherein the flow velocity measuring device is an output circuit that converts the flow velocity value into a flow velocity value and outputs it to the outside.