JPS5826346Y2 - Karman vortex flow meter or current meter - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a flowmeter or current meter that detects Karman vortices using a hot wire, and particularly relates to a device with improved output characteristics that can be pressed in a low flow rate region.

The Karman vortex flow meter utilizes the fact that the frequency of Karman vortex generated by a columnar object placed in a fluid is proportional to the flow rate (flow velocity) of the fluid. For example, a hot wire ( It is configured to detect the frequency of the vortex by detecting the change in resistance of the heating wire using a bridge circuit.

However, with the method described above, in which a constant current is passed through a hot wire and resistance changes due to eddies are detected (hereinafter referred to as constant current method), the frequency response decreases in the range of high flow velocity and large flow rate. This method has drawbacks such as a significant decrease in sensitivity and a relatively large fluctuation in low frequencies, making it difficult to measure with high precision over a wide range.

A constant temperature method has been proposed to solve the above drawbacks.

In this constant temperature method, for example, as shown in FIG.
After the output of the bridge circuit 1 including j is amplified by the amplifier 2, it is negatively fed back to the bridge circuit 1 via the feedback resistor Rf,
This is to control the heat wire Rt so that it always has a constant temperature.

In the constant temperature method described above, low frequency fluctuations are blocked and accurate measurements can be made in a range of high flow velocity and large flow rate.

However, in the conventional constant temperature method, the bridge circuit 1
Since the output of 11 is amplified and negative feedback is provided, in the range where the flow velocity is low, the frequency of Karman vortex generation is low, and the response of the hot wire itself is sufficient to track it, the amplitude of the output voltage will be significantly increased due to the negative feedback. It has the disadvantage of being attenuated.

Note that in FIG. 1, 3 is a counter and 4 is a power supply.

Figure 2 shows an example of the characteristics of the constant temperature method, and it can be seen that the output voltage decreases in the low vortex frequency range (low flow velocity range) (here, the output voltage is the maximum output voltage of amplifier 2). Indicates the minimum difference value.

3A is an output waveform diagram of the constant current method described above, and FIG. 3B is an output waveform diagram of the constant temperature method described above.

As can be seen from FIG. 3, in the constant current method, the output decreases in the range of large flow rates, and in the constant temperature method, the amplitude decreases in the range of small flow rates.

The object of the present invention is to provide a Karman vortex flowmeter (or current meter) that eliminates the drawbacks of the conventional method as described above and can accurately measure flow rates from small to large flow rates.

In order to achieve the above object, the present invention divides the output of the bridge circuit into a DC component and an AC component, and in the low frequency range (small flow range) that can be sufficiently tracked only by the response of the hot wire itself, the DC component and This improves the output characteristics in a small flow rate region by blocking the negative feedback of the AC component among the AC components.

The present invention will be explained in detail below based on the drawings.

FIG. 4 is a diagram showing one embodiment of the present invention.

In FIG. 4, a hot wire R5 and resistors R1 to R3 constitute a bridge circuit 5, and an operational amplifier OP1 and resistors R4 to R3 constitute a bridge circuit 5.
7 constitutes a differential amplifier circuit 6, a capacitor C1 and resistors R8 and R9 constitute a low-pass filter 1, a capacitor C2 and resistors R1o and R11 constitute a bypass filter 8, and a transistor Q1 and a resistor R12pR13 constitute a current It constitutes a control circuit 9.

Further, 10 is a signal output terminal.

The output of the bridge circuit 5 is amplified by a differential amplifier circuit 6 and then sent to a low pass filter 7 and a bypass filter 8.

The cutoff frequency of the bypass filter 8 is set at the upper limit of a frequency range that can be sufficiently followed by the response of the hot wire itself, and cuts off or attenuates the alternating current component in the above frequency range.

Furthermore, the cutoff frequency of the low-pass filter I is set to a sufficiently low value, similar to the cutoff frequency of the bypass filter 8, and mostly outputs only the DC component.

For example, the output of the EVA low-pass filter 7 is shown in Fig. 5A.

The output of the bypass filter 8 has a characteristic as shown in FIG. 5 (here, the output voltage indicates the value of the difference between the maximum and minimum output voltages of the differential amplifier 6.

). Next, both the output of the low-pass filter T and the output of the bypass filter 8 are applied to a current control circuit 9, and the current control circuit 9 sends a current corresponding to the input signal to the bridge circuit 5.

As mentioned above, in the circuit of FIG. 4, the bridge circuit 5
A current proportional to the sum of the DC component and the AC component having a predetermined frequency or higher and the DC component of the amplified signal is fed back to the bridge circuit 5.

Therefore, the direct current component is fed back so that the average temperature is constant, and the amount of feedback of the alternating current component is reduced in the low frequency range.

Therefore, the characteristics of the signal output from the output terminal 10 are:
As shown by the solid line in Figure 6, the low frequency region (small flow region)
It has excellent characteristics with no attenuation.

Note that in Figure 6, the broken line E is the characteristic of the conventional constant temperature method, and the dashed line F is the characteristic of the conventional constant current method when only the DC component is fed back (when the bypass filter 8 is removed). , where E, F
(shows the relationship between the difference between the maximum and minimum voltages generated at the output end of the bridge and the vortex frequency).

Note that since the signal at the output terminal 10 is a signal with a frequency proportional to the flow rate or flow rate, the flow rate or flow rate can be detected by measuring the frequency using a counter or the like.

As explained above, according to the present invention, by blocking and returning the low frequency range of the AC component of the DC and AC components of the output of the bridge circuit, the small flow rate, which was a drawback of the conventional low temperature method, can be reduced. Eliminates signal attenuation in low flow velocity range,
The effect is that stable output can be obtained over a wide range from small flow rate to large flow rate 1.

[Brief explanation of drawings]

Fig. 1 is an example of a conventional device, Fig. 2 is an output characteristic diagram of the conventional device, Fig. 3 is an output waveform diagram of the conventional device, Fig. 4 is an example of an embodiment of the present invention, and Fig. 5 is a low-pass filter. FIG. 6 is an output characteristic diagram of the bypass filter. Explanation of symbols, 1...Bridge circuit, 2...
...Amplifier, 3...Counter, 4...
Power supply, 5... Bridge circuit, 6... Differential amplifier circuit, I... Low pass filter, 8...
... Bypass filter, 9 ... Current control circuit, 10 ... Output terminal.

Claims (2)

[Scope of utility model registration request] (1) Detecting the number of Karman vortices generated using a fluid displacement sensing element that utilizes cooling of a heating element in a through hole provided in a columnar object,
and a Karman vortex flowmeter or current meter that feeds back the detected output so that the temperature of the heating element is constant, comprising means for reducing the alternating current component of the feedback amount when the frequency of occurrence of the Karman vortex decreases. Features Karman vortex flowmeter or current meter. (2) a first means for amplifying the detection output detected by the fluid sensing element; a second means for outputting a DC component of the output of the first means; and an AC component of the output of the first means. a third means for transmitting an output with attenuated low frequency components;
and a fourth means for controlling the amount of feedback in accordance with the sum of the output of the second means and the output of the third means, and the DC component is constantly fed back so that the average temperature of the heating element is constant. The Karman vortex flowmeter or current velocity meter according to claim 1, wherein the amount of feedback of the alternating current component is attenuated in a low frequency range.