Revolutions measuring circuit, selection method and flow meter

Abstract

The application provides a rotation number measurement circuit, a selection method and a flowmeter. The circuit comprises a first circuit composed of an oscillator, an inductor coil and a capacitor in series, a mixer connected with the first circuit, and an analog-to-digital converter connected with the mixer. The oscillator is used for generating an oscillation signal, and the oscillation signal is transmitted to the mixer through the capacitor and the inductor coil. The mixer is used for down-converting the oscillation signal to obtain an intermediate frequency signal. The analog-to-digital converter is used for sampling the intermediate frequency signal to obtain a sampling signal, and sending the sampling signal to an external processor, so that the external processor determines the rotation number of a rotor based on the number of amplitude value changes of the sampling signal. When the oscillator sends an oscillation signal with a high frequency, the amplitude value change range of the final sampling signal is also large, so that the measurement result can be less disturbed by the induction distance. Moreover, the mixer is used for down-converting the oscillation signal to obtain an intermediate frequency signal with a low frequency, and the sampling cost of the measurement circuit can be not increased.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of measurement, and in particular to a rotation number measurement circuit, a selection method and a flow meter. BACKGROUND

[0002] The flow meter usually has a mechanical rotating disc structure, and the liquid flow drives the mechanical rotating disc to rotate. The rotation number of the mechanical rotating disc can be counted to count the liquid flow. At present, an inductor coil is arranged above the rotating disc. When the rotating disc rotates, eddy current is generated in the inductor coil, and the inductance value and the resistance value of the inductor coil change periodically. The number of periodic changes of the inductance value and the resistance value can determine the rotation number of the mechanical rotating disc.

[0003] However, in the current statistical method, the change of the frequency and the voltage amplitude depends on the relative change of the inductance and the resistance. In particular, due to the limitation of the chip area under the consideration of the cost, the relative change of the inductance and the resistance in the existing scheme is usually small, so that the measurement accuracy of the rotation number measurement sensor is easily affected by the sensing distance in actual application. SUMMARY

[0004] Embodiments of the present application provide a rotation number measurement circuit, a selection method and a flow meter to solve the problem that the measurement accuracy of the current rotation number measurement sensor is easily affected by the distance.

[0005] In a first aspect, embodiments of the present application provide a rotation number measurement circuit, the rotation number measurement circuit being used for measuring the rotation number of a rotor, the rotor having regions with different conductive properties; the rotation number measurement circuit comprising:

[0006] a first circuit composed of an oscillator, an inductor coil and a capacitor in series, a mixer connected with the first circuit, and an analog-to-digital converter connected with the mixer; when the rotor rotates, the inductor coil generates different inductance values close to the regions with different conductive properties;

[0007] The oscillator is used for generating an oscillation signal, and the oscillation signal is transmitted to the mixer through the capacitor and the inductor coil.

[0008] The mixer is used for down-converting the oscillation signal to obtain an intermediate frequency signal.

[0009] The analog-to-digital converter is used for sampling the intermediate frequency signal to obtain a sampling signal, and sending the sampling signal to an external processor, so that the external processor determines the rotation number of the rotor based on the number of amplitude value changes of the sampling signal.

[0010] In a possible implementation, the rotation number measurement circuit comprises at least two first circuits, each of the first circuits corresponds to a mixer connected to the first circuit, and each of the mixers is connected to an analog-to-digital converter; when the rotor is stationary, the inductive coils of each of the first circuits are close to different rotor regions, and the rotor regions are regions with different conductive properties of the rotor.

[0011] The analog-to-digital converter corresponding to each of the first circuits is connected to an external processor, so that the external processor receives sampling signals corresponding to each of the first circuits, and determines the rotation number of the rotor and the rotation direction of the rotor according to the change order of each of the sampling signals and the arrangement positions of each of the inductive coils.

[0012] In a second aspect, an embodiment of the present application provides a selection method of a rotation number measurement circuit, which is applied to the rotation number measurement circuit in any of the implementation manners of the first aspect.

[0013] The selection method of the rotation number measurement circuit comprises:

[0014] determining a calculation relationship of a source impedance by taking the impedance of the first circuit as the source impedance;

[0015] taking the impedance of the mixer as a load impedance;

[0016] determining the oscillation frequency of the oscillator and the capacitance value of the capacitor according to the calculation relationship of the source impedance and the load impedance.

[0017] In a possible implementation, the calculation relationship of the source impedance is:

[0018]

[0019] wherein, Z s is the impedance of the first circuit, Z0 is the impedance of the oscillator, C is the capacitance value of the capacitor, is the capacitive reactance of the capacitor, f0 is the frequency of the oscillation signal, j2πf0L v is the impedance of the inductive coil, R v is the resistance value of the inductive coil, L v is the inductance value of the inductive coil.

[0020] In a possible implementation, the determination of the oscillation frequency of the oscillator and the capacitance value of the capacitor according to the calculation relationship of the source impedance, the sampling signal amplitude change condition and the load impedance comprises:

[0021] determining a reflection coefficient relationship between the source impedance and the load impedance, the reflection coefficient relationship being used to calculate the reflection coefficient between the source impedance and the load impedance;

[0022] determining the oscillation frequency of the oscillator and the capacitance value of the capacitor based on the reflection coefficient relationship.

[0023] In a possible implementation, the reflection coefficient relationship is:

[0024]

[0025] wherein Γ is the reflection coefficient of the first circuit and the mixer, Z L is the impedance of the mixer, Z * L is the conjugate of Z L .

[0026] In a possible implementation, the oscillation frequency of the oscillator and the capacitance value are determined according to the calculation relationship of the source impedance and the load impedance, comprising:

[0027] The oscillation frequency of the oscillator and the capacitance value are determined according to the following values of the reflection coefficient:

[0028]

[0029]

[0030] wherein Z s1 represents the source impedance when the rotor rotates to the first position, Z s2 represents the source impedance when the rotor rotates to the second position.

[0031] In a possible implementation, the first position is a position at which the inductance value of the inductor coil is maximum, and the second position is a position at which the inductance value of the inductor coil is minimum.

[0032] In a possible implementation, the first position is a position at which the inductance value of the inductor coil is minimum, and the second position is a position at which the inductance value of the inductor coil is maximum.

[0033] In a third aspect, the embodiment of the present application provides a flowmeter, comprising the revolution measurement circuit according to any one of the implementations of the first aspect.

[0034] The revolution measurement circuit, the selection method and the flowmeter provided by the present application have the following beneficial effects:

[0035] When the oscillator sends an oscillation signal with a large frequency, the impedance of the inductor coil changes greatly when the inductor coil is close to different areas of the rotor, and the amplitude range of the finally obtained sampling signal is also large, which can reduce the interference of the sensing distance on the measurement result. And the mixer down-mixes the oscillation signal to obtain an intermediate frequency signal with a lower frequency, so that the sampling frequency does not need to be improved when the sampling signal is obtained. The measurement accuracy can be improved without increasing the requirements on the analog-to-digital converter, thereby reducing the cost of the measurement circuit. BRIEF DESCRIPTION OF DRAWINGS

[0036] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description only show some of the embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort based on these drawings.

[0037] Figure 1 is a structural schematic diagram of the first circuit provided by the embodiments of the present application;

[0038] Figure 2 is a schematic diagram of the layout position of the inductor coil provided by the embodiments of the present application;

[0039] Figure 3 is an implementation flowchart of the selection method of the revolution measurement circuit provided by the embodiments of the present application;

[0040] Figure 4 is an equivalent circuit diagram of the first circuit provided by the embodiments of the present application. DETAILED DESCRIPTION

[0041] In order to make the person skilled in the art better understand the present scheme, the technical solutions in the embodiments of the present scheme will be clearly described below in combination with the drawings in the embodiments of the present scheme. Obviously, the described embodiments are only some of the embodiments of the present scheme, not all the embodiments. Based on the embodiments in the present scheme, all other embodiments obtained by those skilled in the art without any creative effort should be within the scope of protection of the present scheme.

[0042] The term "comprising" and other any variations thereof in the specification and claims of the present scheme and the above-mentioned drawings means "including but not limited to", which is intended to cover non-exclusive inclusion and is not limited to the examples listed in the text. In addition, the terms "first" and "second" and the like are used to distinguish different objects, not to describe a specific order.

[0043] The revolution measurement circuit provided by the present application is applied to measure the revolution of the rotor, which can be a mechanical turntable in a flowmeter. Different regions of the mechanical turntable are covered with different conductive materials. The liquid flow will drive the mechanical turntable to rotate, and the liquid flow can be determined by counting the revolution of the mechanical turntable. Due to the eddy current phenomenon, the eddy current generated by different conductive materials placed under the inductor coil is different, which in turn causes the inductance and resistance of the inductor coil to be different. Therefore, the revolution of the rotor can be equivalent to the statistics of the number of periodic changes of inductance and resistance.

[0044] At present, the inductance and resistance measurement schemes in flow meter sensors can be roughly divided into four kinds. The first kind is to connect an inductance coil above a mechanical rotating disc, which is covered with different conductive materials, to an oscillation loop containing a capacitor. When the inductance coil has different resistances due to different conductive materials below, the attenuation amplitude of the oscillation signal in the oscillation loop will change accordingly, i.e. the resistance change can be measured by the attenuation amplitude of the oscillation signal. The second kind is to connect an inductance coil above a rotating disc to a charging and discharging loop, and then use the voltage after a certain time of the charging and discharging loop to measure the inductance change. The third kind is to connect an inductance coil above a rotating disc to an oscillation circuit, and use the oscillation frequency change to measure the inductance change. The fourth kind is to connect a coupling inductance coil above a rotating disc to a transformer circuit, and measure the coupling inductance change by the voltage amplitude change of the secondary side of the transformer. However, the frequency and voltage amplitude changes in the above schemes depend on the relative change of inductance and resistance. In particular, in order to reduce the cost, the area of the chip is usually limited, which makes the coil smaller, resulting in that the relative change of inductance and resistance in the above four schemes is usually small, and their measurement signal amplitudes are proportional to the relative change of inductance or resistance, which greatly limits the sensing distance and anti-interference ability of the sensor in practical applications.

[0045] The implementation of the present application will be described in detail below in combination with specific drawings:

[0046] Figure 1 A structure diagram of a revolution measurement circuit provided for an embodiment of the present application is shown in FIG. 1. Referring to FIG. 1, the revolution measurement circuit 1 comprises: Figure 1

[0047] A first circuit 11 composed of an oscillator 111, an inductance coil 112 and a capacitor 113 in series, a mixer 12 connected with the first circuit 11, and an analog-to-digital converter 13 connected with the mixer. When the rotor rotates, the inductance coil 112 generates different inductance values close to regions with different conductive properties.

[0048] The oscillator 111 is used to generate an oscillation signal, which is transmitted to the mixer 12 through the capacitor 113 and the inductance coil 112.

[0049] The mixer 12 is used to down-convert the oscillation signal to obtain an intermediate frequency signal.

[0050] The analog-to-digital converter 13 is used to sample the intermediate frequency signal to obtain a sampling signal, and send the sampling signal to an external processor, so that the external processor determines the revolution of the rotor based on the number of amplitude value changes of the sampling signal.

[0051] In the embodiment, as shown in FIG. 1, the revolution measurement circuit 1 comprises: Figure 1 ​As shown, the oscillation signal generated by the oscillator 111 is transmitted to the RF end of the mixer 12 through the capacitor 113 and the inductive coil 112, and at the same time, a local oscillation signal LO is also transmitted to the mixer 12. After receiving the oscillation signal transmitted through the capacitor 113 and the inductive coil 112 and the local oscillation signal, the mixer 12 performs difference operation on the two signals, obtains an intermediate frequency signal, and outputs the intermediate frequency signal through the IF end. The intermediate frequency signal can reflect the amplitude change of the oscillation signal after the oscillation signal passes through the capacitor 113 and the inductive coil 112, and the frequency of the intermediate frequency signal is much lower than that of the local oscillation signal. The analog-to-digital converter can sample the intermediate frequency signal at a lower frequency, and therefore the rotation number measurement circuit provided in the application has a lower requirement on the performance of the analog-to-digital converter, and the circuit cost can be reduced.

[0052] In a possible implementation, the rotation number measurement circuit 1 comprises at least two first circuits 11, each first circuit 11 corresponds to a mixer 12 connected to the first circuit 11, and each mixer corresponds to an analog-to-digital converter 13 connected thereto. When the rotor is stationary, the inductive coils 112 of the first circuits 11 are close to different rotor regions, and the rotor regions are regions with different conductive properties of the rotor.

[0053] The analog-to-digital converters 13 corresponding to the first circuits 11 are connected to an external processor, so that the external processor receives the sampling signals corresponding to the first circuits, and determines the rotation number of the rotor and the rotation direction of the rotor according to the change order of the sampling signals and the arrangement positions of the inductive coils 112.

[0054] In the embodiment, the plurality of inductive coils are close to different rotor regions respectively, and when the rotor rotates in a certain direction, a certain rotor region will be close to each inductive coil in a fixed order, that is, the inductance values of the inductive coils will change in a fixed order, so that the sampling signals corresponding to the first circuits also change in the fixed order. Therefore, the rotation direction of the rotor can be determined by the change order of the amplitudes of the sampling signals, and the rotation direction of the rotor can be determined by combining the arrangement positions of the inductive coils.

[0055] In addition, the rotation number of the rotor can be determined multiple times according to the change times of the sampling signals corresponding to the first circuits respectively, and finally a more accurate rotation number measurement result can be obtained by calculating an average value or removing abnormal values.

[0056] In a specific embodiment, the rotor region can be a semicircle, and one half of the semicircle on the rotor is covered with metal, and the other half of the semicircle is not covered with metal. Figure 2As shown, three inductance coils are evenly placed above the flowmeter rotating disc, each inductance coil is independently connected with an oscillator, a capacitor, a mixer and an analog-to-digital converter, respectively forming sensors 1, 2 and 3. The three analog-to-digital converters communicate with an external controller through a general-purpose input / output (GPIO) or a serial peripheral interface (SPI). When the metal semicircle of the rotating disc rotates clockwise to below the sensor 1, the inductance coil 1 in the sensor 1 is completely above the metal sheet, the inductance value of the inductance coil 1 changes to the minimum, and the impedance to the oscillation signal is also the minimum, so the intermediate frequency signal corresponding to the sensor 1 can obtain a larger amplitude sampling signal after analog-to-digital conversion. Conversely, the inductance value of the inductance coil 2 below the metal sheet changes to a larger value, at this time the impedance to the oscillation signal is larger, therefore, the intermediate frequency signal corresponding to the sensor 2 can only obtain a smaller amplitude sampling signal after analog-to-digital conversion. The inductance value of the inductance coil in the sensor 3 is between that of the inductance coil 1 and the inductance coil 2, the impedance to the oscillation signal is a medium value, then the intermediate frequency signal corresponding to the sensor 3 can obtain a medium amplitude sampling signal after analog-to-digital conversion. After the external controller obtains the sampling signals corresponding to the three sensors, the amplitudes and time-varying relationships of the sampling signals are compared, the order and number of the metal semicircle of the rotating disc passing through each sensor can be determined in real time, and the rotating direction and number of revolutions of the rotating disc can be determined at the same time.

[0057] Figure 3 is an implementation flowchart of the selection method of the revolution measurement circuit provided by the embodiment of the present application, which is described in detail as follows:

[0058] In step 301, the impedance corresponding to the first circuit is taken as a source impedance, and a calculation relationship of the source impedance is determined.

[0059] In step 302, the impedance of the mixer is taken as a load impedance.

[0060] In step 303, the oscillation frequency of the oscillator and the capacitance value of the capacitor are determined according to the calculation relationship of the source impedance, the sampling signal amplitude change condition and the load impedance.

[0061] In the present embodiment, in the radio frequency circuit, when the source impedance and the load impedance are directly connected and are conjugate impedances, the signal at the source impedance end can be input into the load end without reflection. At this time, the source impedance end and the load end are matched. Conversely, the source impedance end and the load end are not matched, part of the signal at the source end is reflected and cannot be input into the load end.

[0062] Therefore, in order to further increase the change amplitude of the oscillation signal, the impedance matching relationship between the two circuits is utilized in the embodiment, the impedance corresponding to the first circuit is taken as the source impedance, and the impedance of the mixer is taken as the load impedance, so that the reflection coefficient of the oscillation signal when transmitted to the mixer can be calculated, and the amplitude of the oscillation signal received by the mixer can be greatly adjusted by adjusting the reflection coefficient, and the sensitivity of the revolution measurement can be improved when the inductance value of the inductor coil changes slightly.

[0063] In a possible implementation, the calculation relationship of the source impedance is:

[0064]

[0065] wherein, Z s is the impedance of the first circuit, Z0 is the impedance of the oscillator, C is the capacitance value of the capacitor, is the capacitive reactance of the capacitor, f0 is the frequency of the oscillation signal, j2πf0L v is the impedance of the inductor coil, R v is the resistance value of the inductor coil, L v is the inductance value of the inductor coil.

[0066] In the embodiment, in order to facilitate analysis, the revolution measurement circuit is first equivalent to the circuit shown in the following formula (1) in the embodiment, and the impedance of the first circuit in which the oscillator, the capacitor and the inductor coil are connected in series is taken as the source impedance, and the source impedance can be represented as: Figure 4

[0067]

[0068] wherein, Z0 is the impedance of the oscillator, ω0 is the working angular frequency of the oscillator, L v and R v are the variable inductance and resistance of the inductor coil placed above the turntable, and their values are different with different conductive materials on the turntable, and C is the selected series capacitor, which is used to offset the reactance under a certain L v value. It is noted that the impedance Z inL generated by the inductor coil is:

[0069] Z inL = R v +jω0L v = R v +j2πf0L v (2)

[0070] wherein, f0 is the working frequency of the oscillator. Under the condition that the conductive material, the area of the inductor coil, and the distance between the inductor coil and the conductive material are determined, the inductance L v and the resistance R v ​The relative change of the inductance L v and the resistance R v is small. Therefore, in the conventional method of measuring the number of revolutions based on the relative change of the inductance L v and the resistance R v , the change of the signal amplitude is usually small. However, as shown in equation (2), the change of the source impedance is proportional to the frequency f0, and therefore, even if the relative change of the inductance L v and the resistance R v is small, by selecting a large oscillator operating frequency f0, the impedance of the inductance coil can change greatly when the inductance coil is above different conductive materials.

[0071] In one possible implementation, the oscillation frequency of the oscillator and the capacitance value are determined according to the calculation relationship of the source impedance, the sampling signal amplitude change condition, and the load impedance, including:

[0072] determining a reflection coefficient relationship between the source impedance and the load impedance, the reflection coefficient relationship being used to calculate the reflection coefficient between the source impedance and the load impedance;

[0073] determining the oscillation frequency of the oscillator and the capacitance value based on the reflection coefficient relationship.

[0074] In this embodiment, when the reflection coefficient is 0, the oscillation signal can be transmitted to the mixer without reflection and down-converted to an intermediate frequency signal, and the amplitude of the intermediate frequency signal is large. When the reflection coefficient is 1, the oscillation signal is fully reflected, and no signal enters the mixer, and the amplitude of the intermediate frequency signal is 0. Therefore, the amplitude of the intermediate frequency signal changes greatly when the rotor rotates. Therefore, by selecting the oscillation frequency and the capacitance value, the reflection coefficient at the interface between the impedance and the load impedance is approximately 0 or 1 when the inductance value of the inductance coil is different, so that the amplitude of the intermediate frequency signal after down-conversion by the mixer changes greatly. As can be seen, the present application is different from the conventional method based on the measurement of the relative change of the inductance and the resistance. The present application uses the impedance / matching change caused by the inductance coil above different conductive materials under the radio frequency condition to achieve high-sensitivity measurement of the changing inductance and resistance.

[0075] In one possible implementation, the reflection coefficient relationship is:

[0076]

[0077] wherein Γ is the reflection coefficient of the first circuit and the mixer, Z L is the impedance of the mixer, and Z * L is the conjugate of Z L .

[0078] In the embodiment, in order to analyze the signal size on the mixer, the mixer can be regarded as a load, and it is assumed that the impedance of the mixer is Z L Therefore, on the interface of the source impedance and the load impedance, the reflection coefficient caused by the impedance matching problem can be expressed as:

[0079]

[0080] As can be seen from the formula (1) and (3), by selecting appropriate series capacitance C and frequency f0, the maximum or minimum value of the reflection coefficient can be adjusted, so that the reflection coefficient has a large amplitude change when the rotor rotates, so that the amplitude of the sampling signal can also have a large amplitude change, and the sensitivity of the rotation number measurement is ensured.

[0081] In a possible implementation, the oscillation frequency of the oscillator and the capacitance value of the capacitor are determined according to the calculation relationship of the source impedance, the sampling signal amplitude change condition and the load impedance, comprising:

[0082] The oscillation frequency of the oscillator and the capacitance value of the capacitor are determined based on the following values of the reflection coefficient:

[0083]

[0084]

[0085] Wherein, Z s1 represents the source impedance when the rotor rotates to the first position, Z s2 represents the source impedance when the rotor rotates to the second position.

[0086] In the embodiment, by selecting appropriate oscillation frequency and capacitance value, the reflection coefficient between the source impedance and the load impedance can be minimum when the rotor rotates to the first position, and the reflection coefficient between the source impedance and the load impedance can be maximum when the rotor rotates to the second position. Based on the oscillation frequency and the capacitance value, the rotation number measurement circuit is configured, and when the rotation number is measured, it can be determined that the rotor rotates to the first position when the amplitude of the sampling signal is maximum, and it can be determined that the rotor rotates to the second position when the amplitude of the sampling signal is minimum. That is, the rotation number of the rotor is increased by 1 each time the sampling signal appears a maximum value and a minimum value.

[0087] In a possible implementation, the first position is a position at which the inductance value of the inductor is maximum, and the second position is a position at which the inductance value of the inductor is minimum.

[0088] In a possible implementation, the first position is a position at which the inductance value of the inductor is minimum, and the second position is a position at which the inductance value of the inductor is maximum.

[0089] In the embodiment, the maximum and minimum of the inductance value of the inductance coil are respectively brought into the above formula, and the oscillation frequency and the capacitance value are solved.

[0090] As can be seen from the above, when the oscillator sends an oscillation signal with a high frequency, the impedance of the inductance coil changes greatly in different areas close to the rotor, and due to the matching change, the amplitude change range of the final sampling signal is also great, which can reduce the influence of the sensing distance on the measurement result accuracy; and the lower intermediate frequency signal frequency is obtained by mixing the oscillation signal frequency through the mixer, so that the sampling frequency does not need to be improved when the sampling signal is obtained, the measurement accuracy can be improved without increasing the requirement for the analog-to-digital converter, thereby the cost of the measurement circuit is not increased.

[0091] The embodiment of the present application also provides a flow meter, which comprises the revolution measurement circuit shown in the above Figure 1 embodiment or any implementation manner of the embodiment.

[0092] The above embodiments are only used to illustrate the technical solutions of the present application, but not limit the same; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that the technical solutions recorded in the foregoing embodiments can be modified, or some technical features can be replaced by the equivalent ones; and the modification or replacement does not make the essence of the corresponding technical solution deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A revolution measurement circuit for measuring the number of revolutions of a rotor having regions of different electrical conductivity, characterized by, The rotation number measurement circuit comprises: a first circuit composed of an oscillator, an inductor coil and a capacitor in series, a mixer connected with the first circuit, and an analog-to-digital converter connected with the mixer; when the rotor rotates, the inductor coil generates different inductance values near the regions with different conductive properties; the oscillator is configured to generate an oscillation signal, which is transmitted to the mixer through the capacitor and the inductor coil; the mixer is configured to down-convert the oscillation signal to obtain an intermediate frequency signal; the analog-to-digital converter is configured to sample the intermediate frequency signal to obtain a sampling signal, and send the sampling signal to an external processor, so that the external processor determines the rotation number of the rotor based on the number of amplitude value changes of the sampling signal; the oscillation frequency of the oscillator and the capacitance of the capacitor are determined based on the following formula: wherein Z s1 represents the source impedance when the rotor is rotated to the first position, Z s2 represents the source impedance when the rotor is rotated to the second position, Γ1 represents the reflection coefficient of the first circuit and the mixer when the rotor is rotated to the first position, Γ2 represents the reflection coefficient of the first circuit and the mixer when the rotor is rotated to the second position, Z L is the impedance of the mixer, Z * L is the conjugate of Z L . the calculation relationship of the source impedance is: wherein Z s is an impedance of the first circuit, Z0is an impedance of the oscillator, C is a capacitance value of the capacitor, is a capacitive reactance of the capacitor, f0is a frequency of the oscillation signal, j2πf0L v is an impedance of the inductor coil, R v is a resistance value of the inductor coil, L v is an inductance value of the inductor coil.

2. The revolution measurement circuit of claim 1, wherein The rotation number measurement circuit comprises at least two first circuits, each first circuit corresponding to a mixer connected with the first circuit, and each mixer corresponding to an analog-to-digital converter connected therewith; when the rotor is stationary, the inductor coils of the first circuits are close to different regions of the rotor, which are regions with different conductive properties of the rotor; The analog-to-digital converters corresponding to each first circuit are connected with an external processor, so that the external processor receives sampling signals corresponding to each first circuit, and determines the rotation number and the rotation direction of the rotor according to the arrangement positions of the inductor coils and the change order of the sampling signals.

3. A method of selecting a revolution measurement circuit, characterized by: The selection method of the rotation number measurement circuit is applied to the rotation number measurement circuit of claim 1 or 2; The selection method of the rotation number measurement circuit comprises: determining the calculation relationship of the source impedance by taking the impedance of the first circuit as the source impedance; taking the impedance of the mixer as the load impedance; determining the oscillation frequency of the oscillator and the capacitance of the capacitor according to the calculation relationship of the source impedance, the sampling signal amplitude change condition and the load impedance.

4. The method of claim 3, wherein the number of revolutions is measured by counting the number of times the voltage of the signal output from the Hall IC is inverted. The determination of the oscillation frequency of the oscillator and the capacitance of the capacitor according to the calculation relationship of the source impedance and the load impedance comprises: determining the reflection coefficient relationship between the source impedance and the load impedance, which is used to calculate the reflection coefficient between the source impedance and the load impedance; determining the oscillation frequency of the oscillator and the capacitance of the capacitor based on the reflection coefficient relationship; The reflection coefficient relationship is: where Γ is the reflection coefficient of the first circuit and the mixer, Z L is the impedance of the mixer, Z * L is the conjugate of Z L .

5. The method of claim 3, wherein the number of revolutions is measured by counting the number of times the voltage of the signal output from the Hall IC is inverted. The first position is the position where the inductance value of the inductor coil is maximum, and the second position is the position where the inductance value of the inductor coil is minimum.

6. The method of claim 3, wherein the number of revolutions is measured by counting the number of times the voltage of the signal output from the Hall IC is inverted. The first position is the position where the inductance value of the inductor coil is minimum, and the second position is the position where the inductance value of the inductor coil is maximum.

7. A flow meter characterized by, The rotation number measurement circuit of claim 1 or 2 is included.

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