RU2367965C1 - Method for measurement of resonance frequency and device for its realisation - Google Patents

Abstract

FIELD: physics, measurement.

SUBSTANCE: invention is related to measurement equipment and may be used for high-precision remote detection of resonance frequency of resonators applied in different fields of engineering and scientific research. In particular, it may be used in radio-wave resonant sensors of moisture content and level of oil products. In suggested method measurement of resonant frequency based on excitation of electromagnet oscillations with alternating frequency at the inlet, fixation of maximum amplitude of oscillations at the outlet of resonator and measurement of oscillations frequency f₁ corresponding to this maximum amplitude, is achieved by the fact that at the inlet and outlet of resonator they periodically alternate sign of reactive resistance of external circuits connected to resonator, in case of each variation of this sign frequency f₂ is measured, which corresponds to maximum amplitude of oscillations at the outlet of resonator, and resonance frequency is detected as average value of frequencies f₁ and f₂. Also device is suggested for realisation of above described method for measurement of resonant frequency.

EFFECT: development of device for high-precision remote detection of resonant frequency.

3 cl, 5 dwg

Description

The invention relates to measuring equipment and can be used for high-precision remote determination of the resonant frequencies of electromagnetic resonators used in various fields of technology and scientific research. In particular, it can be used in radio wave resonance sensors for the moisture content of oil products, concentration of solutions and resonant level gauges of various media.

There are various methods for determining resonant frequencies, the essence of one of which is that in the resonator a short pulse excites free vibrations and measures the frequency of these vibrations, which is equal to the resonant frequency of the investigated resonator (A.S. No. 1659908 A1, M. class. G01R 27 / 26). The disadvantage of this method, as well as the device implementing it, is that the measurement of the resonant frequency is possible only for resonators with a Q factor of at least 500-1000. This circumstance significantly limits the scope of the indicated method and device, since in practice the value of the Q factor of resonators, especially those used in various sensors, often does not exceed 100.

There is also a technical solution, which, by its technical essence, is closest to the proposed method and device and adopted as a prototype (Novitsky P.V., Knorring V.G., Gutnikov BC Digital devices with frequency sensors. "Energy". 1970. S. 21). In this prototype method, it is proposed to excite electromagnetic oscillations at the resonator input, the frequency of which varies in a predetermined range in which the resonant frequency of the resonator under study is located. The oscillation amplitude is fixed at the resonator output, and when it reaches a maximum, the sweep stops, and at that moment the oscillation frequency is measured, which is equal to the resonant frequency. A prototype device that implements the specified method consists of a tunable generator, a frequency meter and a tuning device. The task of the latter is to tune the generator to the resonant frequency to the maximum of the resonance curve, after which the frequency meter is turned on, which fixes the generator frequency.

The disadvantage of this method and device, adopted as a prototype, is that they are affected by the accuracy of measuring the resonant frequency of the reactance of all external circuits connected to the studied resonator, namely: communication lines, load and excitation generator.

The aim of the invention is to reduce the measurement error of the resonant frequency due to the influence of reactance of all external circuits connected to the resonator under study, both communication lines, and the generator and load connected to these communication lines.

The goal in the proposed method for measuring the resonant frequency, based on the excitation at the input of the resonator of electromagnetic waves with a varying frequency, fixing the maximum amplitude of the oscillations at the output of the resonator and measuring the frequency of oscillations f ₁ corresponding to this maximum amplitude, is achieved by the fact that at the input and output of the resonator periodically change the sign of the reactance of the external circuits connected to the resonator, with each change in this sign measure the frequency f ₂ corresponding to the maximum the amplitude of the oscillations at the output of the resonator, and the resonant frequency is determined as the average value of the frequencies f ₁ and f ₂ .

The goal in the proposed device for measuring the resonant frequency, containing a series-connected tuning device, a tunable generator and a frequency meter, is achieved by the fact that it additionally contains two transforming units, a trigger and a one-shot, the output of which is connected to the reset input of the frequency meter, and the input of the one-shot is connected to the output strobe pulses of the frequency meter, with the control input of the tuning device and with the counting input of the trigger. The trigger output is connected to the control inputs of both transforming units, while the output of the tunable generator through the first transforming unit is connected to the input of the resonator, the output of which through the second transforming unit is connected to the input of the tuning device.

In this case, the transforming unit is made in the form of two switches and a quarter-wave transformer. The first contacts of both switches are connected through a quarter-wave transformer, and the second contacts of these switches are interconnected.

The essence of the invention lies in the fact that due to the periodic change in the sign of the equivalent reactance of all external circuits connected to the resonator, the sign of the increment of the resonant frequency, which is due to the influence of these external circuits, also synchronously changes. Moreover, the absolute value of this increment remains unchanged. Therefore, the average value of the resonant frequencies corresponding to the positive and negative values of the equivalent reactance of the external circuits does not depend on the parameters of these circuits, which allows minimizing the measurement error of the resonant frequency due to the influence of both the communication lines themselves and other devices connected using these lines to the resonator, for example, the generator and the load.

More rigorously this statement can be proved if we consider the general scheme for measuring the resonant frequency, presented in the form of an equivalent circuit (Fig. 1), on which the following notation is adopted: 1 - tunable generator having an internal resistance of Z _g ; 2 - input and output communication lines with wave impedance W; 3 - resonator, presented in the form of an equivalent oscillatory circuit with inductance L, capacitance C and loss resistance R. X _cb1 and X _cv2 are the equivalent resistances of the coupling elements at the input and output of the resonator, respectively; 4 - load having resistance Z _n .

The circuit depicted in figure 1, it is advisable to present in the form of a simpler equivalent circuit (figure 2), in which Z _l - the resistance of the communication line at the point of connection to the resonator. For convenience of analysis in FIG. 2, the resonator is under load, since the influence of both communication lines on the resonant frequency can be studied independently of each other. From figure 2 it can be seen that the total equivalent resistance connected directly to the oscillatory circuit, Z _e is composed of Z _l and X _St , that is

It is advisable to present Z in the form of equivalent conductivity Y _e , which in turn consists of active conductivity G and reactive conductivity B:

It's obvious that

The value of Z _l can be represented as the sum

When measuring the resonant frequency, they always strive to minimize the influence of external circuits on the measurement result, therefore

Given these inequalities, it is easy to obtain an approximate expression for the increment of the equivalent conductivity ΔY _e

The quantity ΔZ _l also consists of active ΔR and reactive ΔX components,

For the case of capacitive coupling of external circuits with a resonator

where f is the frequency.

Substituting (7) and (8) in (6) and isolating the reactive component, we obtain the following expression for the increment of reactance ΔВ _e :

The influence of ΔВ _e on the resonant frequency is equivalent to connecting some capacitance ΔС to the resonant circuit, while

Using (9) and (10), ΔС can be represented as

The relative change in the resonant frequency Δf / f, due to the influence of external circuits, can be determined using (11) and the well-known expression for the resonant frequency of the oscillatory circuit LC:

Then

It can be seen from formula (13) that changes in the resonant frequency Δf can be both positive and negative in sign depending on the sign of the reactance of the external circuit ΔX. Therefore, a periodic change in the sign of ΔX causes a corresponding change in the sign of Δf. It follows that the average value of the resonant frequency does not depend on the reactance of external circuits, which ensures a useful effect formulated for the purpose of the proposal.

For the implementation of the proposed method, it is important to have a resistance transformer that converts polarity, reactance connected to it. As such a resistance transformer, a quarter-wave length of a long line can be used, the operation of which is described by the well-known expression (Lebedev I.V. Technique and microwave devices. - M .: Higher school. 1970. P. 218)

где

Where

Z _o - the complex output resistance of the quarter-wave transformer when connected to the input of this transformer resistance Z _l .

Substituting (4) and (7) into formula (14) and taking into account inequality (5), it is easy to show that for the reactive component X _{o of the} output impedance Z _{o there} is an approximate equality

and, therefore, a quarter-wave transformer, made in the form of a quarter-wave length of a long line, provides the ability to convert the polarity of the reactance.

Figure 3 presents the structural diagram of a device for measuring the resonant frequency. This device consists of a frequency meter 1, a voltage-controlled generator (VCO) 2, a tuner 3, a one-shot 4, a trigger 5, two transforming units 9, each of which contains two switches to two positions 6 and 8 and a quarter-wave transformer 7, and the investigated resonator 10.

This device works as follows. The strobe pulses from the output of the frequency counter 1 (Fig. 4a) are fed to the counting input of the trigger 5, the input of the start of the tuning device 3 and to the input of the one-shot 4. Pulses from the output of the trigger in the form of a square wave (Fig. 4b), having a duration equal to the period of the strobe pulses arrive at the control inputs of the switches 6 and 8, and these pulses provide synchronization of the processes of connecting-disconnecting the quarter-wave transformers 7. Negative pulses of the quarter-wave transformers are included in the measuring circuit, and positive ones are turned off I am. Suppose that in one of the clock cycles the quarter-wave transformers 7 are disconnected and jumpers are turned on instead (the contactors of the switches 6 and 8 in the diagram of Fig. 3 are in the lower position). At time t _{0, the} positive edge of the strobe pulse arrives at the start input of tuning device 3 and initiates the generation of a linearly varying voltage in it, which is supplied to the control input of the VCO 2 and changes its frequency in a given range (Fig. 4c). The voltage input from the output of the resonator 10 is supplied to the signal input of the tuning device 3 (Fig. 4d), and when this voltage reaches its maximum at time t ₁ , the scan stops and in the time interval t ₁ ÷ t _{3 the} voltage at the input of the VCO 2 also as its frequency is maintained at a constant level. The frequency of the VCO in this time interval has a value of f ₁ . At time t _{2, the} negative edge of the strobe pulse opens the input of the frequency meter 1 and in the interval t ₂ ÷ t _{3 the} number of pulses arriving from the output of the VCO 2 and having a frequency f _{1 is} calculated. At time t _{3, the} positive edge of the strobe pulse closes the input of the frequency meter 1 and the counting of the number of pulses stops. At the same time, the negative edge of the trigger pulse 5 (Fig. 46) includes the quarter-wave transformers 7 in the measuring circuit (the contactors of the switches 6 and 8 in the diagram of Fig. 3 are in the upper position) and equivalent reactances connected to the input and output of the resonator 10 , change their polarity. At the same time, tuning device 3 is reset to the initial state and again tuned to the maximum resonance curve, which now corresponds to frequency f ₂ and time t ₄ . The negative front of the strobe pulse at time t ₅ again opens the input of the frequency meter 1, and in the interval t ₅ ÷ t _{6 the} counting of the number of pulses now having a frequency f ₂ continues. Thus, in the intervals t ₂ ÷ t ₃ and t ₅ ÷ t ₆ there is a total count of the number of pulses following in series with a frequency f ₁ and f ₂ . Due to this, averaging of the frequencies f ₁ and f ₂ occurs. At time t _{6, the} process of measuring frequencies f ₁ and f ₂ ends and the obtained average frequency value is stored up to time t ₇ determined by the duration t _{u of a} positive pulse coming from the output of one-shot 4 (Fig. 4e). This one-shot is triggered by the negative edge of the strobe pulse. At time t _7, the frequency counter readings are reset, and at time t ₈ the process of measuring the average frequency begins again. For the correct execution of the averaging operation, it is necessary that in the time interval determined by the duration t _{u there} is always an even number of intervals in which the frequencies f ₁ and f ₂ are measured. Therefore, the value of this duration in the general case must satisfy the condition

(2n-0.5) T <t _u <2nT,

where T is the strobe period.

The proposed method for measuring the resonant frequency and a device for its implementation were implemented when developing a valve displacement sensor for the valve device. The sensitive element (SE) of this sensor is a volume resonator, the natural frequency of which is determined by the position of the valve in the valve cavity. According to the operating conditions, the sensor secondary transducer should be located at a distance of at least 100 m from the SE, so the length of each of the communication cables, both receiving and transmitting, was also 100 m. Figure 5 shows the experimental output characteristics of this sensor, i.e. . the dependence of the resonant frequency of the SE f on the position of the valve x. In this case, curve 1 was obtained when measuring the resonant frequency using a device similar to the prototype, and curve 2 - using the proposed method and device. Figure 5 shows that curve 1 has a wave-like character, due to the influence on the resonant frequency of the SE of communication cables of large length. This effect takes place, especially when the length of these cables exceeds the resonant wavelength of the SE. According to experimental studies, the error in measuring the resonant frequency for the proposed method and device with a communication cable length of 100 m and a resonant frequency of the SE in the range 26 ÷ 32 MHz was 1%, and for the prototype device under the same conditions - 6%.

Claims (3)

1. The method of measuring the resonant frequency, based on the excitation at the input of the resonator of electromagnetic waves with a varying frequency, fixing the maximum amplitude of the oscillations at the output of the resonator and measuring the oscillation frequency f ₁ corresponding to this maximum amplitude, characterized in that at the same time at the input and output of the resonator periodically change the sign of the reactance of the external circuits connected to the resonator, with each change of this sign, measure the frequency f ₂ corresponding to the maximum amplitude fucking at the output of the resonator, and the resonant frequency is defined as the average value of the frequencies f ₁ and f ₂ . 2. A device for measuring the resonant frequency of an electromagnetic resonator containing a tuned device, a tunable generator and a frequency meter, the output of a tunable generator connected to the input of the resonator, the output of which is connected to the input of the tuner, characterized in that it further comprises two transforming units, trigger and one-shot, the output of which is connected to the reset input of the frequency meter, and the input is connected to the output of strobe pulses of the frequency meter, with a control input stroystva settings and to trigger counting input, the output of which is connected to the control inputs of the first and second transforming units, the output of tunable oscillator through a first transforming unit coupled to an input resonator, whose output is via a second transforming unit coupled to an input setting device. 3. The device according to claim 2, characterized in that the transforming unit is made in the form of two switches and a quarter-wave transformer, while the first contacts of both switches are connected through a quarter-wave transformer, and their second contacts are interconnected.

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