KR940008643B1 - Method and apparatus for measuring the distance to an object - Google Patents

Abstract

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Description

Method and measuring device for measuring the distance to the object

1 is a structural diagram of a number measurement device in a container according to the present invention.

2-4 are block diagrams of various embodiments of the measurement unit of FIG.

5 is a block diagram of one embodiment of a reflectometer.

6 shows main functions of the signal processing unit 29. As shown in FIG.

* Explanation of symbols for main parts of the drawings

11 antenna 12 measuring unit

13 channel unit 14 detection unit

15: Courage 17: Sleep

18: microwave oscillator 19: divider (divider)

20: antenna measurement unit 21: frequency correction unit

22: control unit 24A, B: detection unit

28: data processing unit 29: signal processing unit

31 dual circuit 32 mixer

34: interpolation circuit 36: counter unit

The present invention relates to a method and apparatus for measuring the distance to an object, in particular a liquid or liquid medium in a closed container or the like.

Terminology relating to microwave technology as used herein is in accordance with IEC Standard Publication 615, "Terminology for Microwave Devices," published in Geneva, Switzerland, 1976.

Various methods based on the radiation of electromagnetic waves have been used for the above-mentioned purposes.

Pulse radar

The signal is transmitted as a short pulse. The received signal consists of a series of pulses corresponding to echoes from different distances. Signal processing consists of the amplitude and timing of an echo pulse that requires a very fast acting circtit. The resolution increases as the pulse length decreases, and new transmit pulses are not sent until all valid echo signals are returned. If the pulse transmission period is T, then the ratio of peak gain to average gain in the radar transmitter is T / τ. To avoid distortion of the pulse, the radar must have a bandwidth of ˜1 / τ.

Chirp radar

This kind of radar differs from pulse radar only in the generation and detection of pulses. The transmitted signal has a longer duration but is frequency modulated to change frequency from the lowest frequency to the highest frequency within the pulse range or vice versa. At the receiver, the signal passes through a matched filter that delays the low frequency compared to the high frequency and compresses the pulse length of the pulse radar to the same length under the same conditions. Signal processing after the signal is compressed is similar to a pulse radar. This chirp radar should have a lower peak effect than the pulse radar, but requires a matched filter.

Frequency Modulated Radar

In this kind of radar, the transmitted signal has a constant amplitude. Frequency modulation is linear and periodic, from the lowest frequency in the band to the highest frequency, or vice versa, similar to a chirp radar, but more time consuming for frequency scanning. Some of the transmitted signals are branched and used as local oscillator signals for the receive mix circuit and mixed with the reflected signals. The time difference for the reflected signal results in a frequency difference proportional to the distance to the reflective object. Some other reflected signals appear as specific frequency components within the received signal, but they can be filtered out.

Interpretation of the received signal is performed using a filter assembly or one or more tuning filters. The receiver can be designed with a low bandwidth requiring a short scan time operating at a mixed frequency in the low noise region of the receiver.

Reflectometer

이다. 반사계수 ρ(f)(여기서, f는 측정주파수)는 주파수 범위에 걸쳐 균등이 분포된 일련의 분산적 주파수를 구하기 위해 측정된다. 측정된 테이타를 주파수 평면을 시간 평면으로 ρ(f)에 퓨리시키는 변환하는 마이크로 프로세서나 다른 데이타프로세서로 보내진다. 그 결과는 펄스레이다 시스템의 반사신호에 상응하는 시간 함수로 된다. 이러한 방법에 근거한 시스템은 합성된 펄스식 레이다 시스템이라 불리어질 수 있다. 그러나 상기 신호는 실제 시간으로 나타나지 않고 데이타 형태로 존재하며, 이는 그 후의 신호해석에 커다란 도움이 된다.According to this method, the reflection coefficient for the antenna is measured as ρ = ree ^i∮ over the entire frequency range. Where r is the reference dimension of the reflection coefficient (amplitude), ∮ is the phase, e is the natural log

to be. The reflection coefficient, ρ (f), where f is the measured frequency, is measured to find a series of distributed frequencies that are evenly distributed over the frequency range. The measured data is sent to a microprocessor or other data processor that transforms the frequency plane into a time plane at ρ (f). The result is a time function corresponding to the reflected signal of the pulse radar system. A system based on this method may be called a synthesized pulsed radar system. However, the signal does not appear in real time, but in data form, which is of great help in subsequent signal interpretation.

The present invention makes use of the principles described above to make level measurement more accurate and flexible than any existing method, and as described below provides a series of specific aspects of signal generation and signal processing that provide various other advantages. Combined in a way.

Systems based on the reflectometer method cannot be applied in practical time as described above. Therefore, it is appropriate for this system to use digital signal processing, thereby giving the system some unique features.

When using fast DFT-algorithms such as the "Fast Fourier Transform" (FFT), the distributed Fourier transforms (DFTs) perform well with equally spaced frequencies.

If the frequency interval in this equidistant set is Δf, the time representation of the reflection coefficient ρ (f) through the distributed Fourier transform will be periodic according to the period T = 1 / Δf. The two contributions to ρ (f) according to the time delays τ ₁ and τ ₂ will appear in the same position in the time representation if τ ₂ −τ ₁ = nT. Where n is an integer (n = 0, 1, 2, ...). Therefore, it is first necessary to select T sufficiently high so that the largest value tau max of the time delay tau that can occur within a certain range is smaller than T. Also, T should be chosen very high so that no contribution that would interfere with the measurement has a time delay greater than T.

The unification of this system can be performed by measuring only the real part of the complex reflection coefficient p (f), i.e., Re {ρ (f)}, or by measuring only the imaginary part, i.e., Im {ρ (f)}. Considering that the phase of the reference signal, which is a signal for comparing the signals reflected from the antenna, is 0, the reflected signal with the time delay τ gives a contribution proportional to e-j2πfτ with respect to P (f). Will be done. The real part of this contribution is Re {e-j2πfτ} = cos (2πfτ) = 1/2 {ej2πfτ + e-j2πfτ} and gives equal length contributions at time delays τ and -τ. Because of the periodicity described above of the distributed Fourier transform, the contribution of time delay -τ can also be expressed as time delay T-τ. This means that when measuring using only the real or imaginary part of ρ (f), T is chosen to have a time delay as large as possible within the measurement range, τ max should be less than T / 2, otherwise time delay -τ And time delay T-τ are incorrect because they give the same result due to periodicity. That is, in this case, a cosine wave having a periodicity of T / 2 is used.

A disadvantage of measuring the distance to the object using a separate antenna is that the reflected signal is correspondingly reduced with increasing distance to the object. On a flat surface, the amplitude of the reflected signal is inversely proportional to the distance from the antenna, while if the distance exceeds some threshold, the amplitude of the reflected signal for an object with small angular dimensions is squared with distance squared. Inversely This is the case where the medium between the antenna and the object is relatively lossless in the transmission of electromagnetic waves. If the electromagnetic wave is significantly weakened due to the nature of the medium described above, the amplitude of the received signal will be further reduced over the distance described above. Problems with digital signal processing appear especially when the required signal is only a small part of the overall signal. Therefore, to solve such a problem, a large number of units are required in the digital representation of the signal.

이며, 이 도함수는 다음과 같은 계산된다.Instead of using the antenna's firing coefficient values directly in the Fourier transform, significant advances have been made by this application by using Fourier transforms and using a function of the reflection coefficient applicable to the actual measurement task and the actual object to be measured. It was done. When measuring over a plane through a medium with low attenuation, the optimal function is the derivative of the reflection coefficient ρ (f)

This derivative is calculated as

If F (x) is the Fourier transform of x,

...............................................(1)

...............................................(One)

........................................(2)

........................................(2)

Becomes

This relationship uses the derivative of ρ (f), whereby the factor τ for distance (time) is multiplied by R (τ), so that the contribution from the farther distance is proportional to the shorter distance and this distance. It will be emphasized more than the contribution of. Thus, the reflection from the larger, flat surface becomes independent of the distance from the antenna.

In this case, ρ '(f) is not measured, but the differential reflection coefficient Δρ (f) can be found by measuring ρ (f) over the increment f _d and subtracting the value. In other words,

..................................(3)

....................... (3)

This is done on the basis of analogue, which allows to reduce the above-mentioned digital analysis.

The following methods may be used to measure reflection differential coefficients or real or imaginary parts:

The microwave signal input to the antenna varies between two frequencies f ₁ = f−δf and f ₂ = f + δf within the distance δf (where f is the measurement frequency). The signal reflected from the antenna terminal is supplied to a detection circuit having an output voltage V proportional to the real part, for example Re {ρ}. When the frequency changes between f ₁ and f ₂ , V will vary between V ₁ -Re {ρ (f ₁ )} and V ₂ -Re {ρ (f ₂ )}. The voltage ΔV = V ₂ -V ₁ is a measurement for Re {ρ (f ₂ )}-Re {ρ (f ₁ )}. Similarly f ₁ , f ₂ . The frequency is changed between _n values of f _n and corresponding detection voltages V ₁ , V ₂ . Higher-order derivatives are obtained by selecting a suitable linear bond of V _n .

In order to have high accuracy in the measurement according to the present invention, the frequency at which the function value of the antenna input reflection coefficient is obtained must be known with high accuracy. This can be done in principle by using a frequency counter with high accuracy or by combining the measured frequencies. However, since such a method according to the prior art is expensive and time consuming, the following method according to the present invention is preferable.

The microwave signal is generated by a voltage controlled oscillator (VCO) and the control voltage changes to increase or decrease the frequency substantially linearly with time. The measurement is then appropriately performed at equal intervals. The signal is branched and used as a reference impedance for measuring impedance elements, some of which are known functions. The measurement of the reference impedance is made at the same microwave signal frequency used to measure the antenna.

If the reflection coefficient of the reference impedance ρ r at the measurement frequency K, f _k is ρrk, f _k is determined from the following equation.

ρrk = ρr (f _k ). … … … … … … … … … … … … … … … … … … … … … … … … (4)

Where ρr (fk) is the measurement of the known frequency of ρr. The frequency f should in principle be a function of ρ r over the entire measurement frequency range. If f _k is at fb at two frequencies fa at intervals less than the full range of the measurement frequency, then f is _a function of ρ r existing between f _a and f _b .

This method will not normally make it impossible to calibrate equidistant measurement frequencies, as described above, where appropriate routines are used to make interpolation in the FET algorithm when a set of equally spaced measurement frequencies is desired. It is assumed that the object to be measured in the process is stationary. If the object moves, this is the case when the surface of the liquid in the container is filled or emptied, but measurement errors may occur.

The measurement is carried out within the frequency range f ₀ -ΔF / 2, f ₀ + ΔF / 2, ie within the frequency range of the ΔF width around the center frequency f ₀ , and the duration of the measurement is T ₀ . . If the measured distance changes from h ₁ to h ₂ during the measurement period T ₀ , the phase of the reflection coefficient is

(Where C is the speed of light in the medium between the antenna and the object). If the measurement is made at increasing frequency, then the phase change is as follows.

......................(5)

(5)

This should be compared to the phase change for a stationary object at a distance h denoted by 4πΔF h / c when h ₁ = h ₂ = h in equation (5).

At this time, the apparent distance to the moving object is as follows.

이고, 움직이는 물체에 대한 거리는 이에 식(f₀/△F)(h₂-h₁)이 부가되어 있다.Average distance from transplant

The distance to the moving object is added to the equation (f ₀ / ΔF) (h ₂ -h ₁ ).

If the value of f ₀ / ΔF is 10 (which is a possible value), a measurement error occurs, which is 10 times the distance traveled by the object during the measurement time. This error can be eliminated by further measuring by decreasing the frequency from fo + ΔF / 2 to fo−ΔF / 2 during the measurement time To.

If the object moves at a uniform rate, the latter measurement produces the same absolute error as the former, but the sign is reversed. By averaging two measurements, the distance is determined, which corresponds to the distance in the middle of the entire measurement period.

The object of the invention is achieved by carrying out the method according to claim 1.

Another feature of the invention includes an apparatus for carrying out this method, which is described in the other claims.

The invention will be described in more detail with reference to the accompanying drawings.

The apparatus of FIG. 1 comprises an antenna 11, a measuring unit 12, a channel unit 13 and a detection unit 14, all of which are described in detail below.

If the object to be measured is the water surface 17 of the liquid 16 in the vessel 15, the antenna is mounted on top of the vessel, as shown in the figure. In the measuring unit 12, the microwave signal is generated by a microwave oscillator 18 consisting of a voltage controlled oscillator, and in the divider 19, an antenna frequency counter 20 and a frequency oscillator 25, which are antenna measuring units of channel A, Is branched to the frequency correction unit 21 of channel B to which is connected. The control voltage of the microwave oscillator 18 is supplied from a measuring frequency control unit 22 which ensures that the signal is within a set of nominal measuring frequencies. The control voltage is superimposed on the additional voltage from the function control unit 23 to generate a set of frequencies around each measurement frequency necessary to make the above function of the antenna reflection coefficient. This superimposed voltage is also input to the detectors 24A and 24B, giving a weight factor for the contribution of each frequency.

The circuit of FIG. 3 differs from the circuit of FIG. 2 in the generation of a set of frequencies around each measurement frequency. In the phase control shift circuit 26, the phase of the microwave signal is increased by Ψ during the time interval T. The frequency of the microwave signal is then increased to Δf = Ψ / T. By changing Ψ and T, the frequency increase can be determined, resulting in a desirable set of frequencies. However, it is not necessary to increase the phase linearly with time at time interval T. This is because the phase can be increased in one or more steps by using multiple phase modulators.

The circuit of FIG. 4 differs from the circuit of FIG. 3 in that only the signal on the antenna side is processed with reference impedance using two phase control shift units 27A and 28B, respectively. Therefore, the circuit of FIG. 4 can use other functions as described for the A and B channels shown.

The antenna measuring unit 20 and the frequency correction unit 21 can be embodied as reflectometers as described above. 5 is a block diagram of one embodiment of the reflector, in which microwave signals are input from the signal splitter 19 to the second signal splitter 30. Some signals are used as reference signals for the mixer 32 and other signals are input to the duplexing circuit 31. The signal from the duplexing circuit 31 is transmitted and measured with impedance, and the reflected signal is transmitted from the duplexing circuit 31 to the mixer 32, where it is mixed with the reference signal to produce a low frequency signal and then the detector 24, 24A and 24B.

The signals detected in channel A (from antenna frequency counter 20) and channel B (from frequency correction unit 21) are converted from analog signals to digital signals in unit 13 and stored in data storage Stored in a register in the data processing unit 28, the results measured from the two channels are sent to the signal processing unit 29 for further processing.

6 shows the main functions of the signal processing unit 29, which will be described in detail.

The above-mentioned function value of the reflection coefficient for the antenna at a limited number of dispersion frequencies within a certain frequency range from the data processing unit 31 appears in channel A and the above-mentioned function value of the firing coefficient for the reference impedance at about the same measurement frequency. Appears in channel B.

From the data of channel B, the measurement frequency f is determined by the frequency counter unit 33. These frequencies are used in the interpolation circuit 34 to know the above-described function values for channel A in a set of frequencies that can be used in subsequent signal processing if the set frequency set and the measured frequencies do not match. The function value is then Fourier transformed, splits the contribution from the object, and the distance calculation is carried out in the distance calculation circuit 36 using a calculation routine that corresponds to the signal and the setting data according to the measurement path.

The above-described measurements and calculations are performed during the rapid progress of two processes, a process of increasing frequency and a process of decreasing frequency, and averaging at the calculation unit 36 during the measurement period to change the distance of the uniform velocity of the object. Eliminate errors

Data from the distance calculating circuit 36 in the signal processing unit 29 may be used for the corrected distance from the set reference point to be close to the antenna to the object to be measured. In the detection unit 14, other unit and circuits parallel the distance input data from the signal processing unit in the channel unit 13 as well as the data table, the other calculation unit and the sensor, in parallel with the other user data. Are monitored and controlled.

The detector unit 14 uses a multiplexing circuit that operates periodically between the channel unit and the detection unit between the channel unit and the detection unit, and selects a period between channel units connected in parallel to the multiplexing circuit. One channel unit can be used normally.

Similarly, the channel unit 13 may use multiple measuring units by providing a multiplexing circuit between the measuring unit and the channel unit, and the signal processing unit 29 of the channel unit multiplexes between the data processing unit and the signal processing unit. By connecting circuits, several data processing units connected to each measuring unit may be connected and used.

Claims (14)

Generates a microwave signal with a preset frequency, some of which is used as a reference signal and some of the signals are supplied to the antenna that is directed to the object to be measured, so that the surface of the object and other objects A method of measuring the distance to an object, which is a liquid or similar flowable material, by varying the contribution to the distance away from the antenna to affect the input impedance of the antenna and the corresponding unit reflection coefficient. The input reflection coefficient of the antenna at at least two frequencies within a certain period centered on the nominal measurement frequency so that the contribution to the measurement function from the object within is greater than the contribution from the object within any other distance. Create a measurement function of, and collect the value of the measurement function The sum is calculated by a Fourier transform at a predetermined set of measured frequencies, and the contribution of the other object to the Fourier transform function is disadvantageous depending on the distance from the antenna to the other object and at the same time a fixed contribution from the objects The contribution of these objects to the Fourier transform is known and eliminated, and the gears of the surface are separated according to the size or position of the surface to determine the distance to the surface. The method of claim 1, wherein the measurement function is formed as a differential equation with respect to the input reflection coefficient of the antenna defined as the difference between the reflection coefficient value time at two adjacent frequencies within a predetermined interval centered on the nominal frequency. How to measure the distance to the object. The method of claim 1, wherein the measurement function is formed by a second-order differential equation of the input reflection coefficient of the antenna when the object to be measured is within an angle defined with respect to the transmission signal of the antenna, and the second-order differential equation represents the nominal frequency. A method of measuring the distance to an object, characterized in that it is defined as a second derivative of the reflection coefficient at three adjacent frequencies within a certain distance. The method of claim 1, wherein the measurement function is formed as a set of differential equations of different orders of the input reflection coefficient of the antenna when the measurement object is surrounded by a medium in which electromagnetic waves decrease substantially with increasing distance. The general N-based differential equation is defined as the n-th derivative of the antenna input reflection coefficient at n + 1 adjacent frequencies within a range centered on the nominal frequency. The method of claim 1, wherein the value of the measurement function is determined for a defined number of distributed measurement frequencies within a limited frequency range so that a Fourier transform of the function value is made, and the number and average interval of measurement frequencies achieve a set accuracy. A method for measuring the distance to an object, characterized in that it is selected to. The method according to claim 1, wherein at the measurement frequency at which the measurement function of the input reflection coefficient of the antenna is determined, the measurement function value can be calculated at the intermediate frequency by determining the values of the dependent and reference impedances with respect to the measurement function of the selected frequency. The measurement frequencies are selected in close proximity, and these measurement frequencies are determined from the measurement of the reference impedance, and then the measurement function values of the input reflection coefficients are calculated in different sets of frequencies. Way. The method of claim 1, wherein the measurement procedure for providing a value of the measurement function is performed in order to remove an error when the distance to the object during the measurement changes at a constant speed. Method for measuring the distance to the object, characterized in that to be performed twice in a short interval intervals of decreasing frequency. According to claim 1, wherein the frequency component of the signal supplied to the antenna of the reflection coefficient used to determine the measurement function is determined by taking a series of frequency values at a certain interval centered on the process measurement frequency, The detection circuit used to determine the distance to the target object is characterized in that the measurement function is determined after detection by synchronizing with the transmission signal and weighting the reflection coefficient. The method of claim 1, wherein the value of the reflection coefficient used to form the measurement function is determined by combining a microwave signal supplied to the antenna as a component of at least two frequencies, the frequency component being centered on the nominal measurement frequency. The detection circuit used to form the measurement function value is synchronized to the frequency component of the transmitted signal and weighted the reflection coefficient value to determine the measurement function after detection. Method for measuring the distance to the object, characterized in that. In a distance measuring device which is arranged within a distance to be measured from the surface 17 and is directed to the surface, the antenna 11 at a plurality of distributed measurement frequencies within a predetermined frequency range. Measuring unit 12 comprising a frequency controlled microwave oscillator 18, a measuring frequency control unit 22, a function control unit 23 and detectors 24A and 24B to form a measurement function of the input reflection coefficient And a signal processing circuit (28, 29) comprising means (33-36) for calculating a Fourier transform of the function and processing the data provided to calculate the distance to the object. Device. 11. A distance measuring device according to claim 10, wherein the measuring unit (12) comprises one or more control phase shifts (26, 27A, 27B). The data processing unit according to claim 10, wherein the measuring unit (12) converts the signal detected by the antenna measuring unit (20) and the frequency correction unit (21) into a digital signal and supplies it to the signal processing unit (29). And a divider 19 for dividing the signal from the microwave oscillator 18 into the antenna measuring unit 20 and the frequency correction unit 21 respectively connected to the detectors 24A and 24B connected to (28). Distance measuring device characterized in that. 13. The measuring unit according to claim 12, wherein said measuring unit for forming a measuring function of a reflection coefficient comprises means (22, 23, 26, 27A, 27B) for changing the amplitude, frequency or phase of a microwave signal and said detector (24A) 24B) is a distance measuring device characterized in that it is synchronized to each or both of the signal and the reference impedance supplied to the antenna. 13. The antenna measuring unit (20) and the frequency correction unit (21) each have a reflector, and the antenna measuring unit (20) comprises a duplexing circuit (31) connected to the antenna (11). And the duplexing circuit in the frequency correction unit (21) is connected to a frequency dependent reference impedance.

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