A liquid level measurement system based on the principle of transmission line

Abstract

The application provides a liquid level measuring system based on transmission line principle, which comprises a bidirectional radio frequency power detector, a balun impedance transformer, a signal source, a transmission line, a terminal resistance, a microcontroller and a non-metal container for storing liquid; when the non-metal container has liquid or the liquid changes, the impedance of the transmission line changes, a reflected wave signal is generated and transmitted back, the reflected wave signal is transmitted to the bidirectional radio frequency power detector, the power of the reflected wave signal is detected, the power value of the reflected wave signal is transmitted to an analog-to-digital converter, the analog-to-digital converter transmits the incident wave power value and the power value of the reflected wave signal to the microcontroller, and the data of the liquid level is obtained by combining a calibration coefficient, so that fast real-time measurement can be realized, there is no mechanical moving part, and the medium is not contacted, and the application has great advantages in installation, measurement, cost and the like compared with a traditional float type liquid level meter.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of industrial monitoring, and particularly relates to a liquid level measurement system based on transmission line principle. BACKGROUND

[0002] In the traditional industrial field, the liquid level of liquid medium is usually monitored by using a float type liquid level meter. The main problems of the float type liquid level meter are as follows.

[0003] 1. High requirement for installation space, needs to be close to the edge of the container for storing liquid medium, and needs to consider the operation space and position for maintenance.

[0004] 2. Requirements for the object characteristics of the medium, for flammable and explosive and easily volatile medium, the sensor needs to consider a higher explosion-proof level, and the float ball needs to contact the medium, so that the liquid medium with high viscosity cannot be measured.

[0005] 3. The sensor is prone to failure, such as float falling off. SUMMARY

[0006] In order to solve the above technical problems, the present application provides a liquid level measurement system based on transmission line principle, which is used to solve the problems of the prior art, i.e. 1. high requirement for installation space, needs to be close to the edge of the container for storing liquid medium, and needs to consider the operation space and position for maintenance; 2. requirements for the object characteristics of the medium, for flammable and explosive and easily volatile medium, the sensor needs to consider a higher explosion-proof level, and the float ball needs to contact the medium, so that the liquid medium with high viscosity cannot be measured; 3. the sensor is prone to failure, such as float falling off.

[0007] The present application provides the following technical scheme, a liquid level measurement system based on transmission line principle, comprising: a signal source, a bidirectional radio frequency power detector, a balun impedance converter, a transmission line and a terminal resistor which are electrically connected in sequence, the bidirectional radio frequency power detector is electrically connected with an analog-to-digital converter, the analog-to-digital converter is electrically connected with a microcontroller, and one side of the transmission line is provided with a non-metallic container for storing liquid.

[0008] The signal source is used for transmitting incident wave signals and transmitting signals to the bidirectional radio frequency power detector, the bidirectional radio frequency power detector transmits the incident wave signals to the balun impedance converter and is used for detecting the power of the incident wave signals, and transmits the detected incident wave power value to the analog-to-digital converter, the balun impedance converter converts the incident wave signals into differential signals for driving the transmission line, and transmits the differential signals to the transmission line provided on one side of the non-metallic container and having a fixed impedance value, and the transmission line transmits the differential signals to the terminal resistor having a fixed impedance value.

[0009] Wherein, when there is no liquid in the non-metallic container, the impedance of the transmission line does not change, resulting in that the differential signal transmitted to the terminal resistor is fully absorbed by the terminal resistor, and no reflected wave signal is transmitted back; when there is liquid or the liquid changes in the non-metallic container, the impedance of the transmission line changes, and a reflected wave signal is transmitted back, the reflected wave signal is transmitted to the bidirectional radio frequency power detector, the power of the reflected wave signal is detected, and the power value of the reflected wave signal is transmitted to the analog-to-digital converter, the analog-to-digital converter transmits the incident wave power value and the power value of the reflected wave signal to the microcontroller, and the microcontroller is used to calculate the return loss value and obtain the liquid level data in combination with the calibration coefficient.

[0010] Compared with the prior art, the application has the beneficial effects that: by measuring the return loss of the transmission line, a non-contact liquid medium measurement method is found. The liquid level is measured by using the transmission line, and the system structure is much simpler than the traditional mechanical float type liquid level meter, and the volume is also much smaller, only two metal transmission lines are needed, which are fixed outside the liquid medium container, then the transmission line is connected to the acquisition system, and fast real-time measurement can be realized, there is no mechanical moving part, and the medium is not contacted, and the application has great advantages over the traditional float type liquid level meter in terms of installation, measurement, cost and the like.

[0011] Further, a band-pass filter is further included for eliminating external radio interference signals.

[0012] Further, the length of the transmission line is not less than the height of the non-metallic container.

[0013] Further, the signal source adopts a signal source capable of outputting radio frequency signals in a certain frequency range and carrying a certain power, and the internal resistance is 1-100 ohms.

[0014] The power range is generally between 10-20 dBm.

[0015] Further, the length of the transmission line is 1 / 4 of the wavelength of the signal.

[0016] Further, the bidirectional radio frequency power detector includes a directional coupler and a radio frequency power detector, the directional coupler includes a forward coupler and a reverse coupler, the radio frequency power detector includes forward power detection and reverse power detection, the forward coupler is used to pick up the incident wave signal, the reverse coupler is used to pick up the reflected wave signal, and the forward power detection and the reverse power detection are used to convert the incident wave signal and the reflected wave signal into RMS effective value voltage, so that the analog-to-digital converter at the rear end samples and measures.

[0017] Further, the level of the analog-to-digital converter is Ksps level and above.

[0018] Further, the signal source, the bidirectional radio frequency power detector and the band pass filter are single-ended signals.

[0019] Further, the terminal resistance and the impedance of the transmission line are 1-100 ohms, and the terminal resistance is a pure resistive device.

[0020] Further, the microcontroller is also used to store calibration data when measuring different liquid media and different lengths of transmission lines, and upload the data to a computer terminal for further data processing and data display. BRIEF DESCRIPTION OF DRAWINGS

[0021] 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 are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor under the premise of the drawings.

[0022] Figure 1 The structural diagram of the liquid level measurement system based on the transmission line principle of the present application;

[0023] Figure 2 The structural diagram of the bidirectional radio frequency power detector of the present application;

[0024] Figure 3 The structural diagram of the connection of the Barun impedance converter, the transmission line and the terminal resistance of the present application.

[0025] Explanation of reference signs:

[0026]

[0027] The embodiments of the present application will be further described below with reference to the drawings. DETAILED DESCRIPTION

[0028] The embodiments of the present application will be described in detail below, and the examples of the embodiments are shown in the drawings, wherein the same or similar reference signs represent the same or similar elements or elements with the same or similar functions throughout. The embodiments described below by referring to the drawings are exemplary and are intended to explain the embodiments of the present application, and cannot be understood as a limitation of the present application.

[0029] In the description of the embodiments of the present application, it should be understood that the terms "length", "width", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer" and the like indicate the orientation or positional relationship shown in the drawings based on the orientation or positional relationship shown in the drawings, and are only for the purpose of facilitating the description of the embodiments of the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a particular orientation, be constructed and operated in a particular orientation, and therefore cannot be understood as a limitation of the present application.

[0030] In addition, the terms "first", "second" are only for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include one or more of the features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly specified and limited.

[0031] In the embodiments of the present application, unless otherwise explicitly specified and limited, the terms "mounting", "connecting", "connecting", "fixing" and the like should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements. For those skilled in the art, the specific meaning of the above-mentioned terms in the embodiments of the present application can be understood according to the specific circumstances.

[0032] In one embodiment of the present application, as shown in Figure 1 A liquid level measurement system based on transmission line principle comprises:

[0033] The acquisition system, the transmission line 5 and the non-metallic container 9 for storing liquid, the acquisition system comprises a bidirectional radio frequency power detector 1, a band pass filter 2, a balun impedance transformer 3, a signal source 4, a terminal resistance 6, an analog-to-digital converter 7, a microcontroller 8;

[0034] The signal source 4, the bidirectional radio frequency power detector 1, the balun impedance transformer 3, the transmission line 5 and the terminal resistance 6 are electrically connected in sequence, the bidirectional radio frequency power detector is electrically connected with the analog-to-digital converter 7, the analog-to-digital converter 7 is electrically connected with the microcontroller 8, and the non-metallic container 9 is arranged on one side of the transmission line 5;

[0035] The signal source 4 is used to emit incident wave signals and transmit the signals to the bidirectional radio frequency power detector 1, the bidirectional radio frequency power detector 1 transmits the incident wave signals to the balun impedance transformer 3 and is used to detect the power of the incident wave signals and transmit the detected incident wave power value to the analog-to-digital converter 7, the balun impedance transformer 3 converts the incident wave signals into differential signals used to drive the transmission line 5 and transmits the differential signals to the transmission line 5 provided on one side of the non-metal container 9 and having a constant impedance, the transmission line 5 transmits the differential signals to the terminal resistance 6 having a constant impedance;

[0036] When there is no liquid in the non-metal container 9, the impedance of the transmission line 5 does not change, so that the differential signals transmitted to the terminal resistance 6 are all absorbed by the terminal resistance 6 and no reflected wave signals are transmitted back. When there is liquid in the non-metal container 9 or the liquid changes, the impedance of the transmission line 5 changes, so that reflected wave signals are transmitted back. The reflected wave signals transmitted back are transmitted to the bidirectional radio frequency power detector 1, the power of the reflected wave signals is detected, and the power value of the reflected wave signals is transmitted to the analog-to-digital converter 7. The analog-to-digital converter 7 transmits the incident wave power value and the power value of the reflected wave signals to the microcontroller 8, the microcontroller 8 is used to calculate the return loss value and obtain the liquid level data in combination with the calibration coefficient;

[0037] Specifically, when the reflected wave signals transmitted back are transmitted to the bidirectional radio frequency power detector 1, the reflected wave signals are sequentially transmitted through the balun impedance transformer 3 and the band-pass filter 2 and then transmitted to the bidirectional radio frequency power detector 1 for power detection;

[0038] It is worth noting that the transmission line 5 causes the discontinuity of the impedance of the transmission line 5 by sensing the change of the liquid level, thereby causing the reflection of the signals (reflected wave signals). Then, the incident wave power and the reflected wave signals are measured by the bidirectional radio frequency power detector 1 to obtain the power. At this time, the bidirectional radio frequency power detector 4 outputs an analog voltage value (indicating the power) to the analog-to-digital converter 7. The analog-to-digital converter 7 converts the analog signal into a digital signal, which facilitates the microcontroller 8 to calculate the return loss and obtain the liquid level data in combination with the calibration coefficient.

[0039] It can be understood that when the transmission line 5 provided on one side of the non-metal container 9 is used for measurement, the medium of the transmission line 5 is air when the liquid level is above the liquid level. The influence of the non-metal container 9 is not considered, the impedance of the transmission line 5 is basically equal to the impedance in the air, and the impedance of the transmission line 5 is low when the liquid level is below the liquid level because there is liquid medium near the transmission line 5, thereby causing the reflection of the signals and generating the reflected wave signals. With the change of the liquid level, the reflection intensity is linearly related to the liquid level.

[0040] Specifically, since the electromagnetic field is not affected by the non-metallic liquid medium container, it can be close to the outer wall of the non-metallic liquid medium container to sense the influence of the change of the liquid level in the container on the transmission line 5. For example, when there is no liquid, the dielectric constant of the medium around the transmission line 5 is equivalent to that of air, and when there is liquid, the dielectric constant of the medium around the transmission line 5 is equivalent to that of the liquid; since the dielectric constant of the liquid medium is much larger than that of air, the impedance of the part of the transmission line 5 in contact with the liquid will decrease, the attenuation will increase, and therefore the reflected wave power will increase, and the return loss value will decrease (return loss: the ratio of incident wave power to reflected wave power); for example, when the liquid level rises, the reflected wave power also gradually increases, and the return loss continuously decreases; when the liquid level drops, the reflected wave power gradually decreases, and the return loss continuously increases; by calibration, the relationship between the liquid level of a certain liquid medium and the return loss of the transmission line is found to realize accurate measurement of the liquid level.

[0041] Among them, the signal emitted by the signal source 4 is a single-ended incident wave signal, and the signal transmitted to the bidirectional radio frequency power detector and the band pass filter is a single-ended signal.

[0042] Specifically, the band pass filter 2 is used to eliminate radio interference signals such as external WIFI, Bluetooth, cellular mobile communication network, etc. The band pass filter 2 is electrically connected between the balun impedance converter 3 and the bidirectional radio frequency power detector 1. One end of the band pass filter 2 is connected between the balun impedance converter 3 and the bidirectional radio frequency power detector 1 using a single-ended SMA interface, and both require 50 ohm impedance matching. The band pass filter 2 needs to be designed with low insertion loss to make the system performance better.

[0043] Specifically, the transmission line 5 is composed of two metals, and the length of the transmission line 5 is not less than the height of the non-metallic container 9. The transmission line 5 is arranged in parallel on one side of the non-metallic container 9, and the two ends of the transmission line 5 exceed the top and bottom of the non-metallic container 9.

[0044] Specifically, in order to ensure the continuity of the impedance of the transmission line 5, it is necessary to ensure the uniformity of the transmission line and the uniformity of the material of the transmission line.

[0045] Specifically, the signal source 4 uses a signal source that can output a certain frequency range of radio frequency signals and carry a certain power, and the internal resistance is 1-100 ohms. The internal resistance of the signal source 4 can be 50 ohms. The power range is generally between 10-20 dBm, and the frequency of the signal source is selected according to the length of the transmission line 5. The length of the transmission line 5 is 1 / 4 of the wavelength of the signal.

[0046] For example, as shown in FIG. 1, the system for measuring the liquid level of the non-metallic liquid medium container comprises a signal source 4, a band pass filter 2, a balun impedance converter 3, a bidirectional radio frequency power detector 1 and a transmission line 5. Figure 2As shown, the bidirectional radio frequency power detector 1 needs to be able to detect the power of the incident wave and the reflected wave at the same time in order to measure the return loss, so a bidirectional radio frequency power detector composed of a directional coupler and a radio frequency power detector is needed. The bidirectional radio frequency power detector includes a directional coupler and a radio frequency power detector, the directional coupler includes a forward coupler 10 and a reverse coupler 11, the radio frequency power detector includes a forward power detection 12 and a reverse power detection 13, the forward coupler 10 is used to pick up the incident wave signal, the reverse coupler 13 is used to pick up the reflected wave signal, and the forward power detection 12 and the reverse power detection 13 are used to convert the incident wave signal and the reflected wave signal into RMS effective value voltage for sampling and measurement by the analog-to-digital converter in the back end.

[0047] Specifically, in order to ensure measurement accuracy, the analog-to-digital converter 7 is required to have a double-channel synchronous sampling capability. The sampling rate requirement is not high, and generally Ksps level can meet the system requirements, the level of the analog-to-digital converter 7 is Ksps level and above, and the analog-to-digital converter 7 is used to convert the incident wave signal power value and the reflected wave signal power value output by the bidirectional radio frequency power detector 1 into a digital signal, thereby facilitating computer (microcontroller 8) processing. Because the bidirectional radio frequency power detector 1 outputs an analog voltage value (representing power), which cannot be processed by the computer, the analog-to-digital converter 7 is needed to convert the analog signal into a digital signal for easy computer processing. Because the difference between the levels of the two power signals is the return loss value, the analog-to-digital converter needs to have at least two independent conversion kernels to realize synchronous sampling and conversion of the incident signal and the reflected signal.

[0048] As shown in Figure 3 The two ends of the terminal resistor are respectively connected to one end of the two metal wires, the other end of the two metal wires is connected to the balun impedance transformer, the impedance of the terminal resistor and the transmission line is 1-100 ohms, the impedance of the terminal resistor and the transmission line can be 50 ohms, and the terminal resistor is a pure resistive device, the model of the terminal resistor is R193, and the terminal resistor is used to eliminate the signal reflection caused by impedance mismatch, stabilize the working state of the transmission line, and avoid the reflection signal caused by the discontinuous impedance of the transmission line in the liquid medium.

[0049] It is worth mentioning that if the terminal of the two metal wires of the transmission line 5 is not provided with the terminal resistor, the terminal of the two metal wires is either short-circuited together or open-circuited, but the two connections will have reflections, if the transmission line terminal is short-circuited, the reflection wave is equal in size and opposite in direction to the incident wave. If the transmission line terminal is open-circuited, the incident wave is equal in size and same in direction as the reflection wave. Therefore, a terminal resistor is added, so that there is no reflection. In fact, open circuit and short circuit are also an extreme phenomenon of impedance discontinuity, and short circuit and open circuit will cause reflection, short circuit resistance is zero, open circuit resistance is infinite, both of which are total reflection, after connecting a terminal impedance matched with the characteristic impedance of the transmission line, there is no reflection.

[0050] Specifically, the microcontroller is mainly responsible for configuring the analog-to-digital converter, and calculating and converting the data output by the analog-to-digital converter, and uploading the data to a host device. At the same time, the calibration data of the transmission line can also be stored in the microcontroller, and is also used to store the calibration data when measuring different liquid media and different length transmission lines, and upload the data to a computer terminal for further data processing and data display.

[0051] In summary, the liquid level measurement system of the transmission line principle in the above embodiment of the application uses a time domain reflection technology based on the transmission line principle. Since the electromagnetic field is concentrated near the transmission line, changes in the dielectric material near the transmission line will cause changes in the impedance characteristics of the transmission line. For example, near the air and near the liquid medium, the impedance characteristics of the transmission line will change, the impedance of the transmission line near the liquid medium will decrease, increase the loss of the transmission line, i.e. the signal will be reflected. By measuring the size of the return loss, the change of the liquid level is reflected. The application fixes the transmission line outside the non-metallic container of the liquid medium, the measurement system does not contact the medium, the mechanical parts of the system are few, and there is no need to open a hole on the container for fixation like a float type liquid level meter, which ensures the sealing of the container. It is a very suitable non-contact measurement method for industrial liquid media. Since the transmission line principle is used, fast real-time continuous measurement can be achieved.

[0052] The above only describes the preferred embodiments of the application and is not intended to limit the application. Any modification, equivalent replacement and improvement made within the spirit and principle of the application shall be included in the protection scope of the application.

Claims

1. A liquid level measurement system based on the transmission line principle, characterized in that, include: A signal source, a bidirectional radio frequency power detector, a balun impedance transformer, a transmission line, and a terminating resistor are connected in sequence. The bidirectional radio frequency power detector is electrically connected to an analog-to-digital converter, and the analog-to-digital converter is electrically connected to a microcontroller. A non-metallic container for storing liquid is provided on one side of the transmission line. The signal source is used to emit an incident wave signal and transmit the incident wave signal to the bidirectional radio frequency power detector. The bidirectional radio frequency power detector transmits the incident wave signal to the balun impedance transformer and is used to detect the power of the incident wave signal. The detected incident wave power is transmitted to the analog-to-digital converter. The balun impedance transformer converts the incident wave signal into a differential signal for driving the transmission line and transmits the differential signal to the transmission line disposed on one side of the non-metallic container. The transmission line transmits the differential signal to the terminating resistor. When there is no liquid in the non-metallic container, the impedance of the transmission line remains unchanged, causing the differential signal transmitted to the terminating resistor to be completely absorbed by the terminating resistor, preventing the generation of a back-transmitted reflected wave signal. When there is liquid in the non-metallic container or the liquid level changes, the impedance of the transmission line changes, generating a back-transmitted reflected wave signal. The back-transmitted reflected wave signal is transmitted to the bidirectional radio frequency power detector, which detects the power of the reflected wave signal and transmits the power of the reflected wave signal to the analog-to-digital converter. The analog-to-digital converter transmits the incident wave power and the reflected wave signal power to the microcontroller. The microcontroller calculates the return loss value and, in conjunction with a calibration coefficient, obtains the liquid level data.

2. The liquid level measurement system based on the transmission line principle according to claim 1, characterized in that, It also includes a bandpass filter, which is used to eliminate external radio interference signals.

3. The liquid level measurement system based on the transmission line principle according to claim 1, characterized in that, The length of the transmission line is not less than the height of the non-metallic container.

4. The liquid level measurement system based on the transmission line principle according to claim 1, characterized in that, The internal resistance of the signal source is 1 to 100 ohms; The power range of the signal source is between 10 and 20 dBm.

5. The liquid level measurement system based on the transmission line principle according to claim 1, characterized in that, The length of the transmission line is 1 / 4 of the signal wavelength.

6. The liquid level measurement system based on the transmission line principle according to claim 1, characterized in that, The bidirectional radio frequency power detector includes a directional coupler and a radio frequency power detector. The directional coupler includes a forward coupler and a reverse coupler. The radio frequency power detector includes forward power detection and reverse power detection. The forward coupler is used to pick up the incident wave signal, and the reverse coupler is used to pick up the reflected wave signal. The forward power detection and the reverse power detection are used to convert the incident wave signal and the reflected wave signal into an RMS effective value voltage so that the analog-to-digital converter at the back end can sample and measure them.

7. The liquid level measurement system based on the transmission line principle according to claim 1, characterized in that, The analog-to-digital converter is at the Ksps level or higher.

8. The liquid level measurement system based on the transmission line principle according to claim 1, characterized in that, The signal source, bidirectional RF power detector, and bandpass filter are all single-ended signals.

9. The liquid level measurement system based on the transmission line principle according to claim 1, characterized in that, The impedance of the terminating resistor and the transmission line is 1 to 100 ohms, and the terminating resistor is a purely resistive device.

10. The liquid level measurement system based on the transmission line principle according to claim 1, characterized in that, The microcontroller is also used to store calibration data and upload the data to a computer terminal for data processing and display when measuring different liquid media and transmission lines of different lengths.

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