Capacitive Liquid Level Detection System Based on Crystal Oscillator

Abstract

The present invention discloses a capacitive liquid level detection system based on a crystal oscillator method, and belongs to the field of LLD technology. The capacitive liquid level detection system includes a signal generating circuit, a front-end circuit, a back-end signal processing circuit, and a decoupling circuit connected between the front-end circuit and the back-end signal processing circuit. The present invention isolates the front-end and back-end circuits by adding a decoupling circuit between the front-end circuit and the back-end signal processing circuit, thereby increasing the sensitivity of the probe in detecting the liquid level, and at the same time increasing the driving ability of the front-end circuit to the back-end signal processing circuit. In addition, the present invention performs liquid level detection based on the crystal oscillator method, and has a higher detection sensitivity than the existing contact LLD technology, and has a lower use cost than the existing non-contact LLD technology.

Description

Technical Field

[0001] The invention relates to the technical field of probe liquid level detection, and in particular to a capacitive liquid level detection system based on a crystal oscillator method. Background Art

[0002] Most of the fully automatic clinical analysis instruments such as biochemical analyzers, enzyme immunoassay analyzers, urine analyzers, and coagulation analyzers have automatic sample transfer systems. Sample transfer accuracy is one of the determining factors of the analysis accuracy of these instruments.

[0003] The liquid carryover on the outer surface of the pipette needle is the main reason that affects the accuracy of sample transfer. Due to the different liquid surface heights, liquid viscosities and adhesion of the liquid being transferred, as well as the different depths of the pipette needle into the liquid, the amount of liquid carried on the outer surface of the pipette needle is not stable, and the instrument will not produce a stable system error.

[0004] Minimizing the amount of liquid carried on the outer surface of the pipette needle is the most important way to reduce carryover contamination and improve analytical accuracy. There are many ways to reduce carryover, such as coating the outer surface of the pipette needle with materials that are not easily stained by liquids, such as TEFLON; slowing down the speed of the pipette needle when it withdraws from the liquid; using the liquid level detection (LLD) function to control the depth of the pipette needle's penetration into the liquid, etc.

[0005] At present, the most commonly used method is to use the LLD function, which can not only control the depth of the pipette needle into the liquid, but also detect whether the liquid is exhausted or missing, thereby avoiding the instrument from adding samples in vain. The existing LLD technologies mainly include contact LLD technology and non-contact LLD technology. The existing contact LLD technologies mainly include resistance method, capacitance method, air pressure method and mechanical vibration method; non-contact LLD technologies include ultrasonic method, laser method and CCD imaging method, but the existing contact LLD technology has low detection sensitivity. Although the ultrasonic LLD technology in the non-contact method is becoming more mature, the application cost is relatively high. The laser method and imaging method are relatively low in cost but have not yet been widely used. Summary of the invention

[0006] The invention aims to improve the LLD sensitivity and reduce the liquid level detection cost, and provides a capacitive liquid level detection system based on a crystal oscillator method.

[0007] To achieve this object, the present invention adopts the following technical solutions:

[0008] Provided is a capacitive liquid level detection system based on a crystal oscillator method, comprising a signal generating circuit, a front-end circuit, a back-end signal processing circuit, and a decoupling circuit connected between the front-end circuit and the back-end signal processing circuit, wherein the decoupling circuit comprises a common-phase amplifier U5B, a common-phase input end of the common-phase amplifier U5B being connected to a liquid level detection signal output end of the front-end circuit, an inverting input end of the common-phase amplifier U5B being connected to its output end, and an output end of the common-phase amplifier U5B being connected to a signal input end of the back-end signal processing circuit.

[0009] As a preferred solution of the present invention, the front-end circuit includes a probe capacitor voltage divider circuit, a frequency selection circuit and a rectifier filter circuit, the signal generating circuit generates a square wave or sine wave signal of a fixed frequency to drive the probe capacitor voltage divider circuit, the probe capacitor voltage divider circuit is composed of a probe parasitic capacitance, a voltage divider capacitor C1 and a voltage divider capacitor C8, one end of the voltage divider capacitor C1 is connected to the signal output end of the signal generating circuit as the signal input end of the probe capacitor voltage divider circuit, and the other end is connected to the voltage divider capacitor C8, and the other end of the voltage divider capacitor C8 is connected to the probe through a magnetic bead FB7;

[0010] The frequency selection circuit performs band-pass filtering on the signal obtained by voltage division by the voltage-dividing capacitor C1 and the voltage-dividing capacitor C8, and then outputs a sine wave with the same frequency as that of the signal generating circuit;

[0011] The rectifier and filter circuit performs peak detection on the sine wave output by the frequency selection circuit to obtain the peak signal of the sine wave and then outputs it to the back-end signal processing circuit;

[0012] The back-end signal processing circuit converts the received peak signal into a pulse signal and outputs it to the microprocessor. The microprocessor identifies whether the probe detects the liquid level based on the pulse signal.

[0013] As a preferred solution of the present invention, the signal generating circuit includes an active crystal oscillator X2, a resistor R2, a Schmitt trigger U5, a capacitor C37, a resistor R15 and a capacitor C7, the VCC input end of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, the GND grounding end of the active crystal oscillator X2 is grounded, the OUT output end is connected in series with the resistor R2 and then connected to pin 2 of the Schmitt trigger U5; pin 3 of the Schmitt trigger U5 is grounded, pin 4 is connected to one end of the resistor R15, the other end of the resistor R15 is connected to the capacitor C7 and then grounded and serves as the output end of the signal generating circuit, and pin 5 of the Schmitt trigger U5 is connected to a 3.3V operating voltage and connected to the capacitor C37 and then grounded.

[0014] As a preferred solution of the present invention, the signal generating circuit includes an active crystal oscillator X2, resistors R2, R5, R15, R16, capacitors C11, C7, C39, and an operational amplifier U5A. The VCC input terminal of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, the GND grounding terminal of the active crystal oscillator X2 is grounded, and the OUT output terminal is connected in series with the resistor R2 and the resistor R5 in sequence and then connected to the in-phase input terminal of the operational amplifier U5A; one end of the resistor R15 is connected to the operational amplifier U5A. One end of the capacitor C7 is connected to the inverting input terminal of the operational amplifier U5A, and the other end is grounded; one end of the capacitor C11 is connected to the intersection A of the resistor R2 and the resistor R5, and the other end is connected to the output terminal of the operational amplifier U5A; one end of the resistor R16 is connected to the inverting input terminal of the operational amplifier U5A, and the other end is connected to the output terminal of the operational amplifier U5A; the capacitor C39 is connected in parallel to the two ends of the resistor R16.

[0015] As a preferred solution of the present invention, the signal generating circuit includes an MCU microprocessor, a passive crystal oscillator X, load capacitors CL1, CL2 and a common-mode amplifier Ux. The MCU microprocessor, the passive crystal oscillator X, and the load capacitors CL1, CL2 form an oscillation circuit. After power-on, the oscillation circuit starts to oscillate and outputs a sine wave. The sine wave output by the oscillation circuit is amplified by the common-mode amplifier Ux and then drives the probe capacitor voltage divider circuit to work.

[0016] As a preferred solution of the present invention, the MCU microprocessor is externally connected to a 5V operating voltage, and the model of the MCU microprocessor is ATSAMC21E18A.

[0017] As a preferred solution of the present invention, the frequency selection circuit is a crystal oscillator X3, one end of the crystal oscillator X3 is connected to the intersection B of the voltage-dividing capacitor C1 and the voltage-dividing capacitor C8, and the other end is connected to the signal input end of the rectifier and filter circuit as the signal output end of the frequency selection circuit.

[0018] As a preferred solution of the present invention, the rectification and filtering circuit includes a diode D1 and a capacitor C13, the positive electrode of the diode D1 is connected to the signal output end of the frequency selection circuit, the negative electrode is connected to the capacitor C13 and then grounded and serves as the signal output end of the rectification and filtering circuit connected to the signal input end of the back-end signal processing circuit.

[0019] As a preferred solution of the present invention, the front-end circuit also includes a clamping circuit, which includes a diode D2 and a resistor R39, the cathode of the diode D2 is connected to the signal output end of the frequency selection circuit, the anode of the diode D2 is grounded, the resistor R39 is connected in parallel to both ends of the diode D2, and the signal obtained by the voltage division of the voltage-dividing capacitor C1 and the voltage-dividing capacitor C8 is clamped by the diode D2 after filtering by the frequency selection circuit, so that the overall signal level is raised.

[0020] As a preferred solution of the present invention, the rectifier and filter circuit further includes a grounding resistor R load , the grounding resistance R load One end is connected to the cathode of the diode D1, and the other end is grounded.

[0021] As a preferred solution of the present invention, the back-end signal processing circuit includes resistors R21, R7, R38, R18, R25, R5, capacitors C12, C21, MOS tube M1, and operational amplifier U2B, one end of the resistor R21 is connected to the output end of the in-phase amplifier U5B as the signal input end of the back-end signal processing circuit, and the other end is connected to the inverting input end of the operational amplifier U2B;

[0022] One end of the resistor R25 is connected to the inverting input end of the operational amplifier U2B, and the other end is connected to the output end of the operational amplifier U2B;

[0023] The capacitor C21 is connected in parallel to both ends of the resistor R25;

[0024] The non-inverting input terminal of the operational amplifier U2B is connected to the resistor R38 and then to an external power supply Vs, and is connected to an RC parallel circuit and then to ground. The RC parallel circuit is formed by connecting the resistor R7 and the capacitor C12 in parallel.

[0025] The output end of the operational amplifier U2B is connected to the base of the MOS tube M1, the emitter of the MOS tube M1 is grounded, and the collector is connected in series with the resistor R5 and then externally connected to the power supply Vs;

[0026] One end of the resistor R18 is connected to the non-inverting input end of the operational amplifier U2B, and the other end is connected to the collector of the MOS tube M1 and serves as the signal output end of the back-end signal processing circuit.

[0027] As a preferred solution of the present invention, the model of the MOS tube M1 is Nexperia-BSS138P.

[0028] The present invention increases the sensitivity of the probe in detecting the liquid level by adding a decoupling circuit between the front-end circuit and the back-end signal processing circuit, and increases the driving capability of the front-end circuit to the back-end signal processing circuit. In addition, the capacitive liquid level detection system provided by the present invention performs liquid level detection based on the crystal oscillator method, and has a higher detection sensitivity than the existing contact LLD technology, and has a lower use cost than the existing non-contact LLD technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments of the present invention. Obviously, the drawings described below are only some embodiments of the present invention, and for ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0030] Figure 1 is a specific circuit structure diagram of a capacitive liquid level detection system provided by an embodiment of the present invention;

[0031] Figure 2 is a circuit structure diagram of a signal generating circuit provided in Embodiment 1 of the present invention;

[0032] Figure 3 is a circuit structure diagram of a signal generating circuit provided in Embodiment 2 of the present invention;

[0033] Figure 4 is a circuit structure diagram of a signal generating circuit provided in Embodiment 3 of the present invention;

[0034] Figure 5 is a circuit structure diagram of a signal generating circuit provided in Embodiment 4 of the present invention;

[0035] Figure 6 It is a schematic diagram of the specific connection relationship between the probe capacitor voltage divider circuit, the frequency selection circuit and the rectifier filter circuit in the front-end circuit;

[0036] Figure 7 It is a specific circuit structure diagram of a capacitive liquid level detection system based on a crystal oscillator method without adding a decoupling circuit between a front-end circuit and a back-end signal processing circuit;

[0037] Figure 8 It is a block diagram of a capacitive liquid level detection system based on a crystal oscillator method provided by an embodiment of the present invention;

[0038] Fig. 9 It is Figure 3 The measured output waveform of the signal generating circuit after second-order filtering is shown. DETAILED DESCRIPTION

[0039] The technical solution of the present invention is further described below with reference to the accompanying drawings and through specific implementation methods.

[0040] Among them, the drawings are only used for illustrative explanations, and they only represent schematic diagrams rather than actual pictures, and should not be understood as limitations on this patent; in order to better illustrate the embodiments of the present invention, some parts of the drawings may be omitted, enlarged or reduced, and do not represent the size of the actual product; for those skilled in the art, it is understandable that some well-known structures and their descriptions in the drawings may be omitted.

[0041] The same or similar numbers in the drawings of the embodiments of the present invention correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "inner", "outer", etc. appear, the orientation or position relationship indicated is based on the orientation or position relationship shown in the drawings, which is only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation. Therefore, the terms describing the position relationship in the drawings are only used for illustrative purposes and cannot be understood as a limitation on this patent. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0042] In the description of the present invention, unless otherwise clearly specified and limited, if the term "connection" or the like appears to indicate the connection relationship between components, the term should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, it can be the internal connection of two components or the interaction relationship between two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0043] Embodiment 1

[0044] The present invention provides a capacitive liquid level detection system based on the crystal oscillator method, such as Figure 8 As shown, it includes a signal generating circuit, a front-end circuit and a back-end signal processing circuit. The front-end circuit includes a probe capacitance voltage divider circuit, a frequency selection circuit and a rectifier filter circuit. The signal generating circuit is used to generate a square wave or sine wave signal of a fixed frequency (8MHz / 10MHz / 12MHz) to drive the probe capacitance voltage divider circuit. At the above frequency, the impedance of the fixed capacitor on the board and the probe detection system capacitor (including the cable capacitor and the probe detection environment capacitor) is at the kΩ level. When the probe contacts the liquid surface, the parasitic capacitance value of the probe changes, causing its impedance to change, and the signal amplitude obtained by the voltage divider of the fixed capacitor on the board changes accordingly.

[0045] The actual function of the frequency selection circuit is to perform bandpass filtering on the signal obtained by dividing the parasitic capacitance of the probe and the fixed capacitance on the board, that is, only signals with the same frequency as the signal generating circuit are allowed to pass, thereby filtering out interference signals and improving the anti-interference ability of the detection circuit.

[0046] The output signal of the frequency selection circuit is a sine wave, and the back-end signal processing circuit needs to process the amplitude change of the sine wave. Therefore, the peak signal of the sine wave is obtained through the rectifier and filter circuit and then output to the back-end signal processing circuit.

[0047] The back-end signal processing circuit monitors the DC peak signal output by the front-end circuit. Changes in the signal will cause the back-end signal processing circuit to output a pulse of a certain amplitude. The microprocessor detects the pulse signal to identify whether the probe has detected the liquid level.

[0048] In short, the principle of detecting the liquid level by the capacitive liquid level detection system based on the crystal oscillator method provided by the present invention is: the moment the probe touches the liquid surface, the capacitance of the probe to the ground will suddenly increase, and by detecting the change in the capacitance value, it can be known whether the probe touches the liquid surface. For example, after the microprocessor converts the pulse signal into a voltage value, by comparing the voltage value with a preset voltage threshold, when the voltage value is greater than the voltage threshold, it is determined that the probe detects the liquid surface, otherwise it is determined that the probe does not detect the liquid surface, etc.

[0049] Figure 7 A schematic diagram of a specific circuit structure of a capacitive liquid level detection system based on a crystal oscillator method provided in an embodiment of the present invention is shown. Figure 7 The circuit structure shown does not add a decoupling circuit between the front-end circuit and the back-end signal processing circuit, resulting in the problem that the front-end and back-end circuits are coupled to each other and the liquid level detection sensitivity is not ideal. In order to solve this problem, the present invention preferably adds a decoupling circuit between the front-end circuit and the back-end signal processing circuit, such as Figure 1 As shown, the decoupling circuit is preferably a common-phase amplifier U5B, the common-phase input end of which is connected to the page detection signal output end of the front-end circuit (the output signal of the front-end circuit is Figure 1 Use v d Indicated), the inverting input terminal of the common-mode amplifier U5B is connected to its own output terminal, and the output terminal of the common-mode amplifier U5B is connected to the signal input terminal of the back-end signal processing circuit.

[0050] That is to say, after adding the decoupling circuit, the output signal v of the front-end circuit d The back-end signal processing circuit is no longer directly connected, but driven by the common-phase amplifier U5B. By introducing the common-phase amplifier U5B, the front-end and back-end circuits are isolated, and the driving capability of the back-end signal processing circuit is increased due to the signal amplification effect of the common-phase amplifier U5B.

[0051] In addition, to provide a rectifier filter circuit ( Figure 1 The diode D1 and capacitor C13 in the circuit form a rectifier filter circuit) to provide a discharge path, and a grounding resistor R is added to the signal output end of the front-end circuit. load .

[0052] The specific circuit structure of the capacitive liquid level detection system provided in this embodiment is described below:

[0053] In this embodiment, Figure 2 or Figure 7 As shown, the signal generating circuit includes an active crystal oscillator X2, a resistor R2, a Schmitt trigger U5, a capacitor C37, a resistor R15 and a capacitor C7. The VCC input terminal of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, the GND grounding terminal of the active crystal oscillator X2 is grounded, the OUT output terminal is connected in series with the resistor R2 and then connected to the pin 2 of the Schmitt trigger U5 (model 74LVC1G17); the pin 3 of the Schmitt trigger U5 is grounded, the pin 4 is connected to one end of the resistor R15, the other end of the resistor R15 is connected to the capacitor C7 and then grounded and serves as the signal output terminal of the signal generating circuit, and the pin 5 of the Schmitt trigger U5 is connected to the 3.3V operating voltage and connected to the capacitor C37 and then grounded.

[0054] The active crystal oscillator X2 is used to generate a square wave signal with a frequency of 8M, and a Schmitt trigger is added to improve the driving ability. Since the square wave edge output by the Schmitt trigger is very steep, it causes a lot of electromagnetic radiation, so the output adds RC filtering (R15, C7) to reduce electromagnetic radiation. However, RC filtering causes the signal to attenuate, which has a certain impact on the liquid level detection performance. After RC filtering, the radiation emission is greatly improved, but there are still local excesses. In order to solve this problem, the present invention also provides several improved schemes for signal generation circuits, which will be specifically described in the subsequent embodiments 2, 3, and 4.

[0055] like Figure 7 As shown, the probe capacitance voltage divider circuit is composed of the probe parasitic capacitance Ct, the voltage divider capacitor C1 and the voltage divider capacitor C8. One end of the voltage divider capacitor C1 is connected to the signal output end of the signal generating circuit as the signal input end of the probe capacitance voltage divider circuit, and the other end is connected to the voltage divider capacitor C8. The other end of the voltage divider capacitor C8 is connected to the probe through the magnetic bead FB7. When the probe P5 contacts the liquid surface, the capacitance value of the probe parasitic capacitance changes, causing its impedance to change, and the signal amplitude obtained by the voltage divider of the fixed capacitors on the board (voltage divider capacitors C1, C8) changes accordingly. The frequency selection circuit performs bandpass filtering on the signal obtained by the voltage divider capacitors C1 and C8, that is, only allows the signal with the same frequency as the signal generating circuit to pass, thereby filtering out interference signals and improving the anti-interference ability of the detection circuit.

[0056] like Figure 1 or Figure 7 As shown, the frequency selection circuit is a crystal oscillator X3, one end of the crystal oscillator X3 is connected to the intersection point b of the voltage-dividing capacitor C1 and the voltage-dividing capacitor C8, and the other end is connected to the signal output end of the rectifier and filter circuit as the signal output end of the frequency selection circuit. The output signal of the frequency selection circuit is a sine wave (the principle of crystal oscillator frequency selection is not explained here).

[0057] like Figure 1 or Figure 6 As shown, the rectifier and filter circuit includes a diode D1 and a capacitor C13. The positive electrode of the diode D1 is connected to the signal output end of the frequency selection circuit, and the negative electrode is connected to the capacitor C13 and then grounded. The signal output end of the rectifier and filter circuit is connected to the in-phase input end of the in-phase amplifier U5B.

[0058] In order to raise the overall signal level, preferably, Figure 6 As shown, the front-end circuit also includes a clamping circuit, which includes a diode D2 and a resistor R39. The cathode of the diode D2 is connected to the signal output end of the frequency selection circuit, the anode of the diode D2 is grounded, and the resistor R39 is connected in parallel to both ends of the diode D2. The signal obtained by the voltage-dividing capacitor C1 and the voltage-dividing capacitor C8 is clamped by the diode D2 after being filtered by the passive crystal oscillator X3, so that the overall signal level is raised. The diode D1 and the capacitor C13 form a rectifier and filter circuit. Figure 6 The voltage at point d reflects the peak value of the voltage at point c.

[0059] like Figure 1 As shown, the back-end signal processing circuit includes resistors R21, R7, R38, R18, R25, R5, capacitors C12, C21, MOS tube M1, and operational amplifier U2B. One end of the resistor R21 is connected to the output end of the in-phase amplifier U5B as the signal input end of the back-end signal processing circuit, and the other end is connected to the inverting input end of the operational amplifier U2B.

[0060] One end of the resistor R25 is connected to the inverting input terminal of the operational amplifier U2B, and the other end is connected to the output terminal of the operational amplifier U2B;

[0061] Capacitor C21 is connected in parallel across resistor R25;

[0062] The non-inverting input terminal of the operational amplifier U2B is connected to the resistor R38 and then to the external power supply Vs, and is connected to an RC parallel circuit and then to ground. The RC parallel circuit is formed by connecting the resistor R7 and the capacitor C12 in parallel.

[0063] The output end of the operational amplifier U2B is connected to the base of the MOS tube M1, the emitter of the MOS tube M1 is grounded, and the collector is connected in series with the resistor R5 and then connected to the external power supply Vs;

[0064] One end of the resistor R18 is connected to the non-inverting input end of the operational amplifier U2B, and the other end is connected to the collector of the MOS tube M1 and serves as the signal output end of the back-end signal processing circuit.

[0065] In the back-end signal processing circuit, the smaller the transconductance gm of the MOS tube M1, the smaller the output pulse peak value, and the larger the gm, the larger the output pulse peak value. There are large differences in transconductance values ​​between MOS tubes of different manufacturers and models. When selecting, a MOS tube with a larger transconductance should be selected. Therefore, in order to further improve the sensitivity of liquid level detection, in this embodiment, the model of the MOS tube M1 is preferably Nexperia-BSS138P.

[0066] Figure 7 The back-end signal processing circuit shown in is a circuit solution adopted by the present invention when no decoupling circuit is added between the front-end and back-end circuits. The number of components of the back-end signal processing circuit is relatively large. Figure 1 The back-end signal processing circuits used in the system are more, the anti-interference performance is relatively poor, and the transconductance of the MOS tube Q3 used is not as ideal as that of the MOS tube M1. Therefore, it is preferred to use Figure 1 The back-end signal processing circuit shown in .

[0067] In summary, this embodiment improves the sensitivity of liquid level detection by adding a decoupling circuit between the front and rear circuits, and further improves the sensitivity of liquid level detection by correctly selecting the MOS tube M1.

[0068] Embodiment 2

[0069] The difference between the second embodiment and the first embodiment is that the circuit structure of the signal generating circuit is different. Figure 3 As shown, the signal generating circuit provided in the second embodiment includes an active crystal oscillator X2, resistors R2, R5, R15, R16, capacitors C11, C7, C39, and an operational amplifier U5A. The VCC input terminal of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, the GND grounding terminal of the active crystal oscillator X2 is grounded, and the OUT output terminal is connected to the non-inverting input terminal of the operational amplifier U5A after being connected in series with resistors R2 and R5 in sequence; one end of the resistor R15 is connected to the inverting input terminal of the operational amplifier U5A, and the other end is grounded; one end of the capacitor C7 is connected to the non-inverting input terminal of the operational amplifier U5A, and the other end is grounded; one end of the capacitor C11 is connected to the intersection point A of the resistor R2 and the resistor R5, and the other end is connected to the output terminal of the operational amplifier U5A; one end of the resistor R16 is connected to the inverting input terminal of the operational amplifier U5A, and the other end is connected to the output terminal of the operational amplifier U5A; the capacitor C39 is connected in parallel to both ends of the resistor R16.

[0070] The signal generating circuit provided in the second embodiment uses an operational amplifier U5A to replace Figure 2 The Schmitt trigger in Figure 2 The first-order RC low-pass filter in Figure 2 R15, C7) in the filter are changed to second-order filtering to achieve better high-frequency suppression and adjustable signal amplitude.

[0071] like Figure 3 As shown, the active crystal oscillator X2 outputs a square wave signal, which drives the probe capacitor voltage divider circuit after second-order filtering. Fig. 9 The measured output waveform after second-order filtering is shown. It can be seen that its waveform is very close to a sine wave, and the high-order harmonics are effectively filtered out, solving the problem of Figure 2 Even after the first-order RC filtering shown, there is still a problem of excessive local radiation.

[0072] Embodiment 3

[0073] The difference between the third embodiment and the second embodiment is that the square wave generated by the active crystal oscillator X2 not only provides a clock for the MCU, but is also used to provide a driving signal for liquid level detection, thereby reducing the number of components.

[0074] However, the square wave radiation output by the active crystal oscillator is relatively large, and some devices are powered by 3.3V, and some are powered by 5V, and the number of components is relatively large. Therefore, in order to solve this problem, the following embodiment 4 is for Figure 4 The signal generating circuit shown is improved.

[0075] Embodiment 4

[0076] like Figure 5 As shown, the signal generating circuit provided in the fourth embodiment includes an MCU microprocessor, a passive crystal oscillator X, load capacitors CL1, CL2 and a common-mode amplifier Ux. The MCU microprocessor, the passive crystal oscillator X, the load capacitors CL1, CL2 form an oscillation circuit. After power-on, the oscillation circuit starts to oscillate and outputs a sine wave. The sine wave output by the oscillation circuit is amplified by the common-mode amplifier Ux and then drives the probe capacitor voltage divider circuit to work.

[0077] Figure 5 The signal generation circuit shown uses the sine wave signal generated by the oscillation circuit. Since the signal cannot directly drive the external circuit, a common-mode amplifier Ux needs to be added to improve the driving capability and adjust the signal amplitude. At the same time, the original 3.3V powered MCU microprocessor is changed to a 5V powered model ATSAMC21E18A, and the RS232 transceiver is also changed to a 5V powered model. Therefore, the entire development board uses a 5V system instead of the original 3.3V system, reducing the number of components.

[0078] The signal generating circuit used in the fourth embodiment has the following advantages:

[0079] 1. The signal change can be increased by increasing the voltage through the common-mode amplifier Ux;

[0080] 2. Increasing the voltage can improve the anti-interference ability;

[0081] 3. Changing the 5V+3.3V system to a 5V system reduces the number of components and improves the reliability of circuit operation;

[0082] 4. The problem of excessive RE radiation in the first and second embodiments is solved by using an MCU microcontroller to excite a passive crystal oscillator to generate a sine wave.

[0083] It should be noted that the above specific implementations are only preferred embodiments of the present invention and the technical principles used. Those skilled in the art should understand that various modifications, equivalent substitutions, changes, etc. can be made to the present invention. However, as long as these changes do not deviate from the spirit of the present invention, they should be within the scope of protection of the present invention. In addition, some terms used in the specification and claims of this application are not restrictive, but are only for the convenience of description.

Claims

1. A capacitive liquid level detection system based on crystal oscillator method, characterized in that: It includes a signal generating circuit, a front-end circuit, a back-end signal processing circuit, and a decoupling circuit connected between the front-end circuit and the back-end signal processing circuit, wherein the decoupling circuit includes a common-phase amplifier U5B, wherein the common-phase input end of the common-phase amplifier U5B is connected to the liquid level detection signal output end of the front-end circuit, the inverting input end of the common-phase amplifier U5B is connected to its output end, and the output end of the common-phase amplifier U5B is connected to the signal input end of the back-end signal processing circuit; The front-end circuit includes a probe capacitor voltage-dividing circuit, a frequency selection circuit and a rectifier filter circuit. The signal generating circuit generates a square wave or a sine wave signal with a fixed frequency to drive the probe capacitor voltage-dividing circuit. The probe capacitor voltage-dividing circuit is composed of a probe parasitic capacitance, a voltage-dividing capacitor C1 and a voltage-dividing capacitor C8. One end of the voltage-dividing capacitor C1 is connected to the signal output end of the signal generating circuit as the signal input end of the probe capacitor voltage-dividing circuit, and the other end is connected to the voltage-dividing capacitor C8. The other end of the voltage-dividing capacitor C8 is connected to the probe through a magnetic bead FB7. The frequency selection circuit performs band-pass filtering on the signal obtained by voltage division by the voltage-dividing capacitor C1 and the voltage-dividing capacitor C8, and then outputs a sine wave with the same frequency as that of the signal generating circuit; The rectifier and filter circuit performs peak detection on the sine wave output by the frequency selection circuit to obtain the peak signal of the sine wave and then outputs it to the back-end signal processing circuit; The back-end signal processing circuit converts the received peak signal into a pulse signal and outputs it to the microprocessor. The microprocessor identifies whether the probe detects the liquid level based on the pulse signal.

2. The capacitive liquid level detection system based on the crystal oscillator method according to claim 1, characterized in that: The signal generating circuit includes an active crystal oscillator X2, a resistor R2, a Schmitt trigger U5, a capacitor C37, a resistor R15 and a capacitor C7. The VCC input end of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, the GND grounding end of the active crystal oscillator X2 is grounded, the OUT output end is connected in series with the resistor R2 and then connected to pin 2 of the Schmitt trigger U5; pin 3 of the Schmitt trigger U5 is grounded, pin 4 is connected to one end of the resistor R15, the other end of the resistor R15 is connected to the capacitor C7 and then grounded and serves as the output end of the signal generating circuit, and pin 5 of the Schmitt trigger U5 is connected to a 3.3V operating voltage and connected to the capacitor C37 and then grounded.

3. The capacitive liquid level detection system based on the crystal oscillator method according to claim 1 or 2, characterized in that: The signal generating circuit comprises an active crystal oscillator X2, resistors R2, R5, R15, R16, capacitors C11, C7, C39, and an operational amplifier U5A. The VCC input terminal of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, the GND ground terminal of the active crystal oscillator X2 is grounded, and the OUT output terminal is connected to the non-inverting input terminal of the operational amplifier U5A after being connected in series with the resistor R2 and the resistor R5 in sequence; one end of the resistor R15 is connected to the inverting input terminal of the operational amplifier U5A, and the other end is grounded; one end of the capacitor C7 is connected to the non-inverting input terminal of the operational amplifier U5A, and the other end is grounded; one end of the capacitor C11 is connected to the intersection point A of the resistor R2 and the resistor R5, and the other end is connected to the output terminal of the operational amplifier U5A; one end of the resistor R16 is connected to the inverting input terminal of the operational amplifier U5A, and the other end is connected to the output terminal of the operational amplifier U5A; the capacitor C39 is connected in parallel to both ends of the resistor R16.

4. The capacitive liquid level detection system based on the crystal oscillator method according to claim 1 or 2, characterized in that: The signal generating circuit includes an MCU microprocessor, a passive crystal oscillator X, load capacitors CL1, CL2 and a common-mode amplifier Ux. The MCU microprocessor, the passive crystal oscillator X, and the load capacitors CL1, CL2 form an oscillation circuit. After power is turned on, the oscillation circuit starts to oscillate and outputs a sine wave. The sine wave output by the oscillation circuit is amplified by the common-mode amplifier Ux and then drives the probe capacitor voltage divider circuit to work.

5. The capacitive liquid level detection system based on the crystal oscillator method according to claim 4 is characterized in that: The MCU microprocessor is externally connected to a 5V operating voltage, and the model of the MCU microprocessor is ATSAMC21E18A.

6. The capacitive liquid level detection system based on the crystal oscillator method according to claim 1, characterized in that: The frequency selection circuit is a crystal oscillator X3, one end of the crystal oscillator X3 is connected to the intersection B of the voltage-dividing capacitor C1 and the voltage-dividing capacitor C8, and the other end is connected to the signal input end of the rectifier and filter circuit as the signal output end of the frequency selection circuit.

7. The capacitive liquid level detection system based on the crystal oscillator method according to claim 1 or 6, characterized in that: The rectifier and filter circuit includes a diode D1 and a capacitor C13, wherein the positive electrode of the diode D1 is connected to the signal output end of the frequency selection circuit, and the negative electrode is connected to the capacitor C13 and then grounded and connected to the signal input end of the back-end signal processing circuit as the signal output end of the rectifier and filter circuit.

8. The capacitive liquid level detection system based on the crystal oscillator method according to claim 7, characterized in that: The front-end circuit also includes a clamping circuit, which includes a diode D2 and a resistor R39. The cathode of the diode D2 is connected to the signal output end of the frequency selection circuit, the anode of the diode D2 is grounded, and the resistor R39 is connected in parallel to both ends of the diode D2. The signal obtained by the voltage division of the voltage-dividing capacitor C1 and the voltage-dividing capacitor C8 is clamped by the diode D2 after filtering by the frequency selection circuit, so that the overall signal level is raised.

9. The capacitive liquid level detection system based on the crystal oscillator method according to claim 7, characterized in that: The rectifier and filter circuit also includes a grounding resistor R load , the grounding resistance R load One end is connected to the cathode of the diode D1, and the other end is grounded.

10. The capacitive liquid level detection system based on the crystal oscillator method according to claim 1, characterized in that: The back-end signal processing circuit includes resistors R21, R7, R38, R18, R25, R5, capacitors C12, C21, MOS tube M1, and operational amplifier U2B. One end of the resistor R21 is connected to the output end of the in-phase amplifier U5B as the signal input end of the back-end signal processing circuit, and the other end is connected to the inverting input end of the operational amplifier U2B. One end of the resistor R25 is connected to the inverting input end of the operational amplifier U2B, and the other end is connected to the output end of the operational amplifier U2B; The capacitor C21 is connected in parallel to both ends of the resistor R25; The non-inverting input terminal of the operational amplifier U2B is connected to the resistor R38 and then to an external power supply Vs, and is connected to an RC parallel circuit and then to ground. The RC parallel circuit is formed by connecting the resistor R7 and the capacitor C12 in parallel. The output end of the operational amplifier U2B is connected to the base of the MOS tube M1, the emitter of the MOS tube M1 is grounded, and the collector is connected in series with the resistor R5 and then externally connected to the power supply Vs; One end of the resistor R18 is connected to the non-inverting input end of the operational amplifier U2B, and the other end is connected to the collector of the MOS tube M1 and serves as the signal output end of the back-end signal processing circuit.

11. The capacitive liquid level detection system based on the crystal oscillator method according to claim 10, characterized in that: The model of the MOS tube M1 is Nexperia-BSS138P.

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