CN114264351A - Capacitance liquid level detection system based on crystal oscillation method - Google Patents
Capacitance liquid level detection system based on crystal oscillation method Download PDFInfo
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Abstract
The invention discloses a capacitance liquid level detection system based on a crystal oscillation method, and belongs to the technical field of LLD. The capacitance level detection system includes a signal generation 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. According to the invention, the decoupling circuit is added between the front-end circuit and the rear-end signal processing circuit, so that the front-end circuit and the rear-end circuit are isolated, the sensitivity of the probe for detecting the liquid level is increased, and the driving capability of the front-end circuit for the rear-end signal processing circuit is increased. In addition, the liquid level detection is carried out based on the crystal oscillation method, compared with the existing contact-type LLD technology, the detection sensitivity is higher, and compared with the existing non-contact-type LLD technology, the use cost is lower.
Description
Technical Field
The invention relates to the technical field of probe liquid level detection, in particular to a capacitance liquid level detection system based on a crystal oscillation method.
Background
Full-automatic clinical analyzers such as biochemical analyzers, enzyme immunoassay analyzers, urine analyzers, blood coagulation analyzers and the like mostly have automatic sample transfer systems, and the sample transfer precision is one of the determining factors of the analysis precision of the instruments.
The liquid carrying on the outer surface of the pipetting needle is the main reason for influencing the sample pipetting accuracy. Because the liquid level height of the liquid to be transferred is different, the liquid viscosity and the adhesive force are different, and the depth of the liquid transferring needle which is inserted into the liquid is different, the carrying amount of the outer surface of the liquid transferring needle is unstable, and the instrument cannot generate a stable system error.
Minimizing the liquid carrying amount on the outer surface of the pipetting needle is the main method for reducing carrying pollution and improving analysis accuracy. There are various methods for reducing the carrying amount, for example, coating the outer surface of the pipetting needle with a material which is not easy to stick liquid, such as TEFLON; slowing the withdrawal speed of the pipetting needle from the liquid; and a Liquid Level Detection (LLD) function is adopted to control the depth of the liquid-transferring needle in the liquid and the like.
At present, the most common method adopts the LLD function, which not only can control the depth of the liquid transferring needle in the liquid, but also can detect whether the liquid is exhausted or lost, thereby avoiding the false sample adding of the instrument. The existing LLD technology mainly comprises a contact type LLD technology and a non-contact type LLD technology, and the existing contact type LLD technology mainly comprises a resistance method, a capacitance method, an air pressure method, a mechanical vibration method and the like; the non-contact LLD technology includes an ultrasonic wave method, a laser method, a CCD imaging method and the like, but the detection sensitivity of the existing contact LLD technology is not high, while the ultrasonic wave LLD technology in the non-contact type is mature day by day, but the application cost is higher, and the laser method and the imaging method have relatively lower cost but are not generally used at present.
Disclosure of Invention
The invention provides a capacitance liquid level detection system based on a crystal oscillation method, aiming at improving the LLD sensitivity and reducing the liquid level detection cost.
In order to achieve the purpose, the invention adopts the following technical scheme:
the utility model provides a electric capacity liquid level detection system based on crystal oscillator method, include signal generation circuit, front end circuit, rear end signal processing circuit and connect the front end circuit with decoupling circuit between the rear end signal processing circuit, decoupling circuit includes in-phase amplifier U5B, the in-phase input of in-phase amplifier U5B is connected the liquid level detection signal output part of front end circuit, its output is connected to the inverting input of in-phase amplifier U5B, its output is connected to the output of in-phase amplifier U5B the signal input part of rear end signal processing circuit.
As a preferable scheme of the present invention, the front-end circuit includes a probe capacitance voltage-dividing circuit, a frequency-selecting circuit and a rectification filter circuit, the signal generating circuit generates a square wave or sine wave signal with a fixed frequency to drive the probe capacitance voltage-dividing circuit, the probe capacitance voltage-dividing circuit is composed of a probe parasitic capacitance, a voltage-dividing capacitance C1 and a voltage-dividing capacitance C8, one end of the voltage-dividing capacitance C1 is used as a signal input end of the probe capacitance voltage-dividing circuit to connect to a signal output end of the signal generating circuit, the other end of the voltage-dividing capacitance C8 is connected to the voltage-dividing capacitance C8, and the other end of the voltage-dividing capacitance C8 is connected to a probe through a magnetic bead FB 7;
the frequency selection circuit performs band-pass filtering on signals obtained by voltage division of the voltage division capacitor C1 and the voltage division capacitor C8 and outputs sine waves with the same frequency as the signal generation circuit;
the rectification filter circuit performs peak detection on the sine wave output by the frequency selection circuit to obtain a peak signal of the sine wave and outputs the peak signal to the rear-end signal processing circuit;
the rear end signal processing circuit converts the received peak signal into a pulse signal and outputs the pulse signal to the microprocessor, and the microprocessor identifies whether the probe detects the liquid level according to the pulse signal.
As a preferable scheme 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, a VCC input terminal of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, a GND ground terminal of the active crystal oscillator X2 is grounded, and an OUT output terminal 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 working voltage and then connected to the capacitor C37 and then grounded.
As a preferable embodiment of the present invention, the signal generating circuit includes an active crystal oscillator X2, a resistor R2, R5, R15, R16, a capacitor C11, C7, C39, and an operational amplifier U5A, a VCC input terminal of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, a GND ground terminal of the active crystal oscillator X2 is grounded, and an OUT output terminal is connected to a non-inverting input terminal of the operational amplifier U5A after being connected in series to the resistor R2 and the resistor R5 in sequence; one end of the resistor R15 is connected with the inverting input end of the operational amplifier U5A, and the other end is grounded; one end of the capacitor C7 is connected with the non-inverting input end of the operational amplifier U5A, and the other end of the capacitor C7 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 end of the operational amplifier U5A; one end of the resistor R16 is connected with the inverting input end of the operational amplifier U5A, and the other end is connected with the output end of the operational amplifier U5A; the capacitor C39 is connected in parallel across the resistor R16.
As a preferable scheme of the present invention, the signal generating circuit includes an MCU microprocessor, a passive crystal oscillator X, load capacitors CL1, CL2, and an in-phase amplifier Ux, the MCU microprocessor, the passive crystal oscillator X, the load capacitors CL1, and CL2 form an oscillating circuit, the oscillating circuit starts oscillation and outputs a sine wave after being powered on, and the sine wave output by the oscillating circuit is amplified by the in-phase amplifier Ux to drive the probe capacitor voltage dividing circuit to operate.
As a preferable scheme of the invention, the MCU microprocessor is externally connected with a 5V working voltage, and the model of the MCU microprocessor is ATSAMC21E 18A.
As a preferable embodiment 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 division capacitor C1 and the voltage division capacitor C8, and the other end is used as a signal output end of the frequency selection circuit and connected to a signal input end of the rectification filter circuit.
As a preferable aspect of the present invention, the rectifying and filtering circuit includes a diode D1 and a capacitor C13, wherein an anode of the diode D1 is connected to the signal output terminal of the frequency selection circuit, a cathode of the diode D1 is connected to the capacitor C13, and then grounded, and serves as the signal output terminal of the rectifying and filtering circuit and connected to the signal input terminal of the rear-end signal processing circuit.
As a preferable aspect of the present invention, the front-end circuit further includes a clamping circuit, the clamping circuit includes a diode D2 and a resistor R39, a negative electrode of the diode D2 is connected to the signal output end of the frequency selection circuit, an anode of the diode D2 is grounded, the resistor R39 is connected in parallel to both ends of the diode D2, and a signal obtained by voltage division by the voltage division capacitor C1 and the voltage division capacitor C8 is filtered by the frequency selection circuit and then clamped by the diode D2, so that an overall level of the signal is raised.
As a preferable embodiment of the present invention, the rectifying and filtering circuit further includes a ground resistor RloadThe ground resistance RloadOne end of which is connected to the cathode of the diode D1 and the other end of which is grounded.
As a preferred aspect of the present invention, the back-end signal processing circuit includes resistors R21, R7, R38, R18, R25, R5, capacitors C12, C21, a MOS transistor M1, and an operational amplifier U2B, wherein one end of the resistor R21 is used as a signal input end of the back-end signal processing circuit and is connected to an output end of a non-inverting amplifier U5B, and the other end of the resistor R21 is connected to an inverting input end of the operational amplifier U2B;
one end of the resistor R25 is connected with the inverting input end of the operational amplifier U2B, and the other end is connected with the output end of the operational amplifier U2B;
the capacitor C21 is connected in parallel across the resistor R25;
the non-inverting input end of the operational amplifier U2B is connected with an external power supply Vs after being connected with the resistor R38, and is connected with an RC parallel circuit which is grounded after being connected with the resistor R7 and the capacitor C12 in parallel;
the output end of the operational amplifier U2B is connected with the base electrode of the MOS tube M1, the emitter electrode of the MOS tube M1 is grounded, and the collector electrode is connected with the resistor R5 in series and then is externally connected with the power supply Vs;
one end of the resistor R18 is connected to the non-inverting input terminal of the operational amplifier U2B, and the other end is connected to the collector of the MOS transistor M1 and serves as the signal output terminal of the back-end signal processing circuit.
In a preferred embodiment of the present invention, the MOS transistor M1 is of a type Nexperia-BSS 138P.
According to the invention, the decoupling circuit is added between the front-end circuit and the rear-end signal processing circuit, so that the front-end circuit and the rear-end circuit are isolated, the sensitivity of the probe for detecting the liquid level is increased, and the driving capability of the front-end circuit for the rear-end signal processing circuit is increased. In addition, the capacitance liquid level detection system provided by the invention is used for detecting the liquid level based on a crystal oscillation method, and compared with the existing contact-type LLD technology, the detection sensitivity is higher, and compared with the existing non-contact-type LLD technology, the use cost is lower.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required to be used in the embodiments of the present invention will be briefly described below. It is obvious that the drawings described below are only some embodiments of the invention, and that for a person skilled in the art, other drawings can be derived from them without inventive effort.
FIG. 1 is a detailed circuit diagram of a capacitance level sensing system according to an embodiment of the present invention;
fig. 2 is a circuit configuration diagram of a signal generating circuit according to an embodiment of the present invention;
fig. 3 is a circuit configuration diagram of a signal generating circuit according to a second embodiment of the present invention;
fig. 4 is a circuit configuration diagram of a signal generating circuit according to a third embodiment of the present invention;
fig. 5 is a circuit configuration diagram of a signal generating circuit according to a fourth embodiment of the present invention;
FIG. 6 is a schematic diagram showing a specific connection relationship among a probe capacitance voltage divider circuit, a frequency selection circuit and a rectification filter circuit in the front-end circuit;
FIG. 7 is a schematic diagram of a specific circuit structure of a capacitance liquid level detection system based on a crystal oscillation method without adding a decoupling circuit between a front-end circuit and a back-end signal processing circuit;
FIG. 8 is a block diagram of a capacitance level detection system based on a crystal oscillation method according to an embodiment of the present invention;
fig. 9 is a diagram of a second-order filtered actual measurement output waveform from the signal generating circuit shown in fig. 3.
Detailed Description
The technical scheme of the invention is further explained by the specific implementation mode in combination with the attached drawings.
Wherein the showings are for the purpose of illustration only and are shown by way of illustration only and not in actual form, and are not to be construed as limiting the present patent; 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 an actual product; it will be understood by those skilled in the art that certain well-known structures in the drawings and descriptions thereof may be omitted.
The same or similar reference numerals in the drawings of the embodiments of the present invention correspond to the same or similar components; in the description of the present invention, it should be understood that if the terms "upper", "lower", "left", "right", "inner", "outer", etc. are used for indicating the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, it is only for convenience of description and simplification of description, but it is not indicated or implied that the referred device or element must have a specific orientation, be constructed in a specific orientation and be operated, and therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and are not to be construed as limitations of the present patent, and the specific meanings of the terms may be understood by those skilled in the art according to specific situations.
In the description of the present invention, unless otherwise explicitly specified or limited, the term "connected" or the like, if appearing to indicate a connection relationship between the components, is to be understood broadly, for example, as being fixed or detachable or integral; can be mechanically or electrically connected; they may be directly connected or indirectly connected through intervening media, or may be connected through one or more other components or may be in an interactive relationship with one another. The specific meanings of the above terms in the present invention can be understood in specific cases to those skilled in the art.
Example one
The capacitance liquid level detection system based on the crystal oscillation method, as shown in fig. 8, comprises a signal generation circuit, a front end circuit and a rear end signal processing circuit, wherein the front end circuit comprises a probe capacitance voltage division circuit, a frequency selection circuit and a rectification filter circuit, and the signal generation circuit is used for generating square wave or sine wave signals with fixed frequency (8MHz/10MHz/12MHz) to drive the probe capacitance voltage division circuit. Under the frequency, the impedance of the fixed capacitor on the board and the capacitance of the probe detection system (including cable capacitance and probe detection environment capacitance) is in the k omega level, when the probe contacts the liquid surface, the impedance of the probe changes due to the change of the parasitic capacitance value of the probe, and the impedance changes correspondingly to the signal amplitude obtained by voltage division of the fixed capacitor on the board.
The frequency selection circuit is essentially used for performing band-pass filtering on signals obtained by voltage division of the probe parasitic capacitor and the fixed capacitor on the board, namely only allowing signals with the same frequency as the signal generation circuit to pass through, thereby filtering interference signals and improving the anti-interference capability of the detection circuit.
The output signal of the frequency selection circuit is a sine wave, and the rear-end signal processing circuit needs to process the amplitude change condition of the sine wave, so that the peak value signal of the sine wave obtained by the rectification filter circuit is output to the rear-end signal processing circuit.
The back end signal processing circuit monitors a direct current peak signal output by the front end circuit, and the change of the signal will cause the back end signal processing circuit to output a pulse with a certain amplitude. The microprocessor identifies whether the probe detects the liquid level by detecting the pulse signal.
In short, the principle of the capacitance liquid level detection system based on the crystal oscillation method for detecting the liquid level provided by the invention is as follows: at the moment when the probe contacts the liquid surface, the capacitance of the probe to the ground is suddenly increased, and whether the probe contacts the liquid surface or not can be known by detecting the change of the capacitance value. For example, after the microprocessor converts the pulse signal into a voltage value, the voltage value is compared with a preset voltage threshold value, when the voltage value is greater than the voltage threshold value, the probe is judged to detect the liquid level, otherwise, the probe is judged not to detect the liquid level, and the like.
Fig. 7 shows a specific circuit structure schematic diagram of a capacitance liquid level detection system based on a crystal oscillation method according to an embodiment of the present invention. The circuit structure shown in fig. 7 does not add a decoupling circuit between the front-end circuit and the rear-end signal processing circuit, and has a problem that the front-end circuit and the rear-end circuit are coupled with each other, and the liquid level detection sensitivity is not ideal. To solve this problem, the present invention preferably adds a decoupling circuit between the front-end circuit and the back-end signal processing circuit, and as shown in fig. 1, the decoupling circuit is preferably a non-inverting amplifier U5B, and the non-inverting input terminal of the non-inverting amplifier U5B is connected to the page detect signal output terminal of the front-end circuit (the output signal of the front-end circuit is denoted by v in fig. 1dShown), the inverting input of the non-inverting amplifier U5B is connected to its own output, and the output of the non-inverting amplifier U5B is connected to the signal input of the back-end signal processing circuit.
That is, after adding the decoupling circuit, the output signal v of the front-end circuitdThe back-end signal processing circuit is no longer directly connected, but is driven by the non-inverting amplifier U5B. By introducing the non-inverting amplifier U5B, the front-end circuit and the back-end circuit are isolated, and meanwhile, the driving capability of the back-end signal processing circuit is increased due to the signal amplification effect of the non-inverting amplifier U5B.
In addition, in order to provide a discharge path for the rectifying and filtering circuit (the diode D1 and the capacitor C13 in FIG. 1 form the rectifying and filtering circuit), a ground resistor R is added at the signal output end of the front-end circuitload。
The following describes a specific circuit configuration of the capacitance liquid level detection system provided in this embodiment:
in this embodiment, as shown in fig. 2 or fig. 7, 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, a VCC input terminal of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, a GND ground terminal of the active crystal oscillator X2 is grounded, and an OUT output terminal is connected in series with the resistor R2 and then connected to pin 2 of the schmitt trigger U5 (model 74LVC1G 17); pin 3 of schmitt trigger U5 is grounded, pin 4 is connected to one end of resistor R15, the other end of resistor R15 is grounded after being connected to capacitor C7 and serves as a signal output end of the signal generating circuit, and pin 5 of schmitt trigger U5 is grounded after being connected to 3.3V working voltage and capacitor C37.
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 capability. Since the edges of the square wave output by the schmitt trigger are very steep, causing large electromagnetic radiation, the output adds RC filtering (R15, C7) to reduce the electromagnetic radiation. However, the RC filtering attenuates the signal, which has a certain influence on the liquid level detection performance, and the radiation emission after the RC filtering is greatly improved, but still has a local over-standard.
As shown in fig. 7, the probe capacitance voltage dividing circuit is composed of a probe parasitic capacitance Ct voltage dividing capacitor C1 and a voltage dividing capacitor C8, one end of the voltage dividing capacitor C1 is used as a signal input end of the probe capacitance voltage dividing circuit to be connected with a signal output end of the signal generating circuit, the other end of the voltage dividing capacitor C8 is connected with the voltage dividing capacitor C8, and the other end of the voltage dividing capacitor C8 is connected with the probe through a magnetic bead FB 7. When the probe P5 touches the liquid surface, the capacitance value of the probe parasitic capacitance changes to cause the impedance change, and the signal amplitude obtained by voltage division of the fixed capacitance (voltage division capacitance C1, C8) on the plate changes correspondingly. The frequency selection circuit performs band-pass filtering on signals obtained by voltage division of the voltage division capacitor C1 and the voltage division capacitor C8, namely only signals with the same frequency as the signal generation circuit are allowed to pass through, so that interference signals are filtered, and the anti-interference capability of the detection circuit is improved.
As shown in fig. 1 or fig. 7, 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 division capacitor C1 and the voltage division capacitor C8, and the other end is connected to the signal output end of the rectification 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 frequency selection of the crystal oscillator is not described here).
As shown in fig. 1 or fig. 6, the rectifying and filtering circuit includes a diode D1 and a capacitor C13, wherein the anode of the diode D1 is connected to the signal output terminal of the frequency selection circuit, and the cathode thereof is connected to the ground after being connected to the capacitor C13 and is connected as the signal output terminal of the rectifying and filtering circuit to the non-inverting input terminal of the non-inverting amplifier U5B.
In order to raise the overall signal level, preferably, as shown in fig. 6, the front-end circuit further includes a clamping circuit, the clamping circuit includes a diode D2 and a resistor R39, a cathode of the diode D2 is connected to the signal output end of the frequency selection circuit, an anode of the diode D2 is grounded, the resistor R39 is connected in parallel to two ends of the diode D2, and a signal obtained by voltage division by the voltage division capacitor C1 and the voltage division capacitor C8 is filtered by the passive crystal oscillator X3 and then clamped by the diode D2, so that the overall signal level is raised. The diode D1 and the capacitor C13 form a rectifying and filtering circuit, and the voltage at the point D in FIG. 6 reflects the peak value of the voltage at the point C.
As shown in fig. 1, the back-end signal processing circuit includes resistors R21, R7, R38, R18, R25, R5, capacitors C12, C21, a MOS transistor M1, and an operational amplifier U2B, wherein one end of the resistor R21 is used as a signal input end of the back-end signal processing circuit and is connected to an output end of a non-inverting amplifier U5B, and the other end of the resistor R21 is connected to an inverting input end of the operational amplifier U2B;
one end of the resistor R25 is connected with the inverting input end of the operational amplifier U2B, and the other end is connected with the output end of the operational amplifier U2B;
the capacitor C21 is connected in parallel to two ends of the resistor R25;
the non-inverting input end of the operational amplifier U2B is connected with a resistor R38 and then externally connected with a power supply Vs, and is connected with an RC parallel circuit and then grounded, wherein the RC parallel circuit is formed by connecting a resistor R7 and a capacitor C12 in parallel;
the output end of the operational amplifier U2B is connected with the base electrode of the MOS tube M1, the emitter electrode of the MOS tube M1 is grounded, and the collector electrode is connected with the power supply Vs after being connected with the resistor R5 in series;
one end of the resistor R18 is connected to the non-inverting input terminal of the operational amplifier U2B, and the other end is connected to the collector of the MOS transistor M1 and serves as the signal output terminal of the back-end signal processing circuit.
In the rear-end signal processing circuit, the smaller the transconductance gm of the MOS transistor M1 is, the smaller the output pulse peak value is, and the larger the gm is, the larger the output pulse peak value is. MOS tubes of different manufacturers and models have larger difference in transconductance values, and the MOS tube with larger transconductance is selected during model selection. Therefore, in order to further improve the liquid level detection sensitivity, in the embodiment, the model of the MOS transistor M1 is preferably Nexperia-BSS 138P.
The back-end signal processing circuit shown in fig. 7 is a circuit scheme adopted when a decoupling circuit is not additionally arranged between the front-end circuit and the back-end circuit, the number of components of the back-end signal processing circuit is more than that of the back-end signal processing circuit adopted in fig. 1, the anti-interference performance is relatively poor, and the transconductance of the adopted MOS transistor Q3 is not ideal enough compared with that of the MOS transistor M1. Therefore, the back-end signal processing circuit shown in fig. 1 is preferably employed.
In summary, the present embodiment increases the sensitivity of liquid level detection by adding a decoupling circuit between the front and back circuits. And the sensitivity of liquid level detection is further improved through the correct type selection of the MOS tube M1.
Example two
The difference between the second embodiment and the first embodiment is that the circuit structure of the signal generating circuit is different. As shown in fig. 3, the signal generating circuit provided in the second embodiment includes an active crystal oscillator X2, a resistor R2, R5, R15, R16, a capacitor C11, C7, C39, and an operational amplifier U5A, wherein a VCC input terminal of the active crystal oscillator X2 is connected to a 3.3V operating voltage through a magnetic bead FB4, a GND ground terminal of the active crystal oscillator X2 is grounded, and an OUT output terminal is connected to a non-inverting input terminal of the operational amplifier U5A after being sequentially connected to the resistor R2 and the resistor R5 in series; one end of the resistor R15 is connected with the inverting input end of the operational amplifier U5A, and the other end is grounded; one end of the capacitor C7 is connected with the non-inverting input end 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 end of the operational amplifier U5A; one end of the resistor R16 is connected with the inverting input end of the operational amplifier U5A, and the other end is connected with the output end of the operational amplifier U5A; the capacitor C39 is connected in parallel across the resistor R16.
The second embodiment provides a signal generating circuit that uses an operational amplifier U5A to replace the schmitt trigger in fig. 2, and changes the first-order RC low-pass filtering (R15 and C7 in fig. 2) in fig. 2 to the second-order filtering, so as to achieve better high-frequency suppression effect and adjustable signal amplitude.
As shown in fig. 3, the active crystal oscillator X2 outputs a square wave signal, which is second-order filtered to drive the probe capacitance divider circuit. Fig. 9 shows the actually measured output waveform after the second-order filtering, it can be seen that the waveform is very close to a sine wave, and the higher harmonics are effectively filtered, so that the problem that the local radiation exceeds the standard after the first-order RC filtering shown in fig. 2 is solved.
EXAMPLE III
The difference between the third embodiment and the second embodiment is that the square wave generated by the active crystal oscillator X2 is not only used for providing a clock for the MCU, but also used for providing a driving signal for liquid level detection, thereby reducing the number of components.
However, the radiation of the square wave output by the active crystal oscillator is large, and part of the devices are powered by 3.3V, and part of the devices are powered by 5V, and the number of components is relatively large, so in order to solve this problem, the signal generating circuit shown in fig. 4 is improved in the following fourth embodiment.
Example four
As shown in fig. 5, the signal generating circuit provided in the fourth embodiment includes an MCU microprocessor, a passive crystal oscillator X, load capacitors CL1, CL2, and an in-phase amplifier Ux, where the MCU microprocessor, the passive crystal oscillator X, the load capacitors CL1, and CL2 form an oscillating circuit, the oscillating circuit starts oscillating and outputs a sine wave after being powered on, and the sine wave output by the oscillating circuit is amplified by the in-phase amplifier Ux and drives the probe capacitor voltage dividing circuit to operate.
The signal generating circuit shown in fig. 5 utilizes a sine wave signal generated by an oscillating circuit, and since the signal cannot directly drive an external circuit, a non-inverting amplifier Ux is added to improve the driving capability and adjust the amplitude of the signal. Meanwhile, the original MCU microprocessor powered by 3.3V is changed into a 5V power supply model of ATSAMC21E18A, and the RS232 transceiver is also changed into a 5V power supply model, so that the whole development board adopts a 5V system to replace the original 3.3V system, and the number of components is reduced.
The signal generating circuit adopted by the fourth embodiment has the following advantages:
1. the signal variation can be increased by boosting the voltage through the in-phase amplifier Ux;
2. the anti-interference capability can be improved by increasing the voltage;
3. the 5V +3.3V system is changed into the 5V system, so that the number of components is reduced, and the working reliability of the circuit is improved;
4. the MCU microcontroller is used for exciting the passive crystal oscillator to generate sine waves, so that the problem that RE radiation exceeds standard in the first embodiment and the second embodiment is solved.
It should be understood that the above-described embodiments are merely preferred embodiments of the invention and the technical principles applied thereto. It will be understood by those skilled in the art that various modifications, equivalents, changes, and the like can be made to the present invention. However, such variations are within the scope of the invention as long as they do not depart from the spirit of the invention. In addition, certain terms used in the specification and claims of the present application are not limiting, but are used merely for convenience of description.
Claims (12)
1. The utility model provides a electric capacity liquid level detection system based on crystal oscillator method, its characterized in that includes signal generation circuit, front end circuit, rear end signal processing circuit and connects the front end circuit with decoupling circuit between the rear end signal processing circuit, decoupling circuit includes in-phase amplifier U5B, the in-phase input of in-phase amplifier U5B is connected the liquid level detection signal output of front end circuit, its output is connected to the inverting input of in-phase amplifier U5B, the output of in-phase amplifier U5B is connected the signal input part of rear end signal processing circuit.
2. The capacitance liquid level detection system based on the crystal oscillation method as claimed in claim 1, wherein the front end circuit comprises a probe capacitance voltage division circuit, a frequency selection circuit and a rectification filter circuit, the signal generation circuit generates a square wave or sine wave signal with a fixed frequency to drive the probe capacitance voltage division circuit, the probe capacitance voltage division circuit is composed of a probe parasitic capacitance, a voltage division capacitance C1 and a voltage division capacitance C8, one end of the voltage division capacitance C1 is used as a signal input end of the probe capacitance voltage division circuit to be connected with a signal output end of the signal generation circuit, the other end of the voltage division capacitance C8 is connected with the voltage division capacitance C8, and the other end of the voltage division capacitance C8 is connected with a probe through a magnetic bead FB 7;
the frequency selection circuit performs band-pass filtering on signals obtained by voltage division of the voltage division capacitor C1 and the voltage division capacitor C8 and outputs sine waves with the same frequency as the signal generation circuit;
the rectification filter circuit performs peak detection on the sine wave output by the frequency selection circuit to obtain a peak signal of the sine wave and outputs the peak signal to the rear-end signal processing circuit;
the rear end signal processing circuit converts the received peak signal into a pulse signal and outputs the pulse signal to the microprocessor, and the microprocessor identifies whether the probe detects the liquid level according to the pulse signal.
3. The capacitance liquid level detection system based on the crystal oscillation method as claimed in claim 1 or 2, wherein the signal generation circuit comprises an active crystal oscillator X2, a resistor R2, a Schmidt trigger U5, a capacitor C37, a resistor R15 and a capacitor C7, a VCC input end of the active crystal oscillator X2 is connected with a 3.3V working voltage through a magnetic bead FB4, a GND ground end of the active crystal oscillator X2 is grounded, and an OUT output end is connected with a pin 2 of the Schmidt trigger U5 after being connected with the resistor R2 in series; 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 working voltage and then connected to the capacitor C37 and then grounded.
4. The capacitance liquid level detection system based on the crystal oscillation method as claimed in claim 1 or 2, wherein the signal generation circuit comprises an active crystal oscillator X2, resistors R2, R5, R15, R16, capacitors C11, C7, C39 and an operational amplifier U5A, a VCC input end of the active crystal oscillator X2 is connected with a 3.3V working voltage through a magnetic bead FB4, a GND ground end of the active crystal oscillator X2 is grounded, and an OUT output end is connected in series with the resistors R2 and R5 in sequence and then connected with a non-inverting input end of the operational amplifier U5A; one end of the resistor R15 is connected with the inverting input end of the operational amplifier U5A, and the other end is grounded; one end of the capacitor C7 is connected with the non-inverting input end of the operational amplifier U5A, and the other end of the capacitor C7 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 end of the operational amplifier U5A; one end of the resistor R16 is connected with the inverting input end of the operational amplifier U5A, and the other end is connected with the output end of the operational amplifier U5A; the capacitor C39 is connected in parallel across the resistor R16.
5. The capacitance liquid level detection system based on the crystal oscillation method as claimed in claim 1 or 2, wherein the signal generation circuit comprises an MCU microprocessor, a passive crystal oscillator X, load capacitors CL1, CL2 and an in-phase amplifier Ux, the MCU microprocessor, the passive crystal oscillator X, the load capacitors CL1 and CL2 form an oscillation circuit, the oscillation circuit starts oscillation and outputs sine waves after being electrified, and the sine waves output by the oscillation circuit are amplified by the in-phase amplifier Ux to drive the probe capacitor voltage division circuit to work.
6. The crystal oscillation method-based capacitance liquid level detection system as claimed in claim 5, wherein the MCU microprocessor is externally connected with 5V working voltage, and the model of the MCU microprocessor is ATSAMC21E 18A.
7. The system as claimed in claim 2, wherein the frequency-selecting 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 rectifying-filtering circuit as the signal output end of the frequency-selecting circuit.
8. The system as claimed in claim 2 or 7, wherein the rectifying-filtering circuit comprises a diode D1 and a capacitor C13, the anode of the diode D1 is connected to the signal output terminal of the frequency-selecting circuit, the cathode of the diode D1 is connected to the ground after being connected to the capacitor C13, and the diode is connected as the signal output terminal of the rectifying-filtering circuit to the signal input terminal of the back-end signal processing circuit.
9. The capacitance liquid level detection system based on the crystal oscillation method as claimed in claim 8, wherein the front-end circuit further comprises a clamping circuit, the clamping circuit comprises 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 two ends of the diode D2, and a signal obtained by voltage division of the voltage division capacitor C1 and the voltage division capacitor C8 is filtered by the frequency selection circuit and then clamped by the diode D2, so that the overall level of the signal is raised.
10. The crystal oscillator method-based capacitance level detection system according to claim 8, wherein the rectifier filter circuit further comprises a ground resistor RloadThe ground resistance RloadOne end of which is connected to the cathode of the diode D1 and the other end of which is grounded.
11. The system as claimed in claim 1, wherein the back-end signal processing circuit comprises resistors R21, R7, R38, R18, R25, R5, capacitors C12, C21, a MOS transistor M1, and an operational amplifier U2B, wherein one end of the resistor R21 is used as a signal input end of the back-end signal processing circuit and is connected with an output end of a non-inverting amplifier U5B, and the other end of the resistor R21 is connected with an inverting input end of the operational amplifier U2B;
one end of the resistor R25 is connected with the inverting input end of the operational amplifier U2B, and the other end is connected with the output end of the operational amplifier U2B;
the capacitor C21 is connected in parallel across the resistor R25;
the non-inverting input end of the operational amplifier U2B is connected with an external power supply Vs after being connected with the resistor R38, and is connected with an RC parallel circuit which is grounded after being connected with the resistor R7 and the capacitor C12 in parallel;
the output end of the operational amplifier U2B is connected with the base electrode of the MOS tube M1, the emitter electrode of the MOS tube M1 is grounded, and the collector electrode is connected with the resistor R5 in series and then is externally connected with the power supply Vs;
one end of the resistor R18 is connected to the non-inverting input terminal of the operational amplifier U2B, and the other end is connected to the collector of the MOS transistor M1 and serves as the signal output terminal of the back-end signal processing circuit.
12. The system as claimed in claim 11, wherein the MOS transistor M1 is of the type Nexperia-BSS 138P.
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| CN115790777A (en) * | 2023-01-05 | 2023-03-14 | 山东康华生物医疗科技股份有限公司 | Liquid level detection device |
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| CN115790777A (en) * | 2023-01-05 | 2023-03-14 | 山东康华生物医疗科技股份有限公司 | Liquid level detection device |
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