CN110160602B - Oil cup liquid level measuring method of range hood - Google Patents
Oil cup liquid level measuring method of range hood Download PDFInfo
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- CN110160602B CN110160602B CN201810141712.3A CN201810141712A CN110160602B CN 110160602 B CN110160602 B CN 110160602B CN 201810141712 A CN201810141712 A CN 201810141712A CN 110160602 B CN110160602 B CN 110160602B
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01F—MEASURING VOLUME, VOLUME FLOW, MASS FLOW OR LIQUID LEVEL; METERING BY VOLUME
- G01F23/00—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm
- G01F23/22—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water
- G01F23/28—Indicating or measuring liquid level or level of fluent solid material, e.g. indicating in terms of volume or indicating by means of an alarm by measuring physical variables, other than linear dimensions, pressure or weight, dependent on the level to be measured, e.g. by difference of heat transfer of steam or water by measuring the variations of parameters of electromagnetic or acoustic waves applied directly to the liquid or fluent solid material
- G01F23/296—Acoustic waves
- G01F23/2962—Measuring transit time of reflected waves
Abstract
A method for measuring the liquid level of an oil cup of a range hood is characterized by comprising the following steps: (1) the main generation ultrasonic excitation signal satisfies the following conditions:
(2) the main control chip controls the analog-to-digital converter to start at the time when t is NT and samples the ultrasonic reflection signal; (3) the main control chip judges whether the current sampling signal has signal mutation, if so, the next step is executed; if not, the step is circulated; (4) the main control chip calculates the transition time delta t of the analog-to-digital converter when receiving the mutation signal; (5) calculating the height of liquid level
The invention has the advantages that: aiming at the problem of inaccurate detection of the initial pulse of the receiving point in the traditional ultrasonic ranging scheme, a brand new ultrasonic excitation signal is designed
And the mode of phase jump is adopted, and the ultrasonic receiving end can receive the mutation signal, so that the detection of the signal at the receiving end is simpler and more accurate.
Description
Technical Field
The invention relates to a method for measuring the liquid level of an oil cup of a range hood.
Background
The greasy dirt that range hood produced in the use can discharge into in the oil cup, and usually, the liquid level of fluid needs the manual observation to detect in the oil cup, need in time pour the greasy dirt in order to prevent to spill over after accumulating certain liquid level height. In order to observe the liquid level of the oil cup conveniently, most of the traditional range hood oil cups are made of transparent plastic materials, but the oil cup made of the plastic materials has the problems of low strength and easy aging; also have the oil cup that adopts metal material, though intensity is high, but it is inconvenient to observe the oil cup liquid level, even there is the observation window to exist, nevertheless the greasy dirt adhesion on the observation window after long-time the use also can lead to the unable liquid level of seeing the oil cup of seeing clearly of user.
For the above problems, in the prior art, various automatic detection devices and detection methods for the liquid level of the oil cup appear, for example, in the existing chinese patent invention patent ZL201310653491.5, "a detection device for the oil level of the oil cup of the range hood", the oil in the oil cup is heated and the temperature of the oil is detected by a heating thermistor on a detection circuit, and then the height of the oil is obtained by analysis, but the contact detection mode can greatly reduce the service life of the electronic component in the oil contamination environment, thereby reducing the detection precision and affecting the detection accuracy; for example, the chinese invention application "warning oil level advance notice device for oil cup of range hood" with application number 201510386967.2 discloses an oil level detection device which detects the warning liquid level of the oil cup through a floating body and other devices placed in the oil cup, but the device has the disadvantages that the floating body is soaked in oil stain for a long time, the service life is reduced, and the detection precision is not high.
The traditional contact type distance measuring method can greatly reduce the service life of the detection equipment due to oil smoke pollution, in order to adapt to severe environments such as oil smoke pollution and prolong the service life of the detection equipment, the ultrasonic distance measuring method in the prior art is a distance measuring method which is low in cost and wide in application in non-contact short-distance measurement, an ultrasonic transmitter generates ultrasonic waves, the time spent by the ultrasonic waves after the ultrasonic waves are transmitted to a receiver through a medium is the transit time delta t of the ultrasonic waves, and the distance x to be measured can be obtained by combining the transmission speed C of the ultrasonic waves; however, one of the main problems of the current ultrasonic ranging method is: the amplitude of the starting point of the ultrasonic receiving signal is too small, so that the receiving end cannot obtain an accurate starting signal, the detection of the starting point of the receiving signal is difficult, the error of ranging is large, and the accuracy of the final detection result is low.
Disclosure of Invention
The invention aims to solve the technical problem of providing the oil cup liquid level measuring method of the range hood, which has simpler detection and high distance measuring precision, aiming at the current technical situation.
The technical scheme adopted by the invention for solving the technical problems is as follows: the method for measuring the liquid level of the oil cup of the range hood is characterized in that the oil cup liquid level measuring device comprises an ultrasonic transmitting tube, a digital-to-analog converter, an ultrasonic receiving tube, an analog-to-digital converter and a main control chip, wherein the ultrasonic transmitting tube and the ultrasonic receiving tube are arranged in the same horizontal plane, and the method for measuring the liquid level of the oil cup comprises the following steps:
(1) the main control chip controls the digital-to-analog converter to generate an ultrasonic excitation signal
The ultrasonic excitation signal
The following conditions are satisfied:
wherein N is a positive integer, T is a resonance period and satisfies omegas0T2 π, UmFor ultrasonic excitation signals
Maximum voltage of, said ωs0Is the resonant angular frequency;
(2) the master control chip controls the analog-to-digital converter to start at the time when t is NT and samples the ultrasonic reflection signal;
(3) the main control chip judges whether the current sampling signal has signal mutation, if so, the next step is executed; if not, the step is circulated;
(4) the main control chip calculates the transition time delta t of the analog-to-digital converter when receiving the mutation signal;
(5) calculating the height of the liquid level
H is the height of the ultrasonic transmitting tube from the bottom surface of the oil cup, H is the liquid level height of the oil cup, and C is the propagation speed of ultrasonic waves in the air.
Preferably, the ultrasonic transmitting tube and the ultrasonic receiving tube respectively adopt piezoelectric ceramic transducers, and core devices of the transducers are piezoelectric vibrators.
Preferably, the sampling frequency f of the analog-to-digital converter in the step (2)samp＝2NsampF, wherein f is an ultrasonic excitation signal
And satisfy
Said N issampThe number of sampling points in a half sampling period.
More preferably, the number of sampling points N issampThe value range is 2-16, and the value N issampThe best is 8.
Preferably, the sampling signal received by the analog-to-digital converter in step (2) includes a start stage in which the signal amplitude gradually increases, a stable stage in which the signal amplitude is kept stable, and an abrupt stage in which the signal amplitude jumps. After the sampling is started, the signal amplitude before the initial stage is zero because the ultrasonic signal is not received at the beginning.
The ultrasonic receiving signal is approximate to a sinusoidal signal, a current sampling value is set to be ad _ now, a previous sampling value of the current sampling value is ad _ bef, a difference value delta ad _ now between current adjacent sampling values is ad _ now-ad _ bef, a difference value delta ad _ bef between previous adjacent sampling values is ad _ bef-ad _ bef ', wherein ad _ bef' is a next previous sampling value adjacent to the previous sampling value; according to the characteristics of the signal variation, the sampling signal of the initial stage preferably includes the following characteristics:
a. the maximum values ad _ max _ now of the current sample values are all larger than the maximum values ad _ max _ bef of the previous sample values;
b. the minimum value ad _ min _ now of the current sampling value is smaller than the minimum value ad _ min _ bef of the previous sampling value;
c. the maximum difference value delta ad _ max _ now between the current adjacent sampling values is larger than the maximum difference value delta ad _ max _ bef between the previous adjacent sampling values;
d. the minimum value of the difference between the current adjacent sample values Δ ad _ min _ now is greater than the minimum value of the difference between the previous adjacent sample values Δ ad _ min _ bef.
Preferably, the sampling signal in the stable phase includes the following features:
a. the maximum value ad _ max _ now of the current sample value is equal to the maximum value ad _ max _ bef of the previous sample value;
b. the minimum value ad _ min _ now of the current sample value is equal to the minimum value ad _ min _ bef of the previous sample value;
c. the maximum value of the difference value delta ad _ max _ now between the current adjacent sampling values is equal to the maximum value of the difference value delta ad _ max _ bef between the previous adjacent sampling values;
d. the minimum value of the difference between the current adjacent sample values Δ ad _ min _ now is equal to the minimum value of the difference between the previous adjacent sample values Δ ad _ min _ bef.
Preferably, the sampling signal of the mutation stage comprises the following characteristics:
a. the maximum values ad _ max _ now of the current sample values are all smaller than the maximum values ad _ max _ bef of the previous sample values;
b. the minimum value ad _ min _ now of the current sampling value is larger than the minimum value ad _ min _ bef of the previous sampling value;
c. the maximum difference value delta ad _ max _ now between the current adjacent sampling values is smaller than the maximum difference value delta ad _ max _ bef between the previous adjacent sampling values;
d. the minimum value of the difference between the current adjacent sample values Δ ad _ min _ now is greater than the minimum value of the difference between the previous adjacent sample values Δ ad _ min _ bef.
Because sampling at the ultrasonic receiving end may have a certain clock deviation, the time when the signal amplitude is zero is difficult to directly acquire at the sampling point of the abrupt change signal, and in order to reduce the detection deviation of the abrupt change signal acquisition point and further improve the detection accuracy, as further preferable, the transit time Δ t in the step (4) can be obtained by the following method:
(4.1) obtaining two current sampling points A1 and A2 with the maximum difference, wherein the sampling value corresponding to the sampling point A1 is ada1<0, and the value of the sampling point corresponding to the sampling point A1 is MAD(ii) a The sampling value ad corresponding to the sampling point A2a2>0, and the value of the sampling point corresponding to the sampling point A2 is MAD+1；
(4.2) determining the time offset of the time at which the sampled value is 0 from the sampled point A1
(4.3) obtaining the time offset of the starting moment of starting sampling of the analog-to-digital converter relative to the sampling point A1
(4.4) obtaining the transit time
Wherein f issampIs the sampling frequency.
The transition time obtained by the method fully considers the time offset of the signal amplitude at zero relative to the current acquisition point, so that the obtained final transition time is more accurate, and the detection precision can be further improved.
Compared with the prior art, the invention has the advantages that: aiming at the problem of inaccurate detection of the initial pulse of the receiving point in the traditional ultrasonic ranging scheme, a brand new ultrasonic excitation signal is designed
The mode of phase jump is adopted, and the ultrasonic receiving end can receive the mutation signal, so that the detection of the signal at the receiving end is simpler and more accurate; for a traditional high-precision ultrasonic ranging scheme, calculation methods such as Fast Fourier Transform (FFT) and the like are generally needed, the calculation amount is large, the calculation capability of a main control chip is greatly checked, the addition and subtraction operation is only performed on sampled A/D conversion signals (analog-to-digital conversion signals), the calculation speed is high, the calculation can be realized by adopting the main control chip with low cost, and the measured distance precision is high.
Drawings
FIG. 1 is a schematic view of an installation of an oil cup liquid level detection device according to an embodiment of the present invention.
Fig. 2 is a system structure diagram of the oil cup liquid level detection device according to the embodiment of the invention.
FIG. 3 is a flow chart of a method for measuring a liquid level of an oil cup according to an embodiment of the present invention.
FIG. 4 is a graph comparing waveforms of an ultrasonic excitation signal and an ultrasonic reception signal according to an embodiment of the present invention.
Fig. 5 is a schematic diagram of sampling of an ultrasonic received signal in a single sampling period according to an embodiment of the present invention.
Fig. 6 is a waveform diagram and a sampling schematic diagram of an ultrasonic receiving signal from a stable stage to an abrupt stage according to an embodiment of the present invention.
Detailed Description
The invention is described in further detail below with reference to the accompanying examples.
The embodiment relates to an oil cup liquid level measuring device of a range hood and a measuring method thereof, wherein the oil cup liquid level measuring device of the embodiment adopts an ultrasonic ranging principle, and specifically adopts a time of flight method TOF (time of flight): the time of ultrasonic wave from emission to barrier return is measured, and then multiplied by the speed of ultrasonic wave to obtain twice the distance between the sound source and the barrier.
As shown in fig. 1 and fig. 2, which are schematic diagrams of the oil cup liquid level measuring device in this embodiment, the oil cup liquid level measuring device is disposed in a range hood main body located right above an oil cup, and the oil cup liquid level measuring device specifically includes an ultrasonic transmitting tube, a digital-to-analog converter, an ultrasonic receiving tube, an analog-to-digital converter, and a main control chip, wherein the ultrasonic transmitting tube and the ultrasonic receiving tube respectively employ piezoelectric ceramic transducers, and core devices thereof are piezoelectric vibrators;
the digital-to-analog converter is electrically connected with the ultrasonic transmitting tube, the analog-to-digital converter is electrically connected with the ultrasonic receiving tube, the digital-to-analog converter and the analog-to-digital converter are respectively electrically connected with the main control chip, and the ultrasonic transmitting tube and the ultrasonic receiving tube are arranged in the same horizontal plane; the height of the ultrasonic transmitting tube from the bottom surface of the oil cup is set to be H, the liquid level of the oil cup is set to be H, the main control chip controls the digital-to-analog converter to generate an ultrasonic excitation signal, the ultrasonic wave is transmitted through air and then reflected when meeting the liquid plane of the oil cup, and finally the ultrasonic receiving tube transmits the received reflection signal to the analog-to-digital converter for sampling processing.
As shown in fig. 3 to 6, the method for measuring the liquid level of the oil cup in the embodiment includes the following steps:
(1) main control chipControlling a digital-to-analog converter to generate an ultrasonic excitation signal
The ultrasonic excitation signal
The following conditions are satisfied:
wherein N is a positive integer, T is a resonance period and satisfies omegas0·T＝2π，UmFor ultrasonic excitation signals
Maximum voltage of, omegas0Is the resonant angular frequency.
(2) The main control chip controls the analog-digital converter to start at the time when t is NT, and a sampling module of the analog-digital converter performs sampling processing on the ultrasonic reflection signal; setting the sampling frequency f of an analog-to-digital convertersamp＝2NsampF, where f is the frequency of the ultrasonic excitation signal and satisfies
NsampNumber of samples in half a sampling period, see fig. 5, number of samples NsampThe value range is 2-16, and the value N is preferably selected in the embodimentsampI.e. the number of samples in a complete sampling period is 16.
(3) The main control chip judges whether the current sampling signal has signal mutation, if so, the next step is executed; if not, the step is circulated.
(4) And the main control chip calculates the transition time delta t of the analog-to-digital converter when receiving the mutation signal.
(5) Calculating the height of the liquid level
Wherein H is ultrasonic waveThe height of the transmitting tube from the bottom surface of the oil cup, h is the liquid level height of the oil cup, and C is the propagation speed of the ultrasonic wave in the air.
In the step (2), the ultrasonic reflection signal acquired by the analog-to-digital converter includes an initial stage in which the signal amplitude gradually increases, a stable stage in which the signal amplitude is kept stable, and a sudden change stage in which the signal amplitude jumps, and after sampling is started, because the ultrasonic signal is not received at first, the signal amplitude before the initial stage is zero.
According to the waveform of the ultrasonic excitation signal, the ultrasonic receiving signal of the present embodiment is approximately a sinusoidal signal, a current sample value is set to ad _ now, a previous sample value of the current sample value is ad _ bef, a difference Δ ad _ now between current adjacent sample values is ad _ now-ad _ bef, and a difference Δ ad _ bef between previous adjacent sample values is ad _ bef-ad _ bef ', where ad _ bef' is a next previous sample value adjacent to the previous sample value;
if the current sample value is equal to the previous sample value and is zero, that is, ad _ now ═ ad _ bef ═ 0, it indicates that the ultrasonic wave receiving end has not received the ultrasonic wave reflection signal, and the ultrasonic wave receiving signal is before the start stage.
When the ultrasonic receiving end receives the initial signal, the sampling signal of the initial stage comprises the following characteristics:
a. the maximum values ad _ max _ now of the current sample values are all larger than the maximum values ad _ max _ bef of the previous sample values;
b. the minimum value ad _ min _ now of the current sampling value is smaller than the minimum value ad _ min _ bef of the previous sampling value;
c. the maximum difference value delta ad _ max _ now between the current adjacent sampling values is larger than the maximum difference value delta ad _ max _ bef between the previous adjacent sampling values;
d. the minimum value of the difference between the current adjacent sample values Δ ad _ min _ now is greater than the minimum value of the difference between the previous adjacent sample values Δ ad _ min _ bef.
The four characteristics of a, b, c and d show that the ultrasonic wave starts to receive an initial signal of the ultrasonic wave after the ultrasonic wave signal is reflected by the liquid level of the oil cup, and meanwhile, the main control chip stores sampling points of the current two sampling periods.
The signal received in the ultrasonic wave receiving signal stabilization phase is a stable sine wave, and the sampling signal in the stabilization phase comprises the following characteristics:
a. the maximum value ad _ max _ now of the current sample value is equal to the maximum value ad _ max _ bef of the previous sample value;
b. the minimum value ad _ min _ now of the current sample value is equal to the minimum value ad _ min _ bef of the previous sample value;
c. the maximum value of the difference value delta ad _ max _ now between the current adjacent sampling values is equal to the maximum value of the difference value delta ad _ max _ bef between the previous adjacent sampling values;
d. the minimum value of the difference between the current adjacent sample values Δ ad _ min _ now is equal to the minimum value of the difference between the previous adjacent sample values Δ ad _ min _ bef.
The ultrasonic receiving end receives the sudden change signal after receiving the stable sine wave signal, the time domain of the ultrasonic receiving signal is set as r (t), and the difference between the stable stage and the sudden change stage of the ultrasonic receiving signal is as follows: firstly, the maximum sampling value of the stable stage is greater than that of the mutation stage, and the minimum sampling value of the stable stage is less than that of the mutation stage; secondly, for the sine function, the derivative is maximum when the phase is 0 or pi, namely the maximum value of the difference value in the stable stage is larger than the maximum value of the difference value in the abrupt stage, and the minimum value of the difference value in the stable stage is smaller than the minimum value of the difference value in the abrupt stage.
Therefore, during the transition from the stable phase to the abrupt phase, the sampled signal of the abrupt phase contains the following characteristics:
a. the maximum values ad _ max _ now of the current sample values are all smaller than the maximum values ad _ max _ bef of the previous sample values;
b. the minimum value ad _ min _ now of the current sampling value is larger than the minimum value ad _ min _ bef of the previous sampling value;
c. the maximum difference value delta ad _ max _ now between the current adjacent sampling values is smaller than the maximum difference value delta ad _ max _ bef between the previous adjacent sampling values;
d. the minimum value of the difference between the current adjacent sample values Δ ad _ min _ now is greater than the minimum value of the difference between the previous adjacent sample values Δ ad _ min _ bef.
Considering that a certain clock deviation may exist during sampling at an ultrasonic receiving end, when an abrupt change signal occurs, a sampling point is difficult to directly acquire a moment when the signal amplitude is zero, and in order to reduce the detection deviation of an abrupt change signal acquisition point and further improve the detection accuracy, the transit time Δ t in the step (4) can be obtained by the following method:
(4.1) obtaining two current sampling points A1 and A2 with the maximum difference, wherein the sampling value corresponding to the sampling point A1 is ada1<0, and the value of the sampling point corresponding to the sampling point A1 is MAD(ii) a The sampling value ad corresponding to the sampling point A2a2>0, and the value of the sampling point corresponding to the sampling point A2 is MAD+1；
(4.2) determining the time offset of the time at which the sampled value is 0 from the sampled point A1
(4.3) obtaining the time offset of the starting moment of starting sampling of the analog-to-digital converter relative to the sampling point A1
(4.4) obtaining the transit time
Wherein f issampIs the sampling frequency.
Then, the liquid level height formula is combined
The liquid level height of the oil cup can be obtained as follows:
the embodiment aims at the problem that the amplitude of the starting point of the ultrasonic receiving signal is too small in the traditional ultrasonic ranging scheme, so that the detection of the starting pulse of the receiving end is inaccurate,designs a brand new ultrasonic excitation signal
The mode that the ultrasonic receiving end is started in a sampling mode after the ultrasonic transmitting end jumps in phase is adopted, so that the ultrasonic receiving end can detect a sudden change signal with obvious signal characteristic change, and the transition time delta t is accurately obtained, and the accuracy of the final detection result of the liquid level height of the oil cup is ensured.
Compared with the traditional high-precision ultrasonic ranging scheme, calculation methods such as Fast Fourier Transform (FFT) and the like are needed, the calculation capacity of the main control chip is greatly examined due to large calculation amount, and the cost of the selected main control chip is often higher; in the embodiment, a time-domain-based ultrasonic high-precision distance measurement method is designed, only addition and subtraction operations are performed on sampled analog-to-digital signals, and the operations can be realized by adopting a low-cost main control chip, so that the operation speed is high, the measured distance precision is high, and the reliability and the practicability of the product are high.
Claims (9)
1. The method for measuring the liquid level of the oil cup of the range hood is characterized in that an oil cup liquid level measuring device comprises an ultrasonic transmitting tube, a digital-to-analog converter, an ultrasonic receiving tube, an analog-to-digital converter and a main control chip, wherein the ultrasonic transmitting tube and the ultrasonic receiving tube are arranged in the same horizontal plane, and the method for measuring the liquid level of the oil cup comprises the following steps:
(1) the main control chip controls the digital-to-analog converter to generate an ultrasonic excitation signal
The ultrasonic excitation signal
The following conditions are satisfied:
wherein N is a positive integer, T is a resonance period and satisfies omegas0T2 π, UmFor ultrasonic excitation signals
Maximum voltage of, said ωs0Is the resonant angular frequency;
(2) the master control chip controls the analog-to-digital converter to start at the time when t is NT and samples the ultrasonic reflection signal;
(3) the main control chip judges whether the current sampling signal has signal mutation, if so, the next step is executed; if not, the step is circulated;
(4) the main control chip calculates the transition time delta t of the analog-to-digital converter when receiving the mutation signal;
(5) calculating the height of the liquid level
H is the height of the ultrasonic transmitting tube from the bottom surface of the oil cup, H is the liquid level height of the oil cup, and C is the propagation speed of ultrasonic waves in the air.
2. The method for measuring the liquid level of the oil cup of the range hood according to claim 1, wherein: the ultrasonic transmitting tube and the ultrasonic receiving tube respectively adopt piezoelectric ceramic transducers.
3. The method for measuring the liquid level of the oil cup of the range hood according to claim 1, wherein: the sampling frequency f of the analog-to-digital converter in the step (2)samp＝2NsampF, wherein f is an ultrasonic excitation signal
And satisfy
Said N issampWithin half a sampling periodThe number of sampling points.
4. The method for measuring the liquid level of the oil cup of the range hood according to claim 3, wherein: the number of sampling points NsampThe value range is 2-16.
5. The method for measuring the liquid level of the oil cup of the range hood according to claim 1, wherein: the sampling signal received by the analog-to-digital converter in the step (2) comprises an initial stage in which the signal amplitude gradually increases, a stable stage in which the signal amplitude is kept stable, and a sudden change stage in which the signal amplitude jumps.
6. The method for measuring the liquid level of the oil cup of the range hood according to claim 5, wherein: setting a current sampling value to be ad _ now, a previous sampling value of the current sampling value to be ad _ bef, a difference value delta ad _ now between current adjacent sampling values to be ad _ now-ad _ bef, and a difference value delta ad _ bef between previous adjacent sampling values to be ad _ bef-ad _ bef ', wherein ad _ bef' is a previous sampling value adjacent to the previous sampling value; the sampling signal of the initial stage comprises the following characteristics:
a. the maximum values ad _ max _ now of the current sample values are all larger than the maximum values ad _ max _ bef of the previous sample values;
b. the minimum value ad _ min _ now of the current sampling value is smaller than the minimum value ad _ min _ bef of the previous sampling value;
c. the maximum difference value delta ad _ max _ now between the current adjacent sampling values is larger than the maximum difference value delta ad _ max _ bef between the previous adjacent sampling values;
d. the minimum value of the difference between the current adjacent sample values Δ ad _ min _ now is greater than the minimum value of the difference between the previous adjacent sample values Δ ad _ min _ bef.
7. The method for measuring the liquid level of the oil cup of the range hood according to claim 5, wherein: setting a current sampling value to be ad _ now, a previous sampling value of the current sampling value to be ad _ bef, a difference value delta ad _ now between current adjacent sampling values to be ad _ now-ad _ bef, and a difference value delta ad _ bef between previous adjacent sampling values to be ad _ bef-ad _ bef ', wherein ad _ bef' is a previous sampling value adjacent to the previous sampling value; the sampling signal of the stable phase comprises the following characteristics:
a. the maximum value ad _ max _ now of the current sample value is equal to the maximum value ad _ max _ bef of the previous sample value;
b. the minimum value ad _ min _ now of the current sample value is equal to the minimum value ad _ min _ bef of the previous sample value;
c. the maximum value of the difference value delta ad _ max _ now between the current adjacent sampling values is equal to the maximum value of the difference value delta ad _ max _ bef between the previous adjacent sampling values;
d. the minimum value of the difference between the current adjacent sample values Δ ad _ min _ now is equal to the minimum value of the difference between the previous adjacent sample values Δ ad _ min _ bef.
8. The method for measuring the liquid level of the oil cup of the range hood according to claim 5, wherein: setting a current sampling value to be ad _ now, a previous sampling value of the current sampling value to be ad _ bef, a difference value delta ad _ now between current adjacent sampling values to be ad _ now-ad _ bef, and a difference value delta ad _ bef between previous adjacent sampling values to be ad _ bef-ad _ bef ', wherein ad _ bef' is a previous sampling value adjacent to the previous sampling value; the sampling signal of the mutation stage comprises the following characteristics:
a. the maximum values ad _ max _ now of the current sample values are all smaller than the maximum values ad _ max _ bef of the previous sample values;
b. the minimum value ad _ min _ now of the current sampling value is larger than the minimum value ad _ min _ bef of the previous sampling value;
c. the maximum difference value delta ad _ max _ now between the current adjacent sampling values is smaller than the maximum difference value delta ad _ max _ bef between the previous adjacent sampling values;
d. the minimum value of the difference between the current adjacent sample values Δ ad _ min _ now is greater than the minimum value of the difference between the previous adjacent sample values Δ ad _ min _ bef.
9. The method for measuring the liquid level of the oil cup of the range hood according to claim 1, wherein: the transition time Δ t in the step (4) is obtained by the following method:
(4.1) obtaining two current sampling points A1 and A2 with the maximum difference, wherein the sampling value corresponding to the sampling point A1 is ada1<0, and the value of the sampling point corresponding to the sampling point A1 is MAD(ii) a The sampling value ad corresponding to the sampling point A2a2>0, and the value of the sampling point corresponding to the sampling point A2 is MAD+1；
(4.2) determining the time offset of the time at which the sampled value is 0 from the sampled point A1
(4.3) obtaining the time offset of the starting moment of starting sampling of the analog-to-digital converter relative to the sampling point A1
(4.4) obtaining the transit time
Wherein f issampIs the sampling frequency.
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