CN111780980A - Diesel engine rotating speed extraction method based on vibration signal envelope cepstrum analysis - Google Patents
Diesel engine rotating speed extraction method based on vibration signal envelope cepstrum analysis Download PDFInfo
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- G—PHYSICS
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- G01P—MEASURING LINEAR OR ANGULAR SPEED, ACCELERATION, DECELERATION, OR SHOCK; INDICATING PRESENCE, ABSENCE, OR DIRECTION, OF MOVEMENT
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Abstract
The invention discloses a diesel engine rotating speed extraction method based on vibration signal envelope cepstrum analysis.A vibration signal sensor is fixed at the joint part of a valve chamber cover and a cylinder sleeve and is used for obtaining a vibration signal; carrying out low-pass filtering on the vibration signal and then carrying out Hilbert envelope transformation to obtain an envelope spectrum of the vibration signal; obtaining a cepstrum through the envelope spectrum; and obtaining a characteristic spectral line by the curve of the cepstrum, obtaining the corresponding time of the characteristic spectral line, obtaining the working frequency of the corresponding cylinder, and calculating the rotating speed of the diesel engine.
Description
Technical Field
The invention relates to the field of diesel engines, in particular to a diesel engine rotating speed extraction method based on vibration signal envelope cepstrum analysis.
Background
The rotating speed is one of the important contents of diesel engine detection and is also an important calibration condition for diesel engine test and performance analysis and calculation. The conventional method for measuring the rotating speed of the diesel engine is well established, for example, photoelectric, magnetoelectric, hall-type and other sensors are mounted on a connecting part of the diesel engine or an internal rotating part for measurement. The traditional measuring method is complex in operation, and the sensor is difficult to install on certain models due to the limitation of the structure or the space in a vehicle.
In recent years, methods for measuring the rotational speed without disassembly have been proposed, and the engine rotational speed is calculated indirectly mainly by means of vibration measurement. There are roughly three categories that come together: first, vibration-rotation speed fitting method[1]The method is characterized in that an engine appendage vibration signal is converted into a voltage signal by adopting a vibration sensor, single-point virtual vibration power corresponding to a mounting point of the vibration sensor is calculated, and then rotation speed measurement can be realized according to a corresponding relation of rotation speed and single-point virtual vibration power fitted by prior experimental data, but fitting errors are easily introduced by the method. The second method is a time period extraction method, which extracts the peak value of the fuel injector needle valve seating signal from the vibration signal of the engine cylinder head, calculates the working cycle period of the engine, and further realizes the measurement of the engine speed, but the vibration randomness can cause errors in the direct extraction of time points. And thirdly, a vibration frequency extraction method, which adopts a frequency spectrum analysis method to extract vibration fundamental frequency and further converts the vibration fundamental frequency into rotating speed. In order to improve the calculation accuracy, such methods are usually used in combination with some preprocessing techniques, such as short-time processing, autocorrelation, fourier transform, wavelet analysis, etc., or in combination with these methods. The methods have higher precision, but sometimes the spectral lines distributed on the frequency spectrum are not single, and the searching algorithm is more complex when the frequency is extracted by programming.
Aiming at the problem that a rotating speed sensor is not convenient to install when the state of a heavy diesel engine is detected, a method for analyzing envelope cepstrum of a vibration signal is provided, the purpose of obtaining a single spectral line is achieved, and further the rotating speed information of the diesel engine is easier to extract.
Disclosure of Invention
The invention designs and develops a diesel engine rotating speed extraction method based on vibration signal envelope cepstrum analysis, which measures a vibration signal of a cylinder cover of a diesel engine by using a vibration acceleration sensor, performs cepstrum analysis on an envelope signal and successfully extracts rotating speed information.
A diesel engine rotating speed extraction method based on vibration signal envelope cepstrum analysis,
the vibration signal sensor is fixed at the joint of the valve chamber cover and the cylinder sleeve and used for obtaining a vibration signal;
carrying out low-pass filtering on the vibration signal and then carrying out Hilbert envelope transformation to obtain an envelope spectrum of the vibration signal;
obtaining a cepstrum through the envelope spectrum;
and obtaining a characteristic spectral line by the curve of the cepstrum, obtaining the corresponding time of the characteristic spectral line, obtaining the corresponding frequency, and calculating the rotating speed of the diesel engine.
As a further preference, the Hilbert transform essentially transforms a real signal into an analytic signal, the modulus of which is the envelope signal; the method for extracting the real signal envelope by the Hilbert transform is as follows:
complex analytic expressions of real signal s (t):
in the formula, H [ s (t) ] is a Hilbert transformation form of a raw signal s (t), and the transformation process is shown as the following formula:
As a further preference, the mathematical expression of the cepstral function is:
CF(q)＝F{logSx(f)}2
wherein S isx(f) Is the power spectrum of the envelope signal; f { } denotes that the argument q for fourier transforming the signal is the reciprocal frequency.
Preferably, the mathematical expression of the cepstrum function is further transformed,
the invention has the following beneficial effects:
compared with the traditional rotating speed measuring method, the method for indirectly extracting the rotating speed information by using the diesel engine vibration signal has the advantages of simpler and faster detection, higher precision and capability of meeting the requirement of real vehicle detection. The method for extracting the rotating speed of the diesel engine by using the vibration signal envelope cepstrum analysis technology overcomes the defect that the traditional frequency spectrum has many spectral lines and is inconvenient for automatically searching characteristic spectral lines, and test verification shows that the method has higher detection precision; the problem that spectral lines on a general spectrogram are many and difficult to identify is solved. Tests show that the rotating speed calculation method is high in precision.
Drawings
FIG. 1 is a time domain waveform of a vibration signal of a diesel engine according to the present invention.
FIG. 2 is a filtered FFT amplitude spectrum of a vibration signal of a diesel engine according to the present invention.
FIG. 3 is a global envelope of the vibration signal of the diesel engine of the present invention.
Fig. 4 is an envelope spectrum of a diesel engine vibration signal according to the present invention.
FIG. 5 is a cepstrum of an envelope of a diesel engine vibration signal in accordance with the present invention.
Detailed Description
The present invention is further described in detail below with reference to the attached drawings so that those skilled in the art can implement the invention by referring to the description.
The vibration signal of the cylinder cover of the diesel engine is a typical non-stationary signal, the excitation sources are numerous, the action time presents a certain sequence, and a certain explosion sequence exists among cylinders. The diesel engine operating mode according to the crank angle determines the periodicity of the cylinder head vibration signal, so that the frequency of the vibration exciting signal and the crank speed are in a certain proportional relation when the diesel engine is in operation, and the mathematical relation can be expressed as
In the formula (1), n is the crankshaft speed, f0For the fundamental frequency of vibration, τ is the number of cylinders, and i is the number of strokes.
In-cylinder combustion excitation, valve seating and needle valve seating excitation all act directly on the cylinder head as seen in the transmission path, and the cylinder head is therefore the most desirable vibration signal pickup location. Structurally, the upper part of the cylinder head is a valve chamber cover, and the part is of a thin-walled structure, which is likely to cause frequency change, and thus cannot be used as a measurement point position. The cylinder bank is of a V-shaped structure, the exhaust passage is arranged on the outer side of the V-shaped clamp angle, heat radiation is large, and the performance of the sensor is influenced and even the sensor is damaged. In comprehensive consideration, the measuring point is preferably selected at the joint part of the valve chamber cover and the cylinder sleeve.
According to the analysis, the measuring point is selected on the side surface of the joint of the valve chamber cover and the cylinder sleeve for the convenience of sensor installation. Preferably, in order to verify the rotating speed measurement error, a photoelectric sensor is arranged at the output end of the crankshaft at the same time during bench test, and the rotating speed of the engine is measured and measured for comparison.
According to the method and the device, the low-pass filtering is firstly carried out, and then the Hilbert envelope transformation is carried out, so that the high-frequency interference in the signal can be removed, and the information contained in the signal can be more effectively and really obtained.
The Hilbert transform essentially transforms a real signal into an analytic signal, the norm of which is the envelope signal. The method for extracting the real signal envelope by the Hilbert transform is as follows:
the complex analytic expression of the real signal s (t) is shown in formula (2).
In the formula, the left end of the equal sign is capital S which represents S (t), and the right end of the equal sign is lowercase S which represents S (t);
the method is a Hilbert transformation form of an original signal s (t), and the transformation process is shown as formula (3).
And extracting real signal envelope through the Hilbert transform to obtain an envelope spectrum of the vibration signal.
The general spectrogram has excessive frequency difference and is difficult to identify, and the inverse spectrogram is favorable for identification.
The mathematical expression of the cepstral function is
CF(q)＝|F{log Sx(f)}|2(4)
Wherein Sx(f) Is the power spectrum of the envelope signal; f { } denotes that the argument q for fourier transforming the signal is the reciprocal frequency. A commonly used form of engineering is the open-square form of equation (4), called the amplitude cepstrum.
The result of the further transformation is that,
from the above, it can be seen that the cepstrum is a fast fourier transform of the logarithm of the spectrum, which is a spectrum of a spectrum, one of its powerful characteristics being that any periodic or repeating pattern in the spectrum will be identified as one or two specific components in the cepstrum. Therefore, cepstrum analysis can solve the problem of spectral line confusion existing in general spectral analysis, thereby being easier to extract characteristic spectral lines.
A curve of a cepstrum can obtain a remarkable spectral line, the abscissa of the cepstrum is time, and the middle coordinate is amplitude; and obtaining the corresponding time of the obvious spectral line and then obtaining the corresponding frequency, which is the working frequency of the cylinder.
The diesel engine speed is calculated therefrom.
Examples
The test is carried out on a certain heavy vehicle, the rotating speed of a diesel engine is set to be 1000r/min according to a tachometer, a cylinder cover vibration signal of a left 1 cylinder and a photoelectric sensor signal of a crankshaft output end are synchronously collected, the sampling frequency is 51.2kHz, and the time domain waveform of the vibration signal is shown in figure 1.
And (4) performing denoising pretreatment by adopting a low-pass filtering method.
Because the engine is a V-shaped 12-cylinder four-stroke diesel engine, when the highest rotating speed is 2200r/min, f024.44Hz, frequency conversion fnN/60-36.67 Hz, cylinder burst frequency fc＝fn18.33Hz and cylinder burst interval frequency fi＝6fn＝220Hz。
Therefore, when low-pass filtering is performed, the cut-off frequency is taken as 350 Hz. Fig. 2 is a filtered FFT magnitude spectrum with peak line extractions as shown in table 1.
TABLE 1 amplitude Spectroscopy
Theoretically when the rotating speed is 1000r/min, f011.11Hz, frequency conversion fn16.67Hz, cylinder operating frequency fc8.33Hz, cylinder burst interval frequency f i100 Hz. According to the actual spectral analysis, the frequency conversion fn17.7532Hz, cylinder operating frequency fc8.8766Hz, cylinder burst interval frequency fi＝106.5192Hz。 f0Not at the peak frequency. Therefore, the engine speed n is calculated to be 1065 r/min. The main reason is that the rotation speed value controlled by the driver according to the indication of the tachometer is an estimated value during the real vehicle detection, and is not completely consistent with the actual rotation speed.
From the analysis, it is seen that there are many spectral lines in the amplitude spectrum analysis, the amplitude is maximum at the cylinder explosion interval frequency, and the frequency at the maximum peak can be extracted to convert the rotating speed. However, due to the non-stationarity of the vibration signal, there is a phenomenon that the amplitude is not the largest at the frequency of the cylinder explosion interval, so that it is very difficult to find the frequency among many frequencies.
According to the method and the device, the low-pass filtering is firstly carried out, and then the Hilbert envelope transformation is carried out, so that the high-frequency interference in the signal can be removed, and the information contained in the signal can be more effectively and really obtained.
The Hilbert transform essentially transforms a real signal into an analytic signal, the norm of which is the envelope signal. The method for extracting the real signal envelope by the Hilbert transform is as follows:
the complex analytic expression of the real signal s (t) is shown in formula (2).
In the formula (I), the compound is shown in the specification,
the method is a Hilbert transformation form of an original signal s (t), and the transformation process is shown as formula (3).
The envelope of the transformed vibration signal is shown in fig. 3, and the envelope spectrum is obtained as shown in fig. 4. From FIG. 4, it can be found that the frequency f of the cylinder explosion intervaliThis is slightly different from the amplitude spectrum, mainly due to frequency resolution, at 107.03Hz, and still presents the problem of spectral line redundancy.
The general spectrogram has excessive frequency difference and is difficult to identify, and the inverse spectrogram is favorable for identification.
The mathematical expression of the cepstral function is
CF(q)＝|F{log Sx(f)}|2(4)
Wherein Sx(f) Is the power spectrum of the signal x (t), and F { } represents the fourier transform of the signal. A commonly used form of engineering is the open-square form of equation (4), called the amplitude cepstrum.
From the above, it can be seen that the cepstrum is a fast fourier transform of the logarithm of the spectrum, which is a spectrum of a spectrum, one of its powerful characteristics being that any periodic or repeating pattern in the spectrum will be identified as one or two specific components in the cepstrum. Therefore, cepstrum analysis can solve the problem of spectral line confusion existing in general spectral analysis, thereby being easier to extract characteristic spectral lines.
From fig. 5 it is clear that a significant line corresponds to a time of 0.11199s and a frequency of 8.9293Hz, which is the cylinder operating frequency. From this, the frequency f can be calculatedn17.8587Hz, and the calculated rotating speed is n 1071.52 r/min. The engine speed, measured from the crankshaft output, was 1078.13 r/min, from which an error of 0.61% was calculated.
While embodiments of the invention have been described above, it is not limited to the applications set forth in the description and the embodiments, which are fully applicable to various fields of endeavor with which the invention may be practiced, and further modifications may readily be effected therein by those skilled in the art, without departing from the general concept as defined by the claims and their equivalents, which are not limited to the details given herein and the examples shown and described herein.
Claims (4)
1. A diesel engine rotating speed extraction method based on vibration signal envelope cepstrum analysis is characterized in that,
the vibration signal sensor is fixed at the joint of the valve chamber cover and the cylinder sleeve and used for obtaining a vibration signal;
carrying out low-pass filtering on the vibration signal and then carrying out Hilbert envelope transformation to obtain an envelope spectrum of the vibration signal;
obtaining a cepstrum through the envelope spectrum;
and obtaining a characteristic spectral line by the curve of the cepstrum, obtaining the corresponding time of the characteristic spectral line, obtaining the working frequency of the corresponding cylinder, and calculating the rotating speed of the diesel engine.
2. The diesel engine speed extraction method based on vibration signal envelope cepstrum analysis as claimed in claim 1, wherein the Hilbert transform is to transform real signals into analytic signals, the modulus of which is envelope signals; the method for extracting the real signal envelope by the Hilbert transform is as follows:
complex analytic expression of real signal s (t):
in the formula (I), the compound is shown in the specification,
the Hilbert transform of s (t) is an original signal, and the transform process is shown as the following formula:
3. The diesel engine speed extraction method based on vibration signal envelope cepstrum analysis according to claim 1 or 2,
the mathematical expression of the cepstral function is:
CF(q)＝|F{logSx(f)}|2
wherein S isx(f) Is the power spectrum of the envelope signal; f { } denotes that the argument q for fourier transforming the signal is the reciprocal frequency.
4. The diesel engine speed extraction method based on vibration signal envelope cepstrum analysis according to claim 3,
the mathematical expression of the cepstrum function is further transformed,
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