FR2712699A1 - A method of measuring the rotational speed of a rotating machine generating a periodic acoustic signal. - Google Patents

Abstract

Method for measuring the speed of rotation of a rotating machine (1) generating a periodic acoustic signal, in which: - the acoustic signal of the machine comprising components is captured (10) by means of a transducer of various frequencies and - one searches (13) in the signal the component at a particular frequency (fo) of greater energy, to deduce (14) the speed of rotation of the machine (1), process in which, to carry out the search for the particular frequency component, the signal in the frequency domain is transformed (12) into a plurality of frequency components of particular energies and the one of greater energy is isolated.

Description

Method for measuring the rotational speed of a rotating machine

  generator of a periodic acoustic signal The measurement of the speed of rotation, or the determination of the speed, of an internal combustion engine is used to control, in the garage, the good functioning of the engine. On the one hand, with regard to the operation of the engine, this measure makes it possible to regulate the idling and, secondly, in view of the pollution problems, it makes it possible to maintain the engine at a determined speed of rotation to measure, in these conditions, releases of toxic gases. Indeed, and for example, as the effectiveness of a catalytic exhaust pipe is almost zero at low speed, it must, under pain of error, be

  measured at a precise speed of its efficiency range.

  For this speed measurement, two methods are mainly applied.

  The first is applied to gasoline engines. An ammeter clamp is placed on a supply wire of one of the candles and, at each ignition, it provides an electrical pulse. By counting the pulses detected during a determined duration, the frequency of the ignitions of the spark plug is calculated and, knowing the number of engine revolutions per

  ignition, we deduce the speed of rotation.

  However, the detection of ignition pulses is subject to hazards because their shape, and in particular their maximum amplitude, is very variable from one car model to another. In addition, the tendency of manufacturers is to mask the motor and the electric wires and cables, so that this process requires

a lot of manpower.

  The second method applies to diesel engines. The injection pipe is surrounded by two piezoelectric half-shells which are clamped on the

  tubing and detecting the dilatations of the tubing at each injection.

  However, in case of direct injection, there is no tubing to perform

This measure.

  Moreover, and incidentally, the document US-A-4,452,079 teaches a method in which the acoustic noise of the motor is detected by means of a

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  the microphone and performing a scanning frequency search on the obtained electrical signal by scanning the microphone signal by means of a narrow-band, band-pass filter servo-controlled by a scan control signal. When the output of the filter has a maximum amplitude, which corresponds to a value of the scan command signal, this value representing the frequency scanned at this instant is stored. The correspondence between the values of the control signal and the frequencies

  scanned provides the value of the desired rotational speed.

  The latter method makes it possible to overcome the problems of access to the motor but has limitations. Indeed, as the response time, or the inertia, of a low-pass filter increases as one reduces its bandwidth,

  any increase in frequency accuracy by reducing the band

  passing, takes place to the detriment of the necessary sweeping time, which can become prohibitive, that is to say that one will obtain with delay a precise point measurement, but no longer necessarily corresponding to the engine speed, s' he has changed in the meantime. If, on the contrary, the sweeping is too fast, the inertia of the filter creates a slenderness of a desired frequency at the

  next, so that the measured amplitude can be distorted by the previous one.

  The present invention aims at improving the measurement method by microphone and thus the detection of the desired frequency in the acoustic signal of the engine, while extending it to the speed measurement of any rotating machine.

  generator of a periodic acoustic signal.

  For this purpose, the invention relates to a method for measuring the speed of rotation of a rotating machine generating a periodic acoustic signal, in which: - the transducer is used to pick up the acoustic signal of the machine comprising components of various frequencies and the signal is sought in the signal at a particular frequency of greater energy, to deduce the speed of rotation of the machine, characterized in that, to perform the search for the component to In a particular frequency, the signal in the frequency domain is transformed into a plurality of frequency components of particular energies and the one of greater energy is isolated. Signal processing in the frequency domain makes it possible to analyze it by separating its various frequency components without interaction

  temporal, or dragging, between them.

  Preferably, portions of the acoustic signal are subjected to

Fourier transformation.

  It is thus possible, in particular, to use specialized digital circuits

  presenting a high speed of calculations and a high precision.

  Advantageously, the value of the rotational speed of the machine is estimated and the corresponding estimated frequency is deduced therefrom, a range of uncertainty surrounding the value of the estimated frequency is determined and the search for the component with a particular frequency for the components is limited. located

in the beach.

  This eliminates harmonic frequencies of the engine acoustic signal

  and frequencies due to external noise, which reduces the risk of error.

  Advantageously, the method being applied to an internal combustion engine mounted on a motor vehicle having a passenger compartment, at least one microphone is provided in the passenger compartment as a transducer and possibly two microphones, spaced apart from one another, for to emancipate

local acoustic disturbances.

  It is thus possible to have two differently disturbed signals that can be processed, either in combination or singly by switching one of the

microphones on the other.

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  The invention will be better understood with the aid of the following description of the mode of

  preferred embodiment of a measuring device for implementing the method of the invention, with reference to the appended drawing, in which: - Figure 1 is a flowchart illustrating the method of the invention; - Figure 2 is a block diagram of the measuring device and

  FIG. 3 is a spectrum of lines of the acoustic signal of an engine.

  The measuring device of the invention serves, in this example, to measure the rotational speed of a gasoline engine 1, here gasoline, a motor vehicle. The device comprises a microphone 2 picking up, in a step 10, the periodic acoustic signal of the motor 1 and transforming it into an electrical signal representing the periodic acoustic signal comprising various frequency components, due to the motor 1 and noise. This temporal signal is then converted into the frequency domain to analyze the spectrum and identify a fundamental frequency fo which corresponds to the rotational speed of the engine 1. This correspondence depends, in this example, the number of explosions per revolution of 1. The time / frequency conversion is performed by a Fourier transform which, here, is performed numerically, that is to say that it is a discrete Fourier transform. A presentation on the transformation of Fourier in the book "Complements of Mathematics" by A. ANGOT, Technical and Scientific Collection of CNET, Editions of the Review of Optics,

PARIS, 1965.

  Prior to this Fourier transformation, a sampler

  The analog / digital converter 3 receives the signal from the microphone 2 and, in a step 11, supplies a microprocessor 4 in digital samples, in which the component fo of the acoustic signal, of greater amplitude or energy, is sought at a particular frequency for deduce the

speed of rotation of the engine 1.

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  In the microprocessor 4, a circuit 5 groups, at a step 12, the successive samples in blocks, or portions, of the acoustic signal, of predetermined size N. The circuit 5 feeds a calculation block 6 performing, at this step 12, a discrete Fourier transform on each received block and thus providing, in the frequency domain, a spectrum of lines, or frequency components, represented in FIG. temporal acoustic signal of the microphone 2, each having a particular energy E. The line spectrum] 0 contains the same information as the time signal but this information is explicitly presented by the amplitude, or the energy, and the phase of each line. In a step 13 of searching for the frequency component fo of energy E the largest one proceeds in advance, in this example, to a grouping of energies E lines at very close frequencies, or packets of components, so that the The amplitudes of the lines are converted into numerical values of energies which are composed together, here added without weighting. To do this, a comparator 7 identifies the eligible lines (20-26), that is to say those likely to represent the fundamental frequency of the acoustic signal, by identifying those of greater amplitude, or energy. This comparison can be performed iteratively, by retaining each time the line of higher energy, or simultaneously by a tree, to determine the maximum value of energy, then used as a reference to identify eligible lines, whose the energy E exceeds a fraction

  predetermined of this maximum energy.

  In an adder 8, for each of these eligible rays 20

  26, the summation of the energies of all the nearby adjacent lines, 21-22, 23-26, within a reduced uncertainty range, of frequencies surrounding the considered eligible line, and a 30-32 composite energy is determined for each uncertainty range corresponding to a package of components. At the end of step 13, a comparator 9 determines which is the largest 30-32 composite energy, which makes it possible to locate, or isolate, the desired frequency fo,

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  representing the speed of rotation of the motor 1. At a step 14, the value of the frequency marked fo is converted into a value of revolutions per minute, by a

  coefficient of conversion C function of the characteristics of the engine, that is to say

  say the number of explosions per turn.

  It will be understood that, in the absence of composition of the energies, the comparator 7 and the adder 8 would be superfluous. However, their presence allows, if desired, to obtain a more reliable result. In fact, in the case where the rotational speed of the motor 1 fluctuates, or is modulated, for the duration corresponding to a block of samples, the corresponding energy is spread over a modulation band. Similarly, in the case where the ignition angles between candles are slightly different, the composite acoustic signal has a subharmonic modulation corresponding to the angular differences. Therefore, if, as shown in FIG. 3, the spectral analysis is carried out with a high frequency resolution, then on a block representing a relatively long duration during which the time signal may not be stationary, it will be detected, at the place of each line representing the fundamental frequency fo or one of its harmonics or sub-harmonics, a set of very close sub-lines 21-22, 23-26 distant from the pitch P representing the frequency resolution, and amplitudes , or energies, reduced by the spread of energy between these neighboring sub-lines. The ratios of the amplitude 21-22 representing the fundamental frequency fo relative to those representing the harmonics 23-26 and subharmonic 20, the first of which are respectively represented, may then be distorted and

  to cause an error in the speed measurement provided.

  In particular, the modulation width of the subharmonic frequencies is,

  in principle, reduced to a fraction of that of the fundamental frequency fo.

  As a result, during the separation of the frequencies, it is possible to detect, for each subharmonic, only a single line 20 of unchanged amplitude, "covering" the entire reduced modulation range, while several sub-lines 21-22, 23-26, of reduced amplitudes, will be detected for the fundamental frequency f 0 and the harmonics, which could falsify the decision in

  lack of composition of energies.

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  In addition, in order to further reduce the risk of error, it could also have been expected to estimate the speed of rotation of the motor 1 and to limit the search range of the frequency fo to the frequency components situated in a range of uncertainty surrounding the value of the estimated frequency. This estimate may, for example, be made by ear or by means of a tachometer, if

we have it.

  This limitation of the search range can be realized numerically, in the frequency domain, during the Fourier transformation or during the search for energies that follow. It can also be carried out analogically by a bandpass filter input to the converter 3, or by a digital filter at its output. Even in the absence of prior estimation of the desired frequency, a low-pass or bandpass filter makes it possible to eliminate the noises, in particular high-frequency noise, out of the search range, for example 20-200 hertz, and avoids a folding of the parts

  marginal spectrum on the search range.

  In order to limit the parasitic noise picked up by the microphone 2, it is here placed in the passenger compartment of the vehicle, its wire being pinched by a window raised. In addition, since the passenger compartment is a closed volume with stationary acoustic waves, another microphone has been provided, spaced from the microphone 2, in order to permanently have a signal at all frequencies, even when frequency has a vibration node, or resonance, at the position occupied by one of the microphones. In the case of a clearly incorrect measurement, it is then possible to switch from one microphone to another, not located near a vibration node. It is also possible, automatically before or after Fourier transform, to compose, or combine, the signals of the two microphones and to determine a composite signal corresponding, for each frequency, to the value of the largest signal. Thus choosing, for each frequency, the largest "peak" value of the two signals, ie the instantaneous temporal amplitude of the time signals before transformation or the amplitude of the lines representing the energy of the block. of samples at the frequency in question, the amplitude modulation is thus masked as a function of the frequency, due to the

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  nodes and stomachs of vibration and their displacement according to the frequency. An equalization is thus performed without distorting the amplitude of the frequency components, or lines, simultaneously present in the two signals, since this is not an addition of the amplitudes but the permanent choice of the largest signal. Use of more than two microphones

  spaced apart would further improve the equalization above.

  Moreover, in the case where the two microphones pick up the acoustic signal of the motor 1 with attenuations in a known ratio, the comparison of the two signals makes it possible to determine the parasitic noise, not having this

  ratio, and at least partially eliminate the signal used.

  The method of the invention is applicable to any machine producing a periodic acoustic signal. In particular, it can be the noise of the explosions of an internal combustion engine, the mechanical noise of a rotating machine or the noise of a non-rotating but rotating machine, in that it cyclically presents a quasistationary succession of states, some of which are related to a sound program. We can for example think of a carousel

  bottling or a printing press.

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Claims (8)

  1. A method for measuring the rotational speed of a rotating machine (1) generating a periodic acoustic signal, in which: - the transducer is used to pick up the acoustic signal of the machine comprising components of different frequencies and - looking (13) in the signal the component at a particular frequency (fo) of greater energy, to deduce (14) the speed of rotation of the machine (1), characterized by the method the fact that, in order to search for the particular frequency component, the signal in the frequency domain is transformed (12) into a plurality of frequency components of energies   particulars and isolates that of greater energy.   2. Process according to claim 1, in which portions of   acoustic signal at a Fourier transform (12).   3. Method according to one of claims 1 and 2, wherein (8)   the adjacent component frequency energies (21-22) in packets of components, to determine composite energies (31) of packet and we isolate the largest.   4. Method according to one of claims 1 to 3, wherein the estimated   value of the rotational speed of the machine (1) and the corresponding estimated frequency is deduced therefrom, a range of uncertainty surrounding the value of the estimated frequency is determined and the search for the component is limited to   frequency particular to the components in the range.   5. Method according to one of claims I to 4, applied to a motor to internal combustion. 2712699   6. The method of claim 5, wherein, the motor (1) being mounted on a motor vehicle having a passenger compartment, it is arranged in the passenger compartment.   at least one microphone (2) as a transducer.   7. The method of claim 6, wherein there is in the cockpit two microphones spaced apart from each other, to overcome disturbances local acoustics.   8. The method of claim 7, wherein the signals of the O10 microphones are combined to overcome the modulation due to the nodes and bellies of resonance in the cabin.