System for acquiring a vibratory signal of a rotary motor

A method and a system for acquiring a vibratory signal for troubleshooting a rotary motor, including: an input receiving a temporal vibratory signal of the motor and at least one current rotational speed of at least one shaft of the motor, and a sampling mechanism sampling the temporal vibratory signal in real time with at least one sampling signal synchronised with the at least one current rotational speed thus generating a corresponding synchronous vibratory signal.

FIELD OF THE INVENTION

The present invention relates to the field of acquiring vibratory signals of a motor and more particularly, acquiring vibratory signals for on-board motor troubleshooting.

STATE OF THE RELATED ART

A rotary motor is subject to mechanical stress liable to give rise to wear of the rotary elements thereof. One effective way of monitoring the wear or condition of a motor is that of measuring the motor vibrations.

More particularly, in the case of an aircraft motor, the latter comprises vibration sensors of the accelerometer type for detecting the vibrations emitted by the motor. The vibratory signals collected are then analysed to detect anomalies or defects of one or a plurality of rotary components. This analysis comprises a frequency analysis of the signals detected by the vibration sensors.

At the present time, vibratory analysis requires signal sampling operations at very high single constant frequency and signal oversampling operations at frequencies proportional to the harmonics to be analysed and the multiple harmonics thereof. Furthermore, it is necessary to apply very narrow tracking band-pass filters pre-programmed for each harmonic ratio and controlled by the motor rotational speed indicators.

The re-sampling operations essential for filtering harmonics require interpolations which are very costly in computing size and which need to be performed on a very large number of points. Furthermore, in order to ensure sufficient precision on the values of the interpolated filtered signals, it is important to perform very high-frequency acquisitions.

Finally, the interpolated signals are oversampled and involve performing Fourier transforms on a very large number of points. Moreover, the filtering operations require frequency analysis over the entire bandwidth of the signal, which is very costly in computing time.

In this way, the computing capacities of the on-board electronics are considerably monopolised by all these operations.

Consequently, the aim of the present invention is that of providing a system and a method for acquiring a vibratory signal in real time without involving the abovementioned drawbacks and in particular, using simplified computations requiring reduced electronic means.

DESCRIPTION OF THE INVENTION

The present invention is defined by a system for acquiring a vibratory signal for troubleshooting a rotary motor, comprising:

input means for receiving a temporal vibratory signal of said motor and at least one current rotational speed of at least one shaft of said motor, and

sampling means for sampling of said temporal vibratory signal in real time with at least one sampling signal synchronised with said at least one current rotational speed thus generating a corresponding synchronous vibratory signal.

In this way, by directly sampling the vibratory signal at frequencies proportional to the rotation of the motor, this system makes it possible to minimise the computing time and the data storage volume. For example, in the case of an aircraft motor, the acquisition system may thus be advantageously used for on-board motor troubleshooting without monopolising the computing time or the memory space of an on-board computer.

Advantageously, said sampling signal is configured with a predefined maximum harmonic ratio and a predefined sampling ratio.

In this way, it is possible to predefine the maximum harmonics to be retrieved while simplifying the processing of the synchronous vibratory signal.

According to one specific feature of the present invention, the system comprises a buffer for buffering a sample consisting of a predefined number of periods of said synchronous vibratory signal, the temporal length of said buffer being determined according to a minimum harmonic ratio.

Selecting a reduced number of referenced points with respect to the rotation of the motor makes it possible to only occupy a reduced memory space while simplifying frequency signal retrieval.

Advantageously, the system comprises computing means for retrieving, from said buffered sample, frequency signals at multiple harmonics of the minimum harmonic and frequencies proportional to the corresponding current rotational speed.

This enables direct retrieval of the sought harmonics without using resampling or interpolation techniques.

Advantageously, the computing means are configured to retrieve said frequency signals by multiplying said buffered sample with Fourier coefficients of only the harmonics to be retrieved.

In this way, the use of Fourier coefficients on the entire analysis band is avoided.

According to one preferred embodiment of the present invention, the input means are configured to receive first and second current rotational speeds relative to first and second shafts of said motor respectively, and

the sampling means are configured to directly generate first and second synchronous vibratory signals by sampling said temporal signal in real time with, respectively, a first sampling signal synchronised with said first current rotational speed, and a second sampling signal synchronised with said second current rotational speed.

This makes it possible in the case of an aircraft motor to reduce the on-board computing in the event of limited computing powers on an on-board computer on the motor or on the aircraft.

Advantageously, the sampling means are configured to directly generate a third synchronous vibratory signal by sampling said temporal vibratory signal in real time with a third sampling signal synchronised with the sum or the difference of said first and second current rotational speeds, said third sampling signal being reconstituted from a trigonometric combination of said first and second sampling signals.

In this way, a mere trigonometric computation is sufficient to directly generate a synchronous vibratory signal with the sum or the difference of the rotational speeds.

The system comprises first, second and third buffers for buffering respectively, a first sample consisting of a predefined number of periods of said first synchronous vibratory signal, a second sample consisting of a predefined number of periods of said second synchronous vibratory signal, and a third sample consisting of a predefined number of periods of said third synchronous vibratory signal, and in that the computing means are configured to respectively retrieve from said first, second and third buffered samples, first frequency signals at frequencies proportional to said first current rotational speed, second frequency signals at frequencies proportional to said second current rotational speed, and third frequency signals at frequencies proportional to said sum or difference of said first and second current rotational speeds.

The invention also relates to a system for monitoring a rotary motor, comprising the acquisition system according to any of the above features, and further comprising analysis means for analysing the frequency signal(s) in order to troubleshoot the status of the motor.

The invention also relates to a method for acquiring a vibratory signal for troubleshooting a rotary motor, comprising the following steps:

receiving a temporal vibratory signal from said motor and at least one current rotational speed N(t) of at least one shaft of said motor, and

sampling said temporal vibratory signal in real time with at least one sampling signal synchronised with said at least one current rotational speed for generating a corresponding synchronous vibratory signal.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

The underlying concept of the invention is based on the acquisition of vibration signals at frequencies directly synchronised with the rotational signals of the motor.

FIG. 1schematically illustrates a system for acquiring a vibratory signal of a rotary motor, according to the invention.

The acquisition system1comprises input means3and sampling means5.

The input means3are configured to receive a temporal vibratory signal X(t) representing the operating status of the motor7. The vibratory signal is obtained from at least one vibration sensor9of the accelerometer type installed on the motor7.

Furthermore, the input means3are configured to receive at least one current rotational speed N(t) of at least one shaft11of the motor7. It should be noted that the motor7may comprise two or more rotors comprising shafts rotating at different speeds.

The sampling means5are configured to sample the temporal vibratory signal X(t) in real time with a sampling signal synchronised with the current rotational speed N(t) thus generating a corresponding synchronous vibratory signal x(nt).

FIG. 2Aillustrates an example of sampling of a temporal vibratory signal according to the invention.

The temporal vibratory signal X(t) is a continuous signal over time acquired for example at a frequency of the order of 250 kHz.

The sampling signal S is a square signal synchronised with the rotational speed N(t) of the motor7. Furthermore, the sampling signal S is configured with a predefined maximum harmonic ratio kh and a predefined sampling ratio r. In this way, the sampling frequency S has a frequency Sf=r×N×kh. In other words, the sampling frequency varies in real time with the rotational speed of the motor7and is dependent on the maximum order kh of the harmonic to be retrieved and the minimum number of points per period sought (for example, 6 to 8 points). According to the example inFIG. 2A, the maximum harmonic is three (kh=3) and the sampling ratio is eight (r=8).

At each rising edge, the temporal vibratory signal X(t) is sampled to generate the synchronous vibratory signal x(nt). The signal x(nt) is then an undersampled discrete signal at a synchronous frequency with the rotational speed N(t) of the motor7.

In this way, the temporal vibratory signal X(t) is directly converted into a digital signal x(nt) synchronised with the rotational speed N(t) of the motor.

Processing means13are then used to apply a Fourier transform to the synchronous vibratory signal x(nt) in order to retrieve frequency signals proportional to the rotational speed N(t) of the motor7.

The processing means13may be comprised in the acquisition system1as illustrated inFIG. 1. Alternatively, they may be part of another electronic system (not shown) linked with the acquisition system1.

The processing means13comprise computing means15and storage means17comprising at least one buffer19. The storage means17may comprise a computer code program for the implementation of the acquisition method according to the invention.

Advantageously, the buffer19is configured to buffer a sample consisting of a predefined number of periods of the synchronous vibratory signal x(nt). The temporal length of the buffer19is determined according to a minimum harmonic ratio. The example inFIG. 2Aillustrates a sample consisting of two periods of the synchronous vibratory signal x(nt) based on a minimum harmonic having a period equal to 0.1 s. This helps save memory space since the buffer19merely needs to store a very reduced number of points of the synchronous vibratory signal (for example, 8 points per harmonic).

Advantageously, the computing means15are configured to retrieve frequency signals X1, . . . Xkhby multiplying the buffered sample point to point with Fourier coefficients of only the harmonics to be retrieved and not on the entire analysis band. These frequency signals X1, . . . Xkhhave multiple harmonics of the minimum harmonic and frequencies proportional to the corresponding rotational speed N(t) (seeFIG. 2B).

FIG. 2Billustrates an example of the real parts of the Fourier coefficients of harmonics1,2, and3applied to the buffered sample represented inFIG. 2Ato retrieve three frequency signals X11, X12, and X13having the orders kh=1, kh=2 and kh=3 respectively. The imaginary parts (not shown) of the Fourier coefficients are subject to a phase shift of π/2.

In this way, the present invention makes it possible to retrieve the harmonic components directly at multiple frequencies of the rotation of the motor in a very reduced number of operations, without interpolations, and only storing a very reduced number of points in memory. This makes it possible to save considerable computing time and memory space.

It should be noted that the acquisition system and method apply to any type of rotary motor. In the case described hereinafter, the acquisition of a vibratory signal for the on-board troubleshooting of an aircraft motor will particularly be discussed.

FIG. 3illustrates a monitoring system for the on-board troubleshooting of the status of an aircraft motor, according to the invention.

The monitoring system2comprises an acquisition system1and an anomaly detection system21.

The aircraft motor7comprises a low-pressure compressor23upstream from a high-pressure compressor25and a high-pressure turbine27upstream from a low-pressure turbine29. The low-pressure compressor23and turbine29are coupled by a first shaft11ahaving a rotational speed N1. Similarly, the high-pressure compressor25and the turbine27are coupled by a second shaft11bhaving a rotational speed N2. The second shaft11bis a tube which is coaxial with the first shaft11aand the two shafts are separated by an inter-shaft bearing (not shown). The two shafts11a,11bmay be contra-rotating and the bearings then have a rotational speed N1+N2. Alternatively, the two shafts may be co-rotating and the inter-shaft bearings then have a rotational speed N1−N2.

Vibration sensors9of the accelerometer type are placed in the motor7for detecting the vibrations emitted thereby. Furthermore, the motor7comprises censors31for measuring the first and second rotational speeds N1, N2of the first and second shafts11a,11brespectively.

In this way, to correctly diagnose the status of the motor7in operation, the present invention proposes to retrieve directly and in real time three groups of frequency signals respectively proportional to the rotational speeds N1, N2, and N1+N2for detecting in real time any abnormal operation of any of the components of the motor7.

FIGS. 4 and 5illustrate, respectively, an algorithm and a block diagram for acquiring and processing a vibratory signal of a motor according toFIG. 3.

In steps E1-E3(blocks B1-B3), the input means3receive, during a predefined period of operation of the motor7, a temporal vibratory signal X(t) representing the operating status of the motor and the first and second current speed N1(t) and N2(t) relative, respectively, to the first and second shafts11a,11bof the motor7.

The predefined period during which the temporal vibratory signal X(t) and the current speeds N1(t) and N2(t) are obtained may for example correspond to a particular flight phase or to a complete flight.

In steps E4-E9(blocks B4-B9), the sampling means5are configured to directly generate first and second synchronous vibratory signals x1(nt) and x2(nt) by sampling in real time the temporal vibratory signal X(t) with, respectively, a first sampling signal S1synchronised with the first current rotational speed N1(t), and a second sampling signal S2synchronised with the second current rotational speed N2(t).

More particularly, in steps E4and E5(blocks B4and B5), the first and second sampling signals are generated.

The first sampling signal S1has a frequency defined according to the first speed N1, having a predefined maximum harmonic ratio kh and a predefined sampling ratio r. By way of example, the frequency of the first sampling signal is Sf1=8×N1×kh1. The sampling ratio is in this case chosen to be equal to eight in order to facilitate the Fourier transform computations.

Similarly, the second sampling signal S2is a signal wherein the frequency is defined according to the second speed N2, having a predefined maximum harmonic ratio kh2and a predefined sampling ratio r. By way of example, the frequency of the second sampling signal is Sf2=8×N2×kh2.

In steps E6and E7, the rising edges of the first and second sampling signals S1, S2are detected in order to form square signals to sample the temporal vibratory signal X(t).

In step E8(blocks B81, B82), the temporal signal X(t) is first filtered using a first low-pass filter B81wherein the cut-off frequency is dependent on the maximum frequency of the harmonic kh1to be retrieved. Alternatively, the first low-pass filter B81is controlled by the first sampling signal S1wherein the instantaneous frequency is proportional to the first rotational speed N1(t). Filtering the vibratory signal X(t) upstream from the sampling makes it possible to prevent any risk of spectral aliasing.

The vibratory signal X(t) filtered above is then sampled by a first asynchronous DAC analogue-digital converter B82according to each rising edge of the first sampling signal to generate a first synchronous vibratory signal x1(nt).

Similarly, in step E9(blocks B91, B92), the temporary signal X(t) is filtered using a second low-pass filter B92wherein the cut-off frequency is dependent on the maximum frequency of the harmonic kh2to be retrieved, or is controlled by the second sampling signal S2wherein the instantaneous frequency is proportional to the second rotational speed N2(t). The filtered vibratory signal X(t) is then sampled by a second asynchronous DAC B92according to the second sampling signal S2to generate a second synchronous vibratory signal x2(nt).

The signals x1(nt) x2(nt) are discrete signals respectively synchronised with the rotational speeds N1and N2.

In step E10(blocks B10), a first sample consisting of a predefined number of periods of the first synchronous vibratory signal x1(nt) is buffered in a first buffer B10wherein the temporal length is determined according to the minimum harmonic ratio h1.

Similarly, in step E11(blocks B11), a second sample consisting of a predefined number of periods of the second synchronous vibratory signal x2(nt) is buffered in a second buffer B11wherein the temporal length is determined according to the minimum harmonic ratio h2.

The first and second buffers B10, B11are respectively activated at each turn cue of the rotations N1and N2(blocks B101, B111). Indeed, the downstream Fourier transform computations are performed at submultiple frequencies of the buffer refresh rate. The execution frequencies of these computations are synchronised at the rotational speeds of the motor shafts.

In steps E12-E17(block B12-B17), the computing means will retrieve first and second groups of frequency signals X11, . . . X1kh1and X21, . . . X2kh2.

More particularly, in step E12(block B12), the computing means15generate first Fourier coefficients of only the harmonics to be retrieved in respect of the first rotational speed N1: (sin(2πnk)+j cos(2πnk))/8×Nh, the increment of the Fourier analysis nk verifying 0<nk<kh1×8−1; Nh is the number of the harmonic computed where Nh=1,2, . . . , kh1and kh1is the maximum order of the harmonic to be analysed for the rotational speed N1.

In step E13(block B13) the computing means15generate second Fourier coefficients of only the harmonics to be retrieved in respect of the second rotational speed N2: (sin(2πnk)+j cos(2πnk))/8×Nh, the increment of the Fourier analysis nk verifying 0<nk<kh2×8−1; Nh is the number of the harmonic computed where Nh=1,2, . . . , kh2and kh2is the maximum order of the harmonic to be analysed for the rotational speed N2.

In step E14(block B14), the first Fourier coefficients are multiplied in a matrix with the first sample of the first synchronous vibratory signal x1(nt) to generate the first group of frequency signals X11, . . . X1kh1(E16, B16).

In step E15(block B15), the second Fourier coefficients are multiplied in a matrix with the second sample of the second synchronous vibratory signal x2(nt) to generate the second group of frequency signals X21, . . . X2kh2(E17B17).

In steps E19-E27(blocks B19-B27), the sampling means5are further configured to directly generate a third synchronous vibratory signal x3(nt) by sampling the temporal vibratory signal X(t) in real time with a third sampling signal S3synchronised with the sum N1+N2of the first and second current rotational speed.

In steps E19-E21, the third sampling signal S3is reconstituted by the processing means13from a trigonometric combination of the first and second sampling signals S1, S2.

Indeed, in step E19(block B19), a first intermediate synchronisation signal is generated, wherein the frequency is defined according to the first speed N1and a predefined maximum harmonic ratio kh3. In particular, from the first sampling signal (step E4, block B4), a sinusoidal sine signal and a sinusoidal cosine signal having the frequency N1*kh3*8: sin(8×N1×kh3) and cos(8×N1×kh3) are generated.

Similarly, in step E20(block B20), a second intermediate synchronisation signal is generated, wherein the frequency is defined according to the second speed N2and a predefined maximum harmonic ratio kh3. In particular, from the second sampling signal (step E5, bloc B5), a sinusoidal sine signal and a sinusoidal cosine signal having the frequency N2*kh3*8: sin(8×N2×kh3) and cos(8×N2×kh3) are generated.

In step E21, multiplication is performed (B211, B212) of sin(8×N1×kh3) from step E19with cos(8×N2×kh3) from step E20and cos(8×N1×kh3) from step E19with sin(8×N2×kh3) from step E20to respectively form the signals sin(8×N1×kh3)×cos(8×N2×kh3) and cos(8×N1×kh3)×sin(8×N2×kh3). In block B213, these latter two signals are added to form a signal having the format sin(8×kh3×(N1+N2)). On the basis of this signal, a third sampling signal S3is determined wherein the frequency is defined according to the sum of the first speed N1and the second speed N2and a predefined maximum harmonic ratio kh3and a predefined sampling ratio r (in this instance r=8).

It should be noted that if the first and second shafts of the motor11a,11bare co-rotating, it is sufficient to replace the addition of block B213by a subtraction to generate a sampling signal wherein the frequency is defined according to the difference between the first speed N1and the second speed N2.

In step E22(block B22), the rising edges of this sampling signal are retrieved to form a square signal for sampling the temporal signal X(t).

Indeed, in step E23(blocks B231, B232), the temporal vibratory signal X(t) is first filtered using a third low-pass filter B231wherein the cut-off frequency is dependent on the maximum frequency of the harmonic kh3to be retrieved. Alternatively, the third low-pass filter B231is controlled by the third sampling signal S3. The filtered vibratory signal X(t) is then sampled by a third asynchronous DAC B232according to the third sampling signal S3to generate a third synchronous vibratory signal x3(nt). In this way, the signal x3(nt) is a discrete signal synchronised with the rotational speed N1+N2.

In step E24(blocks B24), a third sample consisting of a predefined number of periods of the third synchronous vibratory signal x3(nt) is buffered in a third buffer B24wherein the temporal length is determined according to the minimum harmonic ratio h3. The third buffer B24is activated at each turn cue of the rotation N1+N2(block B241).

In step E25(block B25), the computing means13generate third Fourier coefficients of only the harmonics to be retrieved in respect of the rotational speed N1+N2: (sin(2πnk)+j cos(2πnk))/8×Nh, the increment of the Fourier analysis nk verifying 0<nk<kh3×8−1; Nh is the number of the harmonic computed where Nh=1, 2, . . . , kh3and kh3is the maximum order of the harmonic to be analysed for the rotational speed N1+N2.

In step E26(block B26), the third Fourier coefficients are multiplied in a matrix with the third sample of the third synchronous vibratory signal x3(nt) to generate a third group of frequency signals X31, . . . X3kh3(E27, B27).

The system or method according to the invention does not use under or oversampling operations, and uses FFT simplified Fourier transform computations. Indeed, only the signals with the relevant harmonics are retrieved for the troubleshooting of the motor with minimum computing and memory. Furthermore, tracking filters are not used.

In this way, the required performance of the on-board memory (RAM memory, computing speed) is reduced. Furthermore, the overheating induced is reduced and the operating ranges are increased.

Moreover, the first, second and third groups of frequency signals are suitable for respectively troubleshooting the first shaft, the second shaft and the inter-shaft bearings of the motor.

Indeed, the detection system21(seeFIG. 3) retrieves the first, second and third groups of frequency signals in real time to analyse same. The detection system21comprises analysis means23for, for example, correlating the frequency signals with other signals or for comparing same to predefined threshold values to monitor the status of the motor7in real time. The detection system may, for example, monitor the progression in amplitude of the various harmonics of the frequency signals with respect to corresponding relative thresholds. A threshold overshoot may thus activate alarms or warning messages31.

Alternatively, the analysis of the frequency signals may be carried out offline so as to minimise the computing time further during the flight.

Advantageously, the first, second and third groups of frequency signals may be stored from flight to flight in a database for analysing the progression of the status of the motor7over time.

It should be noted that the monitoring system may be integrated in a specific unit or be part of an existing electronic unit. Advantageously, the acquisition and processing means of an on-board computer in the aircraft or in a computer integrated in the aircraft motor of the EMU (Engine Monitoring Unit) type may be used to operate the system for acquiring vibratory signals and troubleshooting the motor according to the invention.