Measuring and testing – Test stand – For engine
Reexamination Certificate
2000-03-22
2002-05-21
McCall, Eric S. (Department: 2855)
Measuring and testing
Test stand
For engine
Reexamination Certificate
active
06389887
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for the continuous monitoring of a mechanical component of an engine, in particular an important rotary component such as a rolling bearing or a gear of an aircraft engine, and involves monitoring in which the signal from one or more acceleration sensors is processed. The processing of the vibrational signal emanating from the sensor leads to the determination of one or more quantities resulting from or derived from the signal via the processing. These quantities are compared with thresholds so as to detect whether the monitored mechanical component has suffered damage.
2. Summary of the Prior Art
It is known to dispose acceleration sensors in places where these sensors can detect vibrations arising from the mechanical components to be monitored or from the machine in general, then to process the signals originating from the sensors so as to detect a significant abnormality of the signal transmitted by the sensor relative to the signal received in the absence of a fault.
Ideally, the detection of such abnormalities ought to allow a forecast of the remaining lifetime of the component being monitored before the damage becomes serious enough to lead to fracture or to a grave malfunction.
In practice, one generally does not have sufficient experience with regard to a large enough number of components to obtain statistically significant data for arriving at such a forecast.
In practice, the result of the processing is such that damage can be detected early enough so that at the time of detection, the member is still operating satisfactorily and one can be reasonably certain that it will continue to operate satisfactorily, preferably until a next periodic inspection of this member and at least until a next stopover when the member is mounted on a flying machine.
Patent applications EP 0889313 to 0889316 A2 filed on Jul. 3, 1998 disclose a good example of such methods.
In these applications, fifteen acceleration sensors and two azimuth sensors for sensing speeds of rotation are disposed at various sites close to members which together make up a block for transmitting motions to rotary members of a helicopter, in particular to the shaft of the main rotor and to the rear stabilising propeller.
All the signal processing methods described in these four applications comprise a step of acquiring the signal from an acceleration sensor, this involving a step of digitizing the signal from the sensor, carried out for example by means of a sample-and-hold circuit and an analog/digital converter.
The temporal signal thus sensed is transformed into a signal in the frequency domain, in each of the three application Ser. No. 08/89,313, '314 and '315. An initial sequence of values is thus obtained, each determining a vibration frequency and the amplitude associated with this frequency.
The processing described thereafter in application Ser. No. 08/89,313 provides for selecting and processing of the frequency samples so as to obtain a final sequence of samples. After returning to the time domain, an order moment of this temporal signal is calculated on the basis of the final sequence of samples and is compared with a predetermined threshold so as possibly to actuate an alarm. The method described in the '313 application is intended to detect the occurrence of an abnormality located at the level of an exterior shaft.
The processing described thereafter in the '314 application provides for the selecting of a determined frequency sample, the calculating of the amplitude of this sample and the comparison thereof with a reference value, the result being compared with a threshold.: This relatively crude method is intended to detect abnormalities which develop rapidly in flight.
The processing described thereafter in the '315 application provides for the selecting of two groups of frequency samples, the calculating of the energy associated with these two groups, a calculation of the deviation in energy between the two groups and the comparison of this deviation with a threshold. This method is intended to detect a fault on a shaft comprising two gears.
The processing described in the '316 application envisages, after the acquisition phase, a Hilbert transformation of the signal obtained, the defining of a complex number having the signal as its real part and the Hilbert transform as its imaginary part, the calculation of the phase of this complex number and of its derivative with respect to time, and lastly the comparison of this derivative with a threshold value.
In each of these four applications, ratios or variations are sought which impinge on what is called “an engagement frequency” or harmonics of this frequency. It is assumed that this involves the number of revolutions per second of the toothed wheel being monitored.
The reasons why, in these applications, certain quantities are tracked rather than others, are not explained, and therefore it is not known to what extent the teaching of these applications may be used in a different context from that described.
The teaching which may be gleaned from these examples is that the analysis of vibration signals from sensors placed close to rotating components may provide indications about the mechanical condition of these components. In particular, incipient cracking or a spreading crack can be pinpointed by virtue of such analysis. However, for each particular case, one needs to determine which frequencies are the ones which need to be analyzed and which out of all the processing possibilities are the most significant quantities to be monitored in order to obtain significant information about the mechanical components monitored.
SUMMARY OF THE INVENTION
The invention is intended to provide early detection of damage arising in particular in a revolving component of an engine for example a ball or roller bearing, or a gear. It is, however, possible to extend the field of the invention to a nonrevolving component, for example a fixed component such as a cowling, or to a component which is mobile in some other manner, for example a connecting-rod or a valve and its stem.
As in the prior art, the invention employs digital capture of the signal output by at least one acceleration sensor dedicated to the vibrational monitoring of the engine. This temporal signal is thereafter, as in the prior art, transformed into the frequency domain. It has been seen that in the prior art, a filtering is thereafter performed so as to select characteristic frequencies of the sought-after fault, and which in the aforementioned patent applications have been called “engagement frequencies”. Processing operations are then performed on these characteristic frequencies so as to obtain quantities that can be compared with thresholds so as to draw conclusions therefrom relating to possible damage to the mechanical component tracked.
It has been found that the frequency signal obtained by transforming the temporal signal emanating from the sensor is very noisy. It has also been found that the signal corresponding to what has been called the engagement frequency reaches the sensor in a very weakened state, so that it is difficult to isolate any possible presence of this engagement frequency from the ambient noise necessarily present in an engine.
It has been further found that this “engagement frequency” can be present even in the absence of any fault, merely through the fact that the teeth intermesh. What needs to be detected therefore is a significant modification of the spectrum associated with this engagement frequency, for example a modification of the number of harmonic lines of this frequency and/or of the amplitude of vibration at this engagement frequency or at its harmonics.
A hypothesis has been put forward by the inventors that this engagement frequency could be present as a modification frequency of one or more fundamental frequencies of the engine. The expression fundamental frequencies of the engine refers to the frequency of rotation—number of revolutions
Dusserre-Telmon Guy Franck Paul
Flores David
Prieux Frédéric
McCall Eric S.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Snecma Moteurs
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