Device for the activation of an apparatus for measuring acoustic emission by detection of background noise

An apparatus for measuring acoustic emission is activated only at critical periods which are determined by detection of the background noise from a reference level. An acoustic emission transducer continuously transmits a signal to a detection circuit which delivers the top modulation envelope of the background noise to a comparator at a reference level delivered by a control unit. When a mean level of the background noise does not exceed a threshold reference level the AE is sampled at a low rate, f. When the background noise exceeds the threshold reference level the data sampling unit is activated to sample AE at a much higher rate F. Therefore AE is sampled at a high rate only when the pump or other machine under test is running and at a low rate when the article under test is not operating. The AE testing device also includes a frequency spectrum correlator which allows background noise to activate the increased sampling rate only if the noise is in a given frequency range.

This invention relates to a device for the activation of an apparatus which 
serves to measure acoustic emission by detection of background noise. The 
invention finds an application in the field of industrial control by 
acoustic emission. 
In industrial installations subjected to high stresses, there exist a 
certain number of sound-generating phenomena in which sounds are 
propagated in structures and which are of two types: 
percussion impacts of solid portions on stationary obstacles (detachment of 
turbine blade), 
crack formation (in concrete blocks). 
Different types of sounds or noises are also propagated in other frequency 
bands. Noteworthy examples are noises arising from the flow of fluids in 
piping systems, vibrations of unbalanced rotating parts, and so on. 
Systems for acquisition of data by acoustic emission must therefore store 
a large number of unnecessary data. In fact, in the example of a fluid 
circuit with a pump, the potential danger of projection of solid particles 
appears as critical only if the pump is in operation. In acquisition 
systems of the prior art, all that takes place prior to startup of the 
pump will be recorded without distinction. 
In order to overcome this major drawback, the aim of this invention is to 
provide a device for the activation of an apparatus which serves to 
measure acoustic emission by detection of background noise. Said device 
carries out a continuous measurement of environmental background noise by 
means of piezoelectric transducers, compares said noise with a reference 
voltage within a comparator which then activates the measuring apparatus. 
Other features of the invention will be more apparent upon consideration of 
the following description and accompanying drawings, wherein:

The device according to the invention as shown in FIG. 1 comprises a 
comparator 3 which receives at one input a reference voltage delivered by 
a control unit 4 and which receives at the other input a signal delivered 
by a circuit 2 for detecting electrical pulses produced by an acoustic 
emission transducer 1 which may also include a rectifier. This detection 
circuit 2 can consist of a mean noise-level integrator which serves to 
provide the positive peak envelope of the background noise or so-called 
noise level. 
When the noise level is below the level of the reference which has been 
chosen, the comparator is at zero. When the noise level rises above the 
reference, the comparator changes state and initiates the operation of the 
measuring apparatus which then processes the signals received from the 
different transducers. 
The increase in noise level is caused in particular by the commencement of 
a critical phase of operation of the installation which is being 
monitored. In the case of a fluid circuit, startup of the pump induces a 
background noise. This noise is detected by the circuit 2, then turns-on 
the comparator 3, and the control unit 4 delivers a suitably selected 
reference. 
Referring now to FIG. 2a, this figure shows the zero voltage level 10, the 
background noise 8, the noise modulation 7 and the reference voltage 9. 
FIG. 2b shows the zero voltage level 12 and the response of the comparator 
3 when the modulation 7 is higher than the reference 9, whereupon the 
comparator emits a signal. Thus the rectangular wave 15 is the response of 
the comparator 3 at the modulation peak 16. 
In some instances, background noises other than the noise which serves to 
trigger the comparator may conceivably appear. In the example of a fluid 
circuit, the background noise which activates the measuring apparatus is 
the noise emitted by the pump. At some distance away and externally of the 
circuit, another background noise which appears at a sufficient level may 
reactivate the measuring apparatus. In order to forestall such an event, 
discrimination by spectral correlation is carried out by a circuit 2' 
prior to detection of modulation by the detector circuit 2. The acoustic 
emission frequency spectrum is in fact characteristic of the emission 
source to a large extent. It is therefore possible to select the access to 
the detection circuit only in respect of those background noises which are 
correlated with a predetermined reference spectrum. 
The spectrum correlator can be connected to the input of the device 
downstream of the transducer 1 and can either enable or inhibit the 
detection circuit 2 via a special input of said circuit. 
The data acquisition assembly 5 forms an integral part of the complete 
detection apparatus to which the activation device according to the 
invention is coupled. Said assembly comprises in particular a time base 
for acquisition of data which will then be processed in the processing 
assembly 6. The acquisition time base can thus be of the variable 
frequency type. Thus, when the activation device according to the 
invention does not detect any background noise which is capable of turning 
it on, that is to say when the noise level is below the predetermined 
critical threshold value or the reference unit 4, the acquisition time 
base produces a relatively low predetermined frequency f. By way of 
example, the data acquisition assembly 5 then carries out low-rate 
sampling of the signals derived from the transducer 1 at intervals of 1/f 
seconds. When a high noise level is recorded, the signal emitted by the 
comparator 3 switches the acquisition time base to an operating frequency 
F which is distinctly higher than the so-called watch frequency f. The 
sampling operation will then be much faster, namely at intervals of 1/F 
seconds. The flow of data to be processed is then increased by a marked 
extent. 
The present invention therefore makes it possible to perform selective 
monitoring of various acoustic emissions in an industrial installation 
which may be of large size. This may be carried out as a function of 
predetermined hazard thresholds in order to limit the number of monitoring 
data collected by the transducer 1 when there is only a low degree of 
probability that these latter will be accident effects. 
The activation device according to the invention can assume a number of 
different forms according to the various configurations of the apparatus 
for measuring acoustic emission. Thus, if the measuring apparatus 
comprises a large number of transducers whereas the installation under 
control has only a single source of noise which may become critical, the 
activation device according to the invention will be placed on only one of 
the transducers which is suitably chosen for the purpose. The other 
transducers will be directly connected to the assembly 5 for acquisition 
of data to be processed. 
If a number of different background noises are indications of critical 
thresholds, the group of transducers can be arranged in separate 
subassemblies. One activation device and one data acquisition assembly 5 
are associated with each transducer subassembly. 
Furthermore, the present invention applies to different chains for the 
acquisition of data of acoustic origin. The transducer 1 may thus comprise 
a preamplifier in the event of long distances at the transducer location. 
The transducer 1 delivers electrical signals which can be rectified by a 
half-wave or full-wave rectifier device, for example of the semiconductor 
diode type.