FR3012606A1 - METHOD FOR DIAGNOSING LOADING - Google Patents

Abstract

The invention relates to a method for diagnosing a load (10) in which the load (10) to be controlled, equipped with at least one resonator (26) fixed on the surface (S) of said load, is vibrated. resonator (26), the resonance frequency of the resonator (26) is identified, and at least one value representative of the mechanical state of the load (10) is deduced from the resonance frequency.

Description

The invention relates to a method for diagnosing a load. Such a method is especially intended to evaluate the aging of a propellant charge made inaccessible once mounted in a combustion chamber. A preferred field of application of the invention is the aging control of a solid propellant charge in a propellant. In the field of propulsion for example, the mechanical properties of propellant solid loads of a propellant are likely to change as they age. These evolutions are notably caused by the propellant storage environment, for example depending on the presence of oxygen, humidity, salinity, temperature and its variations. Thus, the oxygen present inside a channel of a solid propellant charge can lead to the oxidation of the binder which leads to an over-crosslinking thereof and thus a hardening of the viscoelastic mass. In the presence of a high hygrometry, the solid propellant on the contrary tends to soften. Solid propellant shipments can therefore, during storage, soften or harden depending on the storage environment. The degradation of the mechanical properties of a solid propellant charge can cause the firing of the propellant to fail. It is therefore desirable to measure the evolution over time of the mechanical properties of a solid propellant charge in order to decide on the firing ability of the propellant itself. The same type of problem is obviously encountered in the field of military heads explosives.

Attempts have been made to determine the aging of such a mass of viscoelastic material by incorporating piezoelectric sensors connected to measuring power stations, to measure periodically over time the evolution of their mechanical properties, in correlation with aging. . Thus, US Pat. No. 3,937,070 describes a method for measuring the mechanical properties of a solid charge by means of piezoelectric sensors implanted in the mass of the load. One of the problems associated with this type of monitoring is the difficulty of implanting sensors into the viscoelastic material at the time of manufacture of the load. In addition, the presence of such sensors in the viscoelastic material locally changes the properties of the material and can be a source of failure (eg due to cracking). These sensors can also generate safety problems, in particular by local heating. Above all, the location of the sensors in the material itself does not make it possible to follow an evolution of the properties in the "skin" of the mass of said material, this zone being the most exposed to aging because of the surface exchanges between the superficial zones. of the material and the environment. Patent application FR 2 954 498 also discloses a device for subjecting a sample of viscoelastic material contained in a sample holder situated in the vicinity of the load to be tested to vibration, and for analyzing the response of the sample. sample to these solicitations to deduce its state and, thus, the state of loading submitted to the same environment. This method involves cutting a sample in the load to be monitored, or taking a sample of a material strictly identical to said load. The apparatus necessary for its implementation is, moreover, relatively bulky and complex. It is therefore desirable to have a method of controlling the mechanical properties of a propellant charge to evaluate its aging, which can be implemented easily and, where appropriate, in confined or dangerous systems. The invention makes it possible to achieve this objective with a method of diagnosis of a load, in particular a propellant loading, comprising at least the following steps: a) the load to be controlled is provided equipped with at least one resonator fixed on the surface of said loading, b) the resonator is vibrated, c) the resonance frequency of the resonator is identified, and d) at least one value representative of the mechanical state of the charge is deduced from the resonance frequency. The resonance frequency of a resonator is not the same when the resonator is free of its movements and when its degrees of freedom are modified by its environment. It is also a function of the characteristics of this environment. The method according to the invention exploits the sensitivity of the resonant frequency of a resonator to a modification of its environment, and in particular the mechanical characteristics (the rigidity, for example) of its support. By determining the resonance frequency of the resonator fixed on the surface of a propellant charge, it is thus possible to directly or indirectly deduce one or more representative value (s) from the state of the material constituting the charge, in particular its module. , using predefined relationships or curves. The method according to the invention is applied to a load, which may consist of a composite material, for example a viscoelastic material, in particular propellant.

According to the invention, the resonator is fixed on the surface of the load. In other words, it is at least partially flush with the surface of the load, and preferably it projects at least partially from said surface. Since the resonator is fixed on the surface of the load, the method makes it possible, without being intrusive, to directly analyze the load concerned, without prior sampling, and without modifying the properties of the load or weakening it. According to an exemplary implementation, to make the resonator vibrate, a voltage is applied to an electromechanical exciter adapted to excite the resonator, the frequency of said voltage is varied, and the resonator response is detected at the frequency variations of said resonator. voltage. According to an example, in step c) of the method, the intensity of the current traveling through the exciter is measured as a function of the frequency, a current peak representative of the resonance of the resonator is identified and the frequency is determined. corresponding to this intensity peak, said frequency corresponding to the resonance frequency of the resonator. When the resonator, subjected to a given environment (here the support to which it is attached), is excited by a given frequency voltage, it calls a current of intensity proportional to the energy required to allow its displacement at said frequency. When the frequency of the applied voltage is varied over a frequency range close to the resonance frequency, the intensity of the current called by the resonator varies gradually. It goes through a minimum at the resonant frequency, then increases again. By plotting the intensity curve of the current flowing through the resonator as a function of the frequency, the resonance frequency of the resonator, representative of the characteristics - in particular of the rigidity - of the load to which the resonator is fixed is thus identified. Note that here is meant by intensity peak a minimum value of identifiable intensity on the curve and successive to a drop in intensity. The assembly used is simple and inexpensive. A simple electrical wiring and an ammeter connected to the exciter is enough to measure its response by changing frequency of the input voltage.

In one example, the electromechanical exciter is a piezoelectric element. According to a particular embodiment, the resonator and the exciter form an amplified piezoelectric actuator. Such a piezoelectric actuator usually comprises a piezoelectric element forming an electromechanical exciter and a metal structure intended to amplify the displacement of the piezoelectric element forming a resonator. According to one embodiment of the invention, the resonator can be glued to the surface of the load.

According to one variant, the resonator comprises an attachment part embedded in the mass of the load. According to an advantageous implementation example, in step d), the quality factor of the resonator is calculated and deduced from said representative value of the state of the load. The quality factor Q is defined as the ratio between the frequency value at the peak of intensity of the current flowing through the resonator and the width of the intensity curve at mid-height of the peak. It has been established that this parameter is very sensitive to the evolution of the properties of the material constituting the loading. In one example, the representative value of the state of the load is the modulus of the material constituting the load. According to an example, the method comprises a calibration step during which a sample of material equipped on its surface with a control device is provided, the sample is processed to modify the state of the material, and measuring at least one parameter representative of the resonance of the resonator for different mechanical states of said material. The sample is preferably made of the same material as that of the load that is to be controlled or it may be its inert simulant, or simply a material of the same kind. It is generally a composite material, especially viscoelastic, for example propellant. The treatment of the sample may in particular comprise accelerated aging in temperature and / or hygrometry imposed (s). It is known that by varying the temperature of a material, in particular a viscoelastic material such as propellant, its module is also modified. One simulates so to say the aging of the material, causing it to change state (softened state, hardened state, etc.). For each temperature step, the characteristic values of the resonance peak resonance material are measured and recorded, in particular the resonance frequency, the quality factor and the modulus. The method according to the invention is advantageously applied to the diagnosis of a propellant charge in a propellant. The present invention also relates to an assembly comprising a propellant charge and a device for controlling said charge, the control device comprising at least one resonator fixed on the surface of the charge.

In one example, the assembly further comprises an electromechanical exciter adapted to excite the resonator. According to one example, the assembly according to the invention may furthermore comprise a voltage generator connected to the electromechanical exciter and / or an ammeter connected to the resonator.

The invention will be better understood and its advantages will appear better on reading the detailed description which follows, of an embodiment shown by way of non-limiting example. The description refers to the accompanying drawings, in which: FIG. 1 illustrates a propellant load equipped with a device for controlling its mechanical characteristics; Figures 2A and 2B illustrate possible examples of arrangements of a control device on propellant loading; FIG. 3 shows a curve obtained during the implementation of the control method according to the invention, on a propellant charge of the type represented in FIG. 1; Figure 4 illustrates a sample of viscoelastic material on which calibration tests are performed; Figure 5 shows a curve resulting from the tests performed on a sample of the type shown in Figure 4; FIG. 6 shows a propellant comprising a propellant charge equipped with a control device, as illustrated by FIG. 1. FIG. 1 illustrates a propellant charge 10 equipped with a control device 20 making it possible to examine its propellant mechanical characteristics. The control device 20 here comprises a plurality of state sensors 22 fixed at different places on the surface of the load 10. Each sensor 22 comprises an electromechanical exciter 24 adapted to transform an input voltage into a mechanical displacement and a resonator 26 adapted to vibrate at the frequency imposed by said input voltage and secured to the surface S of the propellant charge 10. The control device 20 further comprises a sinusoidal voltage generator 30 with adjustable amplitude and frequency to prime each electromechanical exciter 24 and a measuring device 32 designed to measure the intensity of the current flowing through each electromechanical exciter 24, for example an amperometric clamp supplemented with an amplifier. In the example, the state sensors 22 are amplified piezoelectric actuators of the type marketed by CEDRAT TECHNOLOGIES under the trademark APA®. As is well known, in this type of actuator, a mechanical device forming a resonator amplifies the displacement of a piezoelectric element forming an electromechanical exciter induced by the application to this exciter of a voltage. In the illustrated example, the piezoelectric element 24 is of generally parallelepipedal shape, for example ceramic. The resonator 26 comprises, itself, an outer armature 27 of substantially elliptical shape, for example made of stainless steel, inside which the piezoelectric element 24 is prestressed with the aid of one or more prestressing wedges The piezoelectric element 24 is here compressed in the direction of the major axis X1 of the armature (hereinafter longitudinal direction), the movement of the element in the longitudinal direction thus being amplified by the outer armature in the direction X2 of the minor axis of said armature (hereinafter transverse direction).

For the rest of the description, a lower end 26a and an upper end 26b of the resonator are defined with reference to the transverse direction X2 in which it deforms by vibrating. In the example, the resonator 26 is connected to the propellant charge at its lower end via an attachment portion 23 integral with the frame 27. In one example, the attachment portion 23 can be fixed loading propellant with a thin layer of glue. In another example, the attachment portion 23 may be partially embedded in the mass of said load.

The armature 27, it remains in all cases free to move over the surface of the load 10. In the example of Figure 1, the propellant charge 10 has a substantially cylindrical shape, particularly in the form of straight cylinder, for example of the solid monolithic block type or disk stack type, defined by a first and a second end face 12a, 12b and an external and internal lateral surface 14 extending between said end faces 12a , 12b. As illustrated in the figure, the resonator 26 can be glued, by one of its lower or upper ends 26a, 26b, to one of the end faces 12a, 12b of the load 10. The resonator 26 can also be glued, by only one of its lower or upper ends 26a, 26b, to the inner face 16 of the load 10. FIGS. 2A and 2B illustrate other possible arrangements of the state sensors 22 on the surface of the propellant load 10.

In the example illustrated in Figure 2A, a state sensor is mounted in the central cavity 18 of the load, its lower and upper ends 26a, 26b being each fixed to diametrically opposite portions of the inner surface 16 of the load. In another example illustrated in FIG. 2B, the propellant charge 10 'has a corrugated inner surface 16, forming a plurality of longitudinal cavities 18' each delimited by a first and a second face 19a, 19b, opposite to each other . A sensor 22 can then be mounted between the first and the second face 19a, 19b of such a cavity 18 '(in the example, its lower end is fixed to the first face 19a of the cavity 18' and its upper end, on the second side 19b). The control method according to the invention is based on the link existing between the resonance frequency of the resonator 26 and the characteristics of its support, here the loading of propellant 10. If the resonator 26 has a natural frequency fi, it has a frequency of different resonance f2 when bound to its support. And its resonant frequency fR still varies according to the mechanical characteristics of the support to which it is connected. Moreover, it has been established that the intensity I of the current called by the resonator 26 reaches a peak at the resonant frequency fR.

To determine the resonance frequency f R, the frequency of the voltage applied by the generator 30 is thus varied, and the curve showing the intensity of the current called by the resonator is plotted as a function of said frequency. The curve resulting from these measurements is illustrated in FIG.

It can be seen that the intensity I of the current measured by the ammeter 32 increases slowly as the frequency increases over a period preceding the resonance phenomenon. Then, from a frequency fA for which the intensity of the current is equal to IA, the curve undergoes a fast decrease until reaching a minimum value IR at the resonance frequency fR. After the resonance, the intensity of the current increases rapidly again to an Ig value at the frequency fB, then resumes a slower progression. The frequency for which the intensity is the weakest therefore corresponds to the resonant frequency.

From the curve, it is possible to determine also the quality factor Q of the resonator 26, which corresponds to the ratio between the intensity peak frequency value (ie the resonance frequency fR) and the width of the curve to half height of the peak. For this purpose, the first place where the curve is plane (i.e. where its derivative is zero) is sought before the resonance frequency has been reached (to the left of the peak in FIG. 4). This point A, which corresponds to a current intensity called IA for a frequency fA, constitutes what is called the foot of the resonance peak. The height of the peak is thus equal to the difference AY = IA-IR. It is easy, therefore, to measure the width L of the curve at half height, that is to say the difference in absolute value between the two frequency values (here fm1 and fm2) for which the intensity of the current is equal to (IA-IR) / 2. The width of the curve L = fm1-fm2 then makes it possible to calculate the quality factor Q = fR / L. These calculations can be performed by an operator, or as illustrated in FIG. 1, automatically by a computing device 34 connected to the measuring device 32. To deduce from the previous values the state of the propellant, it is necessary to establish or have previously established the relationship between the resonance frequency fR resonator 26 and the state of the material constituting its support. For this purpose, tests are carried out on one or more samples 40, of the type illustrated in FIG. 4. A sample 40 may be in the form of a single block of composite material, in particular viscoelastic (propellant or its inert simulant), here parallelepipedic (typically of dimensions 10 mm x 25 mm x 25 mm), which is fixed a control device 20 as defined above. It is well known today that the characteristics of a viscoelastic material such as propellant can be modified by varying the temperature of the material. It is thus possible to simulate the aging of a sample, in particular to modify its modulus, by modifying its temperature. For each temperature, the modulus of the sample is measured and, at the same time, the resonance frequency of the resonator which is fixed on the sample and / or its quality factor.

The curve of FIG. 5 illustrates, for three samples tested, the resonance frequency fR of the resonator 26 as a function of the temperature of the sample. This curve also shows the resonance frequency variations of a resonator in free configuration.

The results thus obtained are usable for the effective control of the propellant charge mounted in a thruster illustrated in FIG. 6. FIG. 6 illustrates the loading of propellant 10 inside a propellant 100. In this position, and before setting propellant loading fire 10, the state sensors 22 are mounted on the load and are, like the load 10, made inaccessible. The generator 30 and the measuring device 32 are only connected during the measurement phases. The invention, however, has a broader field of application than that of the above example.

The propellant loads concerned can be either simple storage or integrated functional purpose in devices. Given the autonomy of the device, its use is particularly suitable for isolated, confined or dangerous devices such as thrusters or military heads.

Claims (19)

REVENDICATIONS1. A method of diagnosing a load, in particular a propellant charge (10), comprising at least the following steps: a) providing the load (10) to be controlled equipped with at least one resonator (26) fixed on the surface ( S) of said loading, b) the resonator (26) is vibrated, c) the resonance frequency of the resonator (26) is identified, and d) the resonance frequency (fR) is derived from at least one representative value of the resonator (26). mechanical state of loading (10). 2. Method according to claim 1, wherein, to vibrate the resonator (26), a voltage is applied to an electromechanical exciter (24) adapted to excite the resonator (26), the frequency of said voltage is varied, and the response of the resonator (26) to the frequency variations of said voltage is detected. 3. The method according to claim 2, wherein, in step c), the intensity of the current traversing the exciter (24) is measured as a function of frequency, a current peak representative of the resonator resonance (26) and the frequency corresponding to this intensity peak is determined, said frequency corresponding to the resonance frequency of the resonator (26). The method of claim 2 or 3, wherein the electromechanical exciter (24) is a piezoelectric element. 5. Method according to any one of claims 1 to 4, wherein the resonator (26) is bonded to the surface (S) of the load (10). 6. Method according to any one of claims 1 to 5, wherein the resonator comprises a fastening portion (23) embedded in the mass of the load (10). 7. Process according to any one of claims 1 to 6, wherein, in step d), the quality factor (Q) of the resonator (26) is calculated and deduced from said representative value of the state of the loading (10). 8. A method according to any one of claims 1 to 7, wherein in step d), the value representative of the state of the load (10) is the modulus of the material constituting the load. 9. A method according to any one of claims 1 to 8, further comprising a calibration step during which: a sample (40) of material equipped on its surface with at least one resonator (26) is provided; ), the sample (40) is processed to modify the mechanical state of said material, and at least one parameter representative of the resonance of the resonator (26) is measured for different mechanical states of said material. The method of claim 9, wherein the processing of the sample (40) comprises a modification of the temperature to which the sample is subjected, resulting in its accelerated aging. 11. The method of claim 9 or 10, wherein the treatment of the sample (40) comprises a change in the hygrometry to which the sample is subjected, causing its accelerated aging. 12. Process according to any one of claims 1 to 11, applied to a propellant charge in a propellant. 13. An assembly comprising a load, in particular a propellant charge (10), and a control device (20) for said charge, the control device (20) comprising at least one resonator (26) fixed on the surface (S) of the loading (10). 3012 606 13 14. The assembly of claim 13, further comprising an electromechanical exciter (24) adapted to excite the resonator (26). 5 15. The assembly of claim 14, further comprising a voltage generator (30) connected to the electromechanical exciter (24). 10 16. The assembly of claim 14 or 15, wherein the electromechanical exciter (24) is a piezoelectric element. An assembly according to any one of claims 13 to 16, wherein the resonator (26) is bonded to the surface (S) of the load (10). 18. An assembly according to any one of claims 13 to 17, wherein the resonator comprises a fastening portion (23) embedded in the mass of the load (10). 20 19. An assembly according to any one of claims 13 to 18, further comprising an ammeter (32) connected to the resonator.