FR2528183A1 - ELASTIC SURFACE WAVE ACCELEROMETER - Google Patents

Abstract

THE INVENTION RELATES TO SURFACE ELASTIC WAVE ACCELEROMETERS. THE SUBJECT OF THE INVENTION IS AN ACCELEROMETER WITH ELASTIC SURFACE WAVES INCLUDING A BULBING TAB CUT IN A 2-SIDED LAMELLA WITH PARALLEL SIDES 4, 5; THE TAB INCLUDES AN AREA OF EQUAL BENDING RESISTANCE 25 OR ARE ARRANGED ACCORDING TO FACES 4, 5 OF MEANS OF MEASURING STRESSES TO ELASTIC SURFACE WAVES 6, 7, 8, 9. THE INVENTION APPLIES IN PARTICULAR TO THE MEASUREMENT OF THE ACCELERATION COMPONENT ACCORDING TO A PREDETERMINED DIRECTION.

Description

b 2528183

ELASTIC WAVE ACCELEROMETER

OF SURFACE

  The present invention relates to elastic wave accelerometers

  surface and more particularly to those which comprise at least two oscillators connected to transducer means located on both sides of a resilient flexural lamella under the action of acceleration Such a structure makes it possible to measure the acceleration according to a component of direc-

  the main faces of the lamella, which generally

  The transducer means are designed to excite and collect surface elastic waves, the propagation speed of which is known to vary according to the mechanical bending stresses. first embodiment, the transducer means form a transmission line whose delay time sets the phase shift of an oscillating loop. According to a second embodiment, the transducer means are located in a network resonant cavity and are connected to means intended to maintain the oscillation of the cavity The frequency representative of the acceleration to be measured results from the subtraction of two oscillation frequencies which have thermal drifts which do not occur.

not exactly compensate.

  In fact, these thermal drifts result from the stresses created by the flush mounting of the lamella and, if appropriate, by the mounting of the seismic mass. These stresses can be added to the mechanical bending stresses when the zones where they originate are close to one another. from the region occupied by the means of measurement to

surface elastic waves.

  When the mechanical bending stresses are those of a beam of constant section, their intensity varies as the bending torque, which requires precise positioning of the surface elastic wave measuring means. In addition, the variation of the stresses causes a variation of velocity of propagation of surface elastic waves in the extent of the

  measurement zone, which can lead to distortion of the wave fronts.

  In order to overcome the drawbacks mentioned above, the invention proposes to use a parallel face sipe cut to have a zone of equal flexural strength in which the means are located.

  measurement with elastic surface waves.

  The subject of the invention is a surface elastic wave accelerometer in which a parallel-sided plate flexes under the action of the acceleration to be measured; the bending stresses created in said faces being detected by means of surface wave elastic oscillator means, characterized in that said measuring oscillator means are located in an area of equal resistance to bending of said slat.

  The invention will be better understood by means of the following description and

  annexed figures, among which: FIG. 1 is an isometric view of a wave accelerometer

  surface elastics of known type.

  Figure 2 is an explanatory figure.

  FIG. 3 is an isometric view showing structures of equal flexural strength equipped with elastic wave transducer means

of surface.

  FIG. 4 represents a plate for an accelerometer according to the invention

  tion. FIG. 5 is a partial isometric view of an accelerometer with

  elastic surface waves according to the invention.

  FIG. 6 is an isometric view of a first variant of

  realization of an accelerometer according to the invention.

  FIG. 7 is a plan view of a second variant embodiment

  an accelerometer according to the invention.

  FIG. 8 is an elevational view of the variant illustrated in FIG.

figure 7.

  FIG. 9 is a partial isometric view of a third variant

  embodiment of an accelerometer according to the invention.

  FIGS. 10 and 11 illustrate other exemplary embodiments

  accelerometers according to the invention.

  In the following description, the mechanical stresses of

  bending generated by the acceleration are manifested on the main faces of a lamella of constant thickness cut in a material capable of propagating surface elastic waves As a non-limiting example, it is conceivable to use a lamella cut in a crystalline piezoelectric material such as quartz, but with a more

  complex means of surface elastic wave transducers, it is

  It is also necessary to use a silica plate. With regard to the measurement of the acceleration, it is the component according to the normal to the main faces of the lamella which is measured. The contours envisaged are such that the flexural rigidity for parallel stresses to the main faces of the lamella is greater than the rigidity entering line of

  account for the measurement of the normal component of acceleration.

  lerometer necessarily includes a lamella holder allowing its

  sag The mass of measurement may consist of the lamella itself

  same, or by a seismic mass In the latter case, the coverslip constitutes an elastic link between the support and the seismic mass and all the elements

  are supposed to undergo the same acceleration.

  In FIG. 1, a known surface type accelerometer with surface wave accelerometer can be seen in relation to an Oxyz trirectangular trihedron. The acceleration to be measured Y> of direction parallel to oy is applied to a support 1, to a seismic mass 3 and to an elastic lamella 2 connecting the support 1 to the mass 3 The cross-section of the lamella corresponding to a section parallel to the yox plane is rectangular of width b and of height h Its moment of inertia, when it fades under action of the acceleration t Y is given bh by the expression 12 oh is substantially less than b For a solicitation following ox the moment of inertia is much higher and for

  a solicitation following the flexion is not taken into account.

  The deformations of the lamella 2 are detected by means of surface elastic wave measuring means Si, for example, the lamella 2 is an ST-cut quartz lamella which offers zero heat drift at a temperature of 25 ° C., the main faces 4 and 5 are provided with interdigitated comb electrodes 6, 7, 8, 9, 10, 11, 12 and 13 which delimit radially oriented radially spaced intervals. The electrodes 6 and 7 form an elastic wave emission transducer of surface that progress in the direction oz before being picked up by a set of electrodes 8 and 9

  constituting the receiving transducer The set of conductive

  The conductors carried by the face 4 thus form a surface-elastic wave delay line which acts as a loop of an amplifier 14. The set of conductive elements carried by the face 5 forms another resilient-wave delay line with a looping surface. The input and the output of another amplifier 15 is thus in the presence of a pair of oscillators whose oscillation frequencies f 1 and f 2 are related to the transit times t 1 and t 2 of the elastic waves of surface exchanged along faces 4 and 5 The outputs of amplifiers 14 and 15 are connected to the inputs of a subtractive mixer

  16 whose output 17 provides an alternating signal of frequency f = fi f 2.

  To lighten the design of the connections, grounding G have been

  added in the electrical diagram of Figure 1.

  In the presence of an acceleration Y, the simple bending of the lamella 2 causes the appearance of mechanical bending stresses at the faces 4 and 5, which has the effect of modifying in opposite directions the transit times t 1 and t 2 This results in new oscillation frequencies f 1 + AF and f 2 AF and a signal on the output 17 having a frequency f + 2 AF. It can thus be seen that the acceleration Y is measured by a

frequency variation 2 AF.

  The deformation of the lamella 2 by simple bending also produces mechanical shear stresses, but these are zero according to the free faces 4 and 5 which convey the elastic surface waves. Therefore, the simple bending can be assimilated to a pure bending To avoid being in the case of a left bending, is given to the lamella 2 and the mass 3 a shape having a plane of symmetry parallel to yoz, thus avoiding the mechanical stresses of torsion of the lamella 2 On the other hand, with the arrangement of FIG. 1, parasitic stresses may be added to the mechanical bending stresses due to the embedding of the lamella 2 in the support 1 and in the seismic mass 3. The thermal expansion of the lamella 2 may present differences compared to that of the notches of the support 1 and the seismic mass 3, which can distort the acceleration measurement Furthermore, as shown in Figure 2, o the same r ferences denote the same elements as in Figure 1, the bending moment in each straight section of the lamella 2 varies sharply The line 18 represents the variation of the bending moment M along the z axis for an end load} = mt and the curve 19 gives the variation of the bending moment M resulting from the intrinsic mass of the lamella The diagram of FIG. 2 makes it possible to understand that the mechanical bending stress in the case of a lamella with a constant moment of inertia section and of constant thickness is a magnitude that varies substantially in the measuring region where the elastic waves of

surface are exchanged.

  The variation of the mechanical stresses implies a corresponding variation

  This can lead to distortions that affect the wave fronts and the delayed electric signal. On the other hand, the position of the transducer means becomes critical since the sensitivity depends on the position of the waveform. these means the

along the coverslip.

  FIG. 3 shows two lamellae embedded in a support 1 which have in the xoy plane a profile which gradually fades towards embedding. In the case of a lamella of constant thickness h, of which only the self-mass meets to the acceleration to be measured, a bending elastic stress of constant value is obtained when the deformed material has a

  constant radius of curvature p For a current value z of the distance

  z, we can integrate the elementary bending moments acting on the lamella cross-section having the width b at the distance z O from the point O bh 3 and the height h If M (z) is the resulting bending moment, I = the moment of inertia of this cross section and E the modulus of elasticity of the material constituting the lamella 2, we have the expression M (z) p E i We see that the condition of equal resistance amounts to adopting a stiffness at the bending EI proportional to the bending moment M (zo) The profile of the lamella 2 in the xoz plane is delimited by two parabolic arcs as illustrated in FIG. 3 as regards the lamella 2 occupying the second position. The interdigital comb transducers 6 , 7, 8 and 9 can be located in the front half of the left z-axis 4 while the interdigital transducers 10, 11, 12 and 13 are located in the right half of the z-axis A such a lateral offset arrangement is advantageous in To reduce the capacitive coupling between the measuring means of the face 4 and those of the face 5 FIG. 3 also shows in the first position a sipe 2 whose condition of equal resistance to bending is provided by a triangular shape of vertex 0 This case corresponds to a lamella whose own mass can be neglected compared to a seismic mass (m) exerting in O the inertial force F = m OY where Y is the acceleration to be measured In the general case, the mass itself of the lamella and the added mass define a profile of equal resistance to bending

  intermediate between the two types shown in Figure 3.

  FIG. 4 shows a lamella 2 for an accelerometer according to the invention which comprises a median zone 25 of equal flexural strength interconnecting an embedding zone 26 having a cross-section 24 of constant width and height, and an end zone 22 of cross section 23 The end zone 22 is traversed by a mounting pin 27 carrying two weights 20 and 21 which, because of the acceleration to be measured, exert a force FM The end zone 22 exerts a force FP La condition of equal resistance applies from section 23 o acts a bending moment that results from forces FM and FP All straight sections of the zone of equal resistance 25 are dimensioned in width taking into account the additional loading due to the lamella proper weight between section 23 and each of the intermediate sections up to section 24 It will be verified in practice that the bending moment M to be resisted by that cross section of the area of equal resistance varies in proportion with the moment of inertia of the section, that is to say in proportion to its width The pace of the zone of equal resistance 25 is trapezoidal, but the sides that connect the bases are slightly curved as shown in FIG. 4, if it is necessary to take account of the mass

clean of the coverslip.

  The partial isometric view of FIG. 5 shows an exemplary embodiment of an accelerometer using a parallel-face piezoelectric strip 2 whose cutout has an area of equal flexural strength. The cutout illustrated in FIG. annular connecting internally to a measuring tongue having at its birth the zone of equal resistance 25 and at its free end a set of weights 20, 21, 27 Preferably, the cutting of the strip 2 is symmetrical relative to the z axis which in the case of Figure 5 is a

  diameter with respect to the circular outer contour of the lamella 2.

  This particular provision is advantageous because it allows for

  the fasteners of the lamella 2 of the measuring zone 25 which contains the surface elastic wave transducers carried by the faces 4 and 5. This separation makes it possible to attenuate the stresses resulting from the attachment mode of the lamella 2 which is diametrically opposed to the connection of the tongue with the annular frame Bending, torsional stresses

  and shearing that arise in the annular frame due to the accelerating

  In order to reinforce the annular frame of the lamella 2, it is possible to provide, as shown in FIG. 5, two stiffeners 28 and 29 in the form of arches which are applied or glued on the faces 4 and 5. 5 of the lamella 2 These stiffeners 28 and 29 are advantageously cut in a blade identical to that which was used to make the lamella 2 while ensuring during assembly to maintain the same crystallographic orientations This measure serves to avoid the creation of stresses due to differences in thermal expansion between the elements 28, 29

and 2.

  All the elements 2, 28 and 29 form a crew which must be fixed to a half box 36 which can be made of a material having

  a coefficient of thermal expansion different from the expansion coefficients

  the material constituting the crew.

  Under these conditions, it is advantageous to adopt fastening means allowing the crew free to expand in the xy plane while having a good seat on the flange 35 of the half-casing 36. FIG. 5 illustrates an advantageous embodiment based on on a set of balls serving as intermediate support A ball 32 takes place in two conical bowls respectively formed in the flange 35 and in the stiffener 29 This fastener allows the pivoting of the housing relative to the crew A second ball 34 is placed in two vee grooves oriented preferably towards the housing of the ball 32 and cut respectively in the collar 35 and in the stiffener 29 This second attachment leaves the crew free to expand but sets its orientation in the xy plane A third ball 33 pinch between two flat surfaces respectively belonging to the flange 35 and the stiffener 29 permanently fixes the stable seating of the crew on the half box first, but leaving free the dilatation of the crew In Figure 5, we have

  omitted to represent another half-casing which overhangs the crew 2, 28, 29.

  This other half-casing rests on the half-casing 36 via columns 37 and its flange is connected to the stiffener 28 by means of balls similar to the balls 32, 33 and 34. FIG. conical housing 30 made in the stiffener 28 to receive two of these three balls Although Figure 5 shows a crew with stiffeners in the form of arches, we can either remove them or make them

  completely cover the annular frame of lamella 2.

  An embodiment variant of the accelerometer according to the invention is shown in FIG. 6 The housing 36 carries, via a bracket 39, a mobile assembly composed of a strip 2. one end connects to a frame

  ring 41 acting as an earthquake mass and the other end of which

  The assembly 38 encloses the oscillator and mixer electrical circuits which cooperate with the elastic wave measuring means of FIG.

located in zone 25.

  The advantageous arrangements illustrated in Figure 6 may apply

  5 Thus, the seismic masses 20, 21, 27 reported on the strip 2 may be replaced by extensions of the strip 2 which occupy the two recesses located on either side of the central tongue. Advantageous arrangements illustrated in FIG. 5 can be applied to FIG. 6 where the bracket 39 could be replaced by a mounting on balls holding an open inside frame cut in the strip 2 from the end of the zone 25. branches

2528,183

  curved of this open inner frame occupying the two empty parts which

frame the central tongue 40.

  As seen above, the measurement of the flexion by surface elastic waves takes place in a zone of equal strength obtained by cutting a lamella with parallel faces The tongue obtained can be one with a frame open or closed which serves as mounting means on a housing and its free end may include extensions that add mass to increase the sensitivity to acceleration that one

wants to measure.

  The invention is not limited to a cutout comprising a single measuring tongue, since the same frame may be common to several tongues each having a measurement zone of equal flexural strength. FIG. 7 shows an alternative embodiment of a measurement unit 2 comprising, inside a parallelogram-shaped frame, two measuring tongues each provided with a zone of equal resistance 251 and 252. tongue is cut symmetrically with respect to an axis 45 or 46 in an approximately trapezoidal shape which terminates in a free end bearing a seismic mass 201 or 202. Each zone of equal resistance 251 or 252 contains on both sides of the lamella 2 a surface elastic wave measuring means In the case of FIG. 7, the measuring means used are surface elastic wave resonators. Each resonator comprises a cavity delimited by two arrays of reflective lines formed on the main faces of the lamella 2. by etching or ion implantation Electrodes are photogravated in the cavity of each resonator to maintain with an amplified circuit 141, 151, 152 or 142

  stationary surface elastic waves.

  Thus, in FIG. 7, four oscillators are available whose oscillation frequencies vary with the bending stresses generated by the acceleration to be measured. Admitting that the two measuring tongues of FIG. 7 react in the same way to a acceleration perpendicular to the figure plane and assuming that the quiescent frequencies of the oscillators 141, 142, 151 and 152 are respectively f 2, f 3 and -1.0 f 4, the mixer 161 provides the difference f 1 f 3 and the mixer 162 difference, f 4 f 2 A mixer 16 receives the signals from the mixers 161 and 162 and supplies a signal whose frequency variation f 1 + f 2 f 3 f 4 represents the acceleration experienced by the crew Indeed if A f is the frequency difference that the acceleration produces, the oscillation frequencies are: f 1 + tf, f 2 + tf, f 3 tf, f 4 tf, which gives

  at the output of the mixer 16 a signal whose frequency is f 1 + f 2 f 3 -

  f 4 + 4 Af If AF 1, AF 2, AF 3 and AF 4 are the temperature drifts of the four oscillators, the thermally disturbed frequencies are fl + t F 1, f 2 + t F 2, f 3 + F and f F 4 The frequency disturbed at the output of the mixer 16 will be: f 1 + f 2 f 3 f 4 AF 1 + t F 2 t F 3 t F 4 We can therefore arrange to cancel the thermal drift: AF 1 + AF 2 AF 3 AF 4 in greater detail than with two measurement oscillators The assembly of the FIG. 7 crew on the half-casing 36 can be done at two points in the middle of the sides of the frame which do not form a part of the Measuring tabs For this

  to do, we use a recessed support 43 and a pivot support 42.

  FIG. 8 is a section along the line 44 of FIG. 7. Crew 2 is locked to the right between two half-casings 361 and 362, but to complete the attachment, the left part is supported between half-casings 362 and 361 by axes on pins 49 and 50 supported between conical housings formed in the half casings 362 and 361 and in thrust plates 47 and 48 which are fixed to the crew 2 This assembly avoids in a wide

  measures the constraints of thermal origin.

  In FIG. 9, an accelerometer of similar design to that of FIGS. 7 and 8 can be seen, but where the measurement tongues form a body with a central spar reinforced by two stiffeners 253 and 254. The seismic masses consist of fins 201 and The zones 51, 52, 53 and 54 indicate the respective positions of the surface elastic wave measuring means which may be interdigitated comb delay lines or grating resonators. of course included in areas of equals

  resistance to bending of the two measuring tongues.

2.528183

  FIG. 10 shows another variant embodiment using a single measuring tab forming the median branch of an E-shaped cutout. The end of the measuring tongue comprises a fin 201 serving as a seismic mass. body with the zone of equal resistance 251 of the tongue is reinforced by bonding on each side of a U-shaped stiffener 253 The monobloc crew thus formed benefits from the same assembly as that of FIGS. 7 and 8. The block 38 contains the oscillator circuits. and mixers which cooperate with the wave measuring means

  surface elastics located in zone 251.

  FIG. 11 illustrates another cut-out of the lamella 2 Two measurement tongues 251 and 252 equipped with seismic masses 201 and 202 form a body with a central tongue 260 which in turn is integral with a rectangular frame. By attaching the frame to the casing of the lamella accelerometer by the edge opposite to that which is integral with the tongue 260, the influence of the fastening stresses in the measurement zones is minimized as much as possible

251 and 252.

Claims (14)

  A surface elastic wave accelerometer in which a lamella (2) with parallel faces (4, 5) flexes under the action of the acceleration to be measured; the bending stresses created in said faces (4, 5) being detected by surface elastic wave measuring oscillator means (6, 7, 8, 9, 10, 11, 12, 13, 14, 15), characterized in said oscillator means are located in an area (25) of equal bending resistance of said lamella (2).   Accelerometer according to claim 1, characterized in that said area of equal flexural strength (25) belongs to a tongue having a free end (22) and an end (26) for attachment to a housing (36, 1) undergoing the acceleration to be measured; the shape of said tongue being symmetrical with respect to the plane of flexion which contains the   direction (y) of measuring the acceleration.   Accelerometer according to claim 2, characterized in that   the free end (22) carries a reported seismic mass (20, 21, 27).   4 accelerometer according to any one of claims 2 and 3,   characterized in that said tongue is integral with a frame which is connected   to the housing (36) by connecting members (32, 33, 34) providing a position   free of thermal stress.   5 accelerometer according to claim 4, characterized in that stiffening means (28, 29) cut in the same material as said lamella   (2) are applied or glued on the parts of said frame requiring reinforcement.   6 accelerometer according to any one of claims 4 and 5,   characterized in that the connecting members are balls (32, 33, 34); one of said balls being housed in a conical housing and another ball being housed in a vee housing.   Accelerometer according to any one of claims 2, 4, 5, 6,   characterized in that the free end (22) of said tongue is integral   with elements (41) of the lamella (2) constituting a seismic mass.   Accelerometer according to claim 7, characterized in that   said elements form a frame (41).   Accelerometer according to claim 7, characterized in that   said elements form a fin (201).   Accelerometer according to one of Claims 1 to 9,   characterized in that the strip (2) is a pair of two measuring tongues each provided with a zone of equal flexural strength (251, 252). 11 Accelerometer according to claim 10, characterized in that said tongues form a body with two opposite sides of a frame; both   other sides of said frame being intended to be connected to a housing (36).   12 accelerometer according to claim 10, characterized in that said tabs are integral with a spar which serves as a means of   suspension and fixing on a housing (36).   Accelerometer according to claim 11, characterized in that   said spar (260) is itself a body with a frame.   Accelerometer according to one of Claims 10 to 13,   characterized in that four surface elastic wave measuring oscillators are provided (141, 142, 151, 152); first mixing means (161, 162) subtracting in pairs the oscillation frequencies of the signals   produced by said oscillators; second mixing means (16) under   milking the frequencies of the signals from said first mixers (161, 162).   Accelerometer according to any one of the preceding claims   dentes, characterized in that said lamella (2) is piezoelectric.   Accelerometer according to one of the preceding claims   characterized in that said oscillator means comprise surface-wave tuning means of the delay line type or   resonator with reflector gratings.   Accelerometer according to claim 16, characterized in that the tuning means carried by the faces of said lamella are offset laterally in opposite directions with respect to the bending plane of said zone. of equal flexural strength.