EP0205381A1 - Electrofluidic jet/flapper transducer, and servo valve equipped with such a transducer - Google Patents

Abstract

The transducer (2) comprises a piezoelectric bimetallic vane (12) whose movements, under the effect of electrical signals, directly control the leakage of one or two nozzle-jets (8,8'). The vane (12 ) is mounted floating, without any recess, between fixed supports (34,34') and elastic supports (36,36') of the swivel type, not hindering the free deformation (deflection or camber) of the pallet. allows to increase the "signal voltage/displacement" ratio of the vane and to reduce the hysteresis to obtain better precision and response fidelity.

Description

The present invention relates to an electro-fluidic transducer of the nozzle / pallet type, in particular for the control of a hydraulic servo-valve from electrical signals.

The invention relates more particularly to a transducer in which the movable pallet is constituted by a piezoelectric bimetallic strip at least one of the faces of which cooperates directly with at least one nozzle for controlling, as a function of the deformations of said bimetallic strip caused by said electrical signals , the leak rate of said one or more nozzles.

The variation in the leakage rate of the nozzle (s) can be used, in known manner, to control the movements of the slide of a servo-valve.

A transducer of this kind, for controlling a servo-valve, has been described, for example, in German patent application DT-25.11.752, published on October 7, 1976.

It is known that a piezoelectric bimetal strip is composed of two layers of materials separated by a common electrode. In one of the layers, the crystals contract while in the other they expand, which produces a camber of the blade.

In the transducers proposed so far, the bimetallic strip is rigidly embedded, at its two ends or else at only one of them (like a cantilever beam), in a casing containing the nozzles, so as to fix the rest position of the bimetallic strip relative to the nozzles. In the case of a bimetallic strip having a circular shape, fixing by embedding can be made over the entire peripheral edge of the bimetallic strip.

Servo-valves fitted with such transducers are appreciated for their reliability, due to the small number of parts of the transducer, for their small volumes and weight, and for their very low electrical consumption in static conditions.

However, the precision and repeatability of this type of servo valve is insufficient for certain applications. In fact, the curve of the movement of the bimetallic strip relative to the associated nozzle, as a function of the tension applied to the bimetallic strip, can have a histeresis of approximately 15%. As a result, the output characteristics of the servo-valve (pressure or flow) are affected by the same imprecision.

The object of the present invention is to remedy this drawback and to improve precision and repeatability, thanks to an improved transducer.

The Applicant has studied the deformation of the piezoelectric bimetallic strips and has observed that the material cannot be held firmly, by embedding, without counteracting the free expansion or contraction of the crystals and thereby influencing the "deformation" of the bimetallic strip. In the embedding zone, the microcrystals undergo permanent compression and, at the edge of the embedding, the material undergoes a shearing force through the end edge of the embedding surface. As a result, the arrow or the camber of an embedded bimetallic strip is less than the deflection or natural camber that this bimetallic strip could freely take and that, moreover, the deformations are not identical during a complete cycle of variation of the electrical control voltage.

The Applicant has discovered that it is possible to remedy the drawbacks described above thanks to a "floating" mounting of the piezoelectric bimetallic strip.

The subject of the invention is a transducer of the nozzle / vane type, with piezoelectric bimetallic strip, cooperating directly with the nozzle (s), in which the piezoelectric bimetallic strip is held only in position, facing the nozzle (s), by substantially punctual or linear support members, acting on the two opposite faces of the bimetallic strip, located in the vicinity of the edges of said bimetallic strip, playing the role of pivots and authorizing the free natural deformation of said bimetallic strip over its entire surface under the effect of said signals electric.

Thanks to this arrangement, any embedding of the bimetallic strip occurring on an appreciable surface thereof is completely eliminated.

All other things being equal, it has been possible to measure that with a transducer according to the invention the hysteresis is only of the order of 1% instead of approximately 15%, as indicated previously.

The invention also relates to hydraulic servo-valves controlled by a transducer / piezo-hydraulic as defined above.

The invention will be better understood using the description and the drawings which show various embodiments of the invention.

Figure 1 is a sectional view of a hydraulic servo valve controlled by a following transducer the invention.

• Figures 2 and 3 are partial views, considerably enlarged, of the fixing of the bimetallic strip, at rest and when the bimetallic strip deforms.
• Figure 4 illustrates the fixing of the bimetal strip by elastic supports.
• FIG. 5 illustrates the fixing of the bimetal strip by elastic mechanical supports.
• FIG. 6 shows the rotary bearings of the bimetallic strip by balls.
• Figure 7 is a schematic view of a nozzle / paddle transducer with a single nozzle.
• Figure 8 is a sectional view of a circular vane transducer, with a single nozzle.
• Figure 9 is a partial view of the transducer of Figure 8, with another embodiment of the pallet fixing system.
• Figures 10-and 11 show the floating attachment of the pallet by hydrostatic supports.
• Figures 12A, 12B, 12C and 13A, 13B, 13C are perspective views of worked piezoelectric paddles.
• Figure 14 is a schematic representation of yet a variant of floating attachment of the pallet in a transducer according to the invention.

The hydraulic servo-valve shown in FIG. 1 essentially comprises an electro-fluidic transducer 2, constituting a pilot stage for a distributor 4. Such a servo-valve can be used to control a receiving device, for example a double-acting hydraulic cylinder 6.

In the embodiment shown, the transducer 2 comprises two nozzles or nozzles 8, 8 ', fixed in a casing 10, which open directly opposite the two faces of a pallet or bimetallic strip piezoelectric 12 receiving the electrical control signal by a conductor 14.

As is known, the deformation of the pallet 12 modifies the leakage ratio between the two nozzles 8, 8 '.

The distributor 4 can be of any known type. It comprises for example a switching slide 16 closing or uncovering the high pressure and low pressure lines 18, 20 as well as the use lines 22, 24 going to the jack 6. The slide 16 is centered by springs 26, 26 'and delimits, at its two ends, pressure chambers 28, 28 'which are in communication, by conduits 30, 32, 30', 32 'with the nozzles 8, 8'.

The operation of such a servo-valve is sufficiently known that it suffices to recall that the electrical signals applied to the piezoelectric paddle 12 make it possible to control the motor member 6.

According to the invention, and as is clearly represented in FIG. 1, the pallet 12 is mounted "floating" in a chamber 33 limited by the walls of the casing 10, that is to say without any embedding of a part of the pallet in a rigid support, for example in the walls of the casing 10.

The pallet 12 is held only in position, facing the nozzles 8, 8 ', by substantially punctual or linear support members forming a suspension of the ball type.

In the case of FIG. 1 (and assuming that the pallet 12 is an elongated blade of rectangular shape), the first supports, 'on one of the faces of the pallet, are constituted by two supports, in the form of an edge , 34, 34 'integral with a first wall of the casing 10. Second supports, on the opposite face of the pallet, are constituted by elastic pressers 36, 36 ', applied against the pallet by springs 38, and whose heads are spherical, or cylindrical, to minimize friction and allow a "rolling" of the pallet on the pressers. The elastic pressers 36, 36 'are supported by the second wall of the casing 10.

The first members 34 form substantially linear supports, the second members 36 form substantially punctual supports, the first and second support members being located opposite one another, on the two opposite faces of the pallet, in the vicinity of the edges from the palette.

Adjustment stops or shims 40, 40 ′ (fixed or adjustable) can be provided in the casing 10 to ensure the longitudinal centering of the pallet 12 in the casing.

There is shown diagrammatically, in FIG. 2, on an enlarged scale, the floating mounting system, on only one of the edges of the pallet 12, the pallet being represented in solid lines in its flat position of rest and in broken lines in its curved position (considerably exaggerated) under the effect of an electric voltage. It can be seen that, when it deforms, the pallet pivots without friction around the edge of the fixed support 34 and pushes the movable elastic pusher 36, without, practically, the free deformation of the pallet being hampered, as happened with the paddles embedded in the housing.

In the variant shown in Figure 3, the fixed support 42 no longer has a sharp edge, but a cylindrical support surface (or spherical in the case of a point support) so that there is a rolling of the surface of the pallet against the surface of the support member.

In the variant shown in Figure 4, the support members may be formed (on one or both sides of the pallet 12) by balls 44 of elastic material or, in the case of the use of a pallet of circular shape, by circular joints in elastic material.

In the variant shown in Figure 5, the support members are constituted by fingers 46, 46 ', with a spherical or cylindrical head, the fingers 46 forming the fixed supports, while the fingers 46' are pressed elastically against the pallet 12 by springs 38.

According to the variant of FIG. 6, the support members of the pallet 12 consist of pairs of two metal balls 48, 48 ′ housed in cavities 50, 50 ′ hollowed out in the walls of the housing 10 of the transducer. The balls form, for the pallet, a ball suspension which does not impede the free deformation of the pallet.

Of course the different types of support members which have been described can be combined together, for example a metal ball (FIG. 6) facing an elastic ball (FIG. 4), or opposite an elastic pusher such as 46 'in Figure 5.

The number of pairs of support members may vary depending on the shape of the pallet. For example, for a rectangular pallet, one can use (FIG. 1) two pairs of support members each comprising a linear support (34) and a point support (36), or else three or four pairs of support organs. occasional support (Figures 4, 5 and 6).

In the case of a circular pallet, three pairs of support members are advantageously used equiangularly distributed, or even an infinite number of continuous support points, forming a continuous support line, as will be seen in connection with the embodiment of FIG. 8.

Above all, reference has been made to transducers comprising a nozzle or nozzle opposite each of the faces of the piezoelectric bimetallic strip, the two nozzles operating differently. But the invention also applies, of course, to the case where the transducer comprises only a single nozzle 8 facing one of the faces of the pallet 12, as shown schematically in FIG. 7. Again , the pallet 12 is held in place by a floating assembly comprising ball bearings 34, 34 'and 36, 36', for example similar to those of FIG. 1.

FIG. 8 shows an embodiment of a transducer comprising a single nozzle 8 facing the pallet 12, the latter having a circular shape and not a rectangular shape.

The casing 10 is formed by two assembled parts 10, 10 '. The part 10 ′ of the casing has a projecting circular lip 52, for example having a rounded profile, which constitutes a continuous line of support points on which bears the lower face of the pallet 12 and against which this lower face can move by rolling during deformation of the pallet.

The second supports, on the opposite face of the pallet 12, are constituted by a circular joint with round section 54, of elastic material, which is positioned in a circular groove 56 hollowed out in the part 10 of the casing.

The circular pallet 12 is thus supported, in a floating manner, without embedding, in the vicinity from its peripheral edge, by two continuous circular support lines facing each other.

A variant of this arrangement, for the same transducer, is partially shown in FIG. 9 where the projecting circular lip 52 ', formed in the part 10' of the casing, has an edge profile. The pallet 12 is then free to pivot around this edge, as has been explained in connection with FIGS. 1 and 2, this edge forming a continuous line of support points. The second support members may be constituted by a plurality (for example 3) of pushers 46 ′, with a spherical head, subjected to the action of springs 38 ′ and similar to what is shown in FIG. 5.

Of course the same suspension mode, with continuous line supports and / or with point supports, can be applied to piezoelectric pallets of polygonal shape.

Mechanical or elastic support members have so far been described and shown to hold the pallet 12, but it is also possible to use, on one of the faces or on both sides of the pallet, 'hydrostatic type support similar to those used for example in hydrostatic bearings.

In FIG. 10, a floating assembly of a pallet 12 is shown schematically in a casing 10 by means of a mechanical support 34 in the form of an edge, on one of the faces of the pallet, and by means of a hydrostatic nozzle. 58, supplied with pressurized liquid through a conduit 60.

In the embodiment of FIG. 11, the suspension is made hydrostatically, by nozzles 58, 58 ′, on the two faces of the pallet 12, the latter being kept centered in the casing 12 by centering shims 40.

Of course, and in particular in the case of a circular-shaped pallet, a plurality of hydrostatic stops (for example three equiangularly distributed) is provided, the nozzles preferably supplying hydrostatic grooves or "pockets" as is usual for fluid bearings.

The hydrostatic suspension system above is advantageous, because it gives a sufficiently stiff suspension, without imposing any mechanical friction on the pallet, which leaves it with its entire natural freedom of deformation. In addition, in a hydraulic servo-valve, the hydraulic fluid under High pressure (for example 100 to 400 bar) is available, to supply the hydrostatic suspension, without requiring any additional installation. Of course, the bimetallic strip operates in the hydraulic fluid (oil) which fills the chamber 33 of the casing, as in the cases where the pallet support system is mechanical or elastic.

Transducers using piezoelectric vanes of rectangular, polygonal or circular shape have been described in the foregoing, these vanes being full.

However, it is also possible to use, as shown in FIGS. 12A, 12B, 12C, an ornate pallet 12 having cutouts 62 leaving at least one free strip 64 between them, on which no support member acts.

The support members have been shown diagrammatically by arrows 66, 66 ', 66 ", these support members possibly being of any of the types described in the foregoing. The nozzle (s), shown diagrammatically by their point of impact 68 on the pallet 12, open opposite the free strips 64 of the pallet. cutouts 62 make it possible to adapt the flexibility of the bimetal pallet while moving the influence of the fixing away from the zone of maximum stress.

It is also possible to provide, as shown in FIGS. 13A, 13B, 13C, more complex shapes, having several slots for forming bimetallic strips 12 which are particularly worked in order to perform multiple functions. Each active strip 64 of the bimetallic strip can be associated with a nozzle or with two nozzles, the locations of which have been indicated 68. The support members have been symbolized by arrows 66.

Each strip 64 can be electrically decoupled from its neighbors by cutting a part of the electrodes according to grooves 70 or 70 'judiciously arranged so as to decouple the mechanical interaction of the bimetallic strips 12 between them, each active strip receiving its electrical signal by a separate conductor 14, 14 ', 14 ". As shown in FIG. 13C, each active strip 64 can be fixed in a two-support version, the location 68 of the nozzle (or nozzles) being located between two supports. But, as it is represented in FIGS. 12A, 12B, 12C, 13A, 13B, each strip 64 can be fixed by simple support, in the manner of a cantilever beam, the location 68 of the nozzle then being at the free end of the blade to benefit, vis-à-vis the nozzle, from the total deformation of the bimetallic strip.

An attachment similar to a cantilever attachment can also be achieved with a pallet 12 of simple, elongated rectangular shape, as is schematically shown in FIG. 14, where the deformation of the pallet 12 has been considerably exaggerated. In this case, two fixed supports 34, 34 'close together, are provided in the vicinity of one of the edges 72 of the pallet and cooperate with opposite elastic supports 36, 36' while the nozzles 8, 8 'cooperate rent with the pallet 12 in the vicinity of its free edge 74, so as to use all of the deformation of the bimetallic strip over substantially its entire length.

It is known that one of the drawbacks of the piezoelectric bimetallic strips used in the nozzle / paddle transducers so far is the high control voltage which they require (more than 100 V and, generally of the order of 200 V ) and that their other drawbacks are the small displacement (a few tens of microns), displacement which is thwarted by the embedding of the bimetallic strip in the casing, as well as the non-reproducibility of the displacements during a deformation cycle.

The floating mounting system of the bimetallic strip according to the invention, without any embedding, makes it possible to obtain a much higher "displacement / control voltage" ratio and to reduce to a very great extent the hysteresis phenomenon.

Claims (7)

1. Electro-fluidic transducer of the nozzle / pallet type, in particular for controlling a hydraulic servo-valve from electrical signals, in which the movable pallet is constituted by a piezoelectric bimetallic strip of which at least one of the faces cooperates directly with at least one nozzle to control, as a function of the deformations of said bimetallic strip caused by said electrical signals, the leakage rate of said one or more nozzles, said piezoelectric bimetallic strip (12) being maintained in position, facing the or nozzles (8, 8 ') by support members (34, 34', 36, 36 '), acting on the two opposite faces of the bimetallic strip, located in the vicinity of the edges of said bimetallic strip, said transducer being characterized: - in that the support members are in substantially punctual contact with the faces of the bimetallic strip; - in that the support members are arranged in pairs (34, 36), facing one another, on the two opposite faces of the bimetallic strip; - in that the support members (34, 42, 46, 48 ') located on one of the faces of the bimetal strip are fixed rigid supports; - in that the support members (36, 36 ', 46') located on the other face of the bimetallic strip are movable in at least one direction perpendicular to the faces of the bimetallic strip; and - In that the support members form, in pairs, rotational pivots allowing the free natural deformations of said bimetallic strip over its entire surface under the effect of said electrical signals. 2. Transducer according to claim 1, characterized in that the support members consist of balls (48, 48 '). 3. Transducer according to claim 1, characterized in that the support members consist of pressing fingers (36-42-46-46 ') with a substantially spherical hard end surface; and in that the fingers (36 - 46 ') located on the aforementioned other face of the bimetallic strip are urged towards the bimetallic strip by springs (38). 4. Transducer according to one of claims 1 to 3, characterized in that the support members are provided by a fluid suspension (58, 58 ') similar to a fluid bearing, without physical support on the bimetallic strip. 5. Transducer according to one of the preceding claims, which comprises a casing (10) forming a closed chamber (33), said casing supporting at least one nozzle (8) of fluid inlet whose nozzle opens facing the 'one of the faces of the bimetallic strip (12), said transducer being characterized in that the abovementioned support members (34, 36) are carried, inside said chamber (33) by the walls of the casing. 6. Transducer according to claim 5, characterized in that the bimetallic strip (12) mounted freely between its support members, is centered laterally in the aforementioned chamber (33) by centering shims (40, 40 ') carried by the casing (10). 7. Hydraulic servo-valve (4), characterized in that it is controlled by an electro-piezo-hydraulic transducer (2) with nozzles / pallet according to one of the preceding claims.

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