CH720396A2 - Shock-resistant piezoelectric rotary motor, particularly for watchmaking - Google Patents

Abstract

The invention relates to a rotary piezoelectric motor (1), in particular for a timepiece, the motor (1) comprising: – a rotor (3) configured to be able to rotate and actuate a mechanical device, – a stator (2) configured for rotating the rotor (3), the stator (2) comprising a piezoelectric actuator, the piezoelectric actuator comprising a movable element (5) whose movement causes the rotor (3) to rotate in a first direction, the piezoelectric actuator comprising only two electrically actuable resonators, the two resonators being connected to the movable element (5) to move it against the rotor (3) in order to make it rotate, the two resonators (6, 7) being arranged relative to the element movable (5) so as to oscillate the movable element (5) in a first and a second direction different from each other, each resonator (6, 7) comprising a center of rotation, the two resonators (6 , 7) being arranged relative to the movable element (5), so that around each center of rotation, the torque resulting from all the acceleration forces applying in the plane of each resonator (6, 7) ) is zero.

Description

Technical field of the invention

[0001] The invention relates to the technical field of rotary piezoelectric motors. The invention also relates to the technical field of timepieces equipped with such a rotary piezoelectric motor.

Technology background

[0002] The electric motors usually used in watchmaking are rotary motors of the “Lavet” type, which operate on electromagnetic physical principles. Such a motor generally comprises a stator equipped with coils and a magnetized rotor, which rotates by out-of-phase actuation of the coils.

[0003] However, these motors have limited resistance to high magnetic fields. From a certain magnetic field value, the motor blocks. In general, they block under the effect of a magnetic field that exceeds 2 mT.

[0004] Thus, to avoid this problem, it is necessary to design motors operating on other physical principles.

[0005] There are, for example, electrostatic comb motors, such as that described in patent CH709512. But the combs take up space, and they consume more energy than “Lavet” type motors.

[0006] Motors based on the piezoelectric effect have also been developed, for example in patent EP0587031. But this is limited to the activation of a date. However, its high consumption and the risk of premature wear do not allow the driving of a seconds hand, which generally requires the most energy.

[0007] To limit consumption, a piezoelectric motor with orbital movement is described in patent application EP21216102.0 filed in the name The Swatch Group Research and Development Ltd. In this motor, the rotor is driven by a movable ring-shaped element describing an orbital movement, so as to come into contact with the rotor, which is arranged inside the ring. To move the movable element, a piezoelectric actuator comprises several piezoelectric resonators formed of oscillating flexible arms, the arms holding and moving the movable element, the arms comprising an actuable piezoelectric material.

[0008] However, despite the great compactness of this solution, a lateral impact directly causes a force on the ring, thus disrupting the orbital movement and potentially causing a chronometric loss of the clock movement equipped with such a piezoelectric motor. .

Summary of the invention

[0009] The aim of the present invention is to provide a rotary piezoelectric motor, which can withstand high electromagnetic fields, which withstands lateral shocks, while keeping energy consumption and a reduced volume.

[0010] For this purpose, the invention relates to a rotary piezoelectric motor, in particular for a timepiece, the motor comprising: – a rotor configured to be able to rotate and actuate a mechanical device, – a stator configured to rotate the rotor, the stator comprising a piezoelectric actuator, the piezoelectric actuator comprising a movable element whose movement causes the rotor to rotate in a first direction.

[0011] The invention is remarkable in that the piezoelectric actuator comprises two electrically actuable piezoelectric resonators, the two resonators being connected to the movable element to move it against the rotor in order to make it rotate, the two resonators being arranged by relative to the movable element so as to cause the movable element to oscillate in a first and a second direction different from each other, each resonator having a center of rotation, the two resonators being arranged relative to the element mobile, so that around each center of rotation, the torque resulting from all the acceleration forces applying in the plane of each resonator is zero.

[0012] A stator having such a configuration makes it possible to easily transmit a rotational movement to the rotor thanks to a piezoelectric actuator. Indeed, the mobile element can be moved in order to be in contact with the rotor to transmit a movement in a first direction. Thus, when the resonators oscillate, the mobile element performs an orbital rotary movement to come into contact with the rotor and transmits a force to it to make it rotate in a first direction.

[0013] As the torque resulting from all the forces exerted on each center of rotation is zero in the plane of each resonator, the effect of lateral shocks is greatly reduced, or even canceled. Thus, the risk of disrupting the orbital movement and therefore chronometric losses in the case of a clock engine is avoided.

[0014] Furthermore, by having two piezoelectric resonators to produce oscillations in significantly different directions, the mobile element can be moved in an orbital movement, without needing to multiply the number of resonators.

According to a particular embodiment of the invention, the first and the second resonator are arranged perpendicular to each other, so that the first and the second directions are substantially perpendicular. We can thus obtain a circular orbital movement.

[0016] According to a particular embodiment of the invention, the first and the second resonator are each arranged on a different side of the movable element, the two sides being preferably adjacent.

[0017] According to a particular embodiment of the invention, the piezoelectric motor comprises a first translation table allowing the mobile element to move in the first direction.

[0018] According to a particular embodiment of the invention, the piezoelectric motor comprises a second translation table allowing the movable element to move in the second direction.

[0019] According to a particular embodiment of the invention, the second translation table is arranged in series with the first translation table, the mobile element being connected to the second translation table.

[0020] According to a particular embodiment of the invention, the first translation table and the second translation table are substantially perpendicular to each other.

[0021] According to a particular embodiment of the invention, a translation table is arranged on the other side of the movable element relative to a resonator.

According to a particular embodiment of the invention, each resonator is provided with an oscillating mass actuated by a pair of flexible blades comprising a piezoelectric material.

[0023] According to a particular embodiment of the invention, the movable element performs an orbital movement in a second direction opposite to the first direction.

According to a particular embodiment of the invention, the movable element is always in contact with the rotor during operation of the rotary motor.

According to a particular embodiment of the invention, the movement of the movable element causes the rotor to rotate continuously.

According to a particular embodiment of the invention, the movable element has the shape of a ring, the rotor being arranged inside the ring.

According to a particular embodiment of the invention, the contact between the movable element and the movable rotor is inside the ring.

[0028] According to a particular embodiment of the invention, the rotor comprises a toothed wheel, the ring comprising internal teeth cooperating with external teeth of the toothed wheel.

[0029] According to a particular embodiment of the invention, the movable element is immobile in rotation on itself.

[0030] According to a particular embodiment of the invention, the movable element is arranged around the rotor.

According to a particular embodiment of the invention, the first and the second resonator are actuated with a phase shift of 90°.

According to a particular embodiment of the invention, the first and the second resonator are each arranged on a different side of the movable element, the two sides being preferably adjacent.

[0033] The invention also relates to a timepiece comprising a timepiece movement comprising a gear transmission configured to rotate at least one hand, and comprising such a piezoelectric motor arranged to actuate the gear transmission .

Brief description of the figures

[0034] Other particularities and advantages will emerge clearly from the description given below, for information only and in no way limiting, with reference to the appended drawings, in which: - Figure 1 schematically represents a top view of an embodiment of the rotary piezoelectric motor according to the invention when stopped, the rotor and the stator not being in contact, - Figure 2 schematically represents a top view of the embodiment of the rotary piezoelectric motor according to the invention in operation, the rotor and the stator being in contact at six o'clock, - Figure 3 schematically represents a top view of the embodiment of the rotary piezoelectric motor according to the invention in operation, the rotor and the stator being in contact at nine o'clock, - Figure 4 schematically represents a top view of the embodiment of the rotary piezoelectric motor according to the invention in operation, the rotor and the stator being in contact at noon, - Figure 5 schematically represents a top view of the embodiment of the rotary piezoelectric motor according to the invention in operation, the rotor and the stator being in contact at three o'clock, and - Figure 6 schematically represents a top view of a resonator of the piezoelectric motor.

Detailed description of the invention

Figures 1 to 5 show an embodiment of a rotary piezoelectric motor 1. The piezoelectric motor 1 can in particular be used in a timepiece to operate a display device, such as hands arranged on a dial. The piezoelectric motor 1 preferably extends substantially in a plane.

The piezoelectric motor 1 comprises a rotor 3 movable in rotation on itself, and configured to be able to rotate and actuate a transmission of mechanical gears, in particular for a display device. The piezoelectric motor 1 includes a stator 2 configured to actuate and rotate the rotor 3.

The rotor 3 is for example a toothed wheel 9 arranged in the center of the piezoelectric motor 1. The toothed wheel 9 is for example mounted on an axis provided with a pivot at each end, these pivots being mounted in bearings allowing the axis to rotate. The toothed wheel 9 comprises an external ring 28 and a hub 27 in the center, the hub 27 being connected to the ring 28 by rigid spokes 19. The axis 13 comprises a pinion 21 parallel to the toothed wheel 9, and arranged to transmit the movement received by the toothed wheel 9 to a gear transmission, for example to a movement of a timepiece. The rotor 3 is provided with peripheral teeth 10 on the ring 28, which makes it possible to operate the toothed wheel 9.

Preferably, the rotor 3 and/or the stator 2 comprises a micro-machinable material, such as silicon, preferably entirely. Alternatively, the rotor 3 may be made of metal so as to limit wear and friction, when the stator 2 is made of silicon, and vice versa.

[0039] Still alternatively, by micro-machining, the rotor 3 and/or the stator 2 comprises, preferably entirely, a material, such as quartz, nickel (obtained by electrodeposition of metal or by a LIGA type process ), or diamond (obtained by an ALD type deposit), or glass (obtained by Selective Laser Etching or SLE).

The stator 2 comprises a stationary fixed element 4 and a movable element 5 configured to actuate the toothed wheel 9 of the rotor 3. The movable element 5 is arranged at a distance from the fixed element 4. The movable element 5 has here a ring shape with a square frame exterior shape, and a circular interior shape.

The movable element 5 is arranged around the rotor 3, the rotor 3 being arranged inside the ring. The movable element 5 is provided with internal teeth 12 on the circular shape of the ring, the internal teeth 12 cooperating with the peripheral teeth 10 of the rotor 3 to make it rotate. The ring is wider than the rotor 3 to be able to insert the rotor 3, and to allow the movement of the mobile element 5.

[0042] Thanks to the movement of the movable element 5, and its contact with the rotor 3, the rotor 3 rotates in a first direction.

[0043] To this end, the stator 2 is equipped with a piezoelectric actuator.

The piezoelectric actuator comprises two electrically actuable resonators 6, 7. A first resonator 6 and a second resonator 7 are connected to the movable element 5 to be able to move it against the rotor 3 in order to rotate the latter.

The resonators 6, 7 are configured to generate an oscillatory movement, so as to guide the mobile element 5 in an orbital movement. The first resonator 6 allows the mobile element 5 to move in a first horizontal direction X, and the second resonator 6 allows the mobile element 5 to move in a second vertical direction Y.

The first resonator 6 and the second resonator 7 each comprise an oscillating mass 20. Each oscillating mass 20 has a longitudinal shape extending along one side 9 of the movable element 5. Each oscillating mass 20 comprises at minus a flyweight at one end.

When a resonator 6, 7 is actuated, the oscillating mass 20 pivots around a center of rotation while oscillating.

The characteristics and operation of the resonators 6, 7 are described in detail later in the description. The oscillations take place transversely to the side of the frame.

Each oscillating mass 20 is connected to the movable element 5 by a secondary flexible blade 11, 12 which is substantially straight. The secondary flexible blades 11, 12 are attached to a weight 21 arranged at the end of the oscillating mass 20, from a stud 22 extending from two adjacent sides of the movable element 5. The secondary flexible blades 11, 12 are substantially perpendicular to the oscillating mass arms 20.

The secondary flexible blades 11, 12 are arranged orthogonally relative to each other, along the two adjacent sides of the movable element 5.

[0051] When the oscillating masses 20 oscillate out of phase, each secondary flexible blade 11, 12 alternately pulls then pushes the mobile element 5.

[0052] Thus, an orbital movement of the mobile element 5 is created. By orbital movement we mean a circular movement of the mobile element 5 around an off-center axis of rotation. In addition, the mobile element 5 does not make any rotary movement on itself, because this degree of freedom is blocked by flexible translation tables tek described below.

[0053] In this invention, only two resonators 6, 7 are used to create this orbital movement. It is not necessary to provide an additional resonator to obtain this displacement.

To actuate the movable element 5, the first and second resonators 6, 7 oscillate in substantially orthogonal directions.

The two resonators are preferably arranged perpendicular to each other, and are arranged on two adjacent sides of the movable element 5. Thus, a substantially circular orbital movement is obtained.

[0056] To accompany and guide the movement of the movable element 5 on a third side, the movable element 5 is further connected to the stator 4 by two flexible translation tables 24, 25. A first translation table 24 and a second translation table 25 are arranged in series, the movable element 5 being attached to the second translation table 25.

[0057] Each translation table 24, 25 is provided with two substantially parallel tertiary flexible blades 31, 32, 33, 34, and with a rigid movable part 35, 36 to which the tertiary flexible blades 31, 32, 33 are connected. , 34.

The tertiary flexible blades 31, 32 of the first translation table 24 are connected to the stator 2 at one end and to a first rigid part 30 at the other end.

The tertiary flexible blades 33, 34 of the second translation table 25 are connected to the first rigid part 35 at one end, and to a second rigid part 36 at the other end. The second rigid part 36 is connected to the frame of the mobile element 5.

The first translation table 24 allows the mobile element 5 to move according to a first degree of freedom, horizontally along the axis X, and the second translation table 25 allows the mobile element 5 to move according to a second degree of freedom, vertically along the Y axis. Preferably, the second degree of freedom is substantially orthogonal to the first degree of freedom.

[0061] To this end, the first translation table 24 and the second translation table 25 are substantially orthogonal to each other. The two tertiary flexible blades 31, 32, 33, 34 prevent the mobile element 5 from pivoting on itself, but allow lateral movement. This characteristic allows the mobile element 5 to transmit a torque to the rotor 3 as described below.

[0062] Each translation table 24, 25 is arranged on the other side of the movable element 5 with respect to one of the resonators 6, 7. In other words, a pair formed of a resonator 6, 7 and d a translation table 24, 25 is arranged on either side of the movable element 5 in the same direction, thus ensuring great compactness of the motor 1.

The resonators 6, 7 are configured to move the mobile element 5 against the rotor 3 to make it rotate. To this end, the resonators 6, 7 are operated out of phase with each other.

[0064] The phase shift between the resonators 6, 7 generates the orbital movement, preferably circular, of the mobile element 5. The mobile element 5 performs a circular movement, while remaining immobile in rotation on itself, thanks to the two translation tables 24.25.

The movement of the movable element 5 is preferably continuous, and causes the rotor 3 to rotate continuously. To this end, the movable element 5 is always in contact with the rotor 3 during engine operation. The contact point P between the movable element 5 and the rotor 3 is movable inside the ring.

[0066] Figures 2 to 5 show different successive moments during which the contact point P between the rotor 3 and the movable element 5 moves inside the ring, here in a clockwise direction. The orbital movement of the ring, the interior space of which is wider than the rotor 3, generates a mobile contact point P between the ring and the rotor 3. A different portion of the internal teeth 12 of the ring meshes the peripheral teeth 10 of the toothed wheel 9 at each moment. Thus, the rotor 3 is rotated on itself in the direction opposite to P, i.e. counterclockwise.

[0067] In Figure 2, the movable element 5 is reassembled, so that the contact point P is at the bottom of the toothed wheel 9, i.e. at six o'clock. In Figure 3, the movable element 5 has shifted to the right, so that the contact point P is to the left of the toothed wheel 9, i.e. at nine o'clock. Then the mobile element 5 is lowered, so that the contact point P is at the top of the toothed wheel 9, i.e. at noon, as shown in Figure 4. Finally, in Figure 5, the mobile element 5 s 'is shifted to the left, so that the contact point P is to the right of toothed wheel 9, i.e. at three o'clock. From one figure to the next, the mobile element 5 has made an orbital movement of a quarter turn. The secondary flexible connecting blades 11, 12, the tertiary flexible blades 31, 32, 33, 34 of the two translation tables 24, 25, and the flexible blades of the resonators 6,7 bend depending on the direction in which the mobile element 5 moves. The oscillating masses 20 also follow the movement: they oscillate sinusoidally with a phase shift of 90° between each of the resonators 6, 7.

The rotor 3 and the mobile element 5 form what is commonly called in mechanics a harmonic reducer. The teeth 10 of the rotor 3 comprise for example 56 teeth, while the teeth 12 of the movable element 5 comprise 60 teeth. Thus, the reduction factor r between the speed of the contact point and the speed of the rotor is given by where Zm designates the number of teeth of the movable element 5, and Zr designates the number of teeth of the rotor 3. Thus, in our example, . This reduction is advantageous because it is directly integrated into the motor, thus reducing the number of additional reduction cogs needed to drive a needle for example.

Preferably, at least one tooth of the teeth 10 of the rotor 3 are in contact with the teeth 12 of the movable element 5 to transmit the movement. Thus, the risk of blocking the rotor 3 is avoided. The mobile element 5 and the rotor 3 can be dimensioned so that only one tooth of the toothing 10 is in contact with the toothing 12 of the rotor 3.

[0070] Preferably, the amplitudes of the alternating voltages applied to the resonators 6, 7, capable of causing the mobile element 5 to oscillate, are variable so as to make the oscillation of the mobile element 5 perfectly circular, with the aim of compensate for any unwanted ovalization of the trajectory, and thus also increase the efficiency of motor 1.

[0071] The electrical signals applied to each of the two resonators 6,7 are preferably sinusoidal and phase shifted by 90°: when one of the amplitudes is at its maximum, the other is zero, and vice versa.

[0072] If we want to rotate the rotor 3 in the other direction, it suffices to reverse the sign of the phase shift of the electrical voltages applied to the resonators 6, 7. Thus, the oscillations of the oscillating masses 20 cause the rotation of the movable element 5 of the stator 2 in the other direction. In the case of operating a needle display, this allows the position of the hands to be adjusted in both directions.

[0073] In the case of a watch, the resonance frequency or natural frequency of each of the resonators 6, 7 of the piezoelectric motor 1 is adapted to the frequency of the quartz, which is used to adjust the running of the movement. By operating at the resonant frequency, a reasonable amplitude is obtained for a given consumption.

[0074] We choose an excitation frequency corresponding not only to the resonance frequency, but also to a submultiple of the quartz frequency, which is generally 32764 Hz. For example, we choose a frequency of 128 Hz or 256 Hz. . The frequency of motor 1 is preferably adjusted and tuned to the excitation frequency so that its oscillation amplitude does not fall below 90-95% of the maximum amplitude at resonance.

[0075] The frequency is adapted by modifying the mass of the mobile element 5 and/or the rigidity of the flexible blades. For example, we can assemble a ring under the mobile element 5 to weigh it down in order to lower its oscillation frequency. The ring, which is not shown in the figures, comprises for example nickel silver, preferably entirely.

[0076] We can also add micro-dots of glue to finely lower the frequency.

[0077] The frequency can also be lowered by removing material from the elastic elements, for example by means of a laser or by milling, to reduce their rigidity.

[0078] To increase the frequency, the mass of the mobile element 5 can be reduced by removing material, for example also by means of a laser or by milling. As these processes allow very precise adjustment, they will preferably be used to tune the quartz motor.

[0079] As the resonators 6, 7 are micro-machined, it is also possible to produce, during construction, small weights to be removed to increase the frequency to a target value.

[0080] The resonance peak of the motor coupled to its load is dimensioned sufficiently large, i.e. much larger than that of quartz. This is why it is possible to slightly vary the speed of the motor by changing its excitation frequency, without losing much amplitude, for example to compensate for a loss of state following a shock or any other disturbance, in order to realign the quartz time base with the position of the hands.

[0081] According to the invention, the two resonators 6, 7 are arranged relative to the movable element 5, so that the torque resulting from all the acceleration forces applying in the plane of each resonator 6, 7 or zero.

This advantage is obtained thanks to the configuration of the piezoelectric motor described previously.

[0083] For example, during a sudden horizontal acceleration (along the axis tend to push them to the left.

[0084] The resonators 6.7 are sized and arranged relative to the movable element 5, such that the torque resulting from all the forces applying around the center of rotation of the resonator 6.7 is zero in the plane of each resonator 6, 7.

[0085] Thus, the resonator 6.7 can oscillate without being disturbed by a lateral impact. This is always true for other shock directions acting on this same resonator 6, 7, if the center of mass of the single resonator is located on a straight line passing through the pivot point.

[0086] Figure 6 shows a resonator 6, 7, such as those used in the piezoelectric motor of Figures 1 to 5. The resonator 6 comprises an oscillating mass 20 provided with a main arm, a first flyweight 44 at a first end , and a second flyweight 45 at a second end, the second flyweight 45 forming an elbow folded under the main arm.

The base 43 has a parallelepiped shape offset towards the first flyweight 44 which is substantially straight, a first corner being oriented towards the folded elbow of the second flyweight 45. The base 43 is arranged between the first flyweight 44 and the folded elbow of the second flyweight 45. The base 43 includes an oblique channel 38 open from the first corner towards the inside of the base 43.

[0088] The resonator comprises a flexible guide provided with a first flexible blade 36 connecting the oscillating mass 20 to the base 43, from the end of the folded elbow, the first flexible blade 36 extending in the oblique channel 38 up to 'at an attachment point at the bottom of the oblique channel 38.

[0089] The flexible guide comprises a second flexible blade 37 extending parallel to the arm of the oscillating mass 20, from a second corner of the base 43 to an attachment point inside the folded elbow of the oscillating mass 20. The second flexible blade 37 is arranged above the first flexible blade 36.

[0090] The first flexible blade 36 and the second flexible blade 37 form a “Y”, and extend so as to form a non-zero angle of between 10° and 80°, preferably between 30° and 60°, or even between 40° and 50°.

[0091] The two flexible blades 36, 37 comprise a piezoelectric material, placed here entirely on the second flexible blade 37, and partly on the first flexible blade 36. The actuation of the flexible blades 36, 37 is identical to that of the previous embodiments, thanks to electrical contacts not shown in the figures.

[0092] The flexible blades have, for example, a layer of piezoelectric material sandwiched between two layers of electrodes. The electrode layers are themselves arranged above a monolithic structural support material, for example monocrystalline or polycrystalline silicon, such as quartz, glass, metal, etc.

[0093] To actuate the flexible blades 36, 37, the base 43 comprises several electrical contacts 9 connected to the electrode layers to receive an electric current and actuate the piezoelectric layers of the flexible blades.

[0094] The piezoelectric layers preferably comprise a crystalline or polycrystalline material, for example solid ceramic (for sodium potassium niobate) or PZT type (for lead titano-zirconates), the flexible blades 36, 37 having a thickness allowing them to to deform.

[0095] Thus, by electrically activating the layers of piezoelectric material, the flexible blades 36, 37 deform alternately laterally towards the center and the outside. Activation is produced with an alternating voltage. Thanks to the actuation of the piezoelectric layers, the flexible blades 36, 37 bend slightly, then straighten alternately at a predefined frequency.

[0096] By choosing to actuate the two flexible blades 36, 37 in opposition to phase, the oscillating mass 20 makes small oscillations around a center of rotation corresponding to the crossing point of the two flexible blades. Thus, the oscillating mass 20 oscillates and the two weights 44, 45 move laterally at a certain frequency.

[0097] The resonators 6, 7 preferably comprise a monocrystalline or polycrystalline material, such as silicon, glass, ceramic, or a metal.

[0098] The resonators 6, 7 are for example obtained by photo-lithographic micro-machining processes of the MEMS type (for micro-electro mechanical systems). The qualities of rigidity, elasticity and the machining precision of such materials confer a high quality of resonance to the resonators 6, 7.

[0099] Furthermore, the non-magnetism and low conductivity characteristics of some of these materials make it possible to obtain excellent resistance to high-value direct and alternating magnetic fields.

[0100] Furthermore, the resonators 6, 7 are configured to oscillate the oscillating mass 20 at the natural frequency of the resonator 6, 7. Thus, the energy consumption of the resonator is limited, in particular by increasing the angular travel of the oscillating mass.

[0101] Other types of resonators are of course possible, such as RCC, double RCC type resonators or spiral resonators. Examples of piezoelectric resonators are described in patent applications EP22216410.5, EP22216418.8 and EP22216423.8.

[0102] It will be understood that various modifications and/or improvements and/or combinations obvious to those skilled in the art can be made to the different embodiments of the invention set out above without departing from the scope of the invention defined by the appended claims.

Claims (18)

1. Rotary piezoelectric motor (1), in particular for a timepiece, the motor (1) comprising: – a rotor (3) configured to be able to rotate and actuate a mechanical device, – a stator (2) configured to rotate the rotor (3), the stator (2) comprising a piezoelectric actuator, the piezoelectric actuator comprising a movable element (5) whose movement causes the rotor (3) to rotate in a first direction, characterized in that the piezoelectric actuator comprises two electrically actuable resonators, the resonators being connected to the movable element ( 5) to move it against the rotor (3) in order to make it rotate, the two resonators (6, 7) being arranged relative to the movable element (5) so as to oscillate the movable element (5) in a first and a second direction different from each other, each resonator (6, 7) comprising a center of rotation, the two resonators (6, 7) being arranged relative to the movable element (5), of so that around each center of rotation, the torque resulting from all the acceleration forces applying in the plane of each resonator (6, 7) is zero. 2. Piezoelectric motor (1) according to claim 1, wherein the first (6) and the second resonator (7) are arranged perpendicular to each other, so that the first and the second direction are substantially perpendicular. 3. Piezoelectric motor (1) according to claim 1 or 2, wherein the first (6) and the second resonator (7) are each arranged on a different side of the movable element (5), the two sides being preferably adjacent. 4. Piezoelectric motor (1) according to any one of the preceding claims, comprising a first translation table (24) allowing the movable element (5) to move in the first direction. 5. Piezoelectric motor (1) according to claim 4, comprising a second translation table (25), allowing the movable element (5) to move in the second direction. 6. Piezoelectric motor (1) according to claim 5, wherein the second translation table (25) is arranged in series with the first translation table (24), the movable element (5) being connected to the second translation table translation (25). 7. Piezoelectric motor (1) according to claim 5 or 6, wherein the first translation table (24) and the second translation table (25) are substantially perpendicular to each other. 8. Piezoelectric motor (1) according to any one of the preceding claims, in which a translation table (24, 25) is arranged on the other side of the movable element (5) relative to a resonator ( 6, 7). 9. Piezoelectric motor (1) according to any one of the preceding claims, in which each resonator (6, 7) is provided with an oscillating mass (20) actuated by a pair of flexible blades (36, 37) comprising a piezoelectric material. 10. Piezoelectric motor (1) according to any one of the preceding claims, wherein the movable element (5) performs an orbital movement, preferably circular, in a second direction opposite to the first direction. 11. Piezoelectric motor (1) according to any one of the preceding claims, wherein the movable element (5) is always in contact with the rotor (3) during operation of the rotary motor. 12. Piezoelectric motor (1) according to claim 10 or 11, wherein the movement of the movable element (5) causes the rotor (3) to rotate continuously. 13. Piezoelectric motor (1) according to any one of the preceding claims, wherein the movable element (5) has the shape of a ring, the rotor (3) being arranged inside the ring. 14. Piezoelectric motor (1) according to claim 13, wherein the contact between the movable element (5) and the movable rotor (3) is inside the ring. 15. Piezoelectric motor (1) according to claim 13 or 14, in which the rotor (3) comprises a toothed wheel (9), the ring comprising internal teeth (10) cooperating with external teeth (12) of the wheel toothed (8). 16. Piezoelectric motor (1) according to any one of the preceding claims, in which the movable element (5) is stationary in rotation on itself. 17. Piezoelectric motor (1) according to any one of the preceding claims, in which the first (6) and the second resonators (7) are actuated with a phase shift of 90°. 18. Timepiece comprising a clockwork movement comprising a gear transmission configured to rotate at least one hand, characterized in that it comprises a piezoelectric motor (1) according to any one of the preceding claims , the piezoelectric motor (1) being arranged to operate the gear transmission.