THE PIEZOELECTRIC LINEAR MOTOR

The present invention relates to a piezoelectric linear motor of the type specified in the preamble of the first claim.

The linear motors are electric motors capable of producing motion directly in linear form. In theory, linear motors can be obtained from equivalent normal rotating motors by “cutting” the stator transversely and “unrolling” it on a plane. If a moving magnet, the equivalent of the rotor, is positioned over a linear sequence of magnetic plates with alternating polarity, the equivalent of the stator, it is sufficient that the moving magnet is always in such a position as to be attracted to the next magnet, in the sense of motion, and rejected by the previous one. To obtain this basic operation, one of the two magnetic fields must be produced by suitably shaped electromagnets in which the windings are powered by the control drive according to precise voltage, amperage and phase sequences. A linear motor can therefore move in both directions of movement and the total stroke can be obtained simply by assembling the necessary number of magnets since the "stator" is composed of modular magnetic plates.

Linear motors therefore allow to directly impart feed forces without the need for transmission elements such as belts, chains or the similar. In fact, the presence of intermediate members, susceptible to wear, introduces mechanical play, friction and inertia which negatively affect the quality of the positioning and the dynamics of the entire drive.

Various construction morphologies are known for linear motors but the most common concern synchronous motors and in particular they include: single-sided structure (Ironcore), double-sided structure (Ironless) and cylindrical structure (Tubular).

The Ironcore motors consist of a track of modular permanent magnets and a slider housing the powered windings. This structure is the simplest but is also affected by magnetic field asymmetry problems. Such structure is the one that offers the best performance in terms of thrust and operating stability thanks to the ability to evacuate the heat generated but it does not represent the best solution for highly dynamic systems due to the attraction force between the armature and the inductor and the inertia of the slider resulting from the high weight.

The Ironless motors recover the symmetry of the magnetic field: the cursor with the windings in fact runs between the magnetic plates. These motors do not need metal cores for the winding of the cursor which is therefore lighter. Such construction morphology is suitable for applications where strong accelerations are required but does not allow high currents to be reached since the closed structure does not favor heat dissipation. Cylindrical motors generally have windings on the outside of the motor and this favors the dispersion of heat by convection. This constructive morphology makes the motors particularly suitable for very fast and precise movements but with limited stroke.

The known technique described comprises some important drawbacks. In particular, traditional rotary motors have significant dimensions.

Furthermore, the transmission parts of the rotating motors, in addition to being subject to wear, introduce inertia and friction which negatively affect the quality of motion.

As for linear motors, they generally require the presence of bearings and therefore can be subject to problems if not regularly overhauled. A further criticality is given by the fact that the dynamics and precision of movement required by a specific application may not be simultaneously achieved by the most widely used linear motors.

In fact, known linear motors are problematic in precision applications since, in order to keep them stationary in one position, it is necessary to feed them continuously so that the active force balances any external forces that cause the change of position. In this situation, the technical task underlying the present invention is to devise a piezoelectric linear motor capable of substantially obviating at least part of the aforementioned drawbacks. Within the scope of said technical task, it is an important object of the invention to obtain a motor with limited overall dimensions which is also simple, resistant and suitable for precision work.

Another important object of the invention is to provide a device suitable for precision applications which is economically convenient. The technical task and the specified aims are achieved by the piezoelectric linear motor as claimed in the annexed claim 1.

Preferred technical solutions are highlighted in the dependent claims.

The characteristics and advantages of the invention are clarified below by the detailed description of preferred embodiments of the invention, with reference to the accompanying figures, in which: the Fig. 1 shows a side view of the piezoelectric linear motor, connected to external elements according to the invention; the Fig. 2 illustrates a top view of a portion of the piezoelectric linear motor according to the invention; the Fig. 3a shows the magnification shown in the rectangle in Fig. 2, in a first configuration; the Fig. 3b shows the magnification shown in the rectangle in Fig. 2, in a second configuration; and the Fig. 4 shows a bottom view of a portion of the piezoelectric linear motor according to the invention.

In the present document, the measurements, values, shapes and geometric references (such as perpendicularity and parallelism), when associated with words like “about” or other similar terms such as “approximately” or “substantially”, are to be considered as except for measurement errors or inaccuracies due to production and/or manufacturing errors, and, above all, except for a slight divergence from the value, measurements, shape, or geometric reference with which it is associated. For instance, these terms, if associated with a value, preferably indicate a divergence of not more than 10% of the value.

Moreover, when used, terms such as “first”, “second”, “higher”, “lower”, “main” and “secondary” do not necessarily identify an order, a priority of relationship or a relative position, but can simply be used to clearly distinguish between their different components.

Unless otherwise specified, as results in the following discussions, terms such as “treatment”, “computing”, “determination”, “calculation”, or similar, refer to the action and/or processes of a computer or similar electronic calculation device that manipulates and/or transforms data represented as physical, such as electronic quantities of registers of a computer system and/or memories in, other data similarly represented as physical quantities within computer systems, registers or other storage, transmission or information displaying devices. The measurements and data reported in this text are to be considered, unless otherwise indicated, as performed in the International Standard Atmosphere ICAO (ISO 2533:1975).

With reference to the figures, the piezoelectric linear motor according to the invention is globally indicated with the number 1.

The piezoelectric linear motor 1 can be used for precision or similar movements in microscopy, laser interferometry, optics, microelectronics and more.

The piezoelectric linear motor 1 comprises, briefly, a first portion 2 and a second portion 3. These portions are mutually separated in the vertical direction 1b. They are capable of being reciprocally moved preferably along a movement direction 1a by motor means 4 of the piezoelectric type. Directions 1 a and 1 b are perpendicular to each other. The two portions 2 and 3 are capable of being constrained to further portions, external to the motor 1 , which must be moved relative to each other along the direction 1a. The first portion 2 preferably comprises a fixed block 20 and movable elements 21 connected to the fixed block 20 by elastic means 22.

The fixed block 20 is capable of being constrained to the center of a base 102 preferably by means of constraining means, more preferably by means of screws. The base 102 can be chosen arbitrarily and constitutes one of the two external portions which must be reciprocally moved by the motor 1. The fixed block 20 is preferably constituted by a body which has the purpose of connecting the movable elements to each other and to the external structures 21. It can be of various forms. For example, it can be a substantially flat plate having a prevalent development plane perpendicular to the vertical direction 1b. It is preferably made of metal, more preferably of steel, aluminum or other. Furthermore, the fixed block 20 comprises the movable elements 21. The movable elements 21 are advantageously separated from the fixed block 20 in such a way that they can be moved, at least in part, in the direction of movement 1a. The movement of the movable elements 21 preferably follows the movement direction 1 a, more preferably this movement follows a limited arc trajectory. Limited in such a way that it can be approximated to a linear trajectory, as illustrated in Figs. 3a and 3b. These elements 21 can have an elongated shape and be of, for example, fins as shown in Fig. 2. The movable elements 21 preferably have the form of single fins elongated in a direction perpendicular to the axial direction 1a. They are preferably present in an even number and more preferably are coupled mirror-like with respect to the movement direction 1a. In particular, these elements 21 are preferably present in a number comprised between 2 and 8, more preferably equal to 4. Furthermore, it is possible, during the construction phase, to increase the number of movable elements 21 at will in order to increase the force transmitted by the motor 1.

The movable elements 21 are connected to the fixed block 20 preferably by means of elastic means 22. These elastic means are capable of returning the movable elements 21 to a predetermined configuration. The movable elements 21 , the elastic means 22 and the fixed block 20 are preferably made from a single piece of material, preferably a metallic material such as steel, aluminum or similar.

The movable elements 21 further comprise pins 23 protruding in the vertical direction 1b from the first portion 2. The pins 23 are preferably made of a material with a high hardness, such as tungsten carbide or similar. These pins 23 are preferably positioned near the end of the movable elements 21 , more preferably each movable element houses a pin 23. The pins 23 allow contact between the first portion 2 and the second portion 3.

The first portion 2 advantageously comprises motor means 4 capable of moving the mobile elements 21. The motor means 4 comprise, and are preferably constituted by, piezoelectric elements 40. The piezoelectric elements 40 are connected to the movable elements 21 and to the main body of the fixed block 20. This connection allows the movable elements 21 to be moved, in direction 1a, with respect to the fixed block 20. In particular, this movement occurs following electrical stress of the piezoelectric elements 40. The piezoelectric elements 40 are preferably capable of expanding in an isotropic way, or rather they exhibit the same deformation in every direction of space. Furthermore, these piezoelectric elements 40 are able to expand in a manner proportional to the applied voltage. The piezoelectric elements 40 preferably have electrical connections 41 mutually independent. The piezoelectric elements 40 preferably have a cubic shape and side having dimensions preferably between 1 mm and 3 mm when not subjected to electrical stress. Furthermore, these elements are preferably present in even numbers, more preferably in a number between 4 and 6 and are present in a number equal to the number of mobile elements 21. Such piezoelectric elements 40 are commonly available on the market and preferably are of the multi -stack type. Preferably, these elements 40 require low supply voltages. Finally, the first portion 2 preferably comprises joining means 5, capable of joining the first portion 2 and the second portion 3 in the vertical direction 1b. The joining means 5 develop an attraction force, preferably adjustable, between the first portion 2 and the second portion 3, in the vertical direction 1b. These joining means 5 preferably comprise magnetic means or elastic means. More preferably, the joining means 5 comprise a permanent magnet, more preferably movable in the vertical direction 1 b. This magnet is preferably controlled by the user, for example by means of a threaded connection, adjustable from head to screw, with a fixed portion. In this way the said bonding force can be adjusted. The joining means 5 are preferably positioned in the center of the fixed block 20. The second portion 3 preferably has a shape and dimensions comparable to those of the base 102 or can have a length in the direction of movement as desired. Said second portion 3 is preferably made of ferromagnetic material, so as to couple to the magnet which constitutes the joining means 5. This second portion is capable of being constrained to an external portion 103 to be moved with respect to portion 102.

The second portion 3 includes rails 30. These rails 30 are preferably parallel to the direction of movement 1a. The rails 30 are preferably present in an even number, more preferably in the number of two, equal to the two sides of symmetry around the axis of movement 1 a, of the pins 23. These rails are adapted to house the ends of the pins 23. Furthermore, each rail 30 preferably houses an even number of pins, more preferably it houses two. In particular, the second portion 3 is constrained to the pins 23 of the first portion 2 preferably by sliding friction. The rails 30 are necessary, but not indispensable, for the movement of the second portion 3 with respect to the first portion 2 along the direction of movement 1a. The movement of the second portion 3 with respect to the first portion 2 preferably takes place by means of a stick-slip mechanism, as detailed below.

Finally, the motor 1 comprises control means, capable of supplying electrical energy to the motor means 4, for monitoring, by means of possible sensors, the motor 1 and for allowing control by a user. They are, for example, of the electronic type and are known to those skilled in the art. The operation of the piezoelectric linear motor 1 previously described in structural terms is as follows.

The first portion 2 and the second portion 3 are respectively constrained to elements 102 and 103 which must be moved relative to each other along the longitudinal direction 1 a. The two portions are then preloaded by means of the joining means 5 which are preferably magnetic means. Furthermore, the pins 23 of the portion 2 are positioned in correspondence with the rails 30 of the portion 3.

The control means, operated by a user, supply electrical energy to the motor means 4 in order to allow the relative movement of the two portions. The motor means 4, consisting of piezoelectric elements 40, expand when electrically stressed, in particular in a manner directly proportional to the applied voltage. The piezoelectric elements 40 are preferably all stressed simultaneously and with a pulse of the same intensity. Alternatively, they could be stressed in sequence as they have electrical connections independent of each other. The expansion of the piezoelectric elements causes the moving elements 21 connected to them to move. The latter move along the direction of movement 1a describing an extremely limited arc trajectory such as to be assimilated to a rectilinear trajectory. The rails 30 also help to keep the trajectory rectilinear. The pins 23 positioned on the movable elements 21 move with them. The movement of the pins 23 therefore takes place along the rails 30 of the second portion 3, along the movement direction 1a.

The movement of the second portion 3 with respect to the first portion 2 along the direction of movement 1a takes place by means of a stick-slip mechanism. In particular, in a first case, the piezoelectric elements 40 are all stressed simultaneously. The electrical impulse is given in such a way that the expansion of the piezoelectric elements 40 is so rapid as to cause an acceleration of the movable elements 21 , and therefore of the pins 23, capable of overcoming the static friction present between these pins and the rails 30. The high acceleration of the movable elements 21 causes the "slip" phenomenon, or rather the sliding of the pins 23 along the rails 30 in the direction of movement 1a. The second portion 3, due to its inertia, is unable to follow the rapid movement of the pins 23 and therefore remains stationary in the initial position. At the end of this movement, the electrical impulse decreases in intensity but more slowly and the piezoelectric elements 40 return to their undeformed state. In this phase the static friction between the pins 23 and the rails 30 is not overcome and therefore the phenomenon of "stick" occurs. This phenomenon involves the dragging of the second portion 3 by the pins 23 positioned on the movable elements 21.

Depending on the use envisaged for this motor 1 , the "stick" phenomenon may precede that of "slip". The electrical impulse is gradually increased so as not to overcome the static friction between the pins 23 and the rails 30 and thus cause the simultaneous movement of these elements. Subsequently, the electrical impulse decreases rapidly causing the pins 23 to slide along the rails 30 which remain stationary.

Furthermore, it is possible that the piezoelectric elements 4 are stressed in sequence, preferably one at a time, or more than one at a time but not all together. The electrical impulse, in the "slip" phase only, arrives in sequence and not simultaneously to all the piezoelectric elements 40, while in the "stick" phase the impulse is simultaneous. This second solution has the advantage of increasing the force transferred during the stick phase.

The joining means 5 generate a magnetic attraction force between the first portion 2 and the second portion 3 which can be adjusted according to the intended use of the motor 1. By adjusting the magnetic force, it is possible to adjust the friction force accordingly established between the pins 23 and the rails 30 and thus obtain the desired type of movement.

The piezoelectric linear motor 1 according to the invention achieves important advantages.

In fact, this motor is compact, has a small footprint and has a low inertia.

This motor has no bearings or other elements subject to wear and therefore requires minimal maintenance.

The piezoelectric linear motor 1 has no longitudinal dimension constraints and ideally, its stroke can be lengthened at will.

Furthermore, it is possible to obtain very reduced movements by applying a low voltage and vice versa, by applying a high voltage in a short time, it is possible to have significant movements.

Thanks to the stick-slip movement mechanism it is possible to obtain displacements in both directions of the longitudinal movement direction 1a.

In particular, this motor is economical with respect to other similar technologies since the piezoelectric elements used are standard.

Finally, in the presence of an external force, the motor 1 remains stationary without the need for power supply thanks to the friction developing between the rails 30 and the pins 23 and this friction being proportional to the preload induced by the joining means 5.

The invention is capable of variants falling within the scope of the inventive concept defined by the claims.

In this context, all the details can be replaced by equivalent elements and the materials, shapes and dimensions can be any.