POSITIONING MOTOR AND METHOD OF OPERATION

Field of the invention

The present invention relates to a motor, more particularly to a piezomotor, which is designed to work in at least a resonant vibration mode, a direct positioning mode and/or a stick-slip mode, simultaneously or sequentially. The present invention also relates to a stator body for such a motor.

Background of the invention

Piezoactuators are often used in a multitude of applications which require very high accuracies. The biggest limiting factor of these positioners however is the very small displacement. To counter this effect several principles of piezomotors have been presented which have limitless travel range.

Three main types of motors have been designed with unlimited strokes (1) stepping motors, (2) stick-slip/inertia driven motors and (3) resonant motors. Stepping motors have a high holding force but are often too slow for many applications. Stick-slip motors achieve speeds in the order of 10 mm/s but often may cause excessive vibrations. Resonant or 'ultrasonic' motors on the contrary achieve the highest speeds, in the order of 100 mm/s. This is because in resonant motors the piezo('s) are excited with sines at a frequency which is close to one or two Eigen frequencies of the motor. A typical piezomotor may for instance have two independent modes, like e.g. a longitudinally and a transversely moving eigenmodes which may nearly coincide in frequency. The modes are often selected so that the respective directions of oscillation are perpendicular to each other. The superposition of the two perpendicular vibrations cause the contact zone or point to move along curves known as Lissajous figures.

Although several ultrasonic motors operating by resonant vibration mode have been designed over the last years, they cannot easily combine the fast resonant mode together with an accurate direct mode. In the direct mode a DC voltage is applied to the piezo('s) to achieve a very high positioning resolution within the limited stroke of the piezo actuators. In addition, a disadvantage of piezoelectric ultrasonic motors known in the art is their limited positioning resolution, as they show strong non-linear behaviour at low speeds, which limits the performance.

Nevertheless US 7,728,488 describes a motor which can work in both the resonant and the direct mode, simultaneously or sequentially. This motor can work in both modes simultaneously by driving one set of piezo' s in resonance and the other set of piezo's in direct mode. Moreover, in US'488 tuning masses are included in the motor structure which mainly influence the longitudinal vibration mode. By making or drilling holes in such a tuning mass the longitudinal vibration mode is basically tuned to a frequency closer to the frequency of the transversal vibration mode. However a drawback of drilling holes for tuning the longitudinal and transverse vibration mode of the motor is that this is a labor-intensive process and the holes need to be positioned carefully to not disturb the modal shapes. This requires an intensive alignment of the work piece on the machine. Furthermore, the removal of material cannot easily be undone and the motor structure can easily get ruined (especially in the flexible hinges). The addition of protuberances is not very convenient because they need to be fixed firmly to avoid loosening. A need still exists for an improved positioning motor and method of operation. Summary of the invention

It is an object of the present invention to provide a good positioning motor and method of operation.

It is another object of the present invention to provide a motor which can supply great traction force, high speed and high accuracy to a load to extend the field of possible applications of piezomotors.

It is yet another object of the present invention to provide a motor which is highly durable and highly reliable. It is an object of the present invention to provide a coarse/fine positioning motor and method of operation. It is an advantage of embodiments of the present invention to provide a good, e.g. an improved motor, more particularly a piezomotor, which can simultaneously or subsequently operate in a resonant vibration mode, a direct positioning mode or a stick-slip mode.

This object is met by the method according to the independent claims of the present invention. The dependent claims relate to preferred embodiments.

In a first aspect the present invention provides motors comprising at least one contacting zone for making contact with a driven part, wherein the motor is arranged to realize a relative motion of the driven part by a stator body, whereby said motor is adapted for being at least operable in a resonant mode, said motor comprising:

- at least one actuator, said actuator adapted to be excited by providing at least one electrical signal ;

- at least one tension element, said at least one tension element adapted to provide a preload on the at least one actuator and adapted to constrain movement of the at least one actuator relative to the stator body;

characterized in that the stator body comprises multiple means for pivoting, whereby said means for pivoting are adapted to act as pivot points for parts or linkages of the stator body located between the means for pivoting upon application of the electrical signal. In another aspect the present invention provides stator bodies, whereby said stator body is adapted to constrain movement of the at least one actuator comprised by the stator body and characterized in that the stator body comprises multiple means for pivoting, whereby said means for pivoting are adapted to act as pivot points for parts or linkages of the stator body located between the means for pivoting upon application of an electrical signal to the at least one actuator.

The motor may furthermore be operable in any of the following modes : a direct positioning mode or stick-slip mode

In preferred embodiments the at least one actuator, e.g. piezoactuator(s), deforms the stator body, whereby said stator body is preferably a metal stator body, mainly in the means for pivoting.

In preferred embodiments the working modes can be performed sequentially or simultaneously. The motor is capable of delivering nanometer accuracy positioning, in addition the motor is able to actuate in nanometer precision in spite of usage of for instance ball bearing guides. Moreover the motor has an ability to drive its load at relatively high speed, at at least 200 mm/s. Resulting, a motor according to embodiments of the invention is a multi-drive motor.

In preferred embodiments a stator body, e.g. a stator module, comprises at least two actuators, whereby at least one actuator may be a non-active actuator, whereby the active actuator preferably is electrically excited, by using an electrical signal, as to provoke the resonant vibration mode or to deform the stator module for the direct positioning mode.

In preferred embodiments where two active actuators, more specifically two piezoelectric actuators, are used, the resonant mode is achieved by exciting the two piezoelectric actuators by two sinusoidal voltages with varying phase difference between the two sinusoidal voltages and/or varying voltage amplitude(s). These voltages excite the horizontal and vertical eigenmodes of the motor' s structure, resulting in an elliptical motion of the contact zone. The phase control may consume more power than compared with the amplitude control, but it leads to an almost deadzone free response. To benefit from both control strategies, preferably a constant low voltage amplitude and phase control are implemented for low velocity and an amplitude control with a phase of 90 or -90 degrees is preferably applied when higher velocity is desired. Using this technique, the motor consumes relatively low power while being able to actuate at high speed, and being relatively free from any deadzone. In preferred embodiments, in order to get a good control performance, the identified velocity response for different phase input is preferably linearized.

Typically the driving frequency of a motor according to the presented invention lies in between 10 and 100 kHz, which can vary depending on the size of the motor. For piezo actuating elements of about 30 mm long, a good driving frequency lies in between 20 and 25 kHz.

Moreover, when or if the contact zone or point is preloaded against a slider, it can also create a stick and slip (or inertial) operational regime or mode that results in macroscopic motion of the slider.

In further embodiments, when the two actuators are supplied with a quasistatic (relatively slowly changing) voltage this will advantageously result in a microscopic displacement with nanometer resolution/accuracy, having an operation stroke of several micrometer (dependant on the measurements of the motor). During this operation the motor is limited to operate only in the stick fricfional region. In this direct-drive mode a Maxwell slip inverse compensation can be implemented like for instance described by Gorka et al in "Asymmetric Hysteresis compensation for Piezoelectric Actuators, Mechanical Systems and Signal Processsing" (2011). In other embodiments, two feedback sensors can be further provided in the motor to measure either the position of each actuator or to measure the position in the final end effector and in one of the actuators. Both the position of the final end effector and the position of each actuator can be easily measured using subtraction or summation of the information of the two sensors.

In preferred embodiments, the means for pivoting are integrated in the stator body or are provided by removing material from the stator body, like for instance flexure hinges. In preferred embodiments, the means for pivoting are placed such that the piezoactuators are restricted in their movement and deform mainly in a specific direction.

In further embodiments the stator body can be of any shape, like for instance rectangular, circular, etc. Advantageously the shape of the stator body can be customized such that different applications for the motor can be realized. Moreover, a motor according to embodiments of the invention comprising an actuator, e.g. piezo, is that the actuating mechanism generally has small dimensions. This enables the miniaturization of the devices built around such actuators. In embodiments, the stator body is a substantially planar body. The stator body in embodiments may engird that at least one actuator. However the means for pivoting are positioned such that the piezoactuators are restricted to a movement and deformation parallel to the length axis of the at least one actuator, whereby as a result unwanted dynamic forces in a direction perpendicular to the length axis on the piezoactuators are negligible. More specifically pivoting can also encompass deflecting, tilting and/or lifting in relation to each other.

In some embodiments the means for pivoting are flexure elements. In more specific embodiments the means for pivoting are flexible hinges or kinematic joints.

In preferred embodiments the at least one actuator of a motor according to the invention is preloaded by the at least one tension element by a pull force exerted by the at least one tension element resulting in a force exerted by the stator body on the at least one actuator and the means for pivoting.

Preferably the means for pivoting are positioned such that a corresponding equivalent linkage system is provided which minimizes the deformation and stresses of the at least one actuator in all other directions except the direction parallel to the length axis of the at least one actuator.

In some embodiments the at least one tension element is a stiffness element. Advantageously, the motor according to embodiments of the invention can be tuned by varying the shape, material, mass and/or stiffness distribution of the at least one tension element. In other embodiments the at least one tension element is a tension bar, a tension bolt or an elastic means, e.g. a serpentine shaped elastic means or a corkscrew shaped elastic means.

In alternative embodiments the at least one tension element may be integrated in the stator body, whereby the stator body is monolithic.

In some embodiments, whereby the motor according to the invention comprises at least two actuators, the at least one tension element is placed in between the at least two actuators. Preferably the at least two actuators are positioned substantially parallel with respect to one another and perpendicular to the driving direction of the motor.

In preferred embodiments of a motor according to the invention, the at least one tension element is positioned substantially parallel with respect to the at least one actuator.

In further embodiments the stator body may comprise at least one mounting feature, whereby said at least one mounting feature is adapted to fix the motor to the environment. Preferably the at least one mounting feature is positioned at nodes of the used resonant vibration mode(s) and/or the direct mode of the motor.

In alternative embodiments, the stator body may comprise the at least one actuator (2) and the at least one tension element (3).

In preferred embodiments the at least one actuator further may comprise a plurality of actuators.

In embodiments of a motor according to the present invention the drive speed of the relative motion between the stator body and a driven part is controllable by varying the phase and/or the amplitude of the electrical signal applied to at least one actuators.

In preferred embodiments of the invention, excitation of the at least two actuators of the motor is adaptable to control the thrust force of the relative motion between the body and the driven part.

In preferred embodiments, the motor and/or the stator body is symmetrical.

In preferred embodiments the at least one actuator is an actuator of the group consisting of piezoelectric actuators, electrostrictive actuators and magnetostrictive actuators.

In yet further embodiments, one or more operating modes of the motor can be performed sequentially or simultaneously.

Embodiments of the present invention also provide an apparatus comprising at least one motor according to the present invention, wherein the apparatus is controllable to allow the simultaneous combination of these motors to position the apparatus in one or more degrees of freedom when they are operating in a mode selected from a group comprising resonant mode, direct mode, stick slip mode and stepping mode, in a sequence or a combination thereof.

In other embodiments the apparatus comprises three motors according to the invention wherein the apparatus is operable by at least three motors (N), which are operable in one or more of the following working modes: a resonant mode, a direct positioning mode, a stepping mode and/or stick-slip mode, and are positioned at a stage within a relative angle of 360/N degrees.

In embodiments a drive function and a bearing function of the apparatus are carried out by the motors of the present invention. Preferably an apparatus according to the invention is a planar drive.

The motor may also be used for driving a flexure stage, both in translational directions (x, Y) as in a rotation direction. The stator could then for example be fixed to the object to be driven.

It is an advantage of embodiments of the present invention that these can be implemented at very small scale, and can for example be implemented as a MEMS device.

In a second aspect the present invention provides stator bodies, whereby said stator bodies are adapted to provide a preload when at least one actuator is enclosed by said stator body, and to constrain movement of the at least one actuator relative to the stator body;

characterized in that the stator body comprises multiple means for pivoting, whereby said means for pivoting are adapted to act as pivot points for parts or linkages of the stator body located between the means for pivoting upon application of an electrical signal on the at least one actuator.

In preferred embodiments the means for pivoting are integrated in the stator body. More specifically, whereby the stator body is monolithic.

In further preferred embodiments the means for pivoting of the stator body are flexure elements. In some embodiments the flexure elements are preferably provided by cut outs in the stator body. In other embodiments the means for pivoting are provided at the sides of the stator body receiving the end faces of the at least one actuator.

In preferred embodiments the means for pivoting of the stator body are positioned such that a corresponding equivalent linkage system is provided which minimizes the deformation and stresses of the at least one actuator in all directions except the direction parallel to the length axis of the at least one actuator.

In further preferred embodiments the stator body comprises at least one mounting feature, whereby said at least one mounting feature is adapted to fix the stator body to an environment.

In some embodiments the stator body is preferably symmetrical.

It is an advantage of embodiments of the present invention that a motor is provided with flexure elements, e.g. hinges, as a result intense stresses in the actuators and smaller amplitudes of motions are prevented. In preferred embodiments, the stator body is a body made from one piece. In further preferred embodiments the stator body comprises a planar surface, more specifically it comprises a two-dimensional characteristic, e.g. situated in a plane. The stator body in addition typically comprises a thickness in the range of 1 to 30 mm. It is an advantage of embodiments of the present invention that with at least two actuators at least two modes, resonant and direct-drive mode, can be accomplished sequentially or simultaneously. Moreover, both working modes can be combined through only one set of actuators, whereas for instance in prior art devices, like US'488, two sets are required resulting in that a motor according to the invention is accurate and cost-effective at the same time. This is achieved by providing flexures, e.g. hinges, which are positioned carefully. In addition, the at least two actuators are driven with driving signals which can be combined AC- and DC-signals.

The stator body may be used in a plurality of different applications such as for example for a clamping means, for a mechanical amplification mechanism, for actuation of valves, for a normal force transducer or exciter, for a generic sonic and/or ultrasonic vibration means such as for use in for example a toothbrush, a shaver, an engraving tools) for optical systems, for friction reduction, for production processes such as drilling, sanding, milling, grinding, etc., for generating Shockwaves, for creating cavitation in fluids, etc.

It is an advantage of embodiments of the present invention to provide a motor which is designed to comprise at least one actuator, wherein a large amount of active material like for instance piezomaterials, per volume can be used which enables one to obtain high power outputs and long strokes of amplitude motion. Moreover, the present motor according to embodiments is preferably designed to hold the at least one actuator which show(s) a good combination of force and stroke per applied signal, e.g. voltage.

In one particular embodiment, the presented motor can achieve minimal stable speeds of 1 nm/s at low voltage amplitudes of IV or less, and maximal speeds up to 500 mm/s, and forces up to 30N at only 22V _rms , while having a range of 11 μιη in the direct mode for a DC voltage change from 0 to 150V when at least two actuators are used, e.g. piezos. Because embodiments of the present invention provide a motor comprising large actuators, e.g. piezostacks, with a short distance between them, preferably in the mm-range with a maximum of 10 mm, the stroke in direct mode is very large compared to other multi-mode piezomotors.. This is advantageous for fine positioning purposes. Moreover, by using a motor according to embodiments of the invention, an improved combination of velocity, accuracy and driving force is possible.

Moreover, in embodiments, the motor is advantageously designed to have as little bending and torsional deformation (e.g. elastic deformation) as possible in the actuators, e.g. piezoelement(s), which is a great advantage for the life expectancy of the motor, as actuators, e.g. piezos, are mostly made from brittle ceramic materials. The stator body on the other hand, preferably a metallic structure, is preferably positioned around the actuators. Moreover, the locations of the flexure elements, e.g. hinges, are chosen to minimize the bending deformation of the actuators, e.g. piezostacks, which are mainly deformed in the longitudinal direction of the stack. The presence of for instance the metallic body of the stator body is also an efficient means for conducting away the generated heat. This improves the motor stability and reliability significantly, because the resonance frequencies will suffer less from shifting due to a change in temperature. As a result, the dynamic characteristics of the motor will not be adversely affected and the duty cycle of the motor can be maintained. As a result, the motor has a main advantage that it is little sensitive to temperature variations due to the design of the stator body and the actuators.

A motor according to embodiments of the invention, can advantageously be tuned, for instance by changing the shape, mass and/or stiffness of the tension element, e.g. preloading bolt. A series of preloading bolts, having different masses and/or stiffnesses, can be made beforehand and the motor is tuned through the choice of the bolt. Furthermore, also the preloading force with which the tension element is fixed, can be used to tune the piezomotor slightly. These tuning processes are much more convenient than other state-of-the-art tuning methods for piezomotors as described above. It is an advantage of embodiments of the present invention to provide a motor which is easy to assemble and where pretensioning by using a tension element, e.g. bolt, of an actuator can be decoupled from the pretension of the motor against a slider, which may provide an additional pretensioning mechanism, in addition to the tension element.

In another aspect the present invention provides motors, more specifically piezomotors, for moving an external body comprising:

- two actuator elements, e.g. piezoelectric, placed parallel with respect to and next to each other and perpendicularly to the driving direction;

- a pretensioning element, preferably positioned in the center between the actuator elements, for preloading the actuator elements;

- a structure, for instance a stator body comprising pivoting means, to transform the motion generated by the actuator elements into motion of the external body, having an equivalent linkage system as to minimize the deformation and stresses of the actuators in all other directions than the direction parallel to the length of the actuator elements; and

- a contacting zone or point, for enabling the motor to contact the external body.

In preferred embodiments, a motor, e.g. piezomotor, has its fixation points between suspension (or environment) and motor body located at the knots of the used resonant modes, to reduce the generation and transfer of vibrations towards the machine structure, and to minimize the influence of the motor fixation on the vibration modes. Therefore the amplitude of the internal motion will not be hindered too much. Moreover, this will have a negligible influence on the mode shapes and resonance frequencies.

In other preferred embodiments, a motor, e.g. piezomotor, is provided with a symmetric geometry, with both ends vibrating in vertically, i.e. parallel to the actuation direction of the actuators, opposite directions, keeping the fixation points to the suspension (or environment) steady.

In preferred embodiments, a motor, e.g. piezomotor, is provided with pivots, e.g. flexure elements, placed on preferably a single line, perpendicular to the actuator elements, limiting translation and vibration of the motor components and links to a direction parallel to the length of the piezos. This advantageously results in minimal sideway vibration and bending of the fixation points and actuators, e.g. piezoactuators.

In preferred embodiments a motor, e.g. piezomotor, is provided with the mass centers of links located in the middle between bordering pivots, e.g. flexure elements, limiting inertia-induced translation of the link and its bordering joints to a direction perpendicular to the line connecting the bordering joints. Combining this with the single line defined in the previous paragraph, translation of all involved joints and links is restricted to a direction parallel to the length of the actuators, e.g. piezos. This advantageously results in minimal sideways vibration and bending of the fixation points and actuators, e.g. piezoactuators.

In preferred embodiments a motor, e.g. piezomotor, is provided which can be tuned by varying the shape, material or preload of the preload elements, e.g. bolt(s). This preload bolt and its preload preferably mainly affect one vibration mode and barely the other vibration mode, thus providing a means to change the eigenfrequency of the affected mode but not significantly the eigenfrequency of the barely affected mode. In further embodiments a motor, for instance piezomotor, is provided with more than one preload element, e.g. bolt. Preferably all bolts may act together to generate a common preload, or with each (set of) bolt(s) generating preload for a separate actuator, e.g. piezoactuator, or with one or more bolts generating a common preload on the motor, and other bolts preloading individual actuators, e.g. piezoactuators. The pretension elements, e.g. bolts can be placed next to the actuators, e.g. piezoactuators, inserted through holes in the actuators, or combinations thereof.

In embodiments, a tension element or preload element is preferably designed such that the stiffness function and the preload function of the preload element, e.g. bolt, can be distributed over different bolts.

In embodiments a motor, e.g. piezomotor, is provided which advantageously can combine high speed and high accuracy by combining resonant and direct modes. Preferably, combining high speed and low tracking error is enabled by applying resonant and direct modes simultaneously by combining the corresponding signals into a single electrical signal applied to each piezoactuator.

In other embodiments, at least one of the actuators, e.g. piezoactuators, is driven with a DC voltage signal, while at least one of the other actuators, e.g. piezoactuators, is driven with an AC signal.

In preferred embodiments a motor, e.g. piezomotor, is provided which can operate simultaneously or sequentially in one or more of the following working modes: resonant mode, direct mode and stick-slip mode. In other embodiments, two or more motors, e.g. piezomotors, according to the invention are put in parallel such that they drive the same load, whereby said motors are suspended either separately or through one suspension mechanism, to achieve higher traction forces and/or to operate in a stepping mode to make the stroke of the direct mode unlimited. The stepping principle can be explained as follows: while one motor pushes the load in the desired direction, the other one retracts to the opposite direction, barely or not touching the load. It is an advantage of embodiments of the invention that a seamless transition between sequentially operated modes can be provided by combining driving signals of different modes.

Some embodiments provide combining (power) signals for different modes of the motor by replacing one or both actuators, each by two or more actuators placed in series, possibly but not necessarily with the inclusion of one or more layers of non-active layers, and preferably having a combined length equal to the length of the replaced actuator, and applying the individual signals for the different modes to different sections of the newly formed serial stack. Preferably the serial combination is realized internally in the actuator.

In other embodiments, at least one so-called actuator, e.g. piezoactuator can for instance consist of both a sensing, e.g. piezosensor, and an actuating element, e.g. piezoactuator. In preferred embodiments, a motor may be linked to a machine frame or slider by means of a flexible suspension mechanism, which preferably is rigid in the driving direction and flexible in the direction normal to the drive face, and which provides a preload for the motor against the opposing member of the drive pair. The preload mechanism can be integrated in the suspension mechanism or be external. In preferred embodiments the preload force can be applied by a mechanical spring (e.g. a mechanical leaf spring or a helicoloidal spring), but this can also be achieved through an actuator producing an electrostatic, magnetic, piezoelectric, etc. force. With an active preload mechanism the preload force can be altered automatically and instantaneously, not only after the installation of the motor but also during operation, for instance in function of the operation mode or required force or speed.

In other embodiments a motor, e.g. piezomotor, is provided where both ends are equipped with driving points. This motor being preferably designed in a way that the driving points generate motion in the same direction and sense.

In some embodiments, the suspension may be configured for obtaining a normal force providing arrangement allowing for example to apply a force between the driven object and the actuating tip.

In preferred embodiments one of the at least two actuators can be a non-active or a dummy actuator, or a body link can be provided, which enables the same function. The direction of motion for the resonant mode can then for instance be altered by applying an electrical signal with a different frequency.

In preferred embodiments the at least two motors, e.g. piezomotors, are combined in a single package to increase force and power output, with their driving tips oriented to actuate the same body. This package preferably provides a common suspension for both motors. In other preferred embodiments the two or more motors, e.g. piezomotors, can be stacked in a single package to increase force and power output, with their driving tips oriented to actuate the same body. This package advantageously may provide individual suspension for each motor, which simplifies the design of the suspension of the different motors, especially for a larger amount of motors put in parallel. Moreover, the preload of each motor against the driven part 6 can then be controlled for each motor individually.

In preferred embodiments the two or more motors, e.g. piezomotors, are stacked in a single package to increase force and power output, with their driving tips oriented to actuate the same body. This package providing individual suspension per couple of motors.

In another aspect the present invention relates to any stack of motors, e.g. piezomotors, or already stacked motors (e.g. a stack of stacks), comprising a motor according to the present invention.

In yet another aspect the present invention relates to 2-DOF versions of a motor according to embodiments of the invention. In embodiments where a 2-DOF version of motor according to the invention is provided, a load can be moved in two directions.

Embodiments of the present invention provide a combined functionality of drive and bearing of one or more of the motors according to embodiments of the invention.

Embodiments of the present invention provide the use of one or more of the piezomotors according to embodiments of the invention, to achieve one or more DOF's of any type of instrument. It is an advantage of embodiments of the present invention to provide a fast nanometer positioning system, which is able to combine the high speed capability of an ultrasonic piezomotor (resonant mode) with the fine positioning capability of an actuator, e.g. piezostack (direct-drive mode). The two modes preferably can be operated simultaneously with the capability of achieving a speed of at least 200 mm/s and a positioning resolution/accuracy of at least 10 nm.

It is an advantage of embodiments of the present invention to provide a motor with a compact construction, because of the orientation of the at least one actuator.

It is an advantage of embodiments of the present invention to provide a motor which can move a driven part with a large travel and high speed while maintaining a high position resolution of the driven part.

Embodiments of the invention provide a motor, preferably a piezoelectric motor, which is designed to work simultaneously or subsequently in a resonant vibration mode and a direct positioning mode. The resonant vibration mode preferably may cause a mechanical element for instance a stator body, e.g. stator module, to vibrate so that a relative movement of a driven part over a large travel is possible while the direct positioning mode preferably may deform the stator module so that at the same time a fine motion control is possible.

A motor according to embodiments of the invention can also operate in a stick-slip mode which can be used for moving at low speeds with small steps. Whereby said speed can be up to several tens of mm/s, depending on the amplitude, frequency and slew rate of the applied electrical signal.

Advantageously a fourth stepping mode is also possible when two motors according to embodiments of the invention are combined in parallel and suspended through one suspension. In the stepping mode both stator bodies alternately push the load in the desired direction preferably in a direct mode operation. While the first is moving the load in the desired direction, the other stator retracts to the opposite direction (without touching the load or at least with less normal force). In this way the stroke of the direct mode becomes unlimited without causing too many vibrations and without losing control at nanometer-level as known in stick-slip or resonant modes known in the art.

In further preferred embodiments a stator body, e.g. a stator module, may comprise several flexure elements, e.g. hinges, as to amplify the amplitude of the contacting point and to minimize internal stresses in the piezos in directions different than the longitudinal one.

Embodiments of the present invention also advantageously provide a novel mechanism to optimize the working efficiency of a motor that can simultaneously operate in a resonant vibration mode and a direct positioning mode. The motor can advantageously be used as a linear motor, a rotational motor or a planar motor.

Particular and preferred aspects of the invention are set out in the accompanying independent and dependent claims. Features from the dependent claims may be combined with features of the independent claims and with features of other dependent claims as appropriate and not merely as explicitly set out in the claims.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

Brief description of the drawings

Further features of the present invention will become apparent from the examples and figures, wherein: Fig. 1 schematically illustrates a motor according to embodiments of the invention.

Fig. 2 schematically illustrates a motor according to embodiments of the invention.

Fig. 3 schematically illustrates a stator body according to embodiments of the invention.

Fig. 4 schematically illustrates a motor according to embodiments of the invention.

Fig. 5 schematically illustrates an equivalent kinematic mechanism of a motor, preferably a piezomotor, according to embodiments of the invention.

Fig. 6 schematically illustrates deformation modes of a motor according to embodiments of the invention. Fig. 7 schematically illustrates an implementation of a motor according to embodiments of the invention, whereby the motor comprises driving contacts on two sides, driving the same load.

Fig. 8 schematically illustrates a cross-sectional view of a motor according to embodiments of the invention whereby three tension elements are provided: two tension elements are each positioned inside the at least two actuators and one tension element is provided in between the at least two actuators each comprising a tension element.

Fig. 9 schematically illustrates a 2-DOF motor according to embodiments of the invention, which can move a load in two direction, for instance in a plane defined by B and C.

Fig. 10 schematically illustrates an apparatus with a platform which can move in a plane perpendicular to direction A, according to embodiments of the invention, with integrated bearing and motor function, comprising at least one 2-DOF version motor according to embodiments of the invention.

Fig. 11 displays a possible layout of an apparatus, for instance a planar positioning system, according to embodiments of the invention, based on 1 -DOF motors according to embodiments of the invention.

Fig. 12 illustrates a scheme for simultaneously controlling the resonant and direct-drive mode of a motor according to embodiments of the invention.

Fig 13 illustrates a motor, not showing a tension element, whereby additional tension can be provided through deformation of the stator body itself.

Figures 14 and 15 illustrate other embodiments of the stator body, not showing a tension element. .

Fig 16, illustrates a motor where some of the flexibility is provided through leaf hinges instead of only through joints, again not showing a tension element..Fig 17 illustrates a motor where all pivoting means, e.g. joints, are located on one line according to embodiments of the invention.

Fig 18 illustrates a stator body which is more easy to manufacture because of an increased free space in the flexure hinges.

Fig. 19 illustrates a motor according to embodiments of the invention, not showing a tension element.

Fig. 20 illsutrates a motor according to embodiments of the invention, not showing a tension element.

Fig 21 illustrates a motor with a serpentine acting as the tension element

Fig 22 illustrates a motor comprising a stator body where both leaf hinges and hinge joints are introduced in the stator body.

The drawings are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Any reference signs in the claims shall not be construed as limiting the scope. In the different drawings, the same reference signs refer to the same or analogous elements.

List of references

[1, la, lb , lc] stator body

[2, 2a, 2b, 2c] actuator

[3, 3b, 3c] pretension element

[4] disk spring

[5] contact zone or point

[6, 6a, 6b] driven part

[7a-7d] mounting feature

[8a-81] means for pivoting

[18] environment

[19] kinematic joints

[20a-20d] side links

[21a-21b] outer links

[23] drive plane

[24] pulling body

[25] a 2 DOF motor

[26] a 1 DOF motor

Detailed description of preferred embodiments

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes. Where the term "comprising" is used in the present description and claims, it does not exclude other elements or steps. Where an indefinite or definite article is used when referring to a singular noun e.g. "a" or "an", "the", this includes a plural of that noun unless something else is specifically stated. The term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B. Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

In the drawings, like reference numerals indicate like features; and, a reference numeral appearing in more than one figure refers to the same element. The drawings and the following detailed descriptions show specific embodiments of a motor, more particularly a piezomotor, which is designed to work in at least a resonant vibration mode, a direct positioning mode and/or a stick-slip mode, simultaneously or sequentially.

Where in embodiments of the present invention reference is made to an actuator, reference is made to an active element, for instance comprising piezoelectric material and electrodes, preferably in a form of a piezoceramic stack. An actuator, used in embodiments of the invention can be of any of the following types: piezoelectric, magnetostrictive, electrostrictive actuators.

Where in embodiments of the present invention reference is made to a motor, e.g. a piezomotor, reference is made to a motor comprising at least one actuator, e.g. piezoactuator, and other mechanical elements to house, preload and support the actuators and transforming the motion of this/these actuator(s) into a useful motion of a load.

Where in embodiments of the present invention reference is made to a stator body, reference is made to a structural part of a motor according to embodiments of the invention, whereby said stator body can be used to hold the at least one actuator, whereby the stator body preferably comprises flexible, such as means for pivoting, and rigid sections, such as linkages in between the means for pivoting, at well-chosen locations on the stator body, to form the mechanism that transforms the deformation of the at least one actuator(s) into a useful motion of a load.

Where in embodiments of the invention reference is made to a flexible hinge, reference is made to a feature of the stator body with narrowed section, and therefore increased flexibility, acting as a pivot or hinge in the structure.

Where in embodiments of the invention reference is made to suspension, reference is made to a component or mechanism that supports the motor and pushes it against the load with a preferably controlled force resulting in a preload.

Where in embodiments of the invention reference is made to contact, reference is made to a location where for instance a tip of a motor, according to embodiments of the invention, touches a surface of the driven object. Where in embodiments of the invention reference is made to strip, reference is made to a part fixed to the load or stage, which makes contact with the tip(s) of at least a motor(s) according to embodiments of the invention. Such a strip may for instance be a linear strip, a curved strip, a circular path or a circular element.

Where in embodiments of the invention reference is made to tip, reference is made to a part of a motor, according to embodiments of the invention, making contact with a load. Such a tip may be a domed point, preferably a ceramic point.

Where in embodiments of the invention reference is made to preload, reference is made to a force acting on an actuator or a motor, according to embodiments of the invention, in rest (e.g. when no electrical signals are applied to excite the actuator). In embodiments where a piezoactuator, e.g. a piezoactuator stack, is used, the preload is coaxial with the (intended) stroke of the piezoactuator. In embodiments where a piezomotor is provided, the preload is perpendicular to the surface of the driven object. Where in embodiments of the invention reference is made to load or stage, reference is made to a component or structure (to be) driven by a motor according to embodiments of the invention.

Where in embodiments of the invention reference is made to monolithic, reference is made to a component consisting of one piece.

Where in embodiments of the present invention reference is made to a voltage signal, it will be clear to the person skilled in the art that also a charge signal or current signal could be used.

Where in embodiments of the invention reference is made to a bolt, reference is made to a fastening means comprising a threaded pin or rod with a head at one end, designed to be inserted through holes in assembled parts.

Embodiments of the present invention provide a motor 25, 26, preferably a piezoelectric motor, whereby the motor comprises a stator body 1, for instance a stator module, comprising at least one actuator. In Figure 1 a 1 DOF motor 26 according to embodiments of the invention is provided, whereby the motor comprises a stator module comprising two actuators 2a, 2b, is provided whereby said two actuators are positioned parallel with respect to each other and whereby, in preferred embodiments, the two actuators are positioned inside the stator body 1, as illustrated in Figure 1. In some embodiments, the two parallel actuators are arranged as close as possible to each other. Moreover, a stator body 1 according to embodiments of the invention further may comprise or contain at least one tension element or a preload element 3, such as for instance a bolt. The performance of the stator body 1 or motor according to embodiments of the invention is advantageously independent of the structure where it is attached to, thus improving design modularity.

In embodiments the at least one tension or pretension element 3 may be integrated as part of the stator body, or said at least one tension element may be a removable item, which can be removed and replaced. The stator body further may comprise left (L) and right (R) arms, said arms preferably are part of the stator body. Said arms can preferably be fixed in position relative to each other by for instance securing at least a mounting feature 7 to the environment.

In embodiments where a piezoelectric element is used as an actuator or as an actuating means, said piezoelectric element may be a piezoelectric stack. Such a piezoelectric stack can comprise at least one piezoactuator placed in series. In further embodiments at least one non-active layer may be introduced in between the at least one piezoactuator or be an integral part of the (piezo)actuator. Preferably when providing a multi-layered piezoelectric stack instead of a piezo comprising one layer, the length of such a multi-layered piezoelectric stack is equal to the length of the piezoelectric element comprising only one layer. In embodiments whereby the actuator comprises at least two actuators, and thus is a multi-layered stack, said layers or sections of the actuator may be excited separately by applying individual signals, to enable the different modes of the motor, to different sections of for instance a serial multi-layered stack. More specifically, as the multi layers are mechanically connected in series, they result in being electrically in parallel to each other. However, the electrical connection can be adapted in function of the modes (and their combinations). Also one part of a stacked type of actuator can for instance be used as a sensor to measure a normal force, while the other part of the actuator can be used as actuating element.

It is an advantage of embodiments of the present invention that highly efficient stacks can be obtained. The piezoelectric devices selected advantageously therefore have a high Q factor, low dielectric losses, a low loss factor, etc.

In preferred embodiments, where at least two actuators are provided, the at least one tension element 3 is positioned in between the at least two actuators. In further embodiments the at least one tension element is positioned in between and parallel relative to the length axes of the at least two actuators, e.g. two piezoelectric elements. The end faces of each of the two actuators 2a, 2b are preferably connected to the stator body 1 for instance via means for pivoting, like e.g. flexible hinges 8 or via kinematic joints 19, for instance ball joints or cylindrical joints. The stator body 1 can be put in contact to a driven part 6a, 6b, for instance a slider 6a or a rotary stage 6b, via at least one contact zone or point 5. Also other configurations or desired trajectories can be thought of such as one where a motor, according to embodiments of the invention, is situated at the central side of the rotary stage or one where the load moves in a somehow atypical way in two or three degrees of freedom. In embodiments where kinematic joints 19 are used as illustrated in Figure 4, said joints can for instance be realized as ball caps or hemispheres mounted on both sides of each of the actuators, e.g. piezoactuator, resting with their spherical surface in conical or spherical recesses in the stator body 1.

A tension or preload element 3, used in embodiments of a motor, preferably can pull both ends of the stator body 1 together, causing a preload on the flexible hinges 8, e.g. joints, and the contacts between the at least one actuator 2a, 2b, joints 8 or 19 and stator body 1. This preload advantageously prevents relative movement of the at least one actuator 2a, 2b flexible connections 8 or joints 19 and the stator body 1. The preload also can apply a compressive stress on the actuator material, more specifically on the piezoactuator. In preferred embodiments the at least one tension element or preload element 3 can advantageously also be used as a tuning element for improving the dynamical behavior of the motor, by for instance changing the material and/or geometry of the tension or preload element and/or the provided preload itself. In one preferred embodiment one or more optional disk springs 4 can be placed between the head of the tension element, e.g. a preload bolt, 3 and the stator body 1. Such disk spring(s) can advantageously provide additional tuning means, generally provide a larger compliance than a bolt, provides a finer adjustment of the preload and prevents loosening of the bolt through vibrations.

A tension element 3 according to embodiments of the invention may be a tension bolt. The use of a bolt allows for simple compensation for dimensional variations in the piezo during assembly to the support. For instance a bolt, with a metrical diameter of 4 mm by Jeveka. Preferably a bolt, according to embodiments, may provide different stiffness and thus a different tuning of the motor, for instance by varying the thread length of a bolt. As a bolt, according to embodiments of the invention advantageously is used as a fine-tuning bolt, one can choose the bolt in function of the specific and desired tuning of the eigenmodes of the motor. In preferred embodiments said tension element 3 is preferably made of a metal, for instance steel or titanium. Depending on the application where a motor according to embodiments of the invention is used, a person skilled in the art can choose the appropriate material for the tension element 3. For instance non-magnetic steel or titanium alloys can be used for a motor implemented in an electron microscope.

In preferred embodiments where at least two actuators are provided, a tension element or a preloading element 3 is positioned in between the at least two actuators to preload the stacks, thus the motor itself preferably does not rely on the general preload of the motor against a load which is performed separately.

Moreover, the stator body 1 of the present invention is a module and very compact. The statory body may be symmetric, horizontally and/or vertically. The stator body advantageously permits a space- and mass-saving setup of the motor according to embodiments of the invention. An advantage of embodiments of the present invention, is that a motor according to the invention comprises fixation points, through for instance mounting feature 7, which can be placed at nodes of the vibration modes. The fixation therefore does not influence the vibration mode shapes or frequencies and does not hamper the deformations in the stator body 1, making it a powerful motor. Secondly, it is an advantage of the present invention, that less or less severe vibrations are transmitted to the environment.

In preferred embodiments of the invention a stator body 1 may be fixed at the sides, for instance through the arms L and R of the stator body, to the environment through at least one mounting feature, e.g. a mounting hole, 7 on for instance each side of motor body 1 as illustrated in Figure 1. One preferred embodiment provides two mounting features, e.g. holes, 7 on each side of the motor body 1, preferably located at nodes of the vibration modes used to drive this motor.

The stator body 1 of preferred embodiments of the invention, for instance when used in a 1 DOF configuration of a motor of the present invention, preferably has a rectangular planar shape as illustrated in the figures. However, the shape is not limited to a rectangular or square type, but can also be elliptic or any other arbitrary shape. Especially the sides of the stator body (L, R), where the stator body is fixed through fixation features 7 can have any type of shape in other embodiments. The shapes of the linkages can also be altered, as long as the eigenmodes of the motor remain more or less similar. The variable shape of the stator body gives the designer a freedom to keep the dimensions within tolerance for a targeted system application. Several embodiments illustrating the latter are provided in Figs. 13-22.

Advantageously, the stiffness (and mass) of the bolt(s) can for instance be altered by changing the length of the thread on this bolt. A bolt used in embodiments of the invention preferably has a socket hex cap and is chosen such that the bolt is longer than the piezoactuator. The cross area of the bolt is also a parameter which can be used for tuning the motor. The thread does not need to be metric but can be of any other standardized or arbitrary shape. The shape of the head has little influence on the function and can be of any type. The bolt may also further comprise a shoulder. In alternative embodiments the bolt can also be a torx screw, a slotted head screw, a crossed recess screw, a tapping screw, a foundation bolt. In other embodiments the bolt can be replaced by fastening elements known in the art, which can provide pretensioning to the at least one actuator, such as for instance rivets, rods, or by means of press-fits or cables.

A motor, e.g. piezoelectric motor, according to embodiments of the invention is preferably designed such that a contact zone, provided by e.g. a point 5, is able to produce a closed trajectory, thereby inducing a relative motion of the driven part 6a, 6b with respect to the motor. The contact zone, provided by e.g. a point 5, is preferably moved by applying an electric signal or field to the at least one actuator 2a and 2b. The motor acts against an element, such as for instance a ruler, or another shape like for instance a ring or arc, to generate a relative motion between both. In preferred embodiments the material of the contact, e.g. a point, and of the counterpart, e.g. ruler, is preferably a wear-resistant ceramic. In one preferred embodiment the at least one actuator, e.g. piezoactuator(s), deforms the stator body 1, whereby said stator body is preferably a metal stator body, mainly in the means for pivoting such as for instance flexible hinges 8, upon application of an electrical signal, e.g. a voltage, applied to the at least one actuator. In some embodiments the means for pivoting 8 are preferably cut out in the stator body or are provided by the stator body. The cut out or opening in the stator body enables a connection between two solid parts of the stator body, in addition allowing only a limited angle of rotation between them. The two solid parts of the stator body connected by the means for pivoting rotate relative to each other about a fixed axis of rotation. In preferred embodiments flexure hinges 8 act as pivot points for the linkages formed by the solid metal structures located between these hinges.

An equivalent kinematic mechanism for a motor according to the present invention is provided in Figure 5. The function of links 21a, between means for pivoting 8b and 8c, and 21b, between means for pivoting 8h and 8i, is to combine the displacement, in embodiments where at least one actuator 2 is used, with the common displacement of said actuators expressed as a translation of said links, and the differential displacement of said actuators expressed as a rotation of said links. Thus, by extending and/or contracting both actuators, e.g. piezo's, with the same amount, the tip on top of link 21a translates towards/away from the contact zone or point. By extending one and contracting the other piezo, link 21a tilts, resulting in a translation of the contact in a direction perpendicular to the contact (parallel to the driving direction). These tip motions can be combined (superimposed) by superimposing the corresponding driving voltages or by adapting the phase difference of the voltage signals. As indicated above, the same may be applied to link 21b. Preferably links 21a and 21b move at the same time and with a similar strength. The main function of links 20a, 20b, 20c, 20d is to connect the mechanism formed by links 21a and 21b and actuators 2a and 2b to the stationary sides of the stator body 1. In addition, the linkage system of a motor according to embodiments of the invention, advantageously is designed with the objective to minimize deformation of the actuators in the direction perpendicular to the length axis of the actuator.

In preferred embodiments flexure hinges which act as means for pivots 8a, 8b, 8c, 8d may be preferably located on a single line as illustrated in Figure 17. On the other opposite side of the stator body, pivots 8g, 8h, 8i, 8j are also preferably on a single line, different from previous one. Both are parallel to each other and perpendicular to the piezoactuators. Pivots 8e and 8f are positioned close to said first line, and pivots 8k and 81 are positioned close to said second line. As a result of this pivot layout, the displacement of the links and pivots belonging to the body is limited to direction A, and is negligible in direction B for the small excitations generated by the piezoactuators. By placing pivots 8e, 8f, 8k, 81 close to said lines, also the piezoactuators restrict their movement and deformation mainly to direction A. As a result, the unwanted static/dynamic forces in direction B on the piezoactuators are negligible, unwanted because they cause bending of the piezoactuators which has a negative influence on the reliability and lifetime of piezoactuators. Also the mass distribution in the links is chosen such that their center of rotation in dynamic use lies close to the line running through pivots 8a, 8b, 8c, 8d or the line running through pivots 8g, 8h, 8i, 8j so as to avoid unwanted dynamic excitations in direction B. A motor according to embodiments of the invention, can be easily scaled without hampering the operating principle of the motor, as well for the resonant mode as for any other operating modes. Besides the obvious isometric scaling, the motor according to embodiments of the invention advantageously is robust against a non- isometric scaling, most peculiarly against a shortening and/or lengthening of for instance the at least one actuator, e.g. piezo, 2a, 2b, and links 22a, 22b with the same amount, and in a symmetric way. This is because in the mass-spring system corresponding to a motor according to embodiments of the invention, the moving mass is mainly located in the links 20 and 21, while the stiffness is mainly determined by the at least one actuator, e.g. piezo, themselves. A shortening or lengthening of the motor as described, changes mainly said stiffness and therefore the resonance frequency but advantageously not so much the modal shapes of the resonant modes used for driving the piezomotor in resonant mode. To provide an advantageous function of the motor, links 22a and 22b are preferably fixed with respect to each other, for instance via at least a mounting or fixation feature(s) 7. As a result this may restrict the possible mode shapes of the motor and increase the stiffness of the motor in the driving direction.

In one preferred embodiment a stator body 1 of the motor, is preferably made from a metal or an alloy with good thermal conductivity, providing an efficient thermally conductive path from the at least one actuator to the machine frame. This may for instance be of importance in vacuum conditions in order to achieve higher duty ratios. For instance when a motor according to embodiments of the invention is used for SEM applications, stator body material 1 is preferably as non-magnetic as possible. However for other applications also other metals could be used. In alternative embodiments, other materials can be used for the stator body 1, such as for instance PEEK or other plastics.

Figure 18 for instance provides a stator body according to embodiments of the invention, where the stator body is machined through water jetting or punching instead of spark erosion processes, for instance to provide the pivoting means 8. This is much cheaper and only feasible if the cutouts are smaller and if the tolerances are not as stringent.

In one preferred embodiment all materials of the motor and suspension mechanism, according to the present invention, are preferentially substantially non-magnetic and thus have a negligible influence on present magnetic fields, like for instance in electron microscopy and other demanding environments.

A motor according to embodiments of the invention advantageously has an open structure in the sense that air or any gas can easily be evacuated from the internals of the motor via holes and/or gaps in the suspension mechanism or motor body. This is for instance important for vacuum applications but it can also improve a convective cooling of the motor in non-vacuum applications.

In further embodiments a motor is provided comprising an external cooling means, whereby said external cooling means is adapted to cool the motor using convection. For instance by using a ventilator or by using external cooling channels, whereby cooling fluids can be introduced in said channels.

The components of the suspension mechanism according to embodiments of the invention are preferably made from metals or alloys with good thermal conductivity, providing an efficient thermally conductive path from the at least one actuator to the motor, and resulting to the machine frame. This advantageously increases the power output and duty ratio of the motor, especially in vacuum environments where no convective cooling is present. Moreover mounting features or fixation points 7 of the stator or motor body 1 are nodes of the excited vibration modes. This minimizes the vibrations transmitted to the environment, for instance a machine frame, and assures that the clamping (force) does not influence the vibration modes or their eigenfrequency.

In a particular embodiment the motor, e.g. piezoelectric motor, according to this invention is characterized in that:

- The actuator body 2a, 2b is preferably symmetric front-to-back, left-to-right, and top-to-bottom. The fixation points of the body are knots of the excited vibration modes. These characteristics minimize the vibrations transmitted to the machine frame, and assure that the clamping (force) does not influence the vibration modes or their eigenfrequency.

- The stator body, e.g. module, 1 comprises at least one actuator, preferably two piezoelectric actuators. This set of piezoactuators can be used in different modes, either sequentially or simultaneously by applying an electrical or driving signal which combines a DC signal with an AC signal for achieving the lowest tracking errors.

- The stator body, e.g. module, 1 can comprise at least one tension element, to achieve a high efficiency in the resonant vibration mode. By changing the stiffness and mass of the tension element e.g. a pretension bolt and/or the pretension force, the horizontal and vertical vibration modes are tuned in order to optimize the dynamic behaviour which guarantees good efficiency. The improvement can be achieved through separation of the two Eigen frequencies or by making them to coincide.

The drive speed and traction force of the relative motion between the stator body, e.g. module, 1 and said driven part 6 is preferably controllable by adapting the phase between and/or the amplitude of the excitation signals of the at least one actuator 2a, 2b accordingly.

The present invention also concerns an apparatus, as illustrated in Figure 10 that comprises one or more of the motors of present invention and that is controllable to allow the simultaneous combination of the motor(s) to position the apparatus in one or more degrees of freedom when they are working in one or more of the working modes. In Figure 10 an apparatus comprising 2-DOF motors 25 according to the invention are shown, however embodiments of an apparatus can also be provided using at least one 1 DOF-motors 26 according to embodiments of the invention (as illustrated in Fig. 11). For instance a motor comprising more degrees of freedom can be provided, for instance a 2 DOF-motor 25 as illustrated in Figure 9, which can be used to drive a mobile unit in two degrees of freedom defined by directions B and C, when the mobile unit is borne in the plane of movement by a separate bearing construction.

Such apparatus may further comprise a mobile unit, wherein the mobile unit is driven by the motors and the drive function and the bearing function may be carried out by the motors of present invention. The errors in the bearing degrees of freedom can be actively compensated by applying a DC voltage signal to any of the at least two actuators 2a, 2b of each respective motor. In a particular embodiment the apparatus is operable by three motors which are operable in a resonant mode and are positioned within a relative angle of 120 degrees. The actuators in the motor can be actuators of the group consisting of piezoelectric actuators, electrostrictive actuators and magnetostrictive actuators.

It is an advantage of the invention that embodiments of the motor have at least three modes of operation: (1) resonant mode, (2) direct mode, and (3) stick-slip mode. As a result embodiments of the invention provide a multi drive motor. Said modes of operation can be performed sequentially or simultaneously.

(1) Resonant mode:

According to a specific embodiment, in the resonant mode, a motor according to embodiments is brought into resonance by driving the at least one piezoactuator, whereby the at least one piezoactuator is adapted to have two eigenmodes on two different frequencies which can alter the direction of motion of the motor. In an alternative embodiment, the resonant mode of a motor structure according to embodiments of the invention can be achieved by driving the at least two piezoactuators each with their sine wave signal of specific amplitude and phase. Preferably, when for instance two actuators are used, the two sine wave signals comprise a varying amplitude and/or phase, but having a same frequency. More specifically, the phase difference between the two sine wave signals may be adjusted to alter the direction and speed of motion, and/or the voltage amplitude of the two sine wave signal may be adjusted.

To acquire the advantage of each mode, phase control may be implemented for low velocity and one can gradually move to amplitude control when high velocity is desired. Adjusting the phase and/or amplitude of the two sine wave signals results in an adjustment of the repeated closed trajectory, for instance an ellipse, of the tip of the motor. By varying the phase difference between both signals and their amplitude, the shape and size of said trajectory can be changed, as well as the direction in which the trajectory is followed, for instance clockwise or anti-clockwise for an ellipse. When said tip is pushed against a surface, this surface is driven relative to the motor, with speed and sense depending on the chosen trajectory. The resonant mode is the fastest mode of all three modes of operation, and gives unlimited stroke.

For instance, to enable a resonant drive mode the at least one actuator, e.g. piezostack, can be driven by exciting the at least one actuator with two sine wave signals, each sine wave signal having a different frequency, in order to alter the direction of motion. In alternative embodiments, two frequencies may be superposed for one actuator, preferably such that one can enable a Lissajous form with only one actuator.

(2) Direct mode:

In direct mode the at least one actuator is driven for tilting the link supporting the tip, resulting in a translation of the contact point parallel to the drive surface, and preferably taking a stage with it. In the case that exactly two actuators are integrated in the motor, both actuators are driven in opposite sense. The actuators do not necessarily have to move exactly opposite and in a synchronous way. For instance, first one actuator can be extended, and then a second actuator contracted. In this mode, the speed is kept low and the contact does not slip, implying high accuracy but limited stroke. In embodiments where two actuators are used to enable a direct mode, two independent (quasi-)static voltages are applied, which results in displacement with nanometer (or even better) resolution , over a stroke of + 5 μπι, depending on the type of actuator used and voltage applied. The maximum stroke of the piezomotor is directly related to the maximum stroke of the used piezoactuators. The direct mode is the most accurate mode of all three, but with limited stroke. Moreover, to compensate the hysteresis of an actuator, e.g. a piezostack, a slip hysteresis compensation algorithm can be implemented, such as one based on the Maxwell slip hysteresis theory.

As indicated before, a motor according to embodiments of the invention, can operate in the direct drive mode and resonant vibration mode, simultaneously or sequentially, by for instance using a global scheme of the control system as illustrated in Figure 12. Figure 12 shows a control scheme combining the modes, which is implemented in a D-Space controller. A more detailed explanation of the scheme and its initial results of the applied scheme can be found in "Simultaneous Resonant and Direct-Drive Control of a Piezomotor, for Combining Fast and Accurate Motion" by Santoso et al presented on the 13 ^th International Conference on New Actuators, 730-733 (2012).

(3) Stick-slip mode:

In stick-slip or inertial mode, the at least one actuator, e.g. piezo, is preferably driven in a similar way as in the direct mode, but the range of the motor is extended by deliberate slipping actions. In the intended driving direction, the piezomotor is driven slowly. When the motor reaches end of stroke (or close to it), the motor is driven fast in the opposite sense such that the motor tip slips against the stage, and the stage remains (more or less) at its position due to its higher inertia. Then the motor is again driven slowly in the intended direction and the cycle continues. The driving signal is typically a saw tooth or any other asymmetrical signal which comprises a fast slope for slipping and a slow slope for the stick-motion.

The stick-slip mode gives unlimited stroke, but with lower resolution than the direct mode. This mode can be very energy efficient when the stage remains stationary over longer periods, as no power is needed to keep its position.

(4 ) Other possible modes, like e.g. stepping or walking mode:

In stepping mode, preferably two stator bodies are used in parallel to drive the load at small steps and low speeds. During stick-slip the fast slope of the driving signal causes a considerable shock which is not always desirable because this can lead to vibrations. In the stepping mode one stator body can move the load forward (or in the opposite direction), similar as to the direct mode as described above, but in the meantime the other stator body may retract and move back without (or barely) touching the load. Next, the other stator body moves the load forward while the first stator body retracts. Both stator bodies alternately move the load in the desired direction. This motion also has an unlimited stroke and is much smoother than stick-slip. In this mode the combination of the two stators is preferably suspended through one suspension mechanism because each stator needs to be able to retract, while the other one remains in contact with the load. Also more than two stator bodies can be put in parallel and the phase difference between the different drive signals does not necessarily need to be 180°.

The direct mode and stick-slip modes can be implemented by for instance using at least one actuator, e.g. a piezoactuator, only instead of two. Where at least two actuators are used, the two actuators do not necessarily have to follow opposite signals, nor have to be active at the same time. For instance, one half of the stroke can be realized by one piezoactuator and the other half of the stroke by the other piezoactuator. On top of these signals, offset voltages can be applied, e.g. to influence certain characteristics of the piezoactuators, to avoid depolarization, to obtain linearization, for hysteresis compensation or to avoid inverting their polarity.

The above discussed modes can, according to embodiments of the invention, be combined, (1) serially or sequentially in time, or (2) in parallel: (1) Combining serially time

Combining serially in time is an easy way of combining the fast resonant mode to move fast to the desired position but with low accuracy, and then correcting the remaining position error using the direct and/or stick- slip mode.

(2) Combining in parallel

By superimposing the sine signals of the resonant mode with low-frequency signals of the direct mode, the tracking error can be reduced.

Depending on the properties of the desired trajectory of the load and on the tracking error, a decision is made towards the choice of the working mode or combination of modes. This decision can for instance be performed through a control module which has one or more sensors as inputs. Examples of sensors which can be used as inputs are: position, speed or acceleration sensors; temperature and/or humidity sensors; electric current and voltage measurements; vibration measurements; etc.

Contact zones, e.g. points, 5, 5a, 5b comprise in preferred embodiments a convex and spherical surface as this works against a wide variety of driven surfaces, for instance planar, cylindrical or sections thereof, and relaxes alignment tolerances of the motor with respect to the driven surface. The contacts can also be planar, cylindrical, or with single or double curvature or any other arbitrary shape. These tips can be made of any type of material; preferably said tip is a ceramic tip. The tip can also be an integral part of the motor or stator body. Or said tip may be part of the at least one tension element 3. It is also possible to use multiple contact points per stator. In a particular embodiment at least one of the actuating elements, e.g. piezoactuator, is replaced by a dummy element or non-active element. The non-active element can be of a material with the same material properties as the active material, such that the dynamics of the motor are not influenced. This material is however cheaper than an actuating element. Such an embodiment would be easier to control or drive because fewer electrical signals need to be provided and/or controlled. Also this lowers the risk of failure because the amount of actuating elements, e.g. piezoactuators, is reduced. In other embodiments the dummy element can be a sensing element, e.g. piezosensor.

In preferred embodiments the motor, e.g. piezomotor, remains stationary in driving direction and the load is put on a slider (i.e. a piece which has one or more degrees of freedom). Therefore the mass of the motor does not need to be moved, and therefore the required force to achieve a desired acceleration can be reduced.

In other embodiments the load is attached to the motor. The motor then moves with respect to a stationary contacting surface. The advantage of this configuration is that it takes up less space for achieving the same stroke. Therefore the system comprising the motor, e.g. piezomotor, will be more compact.

In some embodiments both the motor and the contacting surface are moving with a relative motion with respect to each other.

In other embodiments the thickness of the motor can be made very small. This can be of interest when the available space is limited. In other embodiments the thickness can be increased because this will lead to a higher stall force. Due to the customization possibilities of the stator body according to embodiments of the invention, advantageously shorter actuating elements, e.g. piezo' s, can be used inside the stator body, thus where the stator body is used as a housing for the piezo's. This makes the overall motor smaller and can be of interest when the available space is limited. In alternative embodiments the stator body according to embodiments of the invention may have chamfered edges.

In some embodiments the actuators are piezoactuators and these can be driven or excited based on charge instead of voltage. The amplifiers required for this are termed charge amplifiers. This leads to a more linear displacement behavior of the piezoactuators. Especially at low voltages piezoactuators are known to suffer from effects such as a 'dead zone'.

Double-sided operation and multiple contacts

Instead of having one contact zone, e.g. point 5 only, in one preferred embodiment the piezomotor can be equipped with two contact points 5a and 5b, one on each side of the motor, as illustrated in Figure 7. For this purpose, the head of the at least one tension element, e.g. preload element, 3 - a bolt in the preferred embodiment - is preferably sunk into the link 21b it is acting on, such that the second contact point 5b can be placed on top of it, not necessarily fixed to the head of said preload element 3, but preferably fixed to the link 21b itself, as to follow the motion of said link, in analogy to the first contact point 5a which is fixed to link 21a. Said contact points follow their trajectory in opposite direction: when contact point 5a moves in a clockwise direction, then contact point 5b will move in an anti-clockwise direction, and vice versa. When surfaces are pushed in direction A against both said contacts, driving forces will be generated on both surfaces in direction B and with the same sense. Both contacts can be used to drive the same slider. To assure preload on both contacts, the preload mechanism has to be adapted from the single-contact embodiment. One possible embodiment is shown in Figure 7, with the contacting rulers 6 elastically connected to a single slider.

Alternative preload mechanisms for the piezoactuators according to embodiments of the invention A motor, e.g. piezomotor, according to embodiments of the invention, comprises at least one tension element and thus can have more than one tension or preload element. Two or more tension or preload elements, for instance bolts, can act together to generate a common preload on both piezoactuators, and specific preload elements can generate preload for individual piezoactuators, or combinations thereof. The tension or preload elements 3 can be placed next to the actuators, e.g. piezoactuators, inserted through holes in the actuator, e.g. piezoactuators, or combinations thereof. In other embodiments the tension elements can be wrapped around the at least one actuator, or provided as a coating on the at least one actuator. The actuators, e.g. piezoactuators, can also be inherently preloaded, like a parallel pre-stressed actuator. One particular embodiment is shown in Figure 8: one central tension element, e.g. bolt, 3 gives a common preload, while tensions elements, e.g. bolts, 3b and 3c preload the hollow actuators, e.g. piezoactuator tubes, into which they are inserted. The embodiment of Figure 8 without a central tension element, e.g. bolt 3, can be another variant. The tension or pretension element can be an elastic structure of the stator or motor body 1 , forming one single part with said motor body 1. One particular embodiment provides a tension element 3 comprising elastic means like for instance in a serpentinelike structure in the center of the motor body 1, connecting link 21a with link 21b. Such a serpentine-like or corkscrew-like structure can be produced by using 3D printing technologies known in the art. Using 3D printing technology other, more complex elastic means, can also be manufactured and used in embodiments of the invention. Moreover, one can manufacture a tension element which is a part of the stator body, e.g. where the tension element and stator body are made of one piece, e.g. are monolithic. To insert the at least one actuator, e.g. piezoactuators, the motor body 1 is preferably stretched, extending the serpentine-like structure. After inserting the piezoactuators, the stretching force on the motor is removed, after which the serpentine structure generates a preload force on said piezoactuators. A typical pretension of one piezo, by using a tension element according to embodiments of the invention, may be 20 N/mm ² .

A motor, e.g. piezomotor, according to embodiments of the invention, whereby said motor is a 2 DQF-motor In some embodiments a motor, according to embodiments of the invention can provide two degrees of freedom 25. For instance by merging at least two 1 DOF-motors 26, e.g. piezomotors, according to embodiments of the invention, as illustrated in Figure 9, a planar drive can be realized. In the four-armed design on the left of the picture, at least one actuator, e.g. 4 piezo' s are provided, where actuators 2a and/or 2b generate displacement of tip 5 in a plane defined by directions A and C, while two other actuators, e.g. piezo's 2c and/or 2d, generate displacement of tip 5 in a plane defined by directions A and B, perpendicular to previous said plane. By controlling the signals going to the four actuators, e.g., piezoactuators, motion of the tip can be generated in any combination of directions A, B and C. The tip can be made to follow three-dimensional trajectories. A drive plane is defined as parallel to directions B and C. When the motor is pushed against or brought into contact with a surface parallel to the drive plane, a relative motion in this plane between motor and said surface can be generated. For resonant operation, elliptical trajectories can be used in any plane defined by direction A and any combination of directions B and C. For direct, stick-slip and stepping modes, combinations of directions B and C are used for positioning in the drive plane, while direction A is used to control the distance to the drive plane in bearing mode. In Figure 9, on the right side, a variant with three arms and three actuators, e.g. piezoactuators, is shown.

A planar 2-DOF motor, according to embodiments of the invention, comprises a motor according to embodiments of the invention, whereby the motor comprises at least four actuators which are positioned within a relative angle of 90 degrees (see e.g. left hand side of Figure 15 which illustrates a motor where 4 actuators are used, but where only two actuator are shown) or three actuators which are positioned within a relative angle of 120 degrees (see e.g. right hand side of Figure 15) with respect to at least one other actuator. The actuators 2a, 2c are preferably preloaded by using at least one tension element 3 or one common tension element 3 for all actuators. The relative angle between the actuators is dependent on the amount of actuators used in the motor's design, and is preferably chosen such that the angle between the actuators is [360 degrees/amount of actuators] degrees.

Bearing function of a motor, e.g. piezomotor, according to embodiments of the invention

Both the regular motor 26 as illustrated in Figure 1, providing 1 DOF, and the planar 2-DOF version of the motor 25 as illustrated in Figure 9 can be used for both drive and bearing functions. This functionality makes an active compensation of errors in the bearing degrees of freedom possible. For example, with three motors a positioning system with two translation and one rotational degree of freedom can be constructed without the use of any external bearing function. Moreover, the bearing degrees of freedom can be actively controlled to reduce the motion errors. An example of such a positioning system, comprising 2 DOF-motors, is shown in Figure 10. Three planar 2 DOF-motors, e.g. piezomotors, are preferably fixed to a common body, referred to as table 24, and with their contact zones, e.g. points 5a, 5b, 5c oriented towards a common drive plane 23. These contact points are pressed against the drive plane by the mass of the system and/or additional preload systems pulling body 24 towards drive plane 23. The tip motions in the drive plane are used to drive table 24 with respect to drive plane 23. The tip motions in direction A, perpendicular to the drive plane, can be used to adjust the vertical position (direction A) and tilt of table 24. For embodiments with more than three motors (more than three Tegs'), the tip motion in direction A can be used to perform a stepping motion, in analogy to an animal, with drive plane 23 representing the ground, contacts 5a, 5b, 5c representing the feet, the motors representing the legs, and body 24 representing the animal's body. To move the body 24, one or more contacts (feet) are lifted up from drive plain 23 (ground), followed by displacements of the motors (legs) in a horizontal plane (parallel to the drive plane), followed by the previously lifted contacts (feet) being moved down again into contact with the drive plane (ground). This method allows to perform a stepping operation with no (or minimal) slip between the contact points and the drive plane. Different gaits are possible depending on the number of legs.

Fig. 11 shows the schematic layout whereby a stage is supported by three 1-DOF motors 26, according to embodiments of the invention, and positioned within a relative angle of 120 degrees. The combined operation of the three motors allows a large travel in X, Y and Θ.

Embodiments of a motor according to the present invention, can be used in several applications. For instance in stages for Scanning Electron Microscopy (SEM), wafer steppers, MRI and X-ray applications. Mostly embodiments of the present invention can be used in challenging environments: i.e. vacuum and non-magnetic environments.

Various modifications and variations of the forming process described within embodiments of this invention are possible, which can be made without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the practice of the invention, and the illustrations, examples and specifications described herein can be considered as exemplary only.

It is to be understood that this invention is not limited to the particular features of the means and/or the process steps of the methods described as such means and methods may vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It must be noted that, as used in the specification and the appended claims, the singular forms "a" "an" and "the" include singular and/or plural referents unless the context clearly dictates otherwise. It is also to be understood that plural forms include singular and/or plural referents unless the context clearly dictates otherwise. It is moreover to be understood that, in case parameter ranges are given which are delimited by numeric values, the ranges are deemed to include these limitation values.