Patent Abstract:
An apparatus for actuating a positioning device includes a housing; a piezoelectric element connected to the housing; a driven element configured to move relative to the housing; and a flexible element connected to the piezoelectric element and configured to transfer a motion of the piezoelectric element to the driven element.

Full Description:
FIELD OF DISCLOSURE 
     This disclosure relates to positioning apparatuses, and in particular, to actuators for positioning apparatuses. 
     BACKGROUND 
     Positioning apparatuses are utilized in a variety of applications, such as scanning probe microscopy, micro-scale and nano-scale characterization and testing, and micro-scale and nano-scale fabrication or assembly. In general, a sample resting on a stage is moved approximately into position by a coarse positioning apparatus and then adjusted into a precise position by a precision positioning apparatus having finer resolution. In many cases, positioning apparatuses employ piezoelectric actuators. 
     Referring to  FIG. 1 , one example of a positioning apparatus is a friction-driven actuator  100  used for positioning a sample  199  that rests on a driven element  190 . A piezoelectric (PZT) element  150  is attached to a base  110 . A friction element  170  coupled to the PZT element  150  frictionally engages a bottom surface of the driven element  190 . The PZT element  150  elongates or contracts in the X direction in response to an applied electrical signal, causing the friction element  170  to move along the X axis. This linear motion is transferred to the driven element  190  via the frictional engagement between the friction element  170  and the driven element  190 , thus causing the driven element  190  to slide relative to the base  110  and effecting Xmotion of the sample  199  in the X direction. 
     SUMMARY 
     In one aspect, the invention features an apparatus for actuating a positioning device. Such an apparatus includes a housing; a piezoelectric element connected to the housing; a driven element configured to move relative to the housing; and a flexible element connected to the piezoelectric element. The flexible element is configured to transfer a motion of the piezoelectric element to the driven element. 
     In some embodiments, the flexible element is configured to frictionally engage the driven element. 
     Other embodiments also include a preload element configured to impose a force normal to an interface between the flexible element and the driven element. Among these are those embodiments in which the pre-load element has a spring, and those in which it has a magnet. However, any other that applies a pre-loading force can be used. 
     Yet other embodiments include a friction element disposed between the flexible element and the driven element, the friction element being configured to frictionally engage the driven element. In some of these embodiments, the friction element includes a magnet. However, this is not always the case, as the friction element can be something other than a magnet. 
     Also included among the many alternate embodiments of the apparatus are those that further include a preload element configured to impose a force normal to an interface between the friction element and the driven element. 
     Other embodiments include structures for guiding motion of the drive element relative to the housing. Among these embodiments are those that include a slide guide configured to guide the motion of the driven element relative to the housing. In some of these embodiments, the slide guide is further configured to limit the extent of motion of the driven element. 
     In other embodiments, the driven element is separated from the piezoelectric element. 
     The apparatus also includes many embodiments that cause the driven element to move in a variety of directions relative to the housing. For instance, there are embodiments of the apparatus in which the driven element is configured to move linearly relative to the housing, and there are also embodiments of the apparatus in which the driven element is configured to rotate relative to the housing. 
     Also included are embodiments that vary the way in which the driven element is moved relative to the housing. Among these are those in which the driven element is configured to move relative to the housing via stick-slip motion. 
     In other embodiments, the apparatus also includes a position-sensing element coupled to the driven element; and a detection element configured to detect the position of the position-sensing element 
     A variety of signals can be used to control the motion caused by the apparatus. For example, embodiments of the apparatus include in which the piezoelectric element is controllable by a triangular wave signal, those in which the piezoelectric element is controllable by a saw-tooth electrical signal, those in which the piezoelectric element is controllable by a pulse-width modulated electrical signal, and those in which the piezoelectric element is controllable by any one of the foregoing, whether singly or in combination. 
     Many different kinds of piezoelectric elements can be used in the apparatus. For instance, in some embodiments, the piezoelectric element has a piezoelectric stack. In others, it has a shear mode piezoelectric element. 
     The driven element, in some embodiments of the apparatus, is configured to receive a specimen. For example, the driven element might be a stage of a microscope or coupled to a stage of a microscope to cause movement thereof. 
     In another aspect, the invention features an apparatus for actuating a positioning device. Such an apparatus includes a housing; a piezoelectric element; a flexible element connected to the piezoelectric element; and a driven element configured to move relative to the housing in response to a motion of the piezoelectric element. 
     Among the embodiments of the foregoing apparatus are those in which a friction element is disposed between the piezoelectric element and the driven element. Such a friction element is configured to transfer a motion of the piezoelectric element to the driven element. In some embodiments, the friction element includes a magnet. 
     Yet other embodiments include those having a preload element configured to impose a force normal to an interface between the friction element and the driven element. 
     Other embodiments include those in which the flexible element is also connected to the housing and those in which the piezoelectric element is connected to the driven element. 
     The friction-driven actuator described herein has a number of advantages. Piezoelectric elements are made of fragile ceramics that are generally sensitive to external impacts or shear stresses. Because the driven element does not directly contact the piezoelectric element, the piezoelectric element can be protected from damage that could otherwise result from, for instance, a sudden impact on the driven element or strain deformation of the driven element due to a heavy sample. The lifetime of the piezoelectric element can thus be extended. 
     The friction-driven actuator described herein can be utilized for centimeter-scale, millimeter-scale, nanometer-scale, and sub-nanometer-scale positioning, and thus is suitable for both long-range positioning and high-precision scanning in various scanning probe microscopy applications, such as atomic force microscopy (AFM). 
     Other features and advantages of the invention are apparent from the following description and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective diagram of a prior art positioning apparatus. 
         FIG. 2  is a perspective diagram of a friction-driven actuator. 
         FIG. 3  is a block diagram of the friction-driven actuator of  FIG. 2 . 
         FIGS. 4A and 4B  are block diagrams of the friction-driven actuator of  FIG. 2  with external forces applied. 
         FIGS. 5A and 5B  are block diagrams of a friction-driven actuator employing a magnet. 
         FIG. 6  is a block diagram of a friction-driven actuator with a position sensor. 
         FIG. 7  is a triangular waveform. 
         FIG. 8  is a diagram illustrating elongation and contraction of a piezoelectric stack element. 
         FIG. 9  is a diagram illustrating shear deformation of a shear mode piezoelectric element. 
         FIG. 10  is a continuous saw-tooth waveform. 
         FIGS. 11A and 11B  are block diagrams of the friction-driven actuator of  FIG. 2  under the application of the saw-tooth waveform of  FIG. 10 . 
         FIGS. 12A-12C  are pulse width modulation waveforms. 
         FIGS. 13A and 13B  are block diagrams of an alternative embodiment of a friction-driven actuator. 
         FIGS. 14A-14C  are block diagrams of an alternative embodiment of a friction-driven actuator. 
         FIGS. 15A-15D  are block diagrams of alternative embodiments of a friction-driven actuator. 
         FIGS. 16A and 16B  are block diagrams of an alternative embodiment of a friction-driven actuator. 
         FIGS. 17A-17D  are block diagrams of a friction-driven actuator that generates rotary motion in a driven element. 
         FIG. 18  is a block diagram of an alternative embodiment of a friction-driven actuator that generates X and Y motion in a driven element. 
     
    
    
     DETAILED DESCRIPTION 
     Friction-Driven Actuator 
     Referring to  FIGS. 2 and 3 , a friction-driven actuator  10  includes a piezoelectric (PZT) element  13 , such as a PZT stack, connected at one end to a housing  11 . A second end of PZT element  13  is connected to a flexible element  14 , which frictionally engages a surface  121  of a driven element  12 . In some cases, the flexible element  14  directly contacts a surface  121  of the driven element  12 . In other cases, the flexible element  14  is coupled via a friction element  16  to the driven element  12 . The friction element  16  is anchored to the flexible element  14  and is frictionally coupled to the driven element  12 . The driven element  12  holds a specimen (not shown), such as a specimen for investigation in a scanning probe microscope, or a stage on which a specimen is placed. 
     Application of an electrical signal to the PZT element  13  induces an elongation or contraction of the PZT element in the X direction. As the PZT element  13  elongates and contracts, the flexible element  14  and the friction element  16  are moved in the X direction. Due to the frictional contact between the friction element  16  and the driven element  12 , the driven element  12  is also moved in the X direction relative to the housing  11 . The direction and extent of motion of the driven element  12  are restricted by a slide guide  18 . 
     The driven element  12  does not directly contact the PZT element  13 . Thus, any load, stress, or strain applied to the driven element  12  (e.g., by the weight of a specimen resting on driven element  12 ) or to another part of friction-driven actuator  10  is absorbed by the flexible element  14  rather than by the PZT element  13 . The presence of the flexible element  14  thus protects the PZT element  13  from damage, cracking, malfunction, and stresses that are often induced by the application of external forces to a PZT element. For instance, referring to  FIG. 4A , an external impact torques the driven element  12 , thus tilting it relative to the slide guide  18 , and bending the flexible element  14 , thereby protecting the PZT element  13  from experiencing a torque. Similarly, referring to  FIG. 4B , a downward force applied to the driven element  12  (e.g., by the weight of a specimen) also causes the flexible element  14  to bend, and thus avoids application of a torque to the PZT element  13 . 
     The flexible element  14  may be formed of, for instance, steel, aluminum, carbon fiber, plastic, wood, or another suitable material. The friction element  16  is formed of, for instance, ceramic, copper or copper alloy, sapphire, or another material suitable to establish a frictional contact with the driven element  12 . In some cases, the friction element  16  may be formed of a magnet, a magnetic material, or a conductive material, including a magnetic conductive material. 
     Referring again to  FIGS. 2 and 3 , a preload element  151  is disposed between the flexible element  14  and the housing  11 . The preload element  151  is, for instance, a spiral spring or a spring plate formed of metal, carbon fiber, or plastic. The preload element  151  applies a mechanical force between the friction element  16  and the surface  121  of the driven element  12 , augmenting the frictional force between the friction element  16  and the surface  121 . 
     Referring to  FIG. 5A , as an alternative to a mechanical preload force, a magnetic force can be applied between the friction element  16  and the driven element  12  by a magnetic preload element  152 . In this case, the driven element  12  is formed of a magnetic material or a magnetic conductive material and the magnetic preload element  152  is a magnet. The attractive magnetic force between the driven element  12  and the magnet  152  augments the frictional force between the friction element  16  and the surface  121 . 
     In an alternative embodiment illustrated in  FIG. 5B , a magnetic force causes the friction force. In this embodiment, the driven element  12  is formed of a magnetic material, and a magnet  154  is disposed between the flexible element  14  and the driven element  12 . Motion is transferred from PZT  13  to the driven element  12  via a combination of a frictional coupling between the magnet  154  and the driven element  12  and a magnetic coupling between the magnet  154  and the driven element  12 . 
     Referring to  FIG. 6 , a position sensor  15  is coupled to the driven element  12 . An encoder  17 , which may employ optical, magnetic, resistive, or other encoding mechanisms, is coupled to the housing  11 . The position sensor  15  communicates with the encoder  17  to allow long-range closed-loop positioning control of the friction-driven actuator  10 . 
     Control of the Piezoelectric Element 
     The PZT element used in the friction-driven actuator may include a piezoelectric stack element, a shear mode piezoelectric element, or another type of piezoelectric element. The PZT element may be driven by any of a number of electrical signal formats, such as a triangular signal, a saw-tooth signal, or a pulse width modulation signal. The frequency, amplitude, and shape of the electrical signal applied to the PZT element affect the transfer of motion from the PZT element to the driven element. Appropriate selection of the frequency, amplitude, and shape of the electrical signal can enable rapid long-range (centimeter or millimeter scale) positioning and scanning as well as slower, precision (nanometer or sub-nanometer scale) positioning and scanning. 
     Referring to  FIGS. 7 and 8 , a PZT stack  41  is actuated via the application of the triangular electrical signal shown in  FIG. 7  for sub-nanometer scale high resolution positioning and scanning in one direction. Prior to application of a signal, the PZT stack  41  is not deformed, as shown in  FIG. 8A . Upon application of a first signal C 11 , the PZT stack  41  elongates by a distance ΔX along the X axis to a position +ΔX, as shown in  FIG. 8B . Upon application of a second signal C 12 , the PZT stack  41  contracts along the X axis to a position −ΔX, as shown in  FIG. 8C . In response to a third signal C 13 , the PZT stack  41  elongates to its original configuration, as shown again in  FIG. 8A . 
     Referring to  FIGS. 7 and 9 , a shear mode PZT element  42  is actuated via the application of the same triangular electrical signal. Prior to application of a signal, the PZT element  42  is not deformed (position A). Upon application of the first signal C 11 , the PZT element  42  deforms and a top surface of the PZT element  42  shifts along the X axis to a position +ΔX (position B). Upon application of the second signal C 12 , the PZT element  42  deforms in the opposite direction and the top surface of the PZT element  42  shifts along the X axis to a position −ΔX (position C). In response to the third signal C 13 , the PZT element  42  returns to its original configuration (position A). 
     Referring to  FIGS. 10 and 11A , the PZT element  13  is actuated via the continuous saw-tooth waveform to drive the driven element via “stick-slip” motion (also known as “inertial drive”) for long-range motion. Prior to the application of an electrical signal, the PZT element  13  is not deformed, and a distal end  19  of the driven element  12  is at its initial position X 1  (shown in  FIG. 3 ). When a voltage signal C 1  is applied to the PZT element  13 , the PZT element  13  elongates in the X direction, causing the flexible element  14  and the friction element  16  to move in the X direction. This motion is transferred to the driven element  12  via the frictional coupling between the friction element  16  and the driven element  12 , causing a distal end  19  of the driven element  12  to move a distance ΔX to position X 2 . 
     Referring now to  FIGS. 10 and 11B , a second voltage signal C 2  is then applied to the PZT element  13 , causing the PZT element  13  to contract to its original configuration. This contraction causes the flexible element  14  and the friction element  16  to move back along the X axis to their respective original positions. However, if the dynamic acceleration of the flexible element  14  and the friction element  16  caused by the sudden contraction of the PZT element  13  is sufficiently large, relative motion may occur between the friction element  16  and the driven element  12 . For example, the friction element  16  may slide relative to the driven element  12 , causing the driven element  12  to stay in position X 2  (as shown) or to move back along the X axis by a distance less than ΔX. 
     When applying a continuous saw-tooth or inverted saw-tooth waveform to the PZT element  13 , the driven element  12  may be moved by this stick-slip mechanism in the range of a millimeter in the X direction relative to housing  11 . The frequency and/or amplitude of the saw-tooth waveform can be adjusted to achieve a desired response from the PZT element. 
     Referring to FIGS.  2  and  12 A- 12 C, the PZT element  13  may also be controlled by an electrical pulse width modulated (PWM) signal for high-speed, centimeter-scale long range movement via a stick-slip mechanism. No movement of the driven element  12  occurs when a selective frequency square wave with 50% duty cycle (i.e., t/T=0.5;  FIG. 12A ) is applied to the PZT element  13 . When a square wave with less than 50% duty cycle (t/T&lt;0.5;  FIG. 12B ) is applied to the PZT element  13 , the driven element  12  moves in the +X direction. When a square wave with greater than 50% duty cycle (t/T&gt;0.5;  FIG. 12C ) is applied to the PZT element  13 , the driven element  12  moves in the −X direction. In general, stick-slip motion driven by a PWM signal is faster but less precise than motion driven by a triangular or saw-tooth electrical signal. 
     Alternative Configurations 
     Referring to  FIG. 13A , in an alternative configuration, a friction-driven actuator  20  includes a housing  21 , and a PZT element  23  connected at a first end to a driven element  22  and at a second end to a flexible element  24 . A friction element  26  is anchored to flexible element  24  and slidably frictionally engages a top surface of a slide guide  28 . The elongation and contraction of the PZT element  23  causes driven element  22  to move in the ±X direction along slide guide  28  by a stick-slip mechanism. In this embodiment, the distance that driven element  22  can be moved is limited by the length of slide guide  28  rather than by the length of driven element  22 . This embodiment is well suited to millimeter- or centimeter-scale long range motion. 
     In an alternative embodiment shown in  FIG. 13B , slide guide  28  is formed of a magnetic material, and a magnet  25  is disposed between flexible element  24  and slide guide  28 . Magnet  25  and slide guide  28  are engaged via both a frictional coupling and an attractive magnetic force. 
     Referring to  FIGS. 14A-14C , in another alternative configuration, a friction-driven actuator  70  includes a flexible element  74  connected at a first end to a housing  71  and at a second end to a PZT element  73 . A driven element  72  is mounted on a slide guide  78 , which is connected to the housing  71 . As the PZT element elongates and contracts, this linear motion is transferred to the driven element  72  via a friction element  76 , which is slidably frictionally coupled to the driven element  72 . In some cases, a mechanical or magnetic preload force may be applied. In this configuration, the flexible element  74  protects the PZT element  73  from potentially damaging loads, stresses, and strains, such as a torque from the weight of a specimen, as shown in  FIG. 14C . 
     Referring to  FIGS. 15A-15D , in some embodiments, a slide guide is not present. Referring specifically to  FIGS. 15A and 15C , in friction-driven actuators  60   a  and  60   c , a PZT stack  63   a  and a shear PZT element  63   c , respectively, are anchored to a housing  61 . Motion of the PZT stack  63   a  and the PZT element  63   c  is transferred to a driven element  62  via a flexible element  64  and a friction element  66 . A mechanical or magnetic preload force may also be applied. 
     Referring now to  FIGS. 15B and 15D , in friction-driven actuators  60   b  and  60   d , a flexible element  64 ′ is anchored to housing  61 . A PZT stack  63   b  and a shear PZT element  63   d , respectively, are connected to the flexible element  64 ′. Motion of the PZT stack  63   b  and the shear PZT element  63   d  is transferred to a driven element  62  via a friction element  66 ′. A mechanical or magnetic preload force may also be applied. 
     Referring to  FIG. 16A , in another alternative embodiment, a friction-driven actuator  50  includes two shear mode PZT elements  53   a ,  53   b  anchored at one end to a housing  51 . Application of an electrical signal to the PZT elements  53   a ,  53   b  induces shear deformation in the PZT elements  53   a ,  53   b . Second ends of the PZT elements  53   a ,  53   b  are connected to flexible elements Ma,  54   b , which frictionally engage a driven element  52  via two friction elements  56   a ,  56   b . In some instances, the flexible elements  54   a ,  54   b  directly frictionally engage the driven element  52 . Preload elements  551   a ,  551   b , such as springs, apply forces between the friction elements  56   a ,  56   b  and the driven element  52 , increasing the strength of the coupling between the friction elements  56   a ,  56   b  and the driven element  52 . The shear deformations of the PZT elements  53   a ,  53   b  are transferred to the flexible elements  54   a ,  54   b  and the friction elements  56   a ,  56   b  as linear motion along the X axis, which in turn causes the driven element  52  to move in the X direction along a slide guide  58 . 
     Referring to  FIG. 16B , in another example, a magnet  552  is added to a friction-driven actuator  50  between a flexible element  54  and a friction element  56 . The driven element  52  is formed of a magnetic material. The attractive magnetic force between a magnet  552  and the driven element  52  enhances the frictional coupling between the friction element  56  and the driven element  52 . 
     Referring to  FIG. 17A , in another embodiment, a friction-driven actuator  30  induces rotary motion in a ring-shaped driven element  32  related to the ring shape rotary guide  38 . A PZT element  33  is connected at one end to a housing  31  (not shown). A second end of the PZT element  33  is connected to a flexible element  34 . The flexible element  34  is frictionally coupled to a side face  321  of the driven element  32  via a friction element  36 . A preload force P, generated by a spring, a magnet, or another mechanism, enhances the coupling between the friction element  36  and the driven element  32 . Application of an electrical signal to the PZT element  33  induces an elongation or contraction of the PZT element  33 , which in turn causes the flexible element  34  and the friction element  36  to move in the X direction. Through the frictional coupling between the friction element  36  and a side face  321  of the driven element  32 , the X direction motion of the friction element  36  induces rotation of the driven element  32  about its center. 
     Referring to  FIG. 17B , in some cases, the driven element  32  is formed of a magnetic material, and a magnet  35  is employed in place of the friction element  36 . The attractive magnetic force between the driven element  32  and the magnet  35  enhances the frictional force between the driven element  32  and the magnet  35 . 
     Referring to  FIG. 17C , in other instances, the driven element  32  is formed of a magnetic material, and a magnet  35 ′ is coupled to a side surface  321  of the driven element  32 . Elongation or contraction of the PZT element  33  causes a flexible element  34  and the magnet  35 ′ to move in the X direction, inducing rotation of the driven element  32  about its center. 
     Referring to  FIG. 17D , a friction-driven actuator  30 ′ induces rotary motion in a driven element  32 ′. A first portion of the friction-driven actuator  30 ′ includes a piezoelectric element  33   a  connected at one end to a housing  31   a . A second end of the piezoelectric element  33   a  is connected to a flexible element  34   a . The flexible element  34   a  directly engages a bottom edge of the driven element  32 ′. A preload force Pa enhances the frictional force between the flexible element  34   a  and the driven the element  32 ′. A second portion of the friction-driven actuator  30 ′ includes a piezoelectric element  33   b  connected at one end to a housing  31   b . A second end of piezoelectric element  33   b  is connected to a flexible element  34   b . The flexible element  34   b  directly engages a top edge of the driven element  32 ′. A preload force Pb enhances the frictional force between the flexible element  34   b  and the driven element  32 ′. Rotation of the driven element  32 ′ is controlled by both piezoelectric elements  33   a  and  33   b . In some cases, a friction element (not shown) is disposed between the flexible element  34   a  and the driven element  32 ′ and/or between the flexible element  34   b  and the driven element  32 ′. 
     Referring to  FIG. 18 , a friction-driven actuator  80  induces X and Y linear motion in a driven element  82 . The friction-driven actuator  80  includes a first piezoelectric element  83   a  connected at one end to an X, Y slide guide frame  88 . The other end of the piezoelectric element  83   a  is connected to a flexible element  84   a . The flexible element  84   a  is connected at the other end to a friction element  86 . The second piezoelectric element  83   b  is connected at one end to the X, Y slide guide frame  88  and the other end of the piezoelectric element  83   b  is connected to a flexible element  84   b . The flexible element  84   b  is connected at the other end to the friction element  86 . The friction element  86  engages a bottom face of the driven element  82 . A preload force P enhances the frictional force between the friction element  86  and the driven element  82 . X axis movement of the driven element  82  is driven by the piezoelectric element  83   a . Y axis movement of the driven element  82  is driven by the piezoelectric element  83   b.    
     In general, a shear mode PZT can be used in place of a PZT stack in both the linear and rotational motion embodiments described above. 
     It is to be understood that the foregoing description is intended to illustrate and not to limit the scope of the invention, which is defined by the scope of the appended claims. Other embodiments are within the scope of the following claims.

Technology Classification (CPC): 7