Abstract:
A stick-slip piezo motor. A piezo housing holds at least two piezo elements. The piezo elements are both rigidly connected to the piezo housing. At the end of each of the piezo elements is a piezo friction element. Each of the piezo friction elements is in friction contact with a moving friction element. While oscillating between a stick phase and a slip phase, both of the friction elements operate in conjunction to move the moving friction element in a desired travel direction. The piezo elements oscillate out of phase such that when one of the oscillating piezo elements is operating in the slip phase and moving in a direction opposite to the desired travel direction, the other oscillation piezo element is operating in the stick phase and is moving in the travel direction in order to counteract and overcome unwanted dragging of the moving piezo element.

Description:
This application claims the benefit of Provisional Application No. 61/268,322, filed Jun. 11, 2009. The present invention relates to piezo motors, and in particular, to piezo motors that utilize a stick-slip mode of operation. 
    
    
     BACKGROUND OF THE INVENTION 
     Piezo motors are known. Typically, piezo motors are comprised of motors with many mm of available linear travel or any degree of rotational travel. Generally, a piezo element actuates a friction element that in turn moves a second friction element (sliding element). These piezo motors can be roughly separated into resonant and non-resonant types. Resonant type piezo motors exhibit high-speed, but are less stable at very high resolutions (nanometer to sub-nanometer range). Resonant piezo motors operate in the resonant frequency range of the piezo. Non-resonant piezo motors operate below the resonant frequency range of the piezo (and are often audible). Some of the non-resonant type piezo motors are based on the inertial or stick-slip principle and sometimes are able to achieve nano-meter resolutions. 
     The main problem with conventional piezo motors based on the non-resonant, stick-slip principle is that the moving part of the actuator retracts slightly during the “slip” part of the actuation cycle which results in poor constant velocity behavior, lost efficiency and a decrease of the position control of the actuator. This behavior is especially pronounced at slow velocities. Another problem with the conventional piezo motor is that the available actuation force is limited to the achievable friction of the friction element attached to the piezo element, which needs to be limited to not cause significant retraction during the slip phase of the actuator. 
     For example,  FIG. 1A  shows prior art stick-slip piezo motor  140 . AC voltage source  142  provides alternating current to piezo element  141 . Piezo element  141  is rigidly connected to piezo base  146 . Friction element  143  is rigidly attached to piezo element  141 . Friction element  143  is pressed against sliding friction element  145 . During the stick phase of the cycle, piezo element  141  expands relatively slowly to the right so that friction force is not overcome and there is no slipping. During the slip phase of the cycle, piezo element  141  contracts to the left at a much faster rate to overcome the friction between friction element  143  and sliding friction element  145 . The inertia of sliding friction element  145  is not overcome and there is slipping between friction element  143  and sliding friction element  145 . Slipping is desired so that friction element  143  does not drag sliding friction element  145  backwards to the left. Stated differently, sliding friction element  145  presses against friction element  143  with sufficient force so that friction element  143  moves sliding friction element  145  during the stick phase of the oscillation yet also with such force so that friction element  143  does not significantly drag sliding friction element  145  backwards during the slip phase of the oscillation. 
     With prior art stick-slip piezo motors, there has been a problem with eliminating unwanted dragging during the slip phase.  FIG. 1B  shows a graphical representation of the resultant motion of a prior art stick-slip piezo motor as a function of time. As is clearly shown there is significant undesired retraction  153  during the slip phase of the cycle. 
     What is needed is a better stick-slip piezo motor. 
     SUMMARY OF THE INVENTION 
     The present invention provides a multi-phase, stick-slip piezo motor. A piezo housing holds at least two piezo elements. The piezo elements are both rigidly connected to the piezo housing. At the end of each of the piezo elements is a piezo friction element. Each of the piezo friction elements is in friction contact with a moving friction element. While oscillating between a stick phase and a slip phase, both of the friction elements operate in conjunction to move the moving friction element in a desired travel direction. The piezo elements oscillate out of phase such that when one of the oscillating piezo elements is operating in the slip phase and moving in a direction opposite to the desired travel direction, the other piezo element is operating in the stick phase and is moving in the travel direction in order to counteract and overcome unwanted dragging of the moving friction element. In one preferred embodiment, the travel direction is linear. In another preferred embodiment the travel direction is rotational. In another preferred embodiment, more than two piezo elements are utilized to operate in conjunction to move the sliding friction element in a desired travel direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a prior art stick-slip piezo motor. 
         FIG. 1B  shows a graph depicting the resultant motion caused by a prior art stick-slip piezo motor. 
         FIG. 2  shows a preferred embodiment of the present invention. 
         FIGS. 3A-3E  depict the resultant motion of the sliding friction element of the piezo motor of  FIG. 2 . 
         FIG. 4  shows a graphical representation describing the resultant motion of the sliding friction element of the piezo motor of  FIG. 2 . 
         FIG. 5  shows a linearized graphical representation of the resultant motion of the sliding friction element of the piezo motor of  FIG. 2 . 
         FIG. 6  shows another preferred embodiment of the present invention. 
         FIG. 7  shows another preferred embodiment of the present invention. 
         FIGS. 8A-8E  depict the resultant motion of the sliding friction element of the piezo motor of  FIG. 6 . 
         FIGS. 9A-9E  depict the resultant motion of the sliding friction element of the piezo motor of  FIG. 7 . 
         FIG. 10  shows another preferred embodiment of the present invention. 
         FIG. 11  shows another preferred embodiment of the present invention. 
         FIG. 12  shows another preferred embodiment of the present invention. 
         FIG. 13  shows another preferred embodiment of the present invention. 
         FIG. 14  shows another preferred embodiment of the present invention. 
         FIG. 15  shows another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 2  shows a simplified drawing of a preferred piezo motor  10 . Piezo elements  1  and  2  are both rigidly connected to holding element  6 . Friction elements  3  and  4  are both connected to piezo elements  1  and  2 , respectively. Friction element  5  is pressed against friction elements  3  and  4 . Sliding friction element  5  is the object being moved by piezo motor  10 . Voltage source  12  is connected to piezo element  1 . Voltage source  13  is connected to piezo element  2 . Computer  14  is connected to voltage sources  12  and  13  and is programmed to control the output of voltage sources  12  and  13 . 
     Piezo Element 
     Piezo elements  1  and  2  are multi-layer piezo elements that exhibit longitudinal expansion when a voltage is applied. A preferred piezo element is a 3×3×3 mm stack with a 1 micrometer expansion at 150V applied voltage. 
     Holding Element 
     As stated above piezo elements  1  and  2  are rigidly attached to holding element  6 . Holding element  6  may be fixed or moving depending on the arrangement of the motor. 
     Oscillating Friction Elements 
     Friction elements  3  and  4  are fabricated from any material that causes friction when applied to sliding friction element  5 . In a preferred embodiment, friction elements  3  and  4  are ceramic friction elements. When voltage is applied to piezo elements  1  and  2 , the resultant oscillation of piezo elements  1  and  2  will cause friction element  5  to move in a predetermined manner. 
     Sliding Friction Element 
     Sliding friction element  5  is the object being moved by friction elements  3  and  4 . Sliding friction element  5  is pressed against friction elements  3  and  4  with sufficient force so that friction elements  3  and  4  move friction element  5  during the stick phase of the oscillation yet also with such force so that friction elements  3  and  4  do not significantly drag friction element  5  backwards during the slip phase of the oscillation. 
       FIG. 3  shows a graphical representation illustrating the operation of the preferred embodiment of the present invention shown above in  FIG. 2 . 
     Cycle phase  1 : Voltage sources  12  and  13  are applying voltage to elements  1  and  2  so that elements  1  and  2  are both expanding in the same direction ( FIG. 3A ). The applied voltage increases at a low enough rate so that the speed of the expansion is slow enough so that the friction force between friction elements  3  and  4  and sliding friction element  5  is not overcome. Therefore, there is no slipping between friction elements  3  and  4  and sliding friction element  5 . Hence, both piezo element  1  and piezo element  2  are in the stick phase of motion causing sliding friction element  5  to move in a linear motion consistent with the linear motion of piezo elements  1  and  2  ( FIG. 4 ). 
     Cycle Phase  2 : Voltage source  12  is continuing to apply voltage with a slope slow enough to piezo element  1  so that it continues to expand in the same direction. However, the voltage from voltage source  13  drops to zero at a rapid rate causing piezo element  2  to contract at a rapid rate ( FIG. 3B ). Piezo element  2  contracts at such a rapid rate that the friction force between friction element  4  and sliding friction element  5  is significantly overcome. Hence, during cycle phase  2  piezo element  1  is still in the stick phase but piezo element  2  is now in the slip phase. The inertia of sliding friction element  5  and the forward motion of piezo element  1  counteracts and overcomes most of the reverse motion imparted by piezo element  2 . Hence during Cycle Phase  2 , there is only a very slight dip  15  to the resultant motion curve ( FIG. 4 ). 
     Cycle Phase  3 : Voltage sources  12  and  13  are applying voltage to elements  1  and  2  so that elements  1  and  2  are both expanding in the same direction ( FIG. 3C ). The applied voltage increases at a low enough rate so that the speed of the expansion is slow enough so that the friction force between friction elements  3  and  4  and sliding friction element  5  is not overcome. Therefore, there is no slipping between friction elements  3  and  4  and sliding friction element  5 . Hence, both piezo element  1  and piezo element  2  are in the stick phase of motion causing sliding friction element  5  to move in a linear motion consistent with the linear motion of piezo elements  1  and  2 . 
     Cycle Phase  4 : Voltage source  13  is continuing to apply voltage with a slope slow enough to piezo element  2  so that it continues to expand in the same direction. However, the voltage from voltage source  12  drops to zero at a rapid rate causing piezo element  1  to contract at a rapid rate ( FIG. 3D ). Piezo element  1  contracts at such a rapid rate that the friction force between friction element  3  and sliding friction element  5  is significantly overcome, but not entirely. Hence, during cycle phase  2  piezo element  2  is still in the stick phase but piezo element  1  is now in the slip phase. The inertia of sliding friction element  5  forward motion of piezo element  2  counteracts and overcomes most of the reverse motion imparted by piezo element  1 . Hence during Cycle Phase  4 , there is only a very slight dip  16  to the resultant motion curve ( FIG. 4 ). 
     Cycle Phase  5 : The motion in cycle phase  5  is similar to that described above in reference to cycle phase  1 . Accordingly the cycles continue to repeat until the command signals are altered. 
     Command Signal Linearized 
       FIG. 5  shows a graphical representation similar to that depicted in  FIG. 3 . However, in  FIG. 5  the command signals from computer  14  ( FIG. 2 ) have been linearized. Linearization of the command signals is preferred because the resultant motion is more linear with less slippage. 
     OTHER PREFERRED EMBODIMENTS 
     Multi-Element Stick-Slip Piezo Motor, Opposite Phase Version, Linear Motion 
       FIG. 6  shows another preferred embodiment of the present invention. 
     Piezo elements  21  and  22  are both rigidly connected to holding element  26 . Friction elements  23  and  24  are both connected to piezo elements  21  and  22 , respectively. Friction element  25  is pressed against friction elements  23  and  24 . Sliding friction element  25  is the object being moved by piezo motor  20 . Voltage source  28  is connected to piezo element  21 . Voltage source  29  is connected to piezo element  22 . Computer  27  is connected to voltage sources  28  and  29  and is programmed to control the output of voltage sources  28  and  29 . 
     Cycle phase  1 : Voltage sources  28  and  29  are applying voltage out of phase with respect to elements  21  and  22  so that element  21  is contracting to the right and element  22  is expanding to the right ( FIG. 8A ). The rate of change of the applied voltage is low enough so that the speed of the piezo elements  21  and  22  is slow enough so that the friction force between friction elements  23  and  24  and sliding friction element  25  is not overcome. Therefore, there is no slipping between friction elements  23  and  24  and sliding friction element  25 . Hence, both piezo element  21  and piezo element  22  are in the stick phase of motion causing sliding friction element  25  to move in a linear motion consistent with the linear motion of piezo elements  21  and  22 . 
     Cycle Phase  2 : Voltage source  29  is continuing to apply voltage with a slope slow enough to piezo element  22  so that it continues to expand to the right. However, the voltage from voltage source  28  has reversed at a rapid rate causing piezo element  21  to expand to the left at a rapid rate ( FIG. 8B ). Piezo element  21  expands at such a rapid rate that the inertia of sliding friction element  25  overcomes the friction force between friction element  23  and sliding friction element  25 . Hence, during cycle phase  2  piezo element  22  is still in the stick phase but piezo element  21  is now in the slip phase. The forward motion of piezo element  22  counteracts and overcomes most of the reverse motion imparted by piezo element  21 . Hence during Cycle Phase  2 , there is only a very slight dip to the resultant linear motion curve. 
     Cycle Phase  3 : Voltage sources  28  and  29  are applying voltage out of phase with respect to elements  21  and  22  so that element  21  is contracting to the right and element  22  is expanding to the right ( FIG. 8C ). The rate of change of the applied voltage is low enough so that the speed of the piezo elements  21  and  22  is slow enough so that the friction force between friction elements  23  and  24  and sliding friction element  25  is not overcome. Therefore, there is no slipping between friction elements  23  and  24  and sliding friction element  25 . Hence, both piezo element  21  and piezo element  22  are in the stick phase of motion causing sliding friction element  25  to move in a linear motion consistent with the linear motion of piezo elements  21  and  22 . 
     Cycle Phase  4 : Voltage source  28  is continuing to apply voltage with a slope slow enough to piezo element  21  so that it continues to contract to the right. However, the voltage from voltage source  29  has reversed at a rapid rate causing piezo element  22  to contract at a rapid rate ( FIG. 8D ). Piezo element  22  contracts at such a rapid rate that the inertia of sliding friction element  25  overcomes the friction force between friction element  24  and sliding friction element  25 . Hence, during cycle phase  4  piezo element  21  is still in the stick phase but piezo element  22  is now in the slip phase. The right moving motion of piezo element  21  counteracts and overcomes most of the reverse motion imparted by piezo element  22 . Hence during Cycle Phase  4 , there is only a very slight dip to the resultant motion curve. 
     Cycle Phase  5 : The motion in cycle phase  5  is similar to that described above in reference to cycle phase  1 . Accordingly the cycles continue to repeat until the command signals are altered. 
     Multi-Element Stick-Slip Piezo Motor, Opposite Phase Version, Rotational Motion 
       FIG. 7  shows another preferred embodiment of the present invention. 
     Piezo elements  31  and  32  are both rigidly connected to holding element  36 . Friction elements  33  and  34  are both connected to piezo elements  31  and  32 , respectively. Rotational friction element  35  is pressed against friction elements  33  and  34 . Rotational friction element  35  is the object being moved by piezo motor  30 . Voltage source  38  is connected to piezo element  31 . Voltage source  39  is connected to piezo element  32 . Computer  37  is connected to voltage sources  38  and  39  and is programmed to control the output of voltage sources  38  and  39 . 
     Cycle phase  1 : Voltage sources  38  and  39  are applying voltage out of phase with respect to elements  31  and  32  so that element  31  is contracting to the right and element  22  is expanding to the right ( FIG. 9A ). The rate of change of the applied voltage is low enough so that the speed of the piezo elements  31  and  32  is slow enough so that the friction force between friction elements  33  and  34  and rotational friction element  35  is not overcome. Therefore, there is no slipping between friction elements  23  and  24  and rotational friction element  35 . Hence, both piezo element  31  and piezo element  32  are in the stick phase of motion causing rotational friction element  35  to move in a rotational motion consistent with the motion of piezo elements  31  and  32 . 
     Cycle Phase  2 : Voltage source  39  is continuing to apply voltage with a slope slow enough to piezo element  32  so that it continues to expand to the right. However, the voltage from voltage source  38  has reversed at a rapid rate causing piezo element  31  to expand to the left at a rapid rate ( FIG. 9B ). Piezo element  31  expands at such a rapid rate that the inertia of rotational friction element  35  overcomes the friction force between friction element  33  and rotational friction element  35 . Hence, during cycle phase  2  piezo element  32  is still in the stick phase but piezo element  31  is now in the slip phase. The rightward motion of piezo element  32  counteracts and overcomes most of the reverse motion imparted by piezo element  31 . Hence during Cycle Phase  2 , there is only a very slight dip to the resultant linear motion curve. 
     Cycle Phase  3 : Voltage sources  38  and  39  are applying voltage out of phase with respect to elements  31  and  32  so that element  31  is contracting to the right and element  32  is expanding to the right ( FIG. 9C ). The rate of change of the applied voltage is low enough so that the speed of the piezo elements  31  and  32  is slow enough so that the friction force between friction elements  33  and  34  and rotational friction element  35  is not overcome. Therefore, there is no slipping between friction elements  33  and  34  and rotational friction element  35 . Hence, both piezo element  31  and piezo element  32  are in the stick phase of motion causing rotational friction element  35  to move in a linear motion consistent with the linear motion of piezo elements  31  and  32 . 
     Cycle Phase  4 : Voltage source  38  is continuing to apply voltage with a slope slow enough to piezo element  31  so that it continues to contract to the right. However, the voltage from voltage source  39  has reversed at a rapid rate causing piezo element  32  to contract at a rapid rate ( FIG. 9D ). Piezo element  32  contracts at such a rapid rate that the inertia of rotational friction element  35  overcomes the friction force between friction element  34  and rotational friction element  35 . Hence, during cycle phase  4  piezo element  31  is still in the stick phase but piezo element  32  is now in the slip phase. The right moving motion of piezo element  31  counteracts and overcomes most of the reverse motion imparted by piezo element  32 . Hence during Cycle Phase  4 , there is only a very slight dip to the resultant motion curve. 
     Cycle Phase  5 : The motion in cycle phase  5  is similar to that described above in reference to cycle phase  1 . Accordingly the cycles continue to repeat until the command signals are altered. 
     Linear Motion 
       FIG. 10  shows another preferred embodiment that provides for linear motion. Piezo motor  50  is also similar to the embodiment shown in  FIG. 6 . Piezo element  51  and piezo element  52  are housed in piezo housing  56 . Ceramic friction elements  53  and  54  are rigidly connected to piezo elements  51  and  52 , respectively. Voltage is applied to piezo elements  51  and  52  so that ceramic friction elements  53  and  54  operate to move friction plate  55  in the direction shown by the arrows in  FIG. 10 . Ceramic friction plate  55  is rigidly connected to plate  58 . A user of motor  40  may attach devices to plate  58 , as preferred. 
     Rotary Motion 
       FIG. 11  shows a preferred embodiment that provides for rotational motion. Piezo motor  60  is similar to the embodiment shown in  FIG. 7 . Piezo element  61  and piezo element  62  are housed in piezo housing  66 . Ceramic friction elements  63  and  64  are rigidly connected to piezo elements  61  and  62 , respectively. Voltage is applied to piezo elements  61  and  62  so that ceramic friction elements  63  and  64  operate to rotate disc  68  clockwise or counterclockwise, as preferred. Surrounding disc  68  is rigidly connected ceramic friction band  69 . A user of motor  60  may attach devices to  68 , as preferred. Pressure is applied to ceramic friction elements  63  and  64  by springs  67   a  and  67   b , respectively. The degree of pressure force applied is such that elements  63  and  64  move band  69  during the stick phase of the oscillation yet also with such force so that friction elements  63  and  64  do not significantly drag friction band  69  in the undesired direction during the slip phase of the oscillation. 
       FIG. 12  shows another preferred embodiment that provides for rotational motion. The piezo arrangement for motor  70  is similar to that shown for motor  60 . However, instead of rotating a circular disc, two piezo elements act in conjunction to turn rotor  71 . An operator may rigidly attach an axis to rotor  71  to spin the axis as desired. 
     Planar Motion 
       FIG. 13  shows a preferred embodiment of the present invention that provides for planar motion. Two piezo elements operate to move plate  80  in a linear fashion indicated by arrows  81  and  82 . The linear motion of plate  80  is similar to the linear motion described above in reference to  FIG. 6  and  FIGS. 10A-10B . Plate  80  slides on tracks  83  and  84 . Likewise, two other piezo elements operate to move plate  81  in a linear fashion indicated by arrow  85 . The linear motion of plate  80  is also similar to the linear motion described above in reference to  FIG. 6  and  FIGS. 10A-10B . Plate  81  slides on tracks  87  and  88 . 
     Multiple Piezo Elements 
     Although the above preferred embodiments disclosed stick-slip piezo motors that utilized two piezo elements, it is also possible to increase the number of piezo elements. For example,  FIG. 14  shows a piezo motor that utilizes three piezo elements and  FIG. 15  shows a piezo motor that utilizes four piezo elements. As the number of piezo elements increases, there are a greater number of piezo elements operating in the stick phase to counteract any reverse motion due to a piezo element operating in the slip phase. 
     For example, in  FIG. 14  voltage has been applied to piezo elements  161  and  162  so that they are expanding relatively slowly to the right. Also, the voltage applied to piezo element  163  is rapidly reduced to zero causing it to contract to the left at a rapid rate. Piezo element  163  contracts at such a rapid rate that the friction force between friction element  165  and sliding friction element  164  is significantly overcome. Hence, piezo elements  161  and  162  are in the stick phase but piezo element  163  is now in the slip phase. The rightward motion of piezo elements  161  and  162  counteracts and overcomes most of the reverse motion imparted by piezo element  163 . Hence, there is only a very slight dip to the resultant linear motion curve. 
     Also, in  FIG. 15  voltage has been applied to piezo elements  171 ,  172 , and  174  so that they are expanding relatively slowly to the right. Also, the voltage applied to piezo element  173  is rapidly reduced to zero causing it to contract to the left at a rapid rate. Piezo element  173  contracts at such a rapid rate that the friction force between friction element  175  and sliding friction element  176  is significantly overcome. Hence, piezo elements  171 ,  172 , and  174  are in the stick phase but piezo element  173  is now in the slip phase. The rightward motion of piezo elements  171 ,  172 , and  174  counteracts and overcomes most of the reverse motion imparted by piezo element  173 . Hence, because there is a greater number of piezo elements moving rightward than shown in  FIG. 14  there is an overall decrease to the effect of leftward motion imparted by piezo element  173 . 
     Although the above-preferred embodiments have been described with specificity, persons skilled in this art will recognize that many changes to the specific embodiments disclosed above could be made without departing from the spirit of the invention. For example, it should be understood that the sliding friction elements and rotational friction elements described above are just some of the examples of moving friction elements. Other types of moving friction elements are also possible. Furthermore, it should be noted that the length of the sliding friction element can be varied as desired. There is no limit to the length of this element. For rotational motion, the radius of the rotational friction element may also be of any dimension required. Also, it should be noted that although the above descriptions referred to resultant motion in one direction (i.e., from left to right) it should be recognized that the opposite resultant motion (i.e., from right to left) can be easily achieved by merely reversing the applied voltage in order to reverse the motion of the piezo elements. Therefore, the attached claims and their legal equivalents should determine the scope of the invention.