Abstract:
A drive system and methods thereof include at least one actuator assembly that detachably engages a moveable system and at least one control system. The control system voltage biases the actuator assembly in at least one direction at a rate which causes the at least one actuator assembly to slip with respect to the detachably engaged moveable system. The control system also voltage biases the actuator assembly in at least one other direction at a rate which causes the at least one actuator assembly to stick and move with respect to the detachably engaged moveable system.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/895,635, filed Mar. 19, 2007, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to drive systems and methods thereof and, more particularly, to piezoelectric, linear drive systems and methods thereof. 
     BACKGROUND 
     There is a rapidly expanding demand in a variety of different markets, such as in electronic consumer products, electronics locks, and medical devices, for motors that can move tiny optics and other devices with sub-micrometer precision. By way of example, in the electronic consumer products market, the size of mobile phone handsets continues to shrink while the demand for the inclusion of features, such as digital cameras, in the mobile phone handsets continues to grow. To cost effectively add a digital camera feature into these mobile phones, the digital cameras need to be about one tenth the volume and cost of stand alone digital cameras. Unfortunately, existing solutions for trying to meet these radical size and cost reduction requirements in digital cameras have been less than successful. 
     Some prior mobile phone handsets with digital cameras have used fixed optics. Unfortunately, these fixed optics do not meet most consumer expectations for adjustable focusing capabilities in digital cameras in mobile phone handsets. 
     Other prior mobile phone handsets have used miniaturized electromagnetic motors with their digital cameras to provide adjustable focusing capabilities, e.g. the ability to move a lens up to about 0.5 millimeters. In particular, voice coil motors are popular for use as the miniaturized electromagnetic motors in these digital cameras in the mobile phone sets. Voice coil motors are simple, inexpensive, and operate at relatively low voltage. 
     However, voice coil motors have reached the limit of their potential miniaturization and can not satisfy the emerging market demand for ultra-thin phones with more than two mega pixels image sensors. Additionally, even though these voice coil motors operate at relatively low voltage, they still use too much power. Further, these voice coil motors must be supported by fragile flexure guides and produce excessive position noise for optimal image capture when using two mega pixels or higher digital image sensors. 
     SUMMARY 
     A drive system in accordance with embodiments of the present invention includes at least one actuator assembly that detachably engages a moveable system and at least one control system. The control system voltage biases the actuator assembly in at least one direction at a rate which causes the at least one actuator assembly to slip with respect to the detachably engaged moveable system. The control system also voltage biases the actuator assembly in at least one other direction at a rate which causes the at least one actuator assembly to stick and move with respect to the detachably engaged moveable system. 
     A method for making a drive system in accordance with other embodiments of the present invention includes providing at least one actuator assembly that detachably engages a moveable system. At least one control system is coupled to the actuator assembly and voltage biases the actuator assembly in at least one direction at a rate which causes the actuator assembly to slip with respect to the detachably engaged moveable system. The control system also voltage biases the actuator assembly in at least one other direction at a rate which causes the actuator assembly to stick and move with respect to the detachably engaged moveable system. 
     A method for driving a moveable system in accordance with other embodiments of the present invention includes detachably engaging a moveable system with an at least one actuator assembly. At least one control system voltage biases the actuator assembly in at least one direction at a rate which causes the at least one actuator assembly to slip with respect to the detachably engaged moveable system and in at least one other direction at a rate which causes the at least one actuator assembly to stick and move with respect to the detachably engaged moveable system. 
     The present invention provides a linear drive system which effectively meets the radical size and cost reductions which are now required in a variety of different markets. For example, even ultra-thin camera phones can now incorporate a linear drive system which meets these requirements to drive automatic focus or optical zoom. Additionally, the present invention is able to provide a linear drive system with not only reduced size, but also fewer parts which helps to reduce the overall cost of the linear drive system. Further, the present invention reduces operating power by eliminating the need for a separate position sensors, limit stops, or limit switches and in a digital camera applications helps to reduce power by holding the lens assembly using zero power. Even further, the present invention provides a linear drive system which is acoustically quieter than prior linear drive systems. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective view of a lens system with a lens assembly and a linear drive system in accordance with embodiments of the present invention; 
         FIG. 1B  is a perspective view of the lens system with the lens assembly and the linear drive system shown in  FIG. 1A  with a cover removed; 
         FIG. 2A  is a perspective view of the linear drive system with the lens assembly and a drive control system with a lens housing removed; 
         FIG. 2B  is a top view of the linear drive system with the lens assembly shown in  FIG. 2A  and without the drive control system for ease of illustration; 
         FIG. 2C  is a side view of the linear drive system with the lens assembly and the drive control system shown in  FIG. 2A ; 
         FIG. 2D  is another side view of the linear drive system with the lens assembly and the drive control system shown in  FIG. 2A ; 
         FIG. 3  is a perspective view of the linear drive system without the lens assembly; 
         FIG. 4A  is a side view of the of the linear drive system with the lens assembly in a stationary position; 
         FIG. 4B  is a side view of the of the linear drive system with the lens assembly in a fast movement/slip operation; 
         FIG. 4C  is a side view of the of the linear drive system with the lens assembly in a slow movement/stick operation; 
         FIG. 5  is a timing diagram of a first drive waveform along with a series of side views illustrating the corresponding movement of the drive mechanism and the lens assembly; and 
         FIG. 6  is a timing diagram of a second drive waveform along with a series of side views illustrating the corresponding movement of the drive mechanism and the lens assembly. 
     
    
    
     DETAILED DESCRIPTION 
     A lens system  10  with a lens assembly  12  and a linear drive system  14  in accordance with embodiments of the present invention are illustrated in  FIGS. 1A-3 . The present invention provides a number of advantages including providing a linear drive system which effectively meets the radical size and cost reductions which are now required in a variety of different markets. 
     Referring more to  FIGS. 1A-3 , the lens system  10  includes a module  16  which substantially surrounds the lens assembly  12  and the linear drive system  14 , although the lens system  10  could have other types of housings in other configurations. The module  16  includes a cover  18  with an opening  20  to the lens assembly  12 , although other numbers and types of covers and openings could be used. In this particular embodiment, the dimensions of module  16  are 8.5×8.5×4.3 millimeters, although the module  16  could have other dimensions and shapes. The module  16  is sized and shaped to provide space to receive the linear drive system  14  with the lens assembly  12  nested inside, although the module could be sized and shaped to accept other systems, device, and components. 
     The lens assembly  12  includes a lens  22  and a casing  24  secured around an outer, side edge of the lens  22 , although the lens assembly  12  could include other types and numbers of components including other types and numbers of lens in other arrangements. In this particular embodiment, the lens assembly  12  has a cylindrical shape with a diameter of 5.5 millimeters, although other lens assemblies which other shapes and dimensions could be used. 
     The linear drive system  14  provides a space which is substantially symmetric and conforms to an outer shape of the lens assembly  12 , although the linear drive system  14  could engage the lens assembly in other manners. The linear drive system  14  supports and moves the lens assembly  12  parallel to and in either direction along the optical axis A-A when engaged by the drive control system  36 , although other types and numbers of guide elements with other configurations could be used. The linear drive system  14  includes rails  26 ( 1 )- 26 ( 2 ), piezoelectric plates  28 ( 1 )- 28 ( 2 ), piezoelectric plates  30 ( 1 )- 30 ( 2 ), spring flexure supports  32 ( 1 )- 32 ( 2 ), and lens guide elements  34 ( 1 )- 34 ( 2 ), although the linear drive system  14  can include other types and numbers of components in other configurations. 
     The rails  26 ( 1 )- 26 ( 2 ) have a rectangular shape, are spaced apart, and are substantially parallel to each other, although other types and numbers of rails in other shapes and configurations could be used. In this particular embodiment, the rails  26 ( 1 )- 26 ( 2 ) are made of  303  stainless steel which is a strong, corrosion resistant and electrically conductive material, although other types of metals and polymers, such as polycarbonate by way of example only, could be used for the rails  26 ( 1 )- 26 ( 2 ). 
     Piezoelectric plate  28 ( 1 ) is secured to one surface of the rail  26 ( 1 ) and piezoelectric plate  28 ( 2 ) is secured to an opposing surface of the rail  26 ( 1 ), although other numbers and types of plates could be secured to the rail  26 ( 1 ) in other manners. Piezoelectric plate  30 ( 1 ) is secured to one surface of the rail  26 ( 2 ) and piezoelectric plate  30 ( 2 ) is secured to an opposing surface of the rail  26 ( 2 ), although other numbers and types of plates could be secured to the rail  26 ( 2 ) in other manners. The piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) are each bonded to the rails  26 ( 1 )- 26 ( 2 ), respectively, using high strength adhesive with all the poling directions aligned as shown by the arrows  29 ( 1 )- 29 ( 2 ) in  FIGS. 2A and 2C , although other manners for securing the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) and other poling alignments could be used. In this embodiment, the thickness of the securing adhesive is thin enough to insure direct electrical contact with negligible resistance between the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) and the conductive rails  26 ( 1 )- 26 ( 2 ), although other bonding methods that are electrically insulating and other electrical connection methods may also be used. 
     The piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) are made of a hard piezoelectric ceramic with low dielectric losses at ultrasonic frequencies above 20,000 Hz, although piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) made out of other materials and with other types of properties could be used. In this embodiment, the piezoelectric material is a commonly available “hard” composition with low dielectric losses and high depoling voltage. Thus, by way of example only, one may use a piezoelectric material sold as “PZT- 4 ” by the Morgan Matroc company of Bedsford, Ohio. This material typically has several important properties as described below, although the piezoelectric materials could have other properties. 
     This piezoelectric material has a dielectric loss factor of less than about 1 percent at a frequency greater than about 20,000 Hertz, although piezoelectric materials with other dielectric loss factors could be used, particularly lower dielectric loss factors are beneficial. For example, in another embodiment the piezoelectric material has a dielectric loss factor of less that about 0.5 percent at a frequency greater than about 20,000 Hertz and in yet another embodiment the piezoelectric material has a dielectric loss factor of about 0.4 percent at a frequency greater than about 20,000 Hertz. 
     Additionally, the piezoelectric material has a d33 piezoelectric charge coefficient of at least about 250 picoCoulomb/Newton&#39;s, although piezoelectric materials with other beneficial d33 piezoelectric charge coefficients could be used. For example, in other embodiments the piezoelectric material can have one of the following d33 piezoelectric charge coefficients of at least: 90 picoCoulomb/Newton&#39;s; 105 picoCoulomb/Newton&#39;s; 115 picoCoulomb/Newton&#39;s; 270 picoCoulomb/Newton&#39;s; and 285 picoCoulomb/Newton&#39; s. 
     Further, the piezoelectric material is a single crystal piezoelectric material with a d33 piezoelectric charge coefficient of at least about 2500 picoCoulomb/Newton&#39;s, and a d31 piezoelectric charge coefficient of at least about 900 picoCoulomb/Newton&#39;s, although piezoelectric materials with other crystal structures and charge coefficients can be used. 
     In this particular embodiment, each of the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) is about 0.2 millimeters thick, although the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) could have other thicknesses and other shapes and dimensions. Additionally, in this particular embodiment the exposed outer surface of each of the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) are coated with a conductive metal, although the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) could be coupled to other types and numbers of conductors in other manners and locations. Further, in this particular embodiment the ceramic material from which the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) are made in this example is poled to activate the piezoelectric properties, although piezoelectric could be activated in other manners. 
     The spring flexure supports  32 ( 1 )- 32 ( 2 ) are spaced apart form each other, are each secured at opposing ends to rails  26 ( 1 ) and  26 ( 2 ), and are sized to nest within the space for the lens assembly  12  the linear drive system  14  in module  16 , although other types and numbers of force biasing elements could be used. The spring flexure supports  32 ( 1 )- 32 ( 2 ) generate a preload bias force that squeezes the guide elements  34 ( 1 ) and  34 ( 2 ) on opposite sides of the lens assembly  12  and precisely constrain the lens assembly  12  to move in a straight line substantially parallel to the optical axis A-A, although other force biasing systems configured in other manners and with other preloads could be used. The spring flexure supports  32 ( 1 )- 32 ( 2 ) have a low stiffness in the preload direction and high stiffness parallel to the optical axis A-A. By way of example only, in this particular embodiment the spring flexure supports  32 ( 1 )- 32 ( 2 ) generate a preload of about twenty grams on each side of the lens assembly  12  and the coefficient of friction of each of the guide elements  34 ( 1 )- 34 ( 2 ) is 0.2 which produces about eight grams of holding friction, although other amounts of preload and other coefficients of friction could be used. In this particular embodiment, the static friction is at least ten times greater than the mass of the lens assembly  12  so the focus position is maintained precisely with zero power. 
     The guide elements or devices  34 ( 1 )- 34 ( 2 ) are used to support and guide the lens assembly  12  parallel to the optical axis A-A, although other types and numbers of guide elements in other configurations could be used. Lens guide element  34 ( 1 ) is secured to an inner surface of rail  26 ( 1 ) and lens guide element  34 ( 2 ) is secured to an inner surface of rail  26 ( 2 ), although the guide elements can be secured in other manners and locations. 
     In this particular embodiment, the thickness of the lens assembly  12  is approximately 3.0 millimeters and the thickness of the linear drive system  14  is approximately 1.2 millimeters, although the lens assembly  12  and linear drive system  14  could have other dimensions. The total stroke is limited by the engagement of the guide elements  34 ( 1 )- 34 ( 2 ). In this particular embodiment, the maximum stroke is 3.0 millimeters−1.2 millimeters=1.8 millimeters, although the maximum stroke could be designed to have other dimensions. As illustrated by this example, the present invention provides the lens system  10  with the lens assembly  12  and the linear drive system  14  with a stroke which is much greater than the typical auto focus requirement of 0.4 millimeters in a very compact package. 
     Drive control system  36  is coupled to piezoelectric plates  28 ( 1 )- 28 ( 2 ) and piezoelectric plates  30 ( 1 )- 30 ( 2 ), although other types and numbers of control system can be coupled to other types and numbers of elements in other manners to drive the linear drive system  14 . In this particular embodiment, connection Va from the drive control system  36  is coupled to the electrode on the exposed outer surface of piezoelectric plates  28 ( 1 ) and  30 ( 1 ) and connection Vb from the drive control system  36  is coupled to the electrode on the exposed outer surface of piezoelectric plates  28 ( 2 ) and  30 ( 2 ), although the drive control system  36  can be coupled to control the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) in other manners. In this particular embodiment, the drive control system  36  applies a positive electric field by supplying a voltage ranging from about zero volts to thirty-three volts to the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ), although other ranges of voltages and other manners for bending the rails  26 ( 1 )- 26 ( 2 ) can be used. When the drive control system  36  applies this positive voltage bias and corresponding positive electric field (Va-Vb&gt;0) the rails  26 ( 1 )- 26 ( 2 ) bend resulting in tens of nanometers of movement at the center of the rails  26 ( 1 )- 26 ( 2 ), although other manners for bending the rails  26 ( 1 )- 26 ( 2 ) can be used. 
     Examples of the operation of the lens system  10  with the lens assembly  12  and the linear drive system  14  in accordance with embodiments of the present invention will now be described with reference to  FIGS. 1A-6 . With the present invention, when a positive electric field (an electric field in the same direction as the poling voltage) is applied by drive control system  36  to the piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ), the piezoelectric plates  28 ( 1 )- 30 ( 1 ) shrink in length and piezoelectric plates  28 ( 2 )- 30 ( 2 ) grow in length due to the D31 piezoelectric effect. The piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ) on opposite sides of the linear drive system  14  have opposite polarity and strain which causes the rails  26 ( 1 )- 26 ( 2 ) to bend as shown in  FIGS. 4B ,  4 C,  5 , and  6 . When the rails  26 ( 1 )- 26 ( 2 ) bend the movement is symmetrical with the centers of the rails  26 ( 1 )- 26 ( 2 ) moving in one direction and ends of the rails  26 ( 1 )- 26 ( 2 ) moving in the opposite direction, although the rails  26 ( 1 )- 26 ( 2 ) could be bent in other manners. 
     More specifically, when the drive control system  36  does not supply a voltage to piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ), the lens assembly  12  is held in place by the static friction produced by the preload from the spring flexure supports  32 ( 1 )- 32 ( 2 ) acting on the rails  26 ( 1 )- 26 ( 2 ) with the guide elements  34 ( 1 )- 34 ( 2 ) as shown in  FIG. 4A . As discussed earlier, the static friction is at least ten times greater than the mass of the lens assembly  12  so with the present invention the focus position of the lens assembly  12  can be maintained precisely without expanding any power. This helps to reduce overall power consumption in the lens system  10 . 
     When the drive control system  36  applies a rapidly rising or falling voltage (Va-Vb) to piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ), the application of the positive electric field causes the rails  26 ( 1 )- 26 ( 2 ) to rapidly bend. The rate of acceleration at which the rails  26 ( 1 )- 26 ( 2 ) rapidly bend is sufficient to overcome static friction between the guide elements  34 ( 1 )- 34 ( 2 ) and lens assembly  12 . It is well understood to one of ordinary skill in the art that for most material surfaces the static friction is typically twice the dynamic friction. As a result, guide elements  34 ( 1 )- 34 ( 2 ) on rails  26 ( 1 )- 26 ( 2 ), respectively, slip relative to the lens assembly  12  as shown in  FIG. 4B  and also as shown at the designated “Slip” stages in  FIGS. 5 and 6 . 
     When the drive control system  36  applies a more slowly rising or falling voltage to piezoelectric plates  28 ( 1 )- 28 ( 2 ) and  30 ( 1 )- 30 ( 2 ), the application of the positive electric field causes the rails  26 ( 1 )- 26 ( 2 ) to bend at a slower rate. This slower rate of acceleration at which the rails  26 ( 1 )- 26 ( 2 ) bend is insufficient to overcome static friction between the guide elements  34 ( 1 )- 34 ( 2 ) on rails  26 ( 1 )- 26 ( 2 ), respectively, and lens assembly  12 . As a result, the guide elements  34 ( 1 )- 34 ( 2 ) stick relative to the lens assembly  12  resulting in movement of the lens assembly  12  as shown in  FIG. 4C  and also as shown at the designated “Stick” stages in  FIGS. 5 and 6 . Accordingly, by combining these “Slip” and “Stick” stages in one vibration cycle the net movement of the lens assembly  12  with linear drive system  14  is generated. 
     By way of example only, timing diagrams of alternating asymmetric voltage waveforms along with a series of side views illustrating the corresponding slip and stick movement of the linear drive system  14  with respect to the lens assembly  12  in opposing directions are illustrated in  FIGS. 5 and 6 . In this example, a saw tooth waveform is show where the slower voltage rise or drop is the “stick” portion of the cycle and the fast voltage rise or drop is the “slip” portion of the cycle, although other types of waveforms which provide the “stick” portion and the “slip” portion could be used. As illustrated in  FIGS. 5-6 , the direction of motion of lens assembly  12  is determined by changing the orientation of the saw tooth waveform with drive control system  36 . 
     The symmetric reaction forces and small size enable the linear drive system  14  to operate in the ultrasonic frequency range above 20,000 Hertz. In this particular embodiment, the range of ultrasonic operating frequencies is between about 20,000 Hz to about 70,000 Hz, although other operating frequencies can be used. To achieve an asymmetric vibration cycle with fast (slip) and slow (stick) portions, resonant vibrations which are fundamentally symmetric and do not generate net movement of lens assembly  12  should be avoided. 
     Accordingly, as illustrated and described herein present invention provides a linear drive system which effectively meets the radical size and cost reductions which are now required in a variety of different markets. Additionally, the present invention provides a linear drive system with not only reduced size, but also fewer parts which helps to reduce the overall cost of the linear drive system. Further, the present invention reduces operating power by eliminating the need for a separate position sensors, limit stops, or limit switches and in a digital camera applications helps to reduce power by holding the lens assembly using zero power. Even further, the present invention provides a linear drive system which is acoustically quieter than prior linear drive systems. 
     Having thus described the basic concept of the invention, it will be rather apparent to those skilled in the art that the foregoing detailed disclosure is intended to be presented by way of example only, and is not limiting. Various alterations, improvements, and modifications will occur and are intended to those skilled in the art, though not expressly stated herein. These alterations, improvements, and modifications are intended to be suggested hereby, and are within the spirit and scope of the invention. Additionally, the recited order of processing elements or sequences, or the use of numbers, letters, or other designations therefore, is not intended to limit the claimed processes to any order except as may be specified in the claims. Accordingly, the invention is limited only by the following claims and equivalents thereto.