Abstract:
A system for measuring differential gravity at two points is disclosed. In the illustrative embodiment, the system uses a pair of graspers which each repeatedly grasp, raise, and drop a test mass. The accelerations of the two free-falling test masses are monitored using optical interferometry. An output signal is provided that is based on a differential acceleration of the two test masses.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The underlying concepts, but not necessarily the language, of the following case is incorporated by reference: U.S. Pat. No. 5,892,151, issued 6 Apr. 1999. If there are any contradictions or inconsistencies in language between this application and one or more of the cases that have been incorporated by reference that might affect the interpretation of the claims in this case, the claims in this case should be interpreted to be consistent with the language in this case. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to gravimeters in general, and, more particularly, to differential gravimeters. 
       BACKGROUND OF THE INVENTION 
       [0003]    A differential gravimeter measures a gravity variation between two locations. In addition, multiple differential gravimeters can be used to develop a three-dimensional map of gravity. Such 3-D mapping has been proposed in order to monitor fluid flow in-situ in subterranean reservoirs, such as an oil field. In order to be used in an in-situ underground application, however, the gravity sensors of a gravimeter must be directly insertable into boreholes of with the oil field. As a result, the gravity sensors must be small and robust. 
         [0004]    Since the differences in gravity across an oil field are typically very slight, the gravimeter must be extremely sensitive; sensitivity below 1 micro-Galileo is often necessary. Such extreme sensitivity, however, requires very high immunity to noise sources. Error can be introduced into the output signal of a gravimeter from noise sources such as electromagnetic interference, horizontal components in the acceleration of a free-falling mass, mechanical misalignment of sub-components, mechanical shock, and Coriolis forces that arise due to the rotation of the Earth. 
         [0005]    Gravimeters have been developed that are based on the principle of balancing the weight of a fixed mass with forces from a normal or superconducting spring. Gravimeters such as these, however, have gravity sensors that are typically too large to be inserted into a borehole of an oil well. They are also difficult to setup and calibrate. In addition, they are sensitive to environmental influences such as temperature and vibration. 
         [0006]    More recently small gravimeters have been developed that include gravity sensors specifically designed for direct insertion into a borehole. These small gravimeters utilize piezoelectric launchers to vertically launch a pair of test masses upward so that they can subsequently free-fall downward. An interferometer arrangement is used to monitor the acceleration of their falling masses after each reaches its apex. In addition to some of the drawbacks of other prior-art gravimeters, however, noise due to shock and vibration caused by their piezoelectric launchers limits the sensitivity of these gravimeters. 
         [0007]    There exists a need, therefore, for a gravimeter that avoids or mitigates some or all of the problems associated with prior-art gravimeters. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention provides a differential gravity measurement system. Some embodiments of the present invention are particularly well-suited for monitoring oil flow in subterranean oil fields. In particular, the illustrative embodiment of the present invention uses a pair of optically-interrogated, free-falling test masses in a Michelson interferometer arrangement to provide a highly sensitive measurement of the difference in gravity at two locations. 
         [0009]    In the illustrative embodiment, the gravimeter comprises an interferometer and two gravity sensors that are optically interrogated as part of an interferometer arrangement. Each gravity sensor comprises a mass dropper, which includes a grasper and a test mass, and a gimbal for orientating of the mass dropper to vertical. Each grasper is mechanically-coupled to an actuator that moves it from a first position, wherein the grasper passively grasps the test mass, to a second position, wherein the grasper passively releases the test mass thereby allowing it to free-fall. A processor synchronizes the release of the test masses by their respective graspers. An optical beam is reflected off of each of the two falling test masses as they fall. These two optical paths compose the reference and test legs of the interferometer arrangement. The output signal of the interferometer is based on a difference in the path lengths of these optical beams; therefore, the output signal of the interferometer is a function of the difference in the local gravity that acts on each test mass. 
         [0010]    Some embodiments of the invention comprise a tilt sensor that provides a feedback signal used to minimize the tilt of the mass dropper. Some embodiments of the invention comprise a rotation sensor to provide a signal based on a rotation of the test mass as it falls. This signal is used to provide a post-drop correction for mitigating the effects on the gravity sensor due to component misalignment, residual tilt of the mass dropper, effects from the rotation of the earth, and the like. 
         [0011]    An embodiment of the present invention comprises: (1) a mass dropper comprising; (i) a first test mass; (ii) a grasper, wherein the grasper grasps the first test mass when in a first position, and wherein the grasper releases the first test mass when in a second position; and (iii) an actuator for moving the grasper on a path that includes the first position and the second position; and a sensor for providing a first signal based on an orientation of the mass dropper with respect to vertical; and (2) a gimbal for controlling the orientation of the mass dropper. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  depicts a schematic diagram of details of an oil field fluid flow measurement system in accordance with an illustrative embodiment of the present invention. 
           [0013]      FIG. 2  depicts a schematic diagram of details of a gravimeter in accordance with the illustrative embodiment of the present invention. 
           [0014]      FIG. 3  depicts a schematic diagram of details of a gravity sensor in accordance with the illustrative embodiment of the present invention. 
           [0015]      FIG. 4  depicts a cross-sectional diagram of details of a mass dropper, prior to grasping a test mass, in accordance with the illustrative embodiment of the present invention. 
           [0016]      FIG. 5  depicts a cross-sectional view of a mass dropper, after grasping a test mass, in accordance with the illustrative embodiment of the present invention. 
           [0017]      FIG. 6  depicts a cross-sectional view of a mass dropper, after release of a test mass, in accordance with the illustrative embodiment of the present invention. 
           [0018]      FIG. 7  depicts a cross-sectional view of an actuator in accordance with the illustrative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Full-field differential gravity monitoring for modeling the fluid distribution in an oil field can be achieved by means of mapping the relative gravity across the area of an oil field. An effective method for mapping the relative gravity is the application of double differences to detect gravity changes. 
         [0020]    In this method the differential gravity is measured at a plurality of points, referenced to a base location. These differential gravity measurements are used to develop a full-field gravity image versus time, beginning with an initial image at time, t=0. Changes in oil distribution in the oil field can be determined by comparing subsequent images either by referencing each to the initial image, or by comparing sequential images. In order to minimize random errors, each image may include an average of tens or hundreds of individual measurements between set of two points. Each set of points, comprising the differential gravity measurement between two points, can be obtained by means of a differential gravimeter in accordance with the present invention. 
         [0021]      FIG. 1  depicts a schematic diagram of details of an oil field fluid flow measurement system in accordance with an illustrative embodiment of the present invention. Measurement system  100  comprises differential gravimeter  102 , and oil wells  104 - 1  and  104 - 2 . 
         [0022]    Gravimeter  102  is a differential gravity measurement system for monitoring fluid movement in the oil field in order to maximize production yield. Gravimeter  102  comprises gravity sensors  106 - 1  and  106 - 2 , cables  108 - 1  and  108 - 2 , and controller  110 . In order to monitor fluid flow in the oil field, the multiple gravity sensors are inserted directly into the boreholes of oil wells that are distributed around the oil field. Each pair of sensors provides a differential gravity reading between their locations. In total, the sensors provide a measure of the gravity gradient in the oil field, and thus provide an indication of its oil distribution. Gravimeter  102  is described in more detail below and with respect to  FIG. 2 . 
         [0023]    Each of gravity sensors  106 - 1  and  106 - 2  provide an optical signal that is reflected from a free-falling test mass contained within it. The optical signals are carried to controller  110  via optical fibers contained in cables  108 - 1  and  108 - 2 . A change in the relative phase of these two optical signals denotes a difference in the accelerations of the free-falling test masses. This phase information, therefore, denotes a difference in the local gravity experienced by each gravity sensor. Gravity sensors  106 - 1  and  106 - 2  are described in detail below and with respect to  FIGS. 3-7 . 
         [0024]    Controller  110  is a processor/controller for: (1) supplying optical signals to gravity sensors  106 - 1  and  106 - 2 ; (2) detecting optical signals reflected from gravity sensors  106 - 1  and  106 - 2 ; and (3) generating an output based on the phase difference between the optical signals reflected from gravity sensors  106 - 1  and  106 - 2 . Controller  110  is described in more detail below and with respect to  FIG. 2 . 
         [0025]      FIG. 2  depicts a schematic diagram of details of a gravimeter in accordance with the illustrative embodiment of the present invention. Gravimeter  102  comprises gravity sensors  106 - 1  and  106 - 2 , and controller  110 . Controller  110  comprises a general purpose signal processor as well as optical components that, together with gravity sensors  106 - 1  and  106 - 2 , form an interferometer for providing differential phase information used by controller  110  to generate its output. Specifically, controller  110  comprises source  202 , beamsplitter  204 , detector  210 , and processor  214 . 
         [0026]    Source  202  is a source of an optical signal comprising substantially monochromatic light. This light is launched into optical fiber  206 , which conveys the light to beamsplitter  204 . It will be clear to those skilled in the art how to make and use source  202 . 
         [0027]    Beamsplitter  204  is a conventional beamsplitter that is positioned as the central component in a Michelson interferometer configuration. Beamsplitter  204  splits optical energy received from optical fiber  206  into two optical signals (which are in-phase), and launches these optical signals into optical fibers  208 - 1  and  208 - 2 . Beamsplitter  204  also receives optical signals reflected from gravity sensors  106 - 1  and  106 - 2 . The reflected signals are combined and launched onto optical fiber  212 . It will be clear to those skilled in the art how to make and use beamsplitter  204 . 
         [0028]    Detector  210  is a conventional photodetector which generates an electrical signal based on the intensity of light received from optical fiber  212 . Since the signal received by detector  210  is the combined reflected optical signals from gravity sensors  106 - 1  and  106 - 2 , its electrical output exhibits any effects of the interference of these reflected optical signals. This interference is an indication of any difference in the accelerations of free-falling masses in gravity sensors  106 - 1  and  106 - 2 , as will be explained below and with respect to  FIGS. 4-6 . A difference in these accelerations is a function of a difference in gravity between the locations of gravity sensors  106 - 1  and  106 - 2 . It will be clear to those skilled in the art how to make and use detector  210 . 
         [0029]    Processor  214  is a general purpose processor that: (1) generates an output signal based on an electrical signal received from detector  210 ; (2) provides orientation control signals to gravity sensors  106 - 1  and  106 - 2 ; and (3) synchronizes the release of the test masses in gravity sensors  106 - 1  and  106 - 2 . Although the illustrative embodiment comprises a processor that interfaces with only one pair of gravity sensors, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein processor  214  interfaces with multiple pairs of gravity sensors. 
         [0030]    Electrical cables  216 - 1  and  216 - 2  are conventional control cables for conveying electrical signals between processor  214  and gravity sensors  106 - 1  and  106 - 2 , respectively. Electrical cable  216 - 1  is bundled with optical fiber  208 - 1  in cable  108 - 1 . In similar fashion, electrical cable  216 - 2  is bundled with optical fiber  208 - 2  in cable  108 - 2 . 
         [0031]      FIG. 3  depicts a schematic diagram of details of a gravity sensor in accordance with the illustrative embodiment of the present invention. Gravity sensor  106  is representative of either of gravity sensors  106 - 1  and  106 - 2 . Gravity sensor  106  comprises housing  302 , mass dropper  304 , gimbal frame  306 , tilt sensor  308 , gimbal actuators  310 , ballast  312 , and ferrule  314 . 
         [0032]    Housing  302  is a rigid housing for protecting gravity sensor  106  while it is submerged in the harsh environment of an oil well. It will be clear to those skilled in the art, after reading this specification, how to make and use housing  302 . 
         [0033]    Mass dropper  304  is a system for dropping a test mass so that its free-fall acceleration can be determined. Optical signal  316 , which is transmitted to and reflected from the test mass, is coupled to optical fiber  208  via ferrule  314 . Mass dropper  304  will be described below and with respect to  FIGS. 4-6 . 
         [0034]    Gimbal frame  306  is a frame of rigid material that is configured to enable mass dropper  304  to rotate about two orthogonal axes relative to housing  302 . Gimbal frame  306  is connected to mass dropper  304  via a first set of gimbal actuators  310 , such that mass dropper  304  can rotate about the Y-axis (as shown in  FIG. 3 ) with respect to gimbal frame  306 . Gimbal frame  306  is connected to housing  302  via a second set of gimbal actuators  310  which are oriented orthogonally with respect to the first set of gimbal actuators  310 . The second set of gimbal actuators enable the rotation of gimbal frame  306  about the X-axis (as shown in  FIG. 3 ) with respect to housing  302 . As a result, the first and second set of gimbal actuators  310  enable the rotation of mass dropper  304  about two orthogonal axes with respect to housing  302 . 
         [0035]    Tilt sensor  308  is a conventional electrolytic tilt sensor that provides an electrical signal based on the tilt of mass dropper with respect to vertical. It will be clear to those skilled in the art, after reading this specification, how to make and use tilt sensor  308 . Although the illustrative embodiment comprises an electrolytic tilt sensor, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein tilt sensor  308  comprises other types of tilt sensors having sufficient accuracy. Tilt sensors suitable for use in tilt sensor  308  include, without limitation, accelerometers, MEMS accelerometers, mercury-based tilt switches, rotary encoders, and inertial sensors. 
         [0036]    Gimbal actuators  310  are piezoelectric ultrasonic ring motors, which are capable of high-precision rotation. Although in the illustrative embodiment gimbal actuators  310  comprise ultrasonic ring motors, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein gimbal actuators  310  comprise any type of ring motor having sufficient accuracy. Gimbal actuators  310  also comprise slip-ring electrical contacts for providing electrical connectivity between components in mass dropper  304  and electrical cable  216 . 
         [0037]    Deviation of the orientation of mass dropper  304  from vertical results in errors caused by horizontal components in the gravity sensor output signal. It is desirable, therefore, that the orientation of mass dropper  304  be as close to vertical as possible. For the purposes of this specification, including the appended claims, “vertical” means that orientation that causes a test mass in a mass dropper to fall with no horizontal displacement component (i.e., wherein the mass dropper is “plumb”), and “tilt” means a deviation from vertical. Tilt sensor  308 , processor  214 , and gimbal actuators  310  constitute a feedback system for minimizing the tilt of mass dropper  304 . Processor  214  provides control signals to gimbal actuators  310  to minimize the output of tilt sensor  308 . 
         [0038]    Ballast  312  is a solid mass of highly-dense material that is located on the underside of house  302 . Ballast  312  alters the weight distribution for housing  302  and causes gravity sensor  106  to orient itself in a nearly vertical orientation. The presence of ballast  312 , therefore, reduces the amount of travel required of the gimbal actuators  310  to minimize the tilt the gravity sensor. As a result, optical port  318  maintains a rough alignment with ferrule  314  to allow the passage of free-space optical beam  316  through gimbal frame  304 . Since ballast  312  serves to keep optical port  318  roughly aligned with ferrule  314 , optical fiber  208  needs only a small amount of slack to accommodate the relative motion of mass dropper  306 , gimbal frame  304 , and housing  302  required to put mass dropper  306  in vertical orientation. 
         [0039]    Although in the illustrative embodiment, housing  302  comprises ballast that results in rough alignment of optical port  316  and ferrule  314 , some alternative embodiments do not comprise ballast  312 . In some alternative embodiments, optical fiber  108  includes a loose coil of optical fiber to accommodate large rotations of mass dropper  306  with respect to housing  302 . In some alternative embodiments, free space optical signal  316  is routed from ferrule  314  through gimbal frame  304  and dropper frame  402  via a plurality of mirrors located on the inner surface of gimbal frame  304 . It will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein housing  302  does not comprise ballast. 
         [0040]    Ferrule  314  is a conventional fiber optic ferrule for transmitting and receiving optical signal  316  to/from mass dropper  306 . Ferrule  314  includes a lens for efficiently coupling free-space optical signal  316  into and out of optical fiber  208 . Ferrule  314  also includes a facet having an integrated turning element, such as a mirror, wedge, prism, and the like. In some alternative embodiments, ferrule  314  is not integrated with the turning element. In some alternative embodiments, ferrule  314  is oriented to launch optical signal  316  directly at mass dropper  306  without the need for a turning element. 
         [0041]      FIGS. 4-6  depict a cross-sectional diagram of details of a mass dropper: (1) prior to grasping a test mass; (2) after grasping a test mass; and (3) after releasing a test mass, in accordance with the illustrative embodiment of the present invention. Mass dropper  306  comprises dropper frame  402 , grasper  404 , test mass  406 , actuator  412 , wedge  416 , and seat  418 . 
         [0042]    Dropper frame  402  is a frame of rigid material that is configured to enable mass dropper  304  to rotate about an axis with respect to gimbal frame  304 . Dropper frame  402  is connected to gimbal frame  304  via gimbal actuators  310 . Dropper frame  402  also provides a stable platform for positioning grasper  404  and seat  422  such that grasper  404  drops test mass  406  directly into seat  418  when mass dropper  306  is vertically oriented. Dropper frame  402  comprises optical port  422 , which enables optical signal  318  to interrogate test mass  406  throughout its entire range of travel. 
         [0043]    Grasper  404  is a rigid platform having a plurality of tangs  408  projecting from one face. Together, tangs  408  compose a pincer for passively engaging catch  410  of to grasp test mass  406 . Tangs  408  are made of a resilient material, and thus generate a restoring force when forced apart by catch  410 . When grasper  404  is moved into engagement with test mass  406  by actuator  412 , the top of catch  410  forces tangs  408  to separate. As grasper  404  is moved into further engagement with test mass  406 , tangs  408  spread over catch  410 . The restoring force generated within tangs  408  causes grasper  404  to grasp test mass  406 . Tangs  408  exert a substantially uniformly-distributed force on the outer surface of catch  410 . Uniform distribution of the grasping force on catch  410  results in a smooth release of test mass  406  when grasper  404  releases it. As a result, grasper  404  does not induce substantial rotation of test mass  406  as it drops. Although in the illustrative embodiment grasper  404  comprises four tangs, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein grasper  404  comprises any number of tangs. 
         [0044]    Test mass  406  is a circular mass having a shaped bottom surface for mating with seat  418 . Test mass  406  comprises catch  410  and retroreflector  422 . Its shaped bottom surface ensures that test mass  406  will locate in seat  418  in substantially the same orientation each and every time that test mass  406  is dropped. Retroreflector  422  reflects free-space optical signal  318  on a return path parallel to, and preferably coincident with, its path to test mass  406  from ferrule  316 . 
         [0045]    Actuator  412  is a linear actuator that is affixed to grasper  404 . Actuator  412  moves grasper  404  along shaft  414  from a first position, in which grasper  404  grasps catch  410 , to a second position, in which grasper  404  releases catch  410 . Actuator  412  comprises a conventional MEMS-based inch-worm actuator, which is capable of high-precision motion along shaft  414 . Actuator  412  will be discussed in more detail below and with respect to  FIG. 7 . Although the illustrative embodiment comprises an inch-worm linear actuator, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein actuator  412  comprises other linear actuators. Suitable actuators for use in actuator  412  include, without limitation, solenoids, electromagnetic linear motors, lead screw systems, and the like. 
         [0046]    Shaft  414  is a steel shaft suitable for use with actuator  412 . Shaft  414  includes two flats (not shown) on opposite sides. These flats provide a larger contact surface for actuator  412 . In some alternative embodiments, shaft  414  does not include flats. 
         [0047]    Wedge  416  is a conical projection attached to the free end of shaft  414 . The shape of wedge  416  is suitable for smoothly engaging tangs  408  thereby causing their separation. The diameter of wedge  416  is slightly larger than the diameter of catch  410  to ensure that tangs  408  release catch  410  when they are sufficiently engaged with wedge  416 . 
         [0048]    Seat  418  is a recess in dropper frame  402 . Seat  418  is shaped to accept test mass  406  such that test mass  406  locates in substantially the same position and orientation after each drop by grasper  404 . Seat  418  further comprises optical port  422 , which provides access to retroreflector  420  for optical beam  316 . 
         [0049]    Errors in the output of a gravity sensor can arise from sources such as tilt of the mass dropper, component misalignments, and Coriolis forces caused by the Earth&#39;s rotation. In order to mitigate some of the effects of at least some error sources, a post-drop correction can be applied based on the measured rotation of test mass  406  as it free-falls. Mass dropper  304 , therefore, includes an optional rotation sensor, which communicates with processor  214  via electrical cable  216 . 
         [0050]    Rotation sensor  424  comprises laser diodes  426 - 1  and  426 - 2  and position-sensitive detectors (PSDs)  430 - 1  and  430 - 2 . Laser diode  426 - 1  reflects light beam  428 - 1  off of test mass  406  to PSD  430 - 1 . In similar fashion, laser diode  426 - 2  reflects light beam  428 - 2  off of test mass  406  to PSD  430 - 2 . If test mass  406  has not rotated with respect to mass dropper  304  (such as when located in seat  418 ), PSDs  430 - 1  and  430 - 2  receive light beams  428 - 1  and  428 - 2  at the same elevation. The output voltage of each PSD, therefore, will be the same. If, during its free-fall, test mass  406  has rotated with respect to mass dropper  304 , PSDs  430 - 1  and  430 - 2  will receive light beams  428 - 1  and  428 - 2  at different elevations. As a result, the outputs of PSDs  430 - 1  and  430 - 2  will differ as a function of the degree of rotation of test mass  406 . Although in the illustrative embodiment rotation sensor  424  comprises position-sensitive detectors, it will be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein rotation sensor  424  comprises any detector whose output is a function of received beam location, such as charge-coupled-device (CCD) strip detectors, photodetector arrays, and the like. It will also be clear to those skilled in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein rotation sensor  424  comprises sources of optical energy other than laser diodes. 
         [0051]      FIG. 5  depicts a cross-sectional view of a mass dropper, after grasping a test mass, in accordance with the illustrative embodiment of the present invention. Once grasper  404  has grasped test mass  406 , actuator  412  moves grasper  404  upward toward wedge  416 . Tangs  408  are designed to exert evenly-distributed pressure on catch  410  so that no rotation of test mass  406  occurs as it is lifted by grasper  404 . 
         [0052]      FIG. 6  depicts a cross-sectional view of a mass dropper, after release of a test mass, in accordance with the illustrative embodiment of the present invention. Grasper  404  has engaged wedge  416 , which forces tangs  408  to separate. The separation of tangs  408  causes grasper  404  to smoothly release test mass  404 , which allows test mass  404  to begin its free-fall without induced horizontal velocity components or rotation. Free-space optical beam  316  interrogates test mass  406  during its free-fall via retroreflector  420 . Reflected optical signal  316  is coupled into optical fiber  208  via ferrule  314 . As test mass  406  falls, rotation sensor  424  monitors its rotation via light beams  428 - 1  and  428 - 2 . 
         [0053]      FIG. 7  depicts a cross-sectional view of an actuator in accordance with the illustrative embodiment of the present invention. Actuator  412  comprises body  502 , couplings  504 - 1  and  504 - 2  (referred to, collectively, as couplings  504 ), elongation elements  506 - 1  and  506 - 2  (referred to, collectively, as elongation elements  506 ), upper contact pads  508 - 1  and  508 - 2  (referred to, collectively, as upper pads  508 ), and lower contact pads  510 - 1  and  510 - 1  (referred to, collectively, as lower pads  510 ). 
         [0054]    Body  502  is a ring of rigid material and couplings  504  are bars of rigid material that couple grasper  404  to the elongation elements  506 . Suitable materials for body  502  and couplings  504  include, without limitation, metals, ceramics, plastics, and carbon-based materials. 
         [0055]    Elongation elements  506  are bars of piezoelectric material that elongate along their longitudinal axis energized with sufficient voltage. 
         [0056]    Upper pads  508  and lower pads  510  are pads of piezoelectric material whose contact surface has a coefficient of friction with outer surface of shaft  414  suitable for clamping shaft  414  without slipping. When energized with sufficient voltage, upper pads  508  clamp shaft  414 . In similar fashion, when energized with sufficient voltage, lower pads  510  clamp shaft  414 . 
         [0057]    Actuator  412  is capable of high-precision motion along shaft  414 . Actuator  412  moves upward along shaft  414  by means of a voltage cycle comprising the steps: (1) energizing lower pads  510 ; (2) de-energizing upper pads  508 ; (3) energizing elongation elements  506  to increase the separation between lower pads  510  and upper pads  508 ; (4) energizing upper pads  508 ; (5) de-energizing lower pads  510 ; and (6) de-energizing elongation element  506  to reduce the separation between lower pads  510  and upper pads  508 . Motion downward along shaft  414  is accomplished in similar fashion with the appropriate change in the sequence of steps above. 
         [0058]    It is to be understood that the above-described embodiments are merely illustrative of the present invention and that many variations of the above-described embodiments can be devised by those skilled in the art without departing from the scope of the invention. For example, in this Specification, numerous specific details are provided in order to provide a thorough description and understanding of the illustrative embodiments of the present invention. Those skilled in the art will recognize, however, that the invention can be practiced without one or more of those details, or with other methods, materials, components, etc. 
         [0059]    Furthermore, in some instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the illustrative embodiments. It is understood that the various embodiments shown in the Figures are illustrative, and are not necessarily drawn to scale. Reference throughout the specification to “one embodiment” or “an embodiment” or “some embodiments” means that a particular feature, structure, material, or characteristic described in connection with the embodiment(s) is included in at least one embodiment of the present invention, but not necessarily all embodiments. Consequently, the appearances of the phrase “in one embodiment,” “in an embodiment,” or “in some embodiments” in various places throughout the Specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, materials, or characteristics can be combined in any suitable manner in one or more embodiments. It is therefore intended that such variations be included within the scope of the following claims and their equivalents.