Patent Publication Number: US-10323920-B2

Title: Coordinate measuring machine with carbon fiber air bearings

Description:
PRIORITY 
     This patent application is a continuation patent application of U.S. patent application Ser. No. 14/670,580, filed Mar. 27, 2015, entitled, “COORDINATE MEASURING MACHINE WITH CARBON AIR BEARINGS,” and naming Gurpreet Singh, John Langlais, Jessica Zheng, and Joseph Spanedda as inventors, which claims priority from provisional U.S. patent application No. 61/975,045, filed Apr. 4, 2014 entitled, “COORDINATE MEASURING MACHINE WITH CARBON AIR BEARINGS,” and naming Gurpreet Singh, John Langlais, Jessica Zheng, and Joseph Spanedda as inventors. The disclosures of the two above referenced patent applications are incorporated herein, in their entireties, by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention generally relates to coordinate measuring machines and, more particularly, the invention relates to air bearings used in coordinate measuring machines. 
     BACKGROUND OF THE INVENTION 
     Among other things, coordinate measuring machines (“CMMs,” also known as surface scanning measuring machines) measure geometry and surface profiles, or verify the topography of known surfaces. For example, a CMM may measure the topological profile of a propeller to ensure that its surface is appropriately sized and shaped for its specified task (e.g., moving a 24 foot boat at pre-specified speeds through salt water). 
     To that end, conventional CMMs typically have a base directly connected with and supporting a movable assembly having a probe that directly contacts and moves along a surface of an object being measured. The base also may support the object being measured. Commonly, the movable assembly forms an air bearing with a rail to permit movement along the rail—i.e., in a direction that is generally parallel with the longitudinal axis of the rail. If the air gap of the air bearing is not consistent, then the probe can move relative to the object. Undesirably, this movement can significantly skew the results of the measurement, particularly when measuring to the micron level. 
     SUMMARY OF VARIOUS EMBODIMENTS 
     In accordance with one embodiment of the invention, a coordinate measuring machine has a base for supporting an object, a movable assembly having a probe for measuring the object, and a fixed rail movably guiding the movable assembly along its length. The rail includes carbon (e.g., carbon fiber) and has a rail CTE. The coordinate measuring machine also has an air bearing member circumscribing the rail and fixedly coupled with the movable assembly. The air bearing member has a member CTE, which is about equal to the rail CTE. Various embodiments engineer the CTE of the bearing member to have the desired CTE. 
     The air bearing member may have a torroidally shaped member (e.g., a hollow cylinder) completely circumscribing the rail (e.g., the rail also may be a cylinder, among other shapes). Among other things, the torroidally shaped member may include a non-carbon material, such as metal or ceramic. Alternatively, the torroidally shaped member also may be formed from carbon (e.g., a carbon fiber composite). 
     The torroidally shaped member may have a groove for distributing air, and a plurality of through-holes fluidly connected with the groove for directing air toward the rail. 
     Some embodiments of the air bearing include a sleeve and at least one torroidally shaped member supported by the sleeve. The sleeve and at least one torroidally shaped members may be formed from different materials. Moreover, some implementations may form the air bearing by both sides of the rail, and the rail may be formed mostly from carbon fiber material. The air bearing preferably is a radial air bearing. 
     In accordance with another embodiment of the invention, a method provides a coordinate measuring machine having a movable assembly having a probe, a carbon rail movably guiding the movable assembly along its length, and an air bearing member circumscribing the rail and fixedly coupled with the movable assembly. The method also matches the CTE of the air bearing member with the CTE of the rail, and forces air through the air bearing member to form an air bearing with an air gap between the air bearing member and the rail. 
     The air gap preferably remains substantially constant across a plurality of temperatures when air is forced through the air bearing member and the air bearing is not moving along the rail. The method also may move the air bearing along the rail. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Those skilled in the art should more fully appreciate advantages of various embodiments of the invention from the following “Description of Illustrative Embodiments,” discussed with reference to the drawings summarized immediately below. 
         FIG. 1A  schematically shows a coordinate measuring machine that may be configured in accordance with illustrative embodiments of the invention. 
         FIG. 1B  schematically shows an interface panel that may be used with the coordinate measuring machine in accordance with illustrative embodiments of the invention. 
         FIG. 2  schematically shows details of the electromechancial features of the coordinate measuring machine in accordance with illustrative embodiments. 
     
    
    
     DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In illustrative embodiments, a coordinate measuring machine (a “CMM”) has a more reliable air bearing system to move in two, three, or more directions, such as toward or away from its platform, or generally parallel to its platform. To that end, the CMM has carbon rails configured to match the coefficient of thermal expansion of at least part of its air bearing. Among others, the air bearing may include one or both of a radial air bearing and a thrust air bearing. Details of illustrative embodiments are discussed below. 
       FIG. 1A  is a photograph of one type of coordinate measurement machine  100  that may be configured in accordance with illustrative embodiments. As known by those in the art, the CMM  100 , which is supported on a floor  101  in this picture, measures an object on its bed/table/base (referred to as “base  102 ”). Generally, the base  102  of the CMM  100  defines an X-Y plane  110  that typically is parallel to the plane of the floor  101 . 
     To measure an object on its base  102 , the CMM  100  has movable features  122  arranged to move a measuring device  103 , such as a mechanical, tactile probe (e.g., a touch trigger or a scanning probe in a standard CMM), a non-contact probe (e.g., using laser probes), or a camera (e.g., a machine-vision CMM), coupled with a movable arm  104 . Alternately, some embodiments move the base  102  with respect to a stationary measuring device  103 . Either way, the movable features  122  of the CMM  100  manipulate the relative positions of the measuring device  103  and the object (or calibration artifact) with respect to one another to obtain the desired measurement. Accordingly, the CMM  100  can measure the location of a variety of features of the object or artifact. 
     The CMM  100  has a motion and data control system  120  that controls and coordinates its movements and activities. Among other things, the control system  120  includes computer processor hardware  121  and the noted sensors/movable features  122 . The computer processor may include a microprocessor, programmable logic, firmware, advance control, acquisition algorithms, and analysis algorithms. The computer processor  121  may have on-board digital memory (e.g., RAM or ROM) for storing data and/or computer code, including instructions for implementing some or all of the control system operations and methods. Alternately, or in addition, the computer processor  121  may be operably coupled to other digital memory, such as RAM or ROM, or a programmable memory circuit for storing such computer code and/or control data. 
     Alternately, or in addition, some embodiments couple the CMM  100  with an external computer (or “host computer”)  130 . In a manner similar to the control system  120 , the host computer  130  has a computer processor such as those described above, and computer memory in communication with the processor of the CMM  100 . The memory is configured to hold non-transient computer instructions capable of being executed by the processor, and/or to store non-transient data, such as data acquired as a result of the measurements of an object on the base  102 . 
     Among other things, the host computer  130  may be a desktop computer, a tower computer, or a laptop computer, such as those available from Dell Inc., or even a tablet computer, such as the iPad available from Apple Inc. The host computer  130  may be coupled to the CMM  100  via a hardwired connection, such as an Ethernet cable  131 , or via a wireless link, such as a Bluetooth link or a WiFi link. The host computer  130  may, for example, include software to control the CMM  100  during use or calibration, and/or may include software configured to process data acquired during a calibration process. In addition, the host computer  130  may include a user interface configured to allow a user to manually operate the CMM  100 . 
     Because their relative positions are determined by the action of the movable features  122 , the CMM  100  may be considered as having knowledge about data relating to the relative locations of the base  102 , and the object or artifact, with respect to its measuring device  103 . More particularly, the computers  121  or  130  control and store information about the motions of the movable features  122 . Alternately, or in addition, the movable features  122  of some embodiments include sensors that sense the locations of the table and/or measuring device  103 , and report that data to the computers  121  or  130 . The information about the motions and positions of the table and/or measuring device  103  of the CMM  100  may be recorded in terms of a two-dimensional (e.g., X-Y; X-Z; Y-Z) or three-dimensional (X-Y-Z) coordinate system referenced to a point on the CMM  100 . 
     Some CMMs also include a user interface  125  as shown in  FIG. 1A  and as further schematically illustrated in  FIG. 1B . As shown, the user interface  125  may have control buttons  125 A and knobs  125 B that allow a user to manually operate the CMM  100 . Among other things, the interface  125  may enable the user to change the position of the measuring device  103  or base  102  (e.g., with respect to one another) and to record data describing the position of the measuring device  103  or base  102 . 
     In addition, the interface  125  may enable the user to focus a camera (if the measuring device  103 /arm  104  includes a camera) on an object or target and record data describing the focus of the camera. In a moving table CMM, for example, the measuring device  103  may also be movable via control buttons  125 C. As such, the movable features  122  may respond to manual control, or under control of the computer processor  121 , to move the base  102  and/or the measuring device  103  (e.g., a mechanical probe in a mechanical CMM or a camera in a machine vision CMM  100 ) relative to one another. Accordingly, this arrangement permits the object being measured to be presented to the measuring device  103  from a variety of angles, and in a variety of positions. 
       FIG. 2  schematically shows some details of one implementation of the movable features  122  in accordance with illustrative embodiments of the invention. As noted above, the movable features  122  operate to move the arm  104  and its accompanying measurement device  103  in the X-direction (parallel to the base  102 ), the Y-direction (parallel to the base  102  but perpendicular to the X-direction), and in the Z-direction (toward and away from the base  102 ). To that end, the movable features  122  (or other related portion of the CMM  100 ) have at least three sets of rails/guides  200  that movably guide a movable assembly  202  (part of the movable features  122 ) in any of the X, Y, or Z directions. For simplicity,  FIG. 2  shows two of those sets of rails  200 —one for guiding in the Z-direction, and another for guiding in the X-direction. Those skilled in the art should understand that illustrative embodiments also can have a third set for guiding in the Y-direction. Alternative embodiments, however, may guide in different directions. 
     Although not shown in full, the rails  200  support and guide the movable assembly  202 , which correspondingly moves the arm  104  and its measuring device  103  relative to the object being measured. To improve mechanical and functional efficiencies, the movable assembly  202  preferably couples with the rails  200  by means of an air bearing system  204 . Specifically, as shown in  FIG. 2 , each rail  200  has an attendant air bearing member  206  riding on its outer surface (e.g., the bearing member  206  may circumscribe the rail outer surface). Accordingly, during normal use, the air bearing member  206  should not make contact with the rail  200 . 
     To accomplish its primary function, each air bearing member  206  is considered to form a sleeve  208 . To deliver the air and produce the so-called “floating” function of the air bearing, each end of the sleeve  208  supports/contains a rigid ring  210  for receiving high pressure air from an air external source, such as an external air pump. The ring  210  and sleeve  208  form a single, integrated object that makes up at least part of the air bearing member  206 . 
     The air bearing member  206 , including its ring  210  and/or sleeve  208  (housing), has channels  212  for distributing that air, and holes (not shown) for directing the air between the rail  200  and the ring/assembly. For example,  FIG. 2  shows the ring  210  as having a groove for distributing air along the ring  210 , and through the noted plurality of through-holes. These through-holes, which are fluidly connected with the groove, thus direct the pressurized air they receive from the groove through the ring  210  and toward the rail  200 . 
     In this implementation, the CMM  100  has two parallel rails  200  in each of the X, Y, and Z directions to prevent the movable assembly  202  from rotating or pivoting around any its rail  200 . Some embodiments have more than two parallel rails  200  in each direction, while designs with a single rail  200  can also accomplish the same goals. Accordingly, the movable assembly  202  has the same number of corresponding air bearing members  206 , which are coupled with the arm  104  and other portions of the movable features  122 . Movement of the movable assembly  202  thus produces a corresponding movement of the arm  104 , enabling the CMM  100  to measure the object. 
     In accordance with illustrative embodiments, each rail  200  is formed at least in part from an anisotropic material, such as a carbon based material (e.g., graphite or other carbon fiber). During thermal cycling or other thermal changes, the rails  200  expand and contract substantially the same way as the ring  210 . The carbon rails  200  radially react to heat in this manner similar to the radial reaction of a metal rail. Some embodiments, however, may permit a change in shape longitudinally while ensuring no shape changes radially. 
     In addition, each carbon rail  200  is engineered/selected to have a coefficient of thermal expansion (“CTE”) that is generally matched to that of the ring  210 . Accordingly, both the ring  210  and its rail  200  can have about the same CTE, subject to engineering tolerances. For example, both the ring  210  and rail  200  could have CTE values that are equal to within a prescribed decimal place (e.g., 7.1×10−6 per degree C.), or within 2-5 percent. While illustrative embodiments form all ring and rail pairs on a single CMM  100  so that they have the same CTE, some alternative embodiments have pairs of rails  200  and rings  210  with a first CTE, and other pairs of rails  200  and rings  210  with a second, different CTE. 
     As known by those in the art, during use, the annular space/clearance between the outer surface of the rail  200  and the inner surface of the air bearings often is quite small. Some of those spaces can be on the order of microns (e.g., about ten microns). That space should be maintained substantially constant to ensure proper CMM operation. Matching the CTEs thus ensures that this space should stay generally stable (e.g., within 5, 10, or 15 percent of its nominal space). This is in contrast to prior art CMM designs having rails with different CTEs and/or CMM designs having rails formed from anisotropic materials. Maintaining a small but constant annular gap with such prior art rails has been quite challenging and, if not effectively maintained, can reduce the performance of an underlying CMM  100 . 
     The entire air bearing member  206  can be formed from a single material, or only the rings  210  can be made from the CTE matching material. Among other things, the rings  210  and/or remainder of the air bearing member  206  (e.g., the sleeve  208 ) can be formed from a ceramic, or the rings  210  can be formed from ceramic while at least a portion of the rest of the sleeve  208  includes aluminum. Other embodiments may form the rings  210  from metal or carbon. The holes and air movement system within the air bearing member  206  preferably include jets formed from a hard material, such as sapphire. Of course, the rings  210  and rest of the air bearing member  206  can be formed from a wide variety of other materials. Discussion of those specific materials thus is for illustrative purposes only. 
     Various embodiments form the rings  210  to have a shape that corresponds with the outer shape of the rails  200 . For example, if the rail  200  has a round outer cross-sectional shape, then the rings  210  can have a toroidal shape. In fact, the interior of the ring  210  can have the same shape as the outer surface of the rail  200 , but have a different outside shape. Alternatively, if the rails  200  have a square, rectangular, triangular, irregular, or other cross-sectional/external shape, the rings  210  preferably have the same interior shape and preferably are formed on all sides of the rail  200 . Accordingly, use of the term “ring” is not intended to be limited to toroidally shaped members, or other types of round members. 
     Carbon based rails  200  provide a number of benefits. Primarily, carbon can be engineered to be very strong and stiff and yet, be lighter than conventional materials used with prior art CMMs. For example, many conventional CMMs use steel, ceramic, or granite rails, which are heavier and yet, when compared to carbon fiber, provide lower strength or stiffness advantages. The CMM  100  of  FIG. 2 , with its carbon-based rails  200 , thus can deliver the same or better performance with lighter materials. 
     Moreover, carbon is a natural lubricant. Accordingly, if one of the rings  210  does scrape against the rail  200 , such as at startup, shut down, or during an unintended malfunction, the carbon rails  200  should provide some beneficial crash protection to minimize damage. 
     During use, an air source forces air through tubes (not shown) to the air channels  212  on the air bearing member  206 . This preferably creates a substantially constant-thickness layer of air that substantially surrounds a portion of the relevant rail  200 —primarily around the rings  210 . At this point, the movable features  122  (e.g., the arm  104 ) can move as required by the application. During this time, each air bearing continues to maintain a substantially constant clearance with its rail  200 . In fact, the clearance should remain substantially the same both when the air bearing is moving along the rail  200 , and when the air bearing is not moving along the rail  200 . 
     When the CMM  100  is no longer operating, it may power down, thus stopping the flow of air to the air bearing member  206 . As noted above, this may cause contact with between the rail  200  and the ring  210  and/or rest of the bearing member. During normal use, this contact should have less impact than prior art rings known by the inventors due to the lubricating nature of carbon. 
     Although the above discussion discloses various exemplary embodiments of the invention, it should be apparent that those skilled in the art can make various modifications that will achieve some of the advantages of the invention without departing from the true scope of the invention.