Abstract:
A target base ( 10 ) for attaching an apparatus ( 13 ) to an object ( 11 ) has a substantially rigid body ( 12 ) with a mount surface ( 14 ) adapted to couple the apparatus ( 13 ) to the body ( 12 ), and a base surface. A suction cup ( 46 ) affixed to the base surface ( 18 ) of the target base ( 10 ) has a suction cavity ( 62 ) defined by a deformable sealing lip ( 64 ) at the perimeter of the suction cup ( 46 ). The sealing lip ( 64 ) and suction cavity ( 62 ) form with the object ( 11 ) a vacuum chamber ( 66 ). A vacuum system ( 44 ) includes vacuum passages ( 50, 52 ) for communicating a vacuum pressure to the vacuum chamber ( 66 ). One or more contact members ( 60 ) are affixed to the body and have contact surfaces which engage the object ( 11 ) to provide consistent off-set positioning of the target base ( 10 ) with respect to the object ( 11 ).

Description:
STATEMENT OF GOVERNMENT RIGHTS 
     This invention was made with Government support under contract F33657-87-C-2000 awarded by the United States Air Force. The Government has certain rights in this invention. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to the field of measuring devices and more particularly to a target base for a measuring system. 
     BACKGROUND OF THE INVENTION 
     It is common practice when measuring the surface contour of an object to take measurements of the object using an optical or laser measuring apparatus. One such method includes marking predetermined locations on the object and attaching targets to the object at each location. Conventional target bases generally utilize a magnet or multiple suction cups to attach the target to the object. Conventional target bases may also provide proper spacing of the target from the surface of the object by feet that make point contact with the object. In many applications, it is necessary to keep the targets attached to the object being measured for long periods of time, such as when two or more sub-assemblies are being assembled and their relative positions are being established prior to joining them. 
     Conventional target bases that utilize small suction cups generally operate by being pressed onto the surface of the object. The air is squeezed out of the suction cups and the suction cups are released whereupon they partially resile and form a vacuum within the suction cup. Suction cups have several disadvantages. For example, the force that the suction cups exert is relatively small, inasmuch as it is a function of the resiliency of the suction cup. Accordingly, even when several suction cups are used on a target, the force holding the target to the object is relatively low, and there is a chance that the target will change position or even become dislodged. In addition, suction cups must be applied with attention to making sure that all of the suction cups are evenly pressed onto the surface of the object, lest there be an uneven suction force among them, thus allowing the target to tilt in the direction of a greater suction force. Furthermore, suction cups are inherently resilient, thereby displacing the target away from the surface. Because of the displacement, or rebound, of the target from the surface of the object, the legs that set the distance of the target from the surface of the object are often not effectively engaged. 
     The amount of rebound the target experiences is subject to variation that is dependent upon many factors, such as environmental conditions (e.g., temperature and humidity), the orientation of the target (e.g., gravity forces opposing or aiding the suction cups), and the physical condition of the suction cups (e.g. age, wear, and the like). Other problems associated with suction cups may include creep of the sealing lips of the suction cups that allows them to loosen, and leakage that allows the suction cups to lose their vacuum over time. 
     In short, pressed-on suction cups provide scant assurance that the targets on the object are uniformly positioned at predetermined locations and that the offset distance and the target angle accurately reflects the surface of the object. In addition, it cannot be assured that pressed-on suction cups will not move over time or even be dislodged. 
     SUMMARY OF THE INVENTION 
     Accordingly, a need has arisen for an improved target base for a measuring system. The present invention provides a target base that substantially eliminates or reduces problems associated with the prior methods and systems. 
     In accordance with one embodiment of the present invention, a target base for attaching an apparatus to an object includes a substantially rigid body having a mount surface adapted to couple an apparatus or target to the body, and a base surface. A suction cup is affixed to the base surface of the body and has a suction cavity that is defined by a deformable sealing lip at the perimeter of the suction cup. The sealing lip and vacuum cavity in conjunction with the object form a vacuum chamber. A vacuum system communicates a vacuum pressure to the vacuum chamber. The target base also includes one or more contact members which are coupled to the base surface of the body to provide consistent off-set positioning of the target base with respect to the object. 
     In a particular embodiment, the target base includes a sight system used to accurately position the target base with respect to the object. The sight system may include a hole through the target base, a mounting sleeve assembly housed within the hole, and a reticle mounted within the mounting sleeve assembly in sealed relation. The reticle may include a targeting guide, such as a circle and cross hairs, for precise positioning of the target base over the object. 
     In another embodiment, a locating system is associated with the mount surface of the target base to ensure accurate mating of the apparatus with the target base in a predetermined position. A simple but highly accurate locating system may consist of two circular cylindrical bosses that project from the mount surface and have different diameters. The base may also include threaded holes in the body opening at the mount surface and adapted to receive screws by which the apparatus is attached to the target body. 
     In a particular embodiment of the present invention, the contact surfaces of the contact members are located within the suction cavity, primarily to maximize the area of the suction cup. The contact members may include threaded attachment shanks that pass through the suction cup and are threaded into the body and have heads configured to make substantially point contact with the surface of the object. In general, three contact members equidistant from each other and from a center axis perpendicular to a plane created by the mount surface of the target base are used. The sealing lip of the suction cup may be concentric with the center axis. The described configuration of the contact members and sealing lip of the suction cup provides a distribution of forces due to the pressure differential across the suction cup such that the centroid is along the center axis and the reaction forces on the contact members are equal. 
     One technical advantage of the present invention is to provide a target base for an apparatus, such as a target, to be used in optical or laser measuring systems that will support the apparatus with high accuracy over a point on an object. Another technical advantage is to provide a target base that provides accurate positioning of the apparatus relative to the point to be measured on the surface of the object, regardless of the orientation of that surface to which the target base is attached relative to the horizontal—i.e., the accuracy of the positioning of the target base is not affected by gravitational forces acting on the target base and the apparatus that it supports. Other technical advantages include stable positioning of the apparatus without movement over long periods of time. 
     A further technical advantage of the present invention is to provide a suction cup that is substantially coextensive with the mount surface to allow a large retention force of the vacuum created within the suction cup. The retention force of the vacuum created within a suction cup of the present invention having a large area is generally greater than that of conventional suction cups having smaller areas. Furthermore, the larger area of the suction cup of the present invention allows the target base to be attached to the object with a lower vacuum than that required for conventional suction cups. 
     Other technical advantages of the present invention will be readily apparent to one skilled in the art form the following figures, description, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and its advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings, wherein like reference numerals represent like parts, in which: 
     FIG. 1 is a top plan view of a target base in accordance with one embodiment of the present invention; 
     FIG. 2 is a side cross-sectional view taken along line  2 — 2  of FIG. 1 of a target base; and 
     FIG. 3 is a bottom plan view of the target base of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 through 3 illustrate various aspects of one embodiment of a target base that may be used for attaching an optical or laser measuring apparatus onto the surface of an object. The target base may include a relatively large suction cup that forms with the object a vacuum chamber. A number of contact members located within the vacuum chamber contact the surface of the object to assure accurate and consistent off-set positioning of the target with respect to the surface of the object. 
     According to one embodiment of the present invention, illustrated in FIG. 2, a target base  10  includes a body  12  having a mount surface  14  that faces opposite an object  11  being measured, a perimeter wall  16 , and a base surface  18  opposite from the mount surface  14 . The mount surface  14  is configured to mate with an apparatus or target  13 . In a broad sense, the mount surface  14  has a reference plane  15  from the point of view of establishing a position of the target  13  with respect to the body  12 , including distance and orientation, and a center axis  17  that is perpendicular to the reference plane  15 . In a particular embodiment, the mount surface  14  is planar and coincides with the reference plane  15 . In practice, a planar mount surface  14  has the advantage of being easy to form with high accuracy and is preferred for that reason. It will be understood that mount surface  14  may be any surface suitable to mate with target  13  without departing from the scope of the present invention. 
     Although the body  12  may be fabricated in a single piece, the embodiment, as illustrated, includes an upper piece  20  and a lower piece  22  joined at a split line  24  by countersunk cap screws  25  that are equally spaced apart on a circle near the perimeter of the body  12 . The upper and lower pieces,  20  and  22 , may be annular, each having a circular cylindrical perimeter wall  16  of the same diameter with the upper and lower pieces,  20  and  22 , having a hole  26 ,  28 , respectively concentric with the axis  17 . 
     A locating system  29  may be associated with the mount surface  14  of the body  12  to mate the apparatus or target  13  with the target base  10  such that the target  13  can be mated to the base  10  in only one position. The locating system  29  may include locating elements for positioning the target  13  with the target base  10 . In one embodiment, as shown in FIG. 1, the locating elements include two circular cylindrical bosses  30  and  32  of different diameters spaced apart from each other. It will be understood that the locating elements may be any suitable positioning device or system without departing from the scope of the present invention. 
     In addition to the locating system  29 , threaded holes  34  at the mount surface  14  of the body  12  receive screws  36  by which the target  13  may be attached to the body  12  after being correctly positioned. 
     In one embodiment of the present invention, as shown in FIGS. 1 and 2, a sight system  38  may be used to position the target base  10  over the point on the object  11  to be measured. The sight system  38  may include the hole  26  in the upper piece  20  of the body  12 , and the hole  28  in the lower piece  22  of the body  12 . A reticle  40  may be supported by and sealed within a mounting sleeve assembly  42  which is housed within the hole  28 . As shown in FIGS. 1 and 3, the reticle  40  may include a targeting guide  41  having cross hairs for precise positioning of the target base  10  over the point on the object  11  to be measured. 
     As illustrated in FIGS. 2 and 3, a suction cup  46  may be attached to the base surface  18  of the body  12  by screws  48 , the heads of which bear against a reinforcing disc  68  embedded in the relatively thicker body of the suction cup  46  and the shanks of which are threaded into holes in the lower piece  22  of the body  12 . The sealing lip  64  of the suction cup  46  may be concentric with the axis  17  and forms a suction cavity  62  on the base surface  18  side of the target base  10 . It will be understood that target base  10  may include a plurality of suction cups  46  without departing from the scope of the present invention. 
     When in place on the object  11 , the sealing lip  64  and cavity  62  form a vacuum chamber  66 . A vacuum pressure is communicated to the vacuum chamber  66  by a vacuum system  44 . The vacuum system  44  may include a first passage  52  extending from the vacuum chamber  66  to a second passage  50  that extends to a port opening  51  in the perimeter wall  16  of the body  12 . An O-ring  53  seals the passage  52  at the split line  24  between the parts  20  and  22 . As shown in FIG. 1, a vacuum pump  54  may be coupled in a sealed manner to the second passage  50  through the port opening  51 . The vacuum pump  54  may include a gage  56  for measuring the air pressure within the vacuum chamber  66 . The vacuum pump  54  may be a hand pump or an automatic pump, such an electric pump. It will be understood that the vacuum pump  54  may be any vacuum pump suitable to form a vacuum within the suction cup  46  without departing from the scope of the present invention. 
     As shown in FIGS. 2 and 3, one or more contact members  60  establish the off-set spacing and orientation of the reference plane  15  relative to the surface of the object  11  when the target base  10  is in place on the object  11 . In one embodiment, the target base  10  includes three contact members  60 . In this embodiment, the contact members  60  are equally spaced apart from each other and equidistant from the axis  17 . Each contact member  60  may have a rounded head that provides substantially point contact with the surface of the object  11  and may also have threaded shanks that thread into the lower piece  22  of the body  12 . The contact members  60  are installed such that the contact points lie in a plane that is substantially parallel to the reference plane  15 . 
     The target base  10  of the present invention is attached to the object  11  by a vacuum applied to the suction cup  46  by the vacuum pump  54 . The forming of a vacuum by the pump  54  progressively pulls the target base  10  toward the object  11  until the contact members  60  engage the surface of the object  11 . Any additional vacuum beyond that required to engage the contact members  60  only increases the vacuum force acting between the target base  10  and the object  11  to restrain the target base  10  but does not change the position of the mount surface  14 , as the contact members  60  serve as stops that establish the off-set distance from the mount surface  14  from the surface of the object  11  and the orientation of the mount surface  14  relative to the surface of the object  11 . There is no rebound of the base  10  due to the resiliency of the suction cup  46 . Forming a vacuum by the vacuum pump  54  also permits the retention force to be set to a selected value through the use of a pressure gage  56  that may be associated with the pump  54 . Subject to the possibility of slow leakage, which can be corrected by periodic checking of the gage  56  and additional pumping as required, the vacuum provides a holding force that is sustainable for indefinite periods. The present invention also makes it possible and advantageous to use a suction cup  46  having a relatively large area which, for any given level of vacuum formed in the chamber  66 , provides a correspondingly large retention force. 
     In use, the target base  10  is first positioned exactly over a target point marked on the object  11  being measured, using the reticle  40  to establish the position and circumferential orientation. The vacuum pump  54  is operated to draw a vacuum in the vacuum chamber  66  of the suction cup  46 . The force due to the pressure difference between the atmosphere and the vacuum pulls the body  12  toward the object  11  and engages the contact members  60  with the object  11 . After the contact members  60  contact the object  11 , the target base  10  can no longer move toward the object  11 , and the mount surface  14  is located at a predetermined off-set distance from and parallel to the plane defined by the contact points of the contact members  60 . With the aid of the gage  56  of the vacuum pump  54 , the user can draw a vacuum of the desired value to ensure retention of the target base  10  on the object  11  with a desired force. After the target base  10  is checked for position using the reticle  40 ,.the target  13  can be aligned on the mount surface  14  using the locating system  29  and fastened in place with the screws  36 . 
     Ordinarily, the vacuum should hold steady over long periods of time. Prudence suggests that the gage  56  should be checked periodically to ensure that the vacuum is being maintained. 
     Although one embodiment of the present invention has been described; various changes and modifications may be suggested to one skilled in the art. It is intended that the present invention encompass such changes and modifications as fall within the scope of the present appended claims.