Abstract:
Disclosed are electronic plumb indicators for monopoles, survey poles, global positioning antennas and the like. A exemplary plumb indicator comprises a substrate, a plumb bob suspended by a thread (e.g., wire, cable, string, etc.) and disposed over a first surface of the substrate, and a probe. The substrate has an array of conductive pads disposed over its first surface, preferably in a regular pattern. The probe has a first body portion held by the plumb bob but allowed to move freely in the vertical direction, and a conductive tip disposed between the plumb bob and the first surface of the substrate. The probe tip makes contact with the first surface of the substrate, and contacts one or more pads in normal operation. The pads may be scanned electronically to determine the location of the probe tip and plumb bob with respect to the substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit priority under 35 U.S.C. §119(e) to U.S. patent application Ser. No. 60/199,078, filed Apr. 20, 2000, the contents of which is incorporated herein. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to electronic plumb indicators for monopoles, survey poles, global positioning antennas and the like. 
     BACKGROUND OF THE INVENTION 
     There is a need in the surveying art and the global positioning (GPS) art to increase the speed and accuracy at which surveying poles and GPS antenna poles are plumbed to vertical positions. 
     SUMMARY OF THE INVENTION 
     Broadly stated, the present invention encompasses a plumb indicator comprising a substrate, a plumb bob suspended by a thread (e.g., wire, cable, string, etc.) and disposed over a first surface of the substrate, and a probe. The substrate has an array of conductive pads disposed over its first surface, preferably in a regular pattern. The probe has a first body portion in contact with a surface of the plumb bob, and a conductive tip disposed between the plumb bob and the first surface of the substrate. The probe tip makes contact with the first surface of the substrate, and contacts one or more pads in normal operation. The pads may be scanned electronically to determine the location of the probe tip and plumb bob with respect to the substrate. 
     In preferred embodiments of the present invention, the plumb bob comprises an elongated mass and cylindrical bore along a portion of the vertical center axis of the elongated dimension. Also in these preferred embodiments, the first body portion of the probe comprises a cylindrical shaft which is disposed within the cylindrical bore in the plumb bob, and the tip of the probe has a rounded or flat surface for contact with the first surface of the substrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first exemplary embodiment of a plumb indicator according to the present invention. 
     FIG. 2 shows a cross-sectional view of the exemplary plumb bob and probe shown in FIG. 1 according to the present invention. 
     FIG. 3 shows the first exemplary plumb indicator of FIG. 1 using a second exemplary plumb bob and disposed within an exemplary pole housing according to the present invention. 
     FIG. 4 shows a cross-sectional view of a second exemplary plumb bob according to the present invention. 
     FIG. 5 is a top plan view of a portion of the top surface of a substrate according to the present invention showing an exemplary layout of contact pads according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows an exemplary plumb indicator  10  according to the present invention. Plumb indicator  10  comprises a substrate  20  which has a first surface  21  and an array of conductive pads  24  disposed over the first surface  21 . A plurality of mounting holes  26  are provided in substrate  20  for mounting substrate  20  to a surveying pole or monopole, as described below in greater detail with reference to FIG.  3 . Other mounting means may be employed in the present invention, and the particular mounting means employed is not critical to the practice of the present invention. 
     Plumb indicator  10  further comprises a plumb bob  30  suspended by a thread  32  and disposed over first surface  21  of substrate  20 , and further comprises a probe  40  held by plumb bob  30 . The weight and positioning of plumb bob  30 , and the connection of thread  32  to a point on the vertical center axis of bob  30 , enable the vertical center axis of bob  30  to be substantially perpendicular to substrate  20  when indicator  10  is placed in a plumb position. In a preferred embodiment, as shown in the cross-sectional view of FIG. 2, probe  40  has a body portion  42  which fits within a cylindrical bore  34  made at the bottom of plumb bob  30  and along the vertical center axis of plumb bob  30 , and a conductive tip  44  disposed at the lowermost distal end of body portion  42 . Body portion  42  and bore  34  both preferably have circular cross-sections. Body portion  42  freely moves in the vertical direction within bore  34 , and portions of the outer surface of body portion  42  contact with respective portions of the inner surface of bore  34  to keep body portion  42  and probe  40  aligned substantially parallel with the vertical center line of plumb bob  30 . This in turn places the probe  40  in a position which is perpendicular to substrate  20  when indicator  10  is placed in the plumb position with respect to the earth (due to gravity, plumb bob  30  seeks a stationary state where its vertical center axis is perpendicular to the surface of the earth). 
     Probe tip  44  can have a flat surface with beveled edges, or a rounded surface (e.g., spherical, hemispherical, or semi-hemispherical) for contact with surface  21  of substrate  20  and the pads  24  thereon. In the former case, there preferably is a sufficient gap between the sidewalls of probe body  42  and bore  34  to allow probe tip  44  to remain flat through the arc motion of plumb bob  30 . Probe tip  44  may have a conductive rubber coating on its surface to minimize scratching of the conductive pads  24  while plumb bob  30  freely moves around above substrate  20  during the plumbing process. Probe  40  touches pads  24  on accord of its own weight, and does not require a spring behind it. The position of plumb bob  30  relative to substrate  20  is found by determining which pads  24  are being contacted by probe tip  44 . As one example, a voltage (or ground) may be applied to metal bob  30  through thread  32  and coupled to a metal probe  40  by surface contact with bore  34 , and the pads  24  may be scanned electronically to detect which pad or pads have the same potential as bob  30 . A detector  50 , which preferably comprises a microprocessor unit, may be used for the scanning and detection, and digital multiplexers may be used to selectively group the pads for scanning. As another way, the pads  24  may electrically coupled in a row-column format so that probe tip  42  electrically couples at least one row with at least one column. The rows and columns may be scanned by detector  50  to determine which rows are being coupled to which columns by tip  42 . In this case, there is no need to couple an electrical potential to thread  32 , although such can be done. This embodiment is described in greater detail below with respect to FIG.  5 . To achieve a 1 mm accuracy in finding the position of probe  40  on first surface  21 , the pads may be set to square dimensions of 0.2 mm on a side and spaced apart from one another by gaps of 0.2 mm, and the probe tip  44  may be formed to provide a contact area to surface  21  of 0.5 mm. 
     In one embodiment according to the present invention, plumb bob  30  comprises a cylinder of heavy material with a diameter of 10 mm, a height of 100 mm, a center bore  34  of 1 mm to accommodate free vertical motion of a 0.6 mm diameter probe  40 . The top end of thread  32  and substrate  20  may each be fixed to a housing or other support means of any desired type, and substrate  20  and/or the top end of thread  32  may be aligned so that probe tip  44  comes to rest on a particular set of one or more pads when plumb bob  30  is in the plumb (vertical) position with respect to the earth. 
     Referring to FIG. 3, plumb indicator  10  can be housed inside a pole, such as a surveying pole or a monopole used to position a GPS antenna (in the latter case, the GPS antenna is mounted to the top of the monopole). Substrate  20  is affixed to a widened portion of the pole with mounting screws that pass through the mounting holes  26 . The top of thread  32  may be fixed to the pole by a plug assembly disposed within the tube near the top end and secured thereto by screws, glue, or other attachment means. The plug assembly preferably centers thread  32  within the tubing of the pole at the top of the pole. Mounting holes  26  may have sufficient diameter with respect to the securing screws to enable a controlled amount of lateral movement of substrate  20  with respect to the vertical axis of the pole. With this lateral movement, substrate  20  may be aligned so that probe tip  44  contacts a desired set of pads  24  when plumb bob is placed in the plumb position. 
     As shown in FIG. 4, plumb bob can also have side bars to protect it against falling down into the inside of the pole, if the pole is held upside down. An annular plate may also be used for this purpose. The pole may have a typical diameter of 30 mm and plumb bob  30  may have a typical diameter of 10 mm, in which case the motion of the plumb bob and the probe  40  is contained within a circle having a diameter of 20 mm (30 mm diameter of the pole minus 10 mm diameter of the plumb bob). 
     FIG. 5 shows a top plan view of a preferred embodiment for laying out contact pads  24 . A partial array of six rows R 1 -R 6  and six columns C 1 -C 6  is shown. The pads  24  are arranged in a checker-board red/black arrangement: row pads being assigned to red blocks, and column pads being assigned to black blocks. All of the pads of each row are electrically connected together and routed to an input/output port of detector  50 , and all of the pads of each column are electrically connected together and routed to a input/output port of detector  50 . The connection and routing may be readily accomplished by using a multi-layer printed circuit board where the pads are formed on the top metal layer, where another wiring layer of the board is used to couple the pads of respective rows together with a plurality of metal traces and a plurality of vias to the row pads, and yet another wiring layer is used to couple the pads of respective columns together with a plurality of metal traces and vias to the column pads. These metal traces in the underlying metal layers are shown by dashed lines in FIG.  5 . Using a square pad area of 0.2 mm on a side, and a spacing distance of 0.2 mm between pads, 50 rows and 50 columns can cover a 20 mm by 20 mm square area. The surface area covered on substrate  20  by probe tip  44  is shown by a broad dashed circle in FIG.  5 . In one implementation, a voltage (such as either supply V DD  or ground) is applied to probe tip  44 , and each row and column is wired to another voltage (such as either ground or supply V DD ) through a respective resistor. The probe tip will bring the columns and rows which it overlies to its potential, and this potential can be detected by scanning all the rows and columns and looking for a digital logic level that corresponds to the voltage of the probe tip  44 . 
     As another implementation, no voltage is applied to probe tip. Each row is coupled to one of the digital logic potentials (such as either ground or V DD ) through a respective resistor. Detector  50  then selectively applies the opposite digital logic voltage, referred to herein as the “test voltage,” to each column while scanning all the rows to determine if a row has the voltage of the selected column. To speed this process, a binary-tree scan search may be done. For example, the test voltage may be first applied to column  1 - 25  to see if there is a match, and then to columns  26 - 50  if no such match is found. If a matching row is found when the test voltage is applied to columns  1 - 25  (or alternatively to columns  26 - 50 ), then the search is narrowed by applying the test voltage to columns  1 - 12  (or alternatively to columns  26 - 37 ), and then to columns  13 - 25  (alternatively  38 - 50 ) if no match is found in columns  1 - 12  (alternatively  26 - 37 ). This process is continued until a small set of rows and columns are reached, at which point the test voltage can be individually applied to the columns of the small set. Of course, an equivalent implement of the above may be made by interchanging the function of the rows and columns. As a modification, multiplexed row and columns buffers may be used to connect the rows and columns to detector  50 , which may be a microprocessor. The above approach may find that tip  44  covers two rows at a time, and/or two columns at a time. If so, the position may be computed as being between the adjacent rows and columns (i.e., average the positions of the affected rows and columns). 
     Detector  50  does not need to wait for plumb bob  30  to be stationary. It may scan the entire pads at least  50  times per second and determines the center of the motion. The period of the motion of plumb bob  30  (which is a pendulum) is 2*Pi*SQRT(L/g), where L is the length of the thread and g is the gravity constant. For a one meter length L, the period is about 2 seconds. Detector  50  can read at least one pendulum cycle of data (at least 100 snapshots) with a period, and from this determine the center of the pendulum swing relative to the center of the array of pads  24 . 
     Detector  50  can show the results by a LED display or by sending them via serial port (RS-232 connection or other types of connection ports to a handheld device that shows the location of the plumb bob graphically. The operator may use the information to plumb the pole, or the computer can take the offset into account to determine the location of the survey point. In the latter case the pole must be oriented according a convention known to the handheld device. 
     It may be appreciated that pads  24  may be arranged in a radial pattern, with the rows being concentric circles disposed around the center point of substrate  20 , and the columns being disposed radially form the center point. In this implementation, the probe tip  44  resides at the center point when plumb bob  30  is in the vertical plumb position. It may also be appreciated that larger pads may be used at the outer edges of the array, such as by replacing a 2-by-2 grouping of pad by a single pad that occupies the same area. 
     While the present invention has been particularly described with respect to the illustrated embodiments, it will be appreciated that various alterations, modifications and adaptations may be made based on the present disclosure, and are intended to be within the scope of the present invention. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to the disclosed embodiments but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.