Abstract:
A framework encloses a stepper motor, mounting structure, and circuitry for use in calibrating the responses of utility locators or the precise frequency outputs of locating transmitters, and associated tilt, directional, angle, and gradient sensors. The framework contains two Helmholtz or similar field windings embedded in its sides to achieve maximum accuracy in calibration of locating instruments, such that a locator may be precisely situated within the uniform field of the windings for calibration measurement or testing. Calibration and testing may be done manually or by automated means.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of the filing date of the similarly entitled U.S. Provisional Patent Application Ser. No. 60/912,517 filed Apr. 18, 2007, of Mark S. Olsson et al., the entire disclosure of which is hereby incorporated by reference. This application is a continuation of co-pending U.S. patent application Ser. No. 12/103,971 filed Apr. 16, 2008, and is also a continuation-in-part of co-pending U.S. patent application Ser. No. 12/780,311 filed May 14, 2010, which was a division of U.S. patent application Ser. No. 12/243,191 filed Oct. 1, 2008, now U.S. Pat. No. 7,733,077, which was a continuation of U.S. patent application Ser. No. 11/970,818 filed Jan. 8, 2008, now U.S. Pat. No. 7,443,154, which was a division of U.S. patent application Ser. No. 10/956,328 filed Oct. 1, 2004, now U.S. Pat. No. 7,336,078, which in turn claimed benefit of U.S. Provisional Patent Application Ser. No. 60/508,723 filed Oct. 4, 2003. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the technology of underground utility locating receivers and transmitters, and specifically to apparatus for calibrating such instruments for dependable field use. 
     BACKGROUND 
     For many years, utility locating receivers have been used to identify the location of buried pipes and cables underground. These receivers typically detect fields which are imposed onto pipes and cables using a dedicated transmitter at defined frequencies, by induction or direct connection. Locating receivers may also scan for passive signals generated in underground conductors by other sources than a locating transmitter, such as ambient broadcast energy, electrical current from generating plants, etc. The majority of locating instruments use electro-mechanical elements in their circuits, such as potentiometers, for example, which over time may shift out of calibration causing inaccuracies to creep into the locating process. More modern locating instruments may be tuned and calibrated through software only, but even these must be initially calibrated for accuracy in application and their calibration verified at intervals. Because of the potential cost and potential damage that may be incurred through inaccurate locating, precise calibration is critically important both in the manufacture of locating instruments and in their continued field use. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved system for achieving calibration of a locating receiver or a locating transmitter, or similar device, a system for data capture and storage in the calibrating process, and a system for minimizing distortion in the calibrating process that could be caused by uncontrolled environmental electromagnetic perturbations. It provides as well a system for calibrating the depth detection of a locating instrument and calibrating a locator with an embedded compass. The present invention also provides a system that performs a quick check on depth, signal strength, angle balance, alignment, and operation of gradient coils in a single manual operation. 
     One aspect of the preferred embodiment of this invention is the capability of centrally positioning an omnidirectional locator which uses gradient coil antennas in a controlled symmetrical field in order to calibrate the gradient coils. 
     Another aspect of the present invention is the rotation of a locator within a symmetrical and controlled electromagnetic field established by the Helmholtz windings, as a way of testing or calibrating the detections of the locator and the omnidirectional symmetry of the antenna nodes. 
     Another aspect of the present invention is the ability to control such a rotational process automatically from an associated computer which sends control signals to a rotary drive motor which controls the rotational motion of the locator during testing. 
     Another aspect of the present invention allows the rotary calibration to be performed without automation by a manual operator reading the locator screen in order to do a rapid field check and calibration of the locator. 
     Another aspect of the present invention is the ability to rigidly situate a locator along a vertical line perpendicular to the horizontal 4-fold symmetry axis of the field in such a way that it is slightly above the center of the field, with the result that the upper antenna node receives a lower signal strength than the lower antenna node. When this occurs under controlled conditions, with known values of distance, it enables an operator to calibrate the depth-reading capability of the locator based on the differences between the signal strengths received at the upper and lower antenna nodes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a perspective cutaway view of an embodiment of the present invention. 
         FIG. 1B  is a perspective rear view of the system of  FIG. 1A . 
         FIG. 1C  is a front elevation view of the system of  FIG. 1A . 
         FIG. 1D  is a rear elevation view of the system of  FIG. 1A . 
         FIG. 1E  is an enlarged section view of an adjustment collar by which the vertical location of the antenna nodes relative to the generated field may be controlled. 
         FIG. 1F  is an enlarged section view of a drive motor and rotary coupler assembly. 
         FIG. 2  is a block diagram of the system of  FIG. 1A . 
         FIG. 3A  is a logical flow chart illustrating the operation of the system of  FIG. 1A . 
         FIG. 3B  is a logical flow chart illustrating the calibration of a transmitter. 
         FIG. 3C  is a logical flow chart illustrating the testing of a locator&#39;s depth reading in the system of  FIG. 1A . 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1A , a calibration system  102  comprises a rigid calibrator frame or assembly including two side panels  112 ,  132  around each of which are wound the windings (e.g.,  114 ) of the Helmholtz coil (one shown) covered by protective bumpers  110 ,  134 . A single turn of copper tape is used for each winding in this preferred embodiment. The calibrator assembly itself comprises side panels  112 ,  132 , a platform  130 , a front lower panel  128 , a lower floor  123 , and top panel  106 . The panels assembled on a frame  124  of joined members with three height-adjustable foot pads for leveling the system. (Only foot pad  122  is shown in this view.) The calibrator assembly is strengthened and given additional rigidity by a number of threaded stiffening rods  108  and  126 , which hold opposing sides together. Panels, threaded rods, nuts, and bolts are non-magnetic and electrically insulating fiber reinforced plastics. Suitable fibers are glass and Kevlar. Suitable plastics are rigid epoxies, polyesters, and urethanes. 
     The top panel  106  is fitted with a support in the form of an upper clamp assembly  136  into which the vertical shaft of a portable locator  104  may be secured above a platform  130 . The clamp assembly  136  includes a pivoting locking member  137 . 
     By way of example, the locator  104  may be of the type disclosed in U.S. Pat. No. 7,009,399 granted Mar. 7, 2006, for example, the entire disclosure of which is hereby incorporated by reference. The clamp assembly  136  includes a pivoting locking member  137 . The locator support further includes a cup-like locator mount  116  that is fitted to the top end of shaft  118 , into which mount the lower antenna node of the locator may be seated. Locator mount  116  is formed to accommodate the lower antenna node typical of omnidirectional antenna locators. The locator mount  116  is vertically adjustable by means of a threaded shaft fitted to a collar (not illustrated in this figure). In an alternative embodiment, the locator mount  116  may be configured to adjustably accept various locators of different form-factors. The locator mount  116  is coupled by a shaft  118  which passes through a hole in the platform  130 , and which in turn is coupled through a housing  120  to a drive motor and rotary encoder assembly (not illustrated in this figure) mounted on the calibrator assembly flooring  123  beneath the platform  130 . 
     Turning to  FIG. 1B , the circuit formed by the Helmholtz coil windings (wound under the bumpers  134  and  110 ) terminates in attachment posts, such as  146 , to which the standard clips of a locating transmitter may be easily connected. A suitable transmitter is disclosed in published U.S. Patent Application US-2005-0156600-A1 published Jul. 21, 2005, the entire disclosure of which is hereby incorporated by reference. A cable plug and jack may also be used for the same purpose, rather than individual clips and posts. A specially calibrated transmitter or signal generator may be used to drive this circuit at a selected frequency, thus establishing a uniform and standardized magnetic field around and through the locator at the same frequency. A similar configuration using a calibrated locator may be used to test or calibrate a transmitter&#39;s output at various frequency settings. 
     The housing  120  encloses the drive motor and rotary encoder assembly (not illustrated in  FIG. 1B ) which is connected to an interface  148  such as a USB hub, for example, to which a computer may be connected. The computer, which may be a laptop personal computer, for example, can be used to send drive commands to the rotary motor, causing the shaft  118 , locator mount  116  and thus the locator  104  to rotate on command at a selected rate. The process may be automated by computer or it may be done manually. In the process of manually or automatically rotating the locator  104  when the Helmholtz coil windings are excited at a selected frequency, an operator may capture the responses of the locator  104  and compare them to specification values as a means of calibration or of testing the locator  104 . Angle values read from the rotary encoder assembly are returned to the coupled computer through the same USB hub  148 . Signal strength readings from the locator  104  are transmitted through the same USB hub  148  on a separate channel. The locator  104  is connected to the USB hub  148  via flexible cable or a radio link such as Bluetooth. Infrared data links may also be used. When a transmitter is equipped to be controlled remotely it may similarly be connected through the USB hub  148 , and control signals transmitted from the computer to adjust frequency and power settings or signal generator. 
     As illustrated in  FIG. 1C , the locator  104  is secured in clamp assembly  136  and seated in locator mount  116  on shaft  118 . The locator mount  116  is connected by shaft  118  to the drive motor and rotary encoder assembly which is protected by a housing  120 . The locator mount  116  is supported by a bearing inside adjustable sleeve  143 . Adjustable sleeve  143  threads into coupling ring  141 . Coupling ring  141  is rigidly mounted to upper platform  130 . Rotating adjustable sleeve  143  vertically raises or lowers the locator mount  116 . A mounting collar  145  acts to retain the shaft  118  when the locator mount  116  is removed. The front lower panel  128  (partially illustrated) and floor platform  123 , on which the USB hub  148  is situated, are visible in  FIG. 1C . Leveling adjustable foot pieces  122 ,  142 ,  140  are used to establish the system on a level plane. The protective bumpers  110 ,  134  on the Helmholtz windings used in this embodiment are visible in  FIG. 1C . Holding the locator  104  precisely vertical allows partial accelerometer calibration and 2-D compass calibration to also be performed. 
     In  FIG. 1D , the locator  104 , Helmholtz coil bumper covers  134 ,  110 , and rear panel  150  can be seen. The shaft  118  connecting the locator mount ( FIG. 1B ,  116 ) to the drive motor and rotary encoder assembly  160  is visible. The removable outer housing  120  is cut-away, revealing the drive motor and rotary encoder assembly  160  within. The USB hub  148  provides connection means for angle data from the drive motor and rotary encoder assembly  160 , control data for the drive motor and rotary encoder assembly  160 , signal strength information from locator  104 , transmitter data and control, compass, temperature, tilt, and other data to be exchanged with a computer depending on the configuration of the system components. 
       FIG. 1E  illustrates a closer view of the vertical adjustment mechanism in the locator mount  116  which holds the lower end of the locator  104 . In  FIG. 1E , the locator  104  is seated in a locator mount  116  of cup-like form, which may be notched or slotted to accept protrusions in the appropriate antenna node or its connecting shaft. Shaft  118  connects the locator mount with the drive motor and rotary encoder (not illustrated). Platform  130  supports internally-threaded coupler  141 . Internally-threaded coupler  141  in turn supports an externally-threaded adjustable sleeve  143  which encloses a bearing  147 . In use, the adjustable sleeve  143  may be rotated to raise or lower bearing  147  and the locator mount  116  relative to the symmetrical field generated by means of the Helmholtz coil ( 114  in  FIG. 1A ), such as when calibrating or testing depth measurement. Non-magnetic and non-metallic parts are used whenever possible throughout. Screws  162 ,  164  secure the platform  130  to the coupler  141 . 
       FIG. 1F  illustrates a sectional view of the drive motor and rotary couplers used in the rotation of the locator within the symmetrical field generated by means of the Helmholtz coils. In  FIG. 1F , shaft  118  is connected to the drive motor and rotary encoder assembly  160  by means of a shaft drive coupling adaptor  151  supported by a bearing  161  and joined to a high-precision bellows shaft coupling  153 . A motor-drive couple adaptor  155  connects the bellows coupling  153  to the drive motor and rotary encoder assembly  160 . The base plate  157  of the housing  120  is attached to the floor plate  123  by through-bolts  166 ,  168 . The system frame  124  and one of the footpads  142  are shown supporting the floor plate  123  in  FIG. 1F . 
     In  FIG. 2 , the locator  104  is illustrated as physically connected to the drive motor and rotary encoder assembly  160 . A transmitter  202  is connected to the Helmholtz windings  114 ,  138 . A computer  204  that is connected to the USB hub  148  can issue control data and receive data from the locator  104 , transmitter  202 , and drive motor and rotary encoder assembly  160 . The computer  204  may communicate data to remote data store  206  through wired or wireless means. 
       FIG. 3A  illustrates the logical steps in the operation of the system of  FIGS. 1A-1D  and  FIG. 2  in calibrating the locator  104 . As can be seen in the flow chart, an iterative loop is installed in the process by which signal data is captured at a series of rotary angles of the locator. In  FIG. 3A , the emphasis is on sampling signal strength readings from all nodes, at a specific frequency and transmitter output in a series of steps separated by one hundred and twenty degrees of rotation. Calculated corrective values based on any discrepancies are returned and stored in the locator flash memory as part of the process, bringing about calibration of the instrument. Any angular increment step may be used. Finer rotation steps may be used. If the values exceed specification, the unit is passed to an inspection and repair process, to be re-calibrated when corrected. 
       FIG. 3B  illustrates the logic flow of steps in the calibration of a transmitter. The test in  FIG. 3B  assumes a calibrated locator. A series of test steps at the standard output levels and frequencies is defined and the signal samples, as read at the locator are compared to specification values for an array of transmitter frequencies. The difference between the nominal frequency setting of the transmitter and the detected frequency at the locator, expressed as a percentage, is stored for each tested frequency. When the test series is complete, the values of difference are processed for a linear fit. Corrective values can be returned to and stored in the transmitter&#39;s software to effectuate calibration. If the values exceed manufacturer&#39;s specification, the transmitter is passed to an inspection and repair process, to be re-calibrated when corrected. 
       FIG. 3C  illustrates testing a locator&#39;s depth reading. One locator with depth indication capability is disclosed in U.S. Pat. No. 7,332,901 granted Feb. 19, 2008, the entire disclosure of which is hereby incorporated by reference. A locator is set at a pre-determined location above the horizontal axis of the field generated by the calibration system, such that the lower and upper antenna nodes of the locator are not equidistant from the field&#39;s center. Continuous measurements of the top and bottom antenna-node detections are taken during a full rotation of the locator. Based on the degree of variance in computed depth readings, corrective values may be calculated and stored in software to effectuate calibrating the locator. A rapid field test of a locator may be similarly conducted simply by observing depth readings on the locator while rotating it using the present invention. 
     It will be understood by one versed in the art relating to this invention that modifications in configuration or components may be possible to achieve related results, and that additional applications of the present invention may be conceived of to test or calibrate devices not specifically identified herein or using variations in routines. 
     The design of the coil windings ( 114  in  FIG. 1A ) is not restricted in the present invention to a Helmholtz configuration. It will be clear to one versed in these arts that other winding configurations, such as, for example, a single circular, cylindrical, or an approximately prolate spheroidal coil design, could be used. One or more Helmholtz pairs, offset in space or rotated, could be used in alternative embodiments. A three-axis Helmholtz field could alternatively be used. 
     In an alternate embodiment, the locator mount  116  may be configured to accept different designs of locators and antennas without modification to the basic operation of the present invention. In another alternate embodiment, the present invention may be used to calibrate a compass unit which is part of a particular locator at the same time. The system may be used additionally in conjunction with one or more dipole sources or sondes for the purposes of calibration or testing at appropriate frequencies. 
     While we have described a preferred embodiment of our calibration system, modifications and variations thereof will occur to those skilled in the art. For example, the locator could be stationary and the Helmholtz windings could be moved around the locator. 
     Therefore, the protection afforded our invention should only be limited in accordance with the scope of the following claims.