Electricity: measuring and testing – Of geophysical surface or subsurface in situ – For small object detection or location
Reexamination Certificate
2000-03-23
2002-07-09
Snow, Walter E. (Department: 2862)
Electricity: measuring and testing
Of geophysical surface or subsurface in situ
For small object detection or location
C340S853500, C175S045000
Reexamination Certificate
active
06417666
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to apparatus capable of locating and/or monitoring the position (i.e., the depth below a surface and the location within the horizontal plane at that depth) and/or orientation (i.e., yaw, pitch, roll or a combination thereof) of a device located out of view below a surface. More specifically, the present invention is directed to locator/monitor devices that are suitable for use in combination with boring apparatus.
Utilities are often supplied from underground lines. Two techniques are generally used to install such lines. In one technique, the utility line pathway is excavated; the line is installed; and the excavated material is replaced. While this method is suitable for new developments, implementation of this technique is not always practical in previously developed areas. As a result, industry development efforts have been focused on excavating tools capable of installing utilities underground without surface disruption.
Several guided and unguided boring tools are currently on the market. Guided tools require substantially continuous location and orientation monitoring to provide the necessary steering information. A prerequisite of such monitoring is, of course, locating the tool that is to be monitored. Only once the position of the tool is located can a proper depth measurement be obtained, for example, from a measuring position directly above the head of the boring tool which houses a transmitter. Unguided tools would also benefit from periodic locating or substantially continuous monitoring, for example, in prevention of significant deviation from planned tool pathways and close tool approaches to utilities or other below surface obstructions.
Locating or monitoring systems currently used in combination with boring apparatus are either cable locating systems or are based on cable locating technology. Although the more advanced systems perform adequately, limitations on cable locating technology also limit measurement accuracy.
Most cable locators involve receiver detection of an oscillating magnetic field derived from electrical current directly fed or induced onto the cable. The magnetic field lines emanating from a cable are essentially cylindrical in shape, forming concentric circles around the cable. As the current flows along the cable, losses occur as a result of displacement and induced currents into the soil. Consequently, the exact signal strength of the magnetic field emanating from the cable at any point is unknown. Although local signal peaks or nulls (depending on receiver antennas and electronic configuration) are useful to determine the surface position directly above the cable, signal strength (i.e., magnetic field strength) alone is not directly indicative of cable depth. In certain specific circumstances (i.e., when the rate of loss along the cable length is not great), a signal strength ratio can be used to compute depth. If the cable run is straight for a long distance (compared to the depth), the magnetic field strength (B) will be inversely proportional to the distance (d) from the cable to the receiver (i.e., B .alpha. 1/d or B=k/d, where k is a proportionality constant). By taking two signal strength readings at different locations directly above the cable, the proportionality constant can be eliminated and the depth determined.
A simple device for determining the depth of a relatively straight cable is manufactured by Dynatel, a subsidiary of the Minnesota Mining and Manufacturing Company. The Dynatel device includes a single antenna, a gain control knob and a gain doubling switch. The operator determines cable depth by (1) placing the device on the ground above the cable; and (2) adjusting the output displayed on a meter with the gain control knob until the meter needle lines up with a line on the meter scale; (3) doubling the gain with the switch; and (4) vertically elevating the device until the output returns to the original value (i.e., the needle realigns with the meter line referred to in step (2)). Since the magnetic field strength is inversely proportional to the distance, the height of the unit above the ground at step (4) is equal to the depth of the cable. This procedure is accurate, but time consuming. It also becomes impractical for more deeply buried cables, requiring the operator to raise the device above his head.
Other currently used cable locating devices employ two antennas and logic circuitry to determine depth. The antennas are separated by a fixed distance. With this known separation distance and magnetic field strength readings at the antennas, cable depth can be computed. The difficulty with these devices is that there are practical limits regarding antenna separation. If the cable depth is much larger than the antenna separation, which is generally approximately 12 to 18 inches, signal strength measurement accuracy becomes more critical. Measurement accuracy is affected by differential drifting of the electronics associated with the antennas as well as differential responses of the antennas themselves.
Various approaches have been taken to enhance magnetic field strength measurement precision. The accuracy of these approaches increases as the number of components common to the two measurement circuits increases. Current systems accomplish this by taking a magnetic field reading at one antenna; switching the electronics connection from one antenna to the other; and measuring the magnetic field strength at the second antenna. Although this switching methodology eliminates many sources of error, one major error source remains—the antennas. To increase sensitivity, ferrite rods are sometimes employed to enhance the effective capture area of the antennas. As a result of the antenna separation, both antennas may not experience the same thermal environment. The characteristics of ferrite vary measurably with temperature and are not consistent between rods. Alternatively, large diameter air-core coils are employed. Such coils eliminate the inconsistency of the ferrite rods, but still exhibit thermal drift problems. Air-core coils also are generally larger in diameter.
All of these spatially separated two-antenna devices must be periodically calibrated. Any aging or drifting of an antennas will cause rapid loss in cable depth measurement accuracy, particularly at depths that are large compared to antenna separation. In cable locators, this is generally not a serious problem, since most cables are buried at depths of less than 2 or 3 times the separation.
A device conforming to the above-described arrangement is available from Radiodetection Ltd. (Bristol, England), the RD300. The device includes two antennas with horizontal coil axes disposed a fixed vertical distance from each other. In operation, the device is placed on the ground, such that a first receiving antenna sensor is near ground level (e.g., within about 1-2 inches) and a second receiving antenna is located about 16 inches thereabove. The ground therefore serves as a reference surface for depth measurement. One disadvantage of this particular prior art device and other devices that operate similarly thereto manifests itself when the reference surface exhibits an obstruction such as a curb, a rock, landscaping or the like, at a desired measurement location. Under these circumstances, an operator must compensate for the obstruction to obtain the depth below the reference surface. Another disadvantage of this equipment is that the depth measurement process is time consuming even after the device is properly located above the transmitter (i.e., a needle must be aligned with a meter line through a knob-actuated adjustment process). Radiodetection Ltd. applies this technology to cable, sewer and pipe location as well as horizontal boring tool monitoring.
The principal means of locating a boring tool head for guidance purposes is to place a radio frequency transmitter in the tool head, and track the tool from the surface using a radio frequency receiver that detects the alternating magnetic field emanati
Digital Control, Inc.
Pritzkau Michael
Snow Walter E.
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