Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01C—MEASURING DISTANCES, LEVELS OR BEARINGS; SURVEYING; NAVIGATION; GYROSCOPIC INSTRUMENTS; PHOTOGRAMMETRY OR VIDEOGRAMMETRY
  • G01C15/00—Surveying instruments or accessories not provided for in groups G01C1/00 - G01C13/00
  • G01C15/02—Means for marking measuring points
  • G01C15/04—Permanent marks; Boundary markers

Definitions

• the present invention relates to a marker of the electromagnetical resonance type and of the kind com ⁇ prising a cylindrical housing of a dielectric material housing a ferrite core, which, over a part of its length, is provided with an electrical coil in connec ⁇ tion with a condenser, such that these latter parts will form an electric resonance circuit having a predeter ⁇ mined resonance frequency.
• the housing is cast out with a sealing moulding mass so as to be completely sealed towards the surroundings.
• markers of this type are used as subterranean markers, as in a buried posi ⁇ tion they can be localized by means of special search detectors operating on the same frequency as the said resonance frequency. Thereby it is possible to mark and refind objects in the ground, e.g. pipe joints or splic ⁇ ing muffs on cables, or the markers can define base points in a coordinate system, in which the location of subterranean objects or conduits can be measured in, for subsequent easy refinding.
• a marker of this type is known from WO 83/01306, and such markers have been used in practice through several years. For natural reasons it has been endeav ⁇ oured to design the markers with a size as small as possible in view of the requirements from both a produc ⁇ tion and a utility point of view. Hereby it has been a practical and economical starting point that the markers have been based on standard manufactured ferrite cores for rod antennae, these having proved particularly well suited also for the present particular purpose, and thus it has been possible to make use of cheap standard pro ⁇ ducts in the form of rods with a diameter of 9 mm and a length of 20 cm. From a utility point of view this has been quite suitable, because the marker should produce, actuated by the detector, a resonance signal powerful enough to make it detectable in spite of the marker being placed well underneath the ground surface.
• the ferrite rod is mounted in a cylindrical housing having a noticeable wall thickness, this having the purpose of frequency stabilizing the markers, when these are mounted in sur ⁇ roundings with different dielectrical properties.
• the surrounding dielectricum had a clear influence on the resonance frequency, but later on new types of ferrite have been developed which are much less sensitive.
• the preferred resonance frequency is of the magnitude 100 kHz, and with the use of a relatively thin ferrite core it is then neces ary to use resonance coils having quite many windings.
• the coil wire is machine wound directly on the core, and even if there is used a very thin wire it is necessary to utilize almost the entire core length to hold the coil and to wind the coil with several layers of wind- ings.
• the ferrite cores even from the same series pro ⁇ duction, are not exactly uniform with respect to their magnetic properties, and if they are wound with a pre ⁇ scribed number of windings the result will be that the markers will exhibit a considerable spreading in their resonance frequencies. It is a possibility to work out an adjustment to the correct frequency by individually adjusting the number of the windings in the coils, but in a large production of the small and cheap markers this will imply a considerable complication.
• plural layer coils do not contribute to a high quality of the markers, because there will be a gap of non-magnetic material between the outer winding layers and the core. This will more than outbalance the advantage of the inner winding layer being wound direct ⁇ ly on the core.
• the relatively thick ferrite core would be slightly more effective if shorted only to a little more than half its original standard length, but when the starting material is a cheap standard product it is advantageous to select just half the length, because each core member can then be made into two markers.
• the core has a certain overlength, whereby the discussed 'half length', i.e. some 10 cm, has been found to be quite suitable. With the use of noticeably shorter cores it would occur occasionally that an adjustment to the working frequency would not be possible.
• the coil should be produced as a separate unit to be inserted on the core, and it is important that the coil has a rather close fit on the core. It would be possible to wind the coil on a very thin carrier tube, but with the invention it has been found that because of the relatively large wire thickness, e.g. 0.6 mm, it is also possible to produce a self carrying coil, viz. by coiling the wire and heating the coil for melting together the insulation coatings of the respective neighboring windings. If the coils are wound with an inner diameter of 9,5 mm they will be well suited for use on core rods of a standard diameter rang ⁇ ing between 9.2 and 9.4 mm.
• Fig. 1 is a perspective view of the elements of a marker according to the inven ⁇ tion
• Fig. 2 is a fragmentary view of a coil therein
• Fig. 3 is a schematic view illustrating the adjust ⁇ ment of a coil.
• a marker housing 2 shaped as a simple cutting of a plastic pipe, e.g. having inner and outer diameters of 19 and 20 mm, respectively.
• a ferrite core on which there is mounted a coil 6 connected with a condenser 8. The coil is secured to the core by means of adhesive tape 10.
• the unit 4-10 is introduced into the tube 2, where ⁇ after the latter is cast out with a suitable moulding mass, e.g. polyurethane.
• a suitable moulding mass e.g. polyurethane.
• Perforated collar spacers 12 can be used for centering of the core.
• the rod 4 is produced by breaking a ferrite core of a standard length of 20 cm at the middle thereof, upon a kerb being made at that place.
• the coil 6 is wound on a mandrel 14, which is slightly thicker than the core 4.
• the coiled windings 16 are shown with their associated insulating coating 18.
• the coil 6 is mounted on a core member 4 placed next to a search detector 20 operating on the frequency, to which the marker shall be tuned.
• the condenser 8 and the number of windings of the coil are beforehand adapted to an 'average correct' condi ⁇ tion, and the operator may then, by displacing the coil axially on the core, find the coil position by which the particular core gives rise to a maximum reading on a measuring instrument 22 of the detector 20.
• the coil is fixed in that position, e.g. simply with a piece of adhesive tape 10, whereafter the unit is ready for being cast in in a tube casing 2.
• a wire thickness of 0.6 mm which is more than required for the electrical function, but a resulting good stiffness and robustness of the coil will be achieved, and the solder ⁇ ing connection with the condenser will be facilitated. Only it should be ensured that the number of windings sufficient for the marker to be operative, given the thickness of the ferrite core, will not amount to a coil length that spoils the adjustment possibilities, when the windings are placed in one or perhaps only two layers, confer the wording of claim 1. In the above example it is satisfactory when the coil has a length of 4-6 cm on a 9 mm core having a length of 10 cm. It would be possible to select a slightly thinner wire to thereby shorten the coil and even the core, but with a main ⁇ tained or preferred core length of 10 cm there will normally be a sufficient free length of the core for the said overdimensioned wire to be used.

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• Physics & Mathematics (AREA)
• Engineering & Computer Science (AREA)
• General Physics & Mathematics (AREA)
• Radar, Positioning & Navigation (AREA)
• Remote Sensing (AREA)
• Coils Or Transformers For Communication (AREA)

Applications Claiming Priority (2)

Publications (1)

Family

ID=8109513

Family Applications (1)

Country Status (3)

Cited By (2)

Citations (2)

• 1990
  • 1990-08-23 DK DK202090A patent/DK202090D0/da not_active Application Discontinuation
• 1991
  • 1991-08-23 AU AU84033/91A patent/AU8403391A/en not_active Abandoned
  • 1991-08-23 WO PCT/DK1991/000246 patent/WO1992003703A1/en not_active Ceased

Patent Citations (2)

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