Optical angle measuring instrument

{
  "publication_number": "US4958927A",
  "country": "US",
  "kind": "A",
  "title": "Optical angle measuring instrument",
  "abstract": "A manually-rotatable switch knob is provided on the alhidade of a sextant that is connected to a switching pin of hardened steel arranged eccentrically to the mounting of the knob. The switching pin engages the drum-screw mounting and, under the action of a stop spring, permits the drum screw to be switched back and forth between two stable stop locations when the knob is rotated. In the one stop location, the drum screw engages the limbus toothing. In the other location, the drum screw is completely disengaged from the limbus toothing so that the alhidade can be freely pivoted without risk of the drum screw damaging the limbus toothing during the movement of the alhidade. The stop spring also produces precisely defined contact pressure between the drum screw and the limbus toothing. The previously possible introduction of bending moments into the alhidade during rough adjustment is eliminated.",
  "claims": [
    "1. In an optical angle measuring instrument of the type that includes an alhidade that is pivotally mounted on a limbus and has a free end, a measuring drum that is pivotally articulated on the free end of the alhidade perpendicularly to the axis of an alhidade bearing and having a drum screw engaged to a limbus toothing, the improvement comprising: (a) a manually rotatable switching member; (b) said member being mounted on said alhidade and rigidly connected to a switching pin passing through a slot in an alhidade plate; (c) said pin being located eccentrically with respect to a switching member mount; (d) said pin being so located that said drum screw, under the action of a stop element, is switchable between two stable stop locations in response to rotation of said switching member; and (e) said drum screw being arranged to engage said limbus toothing in one stop location and to disengage said limbus toothing in said other location to permit said alhidade to pivot freely.",
    "2. An angle measuring instrument as defined in claim 1 further characterized in that: (a) said switch member is a switching knob on said alhidade; and (b) said stop element is a stop spring.",
    "3. An angle measuring instrument as defined in claim 2 further characterized in that: (a) said stop spring abuts said switching pin in a contact region; (b) said stop spring is a wire-bow spring having a stop cam in the contact region with said switching pin that is displaced against the action of said wire-bow spring by said switching pin when said switching pin is rotated; and (c) said cam is so located as to lock said switching pin into one of two stop locations determined by the length and arrangement of a slot in said alhidade and said alhidade plate.",
    "4. An angle measuring instrument as defined in claim 3 wherein said stop spring is fitted into an adapted clearance of said alhidade.",
    "5. An angle measuring instrument as defined in claim 3 wherein said stop spring is fitted into said alhidade plate connected to said alhidade.",
    "6. An angle measuring instrument as defined in claim 3 further characterized in that: (a) a three-point bearing pivotally secures said measuring drum with drum screw; (b) the bearing point most remote from said measuring drum is formed by a first pivot bearing; (c) a second bearing point is formed by a displaceable bearing pin; and (d) a third bearing point is formed by said switching pin located outside the line joining said other bearing points.",
    "7. An angle measuring instrument as defined in claim 6 wherein said three-point bearing is located on said alhidade.",
    "8. An angle measuring instrument as defined in claim 6 wherein said three-point bearing is located on said alhidade plate."
  ],
  "description_excerpt": "1. Field of the Invention\n\nThe present invention relates to optical angle measuring instruments. More particularly, this invention pertains to an improved sextant.\n\n2. Description of the Prior Art\n\nThe rough setting of the alhidade along the arc of the limbus toothing requires disengagement of the drum screw (connected to the measuring drum) from the limbus. In the prior art, the drum screw bearing articulated at the lower free end of the alhidade is disengaged by a spring-biased catch and pivoted so that the drum screw is disengaged from the limbus toothing. The disengaging member and the catch are relatively far removed from the limbus toothing and the engagement region of the drum screw. As a consequence, a bending moment can act on the alhidade if the actuation force acting on the catch does not act in at least an approximately tangential direction with respect to the limbus arc. As a result, indirectly caused measuring errors may occur. Additional measuring errors or inaccuracies can arise as a result of the fact that the point of engagement of the (normally conical) drum screw is slightly displaced by unavoidable production tolerances of the alhidade toothing. The moment of contact pressure, which acts, for example, through a bent leaf spring in the measuring drum bearing onto the end of the drum screw remote from the region of engagement may change when the catch is (unknowingly) slightly pressed by the user. A 0.1 mm diameter offset in the radial direction in the region of engagement of the conical drum screw can alter the lead angle by about 50\" (sec).",
  "cpc": [
    "G01C 1/08"
  ],
  "ipc": [
    "G01C 1/08"
  ],
  "assignees": [
    "C Plath Fabrik Nautischer Instrumente"
  ],
  "inventors": [
    "Gerd Finnern"
  ],
  "filing_date": "1989-01-23",
  "publication_date": "1990-09-25",
  "grant_date": "1990-09-25",
  "priority_date": "1988-01-20",
  "application_number": "US-29987389-A",
  "family_id": "6345631",
  "cited_by_count": 1,
  "citations": [
    "US2439102A",
    "US3968570A"
  ]
}