Geometrical instruments – Indicator of direction of force traversing natural media – Level or plumb – terrestrial gravitation responsive
Patent
1993-11-30
1995-09-26
Wirthlin, Alvin
Geometrical instruments
Indicator of direction of force traversing natural media
Level or plumb, terrestrial gravitation responsive
33346, 33355R, G01C 906, G01C 1702
Patent
active
054525195
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to attitude sensors and in particular to such sensors as may be used in or to form compasses, attitude and rotation rate measurement devices or combinations of these.
Magnetic compasses and other types of attitude sensors, in a form that can be read by computers and other electronic devices, are important in numerous fields, sea surface and subsea vessels, land vehicles, aircraft, rockets, and so on. Existing sensors have had many forms and paths of development, and are often combined to give specific types or sets of data.
Traditional magnetic compasses use a needle or card which points toward magnetic north, and some form of suspension to allow its surroundings to rotate around it. As compasses have been developed to be electrically readable, this basic arrangement has been modified, with many forms of gimbals and suspension. Other approaches have used various non-moving forms of magnetic sensors, sometimes coupled to other sensors which determine a local gravitational reference from which essentially the same information can be derived.
Physical analysis shows that to obtain a true `magnetic north` reading it is actually also necessary to have a second reference usually the vertical which defines the horizontal plane in which it is defined; even in the simplest case where the compass is simply mounted on a horizontal surface, this reference is implicit since it was used in some form to level the surface. This is so (except on the magnetic equator), because the couple on the needle is such as to make it want to align with the earth's magnetic field lines, which normally dip below the horizontal. A simple compass card is kept level against the dip torque by a very much larger gravitational torque. Any error in levelling of a simple compass in the magnetic east-west direction (or of the reference used in a static magnetic sensor) will lead to quite significant compass reading errors.
Any gravitational reference is however inevitably affected by linear accelerations applied to its package, since these cannot be distinguished from changes in the direction of the gravitational field. This inevitably leads to compass reading errors when the compass package is subject to motion. In many situations the accelerations are short term, and probably reversing or cyclic, so these effects may be reduced by the use of some form of mechanical damping. This solution unfortunately agravates the effects of the other possible type of external motion, where the package is rotated.
These effects are interactive, and can produce complicated compromises in performance.
It is fundamental that no device that is subject to sustained linear acceleration can find a true gravitational reference. It is equally certain that no acceleration can be extended infinitely in time. A solution can therefore be found in the majority of practical cases, by reducing the effects of the influence of acceleration, if the natural period of the device in question can be made very much longer than the duration of the applied acceleration.
In the case of external rotations, the tendency of any object is to stay stationary in space. Any tendency to rotate with the surroundings is due to the coupling of this motion from the outside. In the case of a compass in gimbals this is due to friction in the gimbal bearings.
The situation is slightly different in analysis between the cases of frictional influence on an object with both a magnetic and a gravitational moment (ie a compass card of some form), and that of one with no such moment (a rate sensor), and other cases of where the influence is magnetic only, and/or gravitational only. However in general terms the conclusion of the analysis is a re-statement of the simplest laws of mechanics; for a body to stay still in space (and thus act as the heart of a sensor of this sort) it should have the lowest frictional coupling to the outside world, and the highest moment of inertia possible (within its practical constraints).
According to the invention there is provided an attitude sen
REFERENCES:
patent: 4344235 (1982-08-01), Flanders
Patent Abstracts of Japan, vol. 11, No. 79 (P-555) (2526), Mar. 11, 1987: Abstract of Japanese Published Patent Appln. No. 61-237011 (Oct. 22, 1986).
Soviet Inventions Illustrated, Week E02, Feb., 1982, Derwent Publications, Ltd., Abstract of SU 823 885 (Apr. 23, 1981).
Crocker Timothy R.
Rees Anthony I.
3D Instruments Limited
Wirthlin Alvin
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