Measuring and testing – Navigation – Ship's log
Patent
1985-08-21
1987-03-31
Woodiel, Donald O.
Measuring and testing
Navigation
Ship's log
73187, G01C 2110
Patent
active
046533206
DESCRIPTION:
BRIEF SUMMARY
SUMMARY OF THE INVENTION
The present invention relates to an apparatus for measuring and displaying the speed of a craft, especially of a sailboard, comprising an immersed rotator which is pivotally mounted and arranged to be drawn by the craft, at least one permanent magnet integral with the rotator, its poles being diametrally opposite each other with respect to the axis of rotation of the rotator, and a measuring device equipped with a display unit and arranged to be mounted on the craft.
Various types of known rotator logs for measuring and displaying the speed of a craft are connected to a speed indicator on board the craft by means of a mechanical or electrical transmission line. In this type of apparatus, the transmission line passes through the hull of the craft and this leads to sealing problems and more difficult upkeep. To meet this problem, an apparatus has been designed wherein the rotator is integral with a permanent magnet whose rotating field induces, across a hull of non-ferromagnetic material, a voltage in one or two coils mounted within the hull, that is to say at a relatively great distance from the magnet.
In an apparatus of this type described in U.S. Pat. No. 4,159,644, magnetic field variations are detected by a combination of two magnetometers with alternating current excitation. In the presence of a magnetic field, a voltage is obtained at the magnetometer output, whose amplitude and polarity represent the difference between the axial field components in the two magnetometers. This device allows detection of the frequency of rotation of the magnet by eliminating stray effects of magnetic fields whose components are simultaneously equal in the two magnetometers, for example the terrestrial field. In this embodiment, this complex device is necessary to detect the field variations of the magnet at a sufficient distance and when the latter rotates at slow speed. The magnetic field intensity in fact decreases so rapidly with increasing distance that these variations cannot be detected reliably by conventional induction coils at distances in the order of a decimeter and with relatively low frequencies.
This apparatus thus comprises relatively complicated and costly detecting means. It must moreover be excited with alternating current which is generally not available on a light dinghy, and certainly not on a sailboard.
The object of the present invention is to provide a simple and inexpensive apparatus which is preferably removably mounted and allows measurement and display of the speed of a craft having no electrical energy source, especially a sailboard, and which requires no electrical or mechanical transmission, nor piercing of the hull.
This object is achieved in accordance with the invention by an apparatus of the above mentioned type which is characterized in that the measuring device comprises a coil arranged to induce an electrical signal representing the magnetic field variations generated by rotation of the magnet, at least one amplifying device arranged to amplify the signal while eliminating very low frequencies, and a frequency meter for determining the frequency of the signal after amplification.
This coil should comprise at least 2,000 turns in order to provide a sensitivity which is sufficient for slight magnetic field variations.
According to a preferred embodiment, the apparatus comprises a source of continuous current consisting of photovoltaic cells and the display unit is of the liquid crystal type.
The rotator is preferably made of a synthetic material and the magnet is accommodated within the rotator.
As no external, mechanical or electrical connection is necessary for this measuring device, particularly if it is powered by long-life cells or photovoltaic cells, the apparatus may comprise a sealed box having a transparent window and enclosing the entire measuring device, including the continuous current source. Built in miniaturized form, this box may be removably mounted on the craft by attaching means allowing rapid fixation, and may thus be taken away by the u
REFERENCES:
patent: 4159644 (1979-07-01), Svala
patent: 4507960 (1985-04-01), Hufnagel et al.
patent: 4535626 (1985-08-01), Wakayama
patent: 4586374 (1986-05-01), Saynjakangas
Carrard Jean-Daniel
Laforge Eric
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