Background of the Invention Proposals for an electronic compass, to avoid drawbacks of the conventional needle compass, have been known for many years but have seen little practical application.

The present invention is believed to provide an electronic compass that, because it is simple, inexpensive and reliable, should find widespread application. The invention also provides, more generally, a sensitive, inexpensive approach to detection or interaction with magnetic flux.

Summary of the Invention According to one aspect of the invention, there is provided a device for detecting the presence or absence of magnetic flux in a chosen direction of the type comprising a lux-concentrating element adapted to be disposed in a region of interest, an excitation means for exciting vibrations in the element and a pickup means arranged to detect change in magnetic flux associated with the element, the flux-concentrating element being mounted to make and break mechanical contact with a second flux-concentrating element in the manner that a flux path of greater or lesser flux capacity occurs depending upon the relationship of the flux concentrating elements to each other, the excitation means comprising means to cause motion of one of the elements relative to the other of the elements to cause rapid, repeated making and breaking of mechanical contact between the elements and the pickup means adapted to produce a response to the attendant change of flux associated with the repeatedly contacting elements, the absence of such response denoting the absence of magnetic flux extending in the direction, and the flux-concentrating elements and the

excitation means being cooperatively related to produce irregular modulation of the spacial relationship between the flux-concentrating elements.

In preferred embodiments of this aspect of the invention, at least one of the flux-concentrating elements is loosely mounted to enable it to move in a disorderly fashion.

According to another aspect of the invention, there is provided a device for detecting the presence or absence of magnetic flux in a chosen direction of the type comprising a flux-concentrating element adapted to be disposed in a region of interest, an excitation means for exciting vibrations in the element, the excitation means comprising a hammer adapted to rapidly strike at least one of the flux concentrating elements, and a pickup means arranged to detect change in magnetic flux associated with the element, the flux-concentrating element being mounted to make and break mechanical contact with a second flux- concentrating element in the manner that a flux path of greater or lesser flux capacity occurs depending upon the relationship of the flux concentrating elements to each other, the excitation means comprising means to cause motion of one of the elements relative to the other of the elements to cause rapid, repeated making and breaking of mechanical contact between the elements and the pickup means adapted to produce a response to the attendant change of flux associated with the repeatedly contacting elements, the absence of such response denoting the absence of magnetic flux extending in the direction. In preferred embodiments, at least one of the elements is elongated and is mounted at a region near an end remote from the region of its contact with the other element..

According to another aspect of the invention, there is provided a device for detecting the presence or absence of magnetic flux in a chosen direction of the type comprising a flux-concentrating element adapted to be disposed in a region of interest, an excitation means for exciting vibrations in the element and a pickup means arranged to detect change in magnetic flux associated with the element, the flux-concentrating element being mounted to make and break mechanical contact with a second flux- concentrating element in the manner that a flux path of greater or lesser flux capacity occurs depending upon the relationship of the flux concentrating elements to each other, the excitation means comprising means to cause motion of one of the elements relative to the other of the elements to cause rapid, repeated making and breaking of mechanical contact between the elements and the pickup means adapted to produce a response to the attendant change of flux associated with the repeatedly contacting elements, the absence of such response denoting the absence of magnetic flux extending in the direction, the excitation means including an oscillator which oscillates at a characteristic frequency unsynchronized with the motion of the flux concentrating elements, the characteristic oscillation of the oscillator being at a substantially frequency and the making and breaking of contact between the flux concentrating elements being substantially irregular.

In preferred embodiments, the pickup means picks up a signal having, as a first component, the characteristic regular frequency of the excitation means and, as a second component, an irregular signal related to flux changes during making and breaking of the contact, an output signal from the pickup means consisting of the first component

without the second component denoting the absence of flux in the given direction.

According to yet another aspect of the invention, there is provided an electronic compass constructed in the manner described wherein the flux concentrating element is elongated and movable in a sweeping motion through a range of directions, the absence of the response denoting that the flux concentrating element is aligned in the East-West direction relative to the earth's magnetic field. In preferred embodiments of the invention, the excitation means comprises an electrically driven piezo crystal, or comprises an electromagnet.

In still another aspect of the invention, in a device incorporating a flux-concentrating means adapted to be mechanically excited by an excitation means in the presence of magnetic flux for modulating the flux path, there is provided an improvement wherein the mechanical excitation means and the flux-concentrating means are so related that the flux-concentrating means is excited in an irregular modulation of flux in the flux path, and a sensor responsive to the modulation to produce a noise signal when flux is present in the path.

In preferred embodiments of this aspect of the invention, the excitation of the flux-concentrating means is adapted to modulate the size of a gap in a flux path in an irregular manner; the flux-concentrating means comprises at least one reed of magnetic material, preferably the flux concentrating means comprises a pair of reeds of magnetic material that are loosely mounted and biased into light overlapping contact with each other whereby the excitation causes the reeds to make and break contact with each other as the size of the gap is modulated; and the excitation means is driven in an irregular manner and the reed is

mounted in a decoupled manner relative to the excitation means such that it moves in irregular motion when driven.

By way of example, according to one aspect of the invention, a flux concentrator made of two similar reeds is vibrated to break a closed flux path infinitesimally and at an irregular rate to produce an irregular modulation of the flux that can be readily detected by a pickup coil playing to earphones or a simple amplifier and loudspeaker. Also, in the prior art of electronic compasses with vibrating flux concentrators, it has been suggested that the vibrating element should be driven near or at its natural mechanical frequency. I have discovered, for present purposes, the converse is true. An extremely inexpensive and accurate device can be achieved using a flux concentrator that is mechanically tapped so as to vibrate non-coherently or in an irregular manner.

The term "non-coherent" or "irregular" as used here is intended to imply that energy is dispersed over numerous mechanical frequencies, a condition that can be called "noisy". While the term is not intended to exclude energy present at the fundamental frequency, or its simple harmonics, it does imply that much of the energy is spread over other frequencies so as to give the overall effect of noise to the ear.

According to one aspect of my invention I have, in a sense, discovered that "cheaper is better" for a class of products which previously may have been thought to require precision. I can use inexpensive, loosely fitted parts made without close tolerances and inexpensive entertainment electronic components to achieve the desired effect.

Another example of an aspect of the invention is a sonic device for detecting the boundary or null regions of a magnetic field, by generating sonic noise from ambient flux,

and employing the ear to discriminate between presence and absence of such noise. Leakage currents from the driver of the flux concentrator are a clear tone, amply swamped by sonic noise.

These and numerous other advantageous features are employed in the achievement of the preferred compass.

Another example of the invention, more generally, is that of breaking a closed magnetic circuit non-coherently so as to efficiently generate electronic noise in the infinitesimal temporary gaps thus formed and lost, which may find use not only in compasses and magnetometers, but also in certain classes of amplifiers and current generators where a vibratory motion of a flux concentrator may be employed. Another example is the use of simple piezoelectric or electromagnetic drive systems for the excitation of the sensing elements.

These and other features and advantages will be understood from the following description of presently preferred embodiments of the invention, and from the claims.

Description of Presently Preferred Embodiments

The preferred embodiments will be described with reference to the figures.

Fig. 1 is a side view of an embodiment of an electronic compass; Fig. 2 is a plan view of the embodiment of Fig. 1; and Fig. 3 is a plan view of the piezoelectric transducer employed in the embodiment of Figs. 1 and 2. Fig. 4 is a plan view and Fig. 5 is a side view, partly in section, of another embodiment which has an electromagnetic driver.

Referring now to Figs. 1 and 2, two elongated flux- concentrating reeds 12, 14 are aligned along axis A in a slightly overlapping relationship. Each reed is formed of

magnetic metal (M5 Silicon Steel) of length, L, 1-1/4 inches (3.18 cm); width, W, 1/8 inch (3.18 mm); and thickness, t, 0.010 inch (0.25 mm). The length of overlap, L _0L , is 1/64 inch (0.40 mm) .

The elements are loosely mounted on posts 16, 18 at their remote ends in a manner which biases them into light contact in the region of overlap when the compass is not energized.

A pickup coil 20 surrounds the region of overlap. It is connected, via amplifier 22, to speaker 24. The interior of the coil restrains the reeds from lateral motion.

Post 18, mounting the end of reed 14, is secured to fixed frame 26 while post 16, mounting the end of reed 16, is secured to the margin of piezoelectric transducer 28, which in turn is secured to the frame.

In the operating breadboard design, which these figures represent, the piezoelectric transducer and 2.8 Khz oscillator 30 are taken from an inexpensive piezoelectric alerting device (Sonalert©, SNP 4 from P.R. Mallory & Son) . The pickup coil 20 is the 1000 ohm winding of an output transformer from an inexpensive transistor radio, with armature removed. The amplifier 22 and speaker arrangement, 24, are taken fr^m a portable cassette player (Model 892 from Lennox Sound) .

Referring to Fig. 3, the piezoelectric crystal is of disc form with electrodes provided to enable excitation and feedback to the oscillator. In known manner, the piezoelectric disc and a brass substrate to which it is secured, are caused to vibrate between concave and convex, dished shapes, with excursions in the direction normal to its plane. A hammer 32 is formed on the piezoelectric

transducer in a region of greatest excursion, arranged to contact the underside of reed 12 adjacent the region of overlap, to repeatedly strike the reed 12. Whereas the piezoelectric transducer and feedback controlled oscillator per se form a highly resonant electro-mechanical system, the loosely mounted reeds form a relatively non-resonant, untuned system, comparable to cymbals, for example.

Because of this construction, the transducer taps the reed intermittently and irregularly, because reed 12 continually changes its position relative to the transducer at the time of contact with the hammer. The second reed 14 is jangled through its loose contact with the first reed, its loose constraints preventing it from damping the first reed excessively as well as allowing it to participate in the disorderly motion.

The reeds, thus excited, can be heard to make a slight tinny sound and the intermittent contact between them makes a slight rustling sound. If the reeds are in a magnetic path, the output of the coil, played to the speaker, sounds roughly like noise.

When a very slight flux is present, noise from the loud speaker is heard, but when no flux is present, the noise disappears. The system is thought to be extraordinary (for its class) in detecting small magnetic fields, and is particularly notable for finding the east-west null in the earth's magnetic field by simply scanning the device in a horizontal arc. Discrimination between east and west is readily achieved by sensing the rise of the null towards compass north or the sinking to the south.

As a possible explanation of the sensitivity that has been observed, it is thought that the wide distribution of frequencies so reduce the energy at any one frequency that magnetostrictive effects are abated. Also conversion

of the frequency of the external mechanical force to alien noise energy reduces input and output coupling, to make discrimination by the ear easy. Noise is peculiarly advantageous for nulling by ear, and it can be pulsed 3 times a second, e.g., to fight wind noise.

Referring to Figs. 4 and 5, the reed 12 in this case is driven by an elongated bar 40 mounted at nodes 42 to vibrate in the manner of a xylophone bar. Hammer 32 at one end of the bar strikes the loosely mounted reed 12 in the manner previously described, while the opposite end of the bar is driven by an electromagnetic drive system 44. For this purpose, magnet 46 is carried by the bar, arranged to interact with electromagnet 48 which is driven by a control chip 50 powered, e.g., by 3 volt battery. The bar, for instance, may be 1/8 inch (3.18 mm) diameter brass tubing, 7 inches (17.75 cm) in length, adapted to resonate at 100 Hz. The electromagnet drive system may be taken from a STAR MMB Buzzer sold by Meshna of Lynn, Massachusetts, the pickup coil may be a telephone pickup and the reeds may each be 1- 1/2 inches (3.8 cm) by 1/4 inch (6.35 mm) by .010 inch (0.25 mm) M5 silicon steel.

In this design, the pickup, being quite remote from the drive, does not respond substantially to stray oscillating energy of the drive.

Other embodiments for delivering noisy agitation to a flux path, and in particular to a gap of a flux path, are within the spirit and scope of the claims.

What is claimed is: