Magnetic sensor including a coil surrounding a pair of magnets, for detecting rotary motion

A device for determining rotary motions generally includes a signal generator with inwardly directed teeth or inwardly directed projections and a double-sided measuring coil which is arranged within the surface of the signal generator. The measuring coil has a core of permanent magnets and pole shoes. The pole shoes project from both sides of the coil and extend up to the inner surface of the signal generator. The pole shoes are designed, arranged and conformed to one another so that, whenever the end piece of a pole shoe points to a tooth or a projection, the second, diametrically opposite end of the pole shoe is directed to a tooth gap or depression. The measuring coil core is made up of at least two permanent magnets which are magnetized perpendicularly to the coil axis and respectively embedded between two pole shoes.

BACKGROUND OF THE INVENTION 
The present invention relates to a device for determining rotary motions 
and for generating an electric alternating signal representative of the 
rotary motion, including a signal generator which undergoes the rotary 
motion and, in the form of a ring, wheel, hollow cylinder, or a similar 
element, is provided with inwardly directed teeth or projections, and a 
stationary, inductive transducer which is arranged within the surface of 
the signal generator and is designed as a double-sided measuring coil that 
comprises a core of permanent magnets and elongated pole shoes which 
project from either side of the measuring coil up to the signal generator, 
wherein the inwardly directed teeth or projections and the pole shoes are 
designed, arranged and conformed to each other so that, whenever an end 
piece of a pole shoe points to a tooth or a projection, the second, 
diametrically opposite end piece of the pole shoe faces a tooth gap or 
depression. 
German patent application No. 42 13 977 discloses a device of this type. In 
this device, a double-sided measuring coil is arranged in a signal 
generator ring with inwardly directed teeth. The measuring coil has a core 
made of a permanent magnet magnetized perpendicularly to the axis of the 
measuring coil. Mounted on the pole surfaces of the permanent magnet, i.e. 
between the core and coil, is one elongated pole shoe which projects from 
either side of the coil and reaches up to the inwardly directed teeth. The 
pole shoes and the inwardly directed teeth are offset from each other, or 
rather conformed to each other so that whenever a pole shoe end points to 
a tooth, the diametrically opposite pole shoe end points to a tooth gap. 
Further, a rotational speed measuring device is disclosed in German patent 
application No. 39 27 007 which also has a toothed disc with inwardly 
directed teeth as a signal generator. Inside the signal generator, there 
is a diametrically aligned measuring coil. The coil encloses a magnetic 
flux guide piece the length of which approximately corresponds to the 
inside diameter, reduced by the necessary distance to allow an air gap 
between the ends of the flux guide piece and the teeth of the signal 
generator. The flux guide piece protrudes from both sides of the coil. 
Permanent magnets, oriented such that they are magnetized perpendicularly 
to the magnetic flux guide piece, are placed on the ends of the flux guide 
piece. If there are signal generator teeth opposite the two magnets of a 
coil side, a magnetic flux forms from the respective outside of the 
magnets through the toothed disc to the flux guide piece and then to the 
inside of the magnets. When the tooth gaps are opposite the two magnets, 
the main flux passes through the air gap to the flux guide piece and the 
insides of the magnets. Thus, on the two coil sides the magnetic flux is 
alternating through the magnetic flux guide piece depending on whether 
tooth gaps or teeth are opposite the magnets. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved rotational 
speed measuring device of the type initially referred to, above all, to 
the effect of producing a higher useful signal without increasing the 
mounting space required. Also of special interest is achieving minimum 
possible manufacturing costs. 
It has been found that this object can be achieved by a device including 
the provision of a coil core which, in the present invention, has at least 
two permanent magnets which are arranged parallel to the coil axis, 
magnetized perpendicularly to the measuring coil axis and respectively 
inserted between two elongated pole shoes. Appropriately, the adjacent 
permanent magnets, isolated by a pole shoe, respectively, are magnetized 
with opposed polarity. 
In a preferred aspect of the present invention, the device includes a 
signal generator having an uneven number of teeth or projections and a 
measuring coil having a core which is made up of two elongated permanent 
magnets embedded between three pole shoes. 
If the signal generator has an even number of teeth or an even number of 
projections, in another aspect of the present invention, a measuring coil 
is appropriately used which has a core that includes an uneven number (at 
least three) of elongated permanent magnets. The permanent magnets are 
embedded between a corresponding number of pole shoes (i.e. at least four 
pole shoes). 
Further details of the present invention can be taken from the following 
description of embodiments making reference to the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 explains the principal design and the arrangement of a wheel bearing 
sensor embodying the advantages which are achieved by the measuring device 
as provided by the present invention. 
FIG. 1 shows a wheel bearing 1 intended for use in an automotive vehicle. 
Generally, wheel bearing 1 includes a wheel bearing outside ring 2, a 
wheel bearing inside ring 3 into which a wheel shaft 4 is mounted, and 
wheel bearing balls 5. The axis of rotation of the wheel bearing 1 is also 
indicated by a dash-dotted line in FIG. 1. 
A sensor housing 6 is connected to the stationary outside ring 2 attached 
to the vehicle. A transducer or sensor, designated by reference numeral 7 
as a whole, is inserted or engaged in the sensor housing. A measuring coil 
8 is arranged in the sensor housing. A bowl-type structure is provided as 
a transducer 9 and is seated on the inside ring 3 of the wheel bearing to 
rotate along with this inside ring. At the level of the measuring coil 8, 
the bowl-type structure has a signal generator ring 9 with inwardly 
directed teeth (not shown) or with undulations. 
FIG. 2 shows the design of the measuring coil 8, which is essential for the 
present invention, and the arrangement of the individual components of the 
measuring coil 8 relative to the inwardly directed teeth, or, as in the 
present example, relative to the projections 10 and depressions 11 of an 
undulated structure which is used as signal generator 9. The projections 
10, when viewed from the double-sided measuring coil 8, correspond to the 
teeth of a prior art signal generator of identical function, which is 
provided as a toothed wheel with inwardly directed teeth. The depressions 
11 correspond to the tooth gaps. The illustrated structure of the signal 
generator 9 can be made, for example, in a favorable manner by 
deep-drawing a member made of thin sheet steel. This method is very 
inexpensive compared to the manufacture of a signal generator with 
inwardly directed teeth. 
The most important components of the double-sided measuring coil 8 is a 
coil member 12, on which a coil winding 13 is mounted, and a coil core 14. 
Core 14 in FIG. 2 includes two elongated permanent magnets 15, 16 and 
three pole shoes 17, 18, 19 which are applied to the surfaces of the 
permanent magnets 15, 16. The permanent magnets 15, 16 are magnetized 
perpendicularly to the coil axis 22, indicated by a dash-dotted line, and 
have a reversed polarity. The north poles NN of the two permanent magnets 
15, 16 meet on the intermediate pole shoe 18 in FIG. 2. 
It is important for the function of the measuring device of the present 
invention that the pole shoes 17, 18, 19 project from both sides of the 
measuring coil 8 to extend (almost) as far as the inside of the signal 
generator ring 9. Further, the arrangement and the design should be so 
that when the end piece of a pole shoe is directed to a tooth or a 
projection 10 of the signal generator 9, the second, diametrically 
opposite end of the pole shoe is opposite a tooth gap or a depression 11 
in the undulated structure shown. In the embodiment of FIG. 2 and the 
illustrated position, the upper (with respect to the drawing) end pieces 
of the pole shoes 17 and 19 point to teeth or projections 10, and the 
lower end pieces of the pole shoes point to tooth gaps or depressions 11. 
In the situation shown in FIG. 2, the intermediate pole shoe 18 upwards 
points to a depression 11 and downwards to a projection 10. 
The measuring coil of FIG. 2 is especially appropriate for a signal 
generator having an uneven number of teeth. In this case, the measuring 
coil 8 can be supported centrally in the signal generator ring 9. Then, 
there is no need for a special shaping or bending of the pole shoes to 
position or adapt them to the teeth. 
When use of a signal generator with an even number of teeth is desired, it 
is suitable to provide the measuring coil with a core having an even 
number of pole shoes, for example, four pole shoes and three permanent 
magnets on which the pole shoes abut. 
In some cases, i.e. when a particularly high useful voltage is required, it 
may be necessary to subdivide the coil core still more precisely. FIG. 3 
shows an example. Five pole shoes 21 are provided and project from both 
sides of a measuring coil 8'. In turn, the pole shoes are separated by 
respectively one, i.e., by four permanent magnets 20 in total. In the 
embodiment shown, a relatively small pole shoe distance of only 1/2 tooth 
pitch was chosen. Appropriately, a plastic-embedded magnetic material was 
chosen for the manufacture of the permanent magnets 20 in a like core 
structure. 
In the embodiment of FIG. 3, the permanent magnets 20 are also magnetized 
perpendicularly to the coil axis 22 and polarized so that in each case 
like poles meet each other on the boundaries isolated by the pole shoes 
21. 
The arrangement and design of the present invention permits achieving an 
increased useful signal in a relatively simple fashion, as compared to 
prior art measuring devices of this type. Alternatively, a larger air gap 
or a larger air gap tolerance is rendered possible, if necessary.