Abstract:
A magnetic field sensor is disclosed with which the direction of a magnetic field can be determined by using the planar Hall effect. An active layer of the sensor is made of a ferromagnetic amorphous metal which is magnetically isotropic. Thus, the magnetization in the active layer always has the direction of an external magnetic field. The developed planar Hall voltage is a measure of the direction of the magnetic field.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a magnetic field sensor, and more particularly to a position sensor for a rotatable shaft or the like. 
     PRIOR ART STATEMENT 
     For a long time the Hall effect has been used to measure magnetic fields. When an electric current flows through a conductor, and a magnetic field simultaneously passes through this conductor in a direction perpendicular to the electric current, a field is produced perpendicular to these two directions whose strength can be taken across the surface of the conductor as a voltage. Less known is the fact that a field is also produced in a direction perpendicular to the direction of current flow, in a plane determined by the magnetic field and the direction of current flow. This effect is referred to as the &#34;planar Hall effect&#34;. 
     A magnetic field sensor which makes use of the planar Hall effect consists, for example, of a circular sheet-metal disk showing a magnetic anisotropy whose preferred direction lies in the disk. The same has four terminals evenly spaced along its circumference. They form two intersecting current paths one of which runs parallel to the preferred direction of the magnetic anisotropy. Such magnetic field sensors are commonly constructed using thin film techniques. See, for example, V. D. Ky, &#34;Planar Hall Effect in Ferromagnetic Films&#34;, Phys. Stat. Sol., Vol. 26, page 565 (1968). 
     Prior art magnetic field sensors do not operate well in such a way that the direction of a magnetic field can be determined with it. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a magnetic field sensor predicated on the following discovery: When a magnetic field H is applied across a magnetically anistropic magnetic field sensor in a direction perpendicular to the preferred direction of the magnetic anisotropy, a magnetization results whose direction makes a predetermined angle with that preferred direction. The predetermined angle is dependent on the quotient of the intensity of the external magnetic field, H, and the intensity of the anisotropy field, H k . The Hall voltage obtainable from the sensor electrodes as a result of the planar Hall effect is dependent upon the strength H of the applied magnetic field exclusively via the said predetermined angle. 
     The voltage developed is thus a direct measure of the direction of the magnetization in the sensor. If the direction of the magnetization can be made equal to the direction of the external magnetic field H, it is possible to determine the direction of a magnetic field. This can be done with a magnetic field sensor constructed in accordance with the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top plan view of a magnetic field sensor constructed in accordance with the present invention; 
     FIG. 2 is a diagrammatic cross-sectional view of the sensor of FIG. 1; 
     FIG. 3 is a graph of an output voltage of the sensor of FIGS. 1 and 2 versus the direction of an applied magnetic field; 
     FIG. 4 is a top plan view of another magnetic field sensor constructed in accordance with the invention; and 
     FIG. 5 is a cross-sectional view of a position sensor with a magnetic field sensor constructed in accordance with the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the magnetic field sensor 10 of the present invention will now be described in more detail with the aid of FIGS. 1 and 2. Four pad electrodes 13-16 are deposited on the upper side of a circular disk shaped substrate 11. The pad electrodes 13-16 are arranged radially in the outer region of substrate 11 spaced at 90° intervals. They are made of gold with a chromium bonding layer. A circular active layer 12 is deposited concentrically on the substrate 11 and covers parts of the electrodes 13-16. It is made of Co 75  Fe 5  B 20  and deposited by sputtering in the form of a layer 100 to 500 nanometers thick. Its diameter is approximately 5 to 20 mm. The layer may also be deposited by evaporation. It is magnetically isotropic. The isotropy can be achieved by annealing in a rotating magnetic field (4 h at 260° C., 400 A/m). 
     Two opposite electrodes 13 and 14 of the magnetic field sensor 10 may be connected to a current source. When this current carrying sensor 10 is placed in an external magnetic field H, a parallel magnetization is obtained in the active layer 12. The magnetic field H and the magnetization can then make an angle φ with the direction of current flow. Between the two electrodes 15 and 16, which are not traversed by current, the Hall voltage 
     
         U.sub.H ˜sin 2φ 
    
     is developed. 
     Instead of a quartz substrate 11, a glass, semiconductor or ceramic substrate could be used. The electrodes 13-16 could also be made of any other suitable material. 
     The active layer 12 can also be made of other ferromagnetic amorphous metals. An alloy of Co x  Fe y  B 100-x-y , where 70≦x≦80 and 4≦y≦10 (x, y in atom %), proved especially suitable for this purpose. Experiments were preferably made with Co 75  Fe 5  B 20  and Co 80  B 20 . 
     It is also possible to use other amorphous metals which are alloys based on transition elements of the iron group. Such an alloy may contain one or more metalloids (B, C, Si, Ge, P) as well as titanium, zirconium, hafnium, and/or niobium. Up to five percent (atom %) of the amorphous metal may consist of other elements. 
     FIG. 3 shows the result of a measurement made on a magnetic field sensor such as sensor 10. The ordinate represents the Hall voltage U H  measured between the electrodes 15 and 16. The absicissa represents the angle between the direction of current flow and the direction of the magnetic field. During the measurement, a direct current of 100 mA was flowing between the electrodes 13 and 14. The magnetic field strength H was 40 Oe. In the example shown, the Hall voltage lies between about -120 μV and +100 μV. 
     Both the equation and the curve of FIG. 3 show that the result of the measurement is ambiguous. Unambiguity is obtained only in four angular ranges each including 90°. By using two like magnetic field sensors spaced 45° apart, the direction of a magnetic field can be unambiguously determined, but not the sense of the field. If the sense is to be unambiguously determined, too, two additional magnetic field sensors are necessary. To unambiguously determine the direction, the substrate 11, which is coated with the first active layer 12, is coated with a second active layer of the same kind. The two active layers cover each other. They are isolated from each other, e.g., by being deposited on different sides of the substrate 11. The second active layer has four pad electrodes, too; they are spaced 45° from those of the first active layer. 
     FIG. 4 shows an embodiment of a magnetic field sensor 10&#34; whose active layer 12&#34; is provided with a total of eight pad electrodes 13&#34;-16&#34; and 13&#39; and 16&#39;, which are spaced at 45° intervals. Current is supplied (box 40) to the sensor 10&#34; through one of the four opposite electrode pairs 13&#34;/14&#34;, 13&#39;/14&#39;, 15&#34;/16&#34;, or 15&#39;/16&#39; by means of multiplex controller 42. The Hall voltage is taken across the respective perpendicular electrode pair (via multiplex controller 42) and is measured by sensing circuit 44. By making successive measurements, the direction and the sense or polarity of the external magnetic field can thus be determined. The measurements must, of course, follow one another at such short intervals that the direction of the magnetic field does not change in the meantime. In case of a change in the direction of the magnetic field, it depends on the application whether a complete set of measured values is used to determine the direction anew or whether the direction determined last is evaluated as well. 
     FIG. 5 shows a position sensor 20 with which the angular position of a shaft 21 can be determined. The shaft 21 turns in a ball bearing 23 mounted in an axle box 24. At one end of the shaft 21, a permanent magnet 22 is attached normal to the axis of the shaft. A magnetic field sensor 10&#39;&#34; is attached to the axle box 24. It is located opposite the end of the shaft 21 so that its active layer 12&#39;&#34; is concentric with the shaft 21. Of the magnetic field sensor 10&#39;&#34; the substrate 11&#39;&#34;, the active layer 12&#39;&#34;, and a case 17 are shown. 
     Such a position sensor can be used to advantage for determining rotary motions of an arrangement. In the case of an automobile, for example, the rotary motion of a drive wheel can be evaluated to determine the speed of travel and the number of kilometers covered. The engine speed can also be determined in this manner. 
     Not only the speed of rotation but also the respective angular displacement can be sensed accurately. 
     The arrangement just mentioned or a corresponding arrangement makes it possible to electrically initiate rotation angle dependent processes in an internal combustion engine. Such processes that can be electrically initiated are the ignition and the opening and closing of electrically operated valves.