Abstract:
The invention relates to a magnetic switch having a yoke which has the shape of a rotationally symmetrical cup on the base of which is arranged a permanent magnet. The flux of the permanent magnet is conducted via a core, which has a radial core flange, to the plate-shaped armature. For the operational air gap effective between the armature and the core and the yoke and the magnetic resistance W A  thereof, the following relation applies in relation to the marginal air gap formed between the core flange and the yoke: W A &lt;W R . As a result of the rotational symmetry with a yoke having the shape of a cup, it is possible to generate a high holding force for the armature with a small structural volume.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a magnetic switch having a cup-shaped yoke, a core, a coil, an armature and a fixed contact and a movable contact. 
     DESCRIPTION OF THE PRIOR ART 
     Magnetic switches of this kind are known, for example, from EP 0 442 311 A2, which discloses a magnetic switch in which a non-magnetic armature is moved by a permanent magnet fixedly connected thereto. During this, in the direction of movement of the armature, only an operational air gap between the permanent magnet and the core brings about the effect. Furthermore, the known magnetic switch has a small coupling surface between the permanent magnet and the core, which extends only over the inner region of the cup-shaped yoke. 
     The known magnetic switch has the disadvantage that the coupling surface between the permanent magnet and the core, and thus the holding force of the magnet system, is only very small relative to the structural volume of the magnetic switch. This means that the known magnetic switch is not suitable for use as a battery disconnecting switch in a motor vehicle, where on the one hand large currents of a few thousand amps have to be switched, and where on the other hand vibrations and jolts act on the magnetic switch. To achieve a greater holding force, it would be possible for special magnetic materials which generate a very strong magnetic field to be used. These have the disadvantage that they are expensive to procure and complicated to process. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is therefore to provide a magnetic switch which presses the movable contact onto the fixed contact with a high holding force while having a small structural volume. 
     This object is achieved by a magnetic switch having a yoke that has the shape of a cup with an axis of symmetry. The cup has a base and a wall and is open to one side. Arranged in the cup is a core which has a radial core flange at its end facing the base. This radial core flange forms with the wall of the cup a marginal air gap with a width of H R . This marginal air gap has a magnetic resistance of W R . Furthermore, the cup contains a plate-shaped permanent magnet which is magnetised parallel to the axis of symmetry of the cup. The permanent magnet is arranged between the core flange and the base and is magnetically coupled thereto. Moreover, arranged in the cup is a coil whereof the coil axis is the axis of symmetry of the cup. The cup is covered by a plate-shaped armature which forms with the yoke a yoke air gap with a width of H J  and forms with the air gaps together form, connected serially one behind the other, the operational air gap, which has a magnetic resistance of W A . Moreover, there is arranged on the magnetic switch a means for lifting the armature axially away from the cup. In the closed condition of the magnetic switch, the following applies: W A &lt;W R . 
     As a result of the construction of the magnetic switch, in accordance with the invention, a magnetic switch is produced which is shielded from the outside extremely well and has only extremely low scattering losses for the magnetic fields. Because the armature forms an operational air gap with the core and the cup-shaped yoke, a coupling surface which is as large as possible is formed with a particular predetermined structural volume. The large coupling surface between the armature and the yoke and the core respectively results in a large holding force of the magnetic switch in the closed condition. Because the magnetic resistance of the operational air gap is smaller than the magnetic resistance of the marginal air gap, it is ensured that the majority of the magnetic flux of the permanent magnet is conducted through the armature in the closed condition. The system polarised by the permanent magnet moreover has the advantage that only a short switch pulse has to be sent through the coil in order to close the switch, and the coil current can be returned to zero in the closed condition. This means, in particular, that low-power operation of the magnetic switch is possible. 
     In order to achieve the maximum holding force of the armature, it would be desirable for the ratio W R /W A  to be as large as possible. This would ensure that all of the magnetic flux of the permanent magnet is guided through the armature without losses being produced by way of the marginal air gap. A very large ratio W R /W A  would, however, mean that a high counter-excitation of the coil would be required to open the switch, in order to displace the magnetic field of the permanent magnet out of the core and guide it through the marginal air gap and the wall of the cup, back onto the underside of the permanent magnet. Consequently, for the magnetic switch to have a high sensitivity to switching off, it would be desirable for the ratio W R /W A  to be in the order of magnitude of one. Thus, it is particularly advantageous to set the ratio W R /W A  such that both the holding force of the armature and the sensitivity of the magnetic switch to switching off are in a reasonable range. Tests have shown that this is the case if the ratio W R /W A  is selected to be between 1 and 100. A ratio W R /W A =50 has been shown to be particularly advantageous. 
     A magnetic switch according to the invention may for example readily be produced with the following dimensions: for the cup, a radial extent of between 10 mm and 50 mm is chosen. With a size such as this, a compact construction is possible for the magnetic switch, as is specifically necessary for its use as a battery disconnecting switch in a motor vehicle. For the width of the yoke and core air gaps, purely for reasons of the superficial roughness of the underside of the armature or of the surfaces of the cup wall and the core, the following range can readily be set: 0.005 mm&lt;H J &lt;0.05 mm and 0.005 mm&lt;H K &lt;0.05 mm. If a wall thickness of the cup of approximately 1 mm is chosen, moreover, then a surface for the yoke air gap with a size of approximately 100 mm 2  is obtained. If the core air gap or the marginal air gap is constructed to have approximately the same order of magnitude as regards their surfaces, then to fulfil the relation mentioned above between the magnetic resistances of the operational and the marginal air gaps, the following apply: 1 mm&gt;H R &gt;0.1 mm. The said dimensions of the core and yoke air gaps relate to the closed condition of the switch. With a magnetic switch constructed in accordance with these example dimensions, a holding force of approximately 30 N can be achieved. The axial extent of the switch corresponds approximately to the radial extent. A magnetic switch of this type can be switched at a switching capacity of approximately 0.7 W. 
     Particularly advantageous is a magnetic switch in which the yoke, permanent magnet, core, coil and armature are arranged rotationally symmetrically about the axis of symmetry. As a result of the rotationally symmetrical arrangement according to the invention of the elements of the magnetic switch about the axis of symmetry, an optimum flux concentration is achieved in the interior of the switch without magnetic field losses occurring at corners or edges. Moreover, a magnetic switch with rotational symmetry is easy to produce. 
     Furthermore, particularly advantageous is a magnetic switch in which the permanent magnet is of a material containing barium ferrite. Barium ferrite is a readily available, inexpensive material for a permanent magnet, which greatly simplifies production of the magnetic switch. Because of the high holding force of the magnetic switch according to the invention, expensive highly magnetised special materials for the permanent magnet can be dispensed with. 
     Furthermore, particularly advantageous is a magnetic switch in which the yoke and/or the core are of a material which contains magnetically soft iron. Magnetically soft iron can very readily be demagnetised, which simplifies the displacement of the flux of the permanent magnet out of the core when the magnetic switch is opened. Furthermore, magnetically soft iron has a low magnetic resistance, as a result of which it is ideal for conducting the flux of the permanent magnet to the armature. 
     A high holding force of the magnetic switch may be achieved in particular in that the core and the yoke are dimensioned, at least in the region of the corresponding air gaps, in cross-section such that in the closed condition and in the absence of excitation, that is to say when the coil current is 0, there is almost magnetic saturation. 
     Furthermore, it is particularly advantageous for the armature to be constructed in accordance with the invention with a radially varying plate thickness such that there is almost magnetic saturation in the entire armature in the closed condition of the magnetic switch. For this, it is in particular possible to make the armature thinner at the edge than in the centre, since the cross-sectional surface available for the magnetic flux increases with the distance from the axis of symmetry. The thickness of the armature can be lessened accordingly with increasing distance from the axis of symmetry. This makes it possible to reduce the mass of the armature to the absolutely minimum amount required. A minimised armature mass of this kind has the advantage that the magnetic switch is less sensitive to vibrations or shocks, since in this case it is primarily the inert mass of the armature which determines the extent to which the armature can respond to rapid accelerations. 
     Furthermore, the magnetic switch may be constructed to be particularly compact if, according to the invention, a hole running along the axis of symmetry is provided in the yoke, the permanent magnet, the core and the armature, and through this hole runs a pin. This pin is secured to the armature at one end, and at the other end is coupled to a movable contact which makes contact with a first fixed contact in the closed condition of the magnetic switch. With the aid of the movable contact and the fixed contact, the circuit of a motor vehicle, for example, may be switched. 
     Moreover, particularly advantageous is a switch in which the movable contact is a flange-shaped contact disc which is arranged symmetrically with respect to the axis of symmetry and has a central hole. In this case, the pin is secured in the hole in the armature by means of flange-shaped end portions and is suspended in the contact disc from a collar projecting into the contact disc hole. The contact disc makes contact with a first and a second fixed contact in the closed condition. Arranged in the hole in the permanent magnet is a pressure spring contact supported against the armature and the contact disc. The end of the pin remote from the armature is pre-tensioned in the direction of the armature via a rocker by means of a tension or pressure spring. The rocker according to the invention, which is coupled to the pin, is a simple means of raising the armature axially away from the cup. The pressure spring contact supported against the armature and the contact disc has the advantage that the holding force of the armature can be transmitted over a certain distance, regardless of the spacing between the fixed contacts and the base of the cup. 
     Here, it is particularly advantageous to select the length of the pin, in accordance with the invention, such that when the magnetic switch opens, the lower pin flange moves by an overtravel distance Hu before it engages with the contact disc. This overtravel has the advantage that, when the magnetic switch opens, first the armature is greatly accelerated, together with the pin, as a result of the pressure spring contact, and only once the lower pin flange has engaged with the collar are the contacts pulled open by the forces of inertia thereby gained. The overtravel distance is preferably set in a range of between 0.5 mm and 1 mm. 
     Furthermore, particularly advantageous is a magnetic switch in which the pin is suspended in a spring contact secured to the second fixed contact and bearing the movable contact. The arrangement according to the invention, of a spring contact on the second fixed contact, has the advantage that it is possible to dispense with the pressure spring contact inside the cup. This means that the construction of the spring contact is no longer determined by the dimensions of the cup, but that it can be shaped and arranged outside the cup in almost any desired way. 
     For the spring contact, a possible material is for example spring steel or a nickel/iron alloy. Although these materials have a very good spring action, they are not suitable for conducting electrical current from the second fixed contact to the movable contact, because of their poor conductivity. Furthermore, it is particularly advantageous for the movable contact to be electrically connected to the second fixed contact by means of a litz wire. This litz wire may for example be of copper, which conducts electrical current very well. 
     So that the magnetic switch can be operated even if the circuit fails, it is particularly advantageous to arrange on the rocker an actuating element which can be used to open the switch manually. This actuating element moreover has the advantage that it can be arranged such that its position can be visually recognised on the switch from the outside. This means that the means of manually opening the switch can be used as an indicator of the condition of the switch at the same time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a first embodiment of the magnetic switch according to the invention, in diagrammatic cross-section. 
     FIG. 2 shows a second embodiment of the magnetic switch according to the invention, in diagrammatic cross-section; 
     FIG. 3 shows the contact assembly of the magnetic switch from FIG. 2, in plan view; 
     FIG. 4 shows a magnetic switch installed in a housing; 
     FIG. 5 shows the magnetic switch illustrated in FIG. 4, without the cover; and 
     FIG. 6 shows the armature holding force, the contact force acting on the fixed contacts, and the spring force of the pressure spring on the rocker, as a function of the spacing between the armature and the cup, illustrated in a measurement graph. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a magnetic switch which is constructed to be substantially rotationally symmetrical with respect to the axis of symmetry  2  that has a yoke  1  in the shape of a cup  3 . The cup  3  has a base  4  and a wall  9 . It is made of a magnetically soft material such as iron. A permanent magnet  5  is laid on the base  4 . The permanent magnet  5  is made for example of barium ferrite. It is magnetised parallel to the axis of symmetry  2 . As a result of the permanent magnet  5  coming into contact with the base  4 , a magnetic coupling is produced between these two bodies. 
     A core  6  having a core flange  8  is arranged on the permanent magnet  5 . The core  6  is made of a magnetically soft material such as iron. The core flange  8  comes into contact with the permanent magnet  5  so that there is a magnetic coupling between these two bodies. A coil  7  which is wound onto a coil bobbin  42  is arranged on the core  6 . The cup  3  is covered by a plate-shaped armature  11 . 
     The armature  11  may be made from a magnetically soft material such as iron. In the closed condition of the magnetic switch, the armature  11  is placed on the cup  3 , with a core air gap  12  being formed between the core  6  and the armature  11  and a yoke air gap  13  being formed between the yoke  1  and the armature  11 . 
     The core air gap  12  and the yoke air gap  13  may for example result simply from the superficial roughness of the armature  11  and the yoke  1  or the core  6 . It may, however, also result from a protective coating on the surface of the armature  11 , which is applied at a defined thickness for example by vapour deposition, sputtering or galvanic deposition, and which may be for example of copper or nickel. With the aid of a protective coating of this kind, the width H K  of the core air gap  12  can be matched precisely to the width H R  Of the marginal air gap  10 . 
     For the width H J  of the yoke air gap  13  and the width H K  of the core air gap  12 , the following applies: 0.005 mm&lt;H J &lt;0.05 mm and 0.005 mm&lt;H K &lt;0.05 mm. Between the core flange  8  and the wall  9  of the yoke  1  there is a further air gap, the marginal air gap  10 . The yoke air gap  13  and the core air gap  12  together form, connected serially one behind the other, the operational air gap of the magnetic switch. The operational air gap has a magnetic resistance of W A . The marginal air gap has a magnetic resistance of W R . 
     In order, as already described above, to set a ratio of W R /W A  which is suitable for the holding force and sensitivity to switching off of the magnetic switch, with a typical diameter of the magnetic switch of 30 mm and a thickness of the wall  9  of 1 mm, and with a dimension of the core  6  which can be derived from this with a properly scaled drawing, a width H R  for the marginal air gap of between 1 mm and 0.1 mm is chosen. Thus, a holding force for the armature  11  of 30 N can be achieved for a magnetic switch of this type. 
     The armature  11  is constructed as regards its plate thickness, varying over the radius, such that in the closed condition of the magnetic switch there is as complete as possible magnetic saturation of the armature  11 . This means that the thickness of the armature  11  lessens with increasing distance from the axis of symmetry  2 . This makes it possible to construct the armature  11  to be preferably low in mass, so that the magnetic switch becomes insensitive to those jolts and vibrations which occur in particular in motor vehicles. 
     The opened condition of the switch is achieved by raising the armature  11  by a spacing s in the direction of the axis of symmetry  2 . The armature  11  is coupled to a contact disc  17  by means of a pin  14 . The pin  14  has an upper pin flange  24  by means of which it is secured to the armature  11 . 
     The contact disc  17  is in the shape of a flange with an inwardly projecting collar  25 . The pin  14  is connected axially movably to the contact disc  17  by means of a lower pin flange  21 . The contact disc  17  is a movable contact  15  which bridges a first fixed contact  16  to a second fixed contact  18 . The contact disc  17  may be for example of copper. 
     The fixed contacts  18 ,  16  are made from electrically conductive material and may for example be injection moulded into a plastics housing. In the interior of the cup  3  there is arranged a pressure spring contact  19  which is supported against the armature  11  and the contact disc  17 . Since the lower pin flange  21  is movable within the contact disc  17  by an overtravel distance H U , secure contact between the contact disc  17  and the fixed contacts  16 ,  18  is ensured even if the contacts  15 ,  16 ,  18  are reduced in thickness by contact erosion. The pressure spring contact  19  transmits the holding force of the armature  11  to the contact disc  17 . At the lower end of the pin  14 , there is arranged a rocker  20  (illustrated only diagrammatically here) which presses the armature  11  upwards into the opened position by spring tension. By applying a coil current across the coil  7 , a strong magnetic field is produced which pulls the armature into the closed position. 
     As already described above, the magnetic resistances of the operational and marginal air gaps W A , W R  are set such that in the closed condition almost the entire magnetic flux of the permanent magnet  5  is conducted through the core  6  into the armature  11 . The holding force exerted by the permanent magnet  5  on the armature  11  is sufficiently large that the coil current can be switched off once the closed condition is reached. Thus, low-power operation of the magnetic switch is possible. 
     In an example embodiment having the dimensions mentioned above, it was possible for the armature  11  to be moved from the opened to the closed condition with a switching capacity of 0.7 watts. Low-power operation of this kind for the magnetic switch is particularly important in cases where electrical power is only available to an extremely limited extent, such as in motor vehicles. 
     In order to open the magnetic switch, a current opposed to the coil current used to close the magnetic switch is sent through the coil  7 . This displaces the magnetic field of the permanent magnet  5  out of the core  6  and thus out of the armature  11 . The pressure spring contact  19  tensioned between the armature  11  and the contact disc  17  now raises the armature  11  to such an extent that the lower pin flange  21  engages with the collar  25  of the contact disc  17 . As a result of the force of inertia obtained in this first switching component, the contact disc  17  is now raised away from the fixed contacts  16 ,  18 , whereby the current flowing through the fixed contacts  16 ,  18  is reliably interrupted. Once the contact disc  17  has been raised away from the fixed contacts  16 ,  18 , the armature finally attains the opened position of the magnetic switch, where it is held by the rocker  20  until either the armature  11  is moved manually back into the closed position or else a new switching procedure takes place by means of the coil  7 . 
     FIG. 2 shows a magnetic switch according to the invention, having a yoke  1 , an axis of symmetry  2 , a cup  3  with a base  4  and a wall  9 , a permanent magnet  5 , a core  6  and a coil  7 , with the cup  3  being covered by an armature  11 . The armature  11  forms with the core  6  a core air gap  12  and with the yoke  1  a yoke air gap  13 . As regards the parts mentioned, the magnetic switch is of exactly the same construction as the magnetic switch shown in FIG.  1 . 
     The armature  11  is coupled to the spring contact  22  by means of a pin  14  which runs in a pin sleeve  26  running through the centre of the cup  3 . The spring contact  22  bears the movable contact  15 . In the closed state of the magnetic switch, the movable contact  15  is electrically connected to a first fixed contact  16  which for its part is secured to a contact beam  27 . The spring contact  22  is secured to a second fixed contact. Possible materials for the spring contact  22  are in particular spring steel or a nickel/iron alloy. Since such materials conduct electrical current poorly, the movable contact  15  is brought into contact with a litz wire  23  by means of solder  30 . The litz wire  23  is fixedly connected to the second fixed contact  18  and of flexible construction so that it follows the movement of the spring contact  22 , even if the magnetic switch is actuated frequently, without tearing off as a result of material fatigue. Arranged on the underside of the pin  14  is a rocker  20 , the functioning of which is clear from FIG.  3 . 
     FIG. 3 shows the second fixed contact  18 , to which the spring contact  22  is secured. With the aid of the spring contact  22 , the second fixed contact  18  may be connected to the first fixed contact (not illustrated in FIG.  3 ), which is secured to the contact beam  27 . Arranged on the underside of the spring contact  22  is a movable contact (not illustrated in FIG.  3 ). This movable contact is connected by means of solder  30  to a litz wire  23 , which for its part is secured by means of solder  30  to the second fixed contact  18 . The spring contact  22  is connected to the second fixed contact  18  by riveting  31 . Arranged below the spring contact  22  is a rocker  20  pivotal about a rocker axis  29 . Arranged on the side of the rocker  20  opposite the spring contact  22  is for example a pressure spring  28  which keeps the armature in the opened condition. 
     FIG. 4 shows the magnetic switch according to the invention, installed in a housing  34  having a cover  35 . By means of a diaphragm  36  arranged above the armature  11 , the magnetic switch may be closed manually. Laterally with respect to the magnetic switch, load terminals  33  are guided into the housing  34 . These load terminals  33  may for example be encased inside the housing  34  by injection moulding. Instead of the pressure spring  28  shown in FIG. 3, it is also possible for an actuating element  32  which is guided upwards out of the housing  34  to be secured to the rocker. With the aid of the actuating element  32 , the magnetic switch may be opened manually. 
     FIG. 5 shows the housing  34  in the opened condition. Next to the magnet system  37  of the magnetic switch and the actuating element  32  there is provided in the housing  34  a chamber  38  in which for example switching electronics for the magnetic switch may be accommodated. Laterally with respect to the magnetic switch, load terminals  33  are guided into the housing  34 . 
     FIG. 6 shows the armature holding force  39 , the contact force  40  acting on the fixed contacts, and the spring force of the pressure spring  41  on the rocker as a function of the spacing s between the armature and the cup, illustrated in a measurement graph for the embodiment of the magnetic switch illustrated in FIG. 1, in the closed condition. When s=0, the armature holding force  39  is approximately 35 N. The armature holding force decreases sharply as the spacing s between the armature and the cup increases, as a result of the weakening of the magnetic field as the operational air gap grows larger. Similarly, the contact force  40  decreases as the spacing s between the armature and the cup increases, although only linearly, in accordance with the force/distance curve of a spring. Similarly, the spring force of the pressure spring  41  decreases linearly with increasing spacing s between the armature and the cup, in accordance with the force/distance curve of a spring. From a spacing s between the armature and the cup of approximately 0.8 mm, the contact is opened, as a result of which the contact force  40  goes down to zero. At this spacing, the armature holding force  39  is still only very small, being still approximately 1 N, as a result of which the armature is held in the opened position by the spring force  41  of the pressure spring. 
     The invention is not restricted to the embodiments indicated by way of example, but is defined in its most general form by claim 1.