The present invention relates to a magnetic switch arrangement according to the preamble to claim 1 and a method for obtaining a differential magnetic switch according to claim 15. This magnetic switch arrangement allows for improved magnetically operated switches.

In modern vehicles, there are many functions that are controlled electronically. Some of these functions are of the on/off type, some can be switched to several positions and some are analogue. Directly coupled switches and sensors control most functions, but some functions require a contact-less operation. An example of functions where a contact-less operation is preferred is e.g. ABS-sensors (ABS = Automatic Brake System), chassis height detection or switches that are exposed to weather, pollutions and direct friction. One kind of contact-less switches and sensors are based on a magnetic principle. There exist different types of magnetic detectors, e.g. reed-contacts, hall-sensors and other kinds of integrated magnetic detectors. A magnetic field is used to influence the detector. The detector and the magnet thus form the switch or the sensor.

To obtain a switch or a sensor with a high resolution and which at the same time is insensitive to external magnetic fields, it is desirable to position the magnet and the detector close to each other. In this way, it is possible to use a detector with a low sensitivity, obtaining a switch or a sensor that is insensible to external magnetic fields.

One problem with magnetic switches and sensors is that the sensitivity of the detector must increase with an increased detection distance. For some applications, especially for magnetic switches, it may be possible to overcome the increased distance with a larger or stronger magnet with a stronger magnetic field.

A problem with the detector being very sensitive is that it will more easily be disturbed by an external, interfering magnetic field. This can e.g. occur when the sensor is close to a high current cable or a large transformer. Thus, it is preferred not to raise the sensitivity too much for the detector.

A problem that arises when the magnetic field is increased by using a larger magnet is that the magnetic field is not only stronger, it is also more distributed in space. This gives the effect that, when an analogue detector is used, the resolution will be degraded due to the imprecise magnetic field.

Due to the nature and to the production process of permanent magnets, the magnetic properties for magnets can vary considerably, even if they are manufactured in the same batch and at the same time. Properties that vary are e.g. the magnetic remanence and the direction of the magnetic field. These varying properties in turn can cause magnetic switches and sensors to behave different even if the specifications are equal. In production, this can cause considerably problems with adjustments and rejected parts.

The document " FR-A- 1 404 208 " discloses a magnetic switch arrangement according to the preamble of claim 1.

The object of the invention is therefore to provide an improved magnetic switch arrangement that is less sensitive to variations in the magnetic properties of the used permanent magnets and a method for obtaining a differential magnetic switch.

The solution to this problem according to the invention is described in the characterizing part of claim 1 concerning the magnetic switch arrangement and in claim 15 for the method. The other claims contain advantageous embodiments and further developments of the magnetic switch arrangement according to the invention.

With a magnetic switch arrangement, comprising a first magnetic system, a second magnetic system and a magnetic switching element, wherein the first magnetic system is arranged for biasing the magnetic switching element and the second magnetic system is arranged to interact with the biasing magnetic field from the first magnetic system at the magnetic switching element so that the magnetic switching element is in a predefined state, the object of the invention is achieved in that the second magnetic system comprises two equally polarised permanent magnets positioned at a predefined distance apart.

By this first embodiment of the magnetic switch arrangement according to the invention, a differential magnetic switch is obtained. This allows for single-unit magnetic switches that are less sensitive to deviations in the magnetic properties of the used permanent magnets.

In an advantageous further development of the magnetic switch arrangement according to the invention, the first magnetic system comprises a magnetic field assembler arranged for creating a longitudinal magnetic field inside the assembler. The advantage of this is that the assembler creates a uniform magnetic field for the magnetic switching element. The angular sensitivity of the magnetic switching element is thus compensated for.

In an advantageous further development of the magnetic switch arrangement according to the invention, the space between the magnets and/or the sides opposite the space between the magnets is/are supplied with a ferro-magnetic material. This makes it possible to adapt the magnetic switch to the desired requirements by controlling the magnetic field.

In an advantageous further development of the arrangement according to the invention, the magnets of the second magnetic system are positioned such that any deviation in the magnetic field direction in respect to the symmetry axis for each magnet is symmetric in respect to a central line between the magnets. This compensates for any deviation in the direction of the magnetic field of each magnet.

In an advantageous further development of the magnetic switch arrangement according to the invention, the magnets of the second magnetic system are obtained by dividing a single magnet into two equal parts along a line parallel to the symmetry axis and where one magnet is rotated 180 degrees around its symmetry axis. This compensates for any deviation in the direction of the magnetic field of each magnet and creates a magnetic system with a magnetic field that is symmetric.

In an advantageous further development of the magnetic switch arrangement according to the invention, the magnetic switch arrangement is integrated in one housing. The advantage of this is that an integrated magnetic switch is obtained that does not require an external magnet to function.

In an advantageous further development of the magnetic switch arrangement according to the invention, the magnetic switch arrangement is a normally open switch. The advantage of this is that it can be connected to a suitable electrical logic system.

In an advantageous further development of the magnetic switch arrangement according to the invention, the magnetic switch arrangement is a normally closed switch. The advantage of this is that it can be connected to a suitable electrical logic system.

In an advantageous further development of the magnetic switch arrangement according to the invention, the magnetic switch arrangement is switched by bringing a ferromagnetic material close to the magnetic switch arrangement. The advantage of this is that the magnetic switch arrangement can be used to detect e.g. when a door is closed.

In an advantageous further development of the magnetic switch arrangement according to the invention, the magnetic switch arrangement is switched by removing a ferromagnetic material from the magnetic switch arrangement. The advantage of this is that the magnetic switch arrangement can be used to detect e.g. when a door is opened.

By the first embodiment of the method for obtaining a differential magnetic switch with a predefined state, comprising a first magnetic system, a second magnetic system and a magnetic switching element, the first magnetic system is positioned so that the magnetic switching element is biased, and the second magnetic system is positioned so that the magnetic field from the second magnetic system interacts with the biasing magnetic field at the magnetic switching element. Thus, a differential magnetic switch is obtained.

The invention will be described in greater detail in the following, with reference to the embodiments that are shown in the attached drawings, in which

• Fig. 1a shows a known magnet,
• Fig. 1b shows a cut section of a known magnet with magnetic field lines,
• Fig. 2a shows a magnetic arrangement included in the invention,
• Fig. 2b shows a cut section of the magnetic arrangement according to 2a with magnetic field lines,
• Fig. 3a - 3c shows a schematic relationship between the magnetic flux density B for a magnet and the distance D,
• Fig. 4a shows an embodiment of the magnetic arrangement included in the invention,
• Fig. 4b shows a cut section of the embodiment according to 4a with magnetic field lines,
• Fig. 5a shows an embodiment of the magnetic arrangement included in the invention,
• Fig. 5b shows a cut section of the embodiment according to 5a with magnetic field lines,
• Fig. 6a shows an embodiment of the magnetic arrangement included in the invention,
• Fig. 6b shows a cut section of the embodiment according to 6a with magnetic field lines, and Fig. 7 shows a first embodiment of the inventive magnetic switch according to the invention.

The embodiments of the invention with further developments described in the following are to be regarded only as examples and are in no way to limit the scope of the protection provided by the patent claims.

Fig 1a shows a known permanent magnet 1. Fig 1b shows a cut section of the magnet 1 along a plane 2 through the middle of the magnet with some schematic magnetic lines indicated with dash dotted lines. The shown magnet is rectangular and symmetrically polarised with a north pole, denoted with an N, and a south pole, denoted with an S. The magnet can be made from any suitable material.

Below, when a magnetic arrangement is described and shown as a cut section, it is a similar cut through the middle of the magnetic arrangement that is used to illustrate the magnetic arrangement with schematic magnetic lines, also indicated with dash dotted lines. It is also assumed that the magnetic field is symmetrical along its symmetry axis 7, a centre line running from N to S in the middle of the magnet.

In figure 2a, a magnetic arrangement 3 comprising two permanent magnets 4, 5 is shown. Preferably, the magnets have approximately the same magnetic properties. It is advantageous if the magnets are made out of the same material and have the same geometric outline, but some deviations are acceptable. As the skilled person will appreciate, the terms "equal" or "the same" for the magnetic properties of permanent magnets will have the meaning "as close as possible" or "approximately the same" due to the nature and to the production process of permanent magnets.

The magnets 4, 5 are equally polarised and positioned next to each other in a symmetrical way with their symmetry axes 7 parallel and with the polarisation in the same direction, as can be seen in figure 2a. The distance between the magnets is denoted with D. Positioned in this way, the magnets will repulse each other, and more specific the north pole of magnet 4 will repulse the north pole of magnet 5 and the south pole of magnet 4 will repulse the south pole of magnet 5. Because the magnets are fixed in relation to each other, the magnetic force between the magnets cannot move the magnets. Instead, the magnetic field from the magnets will deform symmetrically in respect to a plane in between the magnets, indicated as the centre line 6 in figure 2b.

In this example, rectangular magnets are used. The size of the magnets depends on e.g. the desired magnetic field strength. Depending on the desired magnetic field, other geometric shapes are also possible. E.g. bars where one side is much longer than the other sides or circular ring magnets are possible to use. It is important that the magnets are positioned so that they repulse each other, preferably with the north pole and south pole positioned next to each other, side by side. The sides closest to each other are preferably flat.

In figure 2b, the magnetic field lines are deformed somewhat. When the distance D between the magnets is decreased, the magnets will repulse each other and the outer magnetic field at the north and south pole will increase, i.e. the magnetic flux density will increase. A schematic relationship between the magnetic flux density B for a magnet and the distance D is shown in figure 3a - 3c. Fig. 3a shows the magnetic flux density B for two magnets at a distance when the magnets do not affect each other.

At a certain distance, the magnetic flux density B will superimpose so that the magnetic field will be approximately equal between the symmetry axes 7 of the magnets. At this distance, the magnetic field will be as wide as possible with an equal density. This distance is denoted the critical distance d. If the distance D is decreased further, the magnetic flux density B will continue to superimpose and when the magnets touch, the magnetic field will equal that of a single magnet with the size of the two magnets combined.

Fig. 3b shows the magnetic flux density B for two magnets at the critical distance d where the magnetic field will be approximately equal and as wide as possible. The resulting magnetic field from Fig. 3b can be seen in Fig. 3c.

The critical distance d depends on various magnetic properties of the magnets. The critical distance d is small compared to the magnets. As an example, the critical distance d for two ceramic type magnets with the size 12*6*4 mm can be approximately 0.9 mm. The easiest way to obtain the critical distance d is by empirical measurements.

The appearance of the magnetic flux density along line 6, i.e. how pointed the magnetic flux density is, can be altered somewhat by adjusting the distance D. At the critical distance d, the magnetic flux density is as flat and wide as possible. In some cases, it may be desirable to have a magnetic flux density that is somewhat wider and not as flat. For instance, if the magnetic arrangement is to be used for a magnetic switch, the switch can obtain a larger tolerance with a magnetic flux density that is somewhat altered. In this case, the distance between the magnets is extended somewhat.

This well-defined magnetic field can be used in a number of applications, of which a few will be described below. Preferably, the magnetic arrangement is used for various contact-less detectors.

One way to improve the magnetic arrangement 3 as shown above is to use pole-pieces. Figure 4a, shows a magnetic arrangement 12 comprising two magnets 4, 5 and two pole-pieces 9, 10. Preferably, the magnets have approximately the same magnetic properties. It is advantageous if the magnets are made out of the same material and have the same geometric outline, but some deviations are acceptable. The resulting effect is a normalisation of the magnetic field.

A pole-piece is made of a ferromagnetic material and is positioned at a side of a magnet. A pole-piece will collect and lead the magnetic field through the pole-piece instead of through the air. This alters the magnetic flux density in that the magnetic field will be concentrated in the pole-piece. Thus, a high magnetic flux density that is embedded in the pole-piece is obtained. The size of a pole-piece corresponds to the magnet at which it is positioned, and the thickness of the pole-piece is configured so that no saturation in the pole-piece occurs.

The pole-pieces 9, 10 are positioned at the outer sides of the magnets, that is pole-piece 9 is in close contact with the right side of magnet 4 and pole-piece 10 is in close contact with the left side of magnet 5, as can be seen in figure 4a. The thickness of the pole-pieces is chosen so that no saturation in the pole-piece occurs.

A schematic view of the resulting arrangement 12 is shown in figure 4b. In comparison with the arrangement 3 from figure 3b, the magnetic flux density around the outer sides of the arrangement is concentrated closer to the arrangement. In combination with the in space-dispersed magnetic field obtained in between the magnets, this concentration of magnetic flux density at the outsides of the magnets also helps to reduce disturbing influences from the magnetic field of the magnets. Since the magnetic field from the two outer sides of the magnets are embedded in the pole-pieces and also symmetric, the resulting magnetic field is very stable in geometry.

Another magnetic arrangement 13 is shown in figure 5a, where the magnetic arrangement 13 comprises two magnets 4, 5 and a pole-piece 11. Preferably, the magnets have approximately the same magnetic properties. It is advantageous if the magnets are made out of the same material and have the same geometric outline, but some deviations are acceptable.

The pole-piece 11 is laminated between, that is in contact with, the two magnets 4, 5. The thickness of the pole-pieces is chosen so that no saturation in the pole-piece occurs.

The pole-piece 11 will collect and lead the magnetic field through the pole-piece instead of through the air. This alters the magnetic field around the centre line 6 in that the magnetic field will be more concentrated. Thus, a high magnetic flux density that is embedded in the pole-piece is obtained. This type of magnetic arrangement can be used e.g. in combination with a linear displacement sensor comprising a coil where a softmagnetic core is to be saturated. The saturation area of the core influences the coil such that the position of the saturated area, and thus e.g. the piston in a hydraulic cylinder, can de detected.

Another magnetic arrangement 14 is shown in figure 6a, where the magnetic arrangement 14 comprises two magnets 4, 5 and three pole-pieces 9, 10 and 11. Preferably, the magnets have approximately the same magnetic properties. It is advantageous if the magnets are made out of the same material and have the same geometric outline, but some deviations are acceptable.

The pole-pieces 9 and 10 are positioned to the outer sides of the magnets, that is pole-piece 9 is in close contact with the right side of magnet 4 and pole-piece 10 is in close contact with the left side of magnet 5. The thickness of the pole-pieces 9, 10 are chosen so that no saturation in the pole-pieces occurs. The pole-piece 11 is laminated between, that is in contact with, the two magnets 4, 5. The thickness of pole-piece 11 is chosen so that no saturation in the pole-piece occurs. With this embodiment, a high magnetic dispersed flux density that is more equally distributed is obtained.

Above, different approaches using a magnetic arrangement for obtaining a well-defined magnetic field are described. These magnetic arrangements are preferably used in magnetic switches.

In the above magnetic arrangements, it is assumed that the magnetic field of a magnet is symmetrical along its symmetry axis 7, a centre line running from N to S in the middle of the magnet. This is, however, rarely the case for normal production permanent magnets. Instead, the direction of the magnetic field deviates with an angle in respect to the symmetry axis 7. This deviation is normally comparably small, in the region up to 10 degrees, but can be as high as 30 degrees. This deviation in turn affects the function of a magnetic switch or a magnetic sensor where such a magnet is used. The described magnetic arrangements can partly compensate for this deviation.

To improve such a magnetic arrangement further, the deviation of the magnetic field direction can be compensated further. This is done by placing the magnets such that the deviation of one magnet compensates for the deviation of the other magnet. In one example, the magnets have a deviation of 20 degrees relative the symmetry axis. By placing the magnets such that the magnetic field of one magnet deviates with 20 degrees in one direction, e.g. away from the centre line in fig. 2b, and the magnetic field of the other magnet deviates with 20 degrees in the other direction, here also away from the centre line in fig. 2b, the resulting magnetic field will be symmetric in respect to the centre line 6, i.e. to the centre of the magnetic arrangement. By placing the magnets so that the deviation of the magnets is in the direction towards the centre line will also create a symmetric magnetic field. The critical distance d may vary slightly depending of the magnetic field deviation of the magnets.

Since it is difficult to detect the deviation of the magnetic field for a single magnet, especially in a production plant, one way of obtaining a symmetric magnetic field is to start with one magnet having the size of the two desired magnets. By splitting the magnet along the centre in a north-south direction and turning one of the resulting magnets 180 degrees around the symmetric axis, the resulting magnetic field from the resulting magnetic arrangement will always be symmetric, regardless of the deviation of the magnetic field in the single starting magnet.

Using the same method, it is also possible to create a magnetic arrangement that resembles a single magnet but where the direction of the magnetic field is parallel with the symmetry axis. This is done as described above, the difference being that the magnets are positioned together after the splitting, i.e. the critical distance is close to or equal to zero. Regardless of the deviation of the magnetic field in the starting magnet, the resulting magnetic field will always be symmetric.

In a first embodiment of an inventive magnetic switch 17, shown in figure 7, the switch comprises a second magnetic system 25 consisting of two magnets 4, 5, a first magnetic system 24 consisting of a biasing magnet 20 and an assembler 19, and a magnetically sensitive switching element 18. The switching element is e.g. a reed-contact or an integrated circuit-based switching element. The switching element is connected to an electrical circuit (not shown) that detects the state of the switching element. The biasing magnet 20 is positioned close to the switching element 18 and biases the switching element. This biasing magnetic field is strong enough to alter the state of the switching element. Because of the close distance to the switching element, the biasing magnet 20 can be relatively small. Preferably, the biasing magnet 20 has a lower magnetic strength than the magnets 4, 5.

The assembler 19 is a device used to assemble all field lines in a uniform way so that the magnetic field from a permanent magnet positioned outside of the assembler is converted into a longitudinal field inside the assembler. The magnetic field inside the assembler displays identical field directionality regardless of the direction of the magnetic field from the used biasing magnet and thus allows for an identical reproducibility of the magnetic field inside the assembler. A magnetic switching element placed inside the assembler will thus always be subjected to the same magnetic field regardless of the angular response of the detector element. This eliminates the need of having to position an asymmetrically responding magnetic switching element in a specific rotational position along its longitudinal axis. The assembler is preferably made of a soft ferromagnetic material. The biasing magnet 20 is positioned close to or in contact with the assembler. This allows for a relatively small biasing magnet and makes the biasing of the magnetic switching element less sensitive for external interference.

The two permanent magnets 4, 5, are positioned at a distance from the magnetic switching element 18 so that the magnetic field from the magnets 4, 5 interacts with the biasing magnetic field at the magnetic switching element. The switch is designed as one unit, with the magnets and the magnetic switching element integrated in the same housing. In the embodiments described here, a normally open reed-contact is used as the magnetic switching element. This is the most common type of reed-contact and it is also the cheapest type. Other types, such as changeover or normally closed reed-contacts, can also be used when required.

In the first embodiment, the switch is switched by disturbing the magnetic field of the magnets 4, 5 with a ferromagnetic material 21. In this embodiment, the magnets 4, 5, are positioned at a distance from the reed-contact so that the magnetic field from the magnets 4, 5 cancels the biasing magnetic field at the reed-contact. This leaves the reed-contact in its normal, open state. The resulting magnetic field over the reed-contact will thus be close to zero, or at least under the threshold level of the reed-contact.

When the ferromagnetic material 21 is introduced into the magnetic field of magnets 4, 5, that is when the ferromagnetic material 21 approaches the magnetic switch, the material 21 will collect some of the magnetic field, which means that the magnetic field from the magnets 4, 5 at the reed-contact will decrease. When the ferromagnetic material is at a certain distance, the magnetic field from magnets 4, 5 has decreased enough for the biasing field to close the reed-contact, i.e. the switch switches. The switch is e.g. suitable for mounting on a truck and the ferromagnetic material can be e.g. a door. In this case, the switch detects that the door is closed. This embodiment provides for a normally open switch that is closed e.g. by bringing the door close to the switch.

In a second embodiment, the switch is also switched by disturbing the magnetic field of the magnets 4, 5 with a ferromagnetic material 21. In this embodiment, the magnets 4, 5, are positioned somewhat closer to the reed-contact so that the magnetic field from the magnets 4, 5 overcomes the biasing magnetic field enough for the reed-contact to close. The resulting magnetic field over the reed-contact is thus at least over the threshold level of the reed-contact.

When the ferromagnetic material 21 is introduced into the magnetic field of magnets 4, 5, that is when the ferromagnetic material 21 approaches the magnetic switch, the material 21 will collect some of the magnetic field, which means that the magnetic field from the magnets 4, 5 at the reed-contact will decrease. When the ferromagnetic material is at a certain distance, the magnetic field from magnets 4, 5 has decreased so much that it is balanced by the biasing magnetic field. The resulting magnetic field over the reed-contact will thus be under the threshold level of the reed-contact, which opens the reed-contact, i.e. the switch switches. The switch is e.g. suitable for mounting on a truck and the ferromagnetic material can be e.g. a door. In this case, the switch detects that the door is closed. This embodiment provides for a normally closed switch that is opened e.g. by bringing the door close to the switch.

In a third embodiment, the switch is switched by removing a ferromagnetic material 21 from the switch. In this embodiment, the balance between the biasing magnetic field and the magnetic field from magnets 4, 5 at the reed-contact is set up with a ferromagnetic material 21 close to the switch. In this embodiment, the magnets 4, 5, are positioned at a distance from the reed-contact so that the magnetic field from the magnets 4, 5 together with the ferromagnetic material 21 cancels the biasing magnetic field at the reed-contact. This leaves the reed-contact in its normal, open state. The resulting magnetic field over the reed-contact will thus be close to zero, or at least under the threshold level of the reed-contact.

When the ferromagnetic material is removed from the switch, that is when the ferromagnetic material 21 is moved away from the switch, the balance between the biasing magnetic field and the magnetic field from magnets 4, 5 at the reed-contact disappears. In this case, the magnetic field of the magnets 4, 5 will increase enough to close the reed-contact, i.e. the switch switches. The switch is e.g. suitable for mounting on a truck and the ferromagnetic material can be e.g. a door. In this case, the switch detects that the door is opened.

In a fourth embodiment, the switch is also switched by removing a ferromagnetic material 21 from the switch. In this embodiment, the balance between the biasing magnetic field and the magnetic field from magnets 4, 5 at the reed-contact is set up with a ferromagnetic material 21 close to the switch. In this embodiment, the magnets 4, 5 are positioned so that the magnetic field from the magnets 4, 5 together with the ferromagnetic material is less than the biasing magnetic field so that the reed-contact is closed by the biasing magnetic field. The resulting magnetic field over the reed-contact is thus lower than the threshold level of the reed-contact.

When the ferromagnetic material is removed from the switch, that is when the ferromagnetic material 21 is moved away from the switch, a balance between the biasing magnetic field and the magnetic field from magnets 4, 5 at the reed-contact is created. In this case, the magnetic field of the magnets 4, 5 will increase enough to open the reed-contact, i.e. the switch switches. The switch is e.g. suitable for mounting on a truck and the ferromagnetic material can be e.g. a door. In this case, the switch detects that the door is opened.

The above-described switches are suitable for contactless detection of the position of metallic parts on e.g. vehicles. Since the magnetic switch is enclosed in a single housing, it is protected against corrosion, dirt etc. Thus, the switch is especially suitable for the detection of safety critical parts. This can e.g. be to detect if the cab is in a locked position, to detect if the storage doors are closed or to detect if a tipper body is in a rest position. If the part to detect is not made of a ferromagnetic material, a ferromagnetic material can easily be fitted to the part, either by applying it on the surface or by integrating it into the part.

In a further embodiment, a single magnet replaces the two magnets 4, 5. The single magnet is positioned in a similar manner as described above for the magnetic arrangement with magnets 4, 5. To use a single magnet requires a good knowledge of the properties of the used magnet. In production, where the magnetic properties of the used magnets vary considerably not only between different batches but also in the same production batch, it can be difficult to ensure that the magnetic field from the single magnet always balances the biasing magnetic field. Thus, in production it is advantageous to use a magnetic arrangement with two magnets to obtain a good reproducibility.

In a further embodiment, the magnetic switching element is used without the assembler. If the angular response of the magnetic switching element is known and it is possible to position the magnetic switching element in a reproducible predefined position, the switch will work as described above without the assembler. In production, it is advantageous to use an assembler. This ensures that the biasing magnetic field will affect the magnetic switching element in a predefined manner.

In the above magnetic switches, any of the magnetic arrangements described above can be advantageous, depending on the requirements.

The invention is not to be regarded as being limited to the embodiments described above, a number of additional variants and modifications being possible within the scope of the subsequent patent claims. The magnetic switch arrangement can be used wherever a contactless detection is required.