EP0028561A1 - Control push button with magnetic return - Google Patents

Abstract

The invention relates to a control button consisting of a fixed housing (1, 3, 4), a push button (2) slidably mounted in the housing and magnetic return means (6, 7) of the button in its position rest. The housing has a hollow central part (4) open upwards and the bottom of which is at least partially closed by a wall (4a) of determined thickness e, forming a housing for a permanent magnet (6). The above-mentioned button (2) is internally hollowed out to cover the part (4), and carries at its lower part, a plate (7) made of magnetic material, which closes the recess of the button beyond the wall (4a). Application to keyboard devices.

Description

The invention relates to a control button of the type comprising a push button with digital actuation, capable of moving relative to a fixed support between a rest position and a working position in which it acts for example on the element. mobile with an electrical contact to close it. Generally, this type of button includes a member for recalling the push button in its rest position.

This return member can be, in known manner, of the spring type. One of the drawbacks of this elastic return results from the characteristic of the spring. Thus, for a given stroke of the button, the return force of the button opposing the depression increases with the stroke. The driving force must therefore always be greater than the maximum restoring force, that is to say that produced by the spring at the end of the button stroke, to be sure that the button has reached its working position. However, one may have exerted too weak a force on the button which however produced a displacement of the latter without this displacement having been of sufficient amplitude to have reached the working position. The expected result is then not obtained although when touching the operator had the feeling of having correctly operated the button. Furthermore, such a device intended to be used many times sees its characteristics change over time under the effect of wear, aging of the spring ...

To overcome such drawbacks, we turned to magnetic recall buttons. The advantage of the magnetic restoring force is that it is of maximum intensity when the air gap is minimal. Thus, to move the mobile part away from the fixed part, it is necessary to overcome at the start this maximum force which then decreases very quickly. So, as soon as the button is pressed, it is certain that the actuating force will be sufficient for it to reach its working position. The operation of such a button is therefore much safer because a feeling of depression to the touch includes the certainty of having reached the working position. In addition, these magnetic return means undergo virtually no deterioration over time.

However, if the principle of magnetic recall is an advantageous solution for this type of device, its realization poses many problems. One of them is due to the very characteristics of the magnetic reminder: the resistance to pushing the button is imposed, which means that the maximum attraction force of the magnet with zero air gap is dictated by this resistance. In addition, the stroke of the button is also dictated by the technology of the controlled organ and cannot be freely chosen. Depending on these constraints, it can be seen that the magnetic return force of the button from its working position to its rest position is very low and may not be enough to overcome the weight of the button increased by a few random forces of friction.

The present invention intends to remedy this drawback by proposing a magnetic recall control key which meets all the operating requirements of this type of product by using only means which are simple to produce and assemble and which lend themselves to very advanced manufacturing automation.

To this end, it therefore relates to a control button consisting of a fixed housing, a push button slidably mounted in the housing between a first non-depressed rest position and a second depressed working position and magnetic return means of the button in its rest position, said means being constituted by a permanent magnet secured to the housing and by a plate of magnetic material secured to the button.

According to one of the main characteristics of the invention, said housing comprises a hollow central part open upwards and the bottom of which is at least partially closed by a wall of determined thickness, this central part forming a housing for said permanent magnet, while the aforementioned button is internally hollowed out for slidingly covering said central part, the depth of the recess being greater than the height of said central part, and carries at its lower part said plate made of magnetic material, the latter closing the recess of the button beyond the bottom wall of said central part thus trapped in said recess.

In a preferred embodiment said central part is integral with a lateral envelope which the housing comprises, delimiting between them a sliding space of the side walls of the button, by means of lugs extending transversely in said space while said side walls of the button are notched at right of these tabs, on a height allowing the sliding of the button relative to the housing.

It is advantageous for the interior shape of the above-mentioned central part to be parallelepipedic, the aforementioned permanent magnet then being of parallelepipedic shape complementary to the interior shape of said central part.

Furthermore, the plate of said magnetic material is in abutment on the bottom wall of said central part, in the rest position of the push button and has on its outer face actuating protuberances of electrical contacts associated in a known manner with the part fixed.

The invention will be better understood during the description given below by way of purely indicative and nonlimiting example which will make it possible to identify the advantages and the secondary characteristics thereof.

• - La figure 1 est une vue en coupe d'un mode préfèré de réalisation d'une touche selon l'invention,
• - La figure 2 est une vue de dessous de la figure 1,
• - La figure 3 est un graphique significatif des efforts développés par les organes de rappel magnétiques de l'invention.

Reference will be made to the appended drawings in which:

• FIG. 1 is a sectional view of a preferred embodiment of a key according to the invention,
• - Figure 2 is a bottom view of Figure 1,
• - Figure 3 is a significant graph of the efforts developed by the magnetic return members of the invention.

Referring to Figures 1 and 2, we see that the key shown is constituted by a housing 1 and a push button 2 capable of sliding in direction A relative to the housing. This box is in fact constituted by an outer casing 3 and an inner central part 4 spaced from the casing 3. The central part is secured to the lateral casing 3 by lugs 5 extending transversely in the space L separating them . The central part 4 is hollow and open at its upper end. Its end lower is closed by a bottom 4a of thickness e determined. In a variant not shown, this bottom may not be complete and only consist of an inner rim at the lower portion of the central part 4. The recess thus defined in part 4 serves as a housing for a permanent magnet 6. The shape of this recess will be complementary to that of the magnet so that it can be introduced there, by the upper opening, without difficulty. A preferred shape will be parallelepipedic, since permanent magnets of parallelepipedic shape are among those which are produced at the best cost.

The push button 2 is hollowed out internally, so as to cover the aforementioned central part 4. Its recess 2a is of complementary shape to that of the outer shape of the central part (cylindrical or para11è1épipédique) the latter then constituting a guide for the sliding of the button. The lower part of the side walls of the button 2 has notches 2b arranged in line with the tabs 5 connecting the parts 3 and 4 of the housing. These notches are of height h such that they allow the desired sliding stroke of the button relative to the housing. Finally, the button 2 carries at its lower end a plate 7 of magnetic material which closes the recess 2a beyond the wall 4a of the central part, the latter then being trapped inside the button. The fixing of this plate on the lower end of the button 2 is carried out by any known means and preferably by riveting an excess of material of the button on the external face of the plate. If this material is a plastic, this riveting (shown in the figures in 8) can be carried out by ultrasound. The plate 7 has protuberances 7a (here curved parts of the plate 7 itself) turned downwards, that is to say its outer face, these protuberances constituting control members of the movable part 9 of a electrical contact known in itself located at a determined distance from the housing 1. It can be seen in particular in FIG. 1 that the depression of the button 2 until it abuts on the top of the part 4 or on the top of the legs 5 by the bottom of the notches 2b makes it possible to move the protrusions 7a at 7'a and move the above-mentioned part 9 to bring it into contact with the fixed part 10 thus closing a circuit. By releasing the button, the magnet 6 attracts the plate 7 and therefore raises the button until the plate 7 is in abutment on the wall 4a of the bottom of the central part 4.

FIG. 3 shows two curves C1 and C2 representing the variations in the magnetic attraction force of a permanent magnet as a function of the value of the air gap. The curve C1 is that relating to a magnet having a maximum attraction force (that is to say with zero air gap) FI1 corresponding to the desired resistance to penetration which the push button must have. We see that for a value d of the air gap corresponding to the travel of the button when it has reached its working position, the restoring force F12 is very low and insufficient to obtain a safe return of the button, taking into account its weight and possible friction forces.

The choice of a stronger magnet with a characteristic curve C2 makes it possible to obtain a much greater attraction at distance F22 which can be satisfactory. On the other hand, the attraction to the zero gap F21 is far too great and the resistance to pressing the button is too great. The solution to this problem proposed by the invention lies in maintaining a minimum air gap of value e corresponding to the thickness of the bottom wall 4a of the part 4 of the housing. Thus, the resistance to penetration is reduced to its desired value FI1. In this case, taking into account the stroke d which it is necessary to respect, one obtains a force of attraction at distance F23 a little lower than that F22 aforesaid but which remains all the same very much greater than the force F12 aforesaid. It can therefore be seen that the above thickness e depends on the magnetic characteristics of the permanent magnet used.

One of the additional advantages of the invention lies in the fact that the magnet is never in direct contact with the recalled part. It follows that this magnet is not directly subjected to repeated shocks many times which avoids the risk of progressive disintegration of the latter which is generally made of sintered material.

Furthermore, even if particles of the magnet manage to detach from them, they are retained in the central part of the housing thus avoiding the formation of parasitic particles placed between the magnet and the attracted magnetic plate which would modify the value of the minimum air gap.

In addition, it is known that the pulling force at a distance from a magnet is dependent, for a given surface on its thickness. The arrangements of the invention advantageously allow the use of a magnet of relatively small section, which is important in order to be able to be incorporated into a touch of fairly small dimension, but of considerable thickness, therefore of relatively strong attractiveness. The large thickness also allows good guidance.

Finally, the construction of the button according to the invention allows easy automatic mounting of the various elements which constitute it. It also makes it possible to avoid having to use pierced magnets which are therefore expensive. In addition, as the magnet is placed along the central axis of the key, there is no need to hold it in place with great precision, in particular by overmolding a plastic cage for its maintenance which constitutes another factor in lowering the cost of manufacturing the fingerboard.

This key can be mounted by clipping into a support having a determined number of cells to form a complete keyboard, the mobile elements of the electrical contacts being able to be pre-assembled on each key housing or, on the contrary, being integral with a support adapting to the built-in keyboard.

The invention finds an interesting application in the information processing industry, in that of telecommunications or more generally in the manufacture of keyboard control members.

Claims (5)

1. Control key consisting of a fixed housing, a push button slidably mounted in the housing between a first non-depressed rest position and a second depressed working position and magnetic return means of the button in its rest position, said means being constituted by a permanent magnet secured to the housing and by a plate of magnetic material secured to the button, characterized in that said housing has a hollow central part open upwards and the bottom of which is at least partially closed by a wall of determined thickness, this central part forming a housing for said permanent magnet, and in that the aforementioned button is internally hollowed out for slidingly covering said central part, the depth of the recess being greater than the height of said central part, and leads to its lower part said plate made of magnetic material, the latter closing the recess of the button beyond the bottom wall of said central part thus trapped in said recess. 2. Key according to claim 1, characterized in that said central part is integral with a lateral envelope that the housing comprises, delimiting between them a sliding space of the side walls of the button, by means of tabs extending transversely in said space and in that said side walls of the button are notched at the right of these tabs on a height allowing the sliding of the button relative to the housing. 3. Key according to claim 1 or claim 2, characterized in that the said central part is internally parallelepipedal while the permanent magnet is of parallelepipedal shape complementary to the internal shape of the central part. 4. Key according to any one of the preceding claims, characterized in that in the rest position, the plate of magnetic material is supported on the bottom wall of said central part. 5. Key according to any one of the preceding claims, characterized in that the plate made of magnetic material comprises on its outer face, actuating protuberances of electrical contacts associated in known manner with the fixed part of the housing.

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