Abstract:
In a keyboard comprising keys (1) arranged in recesses of a supporting  pl (2) and electric contacts of a contact plate (6) disposed below the keys (1) a tilting axis is provided between the key (1) and the supporting plate (2). In order to ensure symmetrical switching behavior a tilting axis is provided on both sides of the key (1). The tilting axes are formed by corners (4, 5) which are abutted at an obtuse angle (W2) by the key (1).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention concerns a keyboard comprising keys arranged in recesses of a supporting plate with electric contacts of a contact plate disposed below the keys. A tilting axis is provided between the key and the supporting plate. The lateral surface of the key top opposite the tilting axis is recessed for tilting action and the base of the key is placed under the resilient force of the contact. 
     2. Description of the Prior Art 
     A keyboard of this kind is described in German published application No. 2,459,464. In this keyboard the keys are embodied in a single unit with the supporting plate. The tilting axis forms a connector between the key and the supporting plate. 
     The keyboard according to German published application No. 2,459,464 has the drawback that the movement of the keys is not symmetrical, with the result that the operational behaviour in the case of pressure on one side of the key is different from that in the case of a pressure on the other side of the key. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the invention to provide a keyboard of the type described above in which the keys tilt symmetrically when pressed from different sides. 
     According to the invention this object is solved in the case of a keyboard of the type described above in that a tilting axis is provided on both sides of the key. The key has a recess on each side, defining an obtuse angle formed between the lateral surface of the key and a stopper surface of the key base projecting opposite the key top below the supporting plate. The tilting axis is formed by the tangent where the stopper surface abuts a corner of the supporting plate. 
     The tilting and hence the switching behaviour of the key is the same irrespective of whether it is operated more on the one side or more on the other side. In the first case the key tilts about one tilting axis without the tilting being affected by the other tilting axis. In the second case the key tilts about the other tilting axis. At the same time a lever action is brought about. The design of the tilting axes does not incur any additional restoring forces which have to be overcome in operation and which might lead to different operating forces of the individual keys in the keyboard. The necessary operating force is dependent only on the design of the contact plate. 
     In a preferred embodiment of the invention the stopper surfaces are at an acute angle to the plane of the supporting plate when the key is not operated. This ensures that the tilting axis will not slip during operation of the key. The acute angles are preferably dimensioned such that the stopper surface abuts the supporting plate when the contact is closed. This does not influence the action point behaviour of the contact plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Further advantageous embodiments of the invention will become apparent from the following description and from the claims. The drawing shows a sectional view of a portion of a keyboard, illustrating the configuration of a key in relation to the supporting plate and contact plate of the keyboard. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The drawing shows only one key 1 on the keyboard. Depending on the application the keyboard may have a plurality of similar keys arranged in a mutual plate 2. 
     The supporting plate 2 is provided with a recess 3 in which the key 1 is seated. The supporting plate 2 comprises corners 4 and 5 in the recess 3. A contact plate 6 is arranged below the supporting plate 2. 
     The contact plate 6 includes an upper rubber membrane 7 with domes 8 and a printed circuit board 9 which is fixed below the upper rubber membrane 7. A bridging contact 1 is glued in position in the dome 8 and it connects two electric contact tracks 11 and 12 of the printed circuit board 9 when the dome 8 is depressed. 
     The key 1 sits in the recess 3 and abuts the dome 8 with a central point of contact 13 of its base 14. 
     The base 14 protrudes below the supporting plate 2. The base 14 extends from the point of contact 13 in bevels 15 and 16 which are at an angle W1 to the plane of the supporting plate 2. 
     The base 14 develops from stopper surfaces 17 and 18 to lateral surfaces 19 and 20 of a top 21 of the key 1. The stopper surface 17 and the lateral surface 19 and the stopper surface 18 and the lateral surface 20 respectively form an obtuse angle W2 in the transition regions. The transition regions are pressed against the corners 4 and 5 respectively by the resilient force of the dome 8. 
     In the embodiment example the corners 4 and 5 extend in a radius r about a point P. In the embodiment example the radius r is substantially larger than that of a curve in the vertex of the angle W2. This ensures that both the stopper surfaces 17 and 18 and also the lateral surfaces 19 and 20 of the key have tangents to the corners 4 and 5 respectively. The tangent K of the stopper surfaces 17 and 18 respectively becomes effective as a tilting axis when the key 1 is pressed. The tangent Z of the lateral surfaces 19 and 20 respectively becomes effective when the key 1 is restored to its initial position and it improves the centering of the key 1 in its initial position. The stopper surfaces 17 an 18 form and acute angle W3 with the plane of the supporting plate 2. The acute angles W1 and W3 are approximately equal and amount, by way of example, to 12°. The lateral surfaces 19 and 20 are bent about the point P in a radius R. The radius R is dimensioned such that the lateral surfaces 19 and 20 do not abut the edge of the recess 3 before the bridging contact 10 has connected the contact tracks 11 and 12. 
     The manner of functioning of the described apparatus is approximately as follows: 
     When the key top 21 is pressed in the region shown on the right of the figure it tilts about the tilting axis formed by the tangent K of the stopper surface 17. In so doing, the nap 8 is pressed by the point of contact 13 until the bridging contact 10 connects the electric contact tracks 11 and 12. It is only after the tilting motion has carried through about the angle W3 that the stopper surface 17 abuts the supporting plate 2 from below. The bevel 16 permits the necessary stroke of the key without allowing the key to strike the upper rubber membrane 7 before the stroke has been completed. After the key top 21 is released the key 1 is moved back into the position as shown by the restoring force of the dome 8 and is centered on the tangent Z of the lateral surfaces 19 and 20. 
     When the key top 21 is pressed on the side shown left in the drawing it tilts about the tangent K of the stopper surface 18 in the same manner as described above. The switching behaviour and the necessary pressing force is the same in both cases. 
     When the key top 21 is pressed in the middle the vertices on both sides may become removed from the corners 4 and 5 or from the two tangents Z. The dome 8 is depressed by the point of contact 13. 
     In a further embodiment of the invention the corners 4 and 5 may be embodied to be sharp-edged in such a way that the tangents K and Z practically coincide. In this case their corners 4 and 5 are directly in the vertex of the obtuse angles W2 formed between the stopper surfaces 17 and 18 and the lateral surfaces 19 and 20 respectively.