Abstract:
A keypad includes at least one push-button switch and a key to operate the switch along a translational axis. The geometric dispersions of the keypad are accounted for, to the lengthening of the travel of the key and to the enhancement of the tactile sensation when the key is pressed to operate the switch. The keypad includes a plunger, interposed between the key and the switch, of which a stiffness along the translational axis increases continuously with an increase in the compression of the plunger. A slight stiffness at the beginning of compression allows a long travel of the key, while a greater stiffness at the end of compression gives a good tactile sensation with an assured contact even when there are off-center pressures on the key.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Stage of International patent application PCT/EP2009/062301, filed on Sep. 23, 2009, which claims priority to foreign French patent application No. FR 08 05986, filed on Oct. 28, 2008, the disclosures of which are incorporated by reference in their entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a keypad comprising at least one push-button switch and a key making it possible to operate the switch. It relates to taking account of the geometric dispersions of the keypad, to the lengthening of the travel of the key and to the enhancement of the tactile sensation when the key is pressed to operate the switch. The invention finds a particular, but not exclusive, utility in the instrument panel of an aircraft. 
     BACKGROUND OF THE INVENTION 
     When it is intended for the instrument panel of an aircraft, but also for other fields, a keypad must satisfy a certain number of requirements, in particular dimensional requirements. A first requirement is the tolerance relating to the key overshoot. The key overshoot is the difference in height between the top surface of the key and the fixed surface of the keypad. This tolerance is usually slim, of the order of two to three tenths of a millimeter. A second requirement relates to the travel of the key. This travel must usually be between seven and ten tenths of a millimeter depending on the application. A third requirement relates to the force to be applied to a key in order to actuate the switch. The force is for example five or six newtons with a tolerance of about one newton. A fourth requirement may also relate to satisfaction from operating the switch, in other words to the tactile sensation obtained when pressing a key. This sensation is notably associated with the resistance put up by the key when it is pushed and with the marked change in resistance observed when the switch passes from the open state to the closed state. This sensation is important in ensuring reliable feedback to the operator operating the switch. 
     The dimensional requirements may be all the more difficult to satisfy because the fixed portion of the keypad is often made of an assembly of parts. An assembly is for example necessary when the keypad is backlit. The keypad then comprises at least one front face, a printed circuit forming a base and a diffuser interposed between the front face and the printed circuit. The keypad may also comprise sealing elements between the fixed portion and the movable portion, that is to say the key or keys. The assembly of various parts of the keypad causes a geometric dispersion usually of the same order of magnitude as the travel of the key and as the tolerance concerning the key overshoot. For a keypad designed for an aircraft instrument panel, the geometric dispersion is ordinarily between six and ten tenths of a millimeter depending on the tolerance of the parts and the care applied to assembling the keypad. 
     Moreover, push-button switches have an insufficient travel to achieve the minimum travel required for the key. In this instance, the travel of a dome switch is rarely greater than three tenths of a millimeter. Even for a switch incorporating elastomers, the travel is usually less than seven tenths of a millimeter. Consequently, it is not usually possible to produce a rigid connection between the key and the movable portion of the switch. 
     SUMMARY OF THE INVENTION 
     Several solutions have been envisaged by the applicant to ensure both the tolerance of key overshoot and the minimum travel of the key.  FIG. 1  illustrates a first exemplary embodiment of a keypad in a view in section of a portion of the keypad along a plane passing through a key. The keypad comprises a printed circuit  10  forming a base, a front face  11  securely attached to the printed circuit  10  by means of a plate  12  and a push-button switch  13  mounted on the printed circuit  10 . The push-button switch  13  is for example a switch of the “dome” or “blister” type that is to say in which the switching takes place by deflection of a conductive elastic blister dome against two conductors to be connected. This type of switch is known in Anglo-Saxon literature as a dome switch. The front face  11  and the plate  12  comprise an opening  14  allowing a key  15  to move in translation along an axis X and to operate the switch  13 . According to this first exemplary embodiment, the overshoot tolerance  16  of the key  15  is ensured by pressing the key  15  against the front face  11 . This pressing can be carried out by a spring  17  prestressed between the key  15  and the assembly consisting of the printed circuit  10 , the front face  11  and the plate  12 . In particular, the spring  17  can press on an internal collar  18  made on the plate  12 . The travel of the key  15  can be limited on the side opposite to the switch  13  by a shoulder  20  made on the key  15  pressing against the bottom of a counterbore  19  made on the front face  11 . The minimum travel of the key can for its part be ensured by the existence of a clearance  21  between the bottom end  151  of the key  15  and the switch  13 . 
     This first exemplary embodiment makes it possible to absorb great dispersions in the assembly and to greatly lengthen the travel of the key  15 . However, it has several drawbacks.  FIG. 2  shows in graph form the change in a force applied to the key  15  according to a travel of this key  15  for the first exemplary embodiment. For the rest of the description, it is considered that the key  15  moves along the axis X, the origin O of the travel being determined by the rest position, that is to say when the key is not pushed and it is pressed against the bottom of the counterbore  19 . It is also considered that the force is applied to the key  15  along the axis X in the direction of the switch  13 . The change in force depending on the travel is represented by a curve  24 . A first portion  241  of this curve  24  is shown by a straight line with a positive gradient. This portion  241  corresponds to the pressure of the spring  17  alone, the gradient of the straight line corresponding to the stiffness of the spring  17 . Beyond a point of travel C 1 , the lower end  151  of the key  15  makes contact with the movable portion of the switch  13 . The stiffness of the switch  13  is then added to the stiffness of the spring  17 . On the graph, the total of the stiffness of the spring  17  and of the switch  13  is reflected by a second portion  242  of the curve  24  shown by a straight line with a steeper gradient and therefore by a discontinuity in the variation of the force for the point of travel C 1 . This discontinuity is a drawback because it takes the form of a hard point that can make an operator operating the key  15  think that the switch  13  has reached the end of travel and has therefore established an electrical contact. A third portion  243  of the curve  24  can be shown by a convex curve portion. This portion  243  corresponds to the beginning of the deflection of the switch  13  and comprises the point of maximum force F max  that can be applied to the key  15  before the switch  13  makes an electrical contact. This maximum force F max  occurs for a point of travel C 2 . A fourth portion  244  of the curve  24  can be represented by a concave curve portion, the force falling sharply after the point of travel C 2  has been passed. This portion  244  corresponds to the continued deflection of the switch  13 . It comprises the point of minimum force F min  that can be applied to the key  15  to keep the switch  13  closed. This minimum force F min  corresponds to a point of travel C 3 . Beyond the point of travel C 3 , the key  15  can still be pushed in for a short distance until the spring  17  is completely compressed for a point of travel C 4  corresponding to the mechanical travel C m  of the key  15 . The key  15  is then at the end of travel. The exemplary embodiment as illustrated in  FIG. 1  therefore exhibits the drawback of introducing a discontinuity of force into the travel of the key  15 . 
       FIG. 3  illustrates a second example envisaged by the applicant for the production of a keypad in a sectional view similar to  FIG. 1 . According to this second exemplary embodiment, the tolerance of overshoot  16  for the key  15  is also ensured by pressing the key  15  against the front face  11 . On the other hand, the pressing is carried out by a deformable element, called a plunger  31 , prestressed against the lower end  151  of the key  15  and the switch  13 . The plunger  31  consists for example of a cylinder of revolution. 
       FIG. 4  represents in graph form similar to  FIG. 2  the change in force applied to the key  15  depending on its travel for the second exemplary embodiment. A first curve  41  shows the change in force for a plunger  31  of slight stiffness and a second curve  42  shows the change in force for a plunger  31  with greater stiffness. In this example, the points of travel C 2  and C 4  defined above are considered to be identical for both curves  41  and  42 . The point of travel C 3  is marked C 31  for the curve  41  and C 32  for the curve  42 . The second exemplary embodiment makes it possible to remove the clearance  21  between the lower end  151  of the key  15  and the switch  13 . Because of this there is no marked change in stiffness when the key  15  is actuated between the origin O and the point of travel C 2 . Moreover, this second exemplary embodiment makes it possible to a certain extent to lengthen the travel of the switch  15  and to absorb the geometric dispersions of the assembly. The lengthening of the travel of the key  15  and the capacity to absorb the dispersions are promoted by a slight stiffness of the plunger  31 , the latter then deforming easily between the key  15  and the switch  13 . However, the plunger  31  with a slight stiffness has a tactile sensation which is not as good. The tactile sensation associated with the transition of the switch  13  from the open position to the closed position can be represented by the ratio R between the difference in force ΔF between the minimum force F min  and maximum force F max  and the difference in travel ΔC between the points of travel C 2  and C 3 . The ratio R can be defined by the following relation: 
     
       
         
           
             R 
             = 
             
               
                 
                   
                     F 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     max 
                   
                   - 
                   
                     F 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     min 
                   
                 
                 
                   
                     C 
                     3 
                   
                   - 
                   
                     C 
                     2 
                   
                 
               
               = 
               
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   F 
                 
                 
                   Δ 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   C 
                 
               
             
           
         
       
     
     The greater this ratio R, in other words the greater the average gradient in absolute value of the curve  24  between the travels C 2  and C 3 , the better the tactile sensation. Specifically, an operator actuating the key  15  feels the transition of the switch  13  more strongly when the force that he applies to the key  15  falls sharply for a slight movement of this key  15 . In  FIG. 4 , the fact that the tactile sensation of a plunger of great stiffness is better than that of a plunger of slight stiffness can be clearly seen. Specifically, for one and the same difference in force ΔF, the difference in travel ΔC 1  between the points of travel C 2  and C 31  is greater than the difference in travel ΔC 2  between the points of travel C 2  and C 32 . This phenomenon is associated with a more rapid delivery of the energy stored in a plunger  31  of great stiffness than in a plunger  31  of slight stiffness. Moreover, when there are off-center pressures on the key  15 , that is to say when there are pressures along an axis forming an angle with the axis X or along an axis parallel to the axis X but on one edge of the key  15 , the switch  13  might not be actuated with a plunger  31  of slight stiffness. Specifically, the plunger  31  might deform by bending and store energy without being able to deliver it along the axis X in order to activate the switch  13 . In conclusion, for this second exemplary embodiment, a compromise has to be found on the stiffness of the plunger  31  in order, on the one hand, to have a sufficient capacity of elongation and of absorption of the dispersions and, on the other hand, to ensure the activation of the switch  13  when there are off-center pressures on the key  15 . In practice, this second exemplary embodiment is mainly suitable for a slight elongation of the travel of the switch  13  and requires an adaptation of the length of each plunger  31  to the geometric dispersions of the keypad at each key  15 . The individual adjustment of lengths of plungers  31  is clearly costly and makes this embodiment inappropriate for the mass production of keypads. 
     One object of the invention is notably to alleviate the aforementioned drawbacks by proposing a keypad of simple design in which the keys  15  have a long travel and a good tactile sensation. Accordingly, the subject of the invention is a keypad comprising a push-button switch, a key making it possible to operate the push-button switch along a translational axis and a plunger interposed between the key and the switch. According to the invention, a stiffness of the plunger along the translational axis increases continuously with an increase in compression of the plunger. 
     A notable advantage of the invention is that it makes it possible to combine the advantages of a keypad comprising a plunger of slight stiffness with those of a keypad comprising a plunger of great stiffness for a low production cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other advantages will appear on reading the detailed description of embodiments given as examples, which description is made with respect to appended drawings which represent: 
         FIG. 1 , already described, a first exemplary embodiment of a keypad in a sectional view along a plane passing through a key of the keypad, 
         FIG. 2 , already described, the change in a force applied to the key of the keypad of  FIG. 1  depending on the travel of this key, 
         FIG. 3 , already described, a second exemplary embodiment of a keypad in a view similar to that of  FIG. 1 , 
         FIG. 4 , already described, the change in force applied to a key of the keypad of  FIG. 3  depending on its travel, 
         FIG. 5 , an exemplary embodiment of a keypad according to the invention in a view similar to that of  FIGS. 1 and 3 , 
         FIG. 6 , the change in force applied to a key of the keypad of  FIG. 5  depending on its travel for a first embodiment of a keypad according to the invention, 
         FIG. 7 , the change in force applied to a key of the keypad of  FIG. 5  depending on its travel for a second embodiment of a keypad according to the invention, 
         FIGS. 8A ,  8 B and  8 C, examples of a configuration of a keypad according to the second embodiment, 
         FIG. 9 , the change in force applied to a key of a keypad of  FIG. 8A ,  8 B or  8 C depending on its travel. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  represents an exemplary embodiment of a keypad according to the invention in a sectional view similar to  FIGS. 1 and 3 . The keypad according to the invention is similar to the second exemplary embodiment, the main difference relating to the plunger  31 . According to the invention, the stiffness of the plunger  31  along the axis X increases continuously when there is pressure on the key  15  before the actuating of the switch  13 . In other words, the stiffness of the plunger  31  increases with an increase in its compression for a travel of the key  15  preceding the actuation of the switch  13 . 
     According to a first embodiment, the stiffness of the plunger  31  increases continuously until it reaches a given compression point, the stiffness remaining constant beyond this compression point. This particular embodiment makes it possible to obtain a still better tactile sensation. The plunger  31  is for example made in a single piece of uniform material. The plunger  31  can therefore be made by molding very cheaply. The material is advantageously an elastomer such as silicone. In order to make it possible to obtain both a good tactile sensation and a great capacity of deformation of the plunger  31 , and therefore of elongation of the travel of the switch  13  and of absorption of the geometric dispersions of the keypad, the hardness of the elastomer may be between 60 and 80 Shore A. It is for example 70 Shore A. The present description relates to a keypad comprising a single key  15 . Naturally, the keypad may have several keys  15  and, in particular, a plunger  31  as described above for each key  15  of the keypad. 
       FIG. 6  represents, in the form of a graph similar to the graphs of  FIGS. 2 and 4 , the change in the force applied to a key  15  of the keypad of  FIG. 5  depending on the travel of this key  15  for the first embodiment of the invention. The change in force is shown by a curve  61 . On this curve  61 , it is possible to see that the plunger  31  is prestressed between the lower end  151  of the key  15  and the switch  13 , the ordinate at the origin F 0  of the curve  61  being greater than zero. It is also possible to see that, on a first portion  611  of the curve  61 , the stiffness of the key  15 , represented by the gradient of the curve  61 , increases progressively without discontinuity. On a second portion  612  of the curve  61 , the reaction of the switch  13  becomes dominant over that of the plunger  31 , the deflection of the switch  13  being initiated. This portion  612  of the curve  61  comprises the point of maximum force F max  that can be applied to the key  15  before the switch  13  makes an electrical contact. This maximum force F max  occurs at the point of travel C 2 . On a third portion  613  of the curve  61 , the force drops suddenly with the continuation of the deflection of the switch  13  until it reaches the minimum force F rain  at the point of travel C 3 . The force can then increase until it reaches the mechanical abutment of the key  15  at the point of travel C 4 . The difference in travel ΔC between the points of travel C 2  and C 3  is of the same order of magnitude as the difference in travel ΔC 2  observed for a plunger  31  of great stiffness. This phenomenon is explained by the fact that, just before the deflection of the switch  13 , the plunger  31  is greatly compressed and is therefore characterized by a great stiffness. Consequently, the ratio R is great and the key  15  has a good tactile sensation. 
     According to a second embodiment, the plunger  31  has two distinct constant stiffnesses. In this instance, it has a slight stiffness k 1  at the beginning of compression and a greater stiffness k 2  at the end of compression. The slight stiffness k 1  makes it possible, through its great capacity for deformation, to absorb the geometric dispersions and to lengthen the travel of the key, and the great stiffness k 2  makes it possible to obtain a good tactile sensation. 
     A plunger  31  of which the stiffness increases with its compression can notably be made by an appropriate shape of the plunger  31 . In this instance, the plunger  31  may comprise a recess  63 , as shown in  FIG. 5 . This recess  63  makes it possible to define an upper portion  31   a  of the plunger  31  and a lower portion  31   b  of the plunger  31 , the upper portion  31   a  corresponding to the portion of the plunger  31  that comprises the recess. The plunger  31  and/or the recess  63  may revolve around the axis X. According to a particular embodiment, shown in  FIG. 5 , the plunger  31  and/or the recess  63  are cylindrical. 
     According to a particularly advantageous embodiment, the recess  63  is used in order to fix the plunger  31  to the key  15 . The key  15  then comprises a lug  152  the shape of which complements that of an upper portion  63   a  of the recess  63 . The plunger  31  is fitted onto the lug  152  and is held there by elastic deformation. The relative heights of the lug  152  and of the recess  63  along the axis X are determined so as to leave an empty space  63   b  between the lug  152  and the bottom of the recess  63 . The height of this empty space  63   b  is for example between five and fifteen tenths of a millimeter for a total height of the plunger  31  for example of between three and four millimeters. The height of the empty space  63   b  is determined by a computation of the average geometric dispersion of the assembly of the keypad and the knowledge of the necessary travel of the key  15 . It is the presence of the empty space  63  that makes it possible to modify the stiffness of the plunger  31  with its compression. 
       FIG. 7  represents, in the form of a graph similar to the graphs of  FIGS. 2 ,  4  and  6 , the change in the force applied to a key of the keypad of  FIG. 5  depending on its travel for the second embodiment of the invention. The change in the force is represented by a curve  71 . At the origin of the travel of the key  15 , the upper portion  31   a  of the plunger  31  supports the majority of the deformation of the plunger  31 . This upper portion  31   a  has specifically an initial stiffness k 1  that is less than a stiffness k 2  of the lower portion  31   b . Beyond a certain point of travel C 1 , corresponding to the height of the empty space  63   b , the lug  152  comes into contact with the bottom of the recess  63 . The additional deformation of the plunger  31  is then essentially supported by the lower portion  31   b  which has the constant stiffness k 2 . In  FIG. 7 , this phenomenon is reflected by a first segment  711  with gradient k 1  between the origin and the point of travel C 1  and by a second segment  712  of gradient k 2  between the point of travel C 1  and the point of travel C 2  for which the maximum force is produced before the switch  13  makes an electrical contact. In  FIG. 7 , the transition between the stiffness k 1  and the stiffness k 2  is sudden. However, it is possible to obtain a smoother transition. 
       FIGS. 8A ,  8 B and  8 C illustrate examples of key and plunger configuration according to the second embodiment and in which the transition between the two stiffnesses k 1  and k 2  is smoothed. According to a first example of configuration, shown in  FIG. 8A , the lug  152  of the key  15  has a convex shape coming into contact with the bottom of the recess  63 . In this figure, the bottom of the recess  63  is flat. According to a second example of configuration, shown in  FIG. 8B , it is the bottom of the recess  63  of the plunger  31  that has a convex shape, the portion of the lug  152  coming into contact with the bottom of the recess  63  having a flat shape. According to a third example of configuration, shown in  FIG. 8C , both the lug  152  and the bottom of the recess  63  have a convex shape. In  FIGS. 8A ,  8 B,  8 C, it has been considered that the smoothing of the transition between the two stiffnesses k 1  and k 2  was provided by a convex shape. Naturally, any shape providing a progressive increase of the contact surface between the key  15  and the bottom of the recess  63  can be produced within the context of the invention. 
       FIG. 9  shows, in the form of a graph similar to the graphs of  FIGS. 2 ,  4 ,  6  and  7 , the change in the force applied to a key of the keypad as a function of its travel according to the examples of configuration of  FIGS. 8A ,  8 B,  8 C. The change in force is represented by a curve  91 . Relative to the curve  71 , the curve  91  differs essentially in that it comprises a portion of curve  92  linking the first segment  711  of gradient k 1  to the second segment  712  of gradient k 2  in the vicinity of the point of travel C 1 . 
     The plunger  31  according to the invention may be deformed elastically to a considerable degree in its upper portion  31   a . It therefore allows a long travel of key  15  and a great capacity of absorption of the dispersions of the keypad. In this instance, it is not necessary to adapt the length of the various plungers  31  to the geometric dispersions of the keypad at each key  15 . The plungers  31  may have standard dimensions. The plunger  31  also has a great stiffness in its lower portion  31   b . It thus provides a good tactile sensation.