Abstract:
A keyboard comprises: several keys operated by a user, the operating forming, for each key, a signal representative, a flat waveguide having a first and second external face for a wave of light to reflect between the faces, the first face of the waveguide comprising an altered zone so part of the wave leaves the waveguide via the altered zone to backlight the keyboard, for each key a plunger moved between two positions, one in contact with the first face and the other distant from the first face, contact of the plunger with the first face causing the reflection of the wave in the waveguide to be locally frustrated, associated with each of the plungers, a sensor arranged in an empty space delimited by the second face, the sensor associated with the plunger and configured to detect the frustration of the reflection, the sensor forming the representative signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to foreign French patent application No. FR 1402876, filed on Dec. 17, 2014, the disclosure of which is incorporated by reference in its entirety. 
     FIELD OF THE INVENTION 
     The invention relates to a keyboard the reliability of which is improved. Keyboards are present in numerous items of electronic equipment. They are used so that a user can input data. 
     BACKGROUND 
     Known keyboards comprise keys intended to be operated by a user. Operation of a key leads to operation of a switch making it possible to make an electrical connection between two points so that an electrical signal can be made to pass or prevent it from passing according to the pressing of the key. The switch for example comprises a metal dome secured to a printed circuit. The dome is intended to collapse under the effect of the pressing of the key. In collapsing, the dome creates an electrical contact on the printed circuit. 
     This type of keyboard imposes numerous restrictions on the equipment. First of all, the life of the keyboard is limited by the mechanics of the keys and of the associated switches. The moving parts and their fixings may reach their fatigue limit. In order to achieve a desired life, certain components may need to be overengineered, leading to additional costs and potentially hampering attempts at miniaturization. 
     SUMMARY OF THE INVENTION 
     The invention provides a solution to this problem by proposing a keyboard that is far simpler to create. The keyboard is created around an optical waveguide performing numerous functions such as notably that of decoupling the movement of the key from the formation of a signal representative of the movement of the key. 
     To that end, the subject of the invention is a keyboard comprising:
         several keys intended to be operated by a user, the operating of the keys making it possible to form, for each key, a signal representative of the operation thereof,   a flat waveguide having a first and a second external face, it being possible for a wave to reflect totally between the two faces, the first face of the waveguide comprising an altered zone altered so that part of the wave leaves the waveguide via the altered zone in order to backlight the keyboard,   associated with each key a plunger that can be moved by the corresponding key between two positions, one in contact with the first face of the waveguide and the other distant from the first face, contact of the plunger with the first face causing the reflection of the wave in the waveguide to be locally frustrated,   associated with each of the plungers, a sensor arranged in an empty space delimited by the second face, the sensor being configured to detect the frustration of the reflection, the sensor forming the respective representative signal.       

     The presence of an optical waveguide is of course used to form the signals representative of the movement of the keys. The waveguide also allows the keyboard to be backlit. It also allows the keys to be separated from the sensors or, more exactly, allows the two parts of the keyboard:
         the mechanical parts of the keyboard that move when the keys are operated,   the part of the keyboard that forms the signals representative of the movement of the keys   to be physically isolated.       

     This physical isolation allows the part that deals with the formation of the signals to be protected from dust or liquid with which the keys may become contaminated. 
     Moreover, protecting the keyboards from electromagnetic perturbations can present problems. Specifically, in conventional keyboards, the electrical signals passing through the switches may become perturbed or may perturb other equipment. The presence of the moving parts of the keyboards may make it difficult to create shielding screens that allow the signals to be isolated from the external environment. The presence of the waveguide allows this problem to be resolved with ease. To this end, the keyboard advantageously comprises an electrical shielding screen arranged on one of the faces of the waveguide, the shielding screen being transparent to the wave propagated in the waveguide. 
     Advantageously, the keyboard comprises at least one cover to protect the sensor from a parasitic wave not originating from the frustration brought about by the contact of the plunger associated with the sensor. 
     The cover may be formed by a mask that is opaque to the wave propagating in the waveguide, the mask covering the second face except for a hole centered facing a zone intended to receive contact from the plunger on the first face. 
     The altered zone may focus the light extracted via the first face around a direction perpendicular to the plane of the waveguide. 
     The keyboard may comprise a first and a second source of wave propagating in the waveguide and each source emitting in a wavelength band, the two bands being distinct. The sensor is then configured to detect a wave in the band of the first source and the band of the second source is used to exit the waveguide via the altered zone. 
     Advantageously, the keyboard comprises a light source controlled by the representative signal and allowing information to be fed back to the user as a function of contact of the plunger with the first face of the waveguide. 
     The light source allowing the feedback of information may be arranged in such a way as to emit light perpendicular to the waveguide and crossing it. 
     The light source allowing the feedback of information may emit in a wavelength band distinct from that for which the sensor is configured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be better understood and other advantages will become apparent from reading the detailed description of one embodiment given by way of example, which description is illustrated by the attached drawing in which: 
         FIG. 1  schematically in section depicts one example of a keyboard according to the invention; 
         FIG. 2  depicts a simulation of a back-scattered wave for pressure applied simultaneously to several keys of the keyboard; 
         FIG. 3  schematically depicts the keyboard in perspective. 
     
    
    
     For the sake of clarity, the same elements will bear the same references in the various figures. 
     DETAILED DESCRIPTION 
       FIG. 1  depicts a keyboard  10  comprising two keys  11  and  12 . In practice, a keyboard according to the invention may comprise a greater number of keys, as is the case for example with a keyboard allowing the input of alphanumeric characters. Conversely, the invention may be implemented on a keyboard having just one key. 
     Returning to the example depicted, the keys  11  and  12  are able to move translationally with respect to a support  13  which is, for example, flat and fixed to a piece of electronic equipment. Secured to each key  11  and  12 , a plunger, respectively  14  and  15 , follows the translational movement of the key concerned. In practice, the key and the plunger associated with it may be formed by the same mechanical component or by two distinct mechanical components joined together. However, it is possible to make a distinction between the function of the key which is to be pressed by a user, and the function of the plunger which is to transmit this pressure. The plungers  14  and  15  may each move translationally along an axis  16  and  17  respectively, these axes being mutually parallel and perpendicular to the support  13 . In  FIG. 1 , the axes  16  and  17  are depicted as vertical and the movement of the keys  11  and  12  is downward along their respective axis. The movement of each of the keys  11  and  12  is limited upward by an end stop  18  and  19  respectively, formed in the support  13 . Each of the plungers  14  and  15  bears against its respective end. A return spring,  20  and  21  respectively, may push the plunger back towards its respective end stop. In this position, pressing against its end stop, the key and its plunger are in a rest position, namely a position not activated by a user. Conversely, when the user presses on one of the keys  11  or  12 , by means of a vertical pressure, the corresponding plunger ceases to bear against its end stop and depresses the associated return spring. The support  13  may be formed by two mechanical parts  13   a  and  13   b  formed together in order to allow the plunger and the corresponding spring to be fitted. 
     In the example depicted, the return spring  20  or  21  applies to the corresponding plunger  14  or  15  a force that is proportional to its movement. It is possible to provide the user with tactile feedback that is not proportional, for example having a force/displacement curve that exhibits a maximum. This type of curve may for example be obtained by means of a “hard point” in the movement of the plunger. This hard point may comprise a cam produced on the plunger. A cam follower such as a ball for example, able to move horizontally with respect to the support  13  is kept bearing against the cam by means of a spring. The shape of the cam defines the force/displacement curve desired. 
     The keyboard  10  comprises a flat waveguide  20  substantially perpendicular to the axes  16  and  17 . The waveguide  20  makes it possible to guide an electromagnetic wave, such as a wave of light for example, in a central layer  24  bounded by two external faces  22  and  23 . The refractive index of the central layer  24  is higher than that of the medium in which the waveguide  20  is placed, such as air in particular. Total reflection of the wave propagated in the central layer  24  off the two external faces  22  and  23  is thus obtained. The waveguide  20  is, for example, made of glass or of polymethylmethacrylate, often abbreviated to PMMA. 
     The plunger  14  or  15  can moved between two positions: one in contact with the face  22  and the other distant from the face  22  and resting against its end  18  or  19 . In  FIG. 1 , the plunger  14  is depicted distant from the face  22  and the plunger  15  is depicted in contact with the face  22 . Contact of the plunger  15  with the face  22  locally frustrates the reflection of the wave in the guide  20 . This frustration leads to a back-scattering of the wave which crosses the guide  20  at right angles to the plane thereof. Part of the wave therefore passes through the face  23  when the plunger  15  is in contact with the face  22 . In the absence of contact between the plunger and the face  22  there is no frustration and the wave is totally reflected off the two faces  22  and  23 . By arranging, under the waveguide  20 , a sensor under each of the plungers  14  and  15 , it is possible to detect the frustration and therefore detect contact of the relevant plunger with the face  22 . A plunger of which the refractice index is higher than that of air frustrates this reflection. For example, a silicone-based plunger white in color performs this function well. It has the advantage of making good contact with the face  22  while in particular avoiding trapping bubbles of air between the plunger and the face  22 . 
     The keyboard  10  comprises a printed circuit  25  arranged parallel to the waveguide  20  on the side of the face  23 . The printed circuit bears a sensor  26  arranged facing the plunger  14  on the axis  16  and a sensor  27  arranged facing the plunger  15  on the axis  17 . The sensors  26  and  27  are, for example, photodiodes sensitive to the wave of light propagated in the waveguide  20 . The sensors  26  and  27  form a signal representative of the pressing of the respective keys  11  and  12  by a user. For photodiodes the representative signal is electrical and can be processed by other components arranged on the printed circuit  25 . The current coming from a photodiode can be processed using a current-voltage converter followed by an operational amplifier that compares the voltage generated by the converter against a threshold. A binary signal is available at the output of the operational amplifier and represents the detection of a pressing of the key in the case of one of the levels, and the absence of pressing in the case of the other binary level. By way of alternative, other types of sensor may be used within the context of the invention, such as a sensor that delivers an optical signal for example. 
       FIG. 2  depicts a simulation of the wave back-scattered for simultaneous pressing of four closely spaced keys of the keyboard  10 . The wave backscattered when a plunger makes contact with the face  22  is emitted around a direction perpendicular to the plane of the light guide  20 . The various plungers are identified as  35  in  FIG. 2 . The intensity of the backscattered wave is at a maximum in the direction perpendicular to the plane of the waveguide  20  and decreases for directions deviating from the perpendicular direction. In  FIG. 2 , lines represent the presence of a back-scattered wave. The more closely spaced the lines, the greater the intensity of the backscattered wave. In order to prevent the sensors  26  and  27  from detecting a wave backscattered by a nearby plunger not associated with it, the keyboard  20  comprises at least one cover to protect the sensor  26  or  27  from a parasitic wave that does not originate from the frustration brought about by contact of the plunger associated with the sensor. Several alternative forms of cover are depicted in  FIG. 1 . The cover may be formed by a wall  28  arranged on the printed circuit  25  between the sensors  26  and  27 . It is also possible to provide a wall surrounding each of the sensors  26  and  27 . The cover may also be formed by an opaque mask  29  arranged on the face  23  preventing an oblique backscattered wave from leaving the waveguide  20 . The mask may cover the entirety of the face  23  with the exception of holes, for example circular holes, centered on the axes  16  and  17  or, in other words, with the exception of a hole if the keyboard comprises just one key or of several holes each one centered facing a zone intended to receive contact of one of the plungers  14  or  15  on the first face  22 . 
     Advantageously, the waveguide  20  can be used to backlight the keyboard  10 . This lighting may be used to display images, text zones or icons present between the keys on the support  13 . These images act for example as indicators for the various keys of the keyboard  10 . Light extracted from the waveguide  20  passes through the support  13  which is transparent or at least partially transparent. It is possible to allow the light to pass through the support  13  only in zones in which the images are present. Outside of these image zones, the support may be covered with opaque, for example black, paint. The support may also have diffuser properties so as to even out the light passing through it. It is also possible to backlight the keys of the keyboard  10 . In order to extract light from the waveguide  20 , the face  22  comprises one or more altered zones  37  altered so that part of the wave propagating in the guide exits the waveguide  20  via the altered zone or zones  37 . 
     The altered zones may be produced using a diffusing paint applied to the face  22 . Advantageously, the altered zones are configured to focus the light extracted via the face  22  around a direction perpendicular to the plane of the waveguide  20 . The focusing is, for example, performed by a micro prism film, well known by its English-language abbreviation BEF which stands for Brightness Enhancement Film. Focusing makes it possible to prevent the light extracted for the backlighting of the keyboard  10  from perturbing the detection of the frustration by the plungers. 
       FIG. 3  schematically depicts a keyboard  10  in perspective. This figure depicts sources allowing the wave propagated in the waveguide  20  to be generated. The keyboard  10  is flat and extends for example over a parallelepipedal surface. The waveguide  20  extends substantially over the entire surface of the keyboard  10 . The sources for waves propagating through the waveguide  20  may be formed of light emitting diodes arranged on the periphery of the waveguide  20  and illuminating it via its edge face. The sources may all be identical and may all emit only on the same wavelength. 
     Alternatively, it is possible to differentiate the sources according to their use. More specifically, sources with different wavelengths can be used for detecting the pressing by the plungers and for the backlighting of the keyboard  10 . For example, diodes emitting in an infrared band may be used to detect the pressing of the plungers and diodes emitting in a visible wavelength band may be used for the backlighting. The two types of diodes are, for example, alternated around the periphery of the waveguide. The proportion of diodes emitting in each of the bands is adapted to suit the requirement. Thus, the risk of interference between the backlighting and the detection of the pressing by the plungers is reduced. The sensors  26  and  27  are also tailored to detecting the wavelength band selected for the diodes intended to detect the pressing of the plungers. In  FIG. 3 , diodes  40  emit in the infrared and diodes  41  emit in a visible waveband. The altered zones  37  may have a special treatment to limit the transmission of the wavelength chosen for the diodes intended to detect the pressing of the plungers. 
     The presence of the waveguide may advantageously be put to use to allow information to be fed back to the user according to contact of one of the plungers with the face  22 . To this end, the keyboard comprises a light source  45  controlled by the representative signal originating from the corresponding sensor. This light source  45  is activated according to the pressing by the user of the corresponding key. The feedback of information may be direct, the light source  45  may be active only when there is pressure. The feedback of information may be indirect, for example when a single-acting key is used to control relay. The light source  45  is activated upon a first pressing of the key activating the relay, and deactivated on a subsequent pressing, deactivating the relay. 
     The light source  45  may be arranged in such a way as to emit in the waveguide  20 . The source is therefore arranged at the periphery of the waveguide  20  like the sources  40  and  41 . Alternatively, the light source  45  is arranged in such a way as to emit light perpendicular to the waveguide  20  and crossing it. This alternative is well suited to a keyboard comprising several keys and for which feedback of a distinct nature is required for each of the keys. The light source  45  is, for example, formed of one or more light emitting diodes arranged on the printed circuit  25  near the sensor corresponding to the key for which the feedback of information is desired. The wall  28  may separate the light source  45  from the corresponding sensor. Alternatively or in addition, like with the segregation between the backlighting and the detection of the pressing, the sensor may be insensitive to the light emitted by the source  45 . Likewise, it is possible to differentiate the color of the backlighting from that of the information feedback. For example, the sensor may operate in the infrared, the backlighting in the red and the feedback of information in the green. It is possible to provide several different colors for feeding back different information, in the case of a keyboard  10  having several keys, or in the case of one and the same key depending on how it is used. 
     A keyboard according to the invention makes it possible to separate the mechanical action on the key from the detection of this mechanical action. Physically, this separation is achieved by the waveguide  20 . This physical separation can be put to use for electromagnetically isolating the keys of the keyboard  10  and the electric signals deflected by the keyboard according to the pressing of the keys. For this purpose, the keyboard  10  comprises an electrical shielding screen  50  arranged on one of the faces of the waveguide, the shielding screen being transparent to the wave (or waves) propagated in the waveguide  20  and, where appropriate, transparent to the light used for the feedback of commands. The shielding screen is, for example, achieved by means of a transparent conducting electrode arranged on one of the faces  22  and  23 . This electrode is, for example, made of tin-doped indium oxide, well known by its abbreviation ITO which stands for indium tin oxide. Other technologies may be used to form the shielding screen  50 , such as, for example, the use of a transparent film incorporating a conductive mesh. This type of film is known as a micromesh film. The shielding screen  50  covers the entire surface of the waveguide  20  and is connected to a ground potential of the keyboard, for example via the printed circuit  25 .