GB2108763A - Capacitive switch - Google Patents

Abstract

A capacitive switch e.g. for a keyboard (10') comprises a panel (11) bearing at least one stationary capacitor plate (12), and a resilient membrane (19') supporting at least one movable capacitor plate (18). A spacing sheet (15') is positioned between the panel (11) and the resilient membrane (19'), with the panel (11), the spacing sheet (15') and the resilient membrane (19') mounted around the capacitor (29) to substantially seal the capacitor from external contamination. A dielectric layer such as the membrane (19') or a separate member or membrane is positioned between the capacitor plates. The switch membrane (19') finger moved, or utilizes a key (30') which includes a resilient compressible member (36'). In an alternative embodiment, a compressible substantially conical coiled spring serves as a movable capacitor plate. <IMAGE>

Description

SPECIFICATION Improved capacitive keyboard Background of the invention The invention relates to improvements in capacitive keyboards. Capacitive keyboards utilize the variation in capacitance between at least two capacitor plates caused by the motion of the plates relative to each other, or by motion of a conductive element relative to the plates, at each keying location on the keyboard to indicate the selection of that keying location. The term "keyboard" as used herein includes capacitive switches having a single key position, as well as units having a pluraluty of key positions.

Capacitive keyboards typically have the following advantages among their attributes: relatively low manufacturing cost, hysteresis which avoids multiple strobes due to operator dither, and the potential for higher reliability and longer life than hard contact keyboards due to the absence of electrical contacts.

Though the potential for high reliability and long life exists in capacitive keyboards, this has generally not been fully realized in the designs of many of the units presently being manufactured.

Some of the main problems being experienced by such capacitive keyboards, and the reasons therefore, are as follows.

Entry of ambient dust and dirt between the capacitor plates of a keying position may prevent proper closure of the plates. This can cause the closed key capacitance of a keying position to be below that required to trigger the sensor connected to the keying position to sense a keyclosed condition. Thus a key-closed condition can go undetected, resulting in errors of operation.

Capacitor plate area is often limited by the center-to-center distance constraints being placed on keyboard size and keying position spacing. The area available to be occupied by a single capacitor plate is further reduced when the two stationary capacitor plates must be positioned next to each other on the keyboard and a moving bridging plate is used to vary the capacitance between the two side-by-side plates, as is done in a widely-used technique of making capacitive keyboards. The resulting small size of the capacitor plates results in the nominal closed capacitance of the keying position being too small to leave an adequate margin for error caused by either variations in the manufacture of the keying positions or by adverse field conditions. Again, the consequence is that a key-closed condition may go undetected, resulting in errors of the operation of the keyboard.

Furthermore, capacitive keyboards are inherently high-impedance devices and hence they are prone to electrical noise. For this reason, at least the detection leads connected to the keying positions are preferably electrically shielded. But the shielding partially shunts the already marginal capacitive signal to ground, further weakening the signal and again resulting in the possibility of a key-closed condition going undetected.

Finally, capacitive keyboards in the past have tended to utilize specialized elements-elements which in the keyboard perform only a single function. Therefore the number of elements comprising a capacitive keyboard has tended to be relatively large, thus adding to the complexity, unreliability, and cost of capacitive keyboards.

Summary of the invention This invention relates to improvements in capacitive keyboards which alleviate the problems faced by the prior art capacitive keyboards and thus overcome their disadvantages.

According to this invention, a preferred embodiment of the improved capacitive keyboard comprises a panel which bears at least one stationary capacitor plate and a resilient membrane which defines at least one movable capacitor plate, the membrane positioned such that the movable capacitor plate faces the stationary capacitor plate to form therewith a capacitor. Spacing means positioned between the panel and the membrane for normally holding the stationary capacitor plate separated from the movable capacitor plate are also provided. The panel, the spacing means and the membrane are mounted around the capacitor to substantially seal the capacitor from external contamination.

In one embodiment of the invention a dielectric layer may also be positioned between the stationary capacitor plate and the movable capacitor plate. In a second embodiment of the invention, the dielectric may be formed by the resilient membrane. Preferably, the resilient membrane further defines at least one conductive trace connected to the movable capacitor plate to form at least one electrical connection to the movable capacitor plate.

In a third preferred embodiment of the improved capacitive keyboard an electrical circuit means has a first and a second capacitive element, with the second capacitive element formed from a plurality of compressible coil means. The coil means are preferably substantially spirally wound coils. The second capacitive element is adapted to selectively receive a force which causes at least some of the coil means to be moved with respect to the first capacitive element, to change the capacitance between the two capacitive elements. Preferably, the force is applied by a force application means which the second capacitive element normally urges toward its original position upon removal of the force. The force application means preferably includes a key.The key is normally biased by the second capacitive element into a position wherein the compressible coil means of the second capacitive element are extended. Upon selectively depressing the key the compressible coiled means of the second capacitive element are preferably compressed into a flat coil defining a capacitor plate facing the first capacitive element, which defines a second capacitor plate.

By forming a seal around or between the pair of capacitor plates of the capacitor defining each key position, the present invention prevents dust, dirt, and other external contaminants from entering between the capacitor plates, and thus it helps ensure that proper closure between the plates can be obtained. Proper closure of the plates results in full, undiminished closed-key capacitance being obtainable from the keying position's capacitor and thus the possibility of not detecting a key-closed condition is iessened.

By stacking the capacitor plates of a key position one on top of the other, as opposed to positioning them on the keyboard next to each other and coupling them with a bridging plate, the present invention essentially doubles the area that can be occupied by each capacitor plate and halves the effective separation between the plates. The doubling of the area of each capacitor plate and the halving of the effective separation between the plates results in approximately four times the capacity change, and hence approximately four times the signal strength than that which is obtainable from a keying position constructed according to the prior art technique of using a moving bridging plate between two side-by-side capacitor plates.

Because the available signal strength is substantially increased, partial shunting to ground of the signal by detection lead shielding is no longer a substantial problem, the signal which remains after shunting having been increased in strength proportionally to the increase in strength of the generated signal.

Furthermore, utilization in the keys of a resilient compressible member normally biased out of contact with the resilient membrane and the moving capacitor plate borne by the membrane, for pressing the movable capacitor plate towards the fixed capacitor plate to establish a closed-key position, provides both a pre-travel and an over-travel margin of safety in the keys. And if the resilient compressible member is positioned such that it is normally supported by the spacing means out of contact with the resilient membrane and the moving capacitor plate borne by the membrane, the need for a separate spring to normally bias the resilient compressible member into the normal position is eliminated.

Additionally, the embodiment of the improved capacitive keyboard which utilizes at each keying position a compressible coil means positioned over the fixed capacitor plate to form the movable capacitor plate as well as to bias the key means into a normal, key-open position, reduces the number of parts which comprise the keyboard, simplifies the keyboard construction, and hence improves keyboard reliability.

These and other advantages of the invention will become apparent in the following description of the presently preferred embodiments of the invention taken in conjunction with the drawings.

Brief description of the drawings Figure 1 is a fragment sectional view of a first preferred embodiment of the improved capacitive keyboard of the present invention, further utilizing a first preferred embodiment of the improved keys of the present invention; Figure 2 is a break-away view of a portion of the improved capacitive keyboard of Fig. 1: Figure 3 is a fragment sectional view of a second preferred embodiment of the improved capacitive keyboard of the present invention, further utilizing a second preferred embodiment of the improved keys of the present invention; and Figure 4 is a fragment sectional view of a third preferred embodiment of the improved capacitive keyboard of the present invention.

Detailed description of the preferred embodiments Turning now to the drawings, Figs. 1 and 2 show a capacitive keyboard, indicated generally as 10, which utilizes a first preferred embodiment of the present invention.

Preferably, mechanically rigid backing for the keyboard is provided by a rigid panel 11, such as a printed circuit board. Alternatively, the rigid panel 11 can be a sheet of plastic, metal, or any other suitably rigid material.

The panel 11 has defined on its surface at least one disc of conducting material, forming a stationary plate 12 of a capacitor. Each stationary plate 12 is connected by a conductive trace 13 (Fig. 2) to a soldering eyelet 14. The stationary plates 12, traces 13 and the eyelets 14 may be produced on the surface of the panel 11 by wellknown photolithographic techniques.

Alternatively, the stationary plate 12, the traces 13 and the eyelets 14 may be defined on a film membrane which is supported by the panel 11, as is done in the folded membrane technique, known to the art. The traces 13 and the eyelets 14 serve to make connections, via leads (not shown) soldered to the eyelets 14, between the stationary plates 12 and external electronics (not shown).

The stationary plates 12 are preferably substantially surrounded by a first spacing sheet 1 5 which supports a dielectric membrane 1 6 that extends over the stationary plates 12. The dielectric membrane 1 6 is in turn covered by a second spacing sheet 1 7 which substantially surrounds discs of conductive material, each of which discs forms a movable plate 18 of a capacitor.

The movable plates 1 8 are preferably defined on the proximal surface, the surface which faces the panel 11 of a resilient membrane 19 which overlies the second spacing sheet 1 7. The movable plates 1 8 are positioned such that each movable plate 1 8 faces a respective stationary plate 12, thus forming a pair of opposed plates 12, 1 8. Each pair of opposed capacitor plates 12, 1 8 forms a capacitor 29 which defines a single keying position of the capacitive keyboard 10.

Each movable plate 1 8 is connected by a conductive trace 20 to a connecting eyelet 21.

The movable plates 18, traces 20, and connecting eyelets 21 may also be produced on the surface of the resilient membrane 19 by photolithographic techniques. The traces 20 and the connecting eyelets 21 serve to make electrical connections between the movable plates 18 and external electronics (not shown). The connections to the connecting eyelets 21 may be made in any suitable manner, such as by clinchers, soldering, or connectors.

To utilize the keyboard 10, a voltage source (not shown) is preferably connected either to the eyelets 14 or to the connecting eyelets 21, and a capacitance is established between the pair of opposed plates 12, 1 8 of capacitor 29 at each keying position. A detection means (not shown) is preferably connected to the remaining set of eyelets 14 or 21. A desired keying position is selected and the resilient membrane 1 9 at the desired keying position is depressed toward the panel 11, thus moving the movable plate 18 toward the stationary plate 12. The change in spacing between the pair of opposed plates 12, 1 8 changes the capacitance between them.The change in capacitance is detected by the detection means, which produces an indication that the particular keying position has been selected, in a manner well known in the art.

Because the opposed plates 12, 18 of each keying position are stacked one over the other, as opposed to lying next to each other with a third bridging plate overlying them (as in prior art capacitive bridges), each keying position occupies only half the space of the keying position employed in such prior art techniques; capacitor plates 12 and 18 may be made twice as large as the plates of the prior art and still occupy substantially the same amount of keyboard space.

Because all of the layers that form the keyboard 10 are attached to each other, for example by adhesives, each keying position of the keyboard 10 is sealed from external contaminants such as dirt, dust and moisture.

Fig. 3 shows a second preferred embodiment of the capacitive keyboard of the present invention, generally designated as 10'. Because the keyboard 10 of Figs. 1 and 2 and the keyboard 10' of Fig. 3 are in many respects similar, the elements of the keyboard 10' which correspond with elements of the keyboard 10 are generally designated by the same numbers.

Elements of the keyboard 10' which differ from elements of the keyboard 10 are generally designated by prime numbers.

The keyboard 1 0' is structurally and functionally similar to the keyboard 10. The primary difference between them is that in the keyboard 10' the resilient membrane 19' functions as both the resilient membrane 1 9 and dielectric membrane 1 6 of the keyboard 10. The membrane 19' is preferably made of a suitable resilient dielectric material, such materials being well known in the art. As in the embodiment of Fig. 1, the one movable plate 1 8 is located on the distal surface of resilient membrane 19', the surface which faces away from the panel 11.

Thus, in the embodiment of Fig. 3 each movable plate 1 8 still faces its associated stationary plate 12, but the pairs of opposed plates 12, 18 are now separated by the resilient membrane 19', rather than by a separate dielectric membrane 1 6.

The membrane 19' is supported on the panel 11 by a single spacing sheet 17' which preferably substantially surrounds the stationary plates 12.

The panel 11, spacing sheet 17', and membrane 19' are again attached to each other by suitable means to seal each keying position of the keyboard 10' from external comtaminants.

To utilize the keyboard 10', connections are made thereto as with the keyboard 10. A desired keying position is selected, and the movable plate 1 8 is depressed at the desired keying position to deflect the resilient membrane 19'. As the movable plate 1 8 moves toward the stationary plate 12, a change in capacitance between the pair of opposed plates 1 2, 18 results. This change is detected in the same manner as in the keyboard 10.

To selectively key a keying position of the keyboard 10 or 10', a variety of means may be used. For example, the keyboard 10 is suited for use as a "touch-sensitive" keyboard, wherein operator finger pressure is applied directly to the resilient membrane 1 9 to move the movable plate 18. Alternatively, the keyboard 10 or 10' will by keyed by means of keys which are mounted over the keyboard with one key lying over an associated keying position. Various keys suitable for this purpose are known to the art. However, Figs. 1 and 2 disclose two embodiments of improved keys which provide advantages over the keys known to the art.

Fig. 1 shows a first embodiment of an improved keyboard key, generally designated as 30. Two keys 30 are shown, one in the normal, key-open position and the other in the actuated, key-closed position.

The keys 30 are mounted over the keyboard 10 on a key panel 31 which is supported on the keyboard 10 by spacers 32. The key 30 is generally comprised of a pushbutton 33 positioned above the key panel 31, a plunger 34 which is coupled at one end to the pushbutton 33 and extends through the key panel 31 terminating in a depending abutment 39. A biasing coiled spring 35 is mounted around the plunger 34 between the key panel 31 and the pushbutton 33 for normally biasing the key 30 into the key-open position. A resilient compressible member 36 is coupled to the other end of the plunger 34 for actuating the movable plate 1 8 by contacting and depressing the resilient membrane 19 to move the movable plate 1 8 towards the fixed plate 12.

In the normal key-open position, the compressible member 36 is not in contact with the resilient membrane 1 9; rather, there is a space 37 between them. Thus the key 30 provides a pre-travel safety margin, equal to the width of the space 37, which protects the key 30 from erroneously establishing a key-closed condition due to accidental partial actuation of the key 30.

The compressible member 36 is made of a resilient compressible material. When the key 30 is depressed, the compressible member 36 moves the movable plate 1 8 into its key-closed position, and then the compressible member 36 deforms or compresses to absorb further downward travel of the plunger 34 until abutment 39 contacts resilient membrane 19, effectively preventing further downward movement of plunger 34. Thus the key 30 provides an over-travel safety margin corresponding to the compressibility of the compressible member 36. The over-travel margin allows the push-button 33 to continue moving toward the panel 11 even after the moving plate 1 8 has moved toward the stationary plate 12 as far as it can. By cushioning the keying of the key 30 the compressible member 36 gives the key 30 a softer feel, as sensed by a keyboard operator.By absorbing vibrations, the compressible member 36 eliminates contact clatter by firmly positioning the movable capacitor plate 1 8 in the key-closed position, and thus the compressible member 36 assures that a steady key-closed signal is generated by the keying position. By resiliently urging the movable plate 18 into the key-closed position for the duration of its compression, compressible member 36 assures a longer, and hence a more certainly detectable, less detection error-prone, key-closed signal.

When the key 30 is released, the biasing spring 30 lifts the compressible member 36 away from the resilient membrane 1 9 and forces the key 30 back into its normal key-open position.

Fig. 3 shows a second embodiment of an improved keyboard key, generally designated as 30'. Again, two keys 30' are shown, one in the normal, key-open position and the other in the actuated, key-closed position. Because the keys 30 of Fig. 1 and the keys 30' of Fig. 3 are in most respects similar, their common elements are generally designated in Fig. 3 with the same numbers as in Fig. 1; certain of those elements which differ are generally designated with prime numbers in Fig. 3.

The essential difference between the key 30 and the key 30' is that in key 31 the resilient compressible member 36' itself provides the biasing force necessary to force the key 30' into the normal, key-open position, thereby eliminating the need for the separate biasing spring 35 of the key 30. The member 36' is supported across the resilient membrane 19' on the spacing sheet 15', on which it presses to provide the biasing force for the key 30'. As shown in Fig. 3, the face of the compressible member 36' which lies directly over the movable plate 1 8 is made substantially concave so as to form a pre-travel margin space 37' between the movable plate 18 and the compressible member 36'. With these modifications, the operation of the key 30' is substantially the same as that of key 30, described above.

Fig. 4 discloses a third preferred embodiment of the improved keyboard which combines the improved feature of both the keyboard and the key. The keyboard, generally designated as 10", comprises many of the same elements as the keyboards and keys shown in Figs. 1, 2, and 3, and therefore the same numerical designations are generally used for these elements in Fig. 4 as were used in Figs. 1,2, and 3. Elements which have not been described hereinbefore are designated by double prime numbers in Fig. 4.

The keyboard 10" utilizes at least one stationary capacitor plate 12 supported by a panel 11, which in turn supports a spacing sheet 1 5. Spacing sheet 1 5 and stationary plate 12 are overlayed with a dielectric membrane 1 6 which maintains dielectric separation between capacitor plates. Unlike the keyboards 10 and 10', however, keyboard 10" does not use the same movable capacitor plate 1 8. Instead the keyboard 10n utilizes compressible coiled means such as a substantially conically-shaped compressible coiled conductive spring 18D to form the equivalent of a movable capacitor plate.Each spring 1 8" is positioned with its base centered over the associated stationary capacitor plate 12.

The base of the spring 1 8" preferably terminates in a lead 38" which extends in a direction perpendicular to the plane of the base into the panel 11 to mount the spring 18" to the panel 11 and to provide an electrical connection to the spring 18". Alternatively, the lead 38" can extend in a different direction and be mounted to an element of the keyboard 10" other than the panel 11. The apex of the spring 18" preferably encircles an abutment 39" depending from plunger 34. The spring 18" is normally extended and biases the key 30" into a key-open position, as shown by the left-hand key in Fig. 4. When the spring 18" is extended, most of its loops lie substantially distanced from the stationary plate 12 and thus create a capacitive condition equivalent to that of the unkeyed state in the keyboards of Figs. 1 and 3.When the key 30" is depressed, the spring 18" becomes compressed into a substantially flat coiled capacitor plate lying in close proximity to the stationary plate 12, as shown by the right-hand key in Fig. 4. Thus a capacitive condition equivalent to that of the keyed state in the keyboards of Figs. 1 and 3 is achieved. When the key 30" is released, the spring 18" again becomes extended and biases the key 30" into its normal, unkeyed position.

Because the length of the spring 18" is large compared with the spacing of the opposed plates 12, 18 in Figs. 1 or 3, slight partial compressions of the spring 18" by accidental partial depressions of the key 30" have no significant effect on the capacitance of the key position. Thus the spring 1 8" provides a pre-travel safety margin. Of course, the downward movement of plunger 34 is stopped when abutment 39" contacts dielectric membrane 1 6.

By using the spring 18" both to form the movable capacitor plate and to bias the key 30" into its normal position, the use of a special biasing spring and the use of a resilient membrane to bear the moving capacitor plate is eliminated. This greatly simplifies the construction of the keyboard 10". Furthermore, the elimination of a resilient membrane carrying a conductive capacitor plate defined thereon by photolithographic means avoids fatigue problems such as permanent stretching and loss of resiliency by the membrane, cracking of the capacitor plate, and peeling of the plate from the membrane that carries it.

Of course, it should be understood that various changes and modifications to the preferred embodiments described above will be apparent to those skilled in the art. For example, the keyboards may be used without keys; also the various key designs may be used interchangeably with the various keyboards. Or more than two capacitor plates stacked one on top of the other may be used. These and other changes can be made without departing from the spirit and the scope of the invention, and without diminishing its attendant advantages. It is therefore intended that such changes and modifications be covered by the following claims.

Claims (14)

Claims

1. An improved capacitive keyboard comprising: a panel bearing at least one stationary capacitor plate; a resilient membrane defining at least one movable capacitor plate, said membrane positioned such that said movable capacitor plate faces said stationary capacitor plate to form therewith a capacitor; spacing means positioned between said panel and said membrane for normally holding said stationary capacitor plate separated from said movable capacitor plate; said panel, said spacing means, and said membrane being mounted around said capacitor to substantially seal said capacitor from external comtamination; and a dielectric layer positioned between said stationary capacitor plate and said movable capacitor plate.

2. The apparatus of Claim 1 wherein said membrane further defines at least one conductive trace connected to said movable capacitor plate to form at least one electrical connection to said movable capacitor plate.

3. The apparatus of Claim 1 wherein said membrane forms said dielectric layer.

4. The apparatus of Claim 1 further comprising key means normally positioned over said movable capacitor plate, said key means adapted to selectively move said movable capacitor plate toward said stationary capacitor plate.

5. An improved capacitive keyboard comprising: a rigid panel bearing at least one stationary capacitor plate; a resilient dielectric membrane having a proximal surface facing said stationary capacitor and a distal surface: at least one movable capacitor plate disposed at said distal surface, said movable capacitor plate positioned to face said stationary capacitor plate.

said movable capacitor plate and said stationary capacitor plate defining a capacitor; spacing means positioned between said rigid panel and said membrane for normally holding said stationary capacitor plate separated from said movable capacitor plate, said spacing means substantially encircling said stationary capacitor plate; said rigid panel, said spacing means and said membrane mounted to each other around said capacitor to substantially seal said capacitor from external contamination; and key means, normally positioned over said movable capacitor plate, adapted to selectively move said movable capacitor plate toward said stationary capacitor plate.

6. An improved capacitive keyboard comprising: a rigid panel bearing at least one stationary capacitor plate; a resilient membrane having a proximal surface facing said stationary capacitor and a distal surface; at least one movable capacitor plate disposed at said proximal surface, said movable capacitor plate positioned to face said stationary capacitor plate; said stationary capacitor plate and said movable capacitor plate defining a capacitor; spacing means positioned between said rigid panel and said membrane for normally holding said stationary capacitor plate separated from said movable capacitor plate, said spacing means substantially encircling said capacitor plates; said rigid panel, said spacing means, and said membrane mounted to each other around said capacitor to substantially seal said capacitor from external contamination;; a dielectric layer positioned between said stationary capacitor plate and said movable capacitor plate; and key means, normally positioned over said movable capacitor plate, adapted to selectively move said movable capacitor plate toward said stationary capacitor plate.

7. The apparatus of Claim 5 or 6 wherein said membrane further defines at least one conductive trace connected to said movable capacitor plate to form at least one electrical connection to said movable capacitor plate.

8. The apparatus of Claim 4 or 5 or 6 wherein said key means comprise: at least one resilient compressible member position to selectively contact said membrane or said movable capacitor plate when depressed, to move said movable capacitor plate toward said stationary capacitor plate; and spring means positioned to normally bias said member out of contact with said membrane and with said movable capacitor plate.

9. The apparatus of Claim 4 or 5 or 6 wherein said key means comprise: at least one resilient compressible member supported by said spacing means and positioned to normally repose out of contact with said membrane and with said movable capacitor plate, said member further positioned to selectively contact said membrane or said movable capacitor plate when depressed, to move said movable capacitor plate toward said stationary capacitor plate.

10. An improved capacitive keyboard comprising: a rigid panel bearing at least one stationary capacitor plate; at least one substantially conically-shaped compressible coiled spring defining, upon compression, at least one substantially flat coiled capacitor plate and positioned over said stationary capacitor plate to form therewith a capacitor; a dielectric layer positioned between said capacitor plates; and key means mounted over said spring including at least one resilient member positioned to contact at least the apex of said spring, at least one plunger extending movably through said mounting means and coupled at one end to said member, and at least one button mounted to the other end of said plunger for manually operating said key means.

11. The apparatus of Claim 10 wherein said spring terminates at its base in a lead substantially perpendicular to the plane of said base, said lead adapted for mounting said spring to said panel and for making electrical connection to said spring.

12. An improved capacitive keyboard key comprising: means for mounting the key over a capacitive keyboard; a resilient member adapted to selectively actuate a movable capacitor plate of a keyboard capacitor to move said movable capacitor plate toward a stationary capacitor plate of the keyboard capacitor; a plunger extending movably through said mounting means and coupled at one end to said member; and a button coupled to the other end of the plunger for manually operating said key.

13. The apparatus of Claim 12 further comprising: a coiled spring positioned around the plunger between said mounting means and said button to normally deactuate said movable capacitor plate.

14. A capacitive keyboard substantially as herein described with reference to the accompanying drawings.