CAPACITANCE-TYPE SWITCH

CAPACITANCE-TYPE SWITCH

1. Field_--ιof_the_^Invention

This invention relates to electrical switches and more particularly to a capacitance type switch. 2. Desc i£tion_of_the_Prior_Art

An example of such a switch embodied in a keypad is disclosed in U.S. Patent No. 4,367,385. A copper pad on the underside of a membrane is in capacitive relationship with a closely spaced underlying first aluminum film. The first aluminum film is in fixed capacitive relationship with a second aluminum film underlying and closely spaced from the first aluminum film. When the membrane flexes downward, the copper pad comes into contact with the first aluminum film, short circuiting the capacitance between them and thereby producing a large change in the capacitance between the copper pad and the second aluminum film. This permits detection of the key closure condition without requiring intermediate signal amplification. However, since this design involves contact of exposed metallic films, it is susceptible to faulty operation due to environmental contamination of the metallic films. In addition, since key operation is signified by a sudden increase in current flow at the instant contact occurs between the metallic films, it is necessary that the metallic film on the flexible membrane be sufficiently thick and/or of sufficiently high conductivity so that the resulting current is adequate for detection of that condition. A further problem with this design is that the sudden transition or jump in capacitance which occurs when the metallic films come into contact produces a transient current surge in the signal detecting circuit unless compensated by additional circuit features, known as "debounce" circuitry. This, of course, adds t.o the expense of the keypad. The debounce problem is described in some detail in ϋ. S. Patent No. 3,699,294. Summary_,_.gf_the..Invention The foregoing problems are solved according the the invention by a switch characterized in that it includes a pair of capacitive members, each comprising 1) insulating material, 2) a conductive element on one surface of the insulating material, and 3) a dielectric film on the conductive element. At least one of the capacitive members is movable. In addition, the capacitive members are supported in spaced generally parallel alignment.

The conductive elements face and are in general registration with one another, and the dielectric films are adjacent to but spaced from one another. Movement of the movable capacitive member into engagement with the other capacitive member causes the dielectric film on the movable capacitive member to move into engagement with the dielectric film of the other capacitive member, resulting in a detectable increase in the capacitance between the conductive elements of the capacitive members.

Since the facing conductive elements never come into contact, no sudden transition in capacitance occurs. Therefore, debounce circuitry is typically not required in the circuit to which the conductive elements are connected for detecting the change in capacitance between them. In addition, by providing extremely thin dielectric coatings on the conductive elements, there is only a minute separation between the conductive elements when full switch closure occurs. Therefore, the change in capacitance between them is sufficient to be detected without intermediate signal amplification.

A further advantage of the conductive elements not coming into contact is that no continuous current flow occurs between them. Consquently, the conductivity of the elements is not a critical factor in attenuation of the switch closure signal. This permits use of aluminum rather than more expensive high conductivity metals, such as silver. Furthermore, since the conductive elements are completely coated with dielectric material, they are totally protected from environmental contamination.

A feature of a capacitance switch in accordance with the invention is that capacitive elements, comprising the combination of dielectric coating and conductive elements, may be formed from metallic foil by a hot stamping process. Metallic foil is both economical and well suited to use in continuous production. It has been used, for example, for forming decorative metallic patterns on automotive parts. However, it has not heretofore been considered for the manufacture of capacitive switches.. The metallic films of such .foils are extremely thin, of the order of 100 to 1000 Angstroms, and the film resistance is therefor too high to permit sufficient current for reliable and economical signal detection if switch closure is signified by a sudden increase in current upon contact between metallic films. In the instant invention, however, there is no such contact and no sudden increase in current through the metallic films. Consequently, the film resistance does not significantly affect detection of the switch closure condition.

Brief Description of the Drawing

Other features and advantages of the invention will be apparent from the following detailed description and accompanying drawings, wherein: FIGS. 1 and 2 are greatly enlarged cross- sectional views of the open and closed condition, respectively, of a switch in a capacitance-type membrane keypad in accordance with the invention;

FIG. 3 is a graph showing the change in capacitance which occur upon operation of the switch shown in FIGS. 1 and 2; FIG. 4 is a perspective view showing the capacitive elements of a capacitance—type membrane keypad in accordance with the invention, the capacitive elements in combination with the supporting insulating sheet comprising capacitive members;

FIGS. 5 and 6 are greatly enlarged cross- sectional view of the open and closed condition, respectively, of a switch in a prior art capacitance-type membrane keypad; FIGS. 7 and 8 are schematic views of the electrical capacitance provided by the switch in FIGS. 4 an '5 respectively;

FIG. 9 is a cross-sectional drawing showing how the capacitive elements may be fabricated from metallic foil by a hot stamping process;

FIG. 10 is a cross-section of the capacitive elements;

FIG. 11 shows a metallic foil for use in fabricating the capacitive elements. Detailed .Descrietipn

The invention may be best understood by first referring to FIG. 5 shoving one of the switches in a prior art capacitance-type membrane keypad. On the lower surface of a five mil thick flexible membrane 1 of polyester insulating material there is a vacuum deposited pad 2 of copper of the order of 1000 Angstroms thick. Membrane 1 is supported on the upper surface of a one mil thick layer 3 of insulating material which has apertures therein dimensioned and arrayed in correspondence with the pattern of the switch positions of the keypad. Adhered to the lower surface of layer 3 is a capacitive element comprising a 1/4 mil thick layer 4 of polyester insulating material having aluminum films 5 and 6, of the order of 1000 Angstroms thick, respectively vacuum deposited of its upper and lower surfaces. This capacitive element is supported on another layer 7 of polyester material about 5 mils thick. Electrical connection (not shown) is provided to pad 2 and to . aluminum film 6 on the lower surface of insulating layer 4. This serves to connect the pad 2 and film 6 to a detecting circuit (not shown) in which a change in the capacitance between the pad 2 and film 6 produces a detectable signal.

FIG. 7 is the schematic circuit equivalent of FIG. 5, the capacitance C_ representing the capacitance between pad 2 and aluminum film 5 on the upper surface of insulating layer 4. Capacitance C . is in series with a fixed capacitance C-, the latter representing the capacitance between aluminum films 5 and 6. When, as depicted in FIG. 5, the switch is in the undepressed or open state, capacitance C. is typically about 30 picoforads. Fixed capacitance C_ is typically about 10,000 picoforads. Since the resultant series capacitance is given by ~ ^C71^C-2, the value thereof is then

1 ^L2 essentially just that of C_. or about 30 picoforads.

FIG. 6 shows the closed state of the switch in

FIG. 5, as when depressed by pressure exerted by an operator's finger. Membrane 1 flexes down through the aperture in insulating layer 3, and copper pad 2 thereon contacts aluminum film 5. As shown schematically in FIG. 8, the capacitance C. between pad 2 and upper aluminum film 5 is thereby short circuited, and the resultant capacitance between pad 2 and lower aluminum film 6 becomes just that of C , i.e., about 10,000 picoforads. The change from the open to the closed state of the switch thus produces a change in capacitance of more than 300:1. The resulting current pulse or signal is detected in the detecting circuit connected to pad 2 and film 6 without requiring additional amplification.

However, inasmuch as the drastic change from open state capacitance D_ή to closed state capacitance C essentially occurs just at the instant that copper pad 2 comes into contact with upper aluminum film 5, a series of spurious current pulses or "ringing" will be produced in the detecting circuit. Consequently, spurious circuit operation occurs unless the circuit includes compensating debounce circuitry.

Referring now to FIG. 1 there is shown a switch of a capacitance-type membrane keypad in accordance with the present invention. The keypad includes an upper sheet 10 and lower sheet 11 separated by a spacing sheet 12. Upper sheet 10, which extends generally parallel to the lower sheet 11, is a membrane of flexible insulating material, such as the polyester sold under the trademark "Mylar" by E. I. Dupont de Nemours and Company, and is preferably 0.5 to 3 mils in thickness. Lower sheet 11 and spacing sheet 12 also comprise and insulating material, which may be the same material as in the upper sheet 10. Lower sheet 11 may have the same thickness as the upper sheet 10, while spacing sheet 12 is advantageously 3 to 5 mils in thickness.

Spacing sheet 12 has apertures 13 (only one of which is shown) that are dimensioned and arrayed in correspondence with a geometric pattern of switch positions of a desired keypad configuration. In each aperture 13, the lower surface of the upper insulating sheeet 10 has a conductive element 14. The conductive element 14 is typically aluminum, of the order of 100 Angstroms in thickness, and may be formed b -vacuum deposition. The surface of the conductive element 14 is coated with a film 15 of dielectric material, such as lacσuer, also of the order of 100 Angstroms thick to provide a capacitive element that in combination with the insulating sheet 10 comprises a capacitive member. Similarly, in each aperture 13, the upper surface of the lower insulating sheet 11 of the keypad has a conductive element 16, coated with a film 17 of dielectric material to provide a capacitive element that in combination with the insulating sheet 11 comprises a capacitive member that is located in registration with the overlying capacitive member. The conductive elements 14 and 16 are therefore in a capacitive relationship, the capacitance between them being dependent on the thickness of the spacing sheet 12 separating them when the sheet 10 is in an undeflected condition. With the thickness so mentioned above, this capacitance is typically of the order of 15 to 20 picoforads.

FIG. 2 shows the closed condition of the switch depicted in FIG. 1A. Flexible insulating sheet 10 has been depressed by pressure exerted thereon by the finger of an operator in the selected switch position. This results in sheet 10 flexing through the aperture 13 in spacing sheet 12 until dielectric film 15 contacts dielectric film 17. This eliminates the air space between them, and since the combined thickness of the dielectric films is advantageously of the order of a few hundred Angstroms, the capacitance between conductive elements 14 and 16 increases by an order of magnitude. In this "closed" or fully actuated condition of the illustrated switch, a typical value of the capacitance between the conductive elements is 200 picoforads. This is about ten times the capacitance in the open condition of the switch, a change which is sufficient to permit detection of the signal thereby produced without requiring signal amplification.

In addition, since the conductive elements 14 and 16 have no exposed surfaces, they are completely protected from environmental contamination and the erosion which would occur if they were brought into contact by switch actuation. The absence of such contact also precluses continuous current flow between the conductive element 14 and 16, so that the conductivity of the elements does not significantly affect the signal produced upon key actuation. Thus it is unnecessary to employ expensive high conductivity metals, such as silver, and aluminum can be used for both conductive elements 14 and 16.

FIG. 3 is a graph showing how the capacitance between conductive elements 14 and 16 typically changes in response to actuation of a switch. The solid curve is for actuation by finger pressure, and the broken line curve is for actuation by a mechanical impact key. The capacitance is C in the open state of the switch, and in the fully 0 closed state, the capacitance increases to C.« Although, as illustrated, the time in which this change occurs will depend on the switch actuating mechanism employed, the impact key producing a more sudden change than direct finger contact, it is apparent that in either case the change occurs without any sudden transition or jump. Consequently, no debounce circuitry is necessary in a detection circuit connected to conductive elements 14 and 16 and in which an electrical signal will be produced by the change in capaciitance. A particularly advantageous economic feature of a capacitance-type membrane keypad in accordance with the invention is that the upper and lower capacitive elements may each be formed using commmercially available metallic foil by a hot stamping process. FIG. 4 is a perspective view of such capacitive elements 19 on the upper insulating sheet 10 of the keypad in FIG.1. The keypad comprises the conductive elements 14 patterned in a 3 x 3 array of switch positions and the dielectric film 15 overlying the conductive elements. Conductive interconnecting paths 20 extend between conductive elements 14 in the same row, and conductive terminating paths 21 at the end of each row connect the conductive elements of each row to external circuits. Interconnecting paths 20 are simply narrow extensions of the associated conductive elements 14, and they advantageously have an overlying layer of dielectric film that is a narrow extension of the associated dielectric film 15. Terminating paths 21 similarly comprise narrow extensions of the associated conductive elements 14. However, in lieu of an overlying dielectric film, the conductive paths are coated with an overlying conductive protective material such as a carbon filled polyester. This permits use of commercially available low insertion force connectors to make electrical contact with the terminating paths 21.

The capacitive elements on the lower insulating sheet 11, are identical with the capacitive elements 19 as shown in FIG. 4. However, when the insulating sheets 10 and 11 are assembled with the spacing sheet 12 to form a complete keypad as shown in FIG. 1, the lower sheet 11 is positioned so that its rows of interconnected switch positions are orthogonal to those of upper sheet 10. This establishes an intersecting matrix whereby the change in capacitannce which occurs upon actuation of a switch is identified to a particular switch position.

FIG. 9 shows how each of the capacitive elements of a keypad in accordance with the invention may be formed by hot stamping of commercially available metallic foil. The foil sheet is shown in cross-section, and comprises a polyester carrier sheet 22, such as Mylar. Coated on carrier sheet 22 is a layer 23 of release material,such as a thermoplastic copolymer, which is adherent thereto but releases and shears cleanly when heated to an elevated temperature in a particular area. Release layer 23, is coated with a film 24 of dielectric material such as lacquer which will become the top surface of the stamped pattern. Dielectric film 24 is itself coated with a metallic layer 25, typically aluminum, formed thereon by vacuum deposition. The final layer 26 of the foil sheet is a thermoset adhesive material which remains nonadhesive until heated to an elevated temperature. The foil sheet is positioned so that thermoset adhesive layer 26 thereof is adjacent and facing a substrate 27 on which the capacitive elements, such as capacitive elements 19 in FIG. 4, are to be formed, substrate 27 being of a flexible insulating material such as Mylar. It is supported on the machine bed 28 of a stamping press which also includes an electrically heated stamping head 29. Attached to head 29 is a block or die 30 which is patterned, by engraving or embossing, in correspondence with the geometric pattern of the desired keypad, such as that depicted in FIG. 4.

Heated stamping head 29 may be pneumatically driven, and when lowered, subjects areas of the metallic foil to heat and pressure in the switch position pattern set by die 30. This causes release layer 23 to separate from carrier sheet 22 and shear from the adjacent unheated areas, and renders thermoset adhesive layer 26 adhesive to substrate 27. As a result, all of the coatings and films which are carried by carrier sheet 22, except release layer 23, are transferred to substrate 27 in accordance with the geometric pattern off the switch positions. When stamping head 29 is raised and carrier sheet 22 stripped off, there is left on substrate 27 capacitive elements such as that shown in FIG. 4. As shown in FIG. 10, each capacitive element comprises the metallic layer 25 adhered to substrate 27 by adhesive layer 26, and the top coat lacquer film 24 completely coating metallic layer 25. A cross-section of the conductive interconnecting paths 20 in FIG. 4 would be the same as, but narrower than, the cross-section of the capacitive elements as shown in FIG. 10.

In order to form the conductive terminating paths 21 in FIG. 4, a corresponding region of the metallic foil may be provided with an overlying conductive coating over metallic layer 25 in place of the dielectric lacquer film 24. As noted above, such a conductive topcoat may be a carbon filled polyester, and is preferably of the same thickness as the lacquer film 24. This is illustrated in FIG. 11, showing the configuration of the metallic foil shown in cross-section in FIG. 9 prior to stamping, but with the carrier sheet 22 and release layer 23 omitted for clarity. The metallic layer 25 and thermoset adhesive layer 26 extend over the entire area of the foil, but the lacquer dielectric film 24 does not extend into the terminating region of the capacitive element where the terminating paths 21 shown in FIG. 4 are to be formed. Instead, in that region the metallic layer 25 is coated with a layer 31 of carbon filled polyester of the same ^• thickness or dielectric film 24. The aluminum layer in commercially available foils generally has a sheet resistance in the range of 1.5 to 2.5 ohms per square inch, but may also have significant levels of aluminum oxide which causes increased resistance. Lower sheet resistance, in the range of about 0.3 ohms per square inch, can be achieved by increasing the thickness of the deposited aluminum layer. Of course, a more conductive metal such as copper would also provide lower sheet resistance, and could be used if the additional cost is acceptable. However, in a keypad in accordance with the present invention, conductivity is of interest primarily in the interconnecting paths 20 and terminating paths 21 of each capacitive element thereof as shown in FIG. 4. As pointed out above, since the conductive elements in each key position- remain insulated from each other even when the key position is in the closed condition, no continuous current and little transient current flow occurs between them. Consequently, the conductivity of the conductive elements in the key positions does not materially effect detection of the key closure signal.

Stamping of metallic foil is well suited to continuous mass production of the capacitive elements of the keypad. A large roll of foil may be supported on a feed roller and intermittently advanced under the stamping press head, the successive capacitive elements stamped from the foil being successively advanced past the stamping head after each stamping operation and wound on a take-up roller.