Capacitive keyswitch membrane with self contained sense-to-ground capacitance

• US 4,920,343 A
• Filed: 09/30/1988
• Issued: 04/24/1990
• Est. Priority Date: 09/30/1988
• Status: Expired due to Fees

• Current Assignee: Key Tronic Corporation, Inc.
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Schwartz, Kenneth E.
• Primary Examiner(s): NOT, DEFINED
• Assistant Examiner(s): Queen, Tyrone
• Application Number:
  US07/252,228
• Time in Patent Office: 571 Days
• Field of Search: 341/33, 341/34, 341/26, 341/22, 340/825.79, 200/5 A, 200/5 R, 200/600, 200/511, 200/512, 200/292, 400/479, 400/479.1, 400/491, 400/491.1, 178/17 C, 178/17 D, 361/277, 361/278, 361/280, 361/281, 361/287, 361/288, 361/290, 361/291
• US Class Current: 341/33
• CPC Class Codes: H03K 17/98 having a plurality of control members, e.g. keyboard

Abstract

A membrane capacitive keyboard of the type including a first flexible dielectric sheet and a second dielectric sheet having electrically conductive patterns thereon which form an array of capacitors and connecting matrices of drive and sense conductors. The patterns on the sheets include a first array of capacitor plates on an inner surface of the first sheet and a second array of capacitor plates on the outer surface of the second sheet. A third array of electrically floating capacitor plates is formed by a third pattern on the inner surface of the second sheet. The sense conductors are capacitively coupled to ground through coupling capacitor plates connected to the sense conductors and a common capacitor plate overlaying the coupling capacitor plates.

Claims

1. A key actuable capacitive switch membrane assembly comprising:

flexible dielectric sheet means having first and second parallel proximate portions each with first and second surfaces arranged so that the first surfaces are adjacent and the second surfaces are separated by at least the first and second portions;

first, second and third patterns of electrically conductive material on the first surface of the first portion of said sheet means, the first surface of the second portion of said sheet means and the second surface of the second portion of said sheet means respectively, portions of said first, second and third patterns being aligned in a direction perpendicular to the portions of the sheet means so as to form first, second and third plates of an array of capacitors, portions of said first and third patterns forming first and second pluralities of conductors respectively arranged in a cross point matrix, the first and second pluralities of conductors being connected to the first and third plates of the capacitors in the array so that each capacitor is connected to a unique pair of conductors, portions of the second and third patterns forming coupling capacitor means for coupling the second plurality of conductors together ; and

spacer means interposed between the first surfaces of the first and second portions of said sheet means so as to normally maintain a predetermined separation between the first surfaces, said spacer means being of a design which permits the spacing of the first and second plates of capacitors in the array to be varied, whereby each capacitor has a variable capacitance between its first and second plates in series with a fixed capacitance between its second and third plates.

2. The capacitive switch membrane assembly of claim 1 further including keyboard actuator means for selectively applying force to the first portion of said sheet means at the locations of the capacitors to vary the capacitances of selected capacitors in the array.

3. The capacitive switch membrane assembly of claim 2 further including:

scanning drive means for sequentially supplying a drive signal to individual conductors of said first plurality of conductors; and

scanning sense means for sequentially sensing signals on individual conductors in said second plurality of conductors to detect the actuation states of capacitors in the array.

4. Capacitive switch membrane apparatus comprising:

a first flexible sheet of dielectric material having a first electrically conductive pattern on a first surface thereof, at least a portion of the first pattern being adapted to form an array of first capacitor plates;

a second sheet of dielectric material having second and third electrically conductive patterns on first and second surfaces thereof respectively, portions of the second and third conductive patterns being adapted to form second and third arrays of capacitor plates, the plates in the first, second and third arrays being arranged in corresponding patterns, the first and third conductive patterns further forming a cross point matrix of first and second pluralities of conductors each connecting a group of capacitor plates, the second and third conductive patterns further forming a plurality of coupling capacitors, each coupling capacitor having a plate connected to a separate one of the second plurality of conductors and a plate electrically connected in common with each of the other coupling capacitors, said first and second sheets being positioned and said second and third conductive patterns being arranged so that the first surfaces of said first and second sheets are in a proximate spaced relationship and so that the capacitor plates in the arrays of the first, second and third capacitor plates are at least partially aligned to form an array of capacitors each having first, second and third plates aligned along an axis perpendicular to the first and second sheets of dielectric material; and

spacer means located between the first surfaces of said first and second sheets of dielectric material so as to permit the spacing between the first and second plates of the array of capacitors to be varied, whereby each capacitor has a variable capacitance between the first and second plates in series with a fixed capacitance between the second and third plates.

5. The apparatus of claim 4 including:

scanning drive means for sequentially supplying a drive signal to individual conductors of the first plurality of conductors; and

scanning sense means for sequentially sensing signals on individual conductors of the second plurality of conductors to detect the state of individual capacitors in the array of capacitors.

6. The apparatus of claim 5 further including key actuation means for selectively varying the capacitances of the variable capacitors in the array of capacitors.

7. Capacitive keyboard apparatus comprising:

drive means a plurality of output terminals at which drive signals are produced in a predetermined sequence;

capacitance detector means having a plurality of input terminals and operable to sequentially detect whether the capacitances of variable capacitors individually connected to the input terminals exceed a reference capacitance, the operation of said capacitance detector means requiring capacitance within a predetermined range of capacitances between the input terminal at which capacitance is being sensed and a source of reference potential of said detector means, said detector means being operable to maintain each of the input terminals at the reference potential when it is not being addressed;

a first flexible dielectric sheet having a first electrically conductive pattern on a first surface thereof, the first electrically conductive pattern including an array of first capacitor plates and a first plurality of conductors connecting the first capacitor plates in groups, each first capacitor plate being part of a single group;

means for electrically connecting the output terminals of said drive means to said first plurality of conductors so that each output terminal is connected only to a separate one of the conductors;

a second dielectric sheet having first and second surfaces thereon;

a spacer layer located between the first surfaces of said first and second sheets, said spacer layer being adapted to permit the spacing between the first surfaces of the first and second dielectric sheets to be varied at the locations of the capacitor plates in the array of first capacitor plates;

a second electrically conductive pattern on the second surface of the second dielectric sheet, the second conductive pattern including an array of second capacitor plates and a plurality of second conductors connecting the second capacitor plates in groups such that each second capacitor plate is part of a single group, the array of second capacitor plates being arranged so that each second capacitor plate is aligned with a first capacitor plate in a direction perpendicular to the surfaces of said first and second dielectric sheets, each of the second plurality of conductors having a coupling capacitor plates electrically connected thereto, the coupling capacitor plates being laterally separated from capacitor plates in the arrays of first and second capacitor plates, the plurality of second conductors being connected to the plurality of input terminals of said capacitance detector means so that each second conductor is connected only to a separate input terminal of said capacitance detector means; and

a third electrically conductive pattern on the first surface of said second dielectric sheet, the third conductive pattern including an array of third capacitor plates of which a third capacitor plate is aligned with each first capacitor plate in a direction perpendicular to the surfaces of said first and second dielectric sheets, the third conductive pattern further including coupling capacitor plate means of which portions overlay the coupling capacitor plates connected to the plurality of second conductors, whereby the capacitance between the input terminals and ground required for proper operation of said capacitance detector means is provided through the coupling capacitor plate connected to the one of the second plurality of conductors being addressed through the coupling capacitor plate means on the first surface of the second dielectric sheet and through the coupling capacitor plates connected to the unaddressed conductors of the second plurality of conductors.