Capacitive touch switch arrangement

BRIEF DESCRIPTION OF THE DRAWINGS

While the specification concludes with claims which particularly point out and distinctly claim that subject matter which is regarded as the invention, it is believed that the invention will be better understood and appreciated when considering the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a perspective view of a capacitive touch control panel for a kitchen appliance;

FIG. 2A is a sectional view of a capacitive attenuator type switch cell known in the prior art;

FIG. 2B is a schematic circuit diagram representing the equivalent circuit for the touch switch cell of FIG. 2A;

FIG. 3A is a sectional view of another capacitive attenuator type touch switch cell known in the prior art;

FIG. 3B is a schematic circuit diagram representing the equivalent circuit for the touch switch cell of FIG. 3A;

FIG. 4A is a sectional view of a capacitive touch switch cell illustratively emboding the touch switch arrangement of the invention;

FIG. 4B is a schematic circuit diagram representing the equivalent circuit for the touch switch cell of FIG. 4A;

FIG. 5A is a plan view of the touch electrodes and associated circuitry carried on the front side of a dielectric substrate for a touch control panel of the type illustrated in FIG. 1, in accordance with one embodiment of the invention including a schematic diagram of the array of transmitting capacitors connected to the substrate circuitry;

FIG. 5B is a plan view of the receiver electrodes and associated circuitry carried on the rear side of the substrate of FIG. 5A;

FIG. 6A is a simplified schematic circuit diagram illustrating the incorporation of a touch switch arrangement embodying the present invention in a control 40 system for an appliance;

FIG. 6B is a schematic circuit diagram illustrating certain details of a portion of the circuit diagram of FIG. 6A;

FIG. 7 is a timing diagram illustrating the scan signals 45 employed in the circuit of FIG. 6;

FIG. 8A is a sectional view of another embodiment of a capacitive touch switch arrangement in accordance with the invention;

FIG. 8B is a schematic circuit diagram representing the equivalent circuit for the touch switch cell arrangement of FIG. 8A; and

FIG. 9 is a partial perspective view of an alternate capacitive touch control panel for a kitchen appliance incorporating an illustrative embodiment of the switch arrangement of the invention.

DETAILED DESCRIPTION OF THE
INVENTION

In FIG. 1, there is illustrated a capacitive touch panel 10 for controlling the four surface units of an electric cooking range. In the example to follow, a capacitive touch panel of the type illustrated generally in FIG. 1, is connected to a microprocessor that controls the operation of a kitchen range appliance. It is to be understood, however, that the illustrative capacitive touch panel has application to many other appliances and equipment subject to user control.

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Capacitive touch panel 10 comprises an array of touch sensitive switch cells 12 responsive to the human touch which in accordance with the invention include relatively small area touch pads 13 which can be located on closely spaced centers providing the relatively high number of switches in a relatively small panel area. In the illustrative example the touch pads enable the appliance user to select one or more surface heating elements and a power setting for'each element.

For ease of illustration, the switch array is shown as a 4x6 matrix of six rows and four columns of touch sensitive switch cells 12 to provide ON, OFF and four power level settings for each of four heating elements. It will be readily apparent that the switch array could be made larger or smaller as desired, depending upon the desired number of control inputs.

The description to this point, except for the relatively small closely spaced touch pads, is generally applicable to capacitive touch panel arrays well known in the art. The switch cell arrangement commonly used in such prior art arrays is of the type referred to as a capacitive attenuator switch type in which the body capacitance of the user, which is added to the switch circuit when a touch pad is touched, attenuates a scan signal, signifying to the associated control circuitry that the pad has been touched.

Referring to FIG. 2A, a prior art capacitive attenuator type switch cell arrangement is illustrated. The prior art switch cell designated 14 comprises a touch electrode 16 of conductive or semi-conductive material which is fabricated on the front or outwardly facing surface 17 of a dielectric substrate 18. A pair of electrodes or pads 20 and 22, commonly referred to as a transmitting electrode and a receiving electrode, respectively, are fabricated of a conductive or semi-conductive material upon the remaining inwardly facing surface 24 of substrate 18. Both transmitting and receiving electrodes are typically of substantially smaller area than and are positioned substantially within the boundaries of the area of touch pad 16. The closest points between transmitting and receiving electrodes 20 and 22 are separated by a preselected distance d, while each of electrodes 20 and 22 are separated from touch pad 16 by a preselected dielectric thickness T derived in accordance with the insulating and structural characteristics to be achieved. Transmitting electrode 20 has an associated conductor 26 coupled to a point thereon for connection of electrode 20 to a signal generator 28 configured to drive transmitting electrode 20 with a pulsating waveform. Receiver electrode 22 has an associated conductor 30 coupled to a point thereon for connection of electrode 22 to signal detection circuitry 32, which circuitry senses the signal coupled from electrode 20 to electrode 22 and detects the decrease in the magnitude of the signal resulting from the body capacitance of the user being introduced into the circuit by a touch of touch pad 16.

FIG. 2B illustrates the equivalent electrical circuit for switch cell arrangement of FIG. 2A. In this circuit, capacitance Ct represents the capacitance between electrode 20 and touch pad 16; capacitance Cr represents the capacitance between touch pad 16 and electrode 22; and capacitance Ctr represents the cross coupled capacitance between electrode 20 and electrode 22. The human touching of pad 16 is represented schematically as the closing of switch 34, with Cb representing the body capacitance of the user which is switched into the circuit by touching to act as a shunt to ground

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which is effective to attentuate the signal at Point A. Cp represents stray parasitic capacitance.

In order to insure proper switch operation, the capacitance of the series capacitors Cr and Cr formed by electrodes 20 and 22, respectively, in cooperation with touch pad 16, must be sufficient to reliably couple the pulsating signal from signal generator 28 to the detection circuitry 32. The larger the coupling capacitance, the greater the current flow which in turn reduces the susceptibility to noise.

The capacitance of the capacitors Crand Cr may be determined in accordance with the well known formula:

„ _ .225 K X A
c _ T

when

C=capacitance (picofarads)

K=dielectric constant of substrate

A=common capacitor plate area (sq. in.)

T=substrate thickness (in.)

The total coupling capacitance should be on the order of 2-5 picofarads (pF). This means Cr and Cr should each be on the order of 4-10 pF. As an example for coupling capacitance on the order of 3.5 pF, Crand Cr would each be on the order of 7 pF. For a typical glass substrate having a thickness of I inch, and a dielectric constant of 8, the minimum area for each of electrodes 20 and 22 would be on the order of 0.5 in2. In order to minimize the cross coupled capcitance Ctr, a minimum spacing between electrodes 20 and 22 on the order of J inch is recommended; Since touch pad 16 must substantially cover both electrodes, a touch pad area in excess of 1 square inch would be required for each touch pad.

An alternative touch switch cell arrangement in the prior art which may increase the capacitance somewhat for a given pad area is shown in FIG. 3A, designated 14'. In this arrangement an additional dielectric outer panel 36 overlies the substrate carrying the touch pad and electrodes. Because of the additional rigidity of outer panel 36, the dielectric substrate 18' carrying the touch pad 16' and electrodes 20' and 22' may be of reduced thickness which in accordance with the abovedescribed formula provides increased capacitance for the same plate areas. Those elements of FIGS. 3A and 3B carrying the (') designator function in the same manner as their counterparts in FIGS. 2A and 2B having the same numerical designators. Touch pad 16' and the transmitter and receiver electrodes 20' and 22', respectively, are driven by signal generator 28' and the coupled signal is sensed by detection circuitry 32'. The major difference functionally is illustrated in the equivalent circuit schematically represented in FIG. 3B, where outer panel 36 provides an additional capacitance Co in series with the body capacitance Cg of the user in the short path to ground when pad 16' is touched, as represented in FIG. 2B by the closure of switch 34'.

While the reduced thickness for substrate 18' enabled by the additional structural rigidity provided by outer panel 36, provides greater capacitance for the same pad and electrode areas, the additional capacitance effect of the glass outer panel 36 reduces the attenuative effect of the touch on the coupling capacitance, thereby requiring greater capacitance for capacitances Crand Cr than that for the touch cell of the FIG. 2A type in order for

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the detection circuitry to operate effectively at the same sensitivity level.

When a large number of touch pads are desired in a relatively small panel area, it is apparent that the mini

5 mum electrode and touch pad areas required to provide the minimum capacitance presents a significant design limitation for conventional capacitive attenuator type switch cells of either of the aforementioned type. Since in either arrangement, both receiver and trans

10 mitter electrodes must share the touch pad, the touch pad area required to provide the minimum capacitance for each of the series capacitances Crand Cr must be more than twice that required for the transmitting or receiving electrode alone.

15 The capacitive touch switch cell arrangement contemplated in the present invention allows the touch pad size to be reduced by more than 50 percent without sacrificing coupling capacitance and also eliminates any problems presented by the cross-coupling capacitance

20 between transmitter and receiver electrodes. This improvement is achieved by removing the transmitter electrode from the substrate and replacing it with a discrete capacitor separate from the touch pad and the receiver electrode. By this arrangement, the required

25 touch pad area can be reduced to the area of the receiver electrode alone without reducing the capacitance of the resulting receiver capacitance. An additional advantage of the discrete transmitter capacitor is the resultant flexibility to choose any capacitance value

30 during the keyboard tuning and trimming process.

Referring now to FIGS. 4A and 4B, an illustrative embodiment of a touch switch cell 40 in accordance with the invention comprises a conductive touch pad 42 fabricated on the outward facing surface 44 of dielectric

35 substrate 46, and a conductive receiver electrode 48 formed on the opposite surface 50 of substrate 46 in an area overlying and bounded by the area of touch pad 42 to enable the capacitive coupling of a signal from pad 42 to receiver electrode 48. A discrete capacitor 52 capaci

40 tively couples the pulsating signal from signal generator 54 to pad 42 via conductor 56 a substantial portion of which is formed on surface 44 of substrate 46. Receiving electrode 48 has an associated conductor 49 coupled to a point thereon for connection of electrode 48 to

45 detection circuitry 58. Detection circuitry 58 senses the signal at point A as in FIG. 2A to detect the change in the signal at A resulting from the human touching of pad 42.

The equivalent circuit for the touch switch cell ar

50 rangement of the present invention is illustrated in FIG. 4B. In this circuit receiver capacitor Cr is formed by touch pad 42 and receiver electrode 48. However, the function of the transmitter capacitor (Crin FIG. 2B) is performed in this arrangement by discrete capacitor 52.

55 As in FIG. 2B, the introduction of user body capacitance by touching of touch pad 42 is represented by switch 34 and capacitor Cb

Since only the receiver capacitor Cr uses pad 42 as a plate thereof, pad 42 need only be large enough to cover

60 electrode 48. For the same glass substrate and minimum capacitance requirements described with reference to FIG. 2A, touch pad 42 need be only on the order of 0.5 square inch rather than the 1.0 square inch of the conventional arrangement.

65 Having described an illustrative embodiment of an individual touch switch cell, reference is now made to FIGS. 5A and 5B to describe an array of such cells 40 such as might be used for a touch panel 10 of FIG. 1.