SENSOR STRUCTURE

BACKGROUND OF THE INVENTION

[001] This invention relates to a structure for a sensor which can be touch or proximity based and which relies on the use of capacitive sensing techniques.

[002] A critical design parameter in a good capacitive sensing circuit is a plate which is used to detect variations in capacitance. The capacitance of the plate can vary due to coupling via a conductive contact or via any material that acts as a dielectric medium. Different materials have different capacitive coupling efficiencies.

[003] In the presence of a main supply, for example if a live or neutral conductor is to be switched, it is desirable to insulate the plate or any other surface that can be touched by a user from the main supply. This precaution should also be taken to safeguard against adverse effects arising from component malfunction. There are however significant advantages in certain touch applications if a surface which is to be touched is conductive.

[004] An electronic circuit is connected to the plate to monitor its operation. The connection of the circuit to the plate can be problematic and it would be advantageous if assembly of the plate and circuit could be done without the use of screws or other techniques, such as for example soldering, which result in permanent or semi-permanent conductive contacts. For example, the plate may be part of an outer surface of a product and the electronic circuit may be located under cover, in a housing. A connection between the plate and the circuit may therefore present practical difficulties.

[005] If a conductive plate is located on one side of a sheet of glass then, when the other side of the glass is touched, the good capacitive coupling characteristic of the glass causes a significant change in capacitance sensed via the plate. The change in capacitance is however dependent on the force which is exerted by the touch action. Also, a small area of touch (resulting, say, from a small finger contacting the glass) does not cause the same result as a large area of touch (resulting, say, from a large finger contacting the glass). Another factor is that it is possible to obtain different results when different areas of the glass plate are touched, even with similar touch actions.

[006] Applications using infra red (IR) technology, instead of capacitive coupling, to sense human proximity are quite common. These have several requirements that may not always be desirable such as high energy usage, location required, cost and cost of installation.

[007] The invention aims to provide a capacitor-based sensor structure which at least partly addresses the aforementioned factors.

SUMMARY OF INVENTION

[008] In a preferred embodiment the invention provides a sensor structure with a first member as part of a sense plate that is electrically conductive but insulated from its surroundings.

[009] The sensor structure may include a second member which is made from electrically non-conductive material and which has a first surface with at least one region which is electrically conductive.

[0010] The second member can be made from any suitable material such as glass or a plasties material. Glass, in particular, has good capacitive coupling characteristics.

[0011] The second member may be of any suitable shape and size and in one example of the invention the first member is a flat sheet.

[0012] The first surface may be treated in any appropriate way so that it is electrically conductive in the at least one region.

[0013] The at least one electrically conductive region may be formed by applying an electrical layer or coating using any suitable technique, e.g. an etching or deposition technique which deposits an electrically conductive coating on the first surface.

[0014] The second member may have a second non-electrically conductive surface which is spaced from the at least one region. At least one insulated conductor, e.g. the first member, may be engaged with the second surface or may at least be positioned adjacent the second surface. The insulated conductor may be of any appropriate form and for example may comprise an insulated wire or other insulated conductor (e.g. painted metal bracket), a flexible printed circuit board or the like which is capacitively coupled to the at least one conductive region on the first surface.

[0015] The insulated conductor need not be permanently attached to the first member and thus the tasks of assembling and disassembling the sensor structure are facilitated.

[0016] It is to be borne in mind that in a typical capacitive sensing circuit a wire, from an electronic circuit, is electrically connected to the surface of a plate structure. Usually this is done in a permanent manner through the use of solder or a loose contact structure such as a spring is used to bias the wire into electrical contact with the plate surface. In the former instance (the fixed connection) the assembly and installation operations are inhibited while, in the latter case, oxidation or variations in the resistance of the contact between the plate and the wire can result in unacceptable changes in the sensor capacitance. These aspects are overcome by the use of the insulated conductor which is engaged with the second surface.

[0017] With the method of the invention a double insulating effect is achieved in that firstly, insulation is provided by the insulating conductor and, secondly, insulation is provided by the insulating effect of the first member.

[0018] The insulated conductor can be in the form of a metal plate or frame which can be painted or covered with an insulating material to obtain a double insulating effect.

[0019] In a variation of the invention a multi-channel capacitive sensing structure is provided by forming the first surface with two or more electrically conductive regions which are separated from one another by at least one non-conductive border or boundary. This form of the invention facilitates the implementation of a matrix configuration in that, for each electrically conductive region on the first surface, two or more insulated conductors (depending on the matrix structure required) can be engaged with the second surface directly spaced from the corresponding region on the first surface at least by the material from which the first member is made.

[0020] If the sensor structure has two or more regions of the kind referred to then, in a proximity sensing application, it is desirable for maximum capacitance variations to be achieved in response to the presence, say, of a user's hand. However, if the sensor circuit is to be used in a touch application then cross coupling between adjacent regions on the first surface should be reduced. This can be done by introducing a ground plane between adjacent regions and by connecting the ground plane via a suitable resistance to earth. For maximum reduction in touch cross coupling the resistance should be low (of the order of 0 ohms). However for a proximity application the ground plane can be connected to earth via a relative high resistance, say 1 k ohm.

[0021] In some applications it may be desirable to register c\ touch only when a certain minimum pressure is applied to a sensing surface, for example by a user's finger. This can be achieved by covering the conductive region with an appropriate material which has a dielectric constant better than that of air and with an air gap between the cover material and the at least one region. Altemat ively compression of the cover material may be sufficient to register a touch, without the air gap being necessary.

[0022] The cover material may be insulating although in certair i applications it may be conductive with a relative high resistance.

[0023] Another possibility is to make use of a component which has good capacitive coupling characteristics and which is movable by a user into cont act with at least part of a conductive region on the first surface of the first member. In the absence of sufficient force the component is separated from the conductive π 3gion by an air gap.

[0024] Audible ^' feedback can be generated by allowing a lever or other mechanism to generate a click or similar sound when the component contacts the conductive region.

[0025] In a further embodiment the use of an active driven shield may be advantageous for preventing unwanted touch activations. For example in the use of capacitive sensing switch technology in a headlight (i.e. a flashlight to be mounted on a user's head such as a flashlight of the type sold under the name Princeton Tec Aurora) it may be desirable to locate a sense plate in a fastener eg. a strap on a side of the user's head. An active driven shield can then be employed to make the headlamp less sensitive to proximity/touch detection. This will enable the user to operate easily, i.e. to give commands to the headlamp by touching the strap on the side of the head, even with a glove. The user will still be able to adjust the headlamp angle by touching the product body. This will require that the body of the product is well shielded by the active driven shield. The concept of using an active driven shield to shield a noise-sensitive conductor is well known in the art.

[0026] In a specific embodiment the headlamp is put into a specific mode that will allow proximity-based operations through activation of an input device e.g. a switch in the form of a push button. In this mode, the headlamp can show its status visibly, or otherwise, to the user without consuming too much power.

[0027] In another embodiment any further operations of the input device, e.g. a normal push button switch of the headlamp, will cause an exit of the mode in which proximity activation and mode selection are allowed.

[0028] In another embodiment it is advantageous to activate the driven shield only once a proximity event has been detected. This may assist in more sensitivity in terms of proximity detection, by directing physical touch detection towards a specified area only. In a headlight application of the kind referred to it may be required that the user must specifically enter a specific mode in order to activate the load (i.e. the light source) by using a capacitive sensing type switch. For example it may be required to press a push button switch a number of times or for a predetermined period of time that is longer than the time a user would normally take to activate such a product.

[0029] As is described in US 6249089 and EP 1120018 any further operation of the push button, after a predefined period of push button switch inactivity, may then deactivate the capacitive sensing operational mode.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The invention is further described by way of examples with reference to the accompanying drawings in which:

Figures 1 and 2 are views of first and second sides of a multi-channel capacitive sensing structure according to one form of the invention;

Figures 3(a) and 3(b) illustrate a second side of sensing structure with a matrix configuration;

Figure 4 shows a first side of a multi-channel capacitive sensing structure which includes a ground plane;

Figures 5 and 6 are side views in cross section of a sensing structure in standby and operative modes respectively which is responsive only to the application of a minimum force;

Figures 7 and 8 are side views in cross section of different sensing structures according to the invention;

Figures 9(a) and 9(b) show the use of a compressible material to assist with registering a "touch" condition.

Figures 10 to 12 show a computer mouse with a capacitive sensing structure replacing a rotary wheel; and

Figures 13(a) and 13(b) show a proximity capacitive sensing application to activate a flow of water when required.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0031] Figure 1 of the accompanying drawings illustrates a sensor structure 8 which includes a first member 10 which is made from an electrically insulating material. The first member has a square or rectangular configuration outline (this is purely by way of example) and a conductive coating is applied to a first side 12 to form four distinct regions 14, 16, 18 and 20 respectively which are separated from each other by an insulating strip 22 which, in this configuration, is cross-shaped.

[0032] The first member can be made from a plastics material, glass or the like. The conductive regions can be formed by any suitable conductive coating, for example by making use of deposition techniques, painting techniques or the like.

[0033] Figure 2 illustrates an opposed second side 24 of the member 12. Perimeters of the strip 22 (on the first side 12) are indicated by dotted lines 26. Four insulated wires 28, 30, 32 and 34 are engaged with respective areas 36, 38, 40 and 42 on the non-conducting second side 24, notionally defined by the strip 22, at locations which are more or less centrally positioned and directly opposing the

respective regions 14 to 20. The insulated wires can be held in position using any suitable technique which dispenses with the use of solder or other permanent or semi-permanent attachment procedures. It is to be noted that the wires are separated from the conductive regions firstly by the insulating material from which the member 12 is made and, secondly, by the insulation on each wire.

[0034] The wires lead to a suitable sensing or control circuit, not shown. This aspect is however known in the art and for this reason is not further described herein. Inputs to the sensing circuit can be insulated and positioned such that the sensing circuit couples, according to requirement, with specific conductive regions 14 to 20 only.

[0035] Due to the geometrical layout of the structure cross-coupling effects between adjacent regions are fairly low.

[0036] The configuration shown in Figures 1 and 2 lends itself readily to the creation of a matrix structure of the kind shown, by way of example only, in Figures 3(a) and 3(b).

[0037] Figures 3(a) and 3(b) illustrate the second side 24 of a sensing structure 44 which has a first side substantially the same as what is shown in Figure 1. Each area 36 to 42 on the second side has two insulated conductors respectively, engaged with it. Insulated conductors 46 and 48 extend from the left of the layout to the areas 38 and 36, and 42 and 40, respectively while insulated conductors 50 and 52 extend from an upper end of the layout to the areas 38 and 42, and 36 and 40, respectively. This arrangement can be extended to increase the size of the matrix according to requirement.

[0038] Positive simultaneous signals on the wires 46 and 50, or on the wires 46 and 52, or on the wires 48 and 50, or on the wires 48 and 52 indicate, respectively, that the corresponding opposed region 14, 16, 18 or 20 has been touched by a user. Clearly this technique provides good discrimination between a touch and a no-touch situation.

[0039] A similar effect can be obtained by using a piece of material, for example in the form of a dome, made from a non-electrically conductive material e.g. a suitable plastic. An insulated wire is conductively attached to a conducting layer on an inner side of the dome which then forms the plate. Alternatively the conductive wire is positioned to achieve a good capacitive coupling to a conductive object such as a metal film or plate on an outer side of the dome.

[0040] A touch to a metallic or conductive surface yields a better result in terms of capacitive sensing than merely touching a non-conductive material. For example a soft touch and a hard touch are seen to be more similar in outcomes. This may not always be desirable but in some applications it is advantageous to offer a sure trigger for any touch (soft or hard) which may occur.

[0041] Figure 4 shows a multi-channel capacitive sensing structure 60 wherein conductive regions 62 and 64 on a first side 66 of the structure are separated by a non-conductive area 68 and by a ground plane 70 which is connected to earth through a resistor 72.

[0042] An opposing second side (not shown) of the structure 60 has electrically insulated conductors (28, 30) of the type shown in Figure 2 fixed to it in a way that results in a capacitive coupling between the members 62 and 64 and the wires 28

and 30. " Thus a touch on the first side is capacitively coupled to the respective insulated conductor. Even though a touch by a finger may be geometrically central in the region 62, say, there could be a degree of capacitive cross-coupling to the region 64. If however the ground plane is grounded with the resistor 72 at a low value (say 0 ohms) then the capacitive cross-coupling is dramatically reduced.

[0043] The ground plane 70 does have an effect on the sensitivity of the capacitive sensing circuit. This is not a problem and in fact is advantageous when dealing with a physical touch situation. If an objective of the circuit is to detect proximity, though, then the use of the ground plane can be detrimental. In the latter case (for proximity sensing) the resistor 72 should have a larger value, say 1 k ohm. This could present a compromise in terms of the sensitivity required for proximity detection and the separation of the touch detection situations between the sensing regions 62 and 64; or the resistor 72 could be large (several k ohms) until a proximity event is sensed upon which the resistance of the resistor is switched to a much lower value to help in sensing for touch, particularly if the separation of touch events on different regions is more important.

[0044] From the aforegoing it is apparent that it falls within the scope of the invention to adapt the sensing structure through the appropriate use of a ground plane specifically to yield better results, according to requirement, for proximity sensing or for touch sensing. In general, better stability and reduction of noise per channel can be obtained, when measuring a specific channel, if other channels are grounded. This step does however reduce sensitivity when proximity sensing is required. Thus when reliable proximity sensing is required it is preferred to leave the channels floating and for the ground planes, if any, to be connected to earth through a higher value resistance, or not at all.

[0045] In some situations it is desirable to register or record a touch only when a certain minimum force has been applied by a user. A tactile feel or feedback may be desirable as well. Because of the substantial differences in the extent of capacitive coupling of materials such as glass, Perspex, nylon, plastic and air it is possible to create a force-dependent touch sensor using capacitive-sensing technology. Figure 5 illustrates a structure 76, in a stand-by (non-switching) mode in which capacitive sensing plates (conductive regions) 78 and 80 on an insulating member 82 are covered with sheet material 84 which has a dielectric constant (capacitive coupling factor) which is higher than that of air. Air gaps 86 and 88 are formed between the cover and each respective region 78 and 80. As is shown in Figure 6, to place the structure in an operative mode, a user's hand must depress the cover directly above a respective region so that the cover is brought into contact with the region or, at least, is brought relatively closer to the region. Only when this occurs will a touch be registered.

[0046] Figure 7 is a cross sectional view of a structure 90 formed from a first insulating member 92 with tracks on a printed circuit board defining conductive regions 94 and 96. A cover material 98 overlies the regions. The cover material may be conductive although with a relatively high resistance. It may also be advantageous to form a ground plane 100 between the regions 94 and 96.

[0047] Figure 8 shows a push button type structure 102 in which a button component 104, located in a suitable recess 106 in a compressible or flexible cover material 108, is positioned directly above a conducting region 110 in a non- conductive first member 112. An air gap 114 is formed between opposing surfaces of the button 104 and the region 110. The button component is made from a material with good capacitive coupling characteristics. The button component is pushed by a

user, against a resilient force generated by the cover 108, until the button reduces the size of the intervening air gap 114 to zero and the button makes contact with the region 110. The button component at this stage couples decisively with the region 110 and a touch is recognised. The movement of the compressible cover, or the button component, can be linked to a mechanism which generates an audible sound to indicate to a user that switching action is taken place.

[0048] In a further embodiment (see Figure 9(a) which shows a no-touch condition, and Figure 9(b) which shows a load condition) compressible material 120 is used to cover sense plates (C _x i ... C _xn ) (122). This material has a low capacitive coupling factor. When uncompressed a touch on the surface (or on a layer of a separate material 124 that is on top of the material) does not lead to registration of a "touch". A proximity event may however be registered and may lead to a response. When a user exerts pressure (126) and the material is compressed beyond a certain minimum thickness (128) the capacitive coupling between the object and the sense plate arising from the pressure 126 (e.g. finger pressure) improves to such an extent, because of the higher material density and/or the smaller geometrical separation, that a "touch" condition is registered (Figure 9(b)). This structure prevents the detection of a "touch" when the user merely rests a finger on a surface (no or little compression) that is designed to detect a "touch".

[0049] It is also possible to have the top outer layer 124 constructed to provide a tactile "click" when enough pressure is exerted on it.

[0050] In a further embodiment, specifically related to the use of capacitive sense technology in computer pointing applications, such as for example a computer mouse, a rotary wheel of the kind which is presently found in a mouse is replaced

with a structure 130 using capacitive sense technology that does not require rotary (e.g. wheel) movement (Figure 10).

[0051] This structure 130 can be similar in outline to the wheel which is currently used, and can follow the profile of the mouse, essentially creating a flat surface upon which a user sweeps a finger in a forward direction or a backward direction.

[0052] In a preferred embodiment the structure 130 is shaped with a raised surface 131 which has a hollow 132 (Figure 11 ) or a hollow 132A (Figure 12(a) - side view, Figure 12(b) - end view, and Figure 12 (c) - plan view, respectively), on an upper side 133 of the mouse.

[0053] This has a natural feel to a user who no longer rotates a wheel but rather sweeps a fingertip in a specified direction. This structure can be designed for movement along or across a single axis, or a dual axis device (e.g. like a cross), or a multiple direction device (i.e. responsive to movement in three or more directions) (e.g. the inside of a hemisphere).

[0054] The single axis structure can also be adapted to achieve three further "click" button positions. If pressure (more than is required in a typical sweep movement) is applied backwards in a direction A (see Figure 11 ) a switch is triggered. Similarly, a switch is triggered if pressure is applied in an opposing direction B.

[0055] A further button activation can be achieved when pressure is applied straight down in a direction C. This corresponds to a push on a wheel as found on current rotary wheel mouse implementations.

[0056] IrI a further variation, there can be a non-linear correlation between the sweep movement of a user's fingertip over a touch sensor, to the movement of a cursor on a screen or to a sensor of a document on a screen which is produced by an output signal of the sense structure. For example, the sweep movement of the user's fingertip over an inner (centre) part of the touch sensor is non-linearly translated into movement of the cursor or to a scroll rate of the scrolling action, using a certain conversion factor. Referring to Figure 11 the backward sweep movement at the same speed nearer to a forward peripheral part 131 A or a backward peripheral part 131 B (also for flat or ball shapes) is translated to a different scroll rate using a different conversion factor. For example, a user moving a fingertip over a touch sensor around a central part at 2 cm per second can effect a scroll rate of, say, 10 lines per second. The same speed of fingertip movement close to a peripheral part of the touch sensor can result in a scroll rate of, say, 20 lines per second.

[0057] As such the user can decide what speed of fingertip movement is comfortable and choose a corresponding part of the touch sensor necessary to achieve the desired scroll rate.

[0058] It may make sense for the forward peripheral part 131 A to be associated with a conversion factor leading to a slower rate of translation and for the backward peripheral part 131 B to be associated with a conversion factor leading to a faster rate of translation. The rate of translation associated with the centre can lie between the faster and the slower rates. The conversion factor for the various parts of the touch sensor can be reversed for sweep movements in different directions, with the factor related to the centre part of the touch sensor being substantially constant.

[0059] Thus movement of a cursor on a screen is related (in this example) to the position on a structure upon which a user's finger operates, and on the direction of such sweep movement. This process of relating finger movement to cursor movement is more convenient than spinning the wheel of a typical mouse, faster or slower, and requires fewer moving parts to implement. Thus it is anticipated that the process will be more reliable and have longer mtbf (mean time between failures), and be cheaper than existing techniques to implement.

[0060] In a specific construction the sense plate on the mouse is constructed from a single electrically conductive region. The region must have some resistive properties which are preferably uniform over its surface. In some embodiments though the surface could have non-linear resistive properties to produce desired effects.

[0061] By using a single sensing channel connected to one side of the conductive region and with the other side connected to a dummy load (or another sensing channel), the position of touch by an object (e.g. a user's finger) can be determined (see other patent applications by Bruwer). It is then also possible to detect and track movement over the sense plate.

[0062] In other embodiments, movement of the mouse is sensed using capacitive sensing plates to reference the location of the mouse against some fixed reference points on a surface upon which the mouse is moved, or a tap or double tap on the touch sensor strip described may latch a movement on the screen with the position of where the tap occurred on the strip conveying information with regards to speed. The tap(s) may also be used for other function selection as per a electromechanical switch. For example a sweep of the finger may set a rate of movement and a tap within a predefined period thereafter may latch the movement until a further tap is

performed later. It may be advantageous to have further taps at various positions of the touch sensor strip increasing or decreasing the rate of movement in predefined steps or halting the movement. In a specific embodiment where the screen is scrolling at a set rate, the scrolling may be paused with a touch on the strip, to be continued upon removal of the touch condition. A double tap or a tap on a certain position thereafter may not only stop the scrolling but may also reset the screen to the position that was on the screen during the pause.

[0063] A further embodiment of this technology lies in the field of urinal flushers where most sensing is currently implemented using infrared (IR) technology.

[0064] It is common practice for an IR sensor mechanism to be placed above an urinal to trigger flushing in a manner which is dependent on movement, or on the presence, of a user. However capacitive sensing technology can be implemented in a system which is located inconspicuously and which can be installed at a lower price and which uses less power.

[0065] Thus, apparatus which uses any of the capacitive sensing mechanisms and sensor plates described herein can be used, embedded in or behind a suitable device section, at a suitable location, which fronts a urinal as is shown from one side in Figure 13(a), and in plan in Figure 13(b).

[0066] Based on proximity detection using capacitive sensing in areas adjacent regions A, B and C of a urinal 135, a control unit of a water dispenser or flusher of the ^' urinal can activate water-flow based on predetermined criteria or on a pattern of proximity detection signals. The urinal has a body with a bowl 137 in which is located a drain 139. For example, proximity (user presence) must be present for a minimum

period of time and must then turn to "no proximity" detected (user absence) for another minimum period of time to cause flushing. Power for an electronic circuit required to operate can be derived from long life batteries. It is also possible, though, to generate energy from water flow in the urinal to charge some energy storage facilities.

[0067] The power generating device can be hidden from the view of a user. For example, an antenna (C _x ) can be positioned behind or on a front lip of the bowl of the urinal, and conveniently can be at the nearest point to which a user is normally positioned. The antenna can be fully, or doubly, insulated to eliminate any chance of electrical shock. The antenna is connected to the electronic circuitry conveniently located out of sight. The antenna could be in the form of a plate that the user must stand on to use the urinal. This plate may be in the form of glass or other material with a conductive layer, or may be conductive wiring that is positioned conveniently between tiles on the floor.

[0068] In this regard use can be made, as necessary, of any of the techniques described in the specification of South African patent application No. 2006/08894, the content of which is hereby fully incorporated.

[0069] In the various examples of the invention which have been described herein, the sensing element, e.g. the insulated wire or sense plate, may in some embodiments be connected to the capacitive sensing electronics with a conductor that is shielded from the outside world by means of an active driven shield. This will help to prevent unwanted proximity or touch event triggers from occurring due to capacitive coupling taking place with areas other than the sense element.

[0070] The active driven shield also helps to improve the sensitivity of the capacitive sensor, particularly when operating from a non-mains power supply and without an earth.