WO1998005025A1 - Capacitive position sensor - Google Patents

Abstract

A multi-electrode capacitive position sensor functions as part of a computer pointing device that can be integrated with the computer's keyboard (20) and that employs ratiometric capacitive sensing techniques. This approach allows a computer user to move a cursor (12) about on a display (14) by skimming his or her fingers (24) over the keyboard (20) without actually depressing any of the keys. The preferred sensing arrangement comprises a circuit for first charging a resistive layer (16) disposed on an active portion of the keyboard and then discharging the layer (16) by simultaneously connecting each of several spaced apart electrodes (36) to respective inputs of a multi-channel charge detection circuit (40). For a one-dimensional pointing device, two or more charge transferring switches (62) and electrodes (36) are required, with two being the preferred number. For a two-dimensional pointing device, three or more charge transferring switches (62) and electrodes (36) are required, with four being a preferred number.

Description

TITLE OF THE INVENTION Capacitive Position Sensor

TECHNICAL FIELD

The sensor and its method of operation pertain to the determination of a position of an object adjacent a predetermined sensing area In a specific embodiment the object is a part of an operator's body and an output from the sensor is used for moving a pointer or cursor on a computer screen under the operator's control Representative similar devices are commonly known as 'mice', trackballs, joysticks, touch screens, touch pads and the like, and are collectively referred to as pointing devices

BACKGROUND ART

Single-electrode capacitive sensors for sensing the proximate presence of an object are commonly used as non-contact electric switches (e g , elevator call buttons), as part of a water valve controller for operating a wash basin valve in a public restroom, for monitoring the flow of objects on an assembly line and the like Sensors of this sort are commonly configured to provide a binary output and operate by measuring a value of electrical capacitance to an electric ground If the sensor is configured as a "proximity sensor" it provides an output determinative of proximate presence when the value of the measured capacitance exceeds a predetermined threshold valve If the sensor is configured as a "motion sensor" it provides the determinative output when the rate of change of capacitance exceeds a predetermined threshold value

Multi-electrode capacitive sensors having a plurality of electrodes disposed about a predetermined sensing area can determine the position of an object adjacent the area and, by making multiple measurements over a period of time, can determine the direction and speed of motion of the object

Although there are many possible applications for multi-electrode capacitive sensors (e g , for controlling the intensity of a light or sound source responsive to an operator moving his or her hand across the sensing area), the application that appears to be of greatest interest is that of providing an input to a computer that uses the input to determine the position of a cursor on an associated display

Over the past decade, pointing devices have become mandatory instruments for dealing with the increasingly graphical nature of computer user interfaces At the same time, small, portable, battery- powered 'laptop' or 'notebook' computers have become increasingly popular This popularity of laptop computers has produced a need for new pointing devices of increasingly small size to cope with the dramatic reduction in available keyboard and surrounding workspace

Popular laptop pointing devices include inter alia (a) small trackballs on or adjacent the keyboard, (b) tiny joysticks interposed among the keyboard keys, (c) capacitive touch pads, and (d) accessory mice to be used near the laptop, but not built into it The joystick has been found wanting by many, as it is a limited movement 'velocity' device That is, small amounts of force applied laterally to the stick cause the pointer to accelerate to a velocity proportional to the force and to move in the direction of the applied force This less than convenient method requires considerable accommodation on the part of the user Capacitive finger touch pads, which may share the same housing as the keyboard but which do not share an active typing area of the keyboard, offer a significant improvement by presenting a more substantial surface area to the user, and allowing movements that are generally more intuitive than those of a joystick. Neither method has the large usable area of a conventional mouse placed on an adjacent desk surface The touch pad device also negatively affects the available keyboard layout, as it requires a large area in front of the keys Users also report that their thumbs can accidentally strike the pad while typing, thus dislodging the pointer from its prior position, causing frustration

Ng et al., in US 4,476,463, disclose a pointing device responsive to the position of a user's finger adjacent a computer display overlaid with a resistive sheet having three or more electrodes spaced thereabout In one embodiment, the position of the user's finger is determined from a sequence of capacitive measurements, each of which involves charging the sheet and then transferring charge from the sheet into an A to D converter In a burst mode of operation this charge /discharge /measure sequence may be repeated a predetermined number of times to obtain an average value from which a distance (relative to the one electrode at which the measurement is taken) is determined After all the electrodes have been interrogated, the position is determined (or over-determined) as the point satisfying all the distance criteria Ng et al 's teaching embraces an arrangement for resetting reference levels in the capacitive measurement to adapt to environmental changes

Yoshikawa et al , in US 4,680,430, teach a pointing device responsive to the position of a user's finger (which is capacitivelv coupled to ground) above a uniformly resistive layer of material Yoshikawa et al teach the use of their device as a coarse-resolution touch-screen input device, but do not disclose integrating their pointing device with a keyboard or keypad

Matzke et al , in US 4,736,191, teach a multi-electrode capacitive pointing device comprising an array of electrodes symmetrically disposed about a central point Their array, which is operated by sequentially charging and discharging the electrodes to capacitively determine the position of a user's finger adjacent the device, is disposed near the keyboard of a computer

Boie et al, in US 5,463,388, teach a computer mouse or keyboard input device utilizing capacitive sensors Their input device employs an array of metallic electrodes and detects the position of a user's finger from a centroid of capacitance values The position information is used to either move the cursor (if the apparatus is being used as a pointing device) or to select a character to be input (if the apparatus is being used as a keyboard) Boie et al do not disclose integrating their pointing device with a keyboard or a keypad

Brandenburg et al , in US 5,499,041, teach a keyswitch-mtegrated pointing device responsive to changes in the force and in the balance of forces applied to a resistor array made from a sheet of a resistive polymeric material Their resistor array is associated with a single key of a plurality of keys in a keyboard, each of the keys having a respective one of an array of parallel plungers associated therewith Their device is usable for pointing purposes only after their modified key has been pushed down to its lowermost position

Selker et al , in US 5,521,596, teach the use of a joystick disposed in the active typing area of a computer keyboard as a poinung device

Gerpheide et al , in US 5,565,658, teach a touch-pad capacitance-based computer pointing device using two orthogonal arrays of metallic stripes separated by an insulator They employ an arrangement tn which the capacitance measurement is made synchronously with a reference frequency selected to not be coherent with the interfering signal

The inventor, in his pending US patent application entitled "Charge Transfer Capacitance Sensor", having serial number 08/578,464 and filed on 12/26/95, teaches a capacitive field sensor employing a single coupling plate to detect change in capacitance to ground This apparatus comprises both a circuit for charging a sensing electrode as well as a switching element to remove charge from the sensing electrode and to transfer it to a charge detection circuit

Baxter, in a book entitled , "Capacitive Sensors, Design and Applications" (IEEE Press 1997

ISBN 0-7803-1130-2 ), describes a capacitive position measuring device having shaped electrodes and using a "slider" pickoff electrode Baxter also describes the use of a resistive sheet to generate a electric field in a ratiometric posiuon sensor employing a slider pick-off.

DISCLOSURE OF THE INVENTION

A preferred embodiment of the invention provides a multi-electrode capacitive sensor for a computer pointing device, wherein the sensor is integrated with the computer's keyboard and employs ratiometπc capacitive sensing means The preferred sensing means comprises both a circuit for charging a resistive layer and a predetermined number of switching elements, each of which is respectively connected to one of the predetermined number of electrodes, each of which is spaced apart from others of the electrodes The switching elements, hereinafter referred to as "charge transferring switches" or "discharging switches" remove charge from the resisuve layer and transfer it to a multi-channel charge detection circuit For a one-dimensional pointing device, two or more charge transferring switches and electrodes are required, with two being the preferred number For a two-dimensional pointing device, three or more charge transferring switches and electrodes are required, with four being a preferred number

In a particular preferred embodiment, the invention compπses a resistive gπd-shaped member having grid lines intermediate adjacent ones of the keys of at least a portion of a computer's keyboard and having a sheet resistance that is in excess of ten ohms per square but less than one megohm per square. Electrodes are attached adjacent the periphery of the reststive grid at a plurality of widely separated points (e g , the four comers or sides of a rectangular gπd) so as to be conterminous with the sensing area The electrodes provide inputs to a multi-channel charge detector having one channel associated respectively with each electrode The capacitive sensor is used to determine the posiuon of a user's fingers, hand, or other body part, or of a suitable hand-held object adjacent the keyboard or a defined portion thereof

It is a specific object of the invention to provide a computer pointing device comprising a capacitive sensor having a plurality of electrodes disposed adjacent a circuit board or other generally planar fixed support or baseplate underlying a keyboard This pointing device senses the presence of an object adjacent the keycaps and can be operated by a user who skims his or her fingers over the keyboard without having to depress any of the keys

It is an object of the invention to provide a multi-electrode capacitive position sensor compnsing a circuit for charging a sensing layer and a plurality of switching elements, wherein each of the switching elements removes charge from the sensing layer by means of a respective one of the electrodes and transfers the removed charge to a charge detection circuit It is a specific object of the invention to provide such a capacitive position sensor as a portion of a computer pointing device

It is an additional object of the invention to provide a computer pointing device comprising a resistive layer disposed adjacent a keyboard and a capacitive sensing means electrically connected to the resistive layer. It is a specific object of the invention to provide such a pointing device wherein the layer compπses a gπd having resistive gπd lines disposed between adjacent ones of predetermined keys on the computer keyboard It is an additional specific ob_ject of the invention to provide such a pointing device wherein the layer has a resistivity greater than ten ohms per square and less than one million ohms per square.

It is a further object of the invention to provide a computer mouse or keyboard input device utilizing capacitive sensors that automatically compensate for background capacitances, especially those distπbuted unevenly about the capacitive sensing area It is yet a further object of the invention to provide a computer mouse utilizing capacitive sensors that are immune to external radio frequency interference (RFI), including that from conventional multiplex scanning of an adjacent computer keyboard

BRIEF DESCRIPTION OF DRAWINGS

Figure 1 is a partially cut-away elevational view of a computer keyboard with a pointing device of the invention installed therein

Figure 2 is a partial cross-sectional view generally taken along the iine 2-2 of Fig 1.

Figure 3 is a plan view of a preferred configuration of a resistive layer portion of the invention

Figures 4, 5 and 6 are plan views of alternate electrode array configurations

Figure 7 is a partly schematic plan view of a single keycap configured for use with a one-dimensional capacitive position sensor

Figure 8 is a general schematic diagram of a circuit used for charging and discharging a capacitive sensor

Figure 9 is a schematic diagram of a preferred circuit for measuring a capacitance change from a single capacitive sensor, as may be used for sensing a position of a user's hand in the pointing device of the invention

Figure 10 is a schematic diagram of a preferred circuit for sensing the position of a user's hand adjacent an array of capacitive plates

BEST MODE FOR CARRYING OUT THE INVENTION

As is known in the art, a pointing device for a computer 10 allows a user to move a display indicator 12 (which may have the shape of a cross-hairs, arrow, or blinking rectangle, all of which are hereinafter referred to with the conventional term "cursor" ) about on a display 14 In a preferred embodiment, a pointing device of the invention compπses a resistive sheet or layer 16 disposed in a planar configuration beneath the tops of the keycaps 18 of a portion of a keyboard 20, as well as one or more touch pads 22 preferably disposed on a side or front surface of the keyboard 20 In other embodiments a patterned electrode array 17 can be employed instead of the resistive layer 16. Both of these arrangements provide a keyboard 20 that can be used in a conventional fashion, or that can be switched over to a pointing mode of operation when the user touches a predetermined one of the mode selector pads 22 In the preferred pointing mode the user provides the control input bv skimming his or her curled fingers 24 over the tops of the keyboard 20 This can be done without depressing any of the keys in the active typing region of the keyboard, without changing the curl of the fingers, and even without altering the number of fingers proximate to or in contact with the keycaps 18 These user motions, as will be disclosed in detail hereinafter, are sensed by sensing circuitry 40 of the invention, which has an output to cursor location controlling means so that the cursor 12 moves responsive to a motion of the user's hand or fingers 24 It will be understood by those reading the disclosure hereinafter made that although various arrangements are disclosed for a two-dimensional pointing apparatus and method, the same apparatus and method could be employed for one dimensional pointing (e g , in an arrangement having only two electrodes) or for quasi-three-dimensional pointing - e g , in an arrangement in which the position of the user's finger along the Z-axis (l e , perpendicular to the plane of the keyboard) ~ is measured and used to indicate a mouse click, or a Z-axis position in a three-dimensional graphics program or a computer game The requisite Z- axis measurement can be done either by observing variations in signal strength from one or more channels of a multi-channel analyzer or bv obtaining, from the keyboard controller, an indication that one of the keys had been depressed It will also be understood that although it is expected that most users of the pointing device of the invention will employ their fingers or hands for pointing, the user could also move some object or another about over the active region for pointing purposes

The actual active sensing area, e g , the portion of the keyboard 20 encompassed by or conterminous with the sensing layer 16, can be a sizable fraction of total keyboard area In various tests, the invention has functioned using as little as one third to as much as the entire keyboard area In one embodiment, the resistive laver 16 takes the form of a parallelogram encompassing that portion of a conventional QWERTY keyboard bounded bv the "u", "m", "/", and "p" keys Within this active area large scale motions of the screen pointer can be made in response to single motions of the hand Furthermore, it will be understood to those skilled in the art that the surface area can be made to correspond to absolute coordinate positioning instead of differential positioning as is done with most contemporaiy pointing devices Absolute positioning allows the user to mentally map the display screen 14 with hand positions that do not vary Even when errors are made in hand position relative to the screen, as when the user places her fingertips on a section of keyboard thought to correspond to an icon, only small corrections need be made This method is conceptually and practically superior to the differential methods employed currently, which reference relative motions only

A computer mouse, or other pointing device, commonly comprises two or more keys that the user can momentarily actuate, or "click", to select a program feature indicated on the display 14 and pointed to with the cursor 12 In the pointing device of the invention clicking may be provided by the mode selecting means 22 For example, a pointing device of the invention could compπse three touch switches 22, one of which would be used to select the pointing or "mousing" function, and die other two of which could be used analogously to die two buttons on a conventional mouse. It will be understood to those skilled in the art that odier approaches to selecting a program feature could also be used widi die pointing device of the invention once the pointing mode had been selected. For example, specific keys (e.g., the "e" and "c" keys on a conventional "qwerty" keyboard) in a keyboard that had die resistive sensing layer 16 under only a right-hand portion thereof could be used for clicking whenever the pointing mode was selected. Placement of the field electrodes can be under any desired portion of the keyboard, including die left or right portions of the main keyboard, or under a numeric keypad subsection. If desired, near-duplicate field generating structures can be inserted into both left and right portions of a keyboard and means can be provided to select one of these two portions in order to accommodate both left- and right-handed users.

As noted hereinbefore, a preferred sensor comprises a resistive layer 16 disposed across a region encompassing a plurality of keycaps 18 of a keyboard 20 of the conventional type wherein an upper portion of a key, e.g., a keycap 18, is movable along a line generally perpendicular to a fixed support 23 between a resting position in which the keycap is most distal from the fixed support and an actuating position in which die keycap is most proximal to the fixed support. (Because the keyboard is commonly operated in a horizontal attitude, die most distal position will hereinafter be referred to as the "uppermost" position, while the most proximal will be called the 'lowermost"). When the keycap 18 is moved to its lowermost position a characteristic output, uniquely associated widi diat key, is supplied to die computer. In many keyboards, the moving keycaps communicate with respective momentary contact switches 21 disposed on the fixed surface 23 (e.g., a circuit board) by means such as an array of parallel (and generally vertical) plungers 30. In other keyboards, as is well known in the art, non-contact position sensing means (e.g., a Hall effect device or a dedicated capacitive sensor) are used instead of a switch comprising two metal contacts. It will be clear from the disclosure herein provided that the sensing and pointing means of the invention are independent of the arrangement used by the keyboard for supplying character outputs to the computer.

The preferred resistive layer 16 has a plurality of throughholes 28 formed therethrough, each of the throughholes accommodating a portion of a respective key switch 21 so that the resistive layer is disposed thereabout. It will be understood that in some keyswitch designs the diroughhole 28 need only accommodate a relatively narrow plunger 30. In designs that are both more common and more challenging from the viewpoint of trying to dispose a grid-like capacitive sensing surface beneath die keyboard, a relatively wide lower portion of the keyswitch 21 extends upwards from the circuit board 23 through the diroughhole. Embodiments of the invention have been made and tested in which the diroughholes 28 aggregated to 85% of the total surface area covered by the resistive layer 16. It is also noted diat in order to avoid shorting out conducting traces on the printed circuit board 23, an insulating film 35 may be provided between die resistive film 16 and d e fixed substrate 23. It will be recognized to those skilled in die art of capacitive sensing diat conductive sheets (not shown) may be required at various locations on or behind die circuit board in order to shield the back side of die sensing array and to provide a ground plane. Although pointing devices of the invention have been retrofitted into several commercially available keyboards, it is expected that a production version of the sensor will be designed in conjunction wid die associated keys and circuit board.

It will be recognized from the ensuing disclosure of the mediod and apparatus for extracting electrical charge from the resistive layer that although the preferred embodiment may function widi a diaphanous resistive layer 16, the invention could more easily function by using a resistive layer that was free of throughholes. A resistive layer of that sort could, for example, serve as one of the layers of material making up a membrane or capacitive keyboard. Touch-screens and keyboards of this type, because they have little or no perceptible key motion, are generally perceived as being inconvenient to use, but, because they are easily environmentally sealed, are often used in industrial and food service applications.

In order to improve the electrical uniformity of the sensing layer, one may employ a resistive layer of the sort depicted in Fig. 2 that has upstanding wall-like portions 32 filling gaps between adjacent keycaps 18. These upstanding wall-like portions 32 may extend in only one direction along die keyboard (e.g., along the QWERTY keyboard's X-axts as a first ridge disposed between die uiop[ keys and the jkl;' keys and as a second ridge disposed between the jkl;' keys and the nm,./ keys) or may extend in bodi the X and Y directions so as to define an "eggcrate" set of walls surrounding each key in an active portion of die keyboard. Moreover, one may elect to coat the bottom and/or bottom edges of the keycaps 18 wid a conductive film 34 making electrical contact with die filler walls 32. In an embodiment comprising keycaps with conductive coating and an eggcrate-like resistive layer 16 having upstanding wall portions 32 extending along both keyboard axes, it will be recognized that die portion 37 of the resistive layer 16 that is parallel to the printed circuit board 23 is electrically superfluous and may be dispensed widi unless the apparatus is more conveniendv manufactured with the portion 37 present. Thus, the resistive layer 16 may be a generally flat geometrical sheet, or it may comprise a honeycomb array of short tubular segments. Moreover, it will be recognized that a variety of resistive keycap portions can be used instead of the preferred film 34. For example, one could make the entire keycap of a material having an appropriate resistivity, or could fill the underside of a keycap with a diick annular resistive body — e.g., formed by injecting a curable resistive resin into die underside of each keycap. As an additional example, one may consider modifying a prior art membrane keyboard (which already comprises a conductive elastomeric layer having an appropriate sheet resistivity for use in the present invention) by reshaping the conductive layer and /or attaching electrodes thereto.

Although a resistive layer having a honeycomb structure, as described supra, can be made from a plurality of tubular segments joined at points of circumferential contact among adjacent ones of die segments, it will be noted that it is the electrical contact among segments that is of significance, and not die mechanical joining. Thus, one could configure a "phantom layered" keyboard in which each keycap used in a active region of die keyboard had a resistive skirt on its outside lateral surface 33 (e g , if the keycap was made of a material having an appropriate resistivity, or if the lateral walls of the keycap were coated with such an appropriate film 34) and in which all of diose keycaps were in lateral rubbing contact In this arrangement the array of resistive skirts would be electrically and functionally equivalent to the physical honeycomb array forming the layer 16 depicted in Fig 2

It can be appreciated that die mechanical interface between grid and keys as shown in Fig 2 can be altered significandy and still operate in general accord with die invention The intent is merely to make contact substantial enough between grid and keys, with sufficient reliability, so as to allow finger(s) to be capacitively detected in a smooth and uninterrupted fashion without lumpiness in die detection field Non- lineanties in the field occurring over the planar surface can be corrected for in software with a look-up table and an interpolation method, or in a madiematical algonthm designed for the purpose Moreover, although only a single finger 24 is shown in the depiction of Fig 2, if the user skims more than one finger (e g , the depiction of Fig 5, which is believed to be an ergonomically preferable approach) over the active pointing area of the keyboard, he or she will find a smoother response because the centroid of the set of fingers is sensed, thus reducing any effects of key-to-key discontinuity

In the preferred embodiment, die layer 16 is a four-sided plane figure with a metal electrode 36 at each of its four comers (not shown) or at the center points of each of its four sides, as depicted in Fig 3 Each of the electrodes 36 is connected by a wire 38 to a respective input of a four channel capacitive sensing circuit 40 of a sort to be discussed hereinafter with particular reference to Figs 8-10 As a user's fingertips 24 travel across the keys, die resultant increased capacitance is sensed by each channel of the four channel sensor 40 Ratios of the outputs of the four channels are taken, and with further processing, such as two dimensional interpolation and lowpass filtering, the fingertips' position can be ascertained with reasonable precision It will be understood to those skilled in the art that such a ratiometric approach could be made to work with diree or more contacts to the resistive laver 1 Both the simplicity of algorithmic processing and the generally rectangular shape of the display 14 on which d e cursor 12 is to be moved, however, indicate that a four electrode measurement is to be preferred Moreover, it is known in the art that nonlineanties arising at the edges of the array or ansing elsewhere on the sheet due to a given choice of electrode configuration can be compensated for via re-mapping of coordinates in software using a two dimensional lookup table with interpolation, or, by recourse to known calculational procedures

The four-sided preferred layer 16 of the invention, for example, may be disposed under the keycaps 18 of a conventional qwerty keyboard with a first x-axis electrode (denoted as XI) adjacent die "D" key, a second x-axis electrode (denoted as X2) adjacent the "L", a first y-axis electrode adjacent the "7" and die second v-axis electrode adjacent the "N" This arrangement has provided the easiest computation set-up of all the pointing devices of the invention diat have been tested to date The ratiometric processing consists of determining how far to the left or right the user's finger 24 is from the center of the layer by recourse to C(X1), the capacitance measured at XI, and C(X2), the capacitance measured at X2. That is, the relative displacement is C(X1)/C(X2) if C(X1) > C(X2), and is C(X2)/C(X1) if C(X1) < C(X2). The same sort of processing is used for the Y axis. It will be understood to those skilled in the art that one may normalize the capacitance values to compensate for hand size and finger position, e.g., by subtracting a correction term K from each value, so the relative X-axis displacement is calculated from (C(X1) - K)/(C(X2) - K), etc.

As an illustrative case for explaining the ratiometric method of die invention, consider a one- dimensional position measurement arrangement using a single keycap as the active sensing region (see Fig. 7). Two metallic electrodes 36 (labeled XI and X2 in the drawing) are in electrical contact with opposite sides of die keycap 18. A centroid, or effective point of contact 39, of a pointing object (e.g., a user's finger 24) is depicted as being in touching, or near-touching, contact with the keycap 18 and is shown as being coupled to an electrical ground 43 by its capacitance 45 (denoted in Fig. 7 as COBJ )• Prior to discharge, the same potential is applied to bodi electrodes 36 so there is no electric field gradient across the key. When both electrodes 36 are discharged simultaneously die difference in capacitance between die effective point of contact 39 and the two electrodes ensures that a greater amount of the charge from the object 24 flows to the closer of the two electrodes. In the depiction of Fig. 7 the object 24 is shown as having an initial positive charge with respect to ground 43, and the two different quantities of charge ( Qχι and Qχ2 ) are represented by sets of arrows 47a, 47b having different numbers of arrows in the two sets. After the charge has been completely transferred, the electrodes 36 and object contact point 39 are again at the same potential.

It is noted that die ratiometric capacitive measurement, when applied to computer pointing, is not confounded by finger height above the keys, which would odierwise lead to erroneous and unpredictable positioning. By allowing and requiring die amplitude of two capacitive signals to be processed raαometrically for each of the two planar dimensions, signal amplitude (which is related to fingertip surface area and height above the keys) becomes much less of an independent variable. This processing should occur in each of die two dimensions. Should the signal of any one of the sides of die layer fall below a minimally acceptable threshold value, processing may be halted and the screen pointer held stationaiy.

In order to calculate a position centroid using the simple geometry of the preferred embodiment, d e value of die resistance of the resistive film 16 is of some concern. If, for example, this layer were a perfect conductor it would charge and discharge instantaneously and all four capacitance values would be identical, regardless of where the user's finger was placed. More generally, a lower limit on die sheet resistance is dictated by the desired spatial resolution. This lower limit can be extended to below one hundred ohms per square by putting in a summing junction (e.g., using an op amp). It will be understood that one upper limit on the sheet resistivity is set by speed requirements - ι e., the time required to drain charge from the layer should not be so long as to cause a perceptible lag in pointing In most cases, however, the working upper limit on sheet resistivity anses from inhomogeneity - i.e , as sheet resistivity increases many resistive gπd arrangements of the invention become too inhomogeneous to be useful. Generally, the capacitance from film to fingers is less than about 1 picofarad and die relevant RC time constant must be such diat die ratios can be repeatably and accurately determined without severe amplification lest noise affect the measurement It has been found that in order to properly discriminate among the sub-picofarad values associated with a user's fingers spaced shghdy apart from the keyboard one preferably employs a resistive layer 16 having a sheet resistivity on the order of several kilo-ohms per square, a value that can be obtained by making the layer 16 from a graphite-loaded paper or from an appropπate polymer-based conductor

One may note that resistors made from materials with sheet resistivities in the preferred kΩ/square range commonly suffer from problems associated with spatial inhomogeneities across a sample that is several centimeters on a side, with lack of sample-to-sample reproducibility, and with high temperature coefficients of resistance All of these effects could be deleterious to a capacitive or resistive method of moving a cursor to absolute coordinates on a display, but have not been found to be of serious concern for a pointing device One reason for the lack of a problem appears to be that die human operator can adjust to accommodate for such effects - e g , a highly inhomogeneous sheet may be perceived as being die same as a conventional mouse widi a slight acceleration curve Also, ratiometric measurements are not affected by a high temperature coefficient of resistance as long as all portions of the layer experience the same temperature change

Similar conductive laver methods are well described in die patent literature in conjunction with pointing devices, however none describe a mode of operation whereby a group of fingers may be spaced a distance apart from the conductive film by a gap compnsing plastic, or plastic and air

It will be understood to diose skilled in die arts of capacitive proximity sensing that a ratiometric capacitive sensor can also be made using an array of patterned metallic electrodes 42 disposed on an insulating layer or substrate 44, where the electrode geometry is selected to generate a varying capacitive output as a proximate body (e g , a stylus or the user's finger) moves across the electrode array It may be noted that die shaped electrode embodiments also preferably employ an overlay of some sort to set a minimum distance between the pointing object and the electrodes in order to prevent a user from becoming aware of the discrete electrodes. A sufficiendy thick piece of plastic above the shaped electrodes, for example, "blends" the fields from the various electrodes so as to provide a smooth response to the user's pointing motions. As an example, consider the array 42 depicted in Fig 4 This arrangement compπses four interspersed electrode sets, two for each dimension. The x-axis sets, which are triangular, are easier to see and understand A first set of triangles 46 are all electrically connected together to an output buss denoted as XI The second set 48 are also connected together to an output labeled X2 The position of a user's hand with respect to the x-axis can be ascertained from the ratio of signals from XI and X2 Because capacitance is direcdy proportional to surface area, and because the plates connected to XI aggregate to a greater surface area to the left dian do the plates connected to X2 (and vice versa) the ability to take the ratio of X1 /X2 or X2/X1, as discussed supra, is preserved so long as a great enough finger area is over the pattern 42 at a close enough range to provide sufficient signal strength This design also prevents fingertip location on the Y axis from affecting the X measurement A corresponding set of plates are connected to the Yl and Y2 busses The Y-connected set is also ratiometric, although in a manner different from die X sets The Y set consists of alternating Yl-connected and Y2-connected rectangular strips having a y-axis dimension mat varies widi placement in such a manner so as to create a smoothly varying ratio of surface area between Yl and Y2 with location The sum of each pair (e g , die strips labeled 50, 51) of the y-axis strips is made constant so that die sum of die capacitance is the same for anv two paired stripes, I e , C(Y1) + C(Y2) = C(Y) for each pair of strips Then, as the user's fingers move along the v-axis, die detected capacitance ratio is measured in die same manner as the C(X1)/C(X2) ratio, ι e the largest value becomes the numerator

Yet another electrode arrangement is depicted in Fig 5, which also depicts die position of four of a user's fingers in phantom The triangular y-axis electrodes 52, 53 are used for ratiometric surface area determination as described in conjunction with die triangular x-axis electrodes 46,48 depicted in Fig 4 The x-axis determination is done in a yet different manner — it localizes the x-axis position of d e user's hand according to the number of fingers 24 (actually the ratio of finger surface area) disposed over the two x electrodes 54, 55 Should a hand stray outside the boundary zone (either to the left or right) a respective x-axis output will experience a dramatic loss in signal and processing will ordinarily be made to cease and die pointer or cursor will stop moving

Figure 6 depicts still another electrode arrangement in which a pair of triangular x-axis electrodes 46, 48 and a pair of triangular v-axis electrodes 52, 53 are employed to detect the centroid of a set of user's fingers disposed above the patterned electrode array in accordance with die mediods described supra

It will be noted diat aldiough in the preferred embodiment using a resistive sheet 16, the sheet may be charged through any one of die electrodes 17, but all of the electrodes 17 must be discharged simultaneously One may note that if die electrodes were connected to the charge sensor sequentially, rather than simultaneously, the charge would be transferred via the first electrode connected, and die apparatus would fail to provide an indication of position In the embodiments using shaped metal electrodes disposed on an insulating layer it is necessary to connect each of the electrodes to a charging source, but it is not necessary to discharge all of them simultaneously.

Turning now to Fig. 8, one finds a block diagram of a charge transfer measurement circuit similar to one described by the inventor in his pending US application senal number 08/578,464. Here, a charging switch 56, which is preferably a low resistance switching element, such as a transistor controlled by a signal on a charging control line 57, is used to transfer charge from a voltage supply 58 to a capacitive sensing electrode 60, which may be either one of the electrodes 36 disposed on the resistive sheet 16 or one of an array of patterned electrodes 42 disposed on an insulating substrate 44. The capacitive sensing electrode is preferably connected for a first interval, the length of which is selected to be long enough to allow the sensing electrode 60 to reach a saturation voltage, following which the charging switch 56 is opened, and, after waiting for a suitable short period to prevent switch crossover conduction, a signal sent over a discharging control line 63 causes a charge transferring switch 62 to close and thereby to connect the sensing electrode 60 to a charge measurement means 64 so as to rapidly discharge the sensing electrode 60 thereinto Because the switches 56, 62 have intrinsic internal capacitances which inject charge into the charge detector 64, and because the sensing electrode 60 may have a very large inherent capacitance, it is often desirable to cancel these charges as fully as possible to prevent saturating the charge detector 64 with diese background signals. To this end, a charge subtractor or cancellation circuit 66 may be provided in some embodiments of die invention to remove charge from the charge detector 64 Because only the charge detector's offset is affected by the charge subtractor 66, there is no change in gain of the charge detector and the overall system sensitivity remains unaffected. As is conventional in contemporary electronic designs, one may conveniendy provide for the operation and control of the vanous circuit functions discussed widi respect to Fig. 8 by means of a microprocessor 68.

Fig. 9 shows a schematic circuit diagram of an exemplar circuit of the sort described hereinbefore with respect to Fig. 8. In this embodiment die switches 56, 62 are p- and n-channel mosfet transistors of types BSS110 and BSN10, respectively, both of which have integral source-drain diodes. An opuonal resistor 70 adds damping and prevents ringing in the line running to the sensing electrode 60. Although in die preferred embodiment the switches 56, 26 are pulsed direcdy from the microprocessor 68 with lμsec pulses, earlier embodiments of the circuit operated with external pulse networks (not shown). An amplifier circuit 78 is used with an 8-bit DAC 79 to provide an adjustable measurement offset of the signal. The analog to digital converter 80 integral to the preferred PIC16C74 microcontroller 68 is only 8 bits, and thus has limited dynamic range. By adding a large external gain with offset capability, the system obtains an 8-bιt 'window' subrange into a 13-bit measurement space (3 offset bits provide overlap within each 8-bit window subrange). Turning now to Fig 10, one finds a schemauc depiction of a four channel sensor circuit 40 for use with any of die capacitive position sensing electrode arrangements 60 of the invention. In this figure the sensor circuit 40 is depicted as having the four capacitive electrode inputs hereinbefore labeled as XI, X2, Yl, and Y2, as well as having two inputs (labeled as A and B) from respecuve capacitive touch pads 22. A charging control line 57 in this embodiment is used to simultaneously charge all six of the capacitive inputs XI, X2, Yl, Y2, A & B It will be understood to those skilled in the art that in an embodiment of die invention using a single resistive grid or layer 16, a single charging switch 56 could be employed to charge the layer because all four electrodes 36 are connected dirough the layer 16. Moreover, it may be noted that in some embodiments of die invention the charging control line 57 may be omitted and die charging switches 56 replaced with a pull-up resistor constandy connecting the various electrodes to the voltage supply. The pull-up resistor has a resistance chosen to provide an RC time constant greater than die discharge interval used to discharge the layer into the array of charge detectors. In one preferred embodiment diis resistance is between fifteen and twenty five thousand ohms Regardless of how one chooses to charge the layer 16, however, it is preferred that die four channels of charge transfer act simultaneously when transferring charge to the charge detectors - e , as indicated in Fig 10 by the use of a single discharging control line 63 to discharge all the charged electrodes After the transfer, or burst of transfers, has occurred, an analog multiplexer 82 selects which of the six charge detector capacitor outputs is to be fed to die amplifier 84 and ADC 86 and dience to die microcomputer Additionally, an array of reset switches 88 controlled by a reset control line 90 is activated after each pulse or burst of pulses to reset the capacitive inputs to a known reference (e g , to ground) It will be clear to those skilled in the art that many of die circuit elements depicted in the Fig 9 in die detailed schematic for a single channel sensor have been omitted from the schemauc of Fig 10 in the interest of clarity

INDUSTRIAL APPLICABILITY

The invention provides apparatus and method for human-computer interfaces, and is believed to be of particular utility when used with a small, and generally, portable computers of the types commonly referred to as "notebook, 'laptop" or "palmtop" computers, as well as "personal digital assistants" "network computers", and the like.

Claims

1) A capacitive sensor for sensing a position of an object adjacent an acnve sensing region, the sensor comprising

a predetermined number, greater than one, of electrodes disposed in a spaced apart relationship, the electrodes conterminous with die sensing region,

means for transferring electrical charge from a voltage source to at least one of the electrodes; and

means for transferring electric charge from each of the electrodes to a respective one of the predetermined number of charge detectors, each of the charge detectors having an output to a calculation means adapted to calculate a rauo of the outputs and to calculate the position therefrom

2) The sensor of Claim 1 wherein the sensing region comprises a portion of a computer keyboard and wherein the sensor supplies an output representauve of the position to a computer pointing means, die computer pointing means determining the position of a cursor on a display responsive to the position

3) A capacitive sensor for measuring a position of an object adjacent a predetermined sensing area, the presence of the object changing an electric capacitance to a ground, the sensor comprising-

a layer of material having a predetermined number, greater than one, of electrodes disposed thereupon, the electrodes conterminous with die sensing area,

means connecting one of the electrodes to a voltage source,

said predetermined number of charge transferring switches, each having an open state and a closed state, each charge transferring switch, when in its closed state, respectively electrically connecting die electrode associated therewidi to a respecuve charge detector, each charge transferring switch, when in its open state, respectively electrically disconnecting the electrode associated therewith from the respective charge detector, and

means for closing all of the charge transferring switches simultaneously

4) The sensor of Claim 3 wherein the layer of material comprises a resistive sheet having a sheet resistivity greater than ten ohms per square and less than one mdlion ohms per square

5) The computer poinung device of Claim 3 wherein the layer of material is disposed adjacent a keyboard operauvely connected to die computer, die keyboard comprising a plurality of kevs, and wherein the layer of material further compnses an upstanding portion between two adjacent ones of the keys

6) The sensor of Claim 3 further comprising multiplexing means having a separate input from each of the charge detectors, the multiplexing means having an output for sequentially supplying respecuve values representative of each of the inputs to a calculational means

7) The sensor of Claim 6 wherein the calculational means compnses means for calculating d e position of die object from ratios of respective values representative of the inputs

8) The sensor of Claim 3 wherein the means connecting die voltage source to the electrode compπses a charging switch having an open state and a closed state, die charging switch connecting the one of d e electrodes to the voltage source when in d e closed state, die charging switch disconnecting the one of die electrodes from the voltage source when in the open state

9) The sensor of Claim 8 further compnsing a separate charging switch respectively connected to each of the electrodes. 10) A method of measuring a position of an object adjacent a predetermined sensing area, the mediod comprising the steps of

a) charging a layer of resistive matenal by connecting a voltage source to an electrode of a predetermined number of electrodes disposed upon the layer,

b) simultaneously closing each charge transferring switch of said predetermined number of charge transferring switches, each of the charge transferring switches respectively electrically connecting one of the electrodes to a respective charge detector,

c) reading, from each of the charge detectors, a respective output representative of an associated value of a capacitance to an electric ground, and

d) calculating, from the predetermined number of capacitance values, the posiuon of the object

11) The method of Claim 10 wherein the layer of material has a sheet resistivity greater than ten ohms per square and less d an one million ohms per square

12) The method of Claim 10 wherein step d) comprises calculating the position from ratios of ones of the capacitance values

13) The method of Claim 10 wherein each of the charge detectors is separately connected to a respective input to an analog multiplexer and wherein step c) comprises supplying, from the multiplexer, a sequence of values respectively representative of the inputs to a microprocessor

14) The method of Claim 10 wherein step a) is earned out by closing a charging switch electrically connected between the voltage source and the electrode, the mediod further comprising steps intermediate steps a) and b) of

al) holding the charging switch in its closed position for a first predetermined interval,

a2) opening the charging switch for a second predetermined interval and thereby disconnecting die one of die electrodes from the voltage source

15) The method of Claim 10 comprising an additional step after step d) of e) using the position of die object to determine a position of a cursor on a computer display 16) In a computer apparatus compπsing a keyboard compπsing a first predetermined number of keys, each of the first predetermined number of keys compπsing a moveable portion moveable along a line perpendicular to a fixed support between an uppermost position and a lowermost position, each of the keys providing a respective output when the moveable portion of the key is at die respective lowermost position, an improvement compπsing a capacitive means for supplying the computer apparatus an input representauve of a position of an object adjacent an active portion of the keyboard when each respective moveable portion of the first predetermined number of keys is in its uppermost position, the capacitive means comprising a plurality of electrodes adjacent the fixed support , the plurality of electrodes conterminous widi die active portion of the keyboard

17) The apparatus of Claim 16 wherein the capacitive means comprises a resistive layer having a sheet resistivity greater than ten ohms per square and less dian one million ohms per square, the resistive layer disposed adjacent the fixed support, the resistive layer having the plurality of electrodes disposed thereupon in a spaced apart relationship

18) The apparatus of Claim 16 wherein die capacitive means comprises

a charging switch having an open state and a closed state, die charging switch connecting one of die electrodes to a voltage source when in the closed state, die charging switch disconnecting die one of die electrodes from the voltage source when in the open state,

die predetermined number of charge transferring switches, each having an open state and a closed state, each charge transferring switch, when in its closed state, respectively electrically connecting the electrode associated therewith to a respective charge detector, each charge transferring switch, when in its open state, respectively electrically disconnecting the electrode associated therewith from the respective charge detector, and

means for closing all of the charge transferring switches simultaneously

19) The apparatus of Claim 16 wherein each of die electrodes is separately electrically connected to a respective charge detector having a respective output and wherein the capacitive means further compπses a calculation means for calculating die position of die object from a ratio of the outputs 20) In a pointing device connected to a computer, wherein a position of an object is detected by means measuring an electrical capacitance to a ground, wherein a cursor on a display operatively connected to die computer moves responsive to a change in die detected position, and wherein a keyboard operatively connected to die computer comprises a first predetermined number of keys, an improvement comprising-

a second predetermined number, greater than one, of electrodes conterminous with an active portion of the keyboard comprising a third predetermined number of keys, the third predetermined number no greater that d e first predetermined number, each of the electrodes separately electrically connected to the means of measuring electrical capacitance, the means of measuring capacitance having a respective output uniquely associated widi each of die electrodes, and

a calculation means for calculating die position of die object from a ratio of the outputs

21) The pointing device of Claim 20 wherein each key compπses a keycap moveable perpendicular to a fixed support between an uppermost and a lowermost position, and wherein each key provides a respective output when the keycap associated dierewith is in its lowermost position

22) The pointing device of Claim 20 further comprising a resistive layer comprising the diird predetermined number of throughholes, each of he diroughholes accommodating a portion of a respective one of the diird predetermined number of keys

23) The pointing device of Claim 22 wherein the resistive layer comprises an upstanding portion between two adjacent ones of the kevs

24) The pointing device of Claim 23 wherein the upstanding portion comprises a ridge

25) The pointing device of Claim 23 wherein the upstanding portion surrounds one of the keys

26) The pointing device of Claim 22 wherein the resistive layer has a sheet resistivity greater than ten ohms per square and less than one million ohms per square

27) The pointing device of Claim 20 wherein the second predetermined number is four

28) The pointing device of Claim 20 wherein the means measuring the capacitance compπses

a voltage source, means connecting the voltage source to one of the electrodes; and

die second predetermined number of charge transferring switches, each charge transferring switch respectively electrically connecting one of die electrodes to a separate charge detector, each charge detector providing a respective one of the outputs.

29) The pointing device of Claim 28 further comprising an analog multiplexer having a separate input from each of the charge detectors and wherein the calculation means compπses a microprocessor having an input from the analog multiplexer.

30) The pointing device of Claim 28, further comprising means for closing all of the second predetermined number of charge transferring switches simultaneously

31) In a pointing device connected to a computer, wherein a position of an obiect is detected bv electπcal capacitance measurement means having an output representative of a capacitance to an electrical ground, wherein a cursor on a display operatively connected to die computer moves responsive to a change in the detected position of the object, and wherein a keyboard operatively connected to the computer compπses a first predetermined number of keys, an improvement compπsing

a second predetermined number, greater than one, of electrodes conterminous with an active portion of the keyboard comprising a diird predetermined number of kevs, the diird predetermined number no greater that die first predetermined number, each of the electrodes separately electrically connected to the means of measuring electrical capacitance, the means of measuring capacitance having a respective output uniquely associated widi each of die electrodes, and

wherein the capacitance measurement means comprises

a voltage source,

means connecting the voltage source to one of the electrodes, and

the second predetermined number of charge transferring switches, each charge transferring switch respectively electrically connecting one of the electrodes to a separate charge detector

32) The computer pointing device of Claim 31 wherein each key comprises a keycap moveable perpendicular to a fixed support between an uppermost and a lowermost position, and wherein each key provides an output when the respective keycap is at its lowermost position

33) The pointing device of Claim 31 further compπsing a resistive layer comprising the diird predetermined number of throughholes, each of the diroughholes accommodating a portion of a respective one of the diird predetermined number of keys

34) The pointing device of Claim 33 wherein die resistive layer comprises an upstanding portion between two adjacent ones of the keys

35) The pointing device of Claim 34 wherein the upstanding portion comprises a ridge

36) The pointing device of Claim 34 wherein the upstanding portion comprises a wall surrounding one of the keys 37) The pointing device of Claim 33 wherein the resistive layer has a sheet resistivity greater than ten ohms per square and less than one million ohms per square

38) The pointing device of Claim 31 wherein the second predetermined number is four

39) The pointing device of Claim 31 wherein each charge detector has a respective output, the pointing device further comprising a calculation means for calculating die position of the object from a ratio of the outputs.

40) The pointing device of Claim 31, further compπsing means for closing all of the second predetermined number of charge transferring switches simultaneously

41) The pointing device of Claim 31 wherein the means connecting the voltage source to the one of die electrodes comprises a charging switch

42) The pointing device of Claim 31 wherein the means connecting the voltage source to the one of the electrodes comprises a pull-up resistor