Abstract:
A capacitive position sensor for detecting the position of an object, typically an operator&#39;s finger, relative to a resistive sensing element, wherein the sensing element comprises a sensing path that has terminals connected along it that subdivide the sensing path into multiple sections. Each terminal is coupled to its own sensing channel, each of which generates a signal that is sensitive to the capacitance between its terminal and a system ground. The signals are fed to a processor for analysis. The processor determines over which section the object is positioned by comparing the signals from the sensing channels, and determines the position of the object within that section by comparing the signals from the terminals spanning that section. In this way, the sensing path can be formed in a closed loop, such as a circle for a scroll dial, in which the operator&#39;s finger position and movement can be uniquely determined in a straightforward manner.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to capacitive position sensors, more particularly the invention relates to capacitive position sensors for detecting the position of an object around a curved path. 
     Capacitive position sensors are applicable to human interfaces as well as material displacement sensing in conjunction with controls and appliances, mechanisms and machinery, and computing. 
     Capacitive position sensors in general have recently become increasingly common and accepted in human interfaces and for machine control. In the field of home appliances for example, it is now quite common to find capacitive touch controls operable through glass or plastic panels. These sensors are increasingly typified by U.S. Pat. No. 6,452,514 which describes a matrix sensor approach employing charge-transfer principles. 
     Due to increasing market demand for capacitive touch controls, there is an increased need for lower cost-per-function as well as greater flexibility in usage and configuration. Similarly, there is a significant demand for capacitive material displacement sensors (e.g. fluid level sensors, mechanical movement sensors, pressure sensors etc.) at lower price points, which cannot be easily met with current generations of non-mechanical transducers. 
     In many applications there is a need for a human interface having many keys or sensing positions, nearly (but not) akin to the flexibility afforded by 2-D touch screens or touch pads as typified by U.S. Pat. Nos. 4,476,463 and 5,305,017. For example, in a computer monitor, it is desirable to have controls on the screen bezel to allow adjustment of brightness and contrast; ideally, as in former times, a continuously adjustable control (e.g. a potentiometer) is used to control these parameters. Due to price pressures and aesthetic requirements, these have been usually eliminated in favor of just a few bezel-mounted menu selection buttons which are much harder to understand and adjust. 
     In the fields of electronic and medical test instruments, LCD displays are often used in conjunction with rows of bezel buttons to provide software-driven menu functions. Many such applications do not allow the expense, reduced contrast, and fragility of touch screens, yet meanwhile suffer from deep or limited menu options and visual parallax. An example of this type of menu control is to be found in almost any current cash dispenser such as the NCR LCD-5305. Manufacturers would use higher resolution controls on or closer to the edge of the screen if economic considerations could be met. Similar markets exist for domestic appliances, educational games, information/internet kiosks, and the like. 
     In the field of heating ventilation and air conditioning (HVAC), the state of the art in wall-mount thermostatic controls is currently exemplified by the Honeywell model CT8602, a menu-drive system with a small LCD screen. Advanced features in these devices are accessed via deep levels of menus which are often non-intuitive compared with simple dial or slider based controls. 
     Electromechanical human interface controls (such as pushbuttons, membrane switches, and potentiometers) have the noted deficits of being unreliable and subject to water ingress, as well as being only marginally compatible with LCD based menu systems. Classic user controls like dials and resistive potentiometers require panel openings which allow dirt and moisture to enter into the product. They also do not present a ‘clean’ appearance, are considered increasingly quaint, and seriously limit the flexibility of industrial designers. U.S. Pat. No.5,920,131 describes one solution to this problem, in the form of a rotary knob which is magnetically held to a seamless panel surface and which magnetically interacts with position sensing detectors below the panel surface. However this solution still requires a protruding knob and is expensive to manufacture. 
     More recently there has been the appearance of ‘scroll wheels’ as input devices, as typified by the Apple Computer iPod MP3 player, and shown in U.S. D472,245. These devices have either a mechanical input scroll device or a capacitive device based on circuitry from Synaptics, Inc. (San Jose, California, USA). 
     There exists a substantial demand for new human interface technologies which can, at the right price, overcome the technical deficits of electromechanical controls on the one hand, and the cost of touch screens or other exotica on the other. 
     In the field of mechanical displacement sensing, Linear Variable Differential Transformers (LVDTs) exemplified by the Schaevitz Sensors (Slough, UK) MP series exist to provide precision positioning information for feedback in process controls. Other smaller devices such as the Schaevitz Sensors XS-B series are incorporated into machines and instruments. Such devices are usually high-cost solutions albeit very accurate, and rely on magnetic field balance measurements made with expensive signal conditioners to operate. These devices exist to provide highly reliable non-contact sensing that can operate in harsh environments with great precision. They solve the wiper-reliability problem of resistive potentiometric methods by eliminating the use of a physical contact. Similarly, there exist capacitive position sensors as exemplified by the RDP Electrosense Inc. (Pottstown, Pennsylvania, USA) Rotational Capacitive Displacement Transducer (RCDT), which also requires a special, expensive signal conditioner to operate. An example of such technology is described more fully in U.S. Pat. No. 5,461,319 which describes a bridge based circuit. Capacitive based devices can accommodate both linear and rotational position sensing. For example U.S. Pat. No. 5,079,500 describes a linear or rotary ‘potentiometer’having a capacitive wiper, which makes for a highly reliable method of position sensing as it does not use a galvanic wiper. Adaptations are available to measure pressure and by inference, fluid level. The above referenced technologies however suffer the problem of being very complex and expensive to manufacture, limiting their use to high-end or industrial equipment. In addition, U.S. Pat. No. 5,079,500 makes use of an ‘active wiper’, that is, the wiper terminal is connected to amplifier electronics, making that invention unsuited use for human touch. 
     LVDT and RCDT type transducers work very well, but leave untapped a very large market for low cost devices which can be used commercially in automotive and appliance applications. 
       FIG. 1  schematically shows a resistive sensing element  2  for use in a linear capacitive position sensor described in the present inventor&#39;s U.S. Pat. No. 7,148,704, the disclosure of which is incorporated herein by reference. The resistive sensing element  2  extends between first and second terminals  4 ,  6 . Each terminal  4 ,  6  is connected to a respective sensing channel  8 ,  10 . Each one of the sensing channels is operable to generate an output signal which depends on the capacitance between the terminal to which it is connected and a ground  14 . The resistive sensing element  2  has an inherent capacitance Cd to ground  14 . This inherent capacitance Cd is distributed along the length of the resistive sensing element  2 , as shown schematically in  FIG. 1 . When a user&#39;s finger  12  is located close to the resistive sensing element  2 , the user provides an additional capacitive coupling between the resistive sensing element  2  and ground  14 . This additional capacitive coupling is schematically shown by capacitor Cx in  FIG. 1 . The presence of additional capacitance Cx modifies the capacitance between the each of the terminals  4 ,  6  and the ground  14 . In particular, it is found that the ratio of the changes in capacitance associated with each of the terminals depends on the position of the finger along the resistive sensing element. For example, if the ratio is unity, the finger is midway along the resistive sensing element. Using this type of position sensor, the position of a finger can be determined independent of the magnitude of the capacitive coupling provided by the finger. This means the sensor can be used with different kinds of pointer, e.g., fingers and styli, while providing consistent results. This scheme provides for a simple and effective alternative to more complex LDVT sensors. 
     In many cases a rotary position sensor, i.e. one which is capable of determining the position of an object along a circular path, is desired. For example, to provide an equivalent to a mechanical rotary knob which can be used in a touch sensitive control panel. 
       FIG. 2  schematically shows a plan view of a resistive sensing element  16  for use in a rotary capacitive position sensor of the kind proposed in U.S. Pat. No. 7,148,704. The general functions of this device are similar to the device of  FIG. 1 . Many features of  FIG. 2  are similar to and will be understood from correspondingly numbered features of  FIG. 1 . However, the resistive sensing element  16  of  FIG. 2  differs from that of  FIG. 1  in that it is arranged in a curved path, namely an arc. The underlying operation of a rotary capacitive position sensor based on the resistive sensing element of  FIG. 2  is as described above with respect to  FIG. 1 . However, because of the arcuate arrangement of the resistive sensing element, the position at which a finger approaches the resistive sensing element can be translated to an angular position about the center of the resistive sensing element. Although the resistive sensing element  16  of  FIG. 2  can provide for a simple yet effective rotary capacitive position sensor, it is limited as to the angular range over which it can operate. For example, in  FIG. 2  it can be seen that the terminals  4 ,  6  defining the extent of the sensitive area of the position sensor are separated by around 30 degrees. This provides for a sensor with a sensitive angular-span of around 330 degrees and a dead zone of around 30 degrees. The dead zone between the terminals cannot be made arbitrarily small. This is because a finger placed near one terminal would otherwise provide a direct capacitive coupling to ground for the other terminal due to its proximity thereto. This can lead to ambiguity in calculated position. For example, with a finger placed over one terminal which is located too close to another terminal, the respective sensing channels  8 ,  12  can measure broadly similar signals. Similar signals is also what is seen when a finger is placed about midway around the resistive sensing element and hence a reported can be ambiguous. It may be possible to remove the ambiguity based on the overall magnitude of the signals detected, but this requires a priori knowledge of the typical magnitude of capacitive coupling to be expected from the finger, or other pointer. This can mean that different magnitudes of capacitive coupling can lead to differently calculated positions. 
     A similar ambiguity arises whenever a finger can provide a direct capacitive coupling to multiple parts of a resistive sensing element, for example, where the overall scale of the resistive sensing element is small. In such cases it can be possible for a finger placed over one location on the sensing element to provide a direct capacitive coupling to ground for another location, even if the dead zone between the terminals itself is suitably large. This can again leads to ambiguity and so limits how tightly a resistive sensing element  16  of the type shown in  FIG. 2  can be curved. This is a problem where space is at a premium, e.g. when designing control interfaces for small portable devices. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention there is provided a capacitive position sensor for detecting a position of an object comprising:
         a) a sensing element comprising a sensing path;   b) a plurality of at least three terminals connected to the sensing element at different locations along the sensing path to subdivide it into a plurality of sections;   c) a plurality of sensing channels connected to respective ones of the terminals, wherein each sensing channel is operable to generate a signal indicative of capacitance between its terminal and a system ground; and   d) a processor operable to determine over which section the object is positioned by comparing the signals from the sensing channels, and to determine the position of the object within that section by comparing at least the signals from the terminals spanning that section.       

     This arrangement provides for a simple capacitive sensor which does not suffer from the ambiguities which can arise with the above-described capacitive position sensors, for example ambiguities which can arise when an object is capacitively coupled to more than one part of the resistive sensing element at the same time. 
     The arrangement also allows for a resistive sensing element which may be disposed around a closed path. This allows, for example, the position of an object to be detected around a full 360 degrees. By allowing the position of an object to be tracked in this way, an effective alternative to multi-turn potentiometers can be provided for touch-sensitive control panels. It may also be useful for open-path sensor shapes, such as U-shape, or even linear, since it provides a general way of overcoming non-linearities or other problems that may occur with a simple two-terminal sensor. 
     The resistive sensing element may be embodied by a single resistor, for example it may comprise a resistive material deposited on a substrate to form a continuous pattern. This provides for an easy-to-fabricate resistive sensing element which can be deposited on the substrate in any one of a range of patterns. Alternatively, the resistive sensing element may be made from a plurality of discrete resistors. The discrete resistors may be alternately connected in series with a plurality of conducting sense plates, the sense plates providing for increased capacitive coupling between the object and the restive sensing element. This provides for a resistive sensing element which can be fabricated from widely available off-the-shelf items. 
     The resistive sensing element may have a substantially constant resistance per unit length. This provides for a capacitive position sensor having a simple uniform response. 
     Where greater positional resolution is required and/or when employing a relatively long resistive sensing element, the resistive sensing element may include more than three terminals. 
     The sensing channels may each include a sampling capacitor connected in series with a corresponding one of the capacitances between the terminals and the system ground such that when connected to a supply voltage each of the sampling capacitors are supplied with an amount of charge which depends on the capacitance between the corresponding ones of the terminals and the system ground. This effectively provides each sensing channel with a capacitive voltage divider comprising the capacitance of the sampling capacitor and the effective capacitance to ground caused by the object&#39;s capacitive coupling to the resistive sensing element. This allows the capacitances between each of the terminals and the system ground to be determined from the voltage measured on corresponding ones of the sampling capacitors. 
     Charge transfer techniques may be used, for example whereby each of the sensing channels comprises a plurality of switching elements and the capacitive position sensor includes a switch controller configured to allow a switching sequence of the switching elements to be performed such that the switching sequence causes each of the sampling capacitors to be connected to and then disconnected from the supply voltage and the terminals to be subsequently connected to the system ground. This provides for a simple way to transfer an amount of charge to each of the sampling capacitors which depends on the capacitances between each of the corresponding terminals and the system ground. 
     The switching sequence may be executed multiple times, with the terminals being disconnected from the system ground before each sequence execution, such the sampling capacitors are incrementally charged during each sequence execution. This may be done a variable number of times, whereby the number of sequence executions required to charge each of the sampling capacitors to a pre-determined level provides the signals indicative of capacitances between each of the terminals and a system ground, or a fixed number of times, whereby the charge on each of the sampling capacitors after a fixed number of sequence executions provides the signals indicative of capacitances between each of the terminals and a system ground. 
     The processor may be configured to compare the two largest signals, i.e. the two signals which indicate the largest capacitances to system ground of the terminals. This is an easy way of coarsely determining between which of the terminals the object is positioned before more precisely calculating the object&#39;s position between those terminals. Although further precision will generally be desired, it is noted that in some applications it may be sufficient to merely determine between which terminals an object is positioned. 
     To determine the position of an object between the terminals corresponding to the pair of selected signals, the comparing the selected pair of signals may include forming a ratio of the selected pair of signals, the parameter indicative of the position of an object being derived from said ratio. For example, the parameter may comprise the ratio of one of the selected pair signals to the sum of the selected pair of signals. Prior to performing such a ratio, a signal taken from one of the other terminals may be subtracted from the selected pair of signals. 
     If there is a significant level of background capacitive coupling between each of the terminals and the system ground (i.e. not due to the presence of the object), the processor may be configured to subtract respective background signals from each of the signals prior to the comparing. The background signals may correspond to the signals obtained when an object whose position is to be detected is distal from the capacitive position sensor. This means that effects due to the position of the object on the capacitances between the terminals and the system ground can be isolated from those found when the object is not present. The background signals may be calculated regularly during use to account for changing conditions. 
     To determine whether an object is present and to avoid confusion which may occur in attempting to generate a parameter indicative of a position of an object when none is present, the processor may configured to sum the respective signals from the sensing channels and to generate a parameter indicative of a position of an object only if the magnitude of the sum exceeds a detection threshold. The threshold can be set according to how sensitive a designer wishes the capacitive position sensor to be. For example, where there are a number of closely spaced capacitive position sensors on a control panel, the designer may require a high detection threshold to prevent perceived positive detections in one capacitive position sensor when a neighboring capacitive position sensor is being touched. In another case, a lower detection threshold may be preferred to increase the sensitivity of the capacitive position sensor. The processor may be configured to output a status signal indicative of whether the magnitude of the sum of the signals exceeds the detection threshold. This may assist appropriate responses by connected apparatus, e.g. functional equipment being controlled by a control panel. 
     Once a first parameter indicative of the position of an object has been generated, the capacitive position sensor may then generate a second parameter indicative of the position of the object at a later time and output a signal indicative of motion of the object between the first and second times. This allows the capacitive position sensor to be used to measure the relative of a moving displacement of an object rather than its absolute position. 
     The object to be detected may be a pointer, for example a finger or a stylus, which can be freely positioned by a user. Alternatively, the object may be a wiper held in proximity to the resistive sensing element, the position of the wiper along the resistive sensing element being detected by the capacitive position sensor. The position of the wiper may be adjusted by a user, for example by turning a rotary knob, or may be coupled to a shaft driven by connected equipment such that the capacitive position sensor can act as an encoder. 
     In a further embodiment of the invention, the circular element comprises a plurality of discrete resistors electrically connected in series, wherein electrode connections are made in 3 places, and where the junctions of resistor pairs are connected to discrete conductive electrodes to form individual sensing locations. A minimally useful sensor would have 6 resistors along the circle and therefore 6 sensing areas. 
     In a modification of this second embodiment, the discrete conductive electrodes are of sufficiently small size so as to permit field blending and thus position smoothing on the ‘user’ or sensing side of the substrate. 
     It is an object of some embodiments of the invention to provide a one-dimensional positional readout of the location of an object touching a circular element. 
     It is a further object of some embodiments of the invention to provide a coded output indicative of the computed position of touch plus a signal indicative of the detection of sufficient signal to validate the coded output. 
     It is a further object of some embodiments of the invention to provide a relative displacement output indicative of motion of a finger along a circular element. 
     It is yet a further object of some embodiments of the invention to interpret the coded output as one or more discrete touch positions, e.g. buttons. Another object is to provide for a ‘finger scroll wheel’ effect through a plastic surface. 
     It is an additional object of some embodiments of the invention to provide a one-dimensional reading of an amount of angular displacement, due to the angular position of a shaft or object. 
     Further objects of some embodiments of the invention are to provide for a sensor having high reliability, a sealed surface, low power consumption, simple design, ease of fabrication, and the ability to operate using off-the-shelf logic or microcontrollers. 
     Some definitions are now made. Unless otherwise noted hereinafter, the terms ‘connection(s)’ or ‘connected’ refer to either galvanic contact or capacitive coupling. ‘Element’ refers to the physical electrical sensing element made of conductive substances. ‘Electrode’ refers to one of the galvanic connection points made to the element to connect it to suitable driver/sensor electronics. The terms ‘object’ and ‘finger’ are used synonymously in reference to either an inanimate object such as a wiper or pointer or stylus, or alternatively a human finger or other appendage, any of whose presence adjacent the element will create a localized capacitive coupling from a region of the element back to a circuit reference via any circuitous path, whether galvanically or non-galvanically. The term ‘touch’ includes either physical contact between an object and the element, or, proximity in free space between object and element, or physical contact between object and a dielectric (such as glass) existing between object and element, or, proximity in free space including an intervening layer of dielectric existing between object and element. Hereinafter the terms ‘circle’ or ‘circular’ refer to any ellipsoid, trapezoid, or other closed loop of arbitrary size and outline shape having an open middle section. The mention of specific circuit parameters, or orientation is not to be taken as limiting to the invention, as a wide range of parameters is possible using no or slight changes to the circuitry or algorithms; specific parameters and orientation are mentioned only for explanatory purposes. 
     In U.S. Pat. No. 6,466,036 the present inventor teaches a capacitive field sensor employing a single coupling plate to detect change in capacitance to ground. This apparatus comprises a circuit employing repetitive charge-then-transfer or charge-plus-transfer cycles using common integrated CMOS push-pull driver circuitry. This technology forms the basis of some embodiments of the invention and is incorporated by reference herein. 
     One embodiment of the invention includes a sensing element and control circuit designed to provide a circular surface from which can be read the location of a finger capacitively coupled to the sensing element, wherein the control circuit has three sensing channels for measuring capacitance simultaneously at three electrode points along the circle and a computing device, such as a processor comprising processing logic circuitry, computes the ratio of the relative changes in the amount of capacitance measured at the three points. The result of this computation is a 1-dimensional angular co-ordinate number plus a detection state indicator, both of which can be fed to another function, for example an appliance controller, which interprets the co-ordinate and detection state as a command or measurement. 
     In one preferred embodiment of the invention, the sensing element is a circular element with three distinct electrodes thereon. Connections are made between each electrode and a circuit comprised of capacitive signal acquisition and signal processing means. The element is normally disposed on an insulating substrate, and is large enough to accommodate the desired targets for detection purposes. The sense field propagates through the substrate; the other side of the substrate forming the active sensing surface for human touch or a mechanical wiper. Direct touch on the element is also possible in which case the substrate only acts as a mechanical carrier. While it is supposed that an element could be solid enough that no substrate is required, normally the element will be a thin layer requiring mechanical support. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the invention and to show how the same may be carried into effect reference is now made by way of example to the accompanying drawings in which: 
         FIG. 1  schematically shows a plan view of a resistive sensing element for use in a linear capacitive position sensor of the kind proposed in U.S. Pat. No. 7,148,704. 
         FIG. 2  schematically shows a plan view of a resistive sensing element for use in a rotary capacitive position sensor of the kind proposed in U.S. Pat. No. 7,148,704. 
         FIG. 3  schematically shows a section view of part of a resistive sensing element employed in an embodiment of the invention. 
         FIG. 4A  schematically shows a plan view of a resistive sensing element for use in a rotary capacitive position sensor according to an embodiment of the invention. 
         FIG. 4B  schematically shows a section view of the resistive sensing element of  FIG. 4A  bonded to one side of a dielectric surface. 
         FIG. 5  schematically shows a control circuit diagram for use with the resistive sensing element of  FIGS. 4A and 4B . 
         FIG. 6  schematically shows a switching sequence for switching elements in the control circuit of  FIG. 5 . 
         FIG. 7  schematically shows an angular-position co-ordinate system defined for the resistive sensing element of  FIGS. 4A and 4B . 
         FIG. 8  schematically shows example conversion results for a touch proximate the resistive sensing element of  FIGS. 4A and 4B  as a function of position of the touch in the co-ordinate system shown in  FIG. 7 . 
         FIG. 9A  schematically shows a plan view of a resistive sensing element for use in a rotary capacitive position sensor according to another embodiment of the invention. 
         FIG. 9B  schematically shows a section view of the resistive sensing element of  FIG. 4A  bonded to one side of a substrate. 
         FIG. 10  schematically shown an implementation of the control circuit of  FIG. 5  as a micro-controller. 
         FIGS. 11A and 11B  schematically show respective plan and section views of a rotary position encoder employing a capacitive position sensor according to an embodiment of the invention. 
         FIG. 12  schematically shows a section view of a rotary control knob employing a capacitive position sensor according to an embodiment of the invention. 
         FIG. 13  schematically shows a plan view of a resistive sensing element for use in a capacitive position sensor according to another embodiment of the invention. 
         FIG. 14A-C  schematically show plan views of resistive sensing elements for use in capacitive position sensors according to other embodiment of the invention. 
         FIG. 15  schematically shows a plan view of an alternative sensing element for use in capacitive position sensors according to other embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  shows the construction of a resistive sensing strip  100 , also referred to as a sensing element, which is possibly bonded to a control surface composed of a 5 sheet resistor of a specified resistivity, galvanic end terminations  101  and  102 , distributed capacitance Cd  104 , and spot capacitance  106  due to object  105  proximity. Resistive sensing element  100  can be made of any resistive material including carbon film, metal films, ITO or SnO, conductive plastics, screen deposited conductors, sputtered conductors etc. without limitation as to material or method of deposition so long as the result is a resistive strip, rod, line, arc, or other suitable shape in 1, 2, or 3 dimensions. A resistive sensing element formed this way effectively comprises a continuous single resistor. Stray capacitance Cd does not need to be linearly distributed, because a preferred strip functions equally well with ‘lumpy’ capacitances by virtue of the principle of superposition. 
       FIGS. 4A and 4B  show a resistive sensing element  207  according to an embodiment of the invention as a circle, remembering that a circle for the purpose of this description can be any closed shape. The resistive material comprising the resistive sensing element extends between and is connected to electrodes, also known as terminals,  201 ,  202 , and  203 , comprising surfaces  204 ,  205 , and  206 . Cross section view  FIG. 4B  shows this element  207  bonded to a substrate  210 ; either side of the substrate  210  can be made touch sensitive to the fields generated by the element when properly controlled and sensed. However, in human-interface use, normally the upper (when oriented as shown in  FIG. 4B ) surface shown will be employed. 
       FIG. 5  shows a preferred control circuit diagram according to an embodiment of the invention for use with a resistive sensing element  207  of the kind shown in  FIGS. 4A and 4B . As can be seen from  FIG. 5 , the control circuit comprises three sensing channels with one being coupled to each of the electrodes  201 ,  202 ,  203 . The circuitry comprising each individual sensing channel is essentially the same as disclosed in the inventor&#39;s U.S. Pat. No. 7,148,704 and operates in the same basic manner. However, in this example where there are three electrodes arrayed about the element (in other examples there may be more) there are three sensing channels rather than the two employed in U.S. Pat. No. 7,148,704. Further detail as to the operation of this sensing circuit can be read in the aforesaid application and is considered further below. This configuration allows the location of touch (i.e. in this example the location of a user&#39;s finger  105 , which, as can be seen from the figure, creates a capacitive coupling Cx between the resistive sensing element  207  and a system ground  107 ) to be determined by analyzing the location of spot capacitance Cx along one of the three sectors  204 ,  205 , or  206 . More electrodes can be used along with more sensing channels, in order to provide higher resolution or to cover a larger circle than currently envisioned; the principle remains the same. 
     It will be appreciated that any reference potential may be used as system ground. Depending on the power supply used, the system ground may be at Earth potential. However, any other reference potential could equally be used, for example a reference potential defined internal to the device. 
     The control circuit of  FIG. 5  operates using charge transfer techniques. Charge transfer is governed by controlled switching of a number of switching elements in a sequence under the control of a switch controller  307 . This is done at a rate governed by a clock input line to the switch controller labeled  0  in  FIG. 5 . The number of switching sequences executed throughout a measurement cycle is recorded for each sensing channel in one of three counters  311 ,  312 ,  313 , respectively labeled CT 1 , CT 2  and CT 3  in  FIG. 5 . As will be seen further below, it is these numbers of counts which are used as output signals Sig 1 , Sig 2  and Sig 3  which form the basis of further processing by a processor comprising processing logic circuitry (not shown) to determine the location of a touch along the resistive sensing element  207 . As can be seen from  FIG. 5 , each of the three sensing channels of the control circuit are similar. For simplicity only the sensing channel associated with electrode  203  is considered here. The sensing channel is powered from a single rail DC power supply which operates between a system ground and a supply voltage +Vr. The sensing channel includes first  302 , second  303  and third  304  switching elements, respectively labeled A, B and C, a sampling capacitor  305 , labeled Cs 1 , and a voltage comparator  301 , labeled “Comparator 1”. The voltage comparator  301  is configured to provide an output to the switch controller  307  dependent on the result of comparing a voltage VCs 1  present on the sampling capacitor  305  with respect to the system ground with a reference threshold voltage Vt. 
     The switching elements A, B and C are driven by control signal lines from the switch controller  307 . (The three sensing channels are made to operate in time-synchronous fashion so that the three sets of switches A, B, C; A′, B′, C′; and A″, B″, C″ operate in a substantially simultaneous manner, as indicated by their sharing common control lines from the switch controller shown in  FIG. 5 .) An appropriate sequence of switching is shown in  FIG. 6 . The sequence comprises ten steps. As can be seen from  FIG. 6 , the switching sequence is generally the same for each sensing channel. 
     In a first step S 1 , the counters CT 1 , CT 2 , CT 3  associated with each of the sensing channels are reset. In a second step S 2 , switching elements A, A′, A″, C, C′ and C″ are closed (switching elements B, B′ and B″ are open). In step S 2  both sides of each of the sampling capacitors Cs 1 , Cs 2 , Cs 3  are connected to system ground to remove any residual charge they may hold. In step S 3 , all of the switching elements are open. In step S 4  switches B, B′ and B″ are closed. The purpose of floating all switches in step S 3  between step S 2  and S 4  is provide a dead time between switches A, A′ and A″ being closed in step S 2  and switches B, B′ and B″ being closed in step S 3 . This avoids accidental shorting of +Vr to system ground which might otherwise occur during any overlap period were step S 4  to immediately follow from step S 2 . In step S 4 , the sampling capacitors Cs 1 , Cs 2 , Cs 3  are charged by virtue of their being respectively connected to +Vr through switches B, B′ and B″ and their being capacitively coupled to ground through capacitance Cx provided by the user&#39;s finger  105  (or other pointer in other applications). Kirchoff&#39;s current law and the principle of electric charge conservation dictate that the sum of the electric charge on the sampling capacitors Cs 1 , Cs 2  and Cs 3  which occurs due to the presence of Cx is equal to the electric charge on Cx. It is found that this electric charge is shared between the three sampling capacitors depending on the resistance between each of the electrodes  201 ,  202 ,  203  to which the respective sensing channels are coupled and the location of the user&#39;s finger over the resistive sensing element  207 . The capacitance Cx is effectively split into three “virtual” capacitances Cx 1  , Cx 2 , Cx 3  (where Cx=Cx 1  +Cx 2 +Cx 3 ) which are measured by the measurement sensing channels, the ratio of the split is determined by the location of the capacitive coupling to system ground. 
     In step S 5  all of the switching elements are open. In step S 6 , switching elements A, A′ and A″ are closed. This clears the charge on Cx by shorting the resistive sensing element  207  to system ground  107  at each of the electrodes  201 ,  202 ,  203 . In step S 7 , the voltage on each of the sampling capacitors Cs 1 , Cs 2 , Cs 3  is compared with the reference threshold voltage Vt by the respective comparators. For each sensing channel for which the sampling capacitor voltage is found to be less that the reference threshold voltage Vt, the corresponding one of the counters CT 1 , CT 2 , CT 3  is incremented. This function may be performed by logic in the switch controller. In step S 8 , logic in the switch controller  207  determines if any of the voltages on the sampling capacitors Cs 1 , Cs 2 , Cs 3  are less than the reference threshold voltage Vt, and if so, the switching sequence returns to step S 3 . In this way, steps S 3  to S 8  are repeatedly looped until the voltage on all of the sampling capacitors exceeds Vt. During each loop through steps S 3  to S 8  an amount of charge is added to each sampling capacitor as described above. However, once the voltage on any given sampling capacitor exceeds Vt, the counter associated with the corresponding sensing channel ceases to be incremented at step S 7  during subsequent loops. Once it is determined at step S 8  that the voltage on all sampling capacitors exceeds Vt, the sequence moves to step S 9  where all switches are opened. This represents the end of one measurement. At this point the counters CT 1 , CT 2 , CT 3  associated with the three sensing channels store numbers Sig 1 , Sig 2 , Sig 3  which are indicative of how many charge transfer cycles (i.e. loops through steps S 3  to S 8 ) were required to charge the respective sampling capacitors such that their voltage exceeds Vt. This is an inverse measure of how much electric charge each sampling capacitor received during each charge transfer cycle—i.e. a measure of what “fraction” of Cx is effectively connected to the corresponding electrode with which each sampling capacitor is associated. The signals Sig 1 , Sig 2  and Sig 3  may then be processed by a processor (not shown) to estimate the position of a touch as described below. In Step S 10  the switching sequence returns to step S 1  to make a further position estimate (if desired). 
     In the above described sequence of switching, the durations required for the switch closures and openings are usually measured in nanoseconds or microseconds, although the steps involving resetting the sampling capacitors may be in the millisecond range. The actual or optimal timings depend on the choice of specific component values, which include, the sampling capacitor value, switching element resistance, and the sensing strip resistance. For example, a resistive sensing element having a low resistance, such as 10 kOhm between terminals, for example, may need to employ switch closure durations of 100 ns or less to prevent significant cross-bleed of charge between the sampling capacitors through the resistive sensing element itself. 
     It will be appreciated that other charge transfer sequences could also be used. For example, it is not necessary to use the comparator and counter scheme described above. In other schemes, a similar switching sequence to that described above could be executed a fixed number of times (rather than a variable number of times based on whether a reference threshold voltage is exceeded). After the fixed number of times, the voltage on each sampling capacitor can be measured using an, analogue to digital converter and said voltages used to determine the position of a touch in a manner analogous to that described further below for Sig 1 , Sig 2  and Sig 3  (remembering however that said voltages would be directly related to how much charge is transferred during each transfer cycle and not inversely related as Sig 1 , Sig 2  and Sig 3  described above are). The scheme shown in  FIG. 5 , however, relies on relatively easy to implement comparators and counters rather than more complex analogue to digital converters. 
     Although described above as separate circuitry elements, the functionality of the switch controller, the comparators and switching elements may all be provided by a single general purpose programmable microprocessor, microcontroller or other integrated chip, for example a field programmable gate array (FPGA) or application specific integrated chip (ASIC). In addition, the processor (not shown) which is operable to receive the signals Sig 1 , Sig 2 , Sig 3  from the counters CT 1 , CT 2 , CT 3  and to perform the processing described below may also be included in the same single package. 
     Processing may be performed by appropriately configured processing logic circuitry as follows. The functionality of the processor may be provided by a general purpose programmable microprocessor, microcontroller or other integrated chip, for example a field programmable gate array (FPGA) or application specific integrated chip (ASIC). In this example, a first processing step is the subtraction of any background signals (e.g. obtained from calibration readings) which may be present. In other cases, for example where background signals are expected to be small, this may not be considered necessary. 
     During an initial phase of operation, at power up for example, calibration readings can be taken of the baseline or ‘background’ signals from the three sensing channels to obtain ‘reference’ readings, with no object presumed to be present near the sensor area. These readings correspond to charging of the sampling capacitors Cs 1 , Cs 2 , Cs 3  which occurs by virtue of the resistive sensing strip having an inherent capacitance to system ground as well as other stray capacitive couplings which may be present. These readings may be taken using the same above switching sequence. Once a calibration is taken, only differential readings, so called “delta-signals”, from each channel are processed in order to calculate position. Further, slow changes in the background level of signals can be compensated for by using ‘drift compensation’ methods that slowly adjust the ‘reference’ levels in a slew-rate limited manner during intervals of non-detection. Accordingly, the first stage of processing is to obtain the difference between each of Sig 1 , Sig 2  and Sig 3  the corresponding background signals Ref 1 , Ref 2  and Ref 3 . This provides three delta signals ΔSig 1 , ΔSig 2  and ΔSig 3  which relate to the effect of the finger, or other object, whose position is to be determined, and exclude the background capacitive couplings which might otherwise induce skewed results. It is noted that with the scheme shown in  FIG. 5 , the delta signals ΔSig 1 , ΔSig 2  and ΔSig 3  are likely to be negative if one subtracts the reference signals from “real time” signals (i.e. signals obtained during real time operation). This is because the counters count fewer times with increasing amounts of capacitance on the resistive sensing element. Since, as will be seen below, the location of a touch can be determined using appropriately-formed ratios of pairs of delta signals, the sign of the delta signals does not affect the estimated position. However, if only positive values for the delta signals are preferred, for example to allow the use of a simplified processor not having the capability of dealing with negative numbers, the delta signals can be generated by subtracting Sig 1 , Sig 2  and Sig 3  respectively from Ref 1 , Ref 2  and Ref 3  if desired. 
     To compute the position of an object using the circuit of  FIG. 5  the sensor readings from two neighboring electrodes are processed according to the following steps, assuming that the real time acquired signals for the neighboring pair are SigA and SigB (i.e. SigA and SigB are ones of Sig 1 , Sig 2  or Sig 3  according to which pair is being processed), and the baseline reference levels are RefA and RefB (where RefA and RefB are corresponding ones of Ref 1 , Ref 2  or Ref 3 ):
         1) Compute the delta signals ΔSigA, ΔSigB (as described above)
 
ΔSig A =Ref A −Sig A  
 
ΔSig B =Ref B −Sig B  
   2) Compute the ratio Ps indicative of position:
 
 Ps =ΔSig B /(ΔSig A +ΔSig B )
       

     A positive detection is assumed to have occurred when the total incremental delta signal strength for (ΔSig 1 +ΔSig 2 +ΔSig 3 ) rises above a minimum detection threshold value Tx (i.e. when there is enough total difference between the real time signals Sig 1 , Sig 2 , Sig 3  and the reference signals Ref 1 , Ref 2 , Ref 3 ). The processor may be configured to output a signal, for example a status signal, indicative of whether or not this condition is presently satisfied. Tx, i.e. the total incremental delta signal required to indicate a positive detection, will be set depending on how sensitive the designer wishes the position sensor to be. For example, if there are a number of closely spaced position sensors on a control panel, it may be preferable to employ a relatively high value of Tx to avoid one position sensor responding to a finger being placed over a neighboring position sensor. 
     As described above, the three electrodes  201 ,  202 ,  203  divide the resistive sensing element  207  into three sectors  204 ,  205 ,  206  bounded by different neighboring pairs of the electrodes  201 ,  202 ,  203 . Once a positive detection is identified, the location of the touch is assumed to occur within the sector bounded by the two electrodes coupled to the sensing channels showing the two greatest delta signals. These two delta signals are then used as the delta signals ΔSigA and ΔSigB referred to above. 
     Once the appropriate neighboring pair of the electrodes  201 ,  202 ,  203  has been selected and processed as described above, the value Ps defines the calculated position within one sector, referenced from one of the two sector electrodes and is in the range 0 to 1. Only the sector with the largest total touch signal need be processed in this manner to arrive at a position location. The remaining electrode signal can be ignored. If a finger is directly on an electrode so that two sectors are receiving equal amounts of signal, the logic can simply decide to pick one sector over the other, or make an exceptional decision. For example, the fact that one sensing channel measures a large delta signal and the other sensing channels measure equal smaller sensing channels indicates that a touch is made directly over the electrode measuring the large delta signal. 
     Should a touch be made close to but not directly above one terminal, the sensing channels associated with the other two terminals will observe similar but slightly different signals. Where the difference in these signals is small compared to their measurement accuracy it is possible that the processor might determine the touch to occur in the wrong sector. An advantage of capacitive sensors according to the invention is that should this occur the calculated position will nonetheless remain in the vicinity of the terminal being touched and the error does not lead to gross inaccuracy in estimated position. 
     Although not corresponding to a full analytical solution, Ps is found to be an excellent approximation to position. Ps is also found to be remarkably free of effects from differently sized objects (e.g. differently sized fingers used to indicate a position) and, with uniform resistivity around the resistive sensing element  207 , an excellent linearity of response is observed. 
       FIG. 7  is a schematic plan view of the resistive sensing element  207  of  FIG. 5 .  FIG. 7  shows how an angular position may be defined around the resistive sensing element. In this example, the electrode at the boundary between resistive element sectors  206  and  204  is arbitrarily defined to correspond to an angular position of 0 degrees. Since in this example the three electrodes are equi-angularly spaced (this need not always be the case), the remaining two electrodes are at corresponding angular positions of 120 degrees and 240 degrees. Accordingly, if the outputs from the sensing channels indicate that a touch has occurred in sector  204  (i.e. the delta signals for the sensing channels coupled to the electrodes at the ends of this sector are the two largest) and the processing above provides a value of Ps of 0.4, an angular position for the touch of 0.4*120=48 degrees is determined. It will be appreciated that the determined position will be as measured from the electrode corresponding to the sensing channel providing delta signal ΔSigA. If the same value of Ps was determined but where the largest delta signals were found to corresponded to the sensing channels coupled to the electrodes bounding resistive sensing element sector  205 , a position of 120+0.4*120=168 degrees would be determined. If a value of Ps of 0.63 was determined with the largest delta signals corresponding to the electrodes bounding resistive sensing element sector  206 , a position of 240+0.63*120=316 degrees would be determined. These example are for a resistive sensing element  207  having constant resistivity (i.e. constant resistance per unit length). This need not be the case, in other examples, a non-uniform resistivity may be preferred, for example to provide a different angular resolution at different angular positions. In such cases a transform based on the variation of resistivity along the resistive sensing element can be used to transform Ps into an angular position. 
       FIG. 8  shows the sector result of the signals processed according to the inventor&#39;s U.S. Pat. No. 7,148,704 and as described above when applied to the results between any two adjacent electrodes along the element. Three outputs can be provided, one for each sector. As a finger is rotated around the element, the processor logic circuitry processes the received capacitance signals from the electrodes to arrive at a determination of which sector the finger is dominantly located in. Within that sector, the processor performs a ratiometric analysis of the bracketing electrode signals to provide a location of touch along that sector. Signals emanating from other sectors not being touched or only being touched to a lesser degree will be of lower strength, and the processor can therefore act to ignore the weaker signals. Ambiguities will arise when a finger is located directly on an electrode, however such an ambiguity is relatively simple to resolve by logic which the practitioner can readily arrive at. 
     In some embodiments, position estimates may be obtained by combining signals from all three sensing channels (or from more sensing channels where more than three are provided). In the above definition of Ps, the delta signals ΔSigA and ΔSigB corresponding to the two largest of ΔSig 1 , ΔSig 2  and ΔSig 3  define in which sector a touch occurs, signals ΔSigA and ΔSigB are then used to calculate the position of the touch within that sector as described above. In an alternative example, the position of an object is computed whereby the sensor readings from all three electrodes are processed as follows.
         1) Compute the delta signals ΔSigA, ΔSigB, ΔSigC (as described above)
 
ΔSig A =Ref A −Sig A  
 
ΔSig B =Ref B −Sig B  
 
ΔSig C =Ref C −Sig C  
   2) Compute Qs, which is indicative of position:
 
 Qs =(ΔSig A −ΔSig C )/((ΔSig A −ΔSig C )+(ΔSig B −ΔSig C )).
 
Where ΔSigA and ΔSigB are the two largest delta-signals and ΔSigC is the smallest delta-signal. As before, the touch is assumed to occur in the sector bounded by the terminals associated with the two largest delta-signals ΔSigA and ΔSigB. However, the position within this sector is determined by taking into account the smallest delta-signal ΔSigC as well as the two largest. As with the parameter Ps described above, the value Qs defines the calculated position within that sector, referenced from the appropriate one of the two sector electrodes and is in the range 0 to 1.
       

     For the detection of objects having a variable coupling to the strip, the signal processing performed to generate Ps, Qs or any other parameter indicative of position is made only when the total magnitude of the change in all signal strengths with respect to a reference value rises above a minimum threshold value. This object detection process should preferably incorporate hysteresis and filtering to improve onset detection characteristics. 
     Further processing of the result can include:
         segmenting a result to fall into a specific location ‘bin’ for the purpose of defining key regions   signal filtering of Ps to reduce position noise   ‘lift off detection, whereby the removal of the object and subsequent loss of signal is detected and processed so as to permit the memory of the last contacted position with accuracy       

     Although in the above example a uniformly resistive sensing element is described, it will be appreciated that other types of resistive sensing element may be used. 
     Construction of a resistive sensing element  600  as a lumped model is shown in  FIGS. 9   a  and  9   b  with discrete resistors  602  and conductive discrete pads  601 , also known as sense plates. This is operationally identical to the configuration of  FIGS. 4A and 4B , except that the displacement representation of a lumped version is in theory more granular for obvious reasons. Through a substrate, this granularity largely disappears due to field mixing between adjacent pads as shown in  FIG. 9   b  and as explained in the inventor&#39;s U.S. Pat. No. 7,148,704. 
       FIG. 10  shows an embodiment of the invention, wherein the resistive sensing element is driven by simple switching logic from a single logic device  701 . The switching logic provided by switching elements operating according to the switching sequence shown in  FIG. 6  plus all signal processing aspects are subsumed within the logic device  701  to provide a very low cost position detector. The logic device  701  can be a single-chip microcontroller, for example. The I/O ports of the microcontroller may be manipulated in the correct sequence as shown in  FIG. 6 . An I/O port pin of a microcontroller typically has at least the 3 switches necessary to implement the switching sequence of the example shown in  FIG. 6  under software control. The software can readily implement all the switches, control logic, and counters shown in  FIG. 5  and sequencing of  FIG. 6 . The microcontroller may preferably have a push-pull type CMOS pin structure, and an input which can be made to act as a voltage comparator. Most common microcontroller I/O ports are capable of this, as they have a relatively fixed input threshold voltage as well as nearly ideal MOSFET switches. The output of this controller can be either a pulse width modulated (PWM) signal which can be filtered to analog form, or a serial output such as the well known UART, SPI, or I2C formats (or any other type). Such a microcontroller may also be go on to process a useful function as well, for example to control a triac for lighting dimming or motor control in response to a detected angular position, in which case the microcontroller output will be highly processed and application specific. 
       FIGS. 11A and 11B  schematically show respective plan and section views of a rotary encoder according to the invention. A wiper  802  is connected to an AC signal reference via shaft  801 . Resistive sensing element  207  is therefore affected by the angular displacement of shaft  801 , by virtue of the movable capacitance induced by the wiper  802 . This is an example where the position of an object other than a finger is being detected. Such a rotary encoder can be made to operate with a human control, a machine shaft encoder, or the like. 
       FIG. 12  schematically shows a section view of a human operable control knob  901  using a resistive sensing element  207  according an embodiment the invention. Knob  901  is made of either conductive or insulating material. A magnet  801  is bonded to the inside of the knob  901 , and holds the knob  901  against a panel by attraction to a ferromagnetic ‘shaft’  902 . A conductive wiper  802  provides a moving vane which provides a capacitive coupling to element  207 , the location of which can be measured as described above. A spacer  905  holds the knob  901  upright and provides balance (the spacer may be integrally formed with the knob). Capacitive coupling through the panel may be used to effect a return signal path, or, the knob itself may provide this path via the body capacitance of a user (not shown). A conductive path  903  provides a return path to magnet  801  if a human body is not relied upon. It will be appreciated that in other example, the knob may be mounted on a post extending from the panel, or may itself have a post arranged to fit into a recess or opening in the panel, rather than being magnetically mounted. 
     In the above examples, the absolute angular position of an object, e.g. a finger or wiper element, is determined. However, it will be appreciated that, for example in a fingertip scroll wheel application, the angular position output can be simply processed further to translate the angular position into one of ‘change in angular position’, thus providing an output indicative of the direction of scroll and the amount of scrolling distance and speed of scroll etc. Usually in these applications absolute position is not required, and as a result absolute accuracy can be quite poor yet the control will remain acceptable. A resistive sensing element of the kind shown in  FIG. 5  is well suited to this kind of application since a user may scroll continually and smoothly around a full circle and beyond. This would not be possible, for example, using the scheme shown in  FIG. 2 . 
       FIG. 13  schematically shows a plan view of a resistive sensing element  920  according an embodiment of the invention in which a resistive sensing element  920  forms an open path, i.e. does not form a closed path as in the previous embodiment. Three sensing channels  921 ,  922 ,  923  are respectively coupled to three electrodes  925 ,  926 ,  927  connected to the resistive sensing element  901  as shown in the Figure. The resistive sensing element  901  is similar to that of  FIG. 5  except that the electrodes  925 ,  926 ,  927  are not equi-angularly spaced and the resistive sensing element  901  does not extend between one of the pairs of electrodes, namely between electrodes  926  and  927 . This means the resistive sensing element  901  has a dead zone between these electrodes. However, the restive sensing element of  FIG. 13  may be operated as described above to detect the position of a touch between either of the other pairs of electrodes. This scheme can help remove any ambiguity which may occur in a capacitive position sensor caused by an object providing a direct capacitive coupling between a system ground and multiple different parts of a resistive sensing element, but where a “full-circle”sensing element is not required. 
       FIGS. 14A-C  schematically show plan views of other configurations of resistive sensing element which may be used in capacitive position sensors according to other embodiments of the invention. Many others configurations are possible. The particular configuration of a resistive sensing element employed in any given application can be selected according to how a designer wishes a corresponding control interface to appear and/or operate. 
     In  FIG. 14A , the resistive sensing element is configured to form a closed triangular shape. In  FIG. 14B , a circular resistive sensing element is used, but includes four electrodes with four corresponding sensing channels. This may be used, for example, to provide better positional resolution in a large diameter sensor. In  FIG. 14C  a resistive sensing element having the same topology as that of  FIG. 13  is shown but which is differently configured. 
       FIG. 15  shows an alternative sensing element  950  having three terminals  201 ,  202 ,  203  which are coupled to sensing channels  952 ,  954 ,  956  of the kind described above. The sensing element  950  is based on three pairs of tapering triangular electrodes extending adjacent to each other and curving around between respective pairs of the terminals as shown in the figure. The sensing element is thus formed of conductive material, instead of the resistive body used in the above described embodiments. Other geometric forms of the taper can be used. Further details of these kinds of arrangement are given in my earlier US patent U.S. Pat. No. 6,288,707 (see for example FIGS. 4, 5 &amp; 6 and supporting text). 
     Although the above examples have employed charge transfer techniques, it will be appreciated that other kinds of sensing channels or different charge transfer techniques may be also used. For example, the method of switching described herein can be adapted to any of the switching sequences and topologies as described in my U.S. Pat. No. 6,466,036 and these are incorporated herein by reference. However the preferred method is that disclosed in  FIGS. 5 and 6  herein. This particular configuration and switching sequence is robust against external noise and leakage currents, as the signal sampling is done while the strip itself is connected to ground or other form of low-Z reference. 
     There are many variations possible as will become evident to those skilled in the art, involving various combinations of detection methods or switch sequences outlined specifically herein. The method can be combined with methods taught in any number of the inventor&#39;s prior patents including methods for drift compensation, calibration, moisture suppression using short switch closure times, noise immunity via variable timing pulse modulation (spread spectrum), and the like. 
     Although the present invention has been described with respect to several preferred embodiments, many modifications and alterations can be made without departing from the invention. Accordingly, it is intended that all such modifications and alterations be considered as within the spirit and scope of the invention as defined in the attached claims.