Methods and systems for guarding a charge transfer capacitance sensor for proximity detection Number:7,521,942 from the United States Patent and Trademark Office (PTO) owispatent Methods, systems and devices are described for determining a measurable capacitance for proximity detection in a sensor having a plurality of sensing electrodes and at least one guarding electrode. A charge transfer process is executed for at least two executions. The charge transfer process includes applying a pre-determined voltage to at least one of the plurality of sensing electrodes using a first switch, applying a first guard voltage to the at least one guarding electrode using a second switch, sharing charge between the at least one of the plurality of sensing electrodes and a filter capacitance, and applying a second guard voltage different from the first guard voltage to the at least one guarding electrode. A voltage is measured on the filter capacitance for a number of measurements equal to at least one to produce at least one result to determine the measurable capacitance for proximity detection.<H capacitance, detection, Methods, and, sys, 7521942.html, Patent, pto, 7521942

Title: Methods and systems for guarding a charge transfer capacitance sensor for proximity detection

Abstract: Methods, systems and devices are described for determining a measurable capacitance for proximity detection in a sensor having a plurality of sensing electrodes and at least one guarding electrode. A charge transfer process is executed for at least two executions. The charge transfer process includes applying a pre-determined voltage to at least one of the plurality of sensing electrodes using a first switch, applying a first guard voltage to the at least one guarding electrode using a second switch, sharing charge between the at least one of the plurality of sensing electrodes and a filter capacitance, and applying a second guard voltage different from the first guard voltage to the at least one guarding electrode. A voltage is measured on the filter capacitance for a number of measurements equal to at least one to produce at least one result to determine the measurable capacitance for proximity detection.

Patent Number: 7,521,942 Issued on 04/21/2009 to Reynolds

Inventors: Reynolds; Joseph Kurth (Sunnyvale, CA)
Assignee: Synaptics, Inc. (Santa Clara, CA)

Appl. No.: 11/926,411

Filed: October 29, 2007

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
11833828	Aug., 2007	7417441
11445856	Aug., 2007	7262609
60687012	Jun., 2005
60687148	Jun., 2005
60687167	Jun., 2005
60687039	Jun., 2005
60687037	Jun., 2005
60774843	Feb., 2006

Current U.S. Class: 324/688 ; 324/683

Current International Class: G01R 27/26 (20060101)

Field of Search: 324/658,678,688,683

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Primary Examiner: Nguyen; Vincent Q
Attorney, Agent or Firm: Ingrassia, Fisher & Lorenz, P.C.

Parent Case Text

PRIORITY DATA

This application is a continuation application of U.S. patent application Ser. No. 11/833,828, filed Aug. 3, 2007 now U.S. Pat. No. 7,417,441, which is a continuation application of U.S. patent application Ser. No. 11/445,856, which was filed on Jun. 3, 2006, which issued on Aug. 28, 2007, as U.S. Pat. No. 7,262,609, which claims priority to U.S. Provisional Patent Application Ser. Nos. 60/687,012; 60/687,148; 60/687,167; 60/687,039; and 60/687,037, which were filed on Jun. 3, 2005 and Ser. No. 60/774,843 which was filed on Feb. 16, 2006, and are incorporated herein by reference.

Claims

What is claimed is:

1. A device for determining a measurable capacitance for proximity detection, the device comprising: a plurality of sensing electrodes; a guarding electrode proximate the plurality of sensing electrodes; a passive network that includes a filter capacitance; and a controller coupled to the plurality of sensing electrodes, the guarding electrode, and the passive network, the controller configured to apply a pre-determined voltage to at least one of the plurality of sensing electrodes using a first switch, apply a first guard voltage to the at least one guarding electrode using a second switch, share charge between at least one of the plurality of sensing electrodes and the passive network such that shared charge is accumulated on the filter capacitance, apply a second guard voltage different from the first guard voltage to the guarding electrode; and measure a voltage on the filter capacitance for a number of measurements equal to at least one to produce at least one result to determine the measurable capacitance for proximity detection.

2. The device of claim 1 wherein the plurality of sensing electrodes and the guarding electrode are formed as circuits of a printed circuit board.

3. The device of claim 1 wherein the first guard voltage comprises a voltage selected to approximate the predetermined voltage applied to the at least one of the plurality of sensing electrodes.

4. The device of claim 1 wherein the second guard voltage comprises a voltage selected to be in a range between and including a threshold voltage and a reset voltage.

5. The device of claim 1 wherein the second guard voltage comprises a variable voltage with a time constant selected to approximate voltage on the filter capacitance.

6. The device of claim 1 wherein the wherein the second guard voltage comprises a pulse modulated signal.

7. The device of claim 1 wherein the second guard voltage comprises a voltage approximating a predetermined average voltage on the filter capacitance.

8. The device of claim 1 wherein the plurality of electrodes includes at least one transmitting electrode and at least one receiving electrode, and wherein the measurable capacitance comprises a transcapacitance established between the at least one transmitting electrode and the at least one receiving electrode.

9. The device of claim 1 wherein the controller is further configured to share charge using a third switch.

10. The device of claim 1 wherein the controller is further configured to apply the second guard voltage using a third switch.

11. A device for measuring a capacitance value, the device comprising: a passive network coupled to a first capacitance, wherein the passive network is configured to store charge received from the first capacitance; a charge changing circuit coupled to the passive network; a guarding electrode proximate the first capacitance; a controller configured to measure the capacitance value by repeatedly applying a predetermined voltage to the first capacitance, repeatedly storing charge received from the first capacitance on the passive network, and repeatedly comparing a voltage on the passive network to a threshold of a quantizer and changing a charge on the passive network by a quantized amount of charge using the charge changing circuit in response to the voltage on the passive network being past the threshold of the quantizer, the quantized amount of charge determined at least in part as a function of an output of the quantizer; and wherein the controller is configured to apply a guarding signal to the guarding electrode, the guarding signal including at least a first guard voltage level and a second guard voltage level.

12. The device of claim 11 wherein the first capacitance is formed in part from a first of a plurality of sensing electrodes, and wherein the plurality of sensing electrodes and the guarding electrode are formed as circuits of a printed circuit board.

13. The device of claim 11 wherein the first guard voltage level comprises a voltage selected to approximate the predetermined voltage applied to the first capacitance.

14. The device of claim 11 wherein the second guard voltage level comprises a voltage selected to be outside of a range between and including a threshold voltage and the predetermined voltage.

15. The device of claim 11 wherein the wherein the second guard voltage comprises a pulse modulated signal.

16. The device of claim 11 wherein the second guard voltage comprises a voltage approximating a predetermined average voltage on the passive network.

17. The device of claim 11 wherein the second guard voltage level comprises a voltage approximating a threshold voltage of the quantizer.

18. A device for measuring a measurable capacitance, the device comprising: a filter capacitance; a passive impedance statically coupling the filter capacitance to the measurable capacitance; a guarding electrode proximate the measurable capacitance; and a controller coupled to the measurable capacitance, wherein the controller is configured to perform a charge transfer process, wherein the charge transfer process comprises applying a predetermined voltage to the measurable capacitance and allowing the measurable capacitance to share charge with the filter capacitance through the passive impedance, and wherein the controller is further configured to determine a value of the measurable capacitance as a function of an accumulated charge on the filter capacitance; and wherein the controller is configured to apply a guarding signal to the guarding electrode, the guarding signal including at least a first guard voltage level and a second guard voltage level.

19. The device of claim 18 wherein the measurable capacitance is formed in part from a first of a plurality of sensing electrodes, and wherein the plurality of sensing electrodes and the guarding electrode are formed as a circuit of a printed circuit board.

20. The device of claim 18 wherein the first guard voltage comprises a voltage selected to reduce the charge accumulated from the guarding electrode through the sensing electrode and onto the filter capacitance.

21. The device of claim 18 wherein the second guard voltage comprises a voltage selected to be in a range between and including a threshold voltage and a reset voltage.

22. The device of claim 18 wherein the second guard voltage comprises a variable voltage with a time constant selected to approximate voltage on the filter capacitance.

23. The device of claim 18 wherein the wherein the second guard voltage comprises a pulse modulated signal.

24. The device of claim 18 wherein the second guard voltage comprises a voltage approximating a predetermined average voltage on the filter capacitance.

25. The device of claim 18 wherein the controller is further configured to reset a voltage on the filter capacitance using a switch.

Description

TECHNICAL FIELD

The present invention generally relates to capacitance sensing, and more particularly relates to devices, systems and methods capable of detecting a measurable capacitance using switched charge transfer techniques.

BACKGROUND

Capacitance sensors/sensing systems that respond to charge, current, or voltage can be used to detect position or proximity (or motion or presence or any similar information), and are commonly used as input devices for computers, personal digital assistants (PDAs), media players and recorders, video game players, consumer electronics, cellular phones, payphones, point-of-sale terminals, automatic teller machines, kiosks, and the like. Capacitive sensing techniques are used in applications such as user input buttons, slide controls, scroll rings, scroll strips, and other types of inputs and controls. One type of capacitance sensor used in such applications is the button-type sensor, which can be used to provide information about the proximity or presence of an input. Another type of capacitance sensor used in such applications is the touchpad-type sensor, which can be used to provide information about an input such as the position, motion, and/or similar information along one axis (1-D sensor), two axes (2-D sensor), or more axes. Both the button-type and touchpad-type sensors can also optionally be configured to provide additional information such as some indication of the force, duration, or amount of capacitive coupling associated with the input. Examples of 1-D and 2-D touchpad-type sensor based on capacitive sensing technologies are described in United States Published Application 2004/0252109 A1 to Trent et al. and U.S. Pat. No. 5,880,411, which issued to Gillespie et al. on Mar. 9, 1999. Such 1-D and 2-D sensors can be readily found, for example, in input devices of electronic systems including handheld and notebook-type computers.

A user generally operates a capacitive input device by placing or moving one or more fingers, styli, and/or objects, near the input device an in a sensing region of one or more sensors located on or in the input device. This creates a capacitive effect upon a carrier signal applied to the sensing region that can be detected and correlated to positional information (such as the position(s), proximity, motion(s), and/or similar information) of the stimulus/stimuli with respect to the sensing region. This positional information can in turn be used to select, move, scroll, or manipulate any combination of text, graphics, cursors, highlighters, and/or any other indicator on a display screen. This positional information can also be used to enable the user to interact with an interface, such as to control volume, to adjust brightness, or to achieve any other purpose.

Although capacitance sensors have been widely adopted, sensor designers continue to look for ways to improve the sensors' functionality and effectiveness. In particular, engineers continually strive to reduce the effects of spurious noise on such sensors. Many capacitive sensors, for example, currently include ground planes or other structures that shield the sensing regions from external and internal noise signals. While ground planes and other types of shields held at a roughly constant voltage can effectively prevent some spurious signals from interfering with sensor operation, they can also reduce sensor resolution or increase parasitic effects, such as by increasing parasitic capacitance. Therefore, the performance of such devices is by no means ideal.

Accordingly, it is desirable to provide systems and methods for quickly, effectively and efficiently detecting a measurable capacitance while preventing at least some of the adverse effects that can result from spurious noise signals and/or enhance resolution. Moreover, it is desirable to create a scheme that can be implemented using readily available components, such as standard ICs, microcontrollers, and passive components. Other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

Methods, systems and devices are described for determining a measurable capacitance for proximity detection in a sensor having a plurality of sensing electrodes and at least one guarding electrode. A charge transfer process is executed for at least two executions. The charge transfer process includes applying a pre-determined voltage to at least one of the plurality of sensing electrodes using a first switch, applying a first guard voltage to the at least one guarding electrode using a second switch, sharing charge between the at least one of the plurality of sensing electrodes and a filter capacitance, and applying a second guard voltage different from the first guard voltage to the at least one guarding electrode. A voltage is measured on the filter capacitance for a number of measurements equal to at least one to produce at least one result to determine the measurable capacitance for proximity detection.

Using the techniques described herein, a guarded capacitance detection scheme may be conveniently implemented using readily available components, and can be particularly useful in sensing the position of a finger, stylus or other object with respect to a capacitive sensor implementing button, slider, cursor control, or user interface navigation functions, or any other functions.

BRIEF DESCRIPTION OF THE DRAWINGS

Various aspects of the present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and:

FIG. 1A is a flowchart of an exemplary technique for detecting capacitance using switched charge transfer techniques with guarding;

FIG. 1B is a block diagram of an exemplary capacitive proximity sensor that includes guard circuitry;

FIG. 1C is a timing diagram relating to an exemplary technique for operating the capacitive proximity sensor with guard circuitry of FIG. 1B;

FIGS. 2A-B are timing diagrams of exemplary guard signals that can be applied to guarding electrodes.

FIGS. 3A-E are block diagrams of exemplary circuits that could be used to generate guard voltages of a guard signal;

FIGS. 4A-E are more detailed block diagrams of exemplary circuits that could be used to generate guard voltages of a guard signal; and

FIG. 5 is a schematic diagram of a proximity sensor device with an electronic system.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.

According to various exemplary embodiments, a capacitance detection and/or measurement circuit can be readily formulated using two or more switches. Further, a guard signal with two or more guarding voltages can be applied to a guarding electrode using one or more additional switches and one or more passive electrical networks (which can be a simple wire or a complex network); this can be used to shield the sensor from undesired electrical coupling, thereby improving sensor performance. In a typical implementation, a charge transfer process is executed for two or more iterations. In the charge transfer process, a pre-determined voltage is applied to a measurable capacitance using one or more of the switches and a first guarding voltage is applied to a guarding electrode with a second switch, the measurable capacitance then shares charge with a filter capacitance in the passive network and a second guarding voltage is applied to the guarding electrode. With such a charge transfer process, a plurality of applications of the pre-determined voltage and the associated sharings of charge influence the voltage on the filter capacitance. The voltage on the filter capacitance can be the voltage at a node of the circuit that indicates the voltage across the filter capacitance. The voltage on the filter capacitance can also be the voltage across the filter capacitance itself. The charge transfer process thus can be considered to roughly "integrate" charge onto the filter capacitance over multiple executions such that the "output" voltage of the filter capacitance is filtered. The charge transfer process may be done using only switches and passive elements such as resistances, capacitances, and/or inductances. After one or more iterations of the charge transfer process, the voltage on the filter capacitance (which is representative of the charge on the filter capacitance) is measured. One or more measurings can be used to produce one or more results and to determine the measurable capacitance. The measuring of the voltage on the filter capacitance can be as simple as a comparison of the voltage on the filter capacitance with a threshold voltage, or be as complex as a multi-step analog-to-digital conversion extracting charge from the filter capacitance and measuring the voltage multiple times. Using these techniques, capacitive position sensors capable of detecting the presence or proximity of a finger, stylus, or other object can be readily formulated. Additionally, various embodiments of the guard described herein can be readily implemented using only conventional switching mechanisms (e.g. signal pins of a control device) and passive components (e.g. one or more capacitors, resistors, inductors and/or the like), without the need for additional active electronics that would add cost and complexity. The various guarding techniques described herein can use similar components and methods as charge transfer sensing techniques. This, coupled with the ease of multi-channel integration, provide for highly efficient implementation of the guard. As a result, the various guarding schemes (and sensing schemes if desired) described herein may be conveniently yet reliably implemented in a variety of environments using readily-available and reasonably-priced components, as described more fully below.

With reference now to FIG. 1A, an exemplary technique 800 for detecting a measurable capacitance that provides guarding to shield the measurable capacitance from undesired electrical coupling is illustrated. The method 800 uses switched charge transfer to detect measurable capacitances, and is particularly applicable to the detection of capacitances for object position detection. The technique suitably includes the broad steps of performing a charge transfer process with voltage guarding (step 801) for two or more times (as repeated by step 810) and selectively measuring a voltage on the filter capacitance to produce a result (step 824). The charge transfer process 801 includes applying a pre-determined voltage to the measurable capacitance (step 802). Then, a first guard voltage is applied to a guarding electrode (step 804). The first guard voltage is preferably provided before the applying of the pre-determined voltage to the measurable capacitance ceases. Then, charge is shared by the measurable capacitance and a filter capacitance (step 806). "Sharing" charge in this context can refer to actively switching to couple the measurable capacitance and the filter capacitance, actively switching elsewhere in the system, otherwise directing the transfer of charge, or passively allowing the charge to transfer through impedance through quiescence or other inaction. Then, a second guard voltage is applied to the guarding electrode (step 808). The second guard voltage is different from the first guard voltage, and is preferably applied to the guarding electrode before the sharing of charge substantially ends. The charge transfer process repeats at least once (step 810) for at least two performances of the charge transfer process total, and may repeat many more times. The charge transfer process can repeat until the voltage on filter capacitance exceeds a threshold voltage, until the process 801 has executed for a pre-determined number of times, and/or according to any other scheme. Each time the charge transfer process executes, the first and second guard voltages are provided to shield from undesirable electrical coupling.

Measurement of the voltage on the filter capacitance to produce a result (step 824) can take place at any time, including before, after, and during the charge transfer process. In addition, none, one, or multiple measurements of the voltage on the filter capacitance 824 can be taken for each repetition such that the number of measurement results to the number of charge transfer processes performed can be of any ratio, including one-to-many, one-to-one, and many-to-one. Preferably the voltage on filter capacitance is measured when the voltage on the filter capacitance is substantially constant. One or more of the measurement results is/are used in a determination of the value of the measurable capacitance. The value of the measurable capacitance may take place according to any technique. In various embodiments, the determination is made based upon the measurement(s) of the voltage on the filter capacitance (which is indicative of the charge on the filter capacitance), the values of known components in the system (e.g. the filter capacitance), as well as the number of times that the charge transfer process 801 was performed. As noted just previously, the particular number of times that process 801 is performed may be determined according to a pre-determined value, according to the voltage across the filter capacitance crossing a threshold voltage, or any other factor as appropriate.

Steps 802-808 and steps 824 can be repeated as needed (step 810). For example, in a proximity sensor implementation, the measurable capacitance corresponding to each sensing electrode would typically be determined many times per second. This provides the ability to determine the proximity of objects near the sensor, as well as changes to that proximity, and thus facilitates use of the process in a device for user input. Thus, the process can be repeated at a high rate for each sensing electrode each second to enable many determinations of the measurable capacitance per second.

Process 800 may be executed in any manner. In various embodiments, process 800 is executed by software or firmware residing in a digital memory, such as a memory located within or in communication with a controller, or any other digital storage medium (e.g. optical or magnetic disk, modulated signal transmitted on a carrier wave, and/or the like). Process 800 and its various equivalents and derivatives discussed above can also be executed with any type of programmed circuitry or other logic as appropriate.

The steps of applying first and second guard voltages can be implemented with a variety of different techniques and devices. For example, the guard voltages can be provided using switching mechanisms and passive components (e.g. one or more capacitors, resistors, inductors, and/or the like), without the need for additional active electronics that would add cost and complexity (although such active electronics, including DACs and followers, can be used to provide the proper guard voltages at low impedance).

Now with initial reference to FIG. 1B, an exemplary capacitance sensor 100 suitably includes three sensing electrodes 112A-C and one guarding electrode 106. The sensing electrodes 112A-C are directly coupled to switches 116A-C, respectively. The sensing electrodes 112A-C are also directly coupled with a filter capacitance (also "integrating capacitance" or "integrating filter") 110 (C.sub.F) through passive impedances 108A-C, respectively. The filter capacitance 110 is also shown directly coupled to a switch 118. The guarding electrode 106 is coupled to a guarding voltage generating circuit 104 that includes passive guarding network 105 and one or more switch(es) 114. Guarding voltage generating circuit 104 provides an appropriate guard signal (V.sub.G) 103. Also shown in FIG. 1B is stimulus 101 that is not part of capacitance sensor 100 and is detected by capacitance sensor 100. Stimulus 101 can be one or more fingers, styli, objects, and the like, even though one stylus is shown in FIG. 1B.

Although a specific configuration of sensor 100 is shown in FIG. 1B, it is understood that many other configurations are possible. Other embodiments of capacitance sensor 100 may include any number of sensing electrodes, guarding electrode, filter capacitances, passive impedances, switches, guarding voltage generating circuits, and controllers as appropriate for the sensor. They can also be in any ratio appropriate for the sensor; for example, the sensing electrodes may also be coupled to filter capacitance(s) with or without passive impedances in a many-to-one, one-to-many, one-to-one, or many-to-many configuration as allowable by the sensing scheme used. It should be noted that while FIG. 1B shows switch(es) 114, 116A-C, and 118 all implemented using I/Os of a controller 102, that this is just one example embodiment, and that these and other switches could be implemented with a variety of different devices including discrete switches distinct from any controller. As further examples, the sensor may use a passive guarding network that consists of a single wire or a more complex circuit network, or the sensor may also provide the guarding signal using a single switch or multiple switches (which may involve using one or many I/Os of a controller, a multiplexer, a digital-to-analog converter (DAC), etc., since each multiplexer or DAC includes multiple switches). A switch can be used in a multitude of ways to provide the guard signal, including closing the switch, opening the switch, or actuating it in some other manner (e.g. PWM and pulse coded modulating). Therefore, one can apply a voltage by closing a switch as well as by opening a switch, depending on how the circuit is laid out. Additional analog components may also be used (e.g. to buffer the output of the passive guarding network 105).

The sensing electrodes 112A-C provide the measurable capacitances whose values are indicative of the changes in the electric field associated with stimulus 101. Each of the measurable capacitances represents the effective capacitance of the associated sensing electrode(s) 112A-C detectable by the capacitance sensor 100. In an "absolute capacitance" detecting scheme, the measurable capacitance represents the total effective capacitance from a sensing electrode to the local ground of the system. In a "trans-capacitance" detection scheme, the measurable capacitance represents the total effective capacitance between the sensing electrode and one or more driving electrodes. Thus, the total effective capacitance can be quite complex, involving capacitances, resistances, and inductances in series and in parallel as defined by the sensor design and the operating environment. However, in many cases the measurable capacitance from the input can be modeled simply as a small variable capacitance in parallel with a fixed background capacitance.

To determine the measurable capacitances, appropriate voltage signals are applied to the various electrodes 106, 112A-C using any number of switches 114, 116A-C. In various embodiments, the operation of switches 114, 116A-C is controlled by a controller 102 (which can be a microprocessor or any other controller). By applying proper signals using switches 116A-C, the measurable capacitances exhibited by electrodes 112A-C (respectively) can be determined. Moreover, by applying proper signals using switch(es) 114, suitable guarding voltages can be generated to produce a guard signal 103 that is placed on guarding electrode 106 to shield the measurable capacitances from undesired effects of noise and other spurious signals during operation of sensor 100.

Guarding electrode 106 is any structure capable of exhibiting applied guarding voltages comprising guard signal 103 to prevent undesired capacitive coupling with one or more measurable capacitances. Although FIG. 1B shows guarding electrode 106 with a "comb"-type appearance, this appearance is shown for convenience of explanation, and guarding electrode 106 may exhibit any other form or shape, in any number of equivalent embodiments as applicable for the design of sensor 100. For example, the sensing electrodes 112A-C may be laid out in some other pattern or have some other shape, and the shape of guarding electrode 106 can be laid out as appropriate. Guarding electrode 106 can also be routed around all or portions of a perimeter of a set of sensing electrodes to shield the set at least partially from the environment. Guarding electrode 106 can be routed behind at least a portion of the sensing electrodes to shield them from any electronics behind the sensing electrodes. Guarding electrode 106 can also be routed between sensing electrodes to shield them from each other. The guarding electrode does not need to extend the full length between sensing electrodes or cover the full sensing electrodes to offer a useful level of guarding. For example, guarding electrode 106 can parallel only portions of the sensing electrodes 112A-C, or interleave some or all of the sensing electrodes 112A-C. In addition, if a "trans-capacitance" detection scheme is used, guarding electrode 106 may be routed around any areas where guarding electrode 106 may interfere with the capacitive coupling between the sensing electrodes 112A-C and any driving electrode(s), such as some regions between the sensing electrodes 112A-C and the driving electrode(s). As explained below, capacitive coupling between guarding electrode 106 and measurable capacitances can be controlled through application of appropriate guarding voltages via switch(es) 114.

In the exemplary embodiment shown in FIG. 1B, a filter capacitance 110 is provided by one or more capacitors (such as any number of discrete capacitors) to accept charge transferred from sensing electrodes 112A-C. Although the particular filter capacitance value selected will vary from embodiment to embodiment, the capacitance of each filter capacitance 110 will typically be much greater--perhaps by only one to two orders of magnitude but often several orders of magnitude greater--than the capacitance of the measurable capacitances. Filter capacitance 110 may be designed to be on the order of several nanofarads, for example, when expected values of measurable capacitances are on the order of several picofarads or so. Actual values of filter capacitance 110 may vary, however, depending upon the particular embodiment.

The concepts of capacitance sensing in conjunction with guarding can be applied across a wide array of sensor architectures 100, although a particular example is shown in FIG. 1B. In the exemplary embodiment shown in FIG. 1B, each sensing electrode 112A-C, and thus each associated measurable capacitance, is coupled to a common filter capacitance 110 through an associated passive impedance 108A-C. Alternate embodiments may use multiple filter capacitances and/or passive impedances for each measurable capacitance as appropriate. Alternate embodiments may also share a passive impedance and/or a filter capacitance between multiple measurable capacitances. When included, passive impedances 108A-C are typically provided by one or more non-active electronic components, such as any type of diodes, capacitors, inductors, resistors, and/or the like. Passive impedances 108A-C are each generally designed to have an impedance that is large enough to prevent significant current bleeding into filter capacitance 110 during charging of measurable capacitance, as described more fully below. In various embodiments, impedances 108A-C may be on the order of a hundred kilo-ohms or more, although other embodiments may utilize widely different impedance values. Again, however, passive impedances 108A-C need not be present in all embodiments where charge sharing is otherwise implemented.

Operation of sensor 100 suitably involves a charge transfer process and a measurement process facilitated by the use of one or more switches 116A-C, 118 while a guard signal 103 is applied using switch(es) 114. Again, although shown implemented using I/Os of controller 102, switches 114, 116A-C and/or 118 may be implemented with any type of discrete switches, multiplexers, field effect transistors and/or other switching constructs, to name just a few examples. Alternatively, any of switches 114, 116A-C, 118 can be implemented with internal logic/circuitry coupled to an output pin or input/output (I/O) pin of the controller 102, as shown in FIG. 1B. Such I/O pins, if used, can also provide input functionality and/or additional switches. For example, switch 118 can be implemented with I/O 119 that also connects to, or contains, input capability within controller 102. The input capability may be used in measuring the voltage on the filter capacitance 110 directly or indirectly, and might include a multiplexer, comparator, hysteretic thresholds, CMOS threshold, or analog-to-digital converter. Such I/O pins are typically capable of switchably applying one or more logic values and/or a "high impedance" or "open circuit" value by using internal switches coupled to power supply voltages. The logic values may be any appropriate voltages or other signals. For example, a logic "high" or "1" value could correspond to a "high" voltage (e.g. 5 volts), and a logic "low" or "0" value could correspond to a comparatively "low" voltage (e.g. local system ground, -5 volts or the like). The particular signals selected and applied can vary significantly from implementation to implementation depending on the particular controller 102, sensor configuration, and sensing scheme selected. For example, a current source, a pull-up resistance, or a digital-to-analog converter (DAC) also could be used to provide the proper voltages, and may be external or internal to controller 102.

One advantage of many embodiments is that a very versatile capacitance sensor 100 can be readily implemented using only passive components in conjunction with a controller 102 that is a conventional digital controller comprised of any combination of one or more microcontrollers, digital signal processors, microprocessors, programmable logic arrays, integrated circuits, other controller circuitry, and/or the like. A number of these controller products are readily available from various commercial sources including Microchip Technologies of Chandler, Ariz.; Freescale Semiconductor of Austin, Tex.; and Texas Instruments of Richardson, Tex., among others. Controller 102 can contain digital memory (e.g. static, dynamic or flash random access memory) that can be used to store data and instructions used to execute the various charge transfer processing routines for the various capacitance sensors contained herein. During operation of various embodiments, the only electrical actuation on the sensing electrodes 112A-C and their associated measurable capacitances that need take place during operation of sensor 100 involves manipulation of switches 114, 116A-C and 118; such manipulation may take place in response to configuration, software, firmware, or other instructions contained within controller 102.

The charge transfer process, which is typically repeated two or more times, suitably involves using a first switch to apply a pre-determined voltage (such as a power supply voltage, battery voltage, ground, or logic signal) to charge the applicable measurable capacitance(s), and then passively or actively allowing the applicable measurable capacitance(s) to share charge with any filter capacitance (e.g. 110) as appropriate. Passive sharing can be achieved by charge transfer through an impedance such as a resistance, and active sharing can be achieved by activating a switch that couples the applicable measurable capacitance(s) to the appropriate filter capacitance(s).

The pre-determined voltage is often a single convenient voltage, such as a power supply voltage, a battery voltage, a digital logic level, a resistance driven by a current source, a divided or amplified version of any of these voltages, and the like. The value of the pre-determined voltage is often known, and often remains constant; however, neither needs be the case so long as the pre-determined voltage remains ratiometric with the measurement of the voltage on the applicable filter capacitance (e.g. 110). For example, a capacitance sensing scheme can involve resetting the filter capacitance to a reset voltage, and also involve measuring a voltage on the filter capacitance by comparing the voltage (as relative to the reset voltage) on one side of the filter capacitance with a threshold voltage (also as relative to the reset voltage). With such a sensing scheme, the difference between the pre-determined voltage and the reset voltage, and the difference between the threshold voltage and the reset voltage, should remain roughly proportional to each other, on average over the executions of the charge transfer process leading to the determination of the measurable capacitance. Thus, the threshold used to measure the change in voltage on the filter capacitance will be proportional to the change in voltage on the filter capacitance due to the charge shared from the measurable capacitance to the filter capacitance during the executions of the charge transfer process for a determination of the measurable capacitance. In particular, where the pre-determined voltage is V.sub.dd and the reset voltage is GND, the threshold voltage can be ratiometric for a CMOS input threshold, for example (1/2)*(V.sub.dd-GND).

The example shown in FIG. 1B can be operated in a manner as shown by FIG. 1C. In the embodiment shown by FIGS. 1B-C, each switch 116A-C applies a pre-determined voltage with "charging pulses" 201 that typically have relatively short periods in comparison to the RC time constants of impedances 108A-C with the filter capacitance 110, and preferably have relatively short periods in comparison to the RC time constants of impedances 108A-C with their associated measurable capacitances. This is so that the charge added to filter capacitance 110 during the charge transfer process comes mostly from the charge stored on the active measurable capacitance and shared with filter capacitance 110, and less from any flow of current through the associated impedance (e.g. 108A-C) during the applying of the pre-determined voltage. This helps to prevent excessive leakage of current through impedances 108A-C. Also shown in FIG. 1C, each charging pulse 201 additionally provides relatively brief durations of an "opposing" "discharging voltage" (a voltage that have a magnitude opposite that of the pre-determined voltage) before applying the pre-determined voltage. The discharging voltage can compensate for any current leaking through impedances 108A-C during the charge transfer process; it is an optional feature that is not required in all embodiments. More than one level of voltage can be used in the pre-determined voltage in an execution or between executions, and this is also true for the opposing voltage. However, in many cases the pre-determined voltage and the opposing voltage (if used) will have substantially constant voltages.

The following discussion describes the operation with one guarding electrode (e.g. 106), one measurable capacitance (e.g. associated with sensing electrodes 112A-C), one filter capacitance 110, and often one passive impedance (e.g. 108A-C). This is done for clarity of explanation, and it is understood that multiple measurable capacitances, passive impedances, and filter capacitances can be included in the system, and they can be operated in serially (at least partially or completely separate in time) or in parallel (at least partially or completely overlapping in time).

After applying the pre-determined voltage to the measurable capacitance, the measurable capacitance is allowed to share charge with filter capacitance. To allow measurable capacitance to share charge, no action may be required other than to stop applying the pre-determined voltage and pause for a time sufficient to allow charge to passively transfer. In various embodiments, the pause time may be relatively short (e.g. if the filter capacitance is connected directly to the measurable capacitance with a small resistance in series), or some delay time may occur (e.g. for charge to transfer through a larger resistance in series with the measurable capacitance, the filter capacitance, and reference voltage). In other embodiments, allowing charge to transfer may involve stopping the application of the pre-determined voltage and actively actuating one or more switches associated with a controller to couple the measurable capacitance and the filter capacitance, and/or taking other actions as appropriate. For example, charge sharing with the filter capacitance could occur in other embodiments using "sigma-delta" techniques; such as in a process whereby the filter capacitance is charged via a measurable capacitance and discharged by a "delta" capacitance (not shown), or vice versa. As another example, charge sharing with the filter capacitance could occur by actuating switches (not shown) that couple and decouple the measurable capacitance with the filter capacitance or that couple and decouple the filter capacitance with a power supply voltage. In such embodiments, impedances such as those shown as 108A-C shown in FIG. 1B may not be present, may be augmented by passive or active elements, and/or may be replaced by passive or active elements as appropriate.

A charge transfer process where sharing charge between the measurable capacitance and the filter capacitance occurs using one or more active components (e.g. by actively opening or closing a switch) clearly indicates the beginning and the end of a sharing period with these actuations of the active component(s). Similarly, a charge transfer process where the measurable capacitance is directly connected to one side of the filter capacitance, and the other side of the filter capacitance is coupled, by activating a switch, to a low impedance reference voltage, also clearly indicates the beginning and ending of a sharing period. In contrast, charge transfer processes that passively share charge have less clear denotations of the charge sharing periods. In the systems that passively share charge, the charge sharing period can be considered to begin when the applying of the pre-determined voltage ceases; the charge sharing period must end at or before a subsequent charging pulse begins (for a subsequent execution of the charge transfer process) and at or before a reset of the filter capacitance (if a reset is used and indicates an end a set of charge transfer processes). The sharing period may end before a subsequent charging pulse and before any reset because current flow effectively stops when the voltages are similar enough that negligible charge is shared between the measurable capacitance and the filter capacitance; this will be the case when sufficient time has passed while the measurable capacitance and filter capacitance are coupled to each other. However, even if the voltages do not substantially equalize before a subsequent charging pulse or reset signal, charge sharing still ends when the charging pulse or reset signal begins. This is because the applying of the charging pulse or reset signal dominates over any charge sharing between the measurable capacitance and the filter capacitance in a passive sharing system where the filter capacitance is always coupled to the measurable capacitance (such as in sensor 100 of FIG. 1B). The low impedance path of the charging pulse or reset signal means that any charge on the measurable capacitance that would be shared with the filter capacitance is negligible until the low impedance source is removed.

The measurement process may be performed at any point of the charge transfer process as appropriate for the sensor configuration and sensing scheme used, and the number of performances of the measurement process may be in any ratio with the performances of the charge transfer process as appropriate for the sensor configuration and sensing scheme used. For example, the measurement process may take place after the sharing of the charge between the measurable capacitance and the filter capacitance brings the voltage on the filter capacitance to be within some percentage point from an asymptote, or the measurement process may take place every time a charge transfer process is performed. Conversely, the measurement process may take place while the pre-determined voltage is applied (if the filter capacitance is properly prevented from charge sharing with the measurable capacitance at that time). The measurement process may take place only for a set number of repetitions of the charge transfer process, or only after a number of repetitions have already taken place. The measuring of the voltage on the filter capacitance can be as simple as a comparison of a voltage on the filter capacitance with a threshold voltage (such as in a "sigma-delta" scheme), or be as complex as a multi-step analog-to-digital conversion (such as when a known number of charge transfer processes are performed and then the voltage on the filter capacitance is read as a multi-bit value). Multiple thresholds can also be used, such as in an oscillator or other dual-slope sensing system where the voltage on the filter capacitance is driven between low and high thresholds, and in multi-bit ADCs where multiple thresholds are used to measure the voltage on the filter capacitance. One or more measurements can be taken, and stored if appropriate, to determine the measurable capacitance as applicable.

More detail about particular capacitance sensing schemes can be found in various literature, in U.S. Pat. Nos. 5,730,165, 6,466,036, and 6,323,846, as well as in U.S. patent applications entitled Methods and Systems for Detecting a Capacitance Using Switched Charge Transfer techniques, by David Ely et al, filed Jun. 3, 2006 and Methods and Systems for Detecting a Capacitance Using Sigma-Delta Measurement Techniques, by Kirk Hargreaves et al, filed Jun. 3, 2006. Again, the particular capacitance sensing technique and sensor architecture 100 may vary significantly in other embodiments.

A system without any shields or guards will be affected by the environment. Therefore, as discussed earlier, many capacitive sensors include ground planes or other structures that shield the sensing regions from external and internal noise signals. However, ground planes and other types of shields held at a roughly constant voltage are by no means ideal--they can increase the effects of parasitic capacitance (or other parasitic impedance and associated charge leakage) and reduce resolution or dynamic range. In contrast, a driven, low-impedance guard can provide similar shielding without significantly increasing the effect of parasitic capacitance or reducing resolution. This is done by reducing the charge transferred through any parasitic capacitances associated with any guarding electrode(s) onto any filter capacitance(s) during the course of executions of the charge transfer processes leading to the determination of the measurable capacitance(s). The voltages of the guard can be provided by using an output from a charge transfer process similar to the one to be guarded. This output can be provided as an input to a buffer (or other follower circuit) to guard multiple sensing channels with low impedance. Alternatively, these guard voltages can also be directly provided by using a guard-charge transfer process (one performed for guarding purposes) that inherently provides a low impedance guard signal such that no additional buffering is needed; this guard-charge transfer process could also be similar to the charge transfer process used for sensing, but that is not required.

The typical charge transfer sensing scheme will perform the charge transfer processes multiple times (and often hundreds of times or more) to generate the measurement(s) that are used for one determination of the measurable capacitance. This set of charge transfer processes that lead to the measurement(s) used for one determination varies between embodiments. As four examples, the set can be between a reset state and a final-threshold-state for systems that charge to threshold(s); the set can be between an initial state and a final-read-state for systems that perform a set number of charge transfer processes and read one or more multi-bit voltage output(s); the set can be between the low and high thresholds for dual slope or oscillator systems; the set can also be the sample length of a digital filter for sigma-delta systems. This set of charge transfer processes defines a set where the overall guarding effect is considered, or "the course of executions of the charge transfer processes leading to the determination of the measurable capacitance."

To reduce the net charge transferred through the parasitic capacitance associated with the guarding electrode onto the filter capacitance during the course of executions of the charge transfer processes leading to the determination of the measurable capacitance(s), a guard signal with proper guarding voltages can be applied. The applying of the pre-determined charging voltage to the measurable capacitance lasts for some duration of time, and before this duration ends, a first guarding voltage similar to this pre-determined voltage can be applied to the appropriate guarding electrode. Since the pre-determined voltage is typically fairly constant, the first guarding voltage can often be a single, roughly constant voltage. Then, before all the charge is shared (i.e., before charge sharing ends) between the measurable capacitance and associated filter capacitance, the guard signal applied to the guarding electrode may be changed to a second guarding voltage similar to the voltage on the associated filter capacitance. Again, although the singular is used in this discussion, there can be any number of guarding electrodes, measurable capacitances, impedances, filter capacitances, and the like involved.

In the embodiment shown in FIG. 1B, guarding electrode 106 is provided, over a low impedance path, with guarding voltages composing guard signal 103 that at least roughly approximate the voltages on the active electrode (e.g. 112