Methods and systems for detecting a capacitance using switched charge transfer techniques Number:7,521,941 from the United States Patent and Trademark Office (PTO) owispatent

Title: Methods and systems for detecting a capacitance using switched charge transfer techniques

Abstract: Methods, systems and devices are described for detecting a measurable capacitance using charge transfer techniques. According to various embodiments, a charge transfer process is performed for two or more times. During the charge transfer process, a pre-determined voltage is applied to the measurable capacitance, and the measurable capacitance is then allowed to share charge with a filter capacitance through a passive impedance that remains coupled to both the measurable capacitance and to the filter capacitance throughout the charge transfer process. The value of the measurable capacitance can then be determined as a function of a representation of a charge on the filter capacitance and the number of times that the charge transfer process was performed. Such a detection scheme may be readily implemented using conventional components, and can be particularly useful in sensing the position of a finger, stylus or other object with respect to an input sensor.

Patent Number: 7,521,941 Issued on 04/21/2009 to Ely,   et al.

---

Inventors:	Ely; David (Cambridge, GB), Routley; Paul (Cambridge, GB), Reynolds; Joseph Kurth (Sunnyvale, CA), Haines; Julian (Dublin, IE), Hargreaves; Kirk (Mountain View, CA)
Assignee:	Synaptics, Inc. (Santa Clara, CA)
Appl. No.:	11/925,496
Filed:	October 26, 2007

---

Related U.S. Patent Documents

---

	Application Number	Filing Date	Patent Number	Issue Date
	11446323	Jun., 2006
	60687012	Jun., 2005
	60687167	Jun., 2005
	60687148	Jun., 2005
	60687039	Jun., 2005
	60687037	Jun., 2005
	60774843	Feb., 2006

---

Current U.S. Class:	324/678 ; 324/658
Current International Class:	G01R 27/26 (20060101)
Field of Search:	324/678,679,686,688

---

References Cited [Referenced By]

---

U.S. Patent Documents

3898425	August 1975	Crandell et al.
4459541	July 1984	Fielden et al.
5012124	April 1991	Hollaway
5305017	April 1994	Gerpheide
5451940	September 1995	Schneider et al.
5543591	August 1996	Gillespie et al.
5659254	August 1997	Matsumoto et al.
5730165	March 1998	Philipp
5973417	October 1999	Goetz et al.
6323846	November 2001	Westerman et al.
6452514	September 2002	Philipp
6466036	October 2002	Philipp
6536200	March 2003	Schwartz
6593755	July 2003	Rosengren
7288946	October 2007	Hargreaves et al.
2003/0090277	May 2003	Lechner et al.
2004/0104826	June 2004	Philipp
2005/0068712	March 2005	Schulz et al.
2005/0099188	May 2005	Baxter

Foreign Patent Documents

	3413849		Aug., 1985		DE
	1521090		May., 2005		EP
	1593988		Nov., 2005		EP
	2829317		Mar., 2003		FR
	3237594		Oct., 1991		JP
	2004059343		Jul., 2004		WO
	2004066498		Aug., 2004		WO
	2005031375		Apr., 2005		WO

Other References

Touch Sensor IC (B6TS). Omron Corporation [retrieved on Aug. 14, 2006]. Retrieved from Internet: <URL:www.omron.co.jp/ecb/products/touch/pdf/en-keisoku.pdf>. cited by other . Phillip, Hal; Fingering a Problem; New Electronics; Jan. 24, 2006; pp. 51-52; Retrieved from Internet: <URL:www.newelectronics.co.uk/articles/5676/fingering-a-problem.pdf>- ;. cited by other . Cichocki, Andrzej et al.; A Switched Capacitor Interface for Capcaitive Sensors Based on Relaxation Oscillators; IEEE; Oct. 1990; pp. 797-799; vol. 39 No. 5. cited by other . Huang, S.M. et al.; Electronic Transducers for Industrial Measurement of Low Value Capacitances; J. Phys. E.: Sci. Instrum. 21; 1988; pp. 242-250. cited by other . Phillip, H.; Charge Transfer Sensing: Spread Spectrum Sensor Technology Blazes New Applications; Retrieved from Internet: <URL:www.qprox.com/background/white.sub.--paper.php. cited by other . Toth, F.; Design Method. for Low-Cost, High-Perf. Cap. Sensors; Delft Univ. of Tech.; 1997; Retrieved from Internet: <URL:www.exalondelft.nl/download/CapacitiveSensors.PDF>. cited by other . Yamada, M. et al.; A Switched-Capacitor Interface for Capacitive Pressure Sensors; IEEE; Feb. 1992; pp. 81-86; vol. 31, No. 1. cited by other . Johns, D.A.; Analog Integrated Circuit Design; 1997; pp. 542, 543; John Wiley & Sons. cited by other . Banks; Low Cost, Low Speed A/D Conversion for Embedded Systems; Byte Craft Limited (retrieved on Jul. 25, 2006); Retrieved from Internet: <URL: www.bytecraft.com/addaconv.html>. cited by other . Cao, Y. et al.; High-Accuracy Circuits for On-Chip Capacitance Ratio Testing or Sensor Readout; IEEE Transactions; Sep. 1994; pp. 637-639; vol. 41, No. 9. cited by other . Josefsson, O.; Ask the Applications Engineer: Using Sigma-Delta Converters--Part 1; Analog Dialogue 30th Anniversary Reader Bonus; 1997; pp. 29-33. cited by other . Norsworthy, S.; Effective Dithering of Sigma-Delta Modulators; AT&T Bell Laboratories; IEEE; 1992; pp. 1304-1307. cited by other . Martin, K.; A Voltage-Controlled Switch Capacitor Relaxation Oscillator; IEEE Joum. of Solid-State Circuits; Aug. 1981; pp. 412-414; vol. SC-16, No. 4. cited by other . Seguine, D.; Capacitive Key Scan; Cypress MicroSystems; Oct. 2004; version 4.1. cited by other . Seguine, D.; Capacitive Switch Scan; Cypress MicroSystems; Apr. 2005; version 4.2. cited by other . Quantum Research Group, "Secrets of a Successful QTouch.TM. Design", Quantum Research Application Note AN-KD02, Rev. 1.03, Oct. 2005, pp. 1-11. cited by other . Kremin, Victor et al., "Capacitance Sensing--Waterproof Capacitance Sensing--AN2398", Cypress Perform, Document No. 001-14501 Rev., Dec. 8, 2006, pp. 1-11. cited by other.

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

---

Parent Case Text

---

PRIORITY DATA

This application is a continuation of U.S. patent application Ser. No. 11/446,323, filed Jun. 3, 2006, which claims priority of 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 method for measuring a measurable capacitance, the method comprising the steps of: performing a first charge transfer process for a first number of times equal to at least two, wherein the first charge transfer process comprises the steps of: applying a first voltage to the measurable capacitance; and allowing the measurable capacitance to share charge through a passive impedance with a filter capacitance, wherein the passive impedance remains coupled to both the measurable capacitance and the filter capacitance throughout the applying and allowing steps of the first charge transfer process; performing a second charge transfer process for a second number of times equal to at least one, wherein the second charge transfer process comprises the steps of: applying a second voltage to the measurable capacitance; and allowing the measurable capacitance to share charge with the filter capacitance through the passive impedance, wherein the first voltage transfers charge in a first direction, and the second voltage transfers charge in a second direction opposite the first direction, and wherein the passive impedance remains coupled to both the measurable capacitance and the filter capacitance throughout the applying and allowing steps of the second charge transfer process; and determining a value of the measurable capacitance from the first charge transfer process and the second charge transfer process.

2. The method of claim 1 wherein the first charge transfer process further comprises the step of comparing the voltage on the filter capacitance with a first threshold voltage, and wherein the second charge transfer process further comprises the step of comparing the voltage on the filter capacitance with a second threshold voltage, and wherein the first threshold voltage and the second threshold voltage are substantially equal.

3. The method of claim 1 wherein the first charge transfer process further comprises the step of comparing the voltage on the filter capacitance with a first threshold voltage, and wherein the second charge transfer process further comprises the step of comparing the voltage on the filter capacitance with a second threshold voltage, and wherein the first threshold voltage and the second threshold voltage are substantially different.

4. The method of claim 1 wherein the passive impedance comprises a resistance.

5. The method of claim 1 wherein the first charge transfer process further comprises the step of resetting the voltage on the filter capacitance to a first reset voltage, and wherein the second charge transfer process further comprises the step of resetting the voltage on the filter capacitance to a second reset voltage.

6. The method of claim 1 wherein: the first charge transfer process further comprises the steps of: comparing the voltage on the filter capacitance with a first threshold voltage; and resetting the voltage on the filter capacitance to a first reset voltage in response to the voltage on the filter capacitance crossing the first threshold voltage; and the second charge transfer process further comprises the steps of: comparing the voltage on the filter capacitance with a second threshold voltage; and resetting the voltage on the filter capacitance to a second reset voltage in response to the voltage on the filter capacitance crossing the second threshold voltage.

7. The method of claim 6 wherein the step of determining a value of the measurable capacitance comprises: determining the value of the measurable capacitance from a number of first charge transfer processes performed for the voltage on the filter capacitance to cross the first threshold voltage and from a number of second charge transfer processes performed for the voltage on the filter capacitance to cross the second threshold voltage.

8. The method of claim 1 wherein the step of determining a value of the measurable capacitance comprises: determining the value of the measurable capacitance from a quantity of the first charge transfer processes and the second charge transfer processes performed.

9. A proximity sensor comprising: a sensor electrode having a measurable capacitance; a switch coupled to the measurable capacitance; a passive network coupled to the measurable capacitance and the switch, the passive network comprising a passive impedance and a filter capacitance, wherein the passive impedance statically couples the measurable capacitance to the filter capacitance; and a controller coupled to the switch, wherein the controller is configured to perform a first charge transfer process for a first number of times equal to at least two and a second charge transfer process for a second number of times equal to at least one, wherein the first charge transfer process comprises: applying a first voltage to the measurable capacitance; and allowing the measurable capacitance to share charge through a passive impedance with a filter capacitance, wherein the passive impedance remains coupled to both the measurable capacitance and the filter capacitance throughout the applying and allowing of the first charge transfer process; and wherein the second charge transfer process comprises: applying a second voltage to the measurable capacitance; and allowing the measurable capacitance to share charge with the filter capacitance through the passive impedance, wherein the first voltage transfers charge in a first direction, and the second voltage transfers charge in a second direction opposite the first direction, and wherein the passive impedance remains coupled to both the measurable capacitance and the filter capacitance throughout the applying and allowing of the second charge transfer process; and and wherein the controller is further configured to determine a value of the measurable capacitance from the first charge transfer process and the second charge transfer process.

10. The proximity sensor of claim 9 wherein the first charge transfer process further comprises comparing the voltage on the filter capacitance with a first threshold voltage, and wherein the second charge transfer process further comprises comparing the voltage on the filter capacitance with a second threshold voltage, and wherein the first threshold voltage and the second threshold voltage are substantially equal.

11. The proximity sensor of claim 9 wherein the first charge transfer process further comprises comparing the voltage on the filter capacitance with a first threshold voltage, and wherein the second charge transfer process further comprises comparing the voltage on the filter capacitance with a second threshold voltage, and wherein the first threshold voltage and the second threshold voltage are substantially different.

12. The proximity sensor of claim 9 wherein the passive impedance comprises a resistor.

13. The proximity sensor of claim 9 wherein the first charge transfer process further comprises reselling the voltage on the filter capacitance to a first reset voltage, and wherein the second charge transfer process further comprises resetting the voltage on the filter capacitance to a second reset voltage.

14. The proximity sensor of claim 9 wherein: the first charge transfer process further comprises: comparing the voltage on the filter capacitance with a first threshold voltage; and resetting the voltage on the filter capacitance to a first reset voltage in response to the voltage on the filter capacitance crossing the first threshold voltage; and the second charge transfer process further comprises: comparing the voltage on the filter capacitance with a second threshold voltage; and resetting the voltage on the filter capacitance to a second reset voltage in response to the voltage on the filter capacitance crossing the second threshold voltage.

15. The proximity sensor of claim 14 wherein the controller is configured to determine the value of the measurable capacitance from a number of first charge transfer processes performed for the voltage on the filter capacitance to cross the first threshold voltage and from a number of second charge transfer processes performed for the voltage on the filter capacitance to cross the second threshold voltage.

16. The proximity sensor of claim 9 wherein the controller is configured to determine the value of the measurable capacitance from a quantity of the first charge transfer processes performed and the second charge transfer processes performed.

17. The proximity sensor of claim 9 wherein the controller is further configured to perform a second charge transfer process for a second number of times equal to at least one, wherein the second charge transfer process comprises: applying an opposing voltage to the measurable capacitance and allowing the measurable capacitance to share charge with the filter capacitance through the passive impedance, wherein the pre-determined voltage causes charge transfer in a first direction, and the opposing voltage causes charge transfer in a second direction opposite the first direction.

18. The proximity sensor of claim 17 wherein the controller is further configured to reset the voltage of the filter capacitance to a first value after the performing of the charge transfer process for the number of times and to reset the voltage of the filter capacitance to a second value after the performing of the second charge transfer process for the second number of times.

19. The proximity sensor of claim 17 wherein the controller is configured to perform the charge transfer process is at least until the voltage of the filter capacitance crosses a first threshold and to perform the second charge transfer process at least until the voltage of filter capacitance crosses a second threshold.

20. A proximity sensor comprising: a sensor electrode having a measurable capacitance; a switch coupled to the sensor electrode; a passive network coupled to the sensor electrode and the switch, the passive network comprising a passive impedance and a filter capacitance, wherein the passive impedance statically couples the sensor electrode to the filter capacitance; and a controller coupled to the switch, wherein the controller is configured to perform a charge transfer process for a number of times greater than one, wherein the charge transfer process comprises applying a pre-determined voltage to the measurable capacitance using the switch 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 voltage of the filter capacitance and determine a value of the measurable capacitance as a function of the voltage of the filter capacitance.

21. The proximity sensor of claim 20 further comprising a compensation circuit coupled to the filter capacitance configured to add at least a portion of a reference voltage to the filter capacitance.

22. The proximity sensor of claim 20 wherein the controller is further configured to reset the voltage on the filter capacitance.

23. The proximity sensor of claim 20 wherein the controller is further configured to determine a value of the measurable capacitance as a function of the voltage of the filter capacitance by ascertaining the number of times the charge transfer process is performed.

24. The proximity sensor of claim 20 wherein the controller is further configured to determine a value of the measurable capacitance as a function of the voltage of the filter capacitance by ascertaining the number of times the charge transfer process is performed for the voltage of the filter capacitance to pass a threshold voltage.

25. The proximity sensor of claim 20 wherein the controller is further configured to reset the voltage on the filter capacitance in response to the voltage of the filter capacitance passing a threshold voltage.

26. The proximity sensor of claim 20 wherein the measurable capacitance comprises a transcapacitance between a driving electrode and the sensing electrode.

---

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, 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 sensors 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 sensors can be readily found, for example, in input devices of electronic systems including handheld and notebook-type computers.

A user generally operates capacitive input devices by placing or moving one or more fingers, styli, and/or other objects near a sensing region of the sensor(s) 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) or proximity or motion or presences 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 other indicators 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, it is continually desired to simplify the design and implementation of such sensors. Moreover, a need continually arises for a highly versatile yet low cost and easy to implement sensor design. In particular, a need exists for a sensor design scheme that is flexible enough to be easily implemented across a wide variety of applications yet powerful enough to provide accurate capacitance sensing, while at the same time remaining cost effective.

Accordingly, it is desirable to provide systems and methods for quickly, effectively and efficiently detecting a measurable capacitance. Moreover, it is desirable to create a scheme that can be implemented using readily available components, such as standard ICs, microcontrollers, and discrete 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 detecting a measurable capacitance using charge transfer techniques that are implementable on many standard microcontrollers without requiring external, active analog components. According to various embodiments, a charge transfer process is performed two or more times. The charge transfer process comprises applying a pre-determined voltage to the measurable capacitance, and then allowing the measurable capacitance to share charge with a filter capacitance through a passive impedance that remains coupled to both the measurable capacitance and to the filter capacitance throughout the periods of applying of the pre-determined voltage and of allowing of the measurable capacitance to share charge. The value of the measurable capacitance can then be determined as a function of a representation of a charge on the filter capacitance and the number of times that the charge transfer process was performed. The number of times that the charge transfer process is executed can be pre-established or be based on the representation of the charge reaching some threshold. The representation of the charge on the filter capacitance can be obtained by a measuring step that produced a single-bit or multi-bit measurement. These steps can be repeated, and the results of the measuring step can be stored and/or filtered as appropriate.

Using the techniques described herein, a 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 function(s), 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:

FIGS. 1A-D are block diagrams of exemplary implementations of capacitance sensors;

FIG. 2A-B are timing diagrams showing exemplary techniques for operating a capacitance sensor such as that shown in FIG. 1B;

FIG. 3A-B are timing diagrams showing an alternate technique for operating a capacitance sensor such as that shown in FIG. 1B;

FIGS. 4A-C are block diagrams of alternate embodiments of capacitance sensors;

FIG. 5 is a timing diagram showing an exemplary technique for operating a capacitance sensor such as the sensor shown in FIG. 4A;

FIG. 6 is a block diagram showing an alternate embodiment of a multi-channel capacitance sensor incorporating a guard electrode;

FIG. 7 is a block diagram showing another alternate embodiment of a multi-channel capacitance sensor;

FIG. 8 is a flowchart of an exemplary technique for detecting capacitance using switched charge transfer techniques;

FIG. 9 is a schematic diagram of a proximity sensor device using a capacitance sensor coupled 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 a passive electrical network and one or more switches. In a typical implementation, a charge transfer process is executed for two or more iterations in which a pre-determined voltage is applied to the measurable capacitance using one or more of the switches, and in which the measurable capacitance is allowed to share charge with a filter capacitance in the passive network. (The filter capacitance can also be referred to as an "integrating capacitance" or an "integrating filter.") 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 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. After two or more iterations of the charge transfer process (although some embodiments may use only one iteration), the representation of charge on the filter capacitance is read to determine the measurable capacitance. The representation of the charge on the filter capacitance can be a voltage on the filter capacitance, such as a voltage on 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 measuring of the representation of the charge on the filter capacitance can entail comparison with one or more thresholds to generate a single or multi-bit reading. The measuring can entail use of multi-bit analog-to-digital circuitry to generate a multi-bit measure of the representation of the charge.

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 described herein can be readily implemented using only conventional switching mechanisms (e.g. those available through the I/Os 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. As a result, the various schemes 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.

The measurable capacitance is the effective capacitance of any signal source, electrode, or other electrical node having a capacitance detectable by a capacitive sensing system. For capacitive proximity sensors and other input devices accepting input from one or more fingers, styli, and/or other stimuli, measurable capacitance often represents the total effective capacitance from a sensing node to the local ground of the system ("absolute capacitance"). The total effective capacitance for input devices can be quite complex, involving capacitances, resistances, and inductances in series and in parallel as determined by the sensor design and the operating environment. In other cases, measurable capacitance may represent the total effective capacitance from a driving node to a sensing node ("transcapacitance"). This total effective capacitance can also be quite complex. However, in many cases the input can be modeled simply as a small variable capacitance in parallel with a fixed background capacitance.

In input devices using capacitance sensors, the measurable capacitance is often the variable capacitance exhibited by a sensing electrode of the capacitance sensor. The capacitance sensor may include multiple sensing electrodes, and each sensing electrode may be associated with a measurable capacitance. With an exemplary "absolute capacitance" sensing scheme, the measurable capacitance would include the capacitive coupling of the sensing electrode(s) to one or more input objects, such as any combination of finger(s), styli, or other object(s), that are close enough to the sensing electrode(s) to have detectable capacitive coupling with the sensing electrode(s). With an exemplary "transcapacitance" sensing scheme, the measurable capacitance would include the capacitive coupling of the sensing electrode(s) to one or more driving electrodes. This coupling to the input object(s) (for the "absolute capacitance" scheme) or between electrodes (for the "transcapacitance" scheme) changes as the electric field is affected by the input object(s). Thus, the value of the measurable capacitance can be used to ascertain information about the proximity, position, motion, or other positional information of the input object(s) for use by the capacitive input device or by any electronic system in communication with the capacitive input device.

The value of the pre-determined voltage applied to the measurable capacitance is often known, and often remains constant. For example, the pre-determined voltage can be 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. However, the pre-determined voltage can also be unknown or variable, so long as the pre-determined voltage remains ratiometric with the measurement of the charge on the filter capacitance. For example, a capacitance sensing scheme can involve resetting the filter capacitance to a reset voltage, and also involve measuring a voltage across 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 execution(s) 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 execution(s) of the charge transfer process for a determination of the measurable capacitance. In particular, where the pre-determined voltage is V.sub.cc and the reset voltage is GND, the threshold voltage can be ratiometric for a CMOS input threshold, for example (1/2)*(V.sub.cc-GND).

Turning now to the figures and with initial reference to FIG. 1A, an exemplary capacitance sensor 100 for determining a measurable capacitance 112 suitably includes a passive impedance 105 coupled with a filter capacitance 110. Although sensor 100 is driven using switches 101, 103, measurable capacitance 112, filter capacitance 110, and passive impedance 105 still form a passive electrical network that includes no active elements. Passive impedance 105 is provided by one or more non-active electronic components, such as any combination of capacitance(s), inductance(s), resistance(s), and the like. Capacitances, resistances, and inductances can be provided by any combination of capacitive, resistive, and inductive elements, respectively. Some elements exhibit more than one impedance property, such as having both resistive and inductive properties; these elements would thus provide both a resistance and an inductance to the network in which it is used. In various embodiments, passive impedance 105 is a resistance provided by a network or one or more resistors. Additionally, passive impedance 105 can include non-linear components such as diodes. Impedance 105 is generally designed to have an impedance that is large enough to prevent significant charge leaking into filter capacitance 110 during the applying of the pre-determined voltage to the measurable capacitance 112, as described more fully below. In various embodiments, impedance 105 may be a resistance on the order of a hundred kilo-ohms or more, although other embodiments may exhibit widely different impedance values.

Filter capacitance 110 is coupled to node 107 and to passive impedance 105 at node 115. Node 107 can be coupled to a suitable voltage (ground is shown in FIG. 1A although another reference voltage can be used). Filter capacitance 110 can be provided by one or more capacitors (such as a collection of any number of discrete capacitors) configured to accept charge transferred from measurable capacitance 112. Although the particular filter capacitance value selected will vary from embodiment to embodiment, the capacitance value of filter capacitance 110 will typically be an order of magnitude, and often several orders of magnitude, greater than the capacitance value of the measurable capacitance 112. For example, filter capacitance 110 may be designed on the order of several nanofarads or so, whereas measurable capacitance 112 could be on the order of picofarads. As one example that will be described in greater detail below, the filter capacitance 110 is selected such that the time constant of the RC circuit created by filter capacitance 110 and passive impedance 105 is greater than the duration of the pulses used to apply the pre-determined voltage to measurable capacitance 112.

The time constant of measurable capacitance 112 with passive impedance 105 is also preferably greater than the duration of the pre-determined voltage applied by the pulses to measurable capacitance 112. This is so that the charge added to filter capacitance 110 during the charge transfer process comes mostly from the charge stored on the measurable capacitance 112 and shared with filter capacitance 110, and less from any flow of current through passive impedance 105 during the applying of the pre-determined voltage. Since filter capacitance 110 is often orders of magnitude greater than measurable capacitance 112 in order to provide adequate capacitance-sensing resolution, it follows that its time constant with passive impedance 105 is also orders of magnitude greater than the duration of pre-determined voltage applied by the pulses. Thus, a relatively large time constant of the RC circuit allows the charge leakage to the filter capacitance 110 during the applying of the pre-determined voltage to be relatively small.

Sensor 100 also includes a switch 103 in parallel with filter capacitance 110 and coupled to nodes 115 and 107. Switch 103 can be closed to reset the charge on filter capacitance 110 before performing the charge transfer processes for a determination of a value of the measurable capacitance 112. In this case, closing switch 103 clears the charge on filter capacitance 110.

Other options for resetting filter capacitance 110 are readily available. For example, switch 103 can couple node 113 to a voltage such as ground (instead of nodes 107 and 115) such that closing switch 103 would reset the charge on filter capacitance 110 through passive impedance 105. Such a configuration may be implemented using a single digital I/O of a controller (such as shown in FIG. 4A). However, this configuration would reset with the time constant associated with passive impedance 105 and filter capacitance 110, and thus require a reset time greater than placing switch 103 in parallel with filter capacitance 110.

Operation of capacitance sensor 100 suitably involves a charge transfer process and a measurement process facilitated by the use of one or more switches 101, 103. Switches 101, 103 may be implemented with any type of discrete switches, buffered integrated circuits, field effect transistors and/or other switching constructs, to name just a few examples. Alternatively, switches 101, 103 can be implemented with internal logic/circuitry of a controller coupled to an output pin of the controller, as will be discussed in greater detail below. The output pin of the controller may also be coupled to internal logic/circuitry capable of providing input functionality, such that switches 101, 103 can be implemented using one or more I/Os of a controller.

The charge transfer process, which is typically repeated two or more times, suitably applies a pre-determined voltage (convenient voltages for the pre-determined voltage include a power supply voltage, a battery voltage, and a logic signal) to the measurable capacitance 112, and then allows measurable capacitance 112 to share charge with filter capacitance 110 as appropriate. In the example shown in FIG. 1A, closing switch 101 applies the pre-determined voltage to measurable capacitance 112 and opening switch 101 ceases the application of the pre-determined voltage to measurable capacitance 112. Circuit 100 illustrates a configuration where switch 101 is used to apply a voltage when it is closed and the application of the voltage ceases when switch 101 it is open. However, switches can be used to apply voltages when opened or closed, or used to apply a first voltage when open and a different voltage when closed. Thus, switch 101 is used to apply the pre-determined voltage in pulses or other waveforms that have a relatively short period in comparison to the RC time constant associated with impedance 105 and measurable capacitance 112 or filter capacitance 110 to help prevent excessive current leakage through impedance 105 during the applying of the pre-determined voltage. Leakage of charge through impedance 105 can be detrimental to sensor accuracy and/or resolution, since it is often difficult, if not practically impossible, to control or account for the charge leakage in measuring the representation of the charge on the filter capacitance 110 or determining a value of the measurable capacitance 112. Charge leakage through parasitic or stray impedances in addition to impedance 105 can also be detrimental to sensor performance, and this effect can be reduced by having a shorter application of the pre-determined voltage.

After applying the pre-determined voltage to measurable capacitance 112, the applying of the pre-determined voltage for that performance of the charge transfer process is ended by opening switch 101 and the measurable capacitance 112 is allowed to share charge with filter capacitance 110. During charge sharing, charge travels between measurable capacitance 112 and filter capacitance 110 through passive impedance 105. Passive impedance 105 remains coupled to the measurable capacitance 112 and the filter capacitance 110 during both the charging period (when the pre-determined voltage is applied to charge or discharge the measurable capacitance) and the allowing to share period (when the application of the pre-determined voltage is stopped). Although FIG. 1A shows a particular configuration for accomplishing this charge sharing, the sharing of charge through the passive impedance can be done a multitude of ways, and other circuits can be used without departing from the principles disclosed herein. For example, the electrical path from measurable capacitance 112 to ground in FIG. 1A (when the predetermined voltage is not applied and charge sharing is allowed) includes both impedance 105 and filter capacitance 110 in series. Since the impedance 105 and filter capacitance 110 are in series and the principle of the additivity of impedances applies, the relative positions of filter capacitance 110 and impedance 105 could be exchanged for this sensor 100 embodiment without altering the charge transfer process and the operation of the circuit.

To allow measurable capacitance 112 to share charge with the passive network, no action may be required other than to stop applying the pre-determined voltage and to pause (also "delay") for a time sufficient to allow charge to transfer between the measurable capacitance and the filter capacitance. This is true, for example, for the embodiment shown in FIG. 1A, where the pause time required for the charge to share between measurable capacitance 112 and filter capacitance 110 is determined by the time constant of the circuit including the measurable capacitance 112, the passive impedance 105, and the filter capacitance 110. In various embodiments, the pause time required may be relatively short (e.g. if the filter capacitance 110 is connected to the measurable capacitance 112 with a small resistance in series). In other embodiments, the delay required may be longer such that a lengthier pause time may be needed (e.g. if the filter capacitance 110 is connected to the measurable capacitance 112 with more significant passive impedance 105 in series). In other embodiments, allowing charge to transfer may involve actively actuating one or more switches associated with a controller to couple components external to the passive network, and/or taking other actions as appropriate. In such embodiments, the passive impedance 105 may be made smaller.

After performing one or more executions of the charge transfer process that includes the applying of the pre-determined voltage and the allowing of the measurable capacitance 112 to share charge with the filter capacitance 110, a representation of the charge on the filter capacitance 110 can be measured. The representation of the charge on filter capacitance 110 can be conveniently taken as the voltage at node 115 for the embodiment shown in FIG. 1A. The representation of the charge on filter capacitance 110 can also be taken as the voltage at node 113, but the voltage across passive impedance 105 must be accounted for in that case. However, the voltage across passive impedance 105 may be negligible for some embodiments and at certain times; if measurements are taken at these certain times for these embodiments, then the voltage at node 113 would then effectively be equivalent to the voltage at node 115. For example, if passive impedance 105 is a resistance, then negligible charge is stored on passive impedance 105; in addition, if measurable capacitance 112 and filter capacitance 110 have completed sharing, then insignificant current flows through passive impedance 105. In such a case, the voltage across passive impedance 105 is practically zero.

The measurement of the representation of the charge on filter capacitance 110 can be achieved by using a simple comparison with a threshold (such as by using a comparator to produce a single-bit measurement) or by more complex circuitry (such as by using a multi-bit ADC to produce a multi-bit measurement). When a threshold is used to produce a single-bit measurement, typically multiple measurements are taken to ascertain the number of executions of the charge transfer process needed for the representation of the charge to cross this threshold. This number of executions necessary can be used along with known values (e.g. the threshold, the value of filter capacitance 110, etc.) to determine a value of the measurable capacitance 112. When a multi-bit ADC is used to produce a higher resolution measurement, fewer measurements can be taken (one single measurement may be sufficient if the ADC provides sufficient resolution) and the number of executions before taking the measurement(s) can be pre-established. The multi-bit ADC measurement can be used along with known values and the pre-established number to determine the value of the measurable capacitance 112. Additional charge transfer processes can be performed after obtaining the measurement(s) for determining the value of the measurable capacitance 112 to bring the filter capacitance 110 to a reset state, or for convenience of design if the filter capacitance 110 is reset using a different method. These additional charge transfer processes may be especially useful for sensing multiple measurable capacitances.

As stated above, switches 101, 103 can be implemented with separate, discrete switches, or with the internal logic/circuitry coupled to an output or an input/output (I/O) of a controller. Turning now to FIG. 1B, a second exemplary capacitance sensor 150 is illustrated. The capacitance sensor 150 uses a controller 102 with I/Os 104 and 106 to provide switching functionality. I/O 104 can provide the switching associated with switch 101 of FIG. 1A. I/O 106 can provide switching functionality to reset the filter capacitance, but the effect differs from that associated with switch 103. In the case of the embodiment of FIG. 1B, I/O 106 is used to provide a reference voltage as does switch 103, but the filter capacitance 110 is reset to having a set amount of nonzero charge on filter capacitance 110 since node 107 is coupled to a voltage that typically can not be supplied by I/O 106. Digital I/Os of controllers are typically capable of switchably applying one or more logic values and/or a "high impedance" or "open circuit" value. The logic values may be any appropriate voltage or other signal. For example, a logic "high" or "1" value could correspond to a "high" voltage (e.g. +V.sub.cc, which can be +5 volts for some controllers, or the like), and a logic "low" or "0" value could correspond to a comparatively "low" voltage (e.g. ground or 0V). Thus, the particular signals selected and applied using I/Os 104, 106 can vary significantly from implementation to implementation depending on the particular controller 102 selected. Thus, one advantage of these embodiments using controller I/Os is that a very flexible capacitance sensor 150 can be readily implemented using only passive components (e.g., passive resistance 105, filter capacitance 110) in conjunction with a conventional controller 102 such as a microcontroller, digital signal processor, microprocessor, programmable logic array, application specific integrated circuit 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 Dallas, Tex., among others

Further, in some embodiments, the controller 102 includes 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. Because impedance 105 and filter capacitance 110 are statically connected, the only physical action that needs take place during sensor operation involves manipulation of signal levels at I/Os 104 and 106. Such manipulation may take place in response to software, firmware, configuration, or other instructions contained in controller 102.

In some embodiments, the filter capacitance 110 is coupled to an effectively constant voltage such as ground at node 107 such as shown in FIG. 1A. In other embodiments, the filter capacitance 110 is coupled to a varying voltage that improves the performance of the capacitance detection, such as shown in FIG. 1B. The exemplary embodiment shown in FIG. 1B includes such an optional compensation circuit 125 that compensates capacitance sensor 150 for fluctuations in power supply voltages, thereby providing improved resistance to power supply voltage noise effects. Compensation circuit 125 typically couples the side of filter capacitance 110 opposite the measurable capacitance 112 to either or both power supply rails (coupling to +V.sub.cc and ground is shown in FIG. 1B) associated with the implementation of capacitance sensor 150. Although FIG. 1B shows compensation circuit 125 compensating only one filter capacitance 110, the same compensation circuit can be coupled to multiple filter capacitance 110 at node 107. Thus, compensation circuit 125 can easily be used to compensate sensors with multiple sensing channels and multiple filter capacitances. With the configuration shown in FIG. 1B, fluctuations in the supply rails (also "supply voltage ripple") induce similar fluctuations in the voltage at node 107, and therefore can be used to compensate for fluctuations in thresholds associated with controller 102 induced by the same supply voltage ripple.

The exemplary compensation circuit 125 shown in FIG. 1B, includes two impedances 127 and 129 configured in an impedance divider arrangement between the two supply voltages of +V.sub.cc and ground. An impedance divider is composed of two passive impedances in series, where each passive impedance is coupled to at least two nodes. One of these nodes is common to both impedances ("a common node" to which both impedances connect.) The common node serves as the output of the impedance divider. The output of the impedance divider is a function of the voltages and/or currents applied at the "unshared" nodes (the nodes of the two impedances that are not the common node) over time. A simple example of an impedance divider is a voltage divider composed of two capacitances or two resistances. More complex impedance dividers may have unmatched capacitances, resistances, or inductances in series or in parallel. An impedance may also have any combination of capacitive, resistive, and inductive characteristics.

For compensation circuit 125 shown in FIG 1B, the impedance divider can be a voltage divider formed from two resistances or two capacitances coupled to +V.sub.cc and ground. The impedance divider of circuit 125 has a "common node" coupled to the filter capacitance 110 at node 107. Resistive versions of impedances 127 and 129 can comprise resistors to form a resistive divider network, and capacitive versions of impedances 127 and 129 can comprise capacitors to form a capacitive divider network. By selecting appropriate values for impedances 127 and 129, filter capacitance 110 can be biased toward any voltage that lies between the two supply voltages. Moreover, variations in supply voltage will be automatically compensated by the compensation circuit 125. This is because such a voltage divider provides a voltage that reflects the fluctuations in power supply voltage without significant lag. Although advantageous for some embodiments, the use of a compensation circuit 125 may not be desirable in all embodiments.

FIG. 1D shows another sensor circuit 195 that demonstrates another method of coupling the filter capacitance to a varying voltage that improves the performance of the capacitance detection. In circuit 195, voltage compensation is achieved by using a combination filter capacitance 110 formed from a capacitive impedance divider statically coupled to both power supply rails. This combination filter capacitance 110 includes a first filter capacitance 1102 statically coupled to a first power supply rail (+V.sub.cc is shown in FIG. 1D) and a second filter capacitance 1104 statically coupled to the other power supply rail (GND is shown in FIG. 1D). The first and second filter capacitances 1102 and 1104 are coupled to each other at their common node, which is also node 115. Node 115 is further coupled to passive impedance 105 and I/O 106. The passive impedance 105 is also coupled to I/O 104 and measurable capacitance 112 at node 113. Overall, the configuration of circuit 195 is very similar to the configuration of circuit 150 (FIG. 1B) without compensation circuit 125; however, circuit 195 has a split filter capacitance that couples to both power supply rails.

The operation of circuit 195 can be very similar to the operation of circuit 150. I/O 104 can apply the predetermined voltage to measurable capacitance 112 by providing a logic value (e.g. a logic "high"). I/O 104 can then be held at high impedance to allow charge sharing between measurable capacitance 112 and both capacitances 1102-1104 of combination filter capacitance 110. I/O 106 (or some other circuitry) can be used to measure the voltage at node 115, which is still representative of the charge on combination filter capacitance 110 by being representative of the total charge on capacitances 1102-1104. I/O 106 can provide a reset signal (e.g. a logic "low") to reset the charge on both capacitances 1102-1104 after the appropriate number of charge transfer processes has been performed. With the configuration shown in FIG. 1D, fluctuations in the supply rails induce similar fluctuations in the voltages to which combination filter capacitance 110 is referenced (+V.sub.cc for capacitance 1102 and GND for capacitance 1104). Therefore, the embodiment of circuit 195 can compensate for fluctuations in thresholds associated with controller 102 that are induced by the same supply voltage ripple.

Since the circuit 195 achieves compensation by using the filter capacitance, it has the advantage over compensation circuit 125 of fewer components when only one or two filter capacitances are needed by the sensor. In many cases, the compensation illustrated by circuit 195 can be shared by multiple sensing channels that share the same filter capacitance (e.g. circuit 440 of FIG. 4C, discussed below). However, the split-filter-capacitance method illustrated by FIG. 1D is more difficult to share across multiple sensing channels than compensation circuit 125 of FIG. 1B, since sharing the compensation would share the filter capacitance.

With reference to FIG. 2A, an exemplary timing scheme 200 is shown that would be suitable for operating capacitance sensor 150 of FIG. 1B. Specifically, FIG. 2A illustrates the voltages associated with I/Os 104 and 106, and at nodes 113 and 115. Trace 230 shows an exemplary set of charging voltage pulses 210 that can be provided to measurable capacitance 112 using I/O 104. The charging voltage pulses 210 include both logic low (0) output portions 209 and logic high (1) output portions 201. In the embodiment shown in FIGS. 1B, 2A, the logic high output portions apply the pre-determined voltage to the measurable capacitance 112 via node 113 such that the pre-determined voltage is the voltage associated with a logic high output (which is +V.sub.cc for a typical controller 102). Thus, the pre-determined voltage is applied using the appropriate switch (e.g. via circuitry internal to controller 102 that produces the appropriate signal output on I/O 104 in FIG. 1B). The logic high portions 201 of charging voltage pulses 210 are generally selected to have a period shorter than the response time of the RC circuit that includes impedance 105 and filter capacitance 110 such that any charge leakage to the filter capacitance 110 during the applying of the pre-determined voltage will be negligible.

In the embodiment shown in FIG. 2A, the charging voltage pulses 210 also include relatively brief logic low output portions 209 (which is GND for a typical controller 102, although other controllers may output other logic values). Logic low output portions 209 provide an "opposing" voltage that precedes the logic high output portions 201 that apply the pre-determined voltage. Th