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

• US 7,777,503 B2
• Filed: 03/13/2009
• Issued: 08/17/2010
• Est. Priority Date: 06/03/2005
• Status: Active Grant

• Current Assignee: Synaptics Incorporated
• Original Assignee: Synaptics Incorporated
• Inventors: Reynolds, Joseph Kurth
• Primary Examiner(s): Nguyen; Vincent Q
• Application Number:
  US12/404,144
• Publication Number:
  US 20090206852A1
• Time in Patent Office: 522 Days
• Field of Search: 324/678, 324/686, 324/688
• US Class Current: 324/688
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 17/962 Capacitive touch switches
• CPC Class Codes: H03K 17/965 Switches controlled by moving an element forming part of the switch
• CPC Class Codes: H03K 2017/9602 characterised by the type or shape of the sensing electrodes
• CPC Class Codes: H03K 2217/96058 Fail-safe touch switches, where switching takes place only after repeated touch
• CPC Class Codes: H03K 2217/960725 Charge-transfer
• CPC Class Codes: H03K 2217/960775 Emitter-receiver or "fringe" type detection, i.e. one or more field emitting electrodes and corresponding one or more receiving electrodes

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.

Claims

1. A system for determining a capacitance value associated with a sensor electrode, the system comprising:

a guarding electrode proximate to the sensor electrode;

an integrating capacitance configured to store charge received from the sensor electrode;

a charge changing circuit coupled to the integrating capacitance; and

a controller configured to apply a guard signal to the guarding electrode, the guard signal comprising a waveform having at least two different voltages, the controller further configured to determine the capacitance value by repeatedly applying a voltage to the sensor electrode, repeatedly sharing charge between the sensor electrode and the integrating capacitance to accumulate charge on the integrating capacitance, repeatedly measuring a voltage on the integrating capacitance using a quantizer of the controller, generating quantized values responsive to the repeated measurings, repeatedly changing charge on the integrating capacitance based on the quantized values using the charge changing circuit, and determining the capacitance value of the measurable capacitance from the quantized values.

2. The system of claim 1 wherein a first voltage of the at least two different voltages of the guard signal is a voltage approximating the voltage applied to the sensor electrode.

3. The system of claim 1 wherein the waveform of the guard signal has a time constant such that the waveform of the guard signal approximates a waveform on the integrating capacitance.

4. The system of claim 1 wherein a second voltage of the at least two different voltages of the guard signal approximates an average of the voltage on the integrating capacitance.

5. The system of claim 1 wherein the at least two different voltages of the guard signal span a range substantially similar to a range defined by the voltage applied to the sensor electrode and a reset voltage of the integrating capacitance.

6. The system of claim 1 wherein the at least two different voltages of the guard signal span a range substantially less than a range defined by the voltage applied to the sensor electrode and a reset voltage of the integrating capacitance.

7. The system of claim 1 wherein the at least two different voltages of the guard signal comprise voltages that extend beyond the voltage applied to the sensor electrode and a reset voltage of the integrating capacitance.

8. The system of claim 1 wherein the charge changing circuit comprises a resistor, and wherein the controller is further configured to change the charge on the integrating capacitance by passing charge on the integrating capacitance through the resistor for a period of time determined at least in part by the quantized values.

9. A method for determining a capacitance value associated with a sensor electrode, the method comprising the steps of:

applying a guard signal to a guarding electrode proximate the sensor electrode, the guard signal comprising a waveform having at least two different voltages;

providing a voltage to the sensor electrode using a first switch;

allowing the first capacitance to share charge with an integrating capacitance;

producing a quantized value with a quantizer, the quantized value reflective of whether the voltage on the integrating capacitance is past a threshold of the quantizer;

changing a charge on the integrating capacitance by an amount of charge responsive to the voltage on the integrating capacitance being past the threshold of the quantizer, wherein the amount of charge is determined at least in part as a function of the quantized value produced with the quantizer;

repeating each of the providing step, the allowing step, the outputting, and the changing step at least once to generate a plurality of quantized values; and

determining the capacitance value using the plurality of quantized values generated by the repeating step.

10. The method of claim 9 wherein the at least two different voltages of the guard signal comprise a voltage approximating the voltage provided to the sensor electrode.

11. The method of claim 9 wherein the waveform of the guard signal has a time constant such that the waveform of the guard signal approximates a waveform on the integrating capacitance.

12. The method of claim 9 wherein the at least two different voltages of the guard signal comprise a voltage approximating an average of the voltage on the integrating capacitance.

13. The method of claim 9 wherein the at least two different voltages of the guard signal span a range substantially similar to a range defined by the voltage applied to the sensor electrode and a reset voltage of the integrating capacitance.

14. The method of claim 9 wherein the at least two different voltages of the guard signal span a range substantially less than a range defined by the voltage applied to the sensor electrode and a reset voltage of the integrating capacitance.

15. The method of claim 9 wherein the at least two different voltages of the guard signal comprise voltages that extend beyond the voltage applied to the sensor electrode and a reset voltage of the integrating capacitance.

16. The method of claim 9 wherein the step of changing a charge on the integrating capacitance by an amount of charge comprises passing charge on the integrating capacitance to through a resistor for a period of time.

17. The method of claim 9 wherein determining the capacitance value using the plurality of quantized values generated by the repeating step comprises filtering the quantized values.

18. The method of claim 9 wherein determining the capacitance value using the plurality of quantized values generated by the repeating step comprises counting the plurality of quantized values generated within a period of time by the repeating step.

19. The method of claim 9 wherein the repeating of the applying step, the allowing step, and the changing step maintains the voltage on the integrating capacitance near the threshold of the quantizer.

20. A device for determining a measurable capacitance for proximity detection, the device comprising:

a plurality of sensing electrodes, the measurable capacitance associated with at least one of the plurality of sensing electrodes;

a guarding electrode proximate to the sensing electrode;

a passive network that includes an integrating capacitance; and

a controller coupled to the plurality of sensing electrodes, the guarding electrode, and the passive network, the controller configured to apply a guard signal to the guarding electrode, the guard signal comprising a waveform having at least two different voltages, the controller further configured to apply a voltage to at least one of the plurality of sensing electrodes, share charge between the at least one of the plurality of sensing electrodes and the passive network such that shared charge is accumulated on the integrating capacitance; and

measure a voltage on the integrating 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.

21. The device of claim 20 wherein the at least two different voltages of the guard signal span a range substantially similar to a range defined by the voltage applied to the sensor electrode and a reset voltage of the integrating capacitance.

22. The device of claim 20 wherein the at least two different voltages of the guard signal span a range substantially less than a range defined by the voltage applied to the sensor electrode and a reset voltage of the integrating capacitance.

23. The device of claim 20 wherein the at least two different voltages of the guard signal comprise voltages that extend beyond the voltage applied to the sensor electrode and a reset voltage of the integrating capacitance.