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

• US 7,973,542 B2
• Filed: 08/13/2010
• Issued: 07/05/2011
• 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:
  US12/856,416
• Publication Number:
  US 20100308847A1
• Time in Patent Office: 326 Days
• Field of Search: 324/658, 324/688, 324/679
• 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 controller configured to determine a measurable capacitance, the controller configured to:

apply a pre-determined voltage to a first sensing electrode of a plurality of sensing electrodes using a first switch;

apply a first guard voltage to a guarding electrode using a second switch;

share charge between the first sensing electrode and a passive network, such that shared charge is accumulated on a filter capacitance of the passive network;

apply a second guard voltage different from the first guard voltage to the guarding electrode;

measure a voltage on the filter capacitance for a number of measurements equal to at least one to produce at least one result; and

determine the measurable capacitance using the at least one result.

2. The controller of claim 1 wherein the first guard voltage comprises a voltage selected to approximate the pre-determined voltage.

3. The controller 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.

4. The controller of claim 1 wherein the second guard voltage comprises part of a variable voltage waveform, the variable voltage waveform changing with a time constant selected to approximate the voltage on the filter capacitance.

5. The controller of claim 1 wherein the wherein the second guard voltage comprises a pulse modulated signal.

6. The controller of claim 1 wherein the second guard voltage comprises a voltage approximating an average voltage on the filter capacitance.

7. The controller of claim 1 wherein the controller is further configured to share charge between the first sensing electrode and the passive network using a third switch.

8. A controller adapted to determine a capacitance value associated with a sensor electrode, the controller configured to:

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

determine the capacitance value by

repeatedly applying a voltage to the sensor electrode,

accumulating charge on an integrating capacitance by repeatedly sharing charge between the sensor electrode and the integrating capacitance,

generating quantized values by repeatedly measuring a voltage on the integrating capacitance using a quantizer of the controller,

repeatedly changing charge on the integrating capacitance based on the quantized values, and

determining the capacitance value associated with the sensor electrode using the quantized values.

9. The controller of claim 8 wherein a first voltage of the at least two different voltages of the guard signal approximates the voltage applied to the sensor electrode.

10. The controller of claim 8 wherein the waveform of the guard signal has a time constant such that the waveform of the guard signal approximates a waveform of the voltage on the integrating capacitance.

11. The controller of claim 8 wherein a second voltage of the at least two different voltages of the guard signal approximates an average of the waveform of the voltage on the integrating capacitance.

12. The controller of claim 8 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.

13. The controller of claim 8 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.

14. The controller of claim 8 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.

15. A controller including a memory, the memory including executable instructions for determining a measurable capacitance associated with a sensing electrode by:

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

applying a voltage to the sensing electrode;

sharing charge between the sensing electrode and a passive network such that shared charge is accumulated on an integrating capacitance of the passive network;

measuring a voltage on the integrating capacitance for a number of measurements equal to at least one to produce at least one result; and

determining the measurable capacitance using the at least one result.

16. The controller of claim 15 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 sensing electrode and a reset voltage of the integrating capacitance.

17. The controller of claim 15 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 sensing electrode and a reset voltage of the integrating capacitance.

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

19. The controller of claim 15, wherein the at least two different voltages of the guard signal comprises a voltage approximating the voltage applied to the sensing electrode.

20. The controller of claim 15, wherein the guard signal averages to a voltage approximating an average voltage on the integrating capacitance.