Automatic environmental compensation of capacitance based proximity sensors

• US 20070032967A1
• Filed: 07/18/2006
• Published: 02/08/2007
• Est. Priority Date: 07/18/2005
• Status: Active Grant

• Current Assignee: Analog Devices, Inc.
• Original Assignee: Analog Devices, Inc.
• Inventors: Coquerel, Laurent, Feen, Ken, Jeyapaul, Richardson, Cleary, John
• Granted Patent: US 8,050,876 B2
• Time in Patent Office: Days
• US Class Current: 702/47
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/94026 Automatic threshold calibration; e.g. threshold automatically adapts to ambient conditions or follows variation of input

Abstract

Improved capacitive sensor operation is achieved with improved discrimination between environmental drift and apparent drift attributable to human proximity to the sensor. A proximity algorithm detects conditions interpreted as indicating a user is close to, but not touching, a sensor. When such proximity is detected, ambient value calibration is halted, thereby avoiding treating the human'"'"'s proximity as environmental drift requiring compensation and preventing miscalculation of calibration. The proximity algorithm employs two moving-average filters (implemented in hardware or software) to monitor the CDC output values over time and to make appropriate adjustments to a signal representing the ambient, while distinguishing environmental drift from proximity-induced pseudo-drift. Accurate ambient values allow for improved proximity detection by providing this environmentally compensated average value to an adaptive threshold algorithm which features a fast attack, slow decay peak detection to automatically track and compensate for different response characteristics (typically finger sizes).

Claims

1. A method of operating a capacitive sensor assembly comprising at least one capacitive sensor element and a capacitance-to-digital converter (CDC) that supplies output codes corresponding to sensor element capacitance, comprising acts of:

a. determining an ambient value of the CDC output codes;

b. modifying at least one threshold value in response to changes in said ambient value; and

c. detecting proximity of a ground mass to the sensor element and preventing the modification of threshold values while said proximity is detected.

2. The method of claim 1, wherein the ground mass is a portion of a human body.

3. The method of claim 1, further comprising generating an output signal indicating when a CDC output code exceeds in magnitude a respective threshold value.

4. The method of claim 1 wherein modifying at least one threshold value comprises modifying said threshold in dependence on a sensitivity setting to vary an amount of pressure required to signal a valid touch on the sensor element.

5. The method of claim 4 wherein said threshold is modified in dependence on a combination of a sensitivity setting and a history of CDC code variation.

6. The method of claim 4 wherein modifying at least one threshold value further comprises modifying said threshold in dependence on an offset value.

7. The method of claim 4 wherein modifying at least one threshold value further comprises employing ambient, maximum CDC value and minimum CDC value as well as a sensitivity setting to provide a threshold that adapts sensor response to external conditions.

8. The method of claim 7 wherein said external conditions may include one or more of an environmental condition and a user'"'"'s finger size.

9. A method of operating a capacitive sensor assembly comprising at least one capacitive sensor element and a capacitance-to-digital converter (CDC) that supplies output codes corresponding to sensor element capacitance, comprising using a proximity algorithm to detect conditions which are interpreted as indicating a user is close to, but not touching, a sensor element and then halting ambient value calibration while said condition or conditions persist.

10. The method of claim 9 wherein said proximity algorithm employs two moving average filters to monitor the CDC output values over time and to make appropriate adjustments to a signal representing the ambient CDC output value, while distinguishing environmental draft from proximity induced pseudo-drift.

11. The method of claim 10 wherein said filters comprise a Fast Filter and a Slow Filter, the Fast Filter receiving the CDC output values and averaging a predetermined number of CDC output values to produce a sequence of Fast Filter Average values;

the Slow Filter receiving the Fast Filter Average values and generating a new output value only if proximity of a ground mass is not detected and the CDC output values are changing slowly.

12. A method of operating a capacitive sensor assembly having at least one capacitive sensor element and a capacitance-to-digital converter (CDC) that supplies output codes corresponding to sensor element capacitance, comprising:

a. performing an ambient value calibration of the CDC output;

b. using a proximity algorithm, processing the CDC output to detect a condition or conditions interpreted as indicating a user is close to, but not touching, a sensor element; and

c. halting ambient value calibration of the CDC output when the proximity algorithm indicates said condition or conditions persist;

d. the proximity algorithm employing first and second moving average filters to monitor the CDC output values over time and to detect said condition or conditions.

13. The method of claim 12 wherein said first and second filters comprise, respectively, a fast filter and a slow filter, the fast filter receiving the CDC output values and averaging a predetermined number of CDC output values to produce a sequence of Fast Filter Average values;

the Slow Filter receiving the Fast Filter Average values and generating a new output value only if proximity of a ground mass is not detected and the CDC output values are changing relatively slowly.

14. A method of operating a capacitive sensor assembly having at least one capacitive sensor element and a capacitance-to-digital converter (CDC) that supplies output codes corresponding to sensor element capacitance, comprising:

a. using a multi-step proximity algorithm, detecting from output of the CDC a condition or conditions indicative of a user being close to, but not touching, a sensor element; and

b. halting ambient value calibration of the sensor assembly while said condition or conditions persist;

wherein the proximity algorithm employs at least a first step and a second step and said first step and said second step apply different criteria directly or indirectly to the CDC output, to distinguish environmental drift from proximity induced pseudo-drift.

15. The method of claim 14 wherein the different criteria include different averaging of CDC output values.

16. A calibration assembly for use with a capacitive sensor assembly having at least one capacitive sensor element and a capacitance-to-digital converter (CDC) that supplies output codes corresponding to sensor element capacitance, comprising a processor executing one or more sequences of program instructions to:

a. determine an ambient value of the CDC output codes;

b. modify at least one threshold value in response to changes in said ambient value; and

c. detect proximity of a ground mass to the sensor element and prevent the modification of threshold values while said proximity is detected.

17. The assembly of claim 16, wherein the processor further executes instructions to generate an output signal indicating when a CDC output code exceeds in magnitude a respective threshold value.

18. The assembly of claim 16 wherein modifying at least one threshold value comprises modifying said threshold in dependence on a sensitivity setting to vary an amount of pressure required to signal a valid touch on the sensor element.

19. The assembly of claim 18 wherein modifying at least one threshold value further comprises modifying said threshold in dependence on an offset value.

28. The assembly of claim 18 wherein modifying said threshold in dependence on a sensitivity setting further comprises employing ambient, maximum CDC value and minimum CDC value as well as a sensitivity setting to provide a threshold that adapts sensor response to external conditions.

20. A calibration assembly for use with a capacitive sensor assembly having at least one capacitive sensor element and a capacitance-to-digital converter (CDC) that supplies output codes corresponding to sensor element capacitance, comprising a processor executing instructions to perform a proximity algorithm to detect conditions which are interpreted as indicating a user is close to, but not touching, a sensor element and then halting ambient value calibration while said condition or conditions persist.

21. The assembly of claim 20 wherein said proximity algorithm employs two moving average filters to monitor the CDC output values over time and to generate corresponding adjustments to a signal representing the ambient CDC output value, while distinguishing environmental draft from proximity induced pseudo-drift.

22. The assembly of claim 21 wherein said moving average filters comprise a Fast Filter and a Slow Filter, the Fast Filter input receiving the CDC output values and averaging a predetermined number of CDC output values to produce a sequence of Fast Filter Average values;

the Slow Filter input receiving a plurality of the Fast Filter Average values and generating a new output value only if proximity of a ground mass is not detected and the CDC output values are changing relatively slowly.

23. A calibration assembly for a capacitive sensor assembly having at least one capacitive sensor element and a capacitance-to-digital converter (CDC) that supplies output codes corresponding to sensor element capacitance, comprising a processor executing instructions to:

a. perform an ambient value calibration of the CDC output;

b. use a proximity algorithm to processing the CDC output to detect a condition or conditions indicating a user is close to, but not touching, a sensor element; and

c. halt ambient value calibration of the CDC output when the proximity algorithm indicates said condition or conditions persist;

d. the proximity algorithm employing first and second moving average filters to monitor the CDC output values over time and to detect said condition or conditions.

24. The calibration assembly of claim 23 wherein said first and second filters comprise, respectively, a fast filter and a slow filter, the fast filter receiving the CDC output values and averaging a predetermined number of CDC output values to produce a sequence of Fast Filter Average values;

the Slow Filter receiving the Fast Filter Average values and generating a new output value only if proximity of a ground mass is not detected and the CDC output values are changing relatively slowly.

25. A calibration assembly for a capacitive sensor assembly having at least one capacitive sensor element and a capacitance-to-digital converter (CDC) that supplies output codes corresponding to sensor element capacitance, comprising a processor adapted to:

a. use a multi-step proximity algorithm to detect from output of the CDC a condition or conditions indicative of a user being close to, but not touching, a sensor element; and

b. halt ambient value calibration of the sensor assembly while said condition or conditions persist;

wherein the proximity algorithm employs at least a first step and a second step and said first step and said second step apply different criteria directly or indirectly to the CDC output, to distinguish environmental drift from proximity-induced pseudo-drift.

26. The assembly of claim 25 wherein the different criteria include different averaging of CDC output values.

27. The assembly of claim 25 wherein the different criteria include at least two different methods to distinguish environmental drift from proximity-induced pseudo-drift.