Periodic sensor autocalibration and emulation by varying stimulus level

• US 20080309622A1
• Filed: 06/13/2007
• Published: 12/18/2008
• Est. Priority Date: 06/13/2007
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Krah, Christoph Horst
• Granted Patent: US 7,920,134 B2
• Time in Patent Office: Days
• US Class Current: 345/173
• CPC Class Codes: G06F 3/0418 for error correction or compensation, e.g. based on parallax, calibration or alignment
• CPC Class Codes: G06F 3/0445 using two or more layers of sensing electrodes, e.g. using two layers of electrodes separated by a dielectric layer

Abstract

The automatic calibration of a sensor panel is disclosed by varying the amplitude of an input stimulus Vstim to simulate a full-touch condition and calibrating each pixel of the sensor panel in accordance with the difference between the simulated full-touch condition and a baseline full-touch condition. To accomplish this, a baseline full scale output FS_targ_cal can be measured at during pre-delivery calibration for each pixel using a test fixture capable of applying a no-touch to full-touch condition given a nominal Vstim. A full-touch condition can then be emulated for each pixel by lowering Vstim until the current full scale output FS_targ_current equals FS_targ_cal, and determining the Vstim value Vstim_cal at that point. During field calibration, Vstim_cal can be applied to each pixel to simulate a full-touch condition, and FS_targ_current can be obtained and compared against FS_targ_cal. Each analog channel can then be tuned so that in subsequent full-touch conditions, FS_targ_current will approximately equal FS_targ_cal.

Claims

1. A method for calibrating a mutual capacitance touch sensor, comprising:

during pre-delivery calibration, determining a no-touch to full-touch full-scale output value range FS_targ_cal from an analog channel coupled to the sensor, and determining a sensor stimulus voltage amplitude value Vstim_cal at which the full-scale output value of the analog channel is approximately equivalent to FS_targ_cal; and

during field calibration, applying Vstim_cal to the sensor and obtaining a full-scale output value range FS_targ_current from the analog channel, and tuning the analog channel in accordance with any difference between FS_targ_current and FS_targ_cal.

2. The method of claim 1, the step of determining the Vstim value Vstim_cal comprising varying Vstim until the full-scale output value of the sensor is approximately equivalent to FS_targ_cal.

3. The method of claim 1, the step of determining the Vstim value Vstim_cal comprising computing Vstim_cal from the equation Vstim_cal=((Csig−

Δ

Csig)*Vstim)/Csig, wherein Csig is a nominal signal capacitance of the sensor and Δ

Csig is a change in Csig due to a full-touch condition.

4. The method of claim 1, wherein tuning the analog channel comprises tuning a feedback capacitor Cfb in accordance with a difference between FS_targ_current and FS_targ_cal.

5. The method of claim 1, the step of tuning the analog channel in accordance with any difference between FS_targ_current and FS_targ_cal further comprising:

during pre-delivery calibration, calculating a static signal capacitance Csig_stat_cal as Csig_stat_cal=Csig_cal-Δ

Csig_cal, where Csig_cal is a signal capacitance during pre-delivery calibration and Δ

Csig_cal is a change in the signal capacitance due to a full touch during pre-delivery calibration; and

during field calibration,

calculating a static signal capacitance Csig_stat_current as Csig_stat_current=Csig_current−

Δ

Csig_current, where Csig_current is the signal capacitance during field calibration and Δ

Csig_current is a change in the signal capacitance due to Vstim_cal during field calibration,

compensating Csig_stat_current by adjusting an amount of offset compensation in a DAC of the analog channel dVoff as dVoff=Voff_cal*Csig_stat_cal/(Csig_stat_cal-Csig_stat_current), where VOFF_cal is a calibrated offset value, and

adjusting a gain in an ADC dGain as dGain=ADCGAIN_cal*Δ

Csig_cal/(Δ

Csig_cal-dΔ

Csig_cal), where ADCGAIN is a calibrated ADC gain value, and dΔ

Csig_cal is a change in Δ

Csig_cal during field calibration.

6. In a system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, a method for calibrating the output value of the sensor, comprising:

during pre-delivery calibration, determining a no-touch to full-touch full-scale output value range FS_targ_cal from the analog channel connected to the column containing the sensor, and determining a Vstim value Vstim_cal at which the full-scale output value of the sensor is approximately equivalent to FS_targ_cal; and

during field calibration, applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor, and tuning the analog channel in accordance with any difference between FS_targ_current and FS_targ_cal.

7. The method of claim 6, the step of determining the Vstim value Vstim_cal comprising varying Vstim until the full-scale output value of the sensor is approximately equivalent to FS_targ_cal.

8. The method of claim 6, the step of determining the Vstim value Vstim_cal comprising computing Vstim_cal from the equation Vstim_cal=(Δ

Csig*Vstim)/Csig, wherein Csig is a nominal signal capacitance of the sensor and Δ

Csig is a change in Csig due to a full-touch condition.

9. The method of claim 6, wherein field calibration does not commence until any active scanning and touching of the sensor panel has ceased.

10. The method of claim 6, wherein tuning the analog channel comprises tuning a feedback capacitor Cfb in accordance with a difference between FS_targ_current and FS_targ_cal.

11. In a system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, a method for emulating different sensor panel types, comprising:

during pre-delivery calibration, determining a no-touch to full-touch full-scale output value range FS_targ_cal from the analog channel connected to the column containing the sensor, and determining a Vstim value Vstim_cal at which the full-scale output value of the sensor is approximately equivalent to FS_targ_cal; and

during field calibration, applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor, and tuning the analog channel in accordance with any difference between FS_targ_current and FS_targ_cal.

12. In a system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, a method for calibrating the output value of the sensor, comprising:

during pre-delivery calibration,

applying Vstim to the row containing the sensor, applying no-touch and full-touch conditions to the sensor, and obtaining a full-scale output value range of FS_targ_cal from the analog channel connected to the column containing the sensor, and

determining a Vstim value Vstim_cal at which the full-scale output value of the sensor is approximately equivalent to FS_targ_cal; and

during field calibration,

applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor,

comparing FS_targ_current to FS_targ_cal, and

if FS_targ_current is not approximately equal to FS_targ_cal, tuning the analog channel to make FS_targ_current approximately equal to FS_targ_cal.

13. The method of claim 12, the step of determining the Vstim value Vstim_cal comprising varying Vstim until the full-scale output value of the sensor is approximately equivalent to FS_targ_cal.

14. The method of claim 12, the step of determining the Vstim value Vstim_cal comprising computing Vstim_cal from the equation Vstim_cal=(Δ

Csig*Vstim)/Csig, wherein Csig is a nominal signal capacitance of the sensor and Δ

Csig is a change in Csig due to a full-touch condition.

15. The method of claim 12, wherein field calibration does not commence until any active scanning and touching of the sensor panel has ceased.

16. The method of claim 12, wherein tuning the analog channel comprises tuning a feedback capacitor Cfb in accordance with a difference between FS_targ_current and FS_targ_cal.

17. A method for calibrating a mutual capacitance touch sensor, comprising:

during pre-delivery calibration, measuring a plurality of calibration parameters for a nominal sensor stimulus voltage amplitude value Vstim_cal and one or more reduced stimulus levels Vstim_cal or Vstim_cal[ ], where Vstim_cal[ ] is a vector;

during field calibration, measuring a plurality of field parameters for the nominal sensor stimulus voltage amplitude value Vstim_cal and the one or more reduced stimulus levels Vstim_cal or Vstim_cal[ ]; and

applying a correction to calibrated gain and offset values, the correction derived by correlating the calibration parameters and the field parameters.

18. The method of claim 17, further comprising correlating the calibration parameters and the field parameters using lookup tables.

19. The method of claim 17, further comprising correlating the calibration parameters and the field parameters using extrapolation.

20. The method of claim 17, further comprising correlating the calibration parameters and the field parameters using interpolation.

21. In a computing system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, an apparatus for calibrating the output value of the sensor, comprising:

non-volatile memory storing a Vstim_cal value at which a full-scale output value range of the sensor is approximately equivalent to a no-touch to full-touch full-scale output value range FS_targ_cal; and

a panel processor coupled to the non-volatile memory and programmed for, during field calibration, applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor, and tuning the analog channel in accordance with any difference between FS_targ_current and FS_targ_cal.

22. The apparatus of claim 21, the panel processor further programmed for withholding field calibration until any active scanning and touching of the sensor panel has ceased.

23. The apparatus of claim 21, the panel processor further programmed for tuning the analog channel by tuning a feedback capacitor Cfb in accordance with a difference between FS_targ_current and FS_targ_cal.

24. The apparatus of claim 21, the computing system further comprising a panel subsystem coupled to the apparatus, a high-voltage driver coupled to the panel subsystem, and a host processor coupled to the panel processor.

25. The apparatus of claim 24, further comprising a mobile telephone that incorporates the computing system.

26. The apparatus of claim 24, further comprising a digital audio player that incorporates the computing system.

27. In a system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, an apparatus for calibrating the output value of the sensor, comprising:

non-volatile memory storing a Vstim_cal value at which a full-scale output value range of the sensor is approximately equivalent to a no-touch to full-touch full-scale output value range FS_targ_cal; and

a panel processor coupled to the non-volatile memory and programmed for, during field calibration,

applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor,

comparing FS_targ_current to FS_targ_cal, and

if FS_targ_current is not approximately equal to FS_targ_cal, tuning the analog channel to make FS_targ_current approximately equal to FS_targ_cal.

28. The apparatus of claim 27, the panel processor further programmed for withholding field calibration until any active scanning and touching of the sensor panel has ceased.

29. The apparatus of claim 27, the panel processor further programmed for tuning the analog channel by tuning a feedback capacitor Cfb in accordance with a difference between FS_targ_current and FS_targ_cal.

30. A mobile telephone including a computing system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, the mobile telephone containing an apparatus for calibrating the output value of the sensor, the apparatus comprising:

non-volatile memory storing a Vstim_cal value at which a full-scale output value range of the sensor is approximately equivalent to a no-touch to full-touch full-scale output value range FS_targ_cal; and

a panel processor coupled to the non-volatile memory and programmed for, during field calibration, applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor, and tuning the analog channel in accordance with any difference between FS_targ_current and FS_targ_cal.

31. A digital audio player including a computing system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, the digital audio player containing an apparatus for calibrating the output value of the sensor, the apparatus comprising:

non-volatile memory storing a Vstim_cal value at which a full-scale output value range of the sensor is approximately equivalent to a no-touch to full-touch full-scale output value range FS_targ_cal; and

a panel processor coupled to the non-volatile memory and programmed for, during field calibration, applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor, and tuning the analog channel in accordance with any difference between FS_targ_current and FS_targ_cal.

32. In a system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, an apparatus for calibrating the output value of the sensor, comprising:

during pre-delivery calibration, means for determining a no-touch to full-touch full-scale output value range FS_targ_cal from the analog channel connected to the column containing the sensor, and determining a Vstim value Vstim_cal at which the full-scale output value of the sensor is approximately equivalent to FS_targ_cal; and

during field calibration, means for applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor, and tuning the analog channel in accordance with any difference between FS_targ_current and FS_targ_cal.

33. In a system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, an apparatus for calibrating the output value of the sensor, comprising:

during pre-delivery calibration,

means for applying Vstim to the row containing the sensor, applying no-touch and full-touch conditions to the sensor, and obtaining a full-scale output value range of FS_targ_cal from the analog channel connected to the column containing the sensor, and

means for determining a Vstim value Vstim_cal at which the full-scale output value of the sensor is approximately equivalent to FS_targ_cal; and

during field calibration,

means for applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor,

means for comparing FS_targ_current to FS_targ_cal, and

if FS_targ_current is not approximately equal to FS_targ_cal, means for tuning the analog channel to make FS_targ_current approximately equal to FS_targ_cal.

34. In a system including a multi-touch sensor panel having sensors located at intersections of rows and columns, and for which an input stimulus Vstim applied to a row containing a sensor is sensed by an analog channel connected to a column containing the sensor to generate an output value for the sensor, a computer-readable medium comprising program code for calibrating the output value of the sensor, the program code for causing performance of a method comprising:

during pre-delivery calibration, determining a no-touch to full-touch full-scale output value range FS_targ_cal from the analog channel connected to the column containing the sensor, and determining a Vstim value Vstim_cal at which the full-scale output value of the sensor is approximately equivalent to FS_targ_cal; and

during field calibration, applying Vstim_cal to the row containing the sensor and obtaining a full-scale output value range of FS_targ_current from the analog channel connected to the column containing the sensor, and tuning the analog channel in accordance with any difference between FS_targ_current and FS_targ_cal.