Periodic sensor autocalibration and emulation by varying stimulus level

• US 7,920,134 B2
• Filed: 06/13/2007
• Issued: 04/05/2011
• Est. Priority Date: 06/13/2007
• Status: Active Grant

• Current Assignee: Apple Inc.
• Original Assignee: Apple Inc.
• Inventors: Krah, Christoph Horst
• Primary Examiner(s): Liang; Regina
• Assistant Examiner(s): Sheng; Tom V
• Application Number:
  US11/818,263
• Publication Number:
  US 20080309622A1
• Time in Patent Office: 1,392 Days
• Field of Search: 345/173, 345/178, 345/174, 178/18.06, 178/18.05, 178/18.07
• US Class Current: 345/178
• 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 determining a stimulus voltage for a capacitive touch sensor that simulates a touch on the sensor for use in calibrating the sensor, comprising:

during pre-delivery calibration,

applying a predetermined touch to the sensor;

determining a touch calibration output value from an analog channel coupled to the sensor; and

with no touch applied, determining a stimulus voltage at which the output value from the analog channel is approximately equivalent to the touch calibration output value.

2. The method of claim 1, wherein determining the stimulus voltage comprises applying the stimulus voltage to the sensor and varying the stimulus voltage until a simulated touch output value of the sensor is approximately equivalent to the touch calibration output value.

3. The method of claim 1, wherein determining the stimulus voltage comprises subtracting a change in a touch signal capacitance of the sensor from a no-touch signal capacitance of the sensor to form a first intermediate result, multiplying the first intermediate result by a nominal stimulus voltage amplitude value to form a second intermediate result, and dividing the second intermediate result by the no-touch signal capacitance of the sensor.

4. A method for calibrating a capacitive touch sensor, comprising:

during field calibration,

applying a predetermined stimulus voltage to the sensor, the predetermined stimulus voltage selected to cause a simulated touch output value of an analog channel coupled to the sensor to be approximately equivalent to a predetermined touch calibration output value;

measuring the simulated touch output value of the analog channel; and

tuning the analog channel in accordance with any difference between the simulated touch output value and the touch calibration output value.

5. The method of claim 4, wherein tuning the analog channel comprises tuning a feedback capacitor in accordance with a difference between the simulated touch output value and the touch calibration output value.

6. The method of claim 4, wherein calibration of the sensor commences after any active scanning and touching of the sensor has ceased.

7. A method for calibrating a capacitive touch sensor, comprising:

applying a nominal stimulus voltage amplitude value and one or more reduced stimulus voltage amplitude values to the sensor;

measuring a plurality of field parameters after application of the nominal stimulus voltage amplitude value and the one or more reduced stimulus voltage amplitude values;

correlating the plurality of field parameters to a plurality of calibration parameters obtained during a pre-delivery calibration phase by applying the nominal stimulus voltage amplitude value and the one or more reduced stimulus voltage amplitude values to the sensor, the field and calibration parameters being of a same type but having potentially different values;

applying a correction to calibrated gain and offset values of an analog channel coupled to the sensor, the correction derived from the correlation of the field parameters to the calibration parameters.

8. The method of claim 7, further comprising correlating the field parameters and the calibration parameters using lookup tables.

9. The method of claim 7, further comprising correlating the field parameters and the calibration parameters using extrapolation.

10. The method of claim 7, further comprising correlating the field parameters and the calibration parameters using interpolation.

11. An apparatus for calibrating a capacitive touch sensor, comprising:

non-volatile memory storing a stimulus voltage amplitude value at which a simulated touch output value of a capacitive touch sensor is approximately equivalent to a touch calibration output value; and

a processor coupled to the non-volatile memory and programmed for applying the stimulus voltage amplitude value to a drive line containing the sensor and obtaining a current simulated touch output value from an analog channel connected to a sense line containing the sensor, and tuning the analog channel in accordance with any difference between the current simulated touch output value and the touch calibration output value.

12. The apparatus of claim 11, the processor further programmed for withholding calibration until any active scanning and touching of the sensor has ceased.

13. The apparatus of claim 11, the processor further programmed for tuning the analog channel by tuning a feedback capacitor in accordance with a difference between the current simulated full-touch output value and the touch calibration output value.

14. The apparatus of claim 11, 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 processor.

15. The apparatus of claim 14, further comprising a mobile telephone that incorporates the computing system.

16. The apparatus of claim 15, further comprising a digital audio player that incorporates the computing system.

17. An apparatus for calibrating an output value of a capacitive touch sensor, comprising:

non-volatile memory storing a stimulus voltage amplitude value at which a simulated touch output value of the sensor is approximately equivalent to a touch calibration output value; and

a processor coupled to the non-volatile memory and programmed for

applying the stimulus voltage amplitude value to a drive line containing the sensor and obtaining a current simulated touch output value from an analog channel connected to a sense line containing the sensor,

comparing the current simulated touch output value to the touch calibration output value, and

if the current simulated touch output value range is not approximately equal to the touch calibration output value, tuning the analog channel to make the current simulated touch output value approximately equal to the touch calibration output value.

18. The apparatus of claim 17, the processor further programmed for withholding calibration until any active scanning and touching of the sensor has ceased.

19. The apparatus of claim 17, the processor further programmed for tuning the analog channel by tuning a feedback capacitor in accordance with a difference between the current simulated touch output value and the touch calibration output value.

20. A portable computing device including an apparatus for calibrating a capacitive touch sensor, the apparatus comprising:

non-volatile memory storing a stimulus voltage amplitude value at which a simulated touch output value range of the sensor is approximately equivalent to a touch calibration output value; and

a processor coupled to the non-volatile memory and programmed for applying the stimulus voltage amplitude value to a drive line containing the sensor and obtaining a current simulated touch output value from an analog channel connected to a sense line containing the sensor, and tuning the analog channel in accordance with any difference between the current simulated touch output value and the full touch calibration output value.

21. An apparatus for determining a stimulus voltage for a capacitive touch sensor that simulates a touch on the sensor for use in calibrating the sensor, comprising:

means for applying a predetermined touch to the sensor;

means for determining a touch calibration output value from an analog channel coupled to the sensor; and

means for determining, with no touch applied, a stimulus voltage at which the output value from the analog channel is approximately equivalent to the touch calibration output value.

22. A computer-readable storage medium comprising program code for determining a stimulus voltage for a capacitive touch sensor that simulates a touch on the sensor for use in calibrating the sensor, the program code for causing performance of a method comprising:

during pre-delivery calibration,

applying a predetermined touch to the sensor;

determining a touch calibration output value from an analog channel coupled to the sensor; and

with no touch applied, determining a stimulus voltage at which the output value from the analog channel is approximately equivalent to the touch calibration output value.