CAPACITIVE SENSING WITH LOW-FREQUENCY NOISE REDUCTION

• US 20090303203A1
• Filed: 05/14/2009
• Published: 12/10/2009
• Est. Priority Date: 05/19/2008
• Status: Active Grant

• Current Assignee: Atmel Corporation (Microchip Technology Incorporated)
• Original Assignee: Atmel Corporation (Microchip Technology Incorporated)
• Inventors: Philipp, Harald, Yilmaz, Esat
• Granted Patent: US 9,569,037 B2
• Time in Patent Office: Days
• US Class Current: 345/174
• CPC Class Codes: G06F 3/0418 for error correction or compensation, e.g. based on parallax, calibration or alignment
• CPC Class Codes: G06F 3/0443 using a single layer of sensing electrodes
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960705 Safety of capacitive touch and proximity switches, e.g. increasing reliability, fail-safe

Abstract

A method of sensing the presence of a body from a change in an amount of charge present on a capacitively charged key. The method comprises inducing charge onto the key during a drive part of a measurement cycle, coupling a signal measurement capacitor to the key during a signal measurement part of the measurement cycle to the effect that the charge induced on the key during the drive part of the measurement cycle is transferred to the signal measurement capacitor, determining from a noise measurement part of the measurement cycle an amount of charge induced on the key by noise during the signal measurement part of the measurement cycle, and controlling the drive part, the signal measurement part and the noise measurement parts of the cycle to provide the charge sensing circuit with a measurement of the signal from which the noise induced on the key has been or can be substantially cancelled.

Claims

1. A method of sensing the presence of a body from a change in an amount of charge present on a capacitively charged key, the method comprising

inducing charge onto the key during a drive part of a measurement cycle,

coupling a signal measurement capacitor to the key during a signal measurement part of the measurement cycle to the effect that the charge induced on the key during the drive part of the measurement cycle is transferred to the signal measurement capacitor,

determining from a noise measurement part of the measurement cycle an amount of charge induced on the key by noise during the signal measurement part of the measurement cycle, and

controlling the drive part, the signal measurement part and the noise measurement parts of the cycle to provide the charge sensing circuit with a measurement of the signal from which the noise induced on the key has been or can be substantially cancelled.

2. A method as claimed in claim 1, wherein the determining from a noise measurement part of the measurement cycle an amount of charge induced on the key by noise, comprises

coupling a noise measurement capacitor to the key for the noise measurement part of the measurement cycle either after the charge induced by the key has been transferred to the signal measurement capacitor or before the charge has been induced on the key during the drive part of the measurement cycle.

3. A method as claimed in claim 2, wherein the coupling a signal measurement capacitor to the key during a signal measurement part of the measurement cycle, includes

coupling the signal measurement capacitor to the key contemporaneously with the drive part of the measurement cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor at the end of the signal measurement part of the measurement cycle, and

determining an amount of charge present on the signal measurement capacitor.

4. A method as claimed in claim 1, wherein the determining from the noise measurement part of the measurement cycle the amount of charge induced on the key by the noise, includes

coupling the signal measurement capacitor to the key contemporaneously with the drive part of the cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor,

either before the drive part of the measurement cycle or after the signal measurement part of the measurement cycle coupling the signal measurement capacitor to the key in reverse so that charge induced by noise on the key changes the amount of charge induced on the measurement capacitor during the signal measurement part of the cycle.

5. A method as claimed in claim 1, wherein a time period of the signal measurement part and the noise measurement part of the cycle are substantially equal.

6. A method as claimed in claim 1, wherein the noise measurement part of the measurement cycle includes a first period before the signal measurement part of the cycle and a second period after the signal measurement part of the cycle.

7. A method as claimed in claim 6, wherein the first period of the noise measurement part of the cycle and the second period of the noise measurement part of the cycle provide a total noise measurement period which is equal to the signal measurement part of the cycle.

8. A method as claimed in claim 1, wherein the inducing charge onto the key during the drive part of the measurement cycle, occurs before the coupling of the signal measurement capacitor to the key during the signal measurement part of the measurement cycle to the effect that the key is pre-charged before the charge induced on the key is transferred to the signal measurement capacitor.

9. An apparatus for sensing the presence of a body from a change in an amount of charge present on a capacitively charged key, the apparatus comprising

a driving circuit coupled to the key and operable to induce charge onto the key,

a charge sensing circuit which includes a signal measurement capacitor, and

a controller, the controller being operable

to control the drive circuit to induce charge onto the key during a drive part of a measurement cycle,

to control the charge sensing circuit to couple the signal measurement capacitor to the key during a signal measurement part of the measurement cycle to the effect that the charge induced on the key during the drive part of the measurement cycle is transferred to the signal measurement capacitor,

to determine from a noise measurement part of the measurement cycle an amount of charge induced on the key by noise during the signal measurement part of the measurement cycle, wherein the drive part, the signal measurement part and the noise measurement parts of the cycle can provide a measurement of the signal from which the noise induced on the key has been or can be substantially cancelled.

10. An apparatus as claimed in claim 9, wherein the charge sensing circuit includes a noise measurement capacitor and the controller is operable to control the charge sensing circuit

to couple the noise measurement capacitor to the key for the noise measurement part of the measurement cycle either after the charge induced by the key has been transferred to the signal measurement capacitor or before the charge has been induced on the key during the drive part of the measurement cycle.

11. An apparatus as claimed in claim 10, wherein the controller is operable

to control the charge sensing circuit to couple the signal measurement capacitor to the key contemporaneously with the drive part of the measurement cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor at the end of the signal measurement part of the measurement cycle, and

to determine an amount of charge present on the signal measurement capacitor, which represents an amount of charge induced on the key from the signal and the noise.

12. An apparatus as claimed in claim 9, wherein the controller is operable to control the charge sensing circuit

to couple the signal measurement capacitor to the key contemporaneously with the drive part of the cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor, and

either before the drive part of the measurement cycle or after the signal measurement part of the measurement cycle

to couple the signal measurement capacitor to the key in reverse so that charge induced by noise on the key changes the amount of charge induced on the measurement capacitor during the signal measurement part of the cycle.

13. An apparatus as claimed in claim 9, wherein a time period of the signal measurement part and the noise measurement part of the cycle are substantially equal.

14. An apparatus as claimed in claim 9, wherein the noise measurement part of the measurement cycle includes a first period before the signal measurement part of the cycle and a second period after the signal measurement part of the cycle.

15. An apparatus as claimed in claim 14, wherein the first period of the noise measurement part of the cycle and the second period of the noise measurement part of the cycle provide a total noise measurement period which is equal to the signal measurement part of the cycle.

16. An apparatus as claimed in claim 9, wherein the controller is operable to control the charge sensing circuit to induce the charge onto the key during the drive part of the measurement cycle before the signal measurement capacitor is coupled to the key during the signal measurement part of the measurement cycle to the effect that the key is pre-charged before the charge induced on the key is transferred to the signal measurement capacitor.

17. A touch sensitive control panel including a key matrix comprising a first plurality, N of input lines, a second plurality, M, of output lines and N times M keys, each of the N times M keys being arranged to sense the presence of a body and each of the N times M keys is disposed adjacent a respective intersection of an input line and an output line and comprises a drive plate (X) connected to one of the N input lines and a receiving plate (Y) connected to one of the M output lines,

each of the N input lines being connected to a respective drive circuit, and

each of the M output lines being connected to a respective charge sensing circuit, each of the charge sensing circuits including a signal measurement capacitor, wherein the key matrix includes

a controller, the controller being operable

to control the drive circuits to induce charge onto the drive plate of each of the keys during a drive part of a measurement cycle,

to control each of the charge sensing circuits to couple the signal measurement capacitor respectively to the receiving plate of each of the keys during a signal measurement part of the measurement cycle to the effect that the charge induced on the receiving plate of the key during the drive part of the measurement cycle is transferred to each of the respective signal measurement capacitors,

to determine, for each key, from a noise measurement part of the measurement cycle an amount of charge induced on the receiving plate of the key by noise during the signal measurement part of the measurement cycle, wherein the drive part, the signal measurement part and the noise measurement parts of the cycle are arranged to provide a measurement of the signal from which the noise induced on the key has been or can be substantially cancelled.

18. A touch sensitive control panel as claimed in claim 17, wherein each of the charge sensing circuits includes a noise measurement capacitor and the controller is operable to control the charge sensing circuits

to couple the noise measurement capacitor to the key for the noise measurement part of the measurement cycle either after the charge induced by the key has been transferred to the signal measurement capacitor or before the charge has been induced on the key during the drive part of the measurement cycle.

19. A touch sensitive control panel as claimed in claim 18, wherein the controller is operable

to control the charge sensing circuits to couple the signal measurement capacitor to the key contemporaneously with the drive part of the measurement cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor at the end of the signal measurement part of the measurement cycle, and

to determine an amount of charge present on the signal measurement capacitor, which represents an amount of charge induced on the key from the signal and the noise.

20. A touch sensitive control panel as claimed in claim 17, wherein the controller is operable to control the charge sensing circuits

to couple the signal measurement capacitor to the key contemporaneously with the drive part of the cycle to the effect that the charge induced on the key is transferred to the signal measurement capacitor, and

either before the drive part of the measurement cycle or after the signal measurement part of the measurement cycle

to couple the signal measurement capacitor to the key in reverse so that charge induced by noise on the key reduces the amount of charge induced on the measurement capacitor during the signal measurement part of the cycle.

21. A touch sensitive control panel as claimed in claim 17, wherein a time period of the signal measurement part and the noise measurement part of the cycle are substantially equal.

22. A touch sensitive control panel as claimed in claim 17, wherein the noise measurement part of the measurement cycle includes a first period before the signal measurement part of the cycle and a second period after the signal measurement part of the cycle.

23. A touch sensitive control panel as claimed in claim 22, wherein the first period of the noise measurement part of the cycle and the second period of the noise measurement part of the cycle provide a total noise measurement period which is equal to the signal measurement part of the cycle.

24. A touch sensitive control panel as claimed in claim 17, wherein the controller is operable to control the charge sensing circuits to induce the charge onto each of the N times M keys during the drive part of the measurement cycle before the signal measurement capacitor is coupled to the key during the signal measurement part of the measurement cycle to the effect that the key is pre-charged before the charge induced on the key is transferred to the signal measurement capacitor.

25. A method of sensing the presence of a body proximate a touch sensitive control panel, the touch sensitive control panel including a key matrix comprising a first plurality, N of input lines, a second plurality, M, of output lines and N times M keys, each of the N times M keys being disposed adjacent a respective intersection of an input line and an output line and comprises a driven plate (X) connected to one of the N input lines and a receiving plate (Y) connected to one of the M output lines,

each of the N input lines being connected to a respective drive circuit, and

each of the M output lines being connected to a respective charge sensing circuit, each of the charge sensing circuits including a signal measurement capacitor, wherein the key matrix includes

controlling the drive circuits to induce charge onto the drive plate of each of the keys during a drive part of a measurement cycle,

controlling each of the charge sensing circuits to couple the signal measurement capacitor respectively to the receiving plate of each of the keys during a signal measurement part of the measurement cycle to the effect that the charge induced on the receiving plate of the key during the drive part of the measurement cycle is transferred to each of the respective signal measurement capacitors, and

determining, for each key, from a noise measurement part of the measurement cycle an amount of charge induced on the receiving plate of the key by noise during the signal measurement part of the measurement cycle, wherein the drive part, the signal measurement part and the noise measurement parts of the cycle are arranged to provide a measurement of the signal from which the noise induced on the key has been or can be substantially cancelled.