Linear capacitance measurement and touchless switch

• US 20060033508A1
• Filed: 08/12/2005
• Published: 02/16/2006
• Est. Priority Date: 08/16/2004
• Status: Active Grant

• Current Assignee: Ying Lau Lee
• Original Assignee: Ying Lau Lee
• Inventors: Lee, Ying Lau
• Granted Patent: US 7,323,886 B2
• Time in Patent Office: Days
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: H03F 1/34 Negative-feedback-circuit arrangements with or without positive feedback H03F1/02 - H03F1/30, H03F1/38 - H03F1/50, H03F3/50 take precedence ; for rejection of common mode signals H03F3/45479
• CPC Class Codes: H03F 2203/45138 Two or more differential amplifiers in IC-block form are combined, e.g. measuring amplifiers
• CPC Class Codes: H03F 2203/45548 the IC comprising one or more capacitors as shunts to earth or as short circuit between inputs
• CPC Class Codes: H03F 2203/45594 the IC comprising one or more resistors, which are not biasing resistor
• CPC Class Codes: H03F 3/45475 using IC blocks as the active amplifying circuit
• CPC Class Codes: H03F 3/68 Combinations of amplifiers, e.g. multi-channel amplifiers for stereophonics power amplifiers using a combination of several semiconductor amplifiers H03F3/211; combinations of amplifiers using coupling networks with distributed constants H03F3/602
• CPC Class Codes: H03K 17/955 using a capacitive detector

Abstract

Capacitance measurement apparatus that enhances the sensitivity and accuracy of capacitive transducers, proximity sensors, and touchless switches. Each of two capacitors (C1, C2) under measurement has one end connected to ground and is kept at substantially the same voltage potential by operational amplifier (A1) or amplifiers (A0, A1) using negative feedback. The apparatus is driven by a periodic e.g. sinusoidal signal source (G1) or sources (G1, G2) and includes a difference amplifier (A2) operative to produce an electrical signal having a linear relationship with a specified arithmetic function of the capacitances of the two capacitors (C1, C2). A touchless switch is implemented using the capacitance measurement apparatus. The touchless switch includes two sensor electrodes (E1, E2) that correspond to the two capacitors (C1, C2) under measurement and in one embodiment has a front surface in the form of a container.

Claims

1. An apparatus for measuring relative changes in capacitances of at least one first capacitor (C1) and at least one second capacitor (C2), the apparatus comprising:

at least one operational amplifier (A0, A1) operative to employ negative feedback to maintain the at least one first capacitor and the at least one second capacitor at substantially equal voltage potentials, each of the at least one first capacitor and the at least one second capacitor having one end connected to a common ground; and

at least one difference amplifier (A2) operative to provide an electrical signal (Vd), wherein a substantially linear relationship exists between the electrical signal and a specified arithmetic function of the capacitances of the at least one first capacitor and the at least one second capacitor,

wherein one of the at least one operational amplifier (A1) is operative to provide an output (V1) directly or indirectly to an input of the at least one difference amplifier.

2. The apparatus of claim 1 further comprising:

a periodic varying voltage source (G1) operative to drive a first resistor (R1) connected to the first capacitor at a first node (101), and to drive a second resistor (R2) connected to the second capacitor at a second node (102),

wherein the at least one operational amplifier includes a first operational amplifier (A1),

wherein a voltage potential at the first node is provided to an inverting input of the first operational amplifier, and a voltage potential at the second node is provided to a non-inverting input of the first operational amplifier,

wherein a third resistor (R3) is connected between an output of the first operational amplifier and the first node, whereby a feedback current (I3) flowing through the third resistor maintains the voltage potential at the first node substantially equal to the voltage potential at the second node,

wherein the voltage potential at one of the first and second nodes is provided to a first differential input of the difference amplifier, and the output of the first operational amplifier is provided to a second differential input of the difference amplifier, and

wherein at steady state, the electrical signal provided by the difference amplifier is substantially proportional to a signal representing a current (I2) which is a periodic function of time flowing through the second resistor being modulated by a value (c1/c2−

r2/r1), wherein c1 is the capacitance of the first capacitor, c2 is the capacitance of the second capacitor, r1 is the resistance of the first resistor, and r2 is the resistance of the second resistor.

3. The apparatus of claim 2 wherein at steady state, a substantially linear relationship exists between the electrical signal provided by the difference amplifier at a fixed time of a cycle of the electrical signal and the value (c1/c2−

r2/r1).

4. The apparatus of claim 2 wherein at steady state, a substantially linear relationship exists between an average absolute value of a positive and/or negative cycle of the electrical signal provided by the difference amplifier and the value (c1/c2−

r2/r1).

5. The apparatus of claim 2 further including a synchronous demodulator, wherein the periodic varying voltage source is a sinusoidal voltage source, and wherein the synchronous demodulator is operative to extract a signal substantially proportional to the value (c1/c2−

r2/r1) from the electrical signal provided by the difference amplifier by synchronous demodulation.

6. The apparatus of claim 1 further comprising:

a first periodic varying current source (G1) operative to provide a first output current (I1) to the first capacitor, the first capacitor being connected to the first periodic varying current source at a first node (101), and

a second periodic varying current source (G2) operative to provide a second output current (I2) to the second capacitor, the second capacitor being connected to the second periodic varying current source at a second node (102),

wherein a magnitude of the first output current is equal to a constant K times a magnitude of the second output current,

wherein the at least one operational amplifier includes a first operational amplifier (A1),

wherein a voltage potential at the first node is provided to an inverting input of the first operational amplifier, and a voltage potential at the second node is provided to a non-inverting input of the first operational amplifier,

wherein a first resistor (R1) is connected between an output of the first operational amplifier and the first node, whereby a feedback current (I3) flowing through the first resistor maintains the voltage potential at the first node substantially equal to the voltage potential at the second node,

wherein the voltage potential at one of the first and second nodes is provided to a first differential input of the difference amplifier, and the output of the first operational amplifier is provided to a second differential input of the difference amplifier, and

wherein at steady state, the electrical signal provided by the difference amplifier is substantially proportional to a signal representing a current (I2) which is a periodic function of time flowing into the second capacitor being modulated by a value (c1/c2−

K), wherein c1 is the capacitance of the first capacitor, and c2 is the capacitance of the second capacitor.

7. The apparatus of claim 6 wherein at steady state, a substantially linear relationship exists between the electrical signal provided by the difference amplifier at a fixed time of a cycle of the electrical signal and the value (c1/c2−

K).

8. The apparatus of claim 6 wherein at steady state, a substantially linear relationship exists between an average absolute value of a positive and/or negative cycle of the electrical signal provided by the difference amplifier and the value (c1/c2−

K).

9. The apparatus of claim 6 further including a synchronous demodulator, wherein the first and second periodic varying current sources are respective sinusoidal current sources, and wherein the synchronous demodulator is operative to extract a signal substantially proportional to the value of (c1/c2−

K) from the electrical signal provided by the difference amplifier by synchronous demodulation.

10. The apparatus of claim 1

wherein the at least one operational amplifier includes first and second operational amplifiers (A1, A0), and

further comprising:

a periodic varying voltage source (G1) operative to provide an output voltage (Vs) to a non-inverting input of the first operational amplifier and a non-inverting input of the second operational amplifier,

wherein a first resistor (R1) is connected between an output of the first operational amplifier and the first capacitor at a first node (101),

wherein a voltage potential at the first node is provided to an inverting input of the first operational amplifier, whereby a feedback current (I1) flowing through the first resistor maintains the voltage potential at the first node substantially equal to a voltage potential of the output voltage of the periodic varying voltage source,

wherein a second resistor (R2) is connected between an output of the second operational amplifier and the second capacitor at a second node (102),

wherein a voltage potential at the second node is provided to an inverting input of the second operational amplifier, whereby a feedback current (I2) flowing through the second resistor maintains the voltage potential at the second node substantially equal to the voltage potential of the output voltage of the periodic varying voltage source,

wherein the output of the first operational amplifier is provided to a first differential input of the difference amplifier, and the output of the second operational amplifier is provided to a second differential input of the difference amplifier, and

wherein at steady state, the electrical signal provided by the difference amplifier is substantially proportional to a signal representing a time derivative of the output voltage of the periodic varying voltage source which is a periodic function of time being modulated by a value (r1*c1−

r2*C2), wherein c1 is the capacitance of the first capacitor, c2 is the capacitance of the second capacitor, r1 is the resistance of the first resistor, and r2 is the resistance of the second resistor.

11. The apparatus of claim 10 wherein at steady state, a substantially linear relationship exists between the electrical signal provided by the difference amplifier at a fixed time of a cycle of the electrical signal and the value of (r1*c1−

r2*c2).

12. The apparatus of claim 10 wherein at steady state, a substantially linear relationship exists between an average absolute value of a positive and/or negative cycle of the electrical signal provided by the difference amplifier and the value of (r1*c1−

r2*c2).

13. The apparatus of claim 10 further including a synchronous demodulator, wherein the periodic varying voltage source is a sinusoidal voltage source, and wherein the synchronous demodulator is operative to extract a signal substantially proportional to the value (r1*c1−

r2*c2) from the electrical signal provided by the difference amplifier by synchronous demodulation.

14. The apparatus of claim 1 wherein each of the first capacitors is a capacitive transducer.

15. The apparatus of claim 1 wherein each of the second capacitors is a capacitive transducer.

16. The apparatus of claim 1 wherein each of the first capacitors is a capacitive sensor.

17. The apparatus of claim 1 wherein each of the second capacitors is a capacitive sensor.

18. The apparatus of claim 1 wherein each of the second capacitors comprises an internal capacitance associated with an input of an operational amplifier.

19. The apparatus of claim 1 further including first and second sensor electrodes (E1, E2), wherein the first capacitor is formed between a human appendage and the first sensor electrode, wherein the second capacitor is formed between the human appendage and the second sensor electrode, and

further comprising:

a switching decision section (205) operative to generate a first control signal (206) based on the electrical signal provided by of the difference amplifier, and

a switching output section (207) operative to indicate a status condition in response to the first control signal.

20. The apparatus of claim 19 further comprising

a startup delay section (203) operative to delay the generation of the first control signal by the switching decision section until the apparatus is in a steady state condition.

21. The apparatus of claim 19 further comprising a guard electrode (E3),

wherein the guard electrode is electrically isolated from the first and second sensor electrodes, wherein the guard electrode is maintained at substantially the same voltage potential as the first and second sensor electrodes, and wherein the guard electrode is configured to surround a back and sides of the first and second sensor electrodes.

22. The apparatus of claim 19 wherein the first sensor electrode is a center electrode and the second sensor electrode is an outer electrode, and wherein the center electrode is spaced from and at least partly surrounded by the outer electrode.

23. The apparatus of claim 19 further including a surface in a form of a container having a base and a brim, wherein the first and second sensor electrodes are disposed on one of the sides of the surface, and wherein the first sensor electrode is disposed near the base of the container and the second sensor electrode is disposed near the brim of the container.

24. The apparatus of claim 1 wherein the at least one second capacitor is a selected one of a plurality of second capacitors.

25. The apparatus of claim 1 wherein the at least one first capacitor is one of a set of first capacitors (C11-C1n),

wherein the at least one operational amplifier is one of a plurality of first operational amplifiers (A11-A1n), each one of the plurality of first operational amplifiers being associated with a respective one of the set of first capacitors,

wherein the at least one difference amplifier is one of a plurality of difference amplifiers (A21-A2n), each one of the plurality of difference amplifiers being associated with a respective one of the set of first capacitors,

wherein each one of the plurality of first operational amplifiers associated with a respective one of the set of first capacitors uses negative feedback to maintain the respective one of the set of first capacitors and the second capacitor at substantially equal voltage potentials,

wherein each one of the plurality of first operational amplifiers is operative to provide an output directly or indirectly to an input of a respective one of the plurality of difference amplifiers, and

wherein each one of the plurality of difference amplifiers is operative to provide a respective one of a plurality of electrical signals (Vd1-Vdn), wherein a substantially linear relationship exists between each respective electrical signal and a specified arithmetic function of the capacitances of the respective one of the set of first capacitors and the second capacitor.

26. The apparatus of claim 25 wherein the set of first capacitors is a selected one of a plurality of sets of first capacitors.

27. The apparatus of claim 25 further comprising:

a periodic varying voltage source (G1) operative to drive a plurality of first resistors (R11-R1n), each first resistor being connected to a respective one of the set of first capacitors at a respective one of a plurality of first nodes, and to drive a second resistor (R2) connected to the second capacitor at a second node,

wherein a voltage potential at a respective one of the plurality of first nodes is provided to an inverting input of a respective one of the plurality of first operational amplifiers, and a voltage potential at the second node is provided to a non-inverting input of the respective one of the plurality of first operational amplifiers,

wherein a respective one of a plurality of third resistors (R31-R3n) is connected between an output of a respective one of the plurality of first operational amplifiers and a respective one of the plurality of first nodes, whereby a feedback current flowing through the respective one of the plurality of third resistors maintains the voltage potential at the respective one of the plurality of first nodes substantially equal to the voltage potential at the second node,

wherein the voltage potential at the second node is provided to a first differential input of a respective one of the plurality of difference amplifiers, and the output of a respective one of the plurality of first operational amplifiers is provided to a second differential input of the respective one of the plurality of difference amplifiers, and

wherein at steady state, the electrical signal provided by a respective one of the plurality of difference amplifiers is substantially proportional to a signal representing a current which is a periodic function of time flowing through the second resistor being modulated by a respective value (c1/c2−

r2/r1), wherein c1 is the capacitance of a respective one of the set of first capacitors, c2 is the capacitance of the second capacitor, r1 is the resistance of a respective one of the plurality of first resistors, and r2 is the resistance of the second resistor.

28. The apparatus of claim 27 wherein at steady state, a substantially linear relationship exists between the electrical signal provided by a respective one of the plurality of difference amplifiers at a fixed time of a cycle of the electrical signal and a respective value (c1/c2−

r2/r1).

29. The apparatus of claim 27 wherein at steady state, a substantially linear relationship exists between an average absolute value of a positive and/or negative cycle of the electrical signal provided by a respective one of the plurality of difference amplifiers and a respective value (c1/c2−

r2/r1).

30. The apparatus of claim 27 wherein the periodic varying voltage source is a sinusoidal voltage source, and further including a synchronous demodulator operative to extract a respective one of signals substantially proportional to a respective value (c1/c2−

r2/r1) from the electrical signal provided by a respective one of the plurality of difference amplifiers by synchronous demodulation.

31. The apparatus of claim 25 further comprising:

a plurality of first periodic varying current sources (G11-G1n), each of the first periodic varying current sources being operative to provide a first output current (I11-I1n) to a respective one of the set of first capacitors, each of the set of first capacitors being connected to a respective one of the first periodic varying current sources at a respective one of a plurality of first nodes, and

a second periodic varying current source (G2) operative to provide a second output current (I2) to the second capacitor, the second capacitor being connected to the second periodic varying current source at a second node,

wherein a magnitude of a respective one of the first output currents is equal to a respective constant K times a magnitude of the second output current,

wherein a voltage potential at a respective one of the plurality of first nodes is provided to an inverting input of a respective one of the plurality of first operational amplifiers, and a voltage potential at the second node is provided to a non-inverting input of the respective one of the plurality of first operational amplifiers,

wherein a respective one of a plurality of first resistors (R11-R1n) is connected between the output of a respective one of the plurality of first operational amplifiers and a respective one of the plurality of first nodes, whereby a feedback current flowing through the respective one of the plurality of first resistors maintains the voltage potential at the respective one of the plurality of first nodes substantially equal to the voltage potential at the second node,

wherein the voltage potential at the second node is provided to a first differential input of a respective one of the plurality of difference amplifiers, and the output of a respective one of the plurality of first operational amplifiers is provided to a second differential input of the respective one of the plurality of difference amplifiers, and

wherein at steady state, the electrical signal provided by a respective one of the plurality of difference amplifiers is substantially proportional to a signal representing a current which is a periodic function of time flowing into the second capacitor being modulated by a respective value (c1/c2−

K), wherein c1 is the capacitance of a respective one of the set of first capacitors, and c2 is the capacitance of the second capacitor.

32. The apparatus of claim 31 wherein at steady state, a substantially linear relationship exists between the electrical signal provided by a respective one of the plurality of difference amplifiers at a fixed time of a cycle of the electrical signal and a respective value (c1/c2−

K).

33. The apparatus of claim 31 wherein at steady state, a substantially linear relationship exists between an average absolute value of a positive and/or negative cycle of the electrical signal provided by a respective one of the plurality of difference amplifiers and a respective value (c1/c2−

K).

34. The apparatus of claim 31 further including a synchronous demodulator, wherein the plurality of first periodic varying current sources and the second periodic varying current source are respective sinusoidal current sources, and wherein the synchronous demodulator is operative to extract a respective one of signals substantially proportional to a respective value (c1/c2−

K) from the electrical signal provided by a respective one of the plurality of difference amplifiers by synchronous demodulation.

35. The apparatus of claim 25 further comprising:

a second operational amplifier (A0); and

a periodic varying voltage source (G1) operative to provide an output voltage (Vs) to respective non-inverting inputs of the plurality of first operational amplifiers and a non-inverting input of the second operational amplifier,

wherein a respective one of a plurality of first resistors (R11-R1n) is connected between an output of a respective one of the plurality of first operational amplifiers and a respective one of the set of first capacitors at a respective one of a plurality of first nodes,

wherein a voltage potential at a respective one of the plurality of first nodes is provided to an inverting input of a respective one of the plurality of first operational amplifiers, whereby a feedback current flowing through a respective one of the plurality of first resistors maintains the voltage potential at the respective one of the plurality of first nodes substantially equal to a voltage potential of the output voltage of the periodic varying voltage source,

wherein a second resistor is connected between an output of the second operational amplifier and the second capacitor at a second node,

wherein a voltage potential at the second node is provided to an inverting input of the second operational amplifier, whereby a feedback current flowing through the second resistor maintains the voltage potential at the second node substantially equal to the voltage potential of the output voltage of the periodic varying voltage source,

wherein the output of a respective one of the plurality of first operational amplifiers is provided to a first differential input of a respective one of the plurality of difference amplifiers, and the output of the second operational amplifier is provided to a second differential input of the respective one of the plurality of difference amplifiers, and

wherein at steady state, the electrical signal provided by a respective one of the plurality of difference amplifiers is substantially proportional to a signal representing a time derivative of the output voltage of the periodic varying voltage source which is a periodic function of time being modulated by a respective value (r1*c1−

r2*C2), wherein c1 is the capacitance of a respective one of the set of first capacitors, c2 is the capacitance of the second capacitor, r1 is the resistance of a respective one of the plurality of first resistors, and r2 is the resistance of the second resistor.

36. The apparatus of claim 35 wherein at steady state, a substantially linear relationship exists between the electrical signal provided by a respective one of the plurality of difference amplifiers at a fixed time of a cycle of the electrical signal and a respective value (r1*c1−

r2*c2).

37. The apparatus of claim 35 wherein at steady state, a substantially linear relationship exists between an average absolute value of a positive and/or negative cycle of the electrical signal provided by a respective one of the plurality of difference amplifiers and a respective value (r1*c1−

r2*c2).

38. The apparatus of claim 35 further including a synchronous demodulator, wherein the periodic varying voltage source is a sinusoidal voltage source, and wherein the synchronous demodulator is operative to extract a respective one of signals substantially proportional to a respective value (r1*c1−

r2*c2) from the electrical signal provided by a respective one of the plurality of difference amplifiers by synchronous demodulation.

39. The apparatus of claim 25 further including a set of first sensor electrodes (E11-E1n) and a second sensor electrode (E2), wherein each of the set of first capacitors is formed between a human appendage and a respective one of the set of first sensor electrodes and wherein the second capacitor is formed between the human appendage and the second sensor electrode, and

further comprising:

a switching decision section (205) operative to generate a respective one of a plurality of first control signals (206) based on the electrical signal provided by a respective one of the plurality of difference amplifiers, and

a switching output section (207) operative to indicate a respective one of status conditions in response to a respective one of the plurality of first control signals.

40. The apparatus of claim 39 further comprising

a startup delay section (203) operative to delay the generation of each one of the plurality of first control signals by the switching decision section until the apparatus is in a steady state condition.

41. The apparatus of claim 39 further comprising a guard electrode (E3), wherein the guard electrode is electrically isolated from the set of first sensor electrodes and the second sensor electrode,

wherein the guard electrode is maintained at substantially the same voltage potential as the set of first sensor electrodes and the second sensor electrode, and

wherein the guard electrode is configured to surround a back and sides of the set of first sensor electrodes and the second sensor electrode.

42. The apparatus of claim 39 wherein a respective one of the set of first sensor electrodes is a respective one of a set of center electrodes and the second sensor electrode is a common outer electrode, and

wherein each one of the set of center electrodes is spaced from and at least partly surrounded by the common outer electrode.

43. The apparatus of claim 39 further including a surface containing many depressed areas, each depressed area in a form of a container having a base and a brim, wherein the second sensor electrode and the set of first sensor electrodes are disposed on one of the sides of the surface, and

wherein each one of the set of first sensor electrodes is disposed near the base of a respective one of the containers and the second sensor electrode is disposed near the brims of the containers.

44. The apparatus of claim 25 further including a set of first sensor electrodes, wherein each of the set of first capacitors is formed between a conducting object and a respective one of the set of first sensor electrodes, and

wherein the plurality of electrical signals provided by the plurality of difference amplifiers are indicative of proximity of the conducting object with respective to the first sensor electrodes.

45. The apparatus of claim 25 further including a set of first sensor electrodes, wherein each of the set of first capacitors is formed between a conducting object and a respective one of the set of first sensor electrodes,

wherein the set of first sensor electrodes is arranged to form an array, and

wherein the plurality of electrical signals provided by the plurality of difference amplifiers are indicative of a position or movement of the conducting object relative to the first sensor electrodes.

46. The apparatus of claim 25 further including a second sensor electrode, wherein the second capacitor is formed between the conducting object and the second sensor electrode.

47. The apparatus of claim 1 wherein each of the first capacitors comprises an internal capacitance associated with an input of an operational amplifier.

48. The apparatus of claim 1 further including a sensor electrode, wherein the first capacitor is formed between a conducting object and the sensor electrode, and

wherein the electrical signal provided by of the difference amplifier is indicative of a proximity of the conducting object with respect to the sensor electrode.

49. The apparatus of claim 1 further including a sensor electrode, wherein the second capacitor is formed between a conducting object and the sensor electrode, and

wherein the electrical signal provided by the difference amplifier is indicative of a proximity of the conducting object with respect to the sensor electrode.

50. The apparatus of claim 1 further including first and second sensor electrodes (E1, E2), wherein the first capacitor is formed between a conducting object and the first sensor electrode, wherein the second capacitor is formed between the conducting object and the second sensor electrode, and

wherein the electrical signal provided by of the difference amplifier is indicative of a proximity of the conducting object relative to the first and second sensor electrodes.

51. A capacitive sensing apparatus for generating at least one logical signal in response to proximity of a human appendage, the capacitive sensing apparatus comprising:

a surface having a first side and a second side;

at least one first sensor electrode disposed on the first side of the surface;

a second sensor electrode disposed on one of the first and second sides of the surface, the second sensor electrode being adjacent to and at least partly surrounding the at least one first sensor electrode; and

an apparatus configured to maintain the at least one first sensor electrode and the second sensor electrode at substantially equal voltage potentials, and to measure relative changes in a first capacitance between the human appendage and the at least one first sensor electrode with respect to a second capacitance between the human appendage and the second sensor electrode,

wherein the at least one logical signal is indicative of a status condition that the relative changes in the first and second capacitances exceed at least one preset threshold.

52. The apparatus of claim 51 wherein the surface contains at least one depressed area, each depressed area in a form of a container having a base and a brim,

wherein the at least one first sensor electrode is disposed near the base of a corresponding container of the at least one depressed area, and

wherein the second sensor electrode is disposed near the brim of each container.

53. The apparatus of claim 51 further comprising a guard electrode, wherein the guard electrode is electrically isolated from the at least one first sensor electrode and the second sensor electrode,

wherein the guard electrode is maintained at substantially the same voltage potential as the at least one first sensor electrode and the second sensor electrode, and

wherein the guard electrode is configured to at least partly surround the at least one first sensor electrode and the second sensor electrode.