Proximity sensor and proximity sensing method

• US 7,782,220 B2
• Filed: 05/25/2007
• Issued: 08/24/2010
• Est. Priority Date: 05/26/2006
• Status: Expired due to Fees

• Current Assignee: Fujikura Limited
• Original Assignee: Fujikura Limited
• Inventors: Nakamura, Yasushi, Misaki, Nobumasa
• Primary Examiner(s): Wu; Daniel
• Assistant Examiner(s): TUN, NAY L
• Application Number:
  US11/753,935
• Publication Number:
  US 20080252474A1
• Time in Patent Office: 1,187 Days
• Field of Search: 340/562, 340/686.6, 340/539.23, 340/530, 340/545.5, 324658-690, 341/33
• US Class Current: 340/686.6
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960745 Capacitive differential; e.g. comparison with reference capacitance

Abstract

A proximity sensor includes a sense electrode having a first capacitance to ground, wherein the first capacitance varies in response to proximity of an object to be sensed; a first sense circuit which provides a first pulse signal having a pulse width determined in accordance with the first capacitance; a reference capacitor; a second sense circuit which provides a second pulse signal having a pulse width determined in accordance with a second capacitance of the reference capacitor; and a computing circuit which computes a difference pulse by subtracting the second pulse signal from the first pulse signal and provide a pulse of the difference pulse width.

Claims

1. A proximity sensor, comprising:

a sense electrode having a first capacitance to ground, wherein the first capacitance varies in response to proximity of an object to be sensed;

a first sense circuit which provides a first pulse signal having a pulse width determined in accordance with said first capacitance;

a reference capacitor having a second capacitance which does not vary in response to proximity of the object to be sensed to the sense electrode, wherein the reference capacitor has an environmental characteristic equal to that of the sense electrode;

a second sense circuit which provides a second pulse signal having a pulse width determined in accordance with the second capacitance of said reference capacitor;

a computing circuit which computes a difference pulse width by subtracting said second pulse signal from said first pulse signal and provides a pulse having said difference pulse width; and

a delay circuit which delays said first pulse signal relative to said second pulse signal.

2. The proximity sensor according to claim 1, wherein said second pulse signal output from said second sense circuit has an initial pulse width equal to the initial pulse width of said first pulse signal output from said first sense circuit.

3. The proximity sensor according to claim 1, further comprising a trigger signal generator circuit which synchronizes rising edges of said first pulse signal and said second pulse signal with each other.

4. The proximity sensor according to claim 1, further comprising:

a low-pass filter which converts said difference pulse into a DC signal; and

a DC amplifier which amplifies said DC signal created through said low-pass filter.

5. The proximity sensor according to claim 1, wherein said sensor compares said pulse width of said difference pulse with a threshold and provides an ON/OFF signal based on a magnitude relation therebetween.

6. The proximity sensor according to claim 1, wherein said sense electrode is one of a plurality of such sense electrodes, and said sensor further comprises a selector circuit which selects one signal from said plurality of sense electrodes and feeds said one signal to said first sense circuit.

7. A proximity sensor, comprising:

a sense electrode having a first capacitance to ground, wherein said first capacitance varies in response to proximity of an object to be sensed;

a first sense circuit which provides a first pulse signal having a pulse width determined in accordance with said first capacitance;

a reference electrode having a second capacitance which does not vary in response to proximity of the object to be sensed to the sense electrode, wherein the reference electrode has an environmental characteristic equal to that of the sense electrode;

a second sense circuit which provides a second pulse signal having a pulse width determined in accordance with the second capacitance between said reference electrode and ground;

a computing circuit which computes a difference pulse width by subtracting said second pulse signal from said first pulse signal and provides a pulse having said difference pulse width; and

a delay circuit which delays said first pulse signal relative to said second pulse signal.

8. The proximity sensor according to claim 7, wherein said second pulse signal output from said second sense circuit has an initial pulse width equal to the initial pulse width of said first pulse signal output from said first sense circuit.

9. The proximity sensor according to claim 7, further comprising a trigger signal generator circuit which synchronizes rising edges of said first pulse signal and said second pulse signal with each other.

10. The proximity sensor according to claim 7, further comprising:

a low-pass filter which converts said difference pulse into a DC signal; and

a DC amplifier which amplifies said DC signal created through said low-pass filter.

11. The proximity sensor according to claim 7, wherein said sensor compares said pulse width of said difference pulse with a threshold and provides an ON/OFF signal based on a magnitude relation therebetween.

12. The proximity sensor according to claim 7, wherein said sense electrode is one of a plurality of such sense electrodes, and said sensor further comprises a selector circuit which selects one signal from said plurality of sense electrodes and feeds said one signal to said first sense circuit.

13. A proximity sensing method, comprising:

receiving a first pulse signal output from a first sense circuit, said first pulse signal having a first pulse width determined in accordance with a first capacitance between a sense electrode and ground, wherein said first capacitance varies in response to proximity of an object to be sensed and ground to measure said first pulse width;

receiving a second pulse signal output from a second sense circuit, said second pulse signal having a second pulse width determined in accordance with a second capacitance of a reference capacitor to measure said second pulse width, wherein said second capacitance does not vary in response to proximity of the object to be sensed to the sense electrode, wherein said reference capacitor has an environmental characteristic equal to that of the sense electrode;

delaying said first pulse signal relative to said second pulse signal; and

computing a difference pulse by subtracting said second pulse width of said second pulse signal from said first pulse width of said first pulse signal and providing a pulse of said difference pulse width.

14. A proximity sensing method, comprising:

receiving a first pulse signal output from a first sense circuit, said first pulse signal having a first pulse width determined in accordance with a first capacitance between a sense electrode and ground, wherein said first capacitance is variable in response to proximity of an object to be sensed and ground to measure said first pulse width;

receiving a second pulse signal output from a second sense circuit, said second pulse signal having a second pulse width determined in accordance with a second capacitance between a reference electrode and ground to measure said second pulse width, wherein said second capacitance does not vary in response to proximity of the object to be sensed to the sense electrode, wherein said reference electrode has an environmental characteristic equal to that of the sense electrode;

delaying said first pulse signal relative to said second pulse signal; and

computing a difference pulse by subtracting said second pulse width of said second pulse signal from said first pulse width of said first pulse signal and providing a pulse of said difference pulse width.