System and method for interrogating a capacitive sensor

• US 6,469,524 B1
• Filed: 08/25/2000
• Issued: 10/22/2002
• Est. Priority Date: 08/25/2000
• Status: Expired due to Fees

• Current Assignee: Strattec Power Access LLC (Strattec Security Corporation)
• Original Assignee: Delphi Technologies, Inc. (BorgWarner Incorporated)
• Inventors: Oberdier, Larry Max
• Primary Examiner(s): Le, N.
• Assistant Examiner(s): Nguyen, Vincent Q.
• Application Number:
  US09/649,465
• Time in Patent Office: 788 Days
• Field of Search: 324/674, 324/688, 324/649, 324/650, 324/651, 324/662, 324/663, 324/671, 324/683, 324/686
• US Class Current: 324/688
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01D 5/2405 by varying dielectric

Abstract

An exemplary embodiment of the invention is a sensing circuit for use with a proximity sensor having a first electrode and a second electrode and a capacitance between the first electrode and the second electrode. The sensing circuit includes a signal generator for generating an alternating signal. The signal generator is for connection to the first electrode of the sensor. A synchronous detector has a first input for connection to the second electrode of the sensor. A phase shifter is coupled to the signal generator and generates a phase shifted signal in response to the alternating signal. The phase shifted signal is provided to a second input of the synchronous detector. The synchronous detector detects a change in the capacitance between the first electrode and the second electrode in response to a signal from the second electrode and the phase shifted signal.

Claims

1. A sensing circuit for use with a proximity sensor having a first electrode and a second electrode and a capacitance between the first electrode and second electrode, the sensing circuit comprising:

a signal generator for generating an alternating signal, said signal generator for connection to the first electrode of the sensor;

a synchronous detector having a first input and a second input, said first input for connection to the second electrode of the sensor; and

a phase shifter coupled to said signal generator, said phase shifter generating a phase shifted signal in response to said alternating signal, said phase shifted signal being provided to said second input of said synchronous detector;

wherein said synchronous detector detects a change in the capacitance between the first electrode and the second electrode in response to a signal from the second electrode and the phase shifted signal.

2. The sensing circuit of claim 1, further comprising:

a low pass filter coupled to an output of said synchronous detector.

3. The sensing circuit of claim 1 wherein:

said phase shifter is a ninety degree phase shifter.

4. The sensing circuit of claim 1 wherein:

said synchronous detector uses said phase shifted signal as a reference to detect the signal from the second electrode indicative of change in the capacitance between the first electrode and the second electrode.

5. The sensing circuit of claim 1 wherein:

said alternating signal has a frequency less than 100 KHz.

6. The sensing circuit of claim 1 wherein:

said alternating signal has a frequency in the range of 10 KHz to 100 KHz.

7. The sensing circuit of claim 1, further comprising:

an attenuator for receiving the alternating signal and generating a nulling signal for attenuating the signal from the second electrode.

8. The sensing circuit of claim 7, further comprising:

an inverter for inverting said alternating signal and providing the inverted alternating signal to said attenuator.

9. The sensing circuit of claim 1, further comprising:

an amplifier having an inverting input for receiving the signal from the second electrode and said nulling signal and producing an amplifier output signal connected to the first input of the synchronous detector.

10. A method of detecting a response of a proximity sensor having a first electrode and a second electrode and a capacitance between the first electrode and second electrode, the method comprising:

generating an alternating signal and applying said alternating signal to the first electrode of the sensor;

phase shifting said alternating signal to generate a phase shifted signal; and

,

detecting a change in the capacitance between the first electrode and the second electrode in response to a signal from the second electrode and the phase shifted signal.

11. The method of claim 10 wherein:

said phase shifting generates a ninety degree phase shift.

12. The method of claim 10 wherein:

said detecting uses said phase shifted signal as a reference to detect the signal from the second electrode indicative of change in the capacitance between the first electrode and the second electrode.

13. The method of claim 10 wherein:

said alternating signal has a frequency less than 100 Khz.

14. The method of claim 10 wherein:

said alternating signal has a frequency in the range of 10 KHz to 100 Khz.

15. The method of claim 10, further comprising:

attenuating the alternating signal and generating a nulling signal for attenuating the signal from the second electrode.

16. The method of claim 15, further comprising:

inverting said alternating signal prior to attenuating the alternating signal.

17. The method of claim 15, further comprising:

combining the signal from the second electrode and said nulling signal to produce a combined signal; and

,

amplifying the combined signal to produce an amplified signal;

wherein said detecting is performed in response to said amplified signal.