System and method for interrogating a capacitive sensor

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.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of an example, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a proximity sensor based on capacitance;

FIG. 1A is electrode configuration with desirable properties for a capacitive sensor;

FIG. 2 depicts a generic rendition of one embodiment of the invention;

FIG. 2A is a drawing depicting an introduction of dielectric material to that of FIG. 2;

FIG. 3 is an idealized model for analyzing an embodiment of the invention;

FIG. 4 is an idealized model for analyzing an embodiment of the invention including a dielectric layer;

FIG. 5 is an idealized model for analyzing an embodiment of the invention including its concomitant capacitances;

FIG. 6 shows a capacitance C

AB as a function of d;

FIG. 7 shows the dependence of the plateau value ( C

AB at d=10 cm) on the gap g;

FIG. 8 depicts a relative change in capacitance that is increased by introducing a grounded electrode;

FIG. 9 shows a capacitance C

AC as a function of d for various values of g;

FIG. 10 shows a dependence of sensor response |i |/V

O on d;

FIG. 11 shows a number of applications for an embodiment of the invention;

FIG. 12 shows a capacitive sensor for an embodiment of the invention;

FIG. 13 shows a schematic of a circuit in an embodiment of the invention;

FIG. 13A depicts an improvement to the circuit described in FIG. 13;

FIG. 14

a shows, in part, a signal generator;

FIG. 14

b shows, in part, a synchronous detector;

FIG. 14

c shows, in part, a low pass filter;

FIG. 14

d shows, in part, an inverter as well as an attenuator;

FIG. 14

e shows, in part, an extension of an equivalent of the operational amplifier;

FIGS. 15 and 16 depict an example of a sensor for an embodiment of the invention; and

FIG. 17 is a plot of voltage versus distance to an object for an exemplary capacitive sensor.

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:

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

3. The sensing circuit of claim 1 wherein:

4. The sensing circuit of claim 1 wherein:

5. The sensing circuit of claim 1 wherein:

6. The sensing circuit of claim 1 wherein:

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

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

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

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:

11. The method of claim 10 wherein:

12. The method of claim 10 wherein:

13. The method of claim 10 wherein:

14. The method of claim 10 wherein:

15. The method of claim 10, further comprising:

16. The method of claim 15, further comprising:

17. The method of claim 15, further comprising: