Auto-deep scan for capacitive sensing

• US 20050253597A1
• Filed: 05/14/2004
• Published: 11/17/2005
• Est. Priority Date: 05/14/2004
• Status: Active Grant

• Current Assignee: Zircon Corporation
• Original Assignee: Zircon Corporation
• Inventors: Miller, Lawrence F.
• Granted Patent: US 7,148,703 B2
• Time in Patent Office: Days
• US Class Current: 324/662
• CPC Class Codes: G01V 3/088 operating with electric fields G01V3/082 takes precedence

Abstract

A stud or joist sensor and associated sensing method using an amplitude and a ratio of capacitance measurements from a plurality of capacitive sensing elements. The sensor locates a feature of an object or discontinuity behind a surface or wall, such as an edge and/or a center of a stud behind the surface, a joist under a floorboard, a gap behind sheetrock, a metal conductor behind a surface or the like. The sensor may be moved over the surface, thereby detecting changes in capacitance. The change in capacitance is due to the effective dielectric constant caused by the passage over a hidden object such as a stud. When two capacitive sensing elements provide equivalent capacitance measures, the sensor is over a centerline of the stud. When a ratio of the capacitance measurements equals a transition ratio, the sensor is over an edge of the stud. When the sensor is over the stud and the capacitance measurements are low, the sensor is over a deep stud.

Claims

1. A method of finding a feature behind a surface using a sensor having first and second plates, the method comprising the acts of:

moving the sensor and surface adjacent one another;

measuring a first capacitance of a first capacitor including the first plate and the feature;

measuring a second capacitance of a second capacitor including the second plate and the feature;

computing a ratio of the first and second capacitance measurements; and

comparing one of the first or second capacitance measurements with a first threshold, thereby to determine an indication of a depth of the feature.

2. The method of claim 1, further comprising the acts of:

determining a first reference that represents an initial capacitance of the first capacitor; and

determining a second reference that represents an initial capacitance of the second capacitor.

3. The method of claim 2, wherein:

the first capacitance measurement is a difference between the first reference and the first capacitance measurement; and

the second capacitance measurement is a difference between the second reference and the second capacitance measurement.

4. The method of claim 1, further comprising the acts of:

determining whether one or more of the first and second capacitance measurements exceeds a second threshold; and

re-measuring the first and second capacitances if one or more of the first and second capacitance measurements exceeds the second threshold.

5. The method of claim 1, further comprising the act of determining whether the ratio is within a predetermined range.

6. The method of claim 5, further comprising the act of indicating, if the ratio is within the predetermined range, that an edge of an object behind the surface is detected.

7. The method of claim 5, wherein the ratio is a function of a maximum of the first and second capacitance measurements.

8. The method of claim 5, wherein the predetermined range is a range having fixed limits.

9. The method of claim 1, wherein the act of comparing one of the first or second capacitance measurements with the first threshold includes:

determining a largest capacitance measurement of the first or second capacitance measurements; and

determining the feature is deep if the largest capacitance measurement is less than the first threshold.

10. The method of claim 1, further including indicating the feature is deep if the one of the first or second capacitance measurements is less than the first threshold.

11. The method of claim 1, further including indicating the feature is not deep if the one of the first or second capacitance measurements is greater than the first threshold.

12. The method of claim 1, further comprising the acts of:

comparing the first and second capacitance measurements;

determining that an edge of an object behind the surface is closer to a centerline of the first plate than a centerline of the second plate if the first capacitance measurement is greater than the second capacitance measurement; and

determining that the edge is closer to the centerline of the second plate than the centerline of the first plate if the first capacitance measurement is less than the second capacitance measurement.

13. The method of claim 1, further comprising the act of determining whether the ratio is within a predetermined range of a value of one.

14. The method of claim 13, wherein the predetermined range is inclusively 0.9 to 1.

15. The method of claim 13, further comprising the act of indicating that, if the ratio is within the predetermined range, a centerline of an object is detected.

16. The method of claim 1, wherein the first and second capacitances are indicative of a duration of time necessary to charge a respective one of the first and second plates to a respective reference level.

17. The method of claim 2, wherein:

the first reference is indicative of a duration of time necessary to charge the first plate having the initial capacitance to a first reference;

the second reference is indicative of a duration of time necessary to charge the second plate having the initial capacitance to a second reference;

wherein the first capacitance is a difference between the first reference and a duration of time to charge the first plate to the first reference; and

the second capacitance is a difference between the second reference and a duration of time to charge the second plate to the second reference.

18. The method of claim 2, wherein:

the first reference is indicative of a first voltage necessary to charge the first plate having the initial capacitance to a first reference within a determined time;

the second reference is indicative of a second voltage necessary to charge the second plate having the initial capacitance to a second reference within the determined time;

wherein the first capacitance measurement is a difference between the first reference and an indication of the first voltage; and

wherein the second capacitance measurement is a difference between the second reference and an indication of the second voltage.

19. The method of claim 1, the act of moving comprising applying the sensor to the surface.

20. The method of claim 1, the act of moving comprising applying the surface to the sensor.

21. A method of finding a feature behind a surface using a sensor having a first plate and a second plate of approximately equal areas, the method comprising the acts of:

moving the sensor and surface adjacent one another;

measuring a first capacitance of a first capacitor including the first plate and the feature;

measuring a second capacitance of a second capacitor including the second plate and the feature;

comparing the first capacitance measurement to the second capacitance measurement;

comparing one of the first or second capacitance measurements with a first threshold; and

repeating the acts of measuring and comparing.

22. The method of claim 21, wherein the act of comparing the first capacitance measurement to the second capacitance measurement includes:

computing a ratio between the first and second capacitance measurements;

determining whether the ratio is within a predetermined range ratio; and

indicating, if the ratio is within the range, that an edge of an object is detected.

23. The method of claim 21, wherein the act of comparing the first and second capacitance measurements includes:

determining whether the first and second capacitance measurements differ by less than a second threshold;

indicating that, if the first and second capacitances differ by less than the second threshold, a centerline of an object is detected.

24. The method of claim 21, wherein the act of comparing the first and second capacitance measurements includes:

computing a ratio between the first and second capacitance measurements;

determining whether the ratio is within a predetermined range of one; and

indicating, if the capacitance ratio is within the range, that a centerline of an object is detected.

25. A sensor for finding a feature of a structure comprising:

a first plate having a first capacitance and adapted for forming a first capacitor with the structure;

a second plate having a second capacitance and adapted for forming a second capacitor with the structure;

a first measurement circuit coupled to the first plate, the first measurement circuit measuring a first capacitance value of the first capacitor;

a second measurement circuit coupled to the second plate, the second measurement circuit measuring a second capacitance value of the second capacitor; and

a comparison circuit coupled to the first and second measurement circuits, the comparison circuit generating a ratio of the first and second capacitance values and comparing one of the first or second capacitance values to a first threshold.

26. The sensor of claim 25, wherein:

the first capacitance value represents a difference between the first capacitance and an initial capacitance of the first capacitor; and

the second capacitance value represents a difference between the second capacitance and an initial capacitance of the second capacitor.

27. The sensor of claim 25, further comprising threshold circuitry coupled to first and second measurement circuits, the threshold circuitry determining whether the first and second capacitance values are above a second threshold.

28. The sensor of claim 25, further comprising a processing circuit coupled to the comparison circuit and coupled to receive the ratio value.

29. The sensor of claim 28, wherein the processing circuit determines whether the ratio is within a predetermined range.

30. The sensor of claim 29, further including an indicator coupled to the processing circuit, the indicator providing an indication that the sensor is over an edge of the structure when the ratio is within the predetermined range.

31. The sensor of claim 28, wherein the processing circuit determines whether the capacitance ratio is within a predetermined range of one.

32. The sensor of claim 31, further including an indicator coupled to the processing circuit, the indicator providing an indication that the sensor is over a centerline of a portion of the structure when the ratio is within the range of one.

33. The sensor of claim 30, further including a memory storing look-up table and coupled to the processing circuit, the look-up table having a transition ratio for the processing circuit, wherein the transition ratio is used to set the predetermined range.

34. The sensor of claim 28, further comprising:

a source of a reference voltage;

wherein the first measurement circuit includes a first index, the first index indicating a number of clock cycles needed to charge the first plate to the reference voltage level; and

wherein the second measurement circuit includes a second index, the second index indicating a number of clock cycles needed to charge the second plate to the reference voltage level.

35. The sensor of claim 34, wherein the first and second measurement circuits respectively include:

a current source coupled to a respective one of the first or second plates;

a discharge switch coupled to the respective one of the first or second plates;

a digital-to-analog converter (DAC) having an input terminal coupled to receive a data signal from the processing circuit and an output terminal; and

a comparator having a first input terminal coupled to the respective one of the first or second plates, a second input terminal coupled to the DAC, and an output terminal providing the output signal of the measurement circuit.

36. A sensor comprising:

a first plate and a second plate positioned in about the same plane and spaced apart, and adapted to be located adjacent a surface;

a measurement circuit coupled to the first and second plates thereby to measure a capacitance value of each of the plates;

a first comparison circuit coupled to receive the measured capacitance values and determine a ratio between a change in the measured capacitance values; and

a second comparison circuit coupled to receive one of the measured capacitance values and a threshold and to provide a comparison therebetween.

37. The sensor of claim 36, further comprising an indicator coupled to the first comparison circuit thereby to provide an indication of the ratio of the capacitances.

38. The sensor of claim 37, wherein the indication is that the ratio is approximately equal to a predetermined ratio, thereby locating an edge of an object behind the surface.

39. The sensor of claim 37, wherein the indication is that the ratio is approximately equal to one, thereby locating a centerline of an object behind the surface.

40. The sensor of claim 36, further comprising an indicator coupled to the second comparison circuit thereby to provide an indication of a depth of a feature behind the surface.