Fringe field capacitive sensor for measuring the size of an opening

• US 4,935,700 A
• Filed: 03/16/1989
• Issued: 06/19/1990
• Est. Priority Date: 08/09/1985
• Status: Expired due to Term

• Current Assignee: Washington Research Foundation (University of Washington)
• Original Assignee: Washington Research Foundation (University of Washington)
• Inventors: Garbini, Joseph L., Albrecht, Laurence J., Mauer, Georg F., Jorgensen, Jens E.
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): Harvey, Jack B.
• Application Number:
  US07/311,159
• Time in Patent Office: 460 Days
• Field of Search: 324/61 R, 324/61 P, 324/662, 324/687, 340/870.37
• US Class Current: 324/687
• CPC Class Codes: G01B 7/023 for measuring distance between sensor and object G01B7/082 and G01B7/102 take precedence
• CPC Class Codes: G01B 7/12 for measuring diameters
• CPC Class Codes: G01B 7/28 for measuring contours or curvatures

Abstract

A fringe field, capacitive distance sensor for measuring the distance between a probe and the surface of the conductive body. The sensor comprises a conductive plate element mounted by the probe, the thickness of the plate element being substantially less than the height and width of the plate element. The plate element is positioned adjacent to and normal to the body surface, and the fringe field capacitance between the plate element and the body is measured, to thereby determine the distance between the probe and the body surface. The probe may be in contact with and scanned across the body surface, to provide surface profile measurements. A probe including a plurality of plate elements may be positioned inside an opening such as a hole, to measure the geometry of the hole. By maintaining the hole probe adjacent one electrode in contact with the hole wall, both profile and geometry measurements may be made. A noncontacting probe including one or more plate elements may be used to measure surface topography.

Claims

1. An apparatus for measuring the size of an opening in a conductive body along a measurement direction, the apparatus comprising:

a probe having a probe axis and adapted to be inserted in the opening;

two plate elements mounted by the probe such that the plate elements are parallel to one another and normal to the probe axis, the plate elements being spaced apart from one another in a direction normal to the probe axis, the thickness of each plate element being substantially less than the height and width of the plate element, each plate element including an edge extending across the width of the plate element;

positioning means for positioning the probe such that the probe axis is approximately normal to the measurement direction and such that the plate elements are within the opening and positioned adjacent to and normal to respective body surfaces thereof such that each edge is positioned in a generally parallel relationship to its respective body surface, the body surfaces being spaced apart from one another along the measurement direction, the positioning means including means for causing relative movement between the probe and the body such that the probe moves within the opening along a scanning direction parallel to the probe axis; and

measurement means for measuring the fringe field capacitance between each plate element and the body at a plurality of probe positions along the scanning direction, wherein the measurement means alternately selects one plate element and then the other for said measurement selectively energizing only one plate element at a time for each measurement to avoid interference between the measurements, and thereby determining the distance between each plate element and its respective body surface as a function of probe position along the scanning direction at the time that the plate element is selected.

2. The apparatus of claim 1, wherein the measurement means comprises a detection circuit associated with each plate element, each detection circuit being adapted to measure the capacitance between the body and the plate element associated with the detection circuit, and means for selectively activating the detection circuits for making fringe field capacitance measurements to select one of the plate elements for such measurement such that no more than one detection circuit is activated at any given time.

3. The apparatus of claim 1, wherein the positioning means is operable to position the probe such that the probe adjacent one plate element contacts the respective body surface as the probe moves along the scanning direction, whereby the size of the opening along the measurement direction and a measured profile of the body surface associated with said one plate element are measured using a single probe.

4. The apparatus of claim 3, wherein the measurement means comprises dynamic compensation means for processing the measured profile of the body surface to produce an estimate of the actual profile of the body surface.

5. The apparatus of claim 3, wherein the body surfaces have a common lay, and wherein the scanning direction is normal to the lay.

6. The apparatus of claim 5, wherein the width of each plate element parallel to the respective body surface is in the range of 2-5 mm.

7. A method for measuring the size of an opening in a conductive body along a measurement direction, comprising:

providing a probe having a probe axis and adapted to be inserted in the opening, the probe including two plate elements parallel to one another and normal to the probe axis, and spaced apart from one another in a direction normal to the probe axis, the thickness of each plate element being substantially less than the height and width of the plate element, each plate element including an edge extending along the width of the plate element;

positioning the probe such that the probe axis is approximately normal to the measurement direction and such that the plate elements are within the opening and positioned adjacent to and normal to respective body surfaces thereof, such that the edge is positioned in a generally parallel relationship to its respective body surface, the body surfaces being spaced apart from one another along the measurement direction;

causing relative movement between the probe and body such that the probe moves within the opening along a scanning direction parallel to the probe axis; and

measuring the fringe field capacitance between each plate element and the body at a plurality of probe positions along the scanning direction, alternating between the plate elements while making said measurements, and selectively energizing only one plate element at a time for each measurement to avoid interference between the measurements, and thereby determining the distance between each plate element and its respective body surface along the measurement direction as a function of probe position along the scanning direction at the time of the measurement for the plate element.

8. The method of claim 7, wherein the fringe field capacitance is measured by a detection circuit associated with each plate element, and wherein the detection circuits are selectively activated for making the fringe field capacitance measurements such that no more than one detection circuit is activated at any given time to enable the measurement to be made at only one plate element at a time.

9. The method of claim 7, wherein the probe is positioned and moved such that the probe adjacent one plate element contacts the respective body surface as the probe moves along the scanning direction, whereby the size of the opening along the measurement direction and the measured profile of the body surface associated with said one plate element are measured using a single probe.

10. The method of claim 9, comprising the further step of dynamically compensating the measured profile to produce an estimate of the actual profile.

11. The method of claim 10, wherein the body surfaces have a common lay, and wherein the scanning direction is perpendicular to the lay.

12. The method of claim 11, wherein the width of each plate element parallel to the respective body surface in the range of 2-5 mm.

13. An apparatus for measuring the shape of a surface of a body along a measurement direction, comprising:

a probe;

a plurality of conductive plate elements mounted by the probe, the thickness of each plate element being substantially less than the height and width of the plate element, each plate element including an edge extending along the width of the plate element;

positioning means for positioning the probe adjacent the surface such that the plate elements are normal to the surface and such that each edge is positioned in a generally parallel relationship to the body surface, and such that the plate elements are spaced from one another along the measurement direction; and

measurement means for measuring the fringe field capacitance between each plate element and the body in sequence over time by energizing only one plate element at a time for each measurement to avoid interference between the measurements, and thereby measuring the distance between each plate element and the body when said fringe field capacitance measurement for the plate element is made.

14. The apparatus of claim 13, further comprising means for moving the probe parallel to the surface in a scanning direction normal to the measurement direction, to thereby measure the shape of the surface at a plurality of positions along the scanning direction.

15. The apparatus of claim 14, wherein the plate elements are parallel to one another.

16. The apparatus of claim 15, wherein the plate elements are aligned with one another in a plurality of parallel rows, the rows being inclined with respect to the direction normal to the measurement direction such that the plate elements of each row are spaced from one another along the measurement direction.

17. The apparatus of claim 14, wherein the measurement means comprises a detection circuit associated with each plate element, each detection circuit being adapted to measure the capacitance between the body and the plate element associated with the detection circuit, and means for selectively activating the detection circuits in sequence for making the fringe field capacitance measurements such that no more than one detection circuit is activated at any one time.

18. A method for measuring the shape of a surface of a body along a measurement direction, the method comprising:

providing a probe having a plurality of conductive plate elements mounted by the probe, the thickness of each plate element being substantially less than the height and width of the plate element, each plate element including an edge extending along the width of the plate element;

positioning the probe adjacent the surface such that the plate elements are normal to the surface and such that each edge is positioned in a generally parallel relationship to the body surface and such that the plate elements are spaced from one another along the measurement direction; and

measuring the fringe field capacitance between each plate element and the body in sequence over time, selectively energizing only one plate at a time for each measurement to avoid interference between the measurements, and thereby measuring the distance between each plate element and the body when said fringe field capacitance measurement is made.

19. The method of claim 18, comprising the further step of moving the probe parallel to the surface in a scanning direction normal to the measurement direction, to thereby measure the shape of the surface at a plurality of positions along the scanning direction.

20. The method of claim 19, wherein the plate elements are parallel to one another.

21. An apparatus for measuring the size of an opening in a conductive body along a measurement direction, comprising:

a probe having a probe axis and adapted to be inserted in an opening;

a plurality of plate elements mounted by the probe such that the plate elements are parallel to one another, normal to and spaced apart around the probe axis;

the thickness of each plate element being substantially less than the height, H, and width, W, of the plate element measured across an edge thereof;

positioning means for positioning the probe such that the plate elements are within the opening, adjacent to and generally normal to an inner surface of the opening, the edge of each plate element being positioned in a generally parallel relationship to said surface, the positioning means including means for causing relative movement between the probe and the body such that the probe moves within the opening along a scanning direction generally parallel to the probe axis; and

measurement means for measuring the fringe field capacitance, C, between each plate element and the body at a plurality of probe positions along the scanning direction, substantially as defined by ##EQU10## where;

e is the dielectric constant of the probe;

h is the distance between the edge of a plate element and the inner surface of the opening; and

said measuring means being thus further operative to determine the size of the opening as a function of h, at the plurality of probe positions by appropriately combining the measurements made with the plate elements, for each probe position.

22. A method for measuring the size of an opening in a conductive body along a measurement direction, comprising;

providing a probe having a probe axis and adapted to be inserted in the opening, the probe including a plurality of spaced apart plate elements that are substantially parallel to one another and normal to the probe axis, the thickness of each plate element being substantially less than the height, H, and width, W, of the plate element measured across an edge thereof;

positioning the probe such that the probe axis is approximately normal to the measurement direction and such that the plate elements are within the opening, adjacent to and normal to an inner surface thereof, with the edge of each plate element being positioned in a generally parallel relationship to its respective body surface;

causing relative movement between the probe and body such that the probe moves within the opening along a scanning direction parallel to the probe axis;

measuring the fringe field capacitance, C, between each plate element and the inner surface of the opening at a plurality of probe positions along the scanning direction, substantially as defined by ##EQU11## wherein;

e is the dielectric constant of the probe;

h is the edge of a plate element and the inner surface of the opening; and

determining the size of the opening as a function of h, by appropriately combining the measurements made with the plate elements for each of the plurality of probe positions.

23. An apparatus for measuring the shape of a body along a measurement direction, comprising:

a probe;

a plurality of conductive plate elements mounted by the probe, the thickness of each plate being substantially less than the height, H, and width, W, of the plate element, measured along an edge thereof;

positioning means for positioning the probe adjacent the surface such that the plate elements are normal to the surface and such that teach edge is positioned in a generally parallel relationship to the body surface, and such that the plate elements are spaced from one another along the measurement direction; and

measurement means for measuring the fringe field capacitance, C, between each plate element and the body at a plurality of measurement positions, C being substantially defined by ##EQU12## where;

e is the dielectric constant of the probe;

h is the spacing between the edge of a plate element and the body; and

said measurement means including means to determine the shape of the surface as a function of successive values of h for each plate element in respect to variations in the value of h for the plate elements along the measurement direction at each measurement position.

24. A method for measuring the shape of a surface of a body along a measurement direction, comprising the steps of:

providing a probe having a plurality of conductive plate elements mounted by the probe, the thickness of each plate element being substantially less than the height, H, and width, W, of the plate element, measured along an edge thereof;

positioning the probe adjacent the surface such that the plate elements are normal to the surface and such that each edge is positioned in a generally parallel relationship to the body surface and such that the plate elements are spaced from one another along the measurement direction;

at a plurality of measurement positions, measuring the fringe field capacitance, C, between each plate element and the body, substantially as defined by ##EQU13## where;

e is the dielectric constant of the probe;

h is the spacing between the edge of a probe element and the body; and

determining the shape of the surface of the body as a function of successive values of h for each plate element in respect to variations in the value of h for the plate elements along the measurement direction at each measurement position.