Web thickness measurement system

• US 6,281,679 B1
• Filed: 12/21/1998
• Issued: 08/28/2001
• Est. Priority Date: 12/21/1998
• Status: Expired due to Term

• Current Assignee: Honeywell-Measurex
• Original Assignee: Honeywell - Measurex
• Inventors: Haefner, Peter, King, Harriss T.
• Primary Examiner(s): Snow, Walter E.
• Application Number:
  US09/218,266
• Time in Patent Office: 981 Days
• Field of Search: 324/229, 324/230, 324/231, 324/225, 324/226, 324/227, 324/262, 731/59, 356/375, 356/376, 356/381, 356/630, 250/559.27
• US Class Current: 324/229
• CPC Class Codes: G01B 11/0691 of objects while moving G01B11/0616 takes precedence
• CPC Class Codes: G01B 21/08 for measuring thickness
• CPC Class Codes: G01B 2210/44 with detectors on both sides of the object to be measured
• CPC Class Codes: G01B 2210/62 Support for workpiece air film or bearing with positive or negative pressure
• CPC Class Codes: G01B 7/107 for measuring objects while moving G01B7/105 takes precedence

Abstract

A non-contacting thickness measurement apparatus is described, which is capable of on-line, highly accurate web thickness measurements. The apparatus is comprised of a first and second distance determining means which calculate a distance value from each distance determining means to the web surface. The distance determining means are on opposite sides of the web, so each measures the distance to a different side of the web. At the same time, the distance between the two distance determining means is measured to produce a z-sensor spacing value. The two distance values are thereafter subtracted from the z-sensor spacing value to get a web thickness value. An x-y position adjustment means is used to keep the measurement spot on the web for the two distance determining means at the same location on the web, and several techniques are combined to eliminate temperature-induced and/or other unknown variations or drifts in the web thickness value which are not caused by actual changes in the thickness of the web.

Claims

1. A system for precision measurement of the thickness of a web having a first and second side, the system comprising:

a first mounting head disposed adjacent to the first side of the web, the first mounting head having;

a first distance sensor operative to determine a first distance between the first sensor and the first side of the web; and

a first position sensor operative to determine the position of the first distance sensor relative to the first side of the web;

a second mounting head disposed adjacent to the second side of the web, the second mounting head having;

a second distance sensor operative to determine a second distance between the second sensor and the second side of the web; and

a second position sensor operative to determine the position of the second distance sensor relative to the second side of the web;

a sensor positioning mechanism attached to the first and second distance sensors, the sensor positioning mechanism operative to adjust the position of the first and second distance sensors in response to the location of the first and second distance sensors determined by respective first and second position sensors; and

a third distance sensor mounted to the first and second mounting heads and operative to determine a third distance between the first and second mounting heads wherein the thickness of the web is determined by subtracting the first and second distances from the third distance such that any error in measuring the first and second distances is mitigated by the sensor positioning mechanism.

2. The system of claim 1 further comprising a temperature control in communication with the first, second, and third distance sensors, the temperature control operative to maintain a prescribed temperature of the first, second and third distance sensors.

3. The system of claim 2 wherein the temperature control is in communication with the sensor positioning mechanism and the first and second position sensors in order to provide a stable operating temperature of the sensors.

4. The system of claim 1 wherein the web has an x direction and a y direction in the plane of the web and the sensor positioning mechanism is configured to position the first and second distance sensors at a same location in the x direction and the y direction.

5. The system of claim 4 further comprising at least one web stabilizer operative to maintain the web in a prescribed orientation relative to the first and second distance sensors.

6. The system of claim 5 wherein the at least one web stabilizer is an air clamp.

7. The system of claim 6 wherein the air clamp comprises a clamp plate having a circular air channel coincident with the surface of the clamp plate and adjacent to the web.

8. The system of claim 1 wherein the first and second position sensors are selected from the group consisting of:

RF source and receiver coils;

Hall devices; and

magneto resistive devices.

9. The system of claim 1 wherein each of the first and second position sensors comprise:

a source/detector coil;

a reference plate; and

a temperature controller for controlling the temperature of the source/detector coil.

10. The system of claim 9 wherein the temperature controller comprises an annular channel which encloses the coil and through which temperature controlled air is circulated.

11. The system of claim 1 wherein each of the first and second position sensors comprise at least one Hall Effect device and at least one reference magnet.

12. The system of claim 1 wherein the first and second position sensors comprise two Hall Effect devices and a magnet, the Hall effect devices arranged to be affected oppositely by the magnetic field of the reference magnet and affected similarly by temperature changes.

13. The system of claim 1 wherein the first position sensor comprises at least one magneto resistive field-direction device fixedly attached to the first distance sensor and the second position sensor comprises at least one reference magnet fixedly attached to the second distance sensor.

14. The system of claim 1 wherein the sensor positioning mechanism comprises:

an outer sensor mounting frame movably attached to the first mounting head by at least one first movable support member;

an inner sensor mounting frame movably attached to the outer sensor mounting frame by at least one second movable support member, the inner sensor mounting frame movable in a direction generally perpendicular to the outer sensor, mounting frame.

15. The system of claim 14 wherein the first and second support members are selected from the group consisting of:

movable bearings;

roller bearings;

linear bushing bearings;

pivoting structures;

threaded structures; and

air cavity structures.

16. The system of claim 14 wherein the first and second support members are elastic strip members.

17. The system of claim 14 wherein the sensor positioning mechanism comprises first and second magnetic deflection coils, each of the magnetic deflection coils providing a displacement force for a respective sensor mounting frame.

18. The system of claim 14 wherein the sensor positioning mechanism is selected from the group consisting of:

threads;

air cavity;

electric motor; and

pneumatic motor.

19. The system of claim 1 further comprising a calibration device operative to calibrate the first and second distance sensors.

20. The system of claim 18 wherein the calibration device comprises a sample of a known thickness that is measured periodically by the first and second distance sensors to produce an error value.

21. The system of claim 1 wherein the first and second distance sensors are fixedly mounted to respective ones of the first and second mounting heads.

22. The system of claim 1 wherein the first and second distance sensors are selected from the group consisting of:

laser sensors;

nuclear sensors;

IR sensors;

RF sensors;

acoustic sensors;

microwave sensors;

radar sensors; and

pneumatic devices.

23. A web measurement system for measuring a thickness of a web having a first and second side, the system comprising:

a first frame;

at least one first distance sensor mounted to the first frame and operative to determine a distance from the first frame to the first side of the web;

a first sensor positioning mechanism attached to the frame and the at least one first distance sensor, the first sensor positioning mechanism operative to direct the at least one first distance sensor to a prescribed location on the first side of the web;

a second frame;

at least one second distance sensor mounted to the second frame and operative to determine a distance from the second frame to the second side of the web;

a second sensor positioning mechanism attached to the frame and the at least one second distance sensor, the second sensor positioning mechanism operative to direct the at least one second distance sensor to the prescribed location on the second side of the web; and

a third distance center attached to the first frame and the second frame, the third distance sensor operative to determine the distance between the at least one first sensor and the at least one second sensor;

wherein the first and second sensor positioning mechanisms are operative to direct the at least one first and second distance sensors to about the same location on respective first and second sides of the web.

24. The system of claim 23 further comprising a location sensor operative to determine the position of the at least one first and second distance sensors.

25. The system of claim 24 wherein the location sensor comprises a first location sensor attached to the at least one first distance sensor and a second location sensor attached to the at least one second distance sensor, the first and second location sensors being operative to determine whether the first and second distance sensors are coaxially aligned over the first and second sides of the web.

26. The system of claim 25 wherein the location sensor is in electrical communication with the sensor positioning mechanism and the location sensor is operative to align the at least one first and second distance sensors coaxially.

27. A method of measuring the thickness of a web having a first and second side, the method comprising the steps of:

a) providing a first distance sensor on the first side of the web;

b) providing a first position sensor on the first side of the web;

c) determining the position of the first distance sensor relative to the first side of the web with the first position sensor;

d) providing a second distance sensor on the second side of the web;

e) providing a second position sensor on the second side of the web;

f) determining the position of the second distance sensor relative to the second side of the web with the second position sensor;

g) adjusting the respective positions of the first and second distance sensors in response to the positions determined by the respective first and second position sensors such that the first and second distance sensors are aligned and the web is measured at about the same location on both sides of the web;

h) determining a distance between the first distance sensor and the first side of the web to find a first distance;

i) determining a distance between the second distance sensor and the second side of the web to find a second distance;

j) determining a distance between the first distance sensor and the second distance sensor to find a total distance; and

k) computing the thickness of the web by subtracting the first distance and the second distance from the total distance.

28. The method of claim 27 wherein step (g) comprises coaxially aligning the first distance sensor and the second distance sensor in response to the positions of the first and second distance sensors.