Caliper sensor and method using mid-infrared interferometry

Patent metadata

Field	Value
Publication number	US-10072922-B2
Application number	US-201715403197-A
Country	US
Kind code	B2
Filing date	Jan 11, 2017
Priority date	Dec 11, 2013
Publication date	Sep 11, 2018
Grant date	Sep 11, 2018

Abstract

Non-contacting caliper measurements of free standing sheets such as porous polymer and paper detect mid-IR interferometric fringes created by the reflection of light from the top and bottom surfaces of the sheet. The technique includes directing a laser beam at a selected angle of incidence onto a single spot on the exposed outer surface wherein the laser beam comprises radiation having a wavelength in the 3-50 micron range and scanning the laser beam through a selected angle range as the laser beam is directed onto the exposed outer surface and measuring the intensity of an interference pattern that forms from the superposition of radiation that is reflected from the exposed outer surface and from the inner surface. Thickness can be extracted from the fringe separation in the interference pattern. Rotating and focusing elements ensure that the spot position on the sheet remains the same while varying the incident angle.

First claim

What is claimed is:

1. A non-contacting caliper sensor for measuring the thickness of a moving web of scattering material having a first side and a second side, comprising:
a. a substantially monochromatic laser that provides a beam of incident radiation that has a wavelength in the 3-50 micron range;
b. means for directing the incident radiation toward a single spot on an exposed outer surface on the first side of the moving web wherein the incident radiation reaches a fixed position on the exposed surface at an angle of incidence of from 0 to 60 degrees with respect to the moving web surface normal;
c. means for detecting the interference pattern which forms by superposition of a first radiation reflected from the exposed outer surface and a second radiation reflected from an inner surface of the second side; and
d. means for analyzing the interference pattern to calculate the thickness of the moving web at the single spot.

2. The non-contacting caliper sensor of claim 1 wherein the laser is a quantum cascade laser that is tuned to emit at a wavelength substantially transmitted by the moving web.

3. The non-contacting caliper sensor of claim 1 wherein the means for directing the incident radiation comprises a driven closed-loop rotating mirror system that sets the angle of incidence.

4. The non-contacting caliper sensor of claim 3 wherein the means for directing the incident laser radiation comprises:
a. an optically flat front-surface tiltable mirror that is mounted on a first rotational axis and adjusted so that its centerline of the first rotational axis is coincident with a plane of a reflective surface of the optically flat mirror and having a laser beam impingement point that is coincident with the first rotational axis; and
b. a fixed concave front-surface mirror with a figure of revolution such that a laser beam impinging on the optically flat front surface mirror and relayed onto the fixed concave front-surface mirror is focused and imaged onto the first surface of the web.

5. The non-contacting caliper sensor of claim 4 wherein the means for detecting the interference pattern comprises:
a. a fixed concave front-surface mirror with a figure of revolution such that the interference pattern formed from reflected laser light from the moving web is focused and directed toward an optically flat front surface movable mirror; and
b. an optically flat front surface movable mirror that is mounted on a second rotational axis and adjusted so that the centerline of the second rotational axis is coincident with the reflective surface of the optically flat mirror and so that the interference pattern is directed and focused onto an infrared radiation sensitive detector.

6. The non-contacting caliper sensor of claim 3 wherein the means for directing the incident laser radiation comprises:
a. a first optically flat front-surface tiltable mirror that is mounted on a first rotational axis and adjusted so that its centerline of the first rotational axis is coincident with a plane of a reflective surface of the first optically flat front-surface tiltable mirror and having a laser beam impingement point that is coincident with the first rotational axis; and
b. a second optically flat front-surface tiltable mirror that is mounted on a second rotational axis and adjusted so that its centerline of the second rotational axis is coincident with a plane of the reflective surface of the second optically flat front-surface tiltable mirror.

7. The non-contacting caliper sensor of claim 6 wherein the means for detecting comprises:
a. a third optically flat front-surface tiltable mirror that is mounted on a third rotational axis and adjusted so that its centerline of the third rotational axis is coincident with a plane of a reflective surface of the third optically flat front-surface tiltable mirror and directs an image of the interference pattern; and
b. a fourth optically flat front-surface tiltable mirror that is mounted on a fourth rotational axis and adjusted so that its centerline of the fourth rotational axis is coincident with a plane of a reflective surface of the second optically flat front-surface tiltable mirror and that relays the image of the interference pattern from the third optically flat front-surface tiltable mirror to an infrared radiation sensitive detector.

8. The non-contacting caliper sensor of claim 3 wherein the means for directing radiation of the interference pattern comprises a pair of facing concave front-surface mirrors with a figure of revolution such that off-axis specularly reflected laser light forming the interference pattern from the moving web is reflected and directed toward an infrared radiation sensitive detector while the axial laser light forming the interference pattern is transmitted directly toward the infrared radiation sensitive detector without reflection.

9. The non-contacting caliper sensor of claim 3 wherein the means for detecting the interference pattern is received by an infrared radiation sensitive detector array that spans the range of angles of the reflected interference pattern.

10. The non-contacting caliper sensor of claim 3 wherein the means for directing the incident radiation comprises conditioning optics that are coupled to the quantum cascade laser and mounted on an electrically driven translation and rotation stage such as to illuminated a spot position on the outer side of the first surface of the moving web which is stationary while simultaneously positioned at a desired illumination angle and translated to hold the illuminated spot position constant.

11. The non-contacting caliper sensor of claim 10 wherein the means for detecting the interference pattern comprises a second set of conditioning optics that is mounted on a second electrically driven translation and rotation stage and controlled such that the illuminated spot position on the outer surface of the first surface of the moving web is maintained on an axis of the second conditioning optics and the axis is maintained parallel to the beam axis with the output being directed to an infrared radiation sensitive detector.

12. The non-contacting caliper sensor of claim 1 wherein the moving web has a thickness in the range of 10 microns to 200 microns.

13. The non-contacting caliper sensor claim 1 wherein the moving web comprises paper or plastic made of polyethylene, polypropylene, polyethylene terephthalate, polytetrafluoroethylene or polyvinyl chloride.

14. The non-contacting caliper sensor of claim 1 wherein the moving web comprises paper and the radiation has a wavelength of about 22 microns.

15. The non-contacting caliper sensor claim 1 wherein the moving web comprises porous plastic that is made of polyethylene, polypropylene, polyethylene terephthalate, polytetrafluoroethylene or polyvinyl chloride.

16. A non-contacting caliper sensor for measuring the thickness of a moving web of scattering material having a first side and a second side, comprising:
a. a quantum cascade laser that provides a beam of incident radiation that has a wavelength in the 3-50 micron range;
b. means for directing the incident radiation toward a single spot on an exposed outer surface on the first side of the moving web wherein the incident radiation reaches a fixed position on the exposed surface at an angle of incidence of from 0 to 60 degrees with respect to the moving web surface normal;
c. means for detecting the interference pattern which forms by superposition of a first radiation reflected from the exposed outer surface and a second radiation reflected from an inner surface of the second si

Assignees

• Honeywell Ltd

Inventors

• Tixier Sebastien
• Hughes Michael Kon Yew
• Savard Stephane

Classifications

• G01B2290/65

  Spatial scanning object beam · CPC title

• G01B11/0675

  using interferometry · CPC title

• G01B9/02001

  characterised by controlling or generating intrinsic radiation properties · CPC title

• G01B2210/42

  with one or more detectors on a single side of the object to be measured and with a backing surface of support or reference on the other side · CPC title

• G01B11/0633

  using one or more discrete wavelengths · CPC title