Fiber optic sensor for precision 3-D position measurement
View Patent ↗The central system component of the present invention is a flexible “smart cable” which enables accurate measurement of local curvature and torsion along its length. These quantities are then used to infer the position and attitude of one end of the cable relative to the other. Sufficiently accurate measurements of the local curvature and torsion along the cable allow reconstruction of the entire cable shape, including the relative position and orientation of the end points. The smart cable for making these measurements comprises a multicore optical fiber, with individual fiber cores constructed to operate in the single mode regime, but positioned close enough to cause cross-talk (mode coupling) between cores over the length of the fiber. This cross-talk is very sensitive to the distribution of strains (curvature and torsion) along the cable.
1. Apparatus comprising an optical medium having curved sections, said medium including a plurality of single mode cores, said cores being closely spaced to exhibit cross-talk therebetween, said cross-talk exhibiting changed values only at bends in said medium, said apparatus including a broadband source for illuminating said cores sequentially, means for measuring the spectral intensities including the magnitude of light signals at different freguencies of the entire plurality of cores each time a core is illuminated, and means responsive to said spectral intensities for determining the positions of said curved sections.
2. Apparatus as in claim 1 wherein said medium comprises an optical fiber.
3. Apparatus as in claim 1 wherein said optical medium comprises a flexible sheet of material, said sheet having embedded therein said plurality of single mode cores.
4. Apparatus as in claim 3 wherein said cores are organized in groups, wherein only the cores of each group exhibit cross-talk therebetween.
5. A positioning sensor comprising an optical fiber cable having multiple, closely positioned single mode cores;
a broadband illumination source for sequentially illuminating said cores;
a spectral intensity measurement device for measuring the spectral characteristics including the magnitude of light signals at different freguencies of all of said cores after each said core is illuminated by said source; and
an inversion calculation apparatus for determining local curvature and torsion along said fiber cable and calculating cable end-point position and orientation from said spectral intensity measurements.
6. The sensor recited in claim 5 wherein said measurement device comprises a CCD.
7. The sensor recited in claim 5 wherein said measurement device comprises a grating.
8. The sensor recited in claim 5 wherein said measurement device comprises a prism.
9. The sensor recited in claim 5 wherein said single mode cores are positioned for promoting cross-talk therebetween at fiber cable locations subject to local curvature or torsion.
10. A method for measurement of the position of one end of a fiber optic cable relative to a known position of another end of the fiber optic cable, the cable having a plurality of closely-positioned single-mode cores; the method comprising the steps of:
a) illuminating each of said cores with a source of boradband energy;
b) measuring the spectral intensities including the magnitude of light signals at different frequencies of all of said cores as each said core is illuminated;
c) determining the curvature and torsion characteristics of said cable based upon said spectral intensities; and
d) inferring the position of said one end from said curvature and torsion characteristics.
11. The method recited in claim 10 wherein said measuring step b) is carried out by taking into account the cross-talk induced between said cores by said curvature and torsion.
12. The method recited in claim 10 wherein said measuring step b) comprises the steps of spectrally resolving the resulting illumination in each said core and detecting the resolved illumination spectrum in each said core.
13. The method recited in claim 10 further comprising the step of calibrating said fiber optic cable by placing said cable in a selected positional configuration; carrying out steps a) through d); and compensating for known measurement errors along said cable.