An improved version of an optoelectronic apparatus for measuring distances of the order of tens of feet with an error no larger than a small fraction of an inch (a few millimeters) has been built. Like the previous version, the present improved version of the apparatus is designed to measure the distance ≈66 ft (≈20 m) between the axes of rotation of the front and rear tires of the space shuttle orbiter as it rests in a ground-based processing facility. Like the previous version, the present version could also be adapted for similar purposes in other settings: Examples include measuring perpendicular distance from a wall in a building, placement of architectural foundations, and general alignment and measurement operations.
The previous version was described in "Apparatus and Technique for Measuring Distance Between Axles" (KSC-11980), NASA Tech Briefs, Vol. 25, No. 3 (March 2000), page 76. To recapitulate: The major components of the apparatus were (1) a laser range finder and (2) laser line projectors that included two battery-powered laser-diode modules with collimating optics. Each laser-diode module generated a continuous-wave beam with a power of 3 mW at a wavelength of 670 nm. The modules were aimed to point the beams downward, and the beams were made to pass through cylindrical diverging lenses to spread the beams into fans oriented in a nominally vertical plane. The modules were aligned to project coincident vertical lines as viewed from the side and collinear horizontal lines as viewed from the top.
The range finder was aligned precisely with respect to the laser-diode modules and the diverging lenses so that the line of sight of the range finder was perpendicular to the plane defined by the beams from the laser-diode modules. This line of sight was thus nominally horizontal. The apparatus was mounted on a tripod (between the rear tires, in the case of a space shuttle) with the range finder at approximately the height of the distant object of interest (the front tire hub in the case of the space shuttle). Exact matching of heights was not necessary in this application because the geometry was such that even at a height difference as large as a few inches, the difference between the horizontal distance and the measured distance was less than the allowable error of 1/8 in (≈3.2 mm). A target was mounted on the distant object of interest (the front tire hub). The position and orientation of the apparatus were adjusted until the bright lines projected by the fan beams struck the near objects of interest (the hubs of both rear tires in the space-shuttle application) and the beam from the range finder struck the center of the target. Then the distance was measured by use of the range finder, which produced a digital readout. The measurement range was from <1 ft (<0.3 m) to about 300 ft (≈91 m).
The differences between the previous and present versions are the following:
- In the previous version, an optical assembly containing the laser fan-beam generators and the laser range finder was aligned by sliding it on top of a platform attached to a tripod. Because this alignment process proved awkward in practice, rails were added so that the optical assembly could be aligned more precisely and then locked in position. As shown in the left part of the figure, there are two pairs of parallel rails for left/right motion of the assembly and a single rail for forward/backward motion of the assembly.
- The original range finder was replaced with a newer and more accurate one, reducing the measurement error to within a tolerance of l/16 in. (≈1.6 mm).
- Rechargeable batteries that were used in the original version were found to last only a couple of years. They were replaced by batteries of common nonrechargeable AA-size cells.
- The original tripod was replaced with a more rugged one.
- Hinged plates with simple pull pins were installed to afford access, for replacing batteries without need to use tools.
- In the previous version, it was necessary to cut cylindrical lenses and glue them to the laser diodes. During the last few intervening years, much better laser devices arrived on the market. Therefore, the original laser diodes were replaced by laser-diode assemblies that include built-in adjustable focus devices so that the projected lines can be made narrow, increasing the accuracy of apparatus.
This work was done by Douglas E. Willard of Kennedy Space Center and Ivan I. Townsend III of Dynacs, Inc. For further information, contact the Kennedy Technology Programs and Commercialization Office at (321) 867-8130.