An optoelectronic system has been developed for measuring heights, above a floor, of designated points on a large object. In the original application for which the system was conceived, the large object is a space shuttle and the designated points are two front and two rear points for the attachment of jacks for positioning the shuttle at the height and horizontal pitch specified for maintenance operations. The front and rear jacking points are required to be raised to heights of 198±1/4 in. (502.9±0.6 cm) and 120.6±1/4 in. (306.4±0.6 cm), respectively.

Prior to the development of this system, the measurement for each jacking point involved an error-prone, time-consuming procedure in which two technicians were needed to position two telescoping rods with graduations of 1/16 in. (1.6 mm.), an inspector was needed to ensure that the rods were vertical, and the technicians reported the readings to a fourth person who directed the jacking and leveling from a position 30 to 40 ft (9 to 12 m) away. In contrast, the present system can be operated by one person, and once the initial setup of the system has been completed, the system performs and processes the height measurements quickly on command.

Four Laser Rangefinders Measure the Heights of four jacking points above a floor. The rangefinders are controlled and monitored via a computer. A complete measurement cycle takes less than three seconds.

The system (see figure) is based on the use of laser rangefinders to measure the heights. As such, it bears some similarity to the laser-rangefinder-based systems described in "Apparatus and Technique for Measuring Distance Between Axles" (KSC-11980) NASA Tech Briefs, Vol. 24, No. 3 (March 2000), page 76 and "Using Laser Rangefinders To Align Two Structures" (KSC-12040) NASA Tech Briefs, Vol. 25, No. 1 (January 2001), page 16a.

Each laser rangefinder is mounted on a triangular platform that is placed on the floor below one of the jacking points. The laser rangefinder is aimed upward at a target attached temporarily to the jacking point. The platform is equipped with tilt-adjustment screws and a bubble level to enable correction for any deviation of the floor from flatness. The bubble level is accurate to ±0.2° - well within the applicable tolerance of ±2° corresponding to a height error of ±1/8 in. (0.3 cm).

The four laser rangefinders are connected via cables to a multiplexer located in an electronic enclosure. The multiplexer enables serial data communications between the laser rangefinders and a laptop computer. The computer is programmed with special-purpose software for controlling the rangefinders and the multiplexer.

The software generates a graphical display containing an image of the space shuttle with vertical bars superimposed at the jacking points to represent the heights. The height readings are displayed numerically. Each bar is also color-coded: green if the height is within tolerance and either yellow, steady red, or flashing red, depending on the sign and magnitude of the deviation from tolerance. The nominal values, tolerances, and/or other height values for triggering the various colors can be set by use of a normally hidden menu in the computer display. Typically, the system takes less than 3 seconds to perform all four distance measurements and process them into the display. In the event of failure of one of the rangefinders, the software generates a "data drop out" message instead of displaying the corresponding height measurement and color.

This work was done by Robert C. Youngquist (formerly of Dynacs) and Chris Davis of Kennedy Space Center; Jimmy Polk, Brad Burns, William Haskell, and Tim Opalka of Dynacs Engineering Co., Inc; and Michael McClure of United Space Alliance. For further information, access the Technical Support Package (TSP) free on-line at under the Computers/Electronics category.


Photonics Tech Briefs Magazine

This article first appeared in the March, 2003 issue of Photonics Tech Briefs Magazine.

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