An optoelectronic system provides information on the relative position and orientation of two structures that are required to be brought together slowly and gently in a prescribed alignment. In the original intended application, the two structures will be the X-33 launch vehicle and a launch mount. With modifications, the system could likely be used for aligning other paired structures; it may be particularly valuable for aligning such large and/or heavy structures as prefabricated sections of bridges, for docking of ships, and possibly even for coupling of railroad cars.

In the original intended application, the X-33 will be backed horizontally out of its processing bay directly onto the launch mount (see Figure 1). The X-33 and the launch mount will then be tilted, as a unit, until the X-33 faces upward.

Figure 1. Two Structures Must Be Aligned as they approach each other. In this case, the X-33 is backed onto the launch mount and must be aligned with the launch mount at the moment of contact.

The X-33 will make contact with the launch mount at four holddown posts. To prevent damage to the X-33, the angular misalignment between the two structures and the lateral offsets between the nominal contact points on the two structures must be kept within tolerances as the structures are brought together. This means that adjustments will be necessary. The X-33 will be mounted on three air-bearing jacks that will enable the necessary adjustments in all six degrees of freedom: up/down, right/left, backward/forward, roll, pitch, and yaw.

The information provided by the present optoelectronic system will be used to guide the adjustments as the two structures approach. The system (see Figure 2) includes four commercial laser rangefinders fixed to the launch mount. Each laser rangefinder is aimed through one of the holddown posts at a bull's-eye alignment target on the X-33. Each rangefinder measures the distance between its target and the contact face on the end of its holddown post. These measurements can be used to calculate pitch, yaw, and front-to-back distance. Information on roll, right/left offset, and up/down offset can be obtained visually by observation of the displacements of the laser spots from the centers of their respective targets.

Figure 2. Four Laser Rangefinders controlled by a laptop computer measure the distances from the holddown posts to targets at contact points on the X-33.

The rangefinders are controlled and read out digitally via built-in RS-232 interfaces that are, in turn, connected to a laptop computer via RS-232-to-RS-485 protocol converters. The RS-485 protocol includes an addressability feature that makes it possible to connect the rangefinders in a daisy-chain arrangement in which the computer can communicate with any or all rangefinders through a single port. Software that controls the system and displays the rangefinder readings is written in LabView. The distances are displayed to the nearest tenth of an inch (≈ 2.5 mm); they are updated at intervals of about 4 to 5 seconds for a computer containing a Pentium (or equivalent) processor, or about 7 to 8 seconds for a computer containing a '486 (or equivalent) processor. The rangefinder readings can be zeroed by pressing a zero button on the computer screen while holding flat objects against the ends of the holddown posts.

This work was done by Donald E. Burris and Paul A. Schwindt of Kennedy Space Center and Geoffrey K. Rowe, Robert C. Youngquist, William D. Haskell, and Robert B. Cox of Dynacs Engineering Co., Inc. KSC-12040

Photonics Tech Briefs Magazine

This article first appeared in the January, 2001 issue of Photonics Tech Briefs Magazine.

Read more articles from the archives here.