For installation of payloads into the Orbiter cargo bay while at the pad, the payload is delivered to the launch pad in a canister. The Rotating Service Structure (RSS) at the launch pad is retracted and the canister is mated with the Payload Changeout Room (PCR). The payload is transferred from the canister into the PCR, the canister is removed, and the RSS is then mated with the Shuttle. The payload is then transferred into the payload bay of the Shuttle Orbiter. The mechanism responsible for manipulating the payload, the PGHM system, consists of a set of movable J-hooks that lifts the payload by its trunnions.

Successful, safe integration of the payload requires careful position measurement of the J-hooks relative to the payload trunnions and the payload relative to the Orbiter payload bay. Typically, the ground support equipment involved is complexly structured, oddly shaped, and large in scale so that standard toolboxes of methods for distance measurements (theodolites, rulers, laser ranging, etc.) are neither cost-effective, practical, nor efficient. Currently, because of the complex nature of the geometry involved with the flight hardware, rulers are the best way to take these relative position measurements; however, measurement with rulers is difficult, time-consuming, and relatively inaccurate.

The PGHM Vision Measurement System seeks to alleviate this measurement problem by using a standard, inexpensive RS-170 monochrome camera coupled with digital image processing to measure these relative positions accurately in real time to 0.025 inch in x, y, and z. The hardware mounting bracket attaches to the PGHM using existing screw holes and consists of a camera, light-emitting diode (LED) for illumination, and a breakout box. The system is shown in figure 1.

With the data acquired during STS-108, algorithms were developed sufficient to meet the requirements of 0.025-inch accuracy at the working distances of the PGHM. Data acquired by the PGHM Vision Measurement System during STS-108 is shown in figure 2.

This Vision Measurement System consists of inexpensive commercial off-the-shelf (COTS) hardware components and custom-designed software using standard image-processing algorithms. To provide robustness in the initial systems, it is necessary to affix small paper targets to the hardware to measure position. These targets are acquired using highly specialized convolution filters. Development of more advanced pattern matching schemes will eventually lead to acquisition of “landmarks” in the field of view (such as those outlined with the red circles shown in figure 2) instead of targets. Enhancements to the target acquisition software algorithm can easily take place without modification of any hardware.

Key accomplishments:

• Selection and design of hardware compatible with PGHM system using COTS components that require no modification of the PGHM system for integration.
• Development of geometric equations to calculate position of targets and/or landmarks from images.
• Real-time image acquisition, target tracking, and position calculations with accuracy of 0.025 inch at 30 samples per second.
• Successful testing of system including acquisition of image data during STS-108.