Sensors made largely of commercially available optical and electronic components are being developed for use in measuring small horizontal displacements of payloads suspended below cranes at the ends of nominally vertical cables. The outputs of these sensors would be used in computer-based automated and manual crane-control systems to guide the horizontal movements of the cranes with respect to the payloads or with respect to nearby objects. For example, in the original intended application, the horizontal displacement would be produced by manually pushing a heavy payload in a desired direction, and the output of an optoelectronic sensor that measured this displacement would be sent to a computer-based crane-control system, which would apply power to move the crane in that direction.

A sensor of this type includes a source of a well-defined beam of light (e.g., a diode laser and beam-expanding optics) mounted on the crane at the upper end of the cable, nearby photodetectors off to the sides of the source of light, and a corner-cube reflector mounted on the hook or payload at the lower end of the cable. An important advantage of this sensor concept (see figure) is that it is not necessary to run wires along the cable to supply power to a transducer at the payload end.

In this proof-of-concept diagram of Crane-Position Sensor, light is projected toward the retroreflector, which reflects it back to the position-sensitive detector (PSD), monitoring the location of the reflection site.

A corner-cube reflector is a standard, commercially available retroreflector. It sends light back toward the source along a direction parallel to the direction of incidence, laterally offset by the lateral distance between the point of incidence and the point of final reflection. The lateral offset of the reflected beam equals twice the lateral displacement of the apex of the corner cube from the centerline of the beam of light from the source. When one of the photodetectors intercepts the reflected light, that photodetector responds, signifying that the payload has been laterally displaced from the centerline in the same direction and by half the distance of the photodetector from the centerline

An alternative approach utilizes a two-dimensional position-sensitive detector (PSD) to produce a set of four analog signals indicative of the lateral location of the retroreflector. PSDs are solid-state silicon photodetectors, which produce multiple currents whose magnitudes are proportional to the location of the centroid of the light impinging upon them. In this second approach, a high-brightness light-emitting diode (LED) is modulated at a few kilohertz and aimed at a retroreflector attached to the J-hook as shown in the figure. The reflected light is imaged by a lens onto the PSD, which after signal processing, yields four analog signals indicative of the location of the retroreflector. This approach has been demonstrated to yield accurate position location to better than 0.25 in. (6.35 mm) within a 3-ft (91.44-cm) field of view, at a distance of 30 ft (9.14 m).

This work was done by Ivan Velez of Kennedy Space Center and William D. Haskell, Jimmy D. Polk, Robert B. Cox, Jim Strobel, Steve Thayer, and Robert C. Youngquist of I-NET, Inc. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Systems category, or circle no. 192 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).

Inquiries concerning rights for the commercial use of this invention should be addressed to

the Patent Counsel
Kennedy Space Center; (407) 867-6225.

Refer to KSC-11794.

NASA Tech Briefs Magazine

This article first appeared in the March, 1998 issue of NASA Tech Briefs Magazine.

Read more articles from the archives here.