The optical pointing sensor provides a means of directly measuring the relative positions of JPL’s Formation Control Testbed (FCT) vehicles without communication. This innovation is a steerable infrared (IR) rangefinder that gives measurements in terms of range and bearing to a passive retroreflector. Due to its reduced range of motion, the range and bearing measurements are on the order of 10 times better than those of the existing sensor system.
The retroreflector is placed on one robot, and the rangefinder and steering optics are on another robot. The measurements are available on the rangefinder-mounted robot, giving it relative position knowledge to the retroreflector.
The system is composed of an HeNe pointing laser, a SICK IR laser rangefinder, a two-axis fast steering mirror, a shear sensor, and a far-field retroreflector (see figure). The pointing laser is injected into the optical path using a beam splitter and bounces off the steering mirror toward the retroreflector. If the retroreflector is hit by the pointing laser, the beam is returned with the exact opposite direction. When the beam impact with the retroreflector is non-central, the return will be separated (sheared) from the outgoing beam by twice the distance between the impact point and the center of the retroreflector. Provided that shear amount is small enough, the return will hit the aperture of the steering mirror and go back through the beam splitter and be imaged on the back end of the scanner with the shear sensor. A telescope placed in front of the shear sensor serves to compress the image of the return beam to the size of the detector.
To acquire the retroreflector within the field of view of the shear sensor, the system operates by first performing an open loop search for the retroreflector target. Once a return from the retroreflector optic is detected, a servo loop is closed with the fast steering mirror and shear sensor to center the laser beam on the vertex of the retroreflector. Once locked, any motion of the retroreflector will be tracked by keeping the servo error small. Once in track mode, the IR rangefinder can be used to give range measurements. Bearing measurements are available from a local sensor used by the steering mirror.
In comparison to flash LIDAR systems, this work represents a system with much less complexity and a lower cost. The rangefinder used by the sensor system is a low-cost COTS (commercial off-the-shelf) unit. The camera in a flash LIDAR system is replaced with a much lower cost, two-dimensional shear sensor that reports only the center of light of the image. This sensor serves as both a detector for determining whether or not the retroreflector is hit by the pointing laser and as a feedback sensor for the tracking system when the retroreflector is moving.
This work was done by Joel F. Shields and Brandon C. Metz of Caltech for NASA’s Jet Propulsion Laboratory. For more information, contact