Fast Range Scanner using an Optic RAM

A range scanner which calculates ranges by triangulation between the incident angles of laser stripes and the positions of their images on a camera sensor was developed for robotic applications. Its originality resides in the use of a solid state image sensor called \Optic RAM" instead of a CCD sensor. This sensor chip presents three desirable characteristics for the position detection of laser stripes in images; it thresholds the image, detecting only the brighter stripes in binary form; it is an image memory; and pixel values can be addressed randomly in the image. Thus the design does not require an A/D converter or a frame buuer and is consequently inexpensive. For improved performance, only the image region next to the previous stripe location is searched, and a 64 Kbyte lookup table stored in RAM is indexed by incident laser angles and stripe addresses to output range data. A 128256 range image is produced in about 20 seconds. This is reasonably fast considering that this process requires analyzing 256 images. The speed bottleneck is the low sensitivity of the Optic RAM chip which requires a long exposure time per frame (60 ms), corresponding to half the standard video frame rate. We are investigating more sensitive random-access optic chips, which could bring the range image acquisition to a few seconds without any change in our design. Finally, simple calibration methods using planar patterns of parallel lines are presented for calibrating the focal length, the lens distortion and the mirror position with respect to the camera.