Where am I? Sensors and Methods for Mobile Robot Positioning

110 Part I Sensors for Mobile Robot Positioning
Cone shaped object
Laser
Range gate
Timing generator
CCD array
Figure 4.18: Simplified block diagram of a three-camera configuration of the LORDS 3-D laser TOF
rangefinding system. (Courtesy of Robotics Vision Systems, Inc.)
function will be performed by microchannel plate image intensifiers (MCPs) 18 or 25 millimeters in
size, which will be gated in a binary encoding sequence, effectively turning the CCDs on and off
during the detection phase. Control of the system will be handled by a single-board processor based
on the Motorola MC-68040.
LORDS obtains three-dimensional image information in real time by employing a novel time-offlight
technique requiring only a single laser pulse to collect all the information for an entire scene.
The emitted pulse journeys a finite distance over time; hence, light traveling for 2 milliseconds will
illuminate a scene further away than light traveling only 1 millisecond.
The entire sensing range is divided into discrete distance increments, each representing a distinct
range plane. This is accomplished by simultaneously gating the MCPs of the observation cameras
according to their own unique on-off encoding pattern over the duration of the detection phase. This
binary gating alternately blocks and passes any returning reflection of the laser emission off objects
within the field-of-view. When the gating cycles of each camera are lined up and compared, there
exists a uniquely coded correspondence which can be used to calculate the range to any pixel in the
scene.
Transmitted pulse
Schematic of portion
Illuminated vs time
Object to lens delay
Schematic of portion
received vs time
(delayed)
Range gate 1 (A)
Range gate 2 (B)
Range gate 3 (C)
765432 1
Figure 4.19: Range ambiguity is reduced by increasing the number of binary range gates. (Courtesy of
Robotic Vision Systems, Inc.)