Where am I? Sensors and Methods for Mobile Robot Positioning

Chapter 4: Sensors for Map-Based Positioning 111
For instance, in a system configured with only one camera, the gating MCP would be cycled on
for half the detection duration, then off the remainder of the time. Figure 4.19 shows any object
detected by this camera must be positioned within the first half of the sensor’s overall range (half
the distance the laser light could travel in the allotted detection time). However, significant distance
ambiguity exists because the exact time of detection of the reflected energy could have occurred
anywhere within this relatively long interval.
This ambiguity can be reduced by a factor of two through the use of a second camera with its
associated gating cycled at twice the rate of the first. This scheme would create two complete on-off
sequences, one taking place while the first camera is on and the other while the first camera is off.
Simple binary logic can be used to combine the camera outputs and further resolve the range. If the
first camera did not detect an object but the second did, then by examining the instance when the
first camera is off and the second is on, the range to the object can be associated with a relatively
specific time frame. Incorporating a third camera at again twice the gating frequency (i.e., two cycles
for every one of camera two, and four cycles for every one of camera one) provides even more
resolution. As Figure 4.20 shows, for each additional CCD array incorporated into the system, the
number of distance divisions is effectively doubled.
Range gate 1
Range gate 2
Range gate 3
Composite
1 2 3 4 5 6 7
Figure 4.20: Binary coded images from range gates 1-3 are combined to generate
the composite range map on the far right. (Courtesy of Robotics Vision Systems, Inc.)
Alternatively, the same encoding effect can be achieved using a single camera when little or no
relative motion exists between the sensor and the target area. In this scenario, the laser is pulsed
multiple times, and the gating frequency for the single camera is sequentially changed at each new
transmission. This creates the same detection intervals as before, but with an increase in the time
required for data acquisition.
LORDS is designed to operate over distances between one meter and several kilometers. An
important characteristic is the projected ability to range over selective segments of an observed
scene to improve resolution in that the depth of field over which a given number of range increments
is spread can be variable. The entire range of interest is initially observed, resulting in the maximum
distance between increments (coarse resolution). An object detected at this stage is thus localized
to a specific, abbreviated region of the total distance.
The sensor is then electronically reconfigured to cycle only over this region, which significantly
shortens the distance between increments, thereby increasing resolution. A known delay is
introduced between transmission and the time when the detection/gating process is initiated. The
laser light thus travels to the region of interest without concern for objects positioned in the
foreground.