Where am I? Sensors and Methods for Mobile Robot Positioning

Chapter 4: Sensors for Map-Based Positioning 101
For multiple-echo processing, the blanking (BLNK) input must be toggled high for at least 0.44
milliseconds after detection of the first return signal to reset the echo output for the next return.
INIT
TRANSMIT (INT)
16 Pulses
BLNK
BINH
BLANKING (INT)
ECHO
Figure 4.6: Timing diagram for the 6500-Series Sonar Ranging Module executing a
multiple-echo-mode cycle with blanking input. (Courtesy of Polaroid Corp.)
4.1.2 Laser-Based TOF Systems
Laser-based TOF ranging systems, also known as laser radar or lidar, first appeared in work
performed at the Jet Propulsion Laboratory, Pasadena, CA, in the 1970s [Lewis and Johnson, 1977].
Laser energy is emitted in a rapid sequence of short bursts aimed directly at the object being ranged.
The time required for a given pulse to reflect off the object and return is measured and used to
calculate distance to the target based on the speed of light. Accuracies for early sensors of this type
could approach a few centimeters over the range of 1 to 5 meters (3.3 to 16.4 ft) [NASA, 1977;
Depkovich and Wolfe, 1984].
4.1.2.1 Schwartz Electro-Optics Laser Rangefinders
Schwartz Electro-Optics, Inc. (SEO), Orlando, FL, produces a number of laser TOF rangefinding
systems employing an innovative time-to-amplitude-conversion scheme to overcome the subnanosecond
timing requirements necessitated by the speed of light. As the laser fires, a precision
capacitor begins discharging from a known set point at a constant rate. An analog-to-digital
conversion is performed on the sampled capacitor voltage at the precise instant a return signal is
detected, whereupon the resulting digital representation is converted to range using a look-up table.
SEO LRF-200 OEM Laser Rangefinders
The LRF-200 OEM Laser Rangefinder shown in Figure 4.7 features compact size, high-speed
processing, and the ability to acquire range information from most surfaces (i.e., minimum 10-
percent Lambertian reflectivity) out to a maximum of 100 meters (328 ft). The basic system uses a
pulsed InGaAs laser diode in conjunction with an avalanche photodiode detector, and is available
with both analog and digital (RS-232) outputs. Table 4.3 lists general specifications for the sensor's
performance [SEO, 1995a].