= 2 d/c, where c is the
speed of sound, d the distance from the transducer to the reflector
and, , the time for the sensor to detect the first echo exceeding
the threshold value, ignoring a second echo from a further reflector.
The use of ultrasonic sensors as the Polaroid transducer for mobile
robot navigation is very common. These sensors are used for
determining the proximity of objects (refelectors) while the robot is
navigating. For this task, the most conventional implementation is
the time-of-flight (TOF) sensor, which calculates the distance
of the nearest reflector using the speed of sound in air and the
emitted pulse and echo arrival times.
TOF sensors operate on the first echo whose amplitude exceeds a
threshold value, i.e. detecting the nearest reflector present in
the sensor operating range and discarding any other incoming echo from
further reflectors. This can be seen in figure 1. In the
figure, the sensor fires in time = 2 d/c, where c is the
speed of sound, d the distance from the transducer to the reflector
and, , the time for the sensor to detect the first echo exceeding
the threshold value, ignoring a second echo from a further reflector.
Figure 1: Conventional TOF ultrasonic sensor performance.
The advantage of the TOF implementation is simplicity of use in that multiple reflections from the same object (occurring at later times than the first reflection) are ignored. However, there are also some drawbacks. The fact that the sensor fires as soon as an echo reaches the threshold level results in the rest of the incoming data from that echo being lost. This data is crucial for determining properties of the reflector such as shape, motion, etc.