26-06-2012, 01:07 PM
ULTRASONIC DISTANCE MEASUREMENT
[attachment=25565]
What does it do?
Measure distance with an accuracy of a few centimetres, over a range
of a few metres, at a speed of milliseconds to several seconds per
sample.
Why use this technique?
Ultrasonic sensors can offer a number of advantages over optical
methods. Most notably, they can be used for measuring distance to
any surface, including glass and liquids. Ranges vary from a few
millimetres to around ten metres, without increasing the size of the
device.
Typical objects measured:
liquid level measurement
counting objects on a production line
thread or wire break detection
robotic sensing for navigation or obstacle avoidance
wall-to-wall distance measurement
How do ultrasonic rangefinders work?
Typically, an ultrasonic rangefinder sends a 'ping' and waits to hear an
echo. Sound waves propagate from the transmitter and bounce off
objects, returning an echo to the receiver (below left). If the speed of
sound is known, the distance to an object can be calculated from the
time delay between the emitted and reflected sounds.
Operation of an ultrasonic transducer
While the principle of calculating distance from the time of travel is
simple, there are many limiting factors to consider. Sound diverges
very rapidly, so transducers are carefully designed to produce as small
a beam as possible. While some applications require a wide beam, a
narrow beam improves the range and reduces background
interference. There is a direct relationship between beam width and
target surface angle: the wider the beam, the greater the possible
angle between the transducer and the surface. When the angle is too
great (>12 degrees), the reflected beam misses the transducer (above
right). While some surfaces may produce scattered diffuse reflections,
these are much weaker and are not used for distance measuring
purposes.
[attachment=25565]
What does it do?
Measure distance with an accuracy of a few centimetres, over a range
of a few metres, at a speed of milliseconds to several seconds per
sample.
Why use this technique?
Ultrasonic sensors can offer a number of advantages over optical
methods. Most notably, they can be used for measuring distance to
any surface, including glass and liquids. Ranges vary from a few
millimetres to around ten metres, without increasing the size of the
device.
Typical objects measured:
liquid level measurement
counting objects on a production line
thread or wire break detection
robotic sensing for navigation or obstacle avoidance
wall-to-wall distance measurement
How do ultrasonic rangefinders work?
Typically, an ultrasonic rangefinder sends a 'ping' and waits to hear an
echo. Sound waves propagate from the transmitter and bounce off
objects, returning an echo to the receiver (below left). If the speed of
sound is known, the distance to an object can be calculated from the
time delay between the emitted and reflected sounds.
Operation of an ultrasonic transducer
While the principle of calculating distance from the time of travel is
simple, there are many limiting factors to consider. Sound diverges
very rapidly, so transducers are carefully designed to produce as small
a beam as possible. While some applications require a wide beam, a
narrow beam improves the range and reduces background
interference. There is a direct relationship between beam width and
target surface angle: the wider the beam, the greater the possible
angle between the transducer and the surface. When the angle is too
great (>12 degrees), the reflected beam misses the transducer (above
right). While some surfaces may produce scattered diffuse reflections,
these are much weaker and are not used for distance measuring
purposes.