Ultrasonic Air Transducers
Piezoelectric transducers generate, receive, or generate and
receive ultrasonic signals that can be used to measure distances
in air, water, or other fluid media, to determine flow rates, or
for other applications. A single transducer can both generate
and receive a signal, but the two functions often are separated
to optimize the performance of each task.
Apparatus in which piezoelectric transducers are used to
measure distances in air -- impulse-echo devices -- include
level detectors for large containers (e.g., grain silos),
proximity-warning devices (parking aids), and intruder alarms.
The operating range for an impulse-echo device is determined by
the operating frequency and the power of the transducer. The
latter factor, power output, is a function of the mechanical
characteristics and thermal constraints of the device. Operating
frequency requires careful consideration because signal
reflection and absorption are frequency dependent. For long
range applications, low frequency signals are subject to much
less signal damping than high frequency signals. For the best
signal directivity and object resolution, however, the frequency
should be as high as possible. For accurate distance
measurements, especially at short distances, the excitation
pulses must be short.
The range of an impulse-echo device can be extended by
narrowing the bandwidth. A narrow bandwidth also reduces the
input power requirement, and minimizes interference and
reflections from objects outside the signal path. If the signal
is too narrow, however, atmospheric conditions can divert the
signal from its intended path, particularly at longer distances.
Also, a broader band will exhibit less noise.
The mechanical resonance frequency of a flexional ultrasonic
transducer is a function of the characteristics of the
piezoelectric ceramic element, the thickness and diameter of the
metal diaphragm, the manner in which the ceramic / metal
flexional unit is mounted, and the manner in which the
transducer is incited to vibrate.
Directivity of a flexional ultrasonic transducer (see figure)
depends on the wavelength of the emitted signal in air ( ), the
diameter of the area radiating the signal (D), and the
uniformity of the vibrations across the surface of the ceramic
element. As is decreased, or D is increased, the angle of the
beam narrows. Directivity is sharpened and the range is
lengthened. Alternatively, if
is increased or D is decreased,
to the extent that
equals or exceeds D, the directional
characteristic assumes an undesirable spherical form.
In echo sounding applications in which a single,
electronically switched transducer serves both as transmitter
and as receiver, vibrations established during signal
transmission must subside before the transducer can
differentiate a returning signal from background, so there must
be at least a minimum distance between the transducer and the
object. The excitation impulse of the ultrasonic signal should
be as short as practical, which dictates the use of a wide
bandwidth and a high operating frequency. A high operating
frequency, in turn, means the transducer can be smaller. On the
other hand, for a given distance, signal damping increases
significantly as the signal frequency increases. The maximum
range can be increased, and small objects can be detected more
easily, by narrowing the signal beam.
Directivity of a Flexional Ultrasonic Transducer
(distance to receiver > D 2/2
)
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