بحث بعنوان Choosing an Ultrasonic Sensor for Proximity or Distance Measurement

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Choosing an Ultrasonic Sensor for Proximity or Distance Measurement
Donald P. Massa, Massa Products Corp.

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The first step toward identifying the right proximity sensor for your application is to understand the fundamental
ultrasonic properties of the transmission medium and the way they influence the measurement and system operation,
Summary
This two-part article has provided a brief
overview of some of the fundamentals that
influence the operation of ultrasonic sensors. As was shown, the maximum detection
range of an ultrasonic sensor is typically
longer for lower frequencies, while the resolution and accuracy are typically better at
higher frequencies. The strength of the target echo, however, is greatly affected by the
geometry and reflectivity of the target,
thereby affecting the range and resolution
of the distance measuring system.
One of the biggest sources of error in an
ultrasonic position measurement is the variability of sound speed in the transmission
path between the sensor and the target,
largely caused by uncertainty in the average
temperature along the path. Maximum
measurement accuracy is therefore obtained when temperature compensation is
used within the sensor. Note that temperahne uncertainty affects absolute accuracy
substantially more than it does the relative
accuracy of an incremental measurement.
It is not unusual for the amplitude of echo
levels to change by large amounts from
pulse to pulse due to variations in sound
speed in the medium, caused by factors
such as air turbulence or target movement.
Also; long-term changes in humidity can
have a significant effect on the strength of
an echo from a target.
It is usually desirable to use a sensor with
the narrowest possible radiation pattern that
can detect the required targets. For a given
frequency, the narrower the radiation pattern of the sensor, the longer the maximum
range of the sensor and the less susceptibility to unwanted targets at the sides of the
sensor. However, a very narrow radiation
pattern from a sensor will require more
accurate orientation of the sensor's axis with
regard to the acoustic beam's perpendicularity to a flat target. In any event, the user
must understand the effective beam angle
of the sensor when determining which targets will be detected and which will be
ignored. This effective beam angle changes
with the distance of the target and the
strength of the reflection from the target.

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