| Basic Information on Ultrasonic Sensors |
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The 15th part of our series on the “Basic Principles of Sensor Technology” explains important concepts from the field of ultrasonic sensors. |
Ultrasonic sensors are designed for the contactless and wear-free detection of a variety of targets by means of sound waves. A smooth and firm surface is required for the sound waves in order to ensure optimum reflectivity. Environmental conditions such as spray, dust or rain hardly affect the functioning of the sensors.
Coarse material, furs or foam present a challenge since these mainly absorb the sound waves.
Target surfaces with a surface variation of more than 0.15 mm have the advantage that the surface does not have to be aligned to the sensor so precisely. However, this does reduce the possible sensing range.
The colour of the target has no influence on the switching distance, and transparent targets such as glass or plexiglass can also be detected reliably. However, the temperature of the target does influence the sensing range: Hot surfaces do not reflect sound waves as well as cold surfaces. Liquid surfaces reflect the sound waves in the same way as targets that are solid and smooth. However, the correct alignment should be ensured. Fabrics, foams, wool etc. absorb sound waves so this reduces the sensing range.
Sensing modes
Ultrasonic sensors are primarily used in diffuse mode. The distance to the target is calculated on the basis of the echo time and the known speed of sound in air. A target located in front of the sensor reflects part of the sound wave emitted and is thus detected in the same way on the surface of the sonic transducer. To do this, the sensor constantly alternates between emitter and receive mode.
The settling time required by the surface of the sensor transducer to reach the basic state for reception determines the so-called blind zone. Within this zone, located directly in front of the sonic transducer surface, the echo time to the target cannot be measured. This area must therefore be kept clear at all times. Opposed mode and reflective mode barriers can also be operated using ultrasonic waves...
The author:
Dipl.-Phys. Ing. Markus Bregulla
is product specialist for photoelectric and ultrasonic sensors
within the product management team of
Hans Turck GmbH & Co., Mülheim a.d. Ruhr.
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