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Hans Turck GmbH & Co. KG | T +49 (0)208 4952-0 | F +49 (0)208 4952-264 | [email protected] | www.turck.com
Product description
4.3
Functional principle
Ultrasonic sensors are designed for the non-contact and wear-free detection of a variety of ob-
jects by means of sound waves. It does not matter whether the object is transparent or opaque,
metallic or non-metallic, firm, liquid or powdery. Environmental conditions such as spray, dust
or rain also hardly affect the functioning of the sensors.
Ultrasonic sensors emit one or several ultrasonic pulses that are propagated in the air at the
speed of sound. A part of the ultrasonic wave is reflected back to the sensor by the object. The
sensor measures the total time of flight from the sensor to the object and back to the sensor.
The distance to the object is then calculated with the following formula:
D = c × t / 2
D = Distance from the sensor to the object
c = Speed of sound in air
t = Time of flight for the ultrasonic pulse
To improve accuracy, the ultrasonic sensor forms the mean value from the measurement of
several sound pulses before outputting a new value. The ultrasound velocity depends on the
composition and the temperature of the gas in which the sound is propagated. In most ultra-
sound applications, the composition of the gas is stable whereas the temperature may often
fluctuate.
The speed of sound in air varies with the temperature according to the following approxima-
tion formula:
C
m/s
= 20 × √273 + T
C
C
m/s
= Speed of sound in meters per second
T
C
= Temperature in °C
Fluctuations in air temperature therefore affect the speed of sound, which in turn has an effect
on the total time for the echo measured by the sensor. An increase in air temperature shifts
both measuring range limits closer to the sensor. Conversely, both measuring range limits are
shifted further away from the sensor by a drop in air temperature.
This shift is approximately 3.5% of the limit distance with a temperature change of 20°C.
Good ultrasonic reflectors are metals, glass, stone, wood with smooth and hard surfaces, as well
as liquids that are aligned appropriately to the sensor.
Cloth, sand or grains absorb some of the sonic energy. Foams and skins are particularly poor
reflectors.
4.4
Functions and operating modes
The ultrasonic sensors have two outputs that can be adjusted independently of one another.
Output 1 can be used as a switching output, while output 2 can be used as a switching output,
current output (4…20 mA/0…20 mA) or voltage output (0…10 V/0…5 V/1…6 V).
The start and end point of the measuring range can be set for the outputs. The measuring
range must be within the sensing range.
The sensors can be operated as diffuse mode sensors or retro-reflective sensors during normal
operation. Users can set a single switching point or a window or hysteresis function. Additional
operating modes (throughbeam, multiplex, synchronous or release mode) can be parameter-
ized via IO-Link.