V2.0 | 2021/06
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4.3
Operating principle
Ultrasonic sensors are designed for the non-contact and wear-free detection of a variety of tar-
gets by means of sound waves. It does not matter here whether the target is transparent or
non-transparent, metallic or non-metallic, solid, liquid or in powder form. Environmental condi-
tions 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 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 in m
c
Speed of sound in air in m/s
t
Time of flight for the ultrasonic pulse in s
To improve accuracy, the ultrasonic sensor forms the mean value from the measurement of
several sound pulses before outputting a new value. The ultrasonic 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
air
= 20 × √(273 + T)
c
air
Speed of sound in air in m/s
T
Temperature in °C
The speed of sound at an air temperature of 20 °C is approximately 344 m/s
The following formula applies to sensors with integrated temperature compensation:
c
air
= (331.5 × 0.596 + T)
Fluctuations in air temperature 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 and the time of flight of the echo is shorter. A drop
in air temperature shifts both measuring range limits away from the sensor and the time of
flight of the echo is longer. This shift is approximately 3.5 % of the limit distance with a temper-
ature 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.
