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Ultrasonic Sensors
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Consider the General Notes on the Information in the Pepperl+Fuchs Product Catalogs
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Environmental influences
Due to the physical properties of sound propagation, the detection range
and speed of the ultrasonic signal are influenced by the following factors:
-.
Air temperature
-.
Relative humidity
-.
Air pressure
The following diagram illustrates the theoretical relationship between air
temperature, air pressure, and the speed of sound.
Pressure [hPa]
Temperature [˚C]
Sonic speed [m/s]
320
330
340
350
360
370
-20 -10 0
20
10
30 40 50 60
1060
1013
960
Daily air pressure fluctuations have a negligible effect on the speed of
sound.
Temperature
Fluctuations in air temperature in the object area have an indirect effect
upon the propagation of sound waves and therefore upon the accuracy
of the sensor. Typically this changes the speed of sound by 0.17%/K. If,
for example, the temperature increases from 20 °C ... 40 °C, the speed
of sound increases by approx. 3.5% and a fixed object will appear closer
to the sensor. To prevent this from happening, most ultrasonic sensors
include temperature compensation; an electric probe measures the am-
bient temperature.
Ultrasonic sensors incur a certain amount of self-heating, which depends
on the housing design and installation. During the first 60 minutes after
switching on the sensor, this leads to an additional temperature error of
max. 2%.
Erratic temperature changes cannot be compensated for due to the lag
in the temperature probe.
The following diagram illustrates the effect of air temperature and humid-
ity on the detection range of an ultrasonic sensor. In principle, the correla-
tions apply to all ultrasonic sensors, in relation to the respective detection
range.
Relative humidity
Range [m]
Temperature [˚C]
14
12
10
8
6
4
2
0
-40 -20 0 20 40 60 80 100
0 %
5 %
20 %
60 %
100 %
UC4000-30GM-...
Relative humidity
Range [m]
Temperature [˚C]
1,0
0,8
0,6
0,4
0,2
-40 -20 0 20 40 60 80 100
0 %
5 %
20 %
60 %
100 %
UC500-30GM-...
Low temperatures increase the detection range regardless of air humid-
ity. At high temperatures, the detection range is reduced and this is in-
creasingly influenced by relative air humidity.
The detection range details included in the data sheets relate to an ambi-
ent temperature of 20 °C and 50% relative air humidity.
Rain or snow
Light rain or snow does not affect the operation of ultrasonic sensors.
Mounting the sensor at an inclined position prevents water or snow from
collecting on the surface of the converter.
Object properties
Ultrasonic sensors can detect solid, liquid, and granular objects. Mate-
rial properties such as transparency, color, or surface treatment (shiny or
matt) do not affect detection reliability.
Level and flat surfaces located at a right angle to the converter axis pro-
vide an ideal reflection. The maximum angular deviation of the object
from the ideal position is approx. ± 3°.
Rough surfaces result in diffuse reflection of a portion of the sound en-
ergy and the detection range decreases. The surface roughness does,
however, allow for greater angular deviation.
Selecting and Commissioning an Ultrasonic
Sensor
1.
Operating principle
Ultrasonic diffuse mode sensor
The sender and receiver are located in the same housing.The object acts
as a sound reflector.
Features:
•
Simple installation, just one sensing head
•
The detection range depends on the surface properties and the pitch
angle of the object
•
Measuring frequency is lower compared with a thru-beam sensor
•
Background suppression
Introduction
