Page 649 - Sensors and Systems - North American Catalog
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Ultrasonic Sensors
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Ultrasonic Sensors
Introduction
Environmental influences
Due to the physical properties of sound propagation, the sensing range and
speed of the ultrasonic signal are influenced by the following factors:
-.Air temperature
-.Relative humidity
-.Air pressure
The following diagram shows 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
Variations in the target area air temperature directly affect sensor sound
wave propagation rates and subsequent sensor accuracy. A ± 0.17%
change in propagation speed per degree Celsius is typical. For example, an
ambient temperature change from 68 °F (20 °C) to 104 °F (40 °C) increases
the sound wave travel speed by approximately 3.5%, making a stationary
target appear slightly closer as the temperature rises. Most Pepperl+Fuchs
ultrasonic sensors are temperature-compensated, eliminating most
temperature effects on the sensor output. This temperature compensation
is performed by an electrical temperature sensor that is integrated into the
sensor.
The internal temperature rise of the device results in an additional
temperature error of 2% during the first 60 minutes of operation after power
on.
Fast transient temperature changes cannot be compensated for by the
ultrasonic sensor.
The relationships between the sensing range of ultrasonic sensors and
the air temperature, as well as those between the range and the relative
humidity, are shown in the following charts. The charts shown here apply
to sensors of the UC4000-30GM... and UC500-30GM... series, but apply in
principle to all ultrasonic sensors.
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-...
There is a substantial increase in sensing range at low temperatures that
is virtually independent of the relative humidity. The reduced range at high
temperatures, however, is strongly influenced by the relative humidity.
Note:
The sensing ranges stated in the data sheets for our ultrasonic
sensors are based on an ambient temperature of +68 °F (+20 °C) and a
relative humidity of 50%.
Rain or snow
Rainorsnow inmoderateamountswillnotaffecttheoperationofPepperl+Fuchs
ultrasonic sensors. However, the sensor should be mounted at an angle that
does not permit such media to rest on the transducer. Heavy rain, heavy
snowfall or freezing rain will denegrate an ultrasonic sensor’s performance.
Object properties
Ultrasonic sensors can detect solid, liquid, and granular objects. Material
properties such as transparency, color, or surface treatment (shiny or matte)
do not affect detection reliability.
Level and flat surfaces located at a right angle to the converter axis provide
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 energy
and the detection range decreases. The surface roughness does, however,
allow greater angular deviation.
SelectingandCommissioninganUltrasonicSensor
1. Operating principle
Ultrasonic diffuse mode sensor (direct detection)
The emitter and receiver are located in the same housing. The target acts
as a sound reflector.
Features:
•
Simple installation, the complete sensor is a single unit
•
Sensing range depends on surface properties and angle of the target
•
Measuring frequency is lower compared with a thru-beam sensor
•
Background suppression
