Theory of Ultrasonic Proximity Sensors | How Ultrasonic sensors work?



Theory
of Ultrasonic Proximity Sensors | How Ultrasonic sensors work?

Ultrasonic proximity sensors use a transducer to send and receive
high frequency sound signals. When a target enters the beam the sound is
reflected back to the switch, causing it to energize or de energize the output
circuit.


Piezoelectric
Disk

A piezoelectric ceramic disk is mounted in the sensor surface. It can
transmit and receive high-frequency pulses. A high-frequency voltage is applied
to the disk, causing it to vibrate at the same frequency. The vibrating disk
produces high-frequency sound waves. When transmitted pulses strike a
sound-reflecting object, echoes are produced. The duration of the reflected
pulse is evaluated at the transducer. When the target enters the preset
operating range, the output of the switch changes state. When the target leaves
the preset operating range, the output returns to its original state.


The emitted pulse is actually a set of 30 pulses at an amplitude of
200 Kvolts. The echo can be in microvolts.


Blind
Zone

A
blind zone exists directly in front of the sensor. Depending on the sensor the
blind zone is from 6 to 80 cm. An object placed in the blind zone will produce
an unstable output.


Range
Definition 

 The time interval between the transmitted signal and the echo is
directly proportional to the distance between the object and sensor. The
operating range can be adjusted in terms of its width and position within the
sensing range. The upper limit can be adjusted on all sensors. The lower limit
can be adjusted only with certain versions. Objects beyond the upper limit do
not produce a change at the output of the sensor. This is known as “blanking
out the background”. On some sensors, a blocking range also exists. This is
between the lower limit and the blind zone. An object in the blocking range
prevents identification of a target in the operating range. There is a signal
output assigned to both the operating range and the output range.


Radiation
Pattern

The
radiation pattern of an ultrasonic sensor consists of a main cone and several
neighboring cones. The approximate angle of the main cone is 5°.


Parallel
Sensors

 In the first example, two
sonar sensors with the same sensing range have been mounted parallel to each
other. The targets are vertical to the sound cone. The distance between the
sensors is determined by the sensing range. For example, if the sensing range
of the sensors is 6 cm, they must be located at least 15 cm apart.


Sensing  Range(CM)
X  (CM)
6-30
>15
>15
20-130
>60
>60
40-300
>150
>150
60-600
>250
>250
80-1000
>350
>350


Angular
Alignment
The angle of the target entering the sound cone must also be considered.
The maximum deviation from the send direction to a flat surface is ±3°.

If
the angle were greater than 3° the sonic pulses would be reflected away and the
sensor would not receive an echo.

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