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Tuesday, 5 May 2015

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
20-130 >60
40-300 >150
60-600 >250
80-1000 >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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