Ultrasonic Sensor

 

Operation principle of ultrasonic sensor

operaton principle of ultrasonic sensor image

Ultrasonic sensors are composed by: ultrasonic transducer, oscillator, trigger circuit and output circuit. There are three working principles:

  • direct diffusion
Ultrasonic transducer is energized by high voltage pulses and starts to emit an ultrasonic signal. The ultrasonic signal is reflected by the target towards the sensor. Trigger circuit measures the time between the emission and the detection of the signal. Since the speed of the ultrasonic beam in air is known, it is easy to have not only an indication of the presence of the target, but also a measure of the distance between sensor and target. In diffuse models, the ultrasonic transducer works before as emitter and then as receiver. When it is in “emission mode” no signal can be detected. This imply that in front of the sensor there is a blind zone where the detection of objects is not possible or not reliable. The dimension of the blind zone depends on the type of ultrasonic transducer used.

 

direct diffusion image

 

  • retroreflection

To work correctly, it is necessary a background/reflector (any flat, orthogonal, fixed part). The sensor measures the distance between sensor and reflector. Any changes in this measure means that there is an object between sensor and reflector. In this case we don’t have a blind zone, but we have to consider a minimum sensing distance between sensor and reflector: the object can be detected in the whole working area.

  • emitter and receiver

There are two separate units: emitter and receiver. The receiver detects the signal from the emitter. If there is no signal, this means that there is an object between emitter and receiver.

 

 

Considerations on targets tipology

Ultrasonic sensor can detect any kind of targets (more or less). The effective detection distance depends on: target dimensions (more the target is big, more ultrasonic signal is reflected, higher the sensing distance that it is possible to reach), materials (compact targets, as metal, wood, liquids, reflect a lot of the ultrasonic signal – low density materials, as powders, foams, absorb the majority of the ultrasonic beam). Sound absorbing materials can be detected only at very low distance. In the direct diffuse sensors, other factors have to be considered:

  • shape of the object

If the objects are perpendicular to the ultrasonic beam, the beam is reflected towards the sensor, and so the object is well detected. Objects with irregular or tilted shape, scatters the beam, so it is possible that they would not be detected

  • object temperature

Even if MD sensors are fully thermically compensated in the whole sensing range, a high thermal gradient between object and environment, can create turbulent air whirpool that can distort the ultrasonic beam. All the measures indicate in the catalogue are releted to the standard target according EN60947-5-2. Users have to take care of the possible differences with real target used. Switching

 

 

Switching frequencydu

Switching frequency is the maximum output switching frequency performed by the output circuit when standard targets cross the sensing field. It is depending on: sensor’s characteristics, target dimensions and size, distance from the targets.

 

Hysteresis

Hysteresis is the difference between switch on and switch off position. It is necessary to avoid output oscillations if the target is vibrating or fixed close to the switching point.

hysteresis image

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