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Basic knowledge Ultra sonic

Basic knowledge Ultra sonic. 03/2010. Ultra sonic. We all know supersonics from nature. Bats, for instance, use it to “see” in the dark and even to accurately locate insects. We call those sound waves ultrasonic, that exceed the frequencies we are able to hear (approx. 30 Hz – 20 kHz).

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Basic knowledge Ultra sonic

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  1. Basic knowledgeUltra sonic 03/2010

  2. Ultra sonic We all know supersonics from nature. Bats, for instance, use it to “see” in the dark and even to accurately locate insects. We call those sound waves ultrasonic, that exceed the frequencies we are able to hear(approx. 30 Hz – 20 kHz). The frequency of ultrasonic ranges from 20 kHz to 5 MHz. In the automation applications ultrasonic is used for contactless distance measurement. Ultra sonic sensors can be used, since it can detect objects, and also indicate and analyse their absolute distance.

  3. Ultra sonic - Principle The heart of MOBA-Sonic is a piezoceramic ultrasonic converter working as a “sender” as well as a “receiver” of ultrasonic waves.

  4. Ultra sonic – echo running time The MOBA-Sonic works according to the echo propagation time principle. It generates and sends out a very intense sound pulse package cyclically. As soon as this pulse package hits an object, it is partly reflected. This echo comes back to the sensor and – if the impact is strong enough – is converted into an electric signal again. The time between sending and receiving the pulse package – i.e. the “running time” – is directly proportional to the distance between sensor and object. Ultra sonic sensors use the “running time” of the echo to calculate the distance to the object. Sound pulse package Echo

  5. Ultra sonic – blind range After sending the pulse, the sound converter has to settle before it is able to receive an echo. (up to 5 times as long as the pulse package itself) This means that the sending and receiving processes have to be chronologically separated. A) Sending cycle: 25 ms B) Sending pulse package: 150 µs C) Sending pulse package + settling time: ≤ 1 ms D) Stand-by reception time: 24 ms Objects, which are located in a very close distance, cannot bedetected in every case since the echo is already back, before the piezoceramic ultrasonic converter is ready for reception. The so-called “blind range” of MOBA-Sonic ensues from the settling time.

  6. Ultra sonic – double echo If the distance to a object is too close the echo will return before the converter is ready for reception. The housing of the sensor reflects the pulse back into the direction of the object. This will cause a second reflection from the object which will be received by the converter now. The sensor is calculating a incorrect distance by using the time between sending and the reception of the second echo. 3. End of the sending cycle 5. Second Echo • Sending the impulses 4. Reflection from the sensor housing 2. First reflection during the sending cycle

  7. Ultra sonic – frequency Sonic sensors normally work with an ultrasonic frequency of approx. 200 kHz. The higher the ultrasonic frequency that we can use for locating, the more precisely the distance can be determined. Unfortunately, the attenuation caused by the air is proportional to the square of the frequency and the range of the sound pulse decreases significantly as the frequency increases.

  8. Ultra sonic – sound beam Radiation characteristics of a piezoceramic ultrasonic converter in case of undisturbed sound propagation:

  9. Ultra sonic – conditions In order to avoid disturbing reflections ,there has to be a distance of > 150 mm between the axis of the sound beam and a parallel, smooth wall.

  10. Ultra sonic – conditions In order to obtain optimal reflection results, ultra sonic sensors should be aligned in a way that the ultrasonic waves hit the object perpendicularly. The better the sound reflection characteristics of the material, the higher the permitted angular deviation β can be. Basically the following applies: angle of incidence = angle of reflection. In case of coarse bulk material and under ideal conditions, suitable results can even be obtained for angular deviations of up to 45 °.

  11. Ultra sonic – conditions The angular deviation has a decisive influence on the area in which the sound beam is able to reliably detect an object. Here an example with a specified working range of 20 cm – 100 cm. Object oriented for optimum reflection Object with distinct angular deviation gives poor reflection

  12. Ultra sonic – temperature influence The sound velocity depends on the temperature of the air. At a temperature of 0° C, sound waves spread through the air with a speed of 331.6 m/sec. The sound velocity changes with the air temperature by 0.17 %/K. Without compensation a temperature change of e.g. +10 °C will reduce the measured distance by 1.75 %. Sonic sensors should be equipped with a temperature compensating circuit, so that measuring faults caused by thermal fluctuation, can be eliminated. Nevertheless, it is not possible to completely compensate for all the varied thermal fluctuations within the range of the sound beam. High-temperature objects (e.g. red hot metal) cause “streaks” in the air (turbulences caused by temperature). In this case, the ultrasonic pulse is badly disturbed/deviated or scattered; and a suitable echo will not be created.

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