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US and IR sensors

Ultrasonic waves have a large wavelength and a slow rate of propogation when compared with electro-magnetic waves. This makes ultrasound very useful for measuring distances ranging from a few millimetres up to several metres exactly and without the need for contact.

Measuring principle

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A short ultrasonic impulse is sent by an ultrasonic transmitter. This impulse will be reflected by an obstacle. The reflection is received by an ultrasonic receiver. The difference of time Δt between sending an impulse and receiving the reflection can be measured. The distance to the obstacle can thus be calculated using the following formula:

d = ½ * c * Δt

(c = 343m/s ... speed of sound in air at 20°C)
If an ultrasonic transmitter and an ultrasonic receiver are combined in one sensor, then this sensor is called an ultrasonic transducer.

Selection of ultrasonic frequency

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Sound above the range of human hearing is referred to as ultrasound. In general, ultrasound starts above 20kHz and goes up to several megahertz.

If ultrasonic sensors are used to measure distances in air, the upper frequency is limited. This is due to the non-linear effect of air, which absorbs more ultrasound as the frequency increases. This therefore also restricts the maximum distance that can be measured.

On the other hand, the ultrasonic sensor itself limits the upper frequency. The impedance mismatch between the ultrasonic transducer itself and the air as the medium of transmission increasingly prevents the ultrasound generated at the transducer from being emitted into the air.

Therefore the optimal ultrasonic frequency must be selected differently every time in order to suit the task at hand.

Assessment of ultrasound

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advantages

  • no contact required
  • low costs of electronic components
  • low power
  • resolution up to a few millimetres
  • not affected by the colour and in general also the material of the obstacle

disadvantages

  • specific spatial sensitivity range of ultrasonic sensors (distance to but not the position of the obstacle can be detected exactly inside this range)
  • strength of reflected signal depends on the form and material of the obstacle
  • the speed of sound is temperature-dependent
  • total reflectance can occur depending on the surface of the obstacle

Optimisation of ultrasonic sensors

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As mentioned previously, there is a measurement inaccuracy in getting the position of an obstacle inside the sensitivity range. The inaccuracy can be reduced by optimising the directivity of the ultrasonic sensors.