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Many strengths, few limits: How ultrasound technology works

Ultrasonic sensors detect all sound-reflecting objects. To calculate the distance, they measure the time between sending and receiving a reflected sound signal. The target’s colour, transparency and surface reflectivity are irrelevant.

Comparison of ultrasonic sensors with other position sensing products

This chart rates the performance of various technologies with certain influences.

Ultrasonic Inductive Capacitive Photoelectric
Great distance to the object 1
Independent of colour / transparency 11
Independent of material
Temperature fluctuations
Deposits on the sensor 2 3
Extraneous light immunity
High ambient humidity

1) e.g. thin, clear-transparent film/glass
2) if not excessive
3) if non-conductive

Operating principle of ultrasonic sensors

In the operating state, voltage is present at the sound transducer. Due to the piezo-electric effect, the transducer is excited to oscillate and generates a sound burst. At this point, the clock-pulse generator switches the sensor to receive mode and time measurement begins. When the sound burst hits the target, an echo is reflected back at the transducer. The echo of the sound burst causes the transducer to vibrate due to the piezo-electric effect and time measurement stops. Based on the measured time and the speed of sound, the distance to the target can be determined.

Structure of an ultrasonic transducer

  1. Piezo-element sends and receives sound (40…300 kHz range)
  2. Mounting with integral foam allows the piezo-element to vibrate freely
  3. Fixture
  4. Adaptation layer matches the acoustic impedance between air and the piezo-element
  5. Sound wave

When do I use a diffuse mode ultrasonic sensor? What has to be considered?

Diffuse reflection sensors evaluate the sound wave reflected by the object via time measurement to determine the distance to the object. The figure shows the detection range and the switching behaviour of the sensor. The fact that transducers work as emitters and receivers at the same time and need time for switching results in a minimum distance called the blind zone in which the signal cannot be evaluated.

When do I use a retro-reflective mode ultrasonic sensor? What has to be considered?

With retro-reflective sensors, the emitted sound is returned by a defined reflector, e.g. a wall or a metal plate. An object is detected when it interrupts the reflected signal. Retro-reflective mode ultrasonic sensors are used for objects with poor sound reflection performance such as foam, jagged surfaces and for angled shapes. Since they only evaluate whether an echo is received, there is no blind zone with these sensors.

What information do response curves provide about an ultrasonic sensor?

Response curves help the user determine the suitability of a particular ultrasonic sensor to solve a specific application. These curves are shown on the product data sheet. A detailed description can be found in the document “General information on mounting and operation” provided on the product page in the "Downloads" section.

For targets approaching the sensor axially (as in a tank level application), the sensor will detect the target as soon as it reaches the switch-on curve.

For applications where the sound curve is too large, consider using a sound tube to better focus the sound.

Testing in the specific application may be needed to ensure proper function.

What factors can influence the performance of ultrasonic sensors?

Ultrasonic sensors are unaffected by changes in air temperature (they have temperature compensation built in), air pressure and humidity. However, other factors can affect their performance.

  1. Air turbulence – strong air turbulence can negatively influence the sound propagation and, consequently, the measurement. Sources of air turbulence include wind, compressed air and cooling fans. The influence can be minimised by physically shielding the sensor/ measuring distance from the turbulence.
  2. Target material and surface characteristics – materials that absorb sound or surfaces that deflect sound away from the sensor are difficult to sense with diffuse mode ultrasonic sensors. A retro-reflective mode ultrasonic sensor may be the better solution here.
  3. Target speed – diffuse mode ultrasonic sensors typically have switching frequencies of 10Hz or less and are not useful for high-speed applications.
  4. Target size – targets must meet minimum size requirements to be reliably detected. The smaller the target, the shorter the sensing range at which it can be reliably detected by the ultrasonic sensor.
  5. Target orientation – for diffuse ultrasonic applications, the face of the sensor should be parallel to the target. Deviations up to 4° can be tolerated on smooth surfaces.
  6. Soiling of the face – small amounts of dust and moisture can be shed through the vibration of the transducer, but large amounts will degrade performance.
  7. Crosstalk – ultrasonic sensors are prone to crosstalk if they are mounted too close together. Observe the mounting distances specified in the operating instructions.