Air-coupled Ultrasonic Technology

Starting from ultrasound images of an unborn child, over fingerprint recognition in smartphones, up to sonar systems for submarines. Ultrasound is used in a wide variety of applications and in different media due to its mechanical wave characteristics. At the Measurement and Sensor Technology Group, we are specialized on air-coupled ultrasound. Our research focuses on

  • Sonar systems for object detection and industrial imaging,
  • Waveguide technology for flexible array design,
  • Non-contact non-destructive testing of materials and
  • High-velocity flow measurement.

In this context, ultrasonic phased arrays are a common ground for all of these fields of research.

Ultrasonic Phased Arrays

A phased array is a group of multiple ultrasonic transducers. This enables the sound radiation to be steered in a specific direction when transmitting, as well as selective listening in a specific direction when receiving, without requiring the array to be moved mechanically. This technique is called beamforming and is based on phase-shifted excitation of the individual transducers. Transmit and receive beamforming is combined for pulse-echo detection in sonar systems to locate objects in three dimensions.

Sonar Systems and Imaging

Advanced mobile robots must navigate safely in a variety of different environments. Smoke and dust, poor lighting conditions, as well as windows and reflective surfaces are major challenges for conventional monitoring sensors, such as cameras and lidars. Ultrasonic sonar systems reliably locate objects in three dimensions even under these conditions, thus, ensuring precise navigation and mapping.

Our research group investigates the capabilities and limitations of sonar systems regarding multiple optimization goals. A key factor is the viability in terms of real-time capability, computational effort, resolution, space and cost. For flexibility, we create our sonar systems from scratch – starting with array design, electronics, firmware and software development up to GPU-accelerated signal processing. In addition to the development and experimental characterization, we continue to evaluate the actual applications, for example by using a mobile robot.

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The spiral array enables the creation of high-resolution acoustic images.
The spiral array enables the creation of high-resolution acoustic images.

Controlled test environment – anechoic chamber

In order to conduct acoustic characterizations under controlled environmental conditions, we maintain a fully insulated, anechoic chamber. This room is decoupled from the building's foundation, so that mechanical vibrations from the outside do not interfere with our measurements. We use a goniometer setup consisting of two rotational axes and one linear axis to freely position calibrated measurement microphones, senders or targets. This allows us to automatically characterize ultrasonic transmitters, receivers, arrays and sonar systems in three dimensions.

Waveguides – design and simulation

In general, waveguides are channels that carry a sound wave. This could be the sound we perceive through our auditory canal or our voice, which is carried through the throat. The geometry of the channel has a significant impact on the sound wave, for example when generating harmonics as we sing. In addition, waveguides provide a protective function by separating the sound-generating or sound-receiving surface from the free field.

This is exactly the approach we take to protect ultrasonic arrays, to optimize their acoustic properties, and to flexibly design array geometries. We rely on multiphysics simulations to investigate mode conversions, propagation time differences, and acoustic efficiency.

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Reliability for challenging environments

Splashing cooling water, flying splinters and dense soot are not rare in industrial environments. When using arrays under these conditions, our waveguides provide protection for the ultrasonic transducers, however, clogging of the channel openings themselves may occur.

In order to ensure reliability, we investigate the suitability of protection systems, also used in smartphones or Bluetooth speakers, for example. The challenge is to protect the channels without severely impairing acoustic efficiency. Depending on the type of protection, various optimizations must be applied. For example, similar to a guitar string, the pretensioning of the membranes to the correct resonance frequency or the pore size of the hydrophobic textiles are crucial for acoustic efficiency.

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Ultrasonic flow metering

Water vapor and natural gas are just two examples of gases that are passed on at high flow rates in industry. Ultrasonic flow measurement is particularly advantageous here because there is no pressure drop across the measuring device, thus saving energy. In cooperation with Prof. Pelz's Institute for Fluid System Technology, where a test stand with flow velocities of up to 107 m/s exist, we have tested ultrasonic flow measurement at high velocities. However, when one wants to measure such high flow velocities with ultrasound, the problem arises that the sound is blown away by the flow. Our solution to this is to compensate for sound drift with an ultrasonic phased array. We have built a research platform to study this and ensure accurate measurement even at high flow rates with ultrasound.

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Ultrasonic flow metering

Non-destructive testing with ultrasound is a method for inspecting materials for irregularities or defects without damaging the test object. Ultrasonic waves are sent through the material to be tested. Air-coupled ultrasound has the advantage that no direct coupling to the material is required, which enables contactless testing. The recorded ultrasonic signal can be used to draw conclusions about possible defects or structural changes. This non-invasive testing method is used in various industries, from aerospace to the metalworking industry, to ensure the quality and integrity of components.

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