Contactless handling with ultrasound
The contactless handling with ultrasound (also ultrasound levitation ) is used to make objects float in a controlled manner. Various effects of ultrasound are used to improve the handling of objects - for storing, securing and moving. The lower ultrasonic range between 20 and 100 k Hz is used as the frequency . For the following effects it is essential that a gas (air) is used as the medium, since the compressible properties of the gas play an important role in each case. As soon as liquid is involved, the energy is converted in the system and the effects of ultrasonic cleaning occur .
The effects in detail
Standing wave effect in the ultrasonic field
The mostly circular ultrasound source is opposed to a reflector at a distance of an integral multiple of half the wavelength. As a result, the emitted sound wave is reflected on itself, and standing waves of the sound velocity and the pressure change arise , the nodes of the sound velocity being the bellies of the pressure change. Small objects can now be "hooked" into the knot of the sound velocity. Due to the fluid mechanical effects of the high-frequency air movement, the objects keep returning to the center of the node.
This effect was used technically for the first time by ESA and NASA to investigate crystal formation in microgravity . The contactless fixing of the metal spheres and the melting in a protective gas enabled the influence of the walls of the vessel to be eliminated, whereby an unhindered crystallization and solidification of the materials could be investigated.
To this day, the standing wave effect is known as acoustic levitation or containerless processing for reaction observations in chemistry, micro and trace analysis, evaporation and condensation processes as well as various other measurement methods such as B. used spectroscopy.
Near field effect in the ultrasonic field - ultrasonic air bearing
The so-called near-field effect in the ultrasonic field describes an effect in which an object is brought very close to an ultrasonic source. This creates an effect similar to that in a fluid dynamic bearing, the gas is compressed in the gap. The resulting ultrasonic air bearing has roughly the same properties as a "standard air bearing", i.e. a strongly progressive force-displacement curve. However, since a high amplitude (2–15 µm) of the vibration generator is necessary, the gap cannot be very small, the systems are therefore usually operated at a distance of 50–500 µm. This reduces the achievable pressures / forces.
The near field effect was discovered by physicists at the beginning of the 20th century, but was never used technically. In the case of ultrasonic welding, it was even considered a disruptive effect, as it made it difficult to approach the component to be welded.
It was not until 1999 that the Institute for Machine Tools and Industrial Management (iwb) at the Technical University of Munich began to research this effect on the contactless transport of semiconductor wafers. Robotic grippers, linear tracks and storage tables have been successfully examined. By combining the near field effect with a vacuum, it is possible to grip components from above. This was implemented for small components from microsystem technology as well as for wafers with a diameter of 300 mm. It quickly turned out that this new effect made it possible to handle these sensitive components without the creation of scratches and particles and at the same time to avoid the main disadvantage of other non-contact principles such as Bernoulli grippers or air bearings, namely the introduction of additional contaminating particles into the clean room air by the necessary Blown air as well as the expensive air treatment.
In 2006, the iwb team founded the company Zimmermann und Schilp Handlingstechnik GmbH - now ZS-Handling GmbH, which has been marketing these systems ever since. Thanks to continuous development work both at the iwb and in the new company, it is now possible to levitate objects with an edge length of a few millimeters over semiconductor wafers and solar cells up to glass panes up to 2.5 meters in size and thus transport, store and grip them.
Standing wave effect with the component as a reflector
A synthesis of the previous effects is the standing wave effect with the component as a reflector. This effect is much weaker than the near-field effect and only works in the reflector distances already described above. The force that the reflected wave exerts on the reflector is used here to carry the component. In practice, this only works satisfactorily with very light components that are damped or dampen themselves, since the smallest discontinuities in the system (wind or similar) cause the vibration, which is undamped per se, to swing and the effect collapses.
- ZS-Handling GmbH : Description of the principle of the ultrasonic air bearing and applications
- Institute for Machine Tools and Industrial Management ( iwb ) of the Technical University of Munich Research project " Force-controlled , contactless handling and joining by means of power ultrasound "
- Institute for Machine Tools and Industrial Management ( iwb ) of the Technical University of Munich Research project " Contactless ultrasonic vacuum gripper for circuit board assembly "
- Jürgen Höppner: Method for contactless handling using powerful sound transducers , Herbert Utz Verlag Wissenschaft, Munich 2002
- Michael Schilp: Design and design of tools for contactless gripping in micro-assembly , Herbert Utz Verlag Wissenschaft, Munich 2006, ISBN 978-3-8316-0631-3