Ultrasound a brief explanation

Ultrasound is probably best known from medical diagnostics where it is used to take pictures of unborn babies. In industry, ultrasound is used in a wide range of processes, such as cleaning, welding of plastics and metals, cutting, forming, testing of materials, separating, mixing, de-gassing, atomising, localising, measuring and many others.

Ultrasound refers to sound in the range that is not audible for humans. Sound is a propagation of minute fluctuations in pressure and density in an elastic medium (gases, liquids, solids). The number of vibrations within one second is called frequency and is measured in Hertz (Hz). The frequency range that is audible for humans is between 16 Hz and 20,000 Hz. The range above 20,000 Hz is referred to as ultrasound.

Advantages in the industrial field

+ environmentally-friendly – Electrical energy is converted directly in high-frequency mechanical vibrations, using the piezoelectric effect. These vibrations directly affect the target range (welding seam, screen, cleaning medium etc.).

+ energy saving – Ultrasonic processes do not require additives and consumables. The focused energy input in the target range (welding seam, screen, cleaning medium etc.) keep energy loss to a minimum.

+ fast – As the energy input covers the entire target area simultaneously, the ultrasonic process works very rapidly. Typical process speeds in connection technology thus range from tenths of seconds (plastic) to approx. 3 seconds (metal).

+ efficient – Ultrasonic tools suffer comparatively little wear. This means that not much servicing and maintenance work is required. In many fields, ultrasound systems are used in shift work 24 hours a day.

+ cost efficient – Owing to the rapid and efficient process and low energy input it is possible to achieve savings from the first moment.

Ultrasound Technology the basics

The generation of ultrasound is based on the piezoelectrical effect of PZT ceramics. When a PZT ceramic item is charge with a voltage, its length changes. If the item is charged with a (high-frequency) alternating voltage, the PZT ceramic item will lengthen or shorten in response to the changes in voltage – the item will vibrate.

The vibrations of the PZT ceramic item are transferred via acoustically tuned components into the target area, e.g. the welding surface, a screen frame or the cleaning medium. In order to ensure an efficient transfer of the vibrations, the entire vibrating system is acoustically tuned so that its components vibrate in resonance.

The vibrating system comprises the following components:

The converter contains the PZT ceramic part which converts the electrical energy into mechanical vibrations
The booster increases or reduces the vibration amplitude of the converter so that it matches the sonotrode perfectly. Not all applications require a booster.
The sonotrode is the tool that transfers the vibrations to the target area

Types of vibrating systems

Longitudinal vibrating systems

Longitudinal vibration systems are used in many fields, e.g. for welding plastics and non-ferrous metals or for cleaning.

Torsional vibrating systems

The torsional process has the advantage that only a small part of the vibrations is transferred into the area surrounding the weld seam. This means that more of the energy is directed to the actual weld area, achieving greater energy density, and sensitive parts and surfaces are not affected by the vibrations.

Transverse vibrating systems

Transverse vibrating systems are used for exciting screens.

Figure 1: Sonotrode for linear plastic welding

Figure 2: Sonotrode for linear plastic welding

Figure 3: Sonotrode PowerWheel

Figure 4: Screen frame