Birgit Schulze reveals how high-quality piezo elements are used as sound transducers in aeroplane doors
In ancient times flying was only the privilege of the gods, but today travelling by air is taken for granted. The motto is ‘larger, faster and further’. However, the requirements on the components used in aeroplane technology are increasing because above all, safety has the highest priority. On the Airbus A380, this requirement also applies to the sound transducers that generate acoustic signals or warning sounds during automatic processes such as closing the cabin doors.
Monitoring the door locks
The beginning of a long-range flight is the takeoff. But, before a plane taxis to the runway, the various checklists must be processed. Although this may seem to be a trivial matter to the layman, it is of great importance for the safety of the flight. For example, before the plane is allowed to taxi in the direction of the runway, it must be ensured that all outside cabin doors are properly closed.
In the case of the A380, this is not so easy because the doors are large and too heavy for manual closing. Motor force is therefore necessary. However, the automatic closing and locking mechanism must be monitored. For this purpose, 14 sound transducers are built into each plane next to the doors. Holmberg, the company that develops and produces electro-acoustic devices for professional use, has been a specialist for this type of airworthy sound transducers for a long time. Piezo sound transducers for plane doors are not to be compared with mass-produced buzzers and are subject to considerably higher quality requirements.
Basically, sound transducers consist of two piezo discs glued to each other, a plastic membrane and a waterproof case. When electrical voltage is applied, the piezo ceramic begins to deform rhythmically according to the frequency. This is the so-called inverse piezo effect; the piezo element converts the electrical energy to mechanical energy. The motion is transferred to a membrane which then radiates sound waves. Depending on which of the piezo discs is operated, two different tones are generated according to the respective resonant frequency; the OK signal for 'door closed' or the warning tone.
The piezo ceramic used in the sound transducers plays a key role. "We obtain the two piezo discs, which are already glued together, from PI Ceramic,” explains Peter Köppel, technical director at Holmberg. "The piezo discs meet our high standards of quality and are distinguished by a very low tolerance to all piezoelectric parameters relevant for sonic performance such as resonant frequency, electrical capacitance and coupling coefficients.”
The piezo ceramics can also be easily adapted to the respective application requirements because, in addition to the material selected for each application, it is possible to realise different geometric versions and resonant frequencies.
The piezo elements have not only convinced the specialists for electro-acoustic devices from a technical point of view but also from a practical one, and there will most likely be further joint projects in the future.
Birgit Schulze is with Physik Instrumente (PI).