Force sensors, acceleration sensors and vibration sensors have many uses, and exist in many embodiments. Usually, two or three separate sensors are utilized e. g. to sense acceleration in three orthogonal directions, by allowing mas- sive bodies, suspended in spring systems, to move relative to respective reference frames. Rotation is usually sensed with a gyroscope device.

The present invention aims at providing a sensor that, better than previously known solutions, is able to operate with a directional effect and provide good mea- surements regarding translation as well as rotation, by means of one movable body only.

Therefore, in accordance with the invention there is provided a mechano- electrical sensor such as precisely defined in the appended claim 1. Advantageous embodiments of the invention appear from the attached dependent claims.

In the following, the invention shall be illuminated further by describing exemplary embodiments of the invention, and in this connection it is also referred to the appended drawings, wherein Fig. 1 shows a two-dimensional embodiment of the sensor in accordance with the invention; Fig. 2 shows the same embodiment as Fig. 1, however suspended in an outer frame; Fig. 3 shows another two-dimensional embodiment of the sensor in accor- dance with the invention; Fig. 4 shows the same embodiment as Fig. 3, however suspended in an outer frame; Fig. 5 shows a three-dimensional embodiment of the sensor in accordance with the invention, with foil-shaped support structures, in a partially cut-away view; and

Fig. 6 shows another three-dimensional embodiment with filament-shaped support structures, this drawing also in a partially cut-away view.

In Fig. 1 appears a relatively simple, two-dimensional embodiment of the sensor of the invention. An inner body 1 is supported by means of piezoelectric foils 3 in a framework 2, and non-appearing signal wires connected e. g. to respec- tive sides of a foil 3, are able to deliver electrical signals generated when the foils are subject to deformation due to shift of the inner body 1 relative to a relaxed centre position. The figure shows three foils tautened in a hexagonal opening, however one single foil may be used, or a larger number of foils. The choice of inner body will depend on the use field of the sensor. The inner body may, in uses including recording from soft surfaces, consist of e. g. plastic or silicone rubber with various shore values. In other applications, for example industrial diamond mate- rial may be used. Combinations of material and geometrical shape of the inner body is important. The inner body may also exhibit openings to provide a possibi- lity for air passage therethrough, for example in microphone applications. The foils may possibly be attached between two metallic frame parts that are insulated from each other and possibly from other frame parts along the periphery, so that signals can be collected from the metallic frame parts. When foils 3 are used such as indi- cated in the figure, the stretch directions of the foils may be e. g. along the longitu- dinal direction for each foil strip, and this provides an opportunity to collect a hig- her, summed total signal compared to the case of only one single foil, either as a strip across the opening, or as a complete"diaphragm"covering the whole opening.

Centering of the inner body 1 is not necessary, one may visualize embodi- ments with an inner body arranged in an eccentric position. Nor is the shape of framework 2 crucial, as long as the frame is rigid and suitable for attaching the piezoelectric foils.

Such a two-dimensional sensor will clearly be most sensitive with regard to force or vibratory influence in a direction perpendicular to the plane spanned by the sensor, but it will also be possible (when using several foils with separate sig- nal wires) to sense a force in the support plane, i. e. lateral movement of the inner body.

In Fig. 2 appears the same embodiment as in Fig. 1, however the whole basic sensor is suspended in an outside framework 5. The suspension is by means of elastic elements 4, e. g. rubber elements, and such an embodiment of the invention will be particularly favourable e. g. when using the sensor as a sensor element in a microphone. The main purpose of the outside framework 5 is noise attenuation, i. e. attenuation of noise in the form of vibrations that may bring the piezo elements of the sensor into oscillation. When the sensor is attached to an outer frame 5, there will be two oscillatory systems, of which the inner system is the sensor itself. The design must then be so as to give the outer system a reson- ant frequency that is low relative to the resonant frequency of the system compris- ing inner frame/piezoelectric suspension structure/inner body. One will then achi- eve the effect that the frame works as a low pass filter. This relates primarily to the two-dimensional solution.

Further, it will be of great importance whether it is the framework 2 or the inner body 1 that is supposed to oscillate in relation to the surroundings. Ideally, it is desirable to maintain the framework 2 at rest in relation to the surroundings, while the inner body oscillates relative to the framework. In practice, the suspen- sion of the sensor frame will normally provide"good"acoustic coupling between the surroundings and the sensor elements, and normally this is not desirable. Gen- erally, the mass of the inner body will influence the characteristic (the frequency response) most strongly, but design and material choice will also be of importance regarding the coupling between the"sensed medium"and the sensor. Due to the coupled oscillatory systems, the characteristic must be optimized as a function of mass ratios, stiffnesses etc.

In an application in a microphone that is supposed to be good at high fre- quencies, the oscillations in the air will bring the suspension diaphragms (see Fig. 4) into oscillation, and the framework 2 will then oscillate around the inner body 1. In such a case, the vibrating part of the sensor must be as light as possible.

In Fig. 3 appears an alternative embodiment of the sensor in accordance with the invention, still in a two-dimensional version. Here appears an inner body 1 suspended in a number of sector-shaped piezoelectric foils 3, and preferably the stretch direction for every foil sector is arranged in the same manner in relation to the radius in the respective position, e. g. pointing substantially in a radial direction.

There are small openings between foils in this case, which for example in connec- tion with use in a microphone, may be favourable regarding air passage through the openings. Moreover, connection of signal leads is made in a similar manner as mentioned regarding Fig. 1, and it appears that it may be possible to achieve high sum voltages with appropriate coupling of signal leads from each respective foil sector, if this is desirable. Alternatively, of course separate signals can be collec- ted from each respective sector.

Fig. 4 shows suspension in an outer frame 5 in the same manner as in Fig. 2, however in this case the suspension structures are elastic, sector-shaped diaphragms made of e. g. rubber.

In Fig. 5 appears an embodiment of a three-dimensional type. The inner body 1 is held suspended at the centre of a spherical frame 2, by means of piezo- electric foil pieces 3 arranged in such a manner that a relative shifting of the inner body 1, or a rotation for that matter, will be detectable by means of voltages cre- ated in the foils 3, and that can be collected by means of (not shown) signal wires connected to the two sides of the foil pieces projecting out through the frame. Of course, framework 2 does not have to be spherical, nor does it need to be closed, but it is important that it is rigid, in order to constitute a reference for the position of the inner body.

Fig. 6 shows a similar design, but the piezoelectric foils have been replaced by filaments, and the filaments are either of a piezoelectric type with correspond- ing function as the foil pieces in Fig. 5, or the filaments are taut and substantially inelastic, but attached to piezoelectric areas (not shown) of the framework, so that these areas generate voltages depending on the translation or rotation of the inner body relative to framework 2.

Such a three-dimensional force/vibration sensor as shown in Fig. 5 and Fig. 6, is based upon a rigid coupling between the framework and the body for which force or possibly acceleration shall be measured, and thereby the inertia of the inner body will create the measurable voltages in the suspension structures 3 or in their attachment areas. Hence, with signal leads coupled to suitable process- ing equipment, such an acceleration/vibration sensor may constitute a main ele- ment in e. g. an inertia navigation system.

Also the three-dimensional embodiments shown in Figs. 5 and 6 can be suspended in an outer framework via an elastic material in two or three dimen- sions.

The foil pieces shown in the embodiment of Fig. 5 may come in other sha- pes, for example more sector-like or possibly as approximations to full circle areas, and the planes to be spanned, do not necessarily have to be orthogonal like in the figure.

Besides, foil materials or filament materials are not the only possible materi- als in this application, the suspension structures between inner body and frame- work may possibly be piezoelectric bimorph elements or similar elements.

The invention is also intended to accommodate the variant that has already been mentioned, namely the variant with suspension structures that are not piezo- electric, but attached to piezoelectric areas of the framework.