Background of the invention WO 89/00294 discloses a conventional beam type accelerometer of the above kind which comprises two layers, a supporting layer made of silicon and a piezoelectric layer bonded to said layer. This accelerometer is intended to be suitable for mass manufacture from silicon wafers and is also intended to be sensitive in a single plane only relative to its mounting. The accelerometer also is intended to have improved sensitivity.

Although this prior art accelerometer is relatively easy to manufacture, it is sensitive in a plane and it is relatively large, the preferred embodiment having an overall length of about 4 mm.

US-A-5425750 discloses another accelerometer comprising a layered beam. The beam comprises an electrically conducting substrate such as beryllium copper which on each side is covered with a transducing layer of a piezoelectric polymer.

The accelerometer is provided with an additional sensing mass in the free end of the beam which is offset in relation to the plane of the beam in order to ensure that the accelerometer is sensitive in all directions in a plane.

This prior art thus also is sensitive in several directions, particularly in a direction which is oriented along the longitudinal extent of the beam, i. e. in a direction which is oriented orthogonally to the main direction of

sensitivity. The device further is relatively large (a length of about 4mm in the preferred embodiment) and is comparatively complicated to manufacture.

In many applications, such as for instance so-called rate- responsive pacemakers, i. e. pacemakers sensing the physical activity of the patient in order to increase or decrease the stimulation rate applied to the heart in dependence on said activity, there is a need of an accelerometer that may be easy and cheap to manufacture, that has one main sensor sensitivity axis but has a negligible cross sensitivity, i. e. a sensitivity which is negligible in directions orthogonal to said main sensitivity axis, which may be designed to be small, which is very sensitive and which may be designed to be highly resistant to shocks.

This consequently is the problem to be solved by means of the invention.

Description of the inventive concept According to the invention the above problems are solved with an accelerometer of the above kind which is provided with the features set forth in the characterizing part of the attached main claim.

Preferred embodiments of the invention are set forth in the dependent claims.

Short description of the appended drawings Fig 1 illustrates a preferred embodiment of the invention.

Fig 2 illustrates the size of the accelerometer in Fig 1.

Fig 3 illustrates a simplified, schematic embodiment of the invention.

Fig 4 is a detail of the beam in Fig 3.

Fig 5 illustrates a way of manufacturing an accelerometer according to the invention.

Description of preferred embodiments of the invention Fig 1 illustrates a side view of preferred embodiment of the invention.

A beam 1 comprises a piezoelectric layer 2, which for instance may be made of PZT, and a supporting layer 3. The supporting layer preferably should be of an electrically conducting material and should have a high density. A suitable material is tungsten carbide, preferably binderless tungsten carbide. The two layers are joined together by means of a layer 4 of an electrically conducting glue. The supporting layer at the free end of the beam is enlarged to form a sensing mass 5 which thus is made in one piece with the supporting layer 3. The sensing mass 5 has a center of gravity 6.

Binderless tungsten carbide is a particularly suitable material since it has a high density, is very strong and has a very high modulus of elasticity as well as being electrically conducting.

The high density of the material in the supporting layer results in that the mass of the sensing mass will be great.

The supporting layer at the opposite end of the beam 1 is also enlarged to form a support 7 which thus also is made in

one piece with the supporting layer. The support is glued on to the substrate layer 8, which for instance could be a printed circuit board or a thick film substrate. The beam thus will be cantilevered from this support.

The free upper side of the piezoelectric layer is coated with a thin metallic layer serving as an electrode. A lead or band wire 9 connects this electrode with the substrate.

Since the supporting layer 3 is electrically conductive, the layer 3 will, in conjunction with the layer 4 of electrically conductive glue, serve as electrode and conductor for the inner side of the piezoelectric layer.

Fig 2 illustrates the size of the preferred embodiment. In this embodiment the two distances l1 defining the size of the support 7 are 290 pm, the height h1 of the support is 259 pm, the overall length 12 of the beam including support and sensing mass is 2434 pm ( 2.4 mm), and the two heights h2 and h3 defining the size of the sensing mass are 472 respectively 815 pm. The free, deformable length 13 of the beam is 1160 pm, the thickness t1 of the supporting layer is 22 pm, the thickness t3 of the layer bonding the supporting layer and the piezoelectric layer is 10 pm and the thickness t2 of the piezoelectric layer will be 65 pm. The angle a is 14 degrees. The maximal height of the accelerometer over the substrate is 866 pm. The deflection of the free end of the beam under the influence of a force along the main axis of sensitivity is in the size order of one nanometer.

The device according to the invention thus may be designed to be much smaller than the prior art devices described above.

The width of the accelerometer may for instance be 760 pm.

This width will be sufficient to ensure that the accelerometer is insensitive in directions orthogonal to the plane of the drawing. Apart from the desired rigidity in the direction orthogonal to the plane of the drawing, there are no restrictions on the width.

As indicated above, the supporting layer is made of a material with a very high strength and may thus be made thin in comparison to the piezoelectric layer.In conjunction with the high modulus of elasticity which is very high, the supporting layer will guide the deflection of the beam in such a way that the neutral layer of the beam will be located close to the boundary between supporting layer and piezoelectric layer or even in the supporting layer. This will ensure that the piezoelectric layer will be deformed in such a way that a very high sensitivity is obtained for deflections in the main direction of sensitivity.

The presence of the angle a as defined above has the effect that the sum of the deformations along the beam under the influence of a force which is orthogonal to the main direction of sensitivity is negligible. i. e. that the output from the piezoelectric layer as a result of this force will be negligible and that the accelerometer consequently will be insensitive in this direction.

The ratio of the modulae of elasticity for the supporting layer and the piezoelectric layer should be in the range 8 - 12, preferably about 10.

The material in the supporting layer in a preferred embodiment should have a modulus of elasticity 2 400 GPa, preferably 2 500 GPa whereas the material in the

piezoelectric layer should have a modulus of elasticity < 40 GPa @preferably < 50 Gpa, more preferably < 70 Gpa. The binderless tungsten carbide used in the preferred embodiment has an modulus of elasticity of about 670 GPa whereas the modulus of elasticity for PZT is about 67 GPa. Both these materials are ceramic materials and are well matched in regard of their thermal expansion coefficients.

A suitable range for the ratio R between the thicknesses of the supporting layer and the piezoelectric layer in this case might be between 2 and 5, in a preferred embodiment about 3.

In the preferred embodiments of the invention, for instance according to Figs 1 and 2, a may be in the range 5° < a < < cc 300, suitably in the range 100 < cc < 160 and preferably within the range 13.50 < cc < 14".5.

The angle a is to some extent dependent on the geometry of beam and sensing mass. An idea of this functional relationship may be obtained by means of the idealized and simplified model of the accelerometer shown in Figs 3 and 4.

In this model XT and YT are the coordinates for the center of gravity of the sensing mass in the X-Y-system, E1 is the modulus of elasticity of the piezoelectric material, E2 is the modulus of elasticity of the supporting layer, t1 is the thickness of the piezoelectric material, t2 is the thickness of the supporting material, 1 is the deformable length of the beam, F#α

Y the main direction of sensitivity, and X the orthogonal direction with negligible sensitivity.

M is the center of gravity of the sensing mass The weight of the beam is considered negligible in comparison with the weight of the sensing mass.

Under these circumstances the following functional relationship can be found, the requirement being that the mean value of the positive stress in the piezoelectric layer is compensated by the mean value of the negative stress in the layers which are caused by a force directed along the X' -axis.

Under these conditions, F is a fairly good approximation of a. A comparison with the above detailed embodiment described in conjunction with Figs 1 and 2 indicates that another way of expressing the variation of a would be 0. 7F<a<1. 2F.

As illustrated in Fig 5, the device can be manufactured from a bilaminar wafer comprising the supporting material and the piezoelectric material. The wafer may for instance be placed on the vacuum chuck of a so called dice cutter. The dice cutter is programmed to structure the electrically conductive material so that the support, the deflectable part of the beam and the sensing mass is formed. The dice cutter is then used to cut the structured wafer into a large number of individual accelerometers.

By means of the invention an accelerometer is obtained which can be designed to be very small but in spite of this highly sensitive, which has a main direction of sensitivity and a negligible cross-sensitivity and which may be designed to be extremely shock-resistant. The accelerometer may have a generally linear response up to accelerations of 100 G and may be designed to withstand shocks up to 1200G.