EP2789094A1

EP2789094A1 - Printed circuit board for generating energy for an autonomous electronic unit

Info

Publication number: EP2789094A1
Application number: EP12813419.4A
Authority: EP
Inventors: Stefan Klump; Martin Rüttel; Klaus Van Der Linden
Original assignee: Johnson Matthey Catalysts Germany GmbH
Current assignee: Johnson Matthey Catalysts Germany GmbH
Priority date: 2011-12-06
Filing date: 2012-12-06
Publication date: 2014-10-15
Also published as: DE102011087844A1; WO2013083990A1

Abstract

A printed circuit board for generating and supplying an electronic unit with autonomous energy comprises a plate-like carrier (4) of constant thickness and a bending transducer (2) comprising the carrier (4) and at least one piezoelectric layer (16a, b) applied thereto, wherein, in the bending transducer (2), the carrier (4) comprises a forward bending region (14) and extends in the longitudinal direction (10) from a fastening region (6) to a deflecting end (8), wherein the piezoelectric layer (16a, b) is applied in the forward bending region (14) and wherein, adjoining the fastening region (6), the carrier (4) comprises a narrowing region in which a width of the carrier (4) reduces in the direction of the deflecting end (8).

Description

PRINTED CIRCUIT BOARD FOR GENERATING ENERGY FOR AN AUTONOMOUS ELECTRONIC UNIT

The present invention relates to a printed circuit board for generating energy for an autonomous electronic unit, i.e. an electronic unit which has no connection to further electric or electronic components, and in particular is not connected to a further energy source or an energy store.

The exploitation of the piezoelectric effect to generate energy, i.e. the conversion of mechanical energy into electrical energy, is increasingly being used in various technical fields. An autonomous energy supply, which is independent of other energy sources, for an electronic unit is often desired as a result. One field of use is, for example, the measurement of the air pressure within a vehicle tyre with the aid of an appropriate sensor. The energy for operating the sensor and for wireless data transmission of the sensor signal to a receiver unit is provided in this case by a suitably configured piezoelectric element. The production of energy by means of the piezoelectric effect is generally also known as energy harvesting.

On account of the underlying piezoelectric effect, the amounts of energy that can be generated are comparatively small. If an autonomous energy supply of electronic assemblies is desired, both as low as possible energy consumption of the electronic structural units and energy generation by the piezoelectric element which is as efficient as possible should be considered. What are known as bending transducers are frequently used as piezoelectric elements for generating energy, said bending transducers being clamped and secured on one side in a fastening region and having an opposite deflecting end which is forcibly mechanically deflected. US 2009/0322184 Al discloses an assembly for generating energy with the aid of a bending transducer. The deflecting end is deflected in this case with the aid of a push rod on which a plurality of protrusions formed in the manner of teeth is arranged. As a result, when the push rod is actuated once, the deflecting end is deflected a number of times corresponding to the number of teeth, thereby resulting in a multiplication of the amount of energy generated when the push rod is actuated once.

In the case, for example, of devices to be activated by way of a push button, actuation takes place by mechanical actuation of a button.

Proceeding herefrom, the invention is based on the object of specifying a printed circuit board for, inter alia, generating energy, which enables efficient production of energy for an autonomous supply of an electronic unit.

The object is achieved according to a first aspect of the invention by a printed circuit board for generating and supplying an electronic unit with autonomous energy, particularly for an electronic unit to be actuated by a push button, which printed circuit board comprising a plate-like carrier of constant thickness, wherein the printed circuit board comprises a bending transducer comprising the carrier and at least one piezoelectric layer, in particular a PZT ceramic, applied thereto, wherein, in the bending transducer, the carrier comprises a forward bending region and extends in the longitudinal direction from a fastening region to a deflecting end, wherein the piezoelectric layer is applied in the forward bending region and wherein, adjoining the fastening region, the carrier comprises a narrowing region in which a width of the carrier reduces in the direction of the deflecting end. The piezoelectric layer is applied preferably exclusively in the bending region and does not extend into the fastening region. In the bending region, the carrier now has a bending stiffness that changes in the longitudinal direction, said changing bending stiffness being brought about by a narrowing region in which a width of the carrier reduces in the direction of the deflecting end.

This configuration is based on the consideration that homogeneous, uniform bending of the piezoelectric layer along its entire length is of particular significance for energy production which is as efficient as possible. The fundamental desire is therefore that, in the event of mechanical loading, i.e. when the deflecting end is deflected mechanically by a defined deflection distance perpendicularly to the longitudinal direction, the piezoelectric layer has a substantially constant curvature, i.e. a substantially constant radius along its entire length.

At the same time, this configuration is based on the finding that the bending transducer clamped on one side has a bending distortion directly at the edge between the fastening region, at which it is clamped, and the bending region in the case of a mechanical deflection at the deflecting end (such edge also referred to herein as the bearing end point x₀). This is understood to mean that, in the region of this edge, a curvature of the piezoelectric layer that deviates from the rest of the region is formed and in this respect the curvature is not homogeneous.

This lack of homogeneity is compensated according to the invention in that, in the bending transducer, the carrier has a changing bending stiffness along its length in the bending region. The bending stiffness generally provides - in the context of the beam theory to be applied here to the bending transducer - the ratio of an applied bending moment to the curvature that is established. In this case, the bending stiffness is formed by the product of the modulus of elasticity as a material constant and the geometrical moment of inertia of the carrier. A variation in the bending stiffness is therefore possible both as a result of a variation in the modulus of elasticity and as a result of a variation in the geometrical moment of inertia. In the present invention, the geometrical moment of inertia of the carrier is varied in that the width of the carrier is changed.

The geometry, and generally the bending stiffness, is expediently varied in particular such that the piezoelectric layer has an identical radius of curvature along its entire length in the event of a defined deflection at the deflecting end. To this end, it is in particular provided that the carrier has a greater bending stiffness, i.e. in particular a greater geometrical moment of inertia, immediately at the fastening end compared with the rest of the profile of the bending region in the longitudinal direction.

Accordingly, it is provided in a preferred development that on the fastening region there is formed a narrowing region in which the carrier narrows in the direction of the deflecting end. Preferably, only the width of the carrier narrows in this case.

The narrowing region expediently extends as far as the deflecting end, specifically preferably continuously and in particular uniformly. In this configuration, the bending stiffness decreases continuously over the entire bending region as far as the deflecting end. The narrowing region is in this case formed in particular in a trapezoidal manner. In a preferred embodiment, the carrier terminates at the deflecting end in a blunt end.

Alternatively, the narrowing region is followed by a carrier bar having a reduced width, which extends as far as the deflecting end. The piezoelectric layer is in this case arranged on the carrier bar and extends as far as the end of the narrowing region where the fastening region begins and no more bending occurs. Expediently, it is in this case provided that, adjoining the narrowing region, the carrier bar has a constant width. The changing bending stiffness is thus provided only in a region in the vicinity of the fastening region, i.e. at the fastening edge. This is based on the fact that a homogeneous, constant curvature is automatically formed - as viewed in an idealized manner - at a distance from the fastening region. Expediently, the length of the narrowing region in the longitudinal direction is in this case less than or equal to the length of the adjoining carrier bar. The ratio between the carrier bar and the narrowing region is for example in the range of 5 : 1 to 1 : 1. Expediently, the carrier bar is in the form of a central bar, i.e. the carrier bar narrows symmetrically with respect to a central axis. Overall, this results in an approximately T-shaped configuration of the carrier, with the crossbar of the T- shaped carrier defining the fastening region. Preferably, according to the first aspect of the invention, a width of the at least one piezoelectric layer is constant along its entire length.

For generation of energy which is as efficient as possible, in a preferred development there is provided what is known as a trimorphous configuration, in which precisely one piezoelectric layer is applied to each of the two sides of the carrier. The trimorphous configuration has proved to be particularly efficient in such fields of use. Generally, the piezoelectric layers, i.e. the piezoelectric ceramics, are provided on each of their two sides with a metallization layer, which is often applied by sputtering. The layer thickness of this metallization layer is usually in the range of a little under 200 nanometres. These metallization layers usually have a layer structure, with the outermost layer being formed for example by a gold layer. Electrical contact is made by means of contact elements which are electrically connected to the outer side, remote from the carrier, and to the inner side, facing the piezoelectric carrier, of the piezoelectric ceramic. In this case, a planar metal layer, for example in the form of a thin (copper) foil, is usually applied to the carrier, at least in the region of the piezoelectric layer, the piezoelectric ceramic being adhesively bonded directly to said foil by way of the metallization layers applied on both sides.

With regard to the highest possible efficiency in the energy yield, it is furthermore provided in a preferred configuration that the two piezoelectric layers are electrically isolated from one another. As a result of the electrical isolation, a charge carrier exchange between the two layers is prevented. Such a charge carrier exchange reduces in principle the energy efficiency, since not all of the charge carriers generated contribute to the production of energy.

For the same purpose, it is additionally furthermore provided in a preferred configuration that each piezoelectric layer is assigned a rectifier, in particular a bridge rectifier. The charge carriers generated by each piezoelectric layer are therefore converted into direct current in each case in a separate rectifier before they are subsequently combined.

The printed circuit board can be loaded with electronic components of an electronics assembly in the fastening region. Therefore, use can be made of the technologies of printed circuit board manufacture in order to produce the carrier. On account of the direct integration of the electronic structural units, a compact assembly having coordinated units is made possible overall in the smallest possible space with only small electrical losses. According to a second aspect of the invention, there is provided an electronic unit comprising a printed circuit board according to the first aspect of the invention or an assembly according to the third aspect of the invention. The electronics assembly is a completely autonomous assembly, which therefore has no connection to further electric or electronic components, and in particular is not connected to a further energy source or an energy store.

Expediently, the printed circuit board contains an indicator element as electronic component. Said indicator element is configured preferably for electronically indicating a status of each device in which the assembly is installed. For example, the number of actuations already carried out can be indicated.

In order to generate as large an amount of energy as possible per actuation for example actuation of a button, for mechanical deflection according to a third aspect of the invention there is arranged an actuating element which is formed in particular as a push rod and moves perpendicularly to the longitudinal direction when the button is actuated. The push rod has in this case a number of protrusions, with each protrusion mechanically deflecting the deflecting end of the bending transducer as it slides past the latter, so that when the metering button is actuated once, i.e. when the actuating element is displaced perpendicularly to the longitudinal direction, a plurality of deflections of the bending transducer take place. The number of protrusions can in this case vary considerably and is for example between 1 and 100, preferably in the range between 3 and 20.

The protrusions on the actuating elements are in this case provided with a falling and a rising flank, in order to ensure deflection of the deflecting end which is as smooth as possible. The protrusions are in this case formed for example in the manner of teeth or triangles. The rising and falling flanks can also be curved surfaces, however.

In an expedient development, a deflecting element is arranged directly at the deflecting end of the bending transducer part of the printed circuit board, said deflecting element interacting with the actuating element e.g. push rod, in particular with the protrusions. The deflecting element fastened to the deflecting end likewise has rising and falling actuating surfaces which are thus formed so as to extend in an oblique or curved manner with respect to a plate plane of the carrier. In this case, the deflecting element is preferably formed at least as part of a rotational body, such as a sphere, ellipse, cylinder, etc., for example, and forms the end of the bending transducer.

According to a preferred arrangement, at the deflecting end, the at least one piezoelectric layer extends to the deflecting element.

An exemplary embodiment of the invention is explained in more detail in the following text by way of the Figures.

The Figures show, in each case in schematic, simplified illustrations:

Figure 1 a top view of a bending transducer;

Figure 2 a side view of the bending transducer according to Figure 1;

Figure 3 a side view of an assembly for generating energy, in particular for a device actuated by means of a push button and having a bending transducer and an actuating element in the form of a push rod for deflecting the bending transducer;

Figure 4 a qualitative illustration of the profile of the bending stiffness of the carrier in the longitudinal direction; and

Figure 5 a simplified circuit diagram to illustrate the production of energy with two rectifiers.

In the Figures, parts that act in the same way are provided with the same reference signs. The basic structure of the printed circuit board is apparent from Figures 1 and 2. The printed circuit board 2 comprises carrier 4 which also forms part of the bending transducer 2. The bending transducer 2 is in the form of a trimorphous bending transducer and the carrier 4 is composed of a printed circuit board material (FR4), which extends in the longitudinal direction from a rearward fastening region 6 to a forward deflecting end 8. In the fastening region 6, the bending transducer 2 is subsequently clamped or is fastened to a support structure 12 (cf. Figure 3). The fastening region 6 is adjoined in the longitudinal direction 10 by a bending region 14. In the bending region 14, a piezoelectric layer 16a, b is applied to both sides of the carrier 4. In particular, the piezoelectric layer 16a, b is for this purpose fastened directly to the carrier 4 by adhesive bonding. The inner side, directed towards the carrier 4, and the outer side, remote therefrom, of each layer 16a, b is electrically contacted. Fastened in the foremost region of the carrier 4 at the deflecting end 8 there is a deflecting element 18 which is formed in the manner of a sphere in the exemplary embodiment.

In the fastening region 14, the carrier 4 is loaded with a plurality of electronic components 20, 22, which are part of a common, autonomous assembly. The electronic component 22 is in this case in the form of an indicator element. The individual electronic components 20, 22 are connected together and to the piezoelectric layers 16a, b in a manner not illustrated in more detail via conductor tracks. The electronic components 20, 22 form an autonomous assembly to such an extent that they are connected to no further components, energy stores, energy sources, etc.

As is apparent in particular from the top view according to Figure 1, the fastening region 6 is followed in the longitudinal direction towards the deflecting end 8 by a narrowing region 24 in which the width of the carrier 4 narrows uniformly towards the centre. In the exemplary embodiment, the narrowing region 24 is also immediately followed by a carrier bar 26 having a constant width. The narrowing region 24 and the carrier bar 26 together form the bending region 14. As an alternative, the narrowing region 24 forms the bending region 14, and so extends continuously up to the deflecting end 8. In this case - which is indicated by a dashed line in Figure 1 - the width decreases continuously. The narrowing region 24 is in this case formed in particular in a trapezoidal manner.

The piezoelectric layers 16a, b extends at least virtually over the entire bending region 14 as far as the deflecting element 18. The piezoelectric layers 16a, b preferably have - in contrast to the carrier 4 - a constant width along their entire length. The thickness of the carrier 4 is constant along its entire length. It is typically in the range of around 0.2-0.4 mm. The layer thickness of the piezoelectric layers is in each case typically in a similar range. The overall thickness of the layer structure consisting of the carrier 4 and the layers 16a, b is typically around 1 mm. Under the piezoelectric layer 16a, b, a layer structure is produced, formed by a central ceramic layer (PZT layer), to both sides of which a metallization layer is applied, in particular by sputtering.

On account of the specially selected geometry of the carrier 4, a bending stiffness EI of the carrier 4 that changes in the longitudinal direction 10 arises in the bending region 14, as is apparent from Figure 4: In the fastening region 6, the carrier 4 has in the exemplary embodiment a constant cross-sectional area and thus a constant geometrical moment of inertia I. The material of the carrier 4 is homogeneous along its entire length and has the same modulus of elasticity E as a material constant. In this respect, the product, defining the bending stiffness, of the modulus of elasticity E and the geometrical moment of inertia I is constant up to the end of the fastening region 6 at the position x₀. This position defines the bearing end point or the bearing edge at which the carrier 4 is ultimately supported on the support structure 12. Subsequently, the geometrical moment of inertia I decreases up to the position xi. The extent between the points xo and xi defines the length of the narrowing region 24. This is followed by the carrier bar 26 which extends ultimately as far as the position x₂. In this region, the geometrical moment of inertia I and thus the bending stiffness again have a constant value. The length of the narrowing region 24 (distance between xi and xo) is in this case preferably less than the length of the carrier bar 26 (distance of x₂ from xi).

In the alternative configuration, the narrowing region 24 extends along the entire length L, i.e. from the position x₀ to the position x₂. This is illustrated in Figure 4 by a dashed line. The bending stiffness in this case decreases preferably in a linear manner.

The piezoelectric layers 16a, b begin preferably directly at the bearing end point x₀ and extend up to or at least close to the end of the carrier 4 at the position x₂. The piezoelectric layers 16a, b are in this case - apart from any manufacturing tolerances - formed identically. They have a length "1" and a width "b" {cf. Figure 1).

The deflecting end 8 is deflected with the aid of an actuating element which is in the form of a push rod 28 and is provided with protrusions 30 formed in the manner of teeth, as is apparent from Figure 3. The push rod 28 is in this case displaced in the push rod longitudinal direction 32 and thus perpendicularly to the longitudinal direction 10 of the bending transducer 2. The individual protrusions in this case come into contact with the deflecting element 18 and thus deflect the bending transducer 2 in each case with a defined deflection. On account of this forcible mechanical deflection, the bending transducer 2 is elastically bent in its bending region 14.

Of particular significance is the narrowing region 24 having bending stiffness that decreases in the longitudinal direction 10 towards the deflecting end 8. This ensures that, in the event of a forced mechanical deflection, the bending transducer assumes over the entire bending region 14 a radius of curvature R that is as far as possible identical along the entire length of the bending region 14 (cf. Figure 1).

As is further apparent from Figure 3, the individual protrusions 30 are formed in the manner of triangles as seen in cross section. In the exemplary embodiment, these are irregular triangles, the rising flank 34a of which is oriented in a flatter manner than the falling flank 34b. As a result, an actuation of the push rod 28, i.e. a displacement of the push rod in each case by a stroke length in and counter to the push rod longitudinal direction 32, causes a total of six deflections of the deflecting end 8.

Instead of the cross-sectionally viewed triangular configuration of the protrusions 30, the latter can also be provided with a curved surface, in a similar manner to the deflecting element 18. Conversely, the deflecting element 18 can also be formed, in a manner similar to the protrusions 30, with obliquely extending surfaces, in the manner of a wedge. As an alternative to the configuration with the push rod, other types of mechanical actuating elements can also be provided, these preferably being formed such that they cause a rapid deformation and recovery of the bending transducer 2. The rapid deformation and recovery of the bending transducer 2 allows optimal exploitation of the generated charge carriers, in particular in conjunction with the two independent rectifiers 36. The actuating element 38 is in this case formed for example as an in particular resiliently mounted rocker, wherein one end of the rocker deflects the deflecting end 8. Each individual deflection leads in this case to the generation of charge carriers on account of the bending of the piezoelectric layers 16a, b. Said charge carriers are used in particular to feed an energy buffer store which is formed in particular as a capacitor. As is apparent from Figure 5, each piezoelectric layer 16a, b is assigned a separate rectifier 36 in the form of a bridge rectifier. This ensures that the charge carriers generated in the two piezoelectric layers 16a, b are initially separated, until the current has been rectified. Only then, i.e. following the rectifiers 36, are the rectified charge carriers (currents) combined and used to charge the capacitor 38.

The printed circuit board for generating energy which is described here is distinguished by high energy generation efficiency. An essential aspect here can be considered to be the changing bending stiffness of the carrier 4, which is caused in particular by the variation in the width of the carrier 4. As a result, a curvature which is homogeneous along the entire length of the piezoelectric layers 16a, b is achieved, and this has a positive effect on energy production. It is furthermore of particular significance that the two piezoelectric layers 16a, b are electrically isolated, i.e. the two electrodes of each piezoelectric layer are supplied, separately from one another, to the electric circuit, in particular the two separate rectifiers 36. A further particular aspect can be seen in the use of two separate rectifiers 36, since as a result - just as by the electrical isolation - a charge carrier exchange is prevented. Finally, on account of the special configuration of the push rod 28 with the plurality of protrusions 30, multiple deflection of the bending transducer 2 is brought about with only one actuation, and so the amount of energy generated is increased. The amount of energy generated per actuation is preferably several 100 μΐοώε. The integration of all of the electronic components 20, 22 of the autonomous assembly on the carrier 4 furthermore allows cost-effective manufacture. In addition, this measure also results in a very small type of construction, thereby producing a space-saving arrangement in a device.

Claims

A printed circuit board for generating and supplying an electronic unit with autonomous energy, which printed circuit board comprising a plate-like carrier (4) of constant thickness, wherein the printed circuit board comprises a bending transducer (2) comprising the carrier (4) and at least one piezoelectric layer (16a, b) applied thereto, wherein, in the bending transducer (2), the carrier (4) comprises a forward bending region (14) and extends in the longitudinal direction (10) from a fastening region (6) to a deflecting end (8), wherein the piezoelectric layer (16a, b) is applied in the forward bending region (14) and wherein, adjoining the fastening region (6), the carrier (4) comprises a narrowing region in which a width of the carrier (4) reduces in the direction of the deflecting end (8).

Printed circuit board according to claim 1, wherein the narrowing region reduces in width uniformly as far as the deflecting end (8).

Printed circuit board according to claim 2, wherein the carrier (4) terminates at the deflecting end (8) in a blunt end.

Printed circuit board according to claim 1, wherein, in the narrowing region, the width of the carrier (4) narrows, wherein the narrowing region adjoins a carrier bar (26) that extends as far as the deflecting end (8), and the piezoelectric layer (16a, b) is arranged on the carrier bar (26) and the narrowing region.

Printed circuit board according to claim 4, wherein the carrier bar (26) has a constant bar width.

Printed circuit board according to claim 4 or 5, wherein the carrier bar (26) is in the form of a central bar and the carrier (4) narrows symmetrically with respect to a central axis towards the carrier bar (26).

7. Printed circuit board according to any preceding claim, wherein a width of the at least one piezoelectric layer (16a, b) is constant along its entire length. 8. Printed circuit board according to any preceding claim, wherein the at least one piezoelectric layer (16a, b) does not extend into the fastening region (6).

9. Printed circuit board according to any preceding claim, wherein, in the bending transducer (2), a piezoelectric layer (16a, b) is adhesively bonded onto each of the two sides of the carrier (4), wherein the layers (16a, b) are electrically isolated from one another.

10. Printed circuit board according to any preceding claim, wherein each piezoelectric layer (16a, b) is assigned a rectifier (36).

11. Printed circuit board according to any preceding claim, wherein the printed circuit board is loaded with electronic components (20, 22) of an electronics assembly, and the electronic components (20, 22) are applied directly to the carrier (4) in the fastening region (6).

12. Printed circuit board according to Claim 11, wherein the electronics assembly has an indicator element (22) for the electronic indication of a status, wherein the electronics assembly is autonomous.

13. Printed circuit board according to any preceding claim, wherein a deflecting element (18) is arranged at the deflecting end (8), said deflecting element (18) being adapted to interact with an actuating element (28) and for this purpose has an actuating surface which extends in an oblique or curved manner with respect to a plate plane of the carrier, wherein actuation of the actuating element (28) deflects the deflecting end (8).

14. Printed circuit board according to claim 13, wherein the deflecting element (18) is formed at least as part of a rotational body.

15. Printed circuit board according claim 14, wherein the rotational body is a sphere or an ellipse.

Printed circuit board according to claim 13, 14 or 15, wherein, at the deflecting end (8), the at least one piezoelectric layer (16a, b) extends to the deflecting element (18).

An assembly comprising a printed circuit board according to preceding claim and a push rod (28) for deflecting the deflecting end of the bending transducer (2), said push rod (28) being displaceable perpendicularly to the longitudinal direction (10) and having a number of protrusions (30) for repeated deflection of the deflecting end (8). 18. An electronic unit comprising a printed circuit board according to any of claims 1 to 16 or an assembly according to claim 17.