Power generation device

Abstract

A device able to generate electrical power through relative rotational motion of first (370, 380) and second (200, 230) principal components around an axis of rotation (220); wherein: the first and second principal components comprise an arrangement of piezoelectric elements (380) and permanent magnets (230, 370) such that the interaction between these magnets and piezoelectric elements, in use, makes it possible to generate electricity; and wherein: the second principal component (200, 230) comprises a center of mass offset from the axis of relative rotation (220) such that the response of the second principal component (200, 230) to either gravitational or inertial forces is a relative rotation of the second principal component (200, 230) in relation to the first principal component (370, 380); the first principal component (370, 380) being fixedly attached to the moving host structure (100).

Claims

1. A device for generating electricity by acceleration of and/or changes in orientation of the device, the device comprising first and second principle components, wherein: the first principle component is mounted relative to a housing for resilient deformation and to generate electricity when so deformed; the second principle component is rotationally mounted relative to the housing for rotation relative to the housing and the first component, the center of mass of the second component being offset from its axis of rotation; the first component extends through the axis of rotation of the second component; the first component and/or the second component comprises one or more magnets; the or each magnet is arranged such that during at least part of the rotational path of the second component, at least one magnet on one of the first and second component interacts with the other of the first and second component, or a magnet thereon, to deform the first component and thereby generate electricity.

2. A device according to claim 1, wherein the first component is mounted as a cantilever with one end substantially fixedly mounted relative to the housing.

3. A device according to claim 1, wherein the deformation is bending.

4. A device according to claim 1, wherein the first component comprises a substrate of a resiliently deformable material.

5. A device according to claim 1, wherein the first component comprises electricity-generating material that generates electricity when it is subject to the deformation.

6. A device according to claim 1, wherein the one or more magnets comprise a series of magnets.

7. A device according to claim 1, wherein both the first and second components comprise at least one respective magnet, the magnet on each component interacting with that on the respective other component along at least part of the rotational path of the second component to deform the first component.

8. A device according to claim 1, wherein the first component is arranged, upon receiving an initial displacement, to vibrate at the natural frequency of the electricity-generating material.

9. A device according to claim 1, further comprising an electrical circuit operable to regulate electricity generated by the device.

10. A device according to claim 9, wherein the electrical circuit operable to regulate electricity is arranged to carry out one, more or all of the following or any combination thereof: rectification from AC electricity to DC electricity; converting electricity at a varying voltage to electricity at a more constant voltage; storing energy generated by the device; limiting the current supplied to a load so as to prevent damage to the generator or load or for some other purpose; and supplying power to an attached load.

11. A device according to claim 10, wherein the attached load is a wireless sensor.

12. A device according to claim 1, wherein the first principal component is attached internally or externally to a car tire or a human or animal body or to any other source of motion.

13. A device according to claim 5, wherein the electricity-generating material comprises a piezoelectric material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which:

(2) FIG. 1 illustrates an equivalent circuit of a piezoelectric generator;

(3) FIG. 2 is a schematic illustrating the motion of an eccentric mass around an axis of rotation under different external excitations;

(4) FIG. 3 is the voltage output of a piezoelectric beam left to vibrate after initial excitation;

(5) FIG. 4 illustrates a generator for timepieces as known in the art;

(6) FIG. 5 shows a possible implementation of the invention for timepieces;

(7) FIG. 6 illustrates the first embodiment of the device;

(8) FIG. 7 illustrates a device according to an embodiment of the invention connected to an external source of motion;

(9) FIG. 8 illustrates a device according to an embodiment of the invention connected to a power processing circuit.

(10) In FIGS. 4, 5, 6, 7 and 8, like elements are indicated by like reference numerals.

SPECIFIC DESCRIPTION OF CERTAIN EXAMPLE EMBODIMENTS

(11) FIG. 1 shows the simplest possible equivalent circuit of a piezoelectric generator attached to a resistive load R.sub.L. The piezoelectric element can be described as an AC (alternative current) voltage source V in series connection with a capacitor C.sub.P. The application of Kirchhoff's voltage law around this system gives:

(12) $\begin{array}{cc}V=I_{\mathrm{elec}}.Math.\left(\frac{1}{i{C}_p}+R_L\right)& \left(1\right)\end{array}$
where V is the generated voltage, I.sub.elec is the current around the circuit and is the operation frequency. The generated voltage V in this case is directly proportional to the applied force, stress or strain on the material and the electrical power P dissipated in the load resistor is:

(13) $\begin{array}{cc}P=\frac{V^2{R}_L}{\left({\left(\frac{1}{C_p}\right)}^2+R_L^2\right)}& \left(2\right)\end{array}$

(14) The maximum power transfer theorem yields that the maximum power is achieved when the magnitude of the load impedance matches the magnitude of the impedance of the piezoelectric material, i.e.:

(15) $\begin{array}{cc}{R}_L=\frac{1}{C_p}& \left(3\right)\end{array}$

(16) Turning to FIG. 2, the schematics shown in FIG. 2 help to explain the dynamic behaviour of an eccentric point mass m situated at a distance r from its axis of rotation (the z-axis of the depicted coordinate system in the case of FIG. 2) under gravitational and external excitation. The values r.sub.x and r.sub.y describe the coordinates of the mass in the x and y directions respectively and the forces F.sub.x and F.sub.y are inertial reaction forces acting on the proof mass in these directions as a result of external excitation. The gravity g is acting in negative y direction. The angle is the angular deflection of the proof mass in relation to the y-axis, defined in a mathematically positive direction. The angle is representative for rotational base excitation, i.e. rotation of the inertial frame and is the resulting angle between the mass and the inertial frame, wherein the following relationship holds true:
=+(4)

(17) Furthermore, describes the relative angle between the first and second principal components of the device and is thus the angle, relevant for the generation of electrical energy.

(18) The basic equation of motion for rotation states that the angular acceleration {tilde over ()} multiplied by the mass moment of inertia equals the sum of all n external moments M.sub.i acting on the mass:

(19) $\begin{array}{cc}\tilde{y}.Math.I=\underset{i=1}{\overset{n}{.Math.}}{M}_i& \left(5\right)\end{array}$

(20) In the case of a simple point mass at a distance r from the rotational axis, the moment of inertia around this axis is given:
I=mr.sup.2(6)

(21) Application to the system of FIG. 2 gives:
{tilde over ()}.Math.I=F.sub.xr.sub.y+(F.sub.ymg)r.sub.x(7)

(22) The coordinates r.sub.x and r.sub.y may be calculated as:

(23) $\begin{array}{cc}\{\begin{array}{c}{r}_x=r\sin \\ r_y=r\cos \end{array}& \left(8\right)\end{array}$

(24) From which follows:
{tilde over ()}.Math.I=F.sub.xr cos +(F.sub.ymg)r sin (9)

(25) Which may be rewritten (using (4)) as:
({tilde over ()}+{tilde over ()}).Math.I=F.sub.x cos(+)+(F.sub.ymg)r sin(+)(10)

(26) Equation (10) is representative for the relative rotational motion between the first and second principal components in an undamped case and shows the influence of linear and rotational excitation, be it continuous or discontinuous.

(27) FIG. 3 shows one example of what the open circuit voltage at the terminals of a piezoelectric beam can look like when released after an initial excitation, in an arrangement where the force between the magnets is attractive. The first peak in the graph occurs when the magnet on the second principal component approaches the magnet on the tip of the piezoelectric beam. The tip of the beam receives an initial acceleration towards the approaching magnet and is then held close to it due to the magnetic force, travelling together with the second principal component until the beam bending force exceeds the magnetic attraction force and the beam gets released to vibrate at its own natural frequency after the second peak in the graph.

(28) FIG. 4 illustrates a prior art electromagnetic generator as it is currently employed in wristwatches. An eccentric mass 200 is fixedly connected via a shaft 220 to a large gear wheel 230. The assembly of these components 200, 220 and 230 is connected to an inertial frame 100 via a bearing 210 so that a rotational degree of freedom in relation to the inertial frame is maintained. The large gear wheel 230 is acting on a smaller gear wheel 310, thus effectively increasing the rotational speed of a shaft 350 fixedly attached to a rotor 330. The rotor 330 rotates relatively to a stator 300 holding magnets 320. An arrangement of coils on the rotor 330 makes the generation of electricity possible through the electromagnetic interaction with the magnets 320 in a way as it is well known in the art of conventional generators. The stator 300 itself is connected to the inertial frame 100 via a connection 340. Ultimately a load 360 can be powered by the generated electricity.

(29) FIGS. 5 and 6 illustrate a first embodiment of the present invention as it could be used to replace the prior art generator from FIG. 4. In this case a permanent magnet 230 is fixedly attached to an eccentric mass 200. The eccentric mass 200 is fixedly attached to a shaft 220 and, by means of a bearing 210, is free to rotate around the axis of said shaft 220 in relation to the inertial frame 100. A second permanent magnet 370 is fixedly attached to the tip of a piezoelectric beam 380. The piezoelectric beam 380 does not require any external attachment points other than the connection to the inertial frame 100 via the mounting 340. It will be appreciated that the permanent magnets 230 and 370 can be arranged in various different ways in relation to each other, e.g. such that the magnetic force between them is either repulsive or attractive. Given that the beam 380 is fixed to the inertial frame 100 and the eccentric mass 200 can freely rotate around the axis of shaft 220, a relative motion between the eccentric mass 200 and the beam 380 is possible. Under relative motion between these first and second principal components the interaction of the magnets 230 and 370 will lead to a deflection of the piezoelectric beam 380. Consequently, given the inherent properties of piezoelectric materials, electric energy can be extracted from the beam 380. This energy can then be used to power an external load 360. It will be appreciated that the shown external load 360 is present as an example only and can take on various different forms, such as a sensor or any form of electric circuitry. Furthermore, this load 360 is not necessarily situated within the inertial frame 100 but can, as an example, be connected via external connectors and be situated outside the device.

(30) FIG. 7 shows such an arrangement, where the load 360 is connected outside the presented device. A possible arrangement of the generator is shown, where the inertial frame 100 is fixedly connected to a source of motion 400 by a single means of connection 410. The source of motion 400 here is representative for any possible excitations, for example, but not exclusively, linear, rotational, discontinuous or continuous excitation.

(31) FIG. 8 illustrates another possible application of the invention. A load 360 may require a power supply of a particular form, e.g. a DC (direct current) voltage. The voltage output from the invention may not provide a supply to the exact specification needed by the load 360. In this case a circuit 500 may be connected to the inertial frame 100 via electrical connectors 510. The task of this circuit 500 would be to fulfill the requirements of the load and would achieve one or more of the following: conversion from AC (alternative current) to DC voltage, intermediate energy storage or regulation of the output voltage to a suitable level as well as limiting current as to prevent any connected parts from damage.

(32) In the embodiment of FIG. 6, the beam 380 is shown extending through the axis of the eccentric mass 200. In other embodiments it is envisaged that the beam be mounted to the side of the axis (that is, displaced laterally with respect to the beam in the FIG. 6 embodiment) such that the beam does not extend generally along a radius. This may give packaging advantages in certain applications. The second permanent magnet 270 would still be engaged in the same way in order to deflect the beam 380.

(33) In still other embodiments, the beam 380 may be larger than that of the FIG. 6 embodiment such that it extends across as much of the diameter of the device as is possible. As before, the beam would be fixed at one end and free at the other such that the beam is deflected in substantially the same way.

(34) In another embodiment, the beam may be provided with a non-magnetised material that can be attracted and/or repelled by a magnet, for example a quantity of iron, at its free end in substitution for the second permanent magnet 270 to interact with the first permanent magnet 230 on the rotatable mass 200. Alternatively, this arrangement may be reversed, with the mass 200 being provided with the non-magnetised material and the beam being provided with the permanent magnet.

(35) In another embodiment no permanent magnets may be provided on either the eccentric mass 200 or the beam 380 and instead the beam and the mass may be arranged such that the mass abuts structure of the beam during rotation to deflect the beam to a point at which the abutment ceases and the beam is released to recover its deflection. In other words, the arrangement may make use of mechanical plucking.

(36) In at least some of the previously described embodiments, the device is in the form of a rotational, inertial harvester using piezoelectric beam transduction. Despite the focus on having an eccentric proof mass, the device would still work in a purely inertial way under alternating rotational excitation with a proof mass that has its centre of gravity on the axis of rotation. Embodiments arranged in this way are therefore also envisaged.