WO2009034321A1

WO2009034321A1 - Electrical generator

Info

Publication number: WO2009034321A1
Application number: PCT/GB2008/003071
Authority: WO
Inventors: Eric Morgan Yeatman; Paul David Mitcheson
Original assignee: Imperial Innovations Limited
Priority date: 2007-09-14
Filing date: 2008-09-10
Publication date: 2009-03-19
Also published as: GB0718008D0

Abstract

A device for generating electrical power, the device comprising first and second principal components, the device being operable to generate electrical power as a result of relative rotational motion of the first and second principal components about an axis of relative rotation, wherein: the first principal component is fixedly attachable to a rotating structure in use; the second principal component is rotatable in use with respect to the first principal component; and the second principal component has a centre of gravity offset from the axis of relative rotation, such that the action of gravity on the second principal component impedes rotation of the second principal component relative to the first principal component.

Description

ELECTRICAL GENERATOR

This invention relates to an electrical generator operable to generate electricity from rotational motion. The rotational motion may be continuous or discontinuous (e.g. oscillatory). The invention is particularly applicable, but by no means limited, for use as a micro-generator in applications in which space is limited, and/or in self-powered sensing applications.

Background to the Invention The use of ambient energy harvesting to replace or supplement batteries in wireless sensors, and other electronic applications, has attracted a great deal of research attention. Motion and vibration provide possible power sources, and a wide range of devices have been developed which exploit them [I]. So far, reported devices have generally used internal vibration of a proof mass in a frame, with the transduction mechanism (e.g. piezoelectric) damping the internal motion to extract power. Devices using rotation of a proof mass have also been demonstrated, but these also rely on oscillating source motion [2]. All these devices rely on interaction between the acceleration implicit in oscillations, and linear or rotational inertia of an internal mass, to generate internal forces. Thus they are typically classed as inertial generators.

There are many applications where it could be desirable to mount a self-powered sensor on a rotating body, such as a turbine or pump, or within a vehicle tyre. Since there is no appropriate acceleration inherently associated with continuous rotating motion, inertial generators in this case must rely on speed variations, or vibration associated with the rotation.

A conventional DC (direct current) generator comprises two principal components: a stator which is normally held stationary by attachment to some fixed structure, and a rotor, which rotates during operation relative to the stator, as a result of an applied mechanical torque, for example from a fluid-flow driven turbine. Electromagnetic coupling between the stator and rotor creates a torque, and the relative rotating motion, by doing work against this torque, generates electrical power. Such a generator in its conventional form is clearly well suited to generating electricity from a rotating structure. However, this power cannot be produced without the generator having mechanical attachments to two structures rotating relative to each other. Typically, in order to produce a counter-torque to oppose the electromagnetic torque, one of the two parts (the rotor) is fixed to the rotating structure, and the other part (the stator) is fixed to some non-moving structure. This has implications on the necessary size, and convenience of installation, of the generator. For example, for a small device installed within an automotive tyre, an attachment to the rotating structure (the tyre) is straightforwardly achieved, but simultaneous attachment to a non-rotating structure such as the automobile body is not feasible.

There is therefore a desire for a generator operable to generate electricity from rotational motion of a rotating structure, and which requires mechanical attachment to only a single point on the rotating structure.

A number of devices for generating electrical power from ambient motion are known from background art. For example, in US 6 725 713, a device is described for generating power from the motion of an automobile tyre. This invention is based on the cyclic flexing of a piezoelectric structure in response to the periodic deformation of the tyre during motion. US 6 984 902 describes a device for generating power from motion. This device relies on relative motion between a permanent magnet and a magnetic field sensing element, and thus does not allow an implementation in which attachment to only a single point is sufficient. US 4 644246 describes another device for generating power from its own motion, for incorporation in articles such as wristwatches.

According to a first aspect of the present invention there is provided a device as defined in Claim 1 of the appended claims. Thus there is provided a device for generating electrical power, the device comprising first and second principal components, the device being operable to generate electrical power as a result of relative rotational motion of the first and second principal components about an axis of relative rotation, wherein: the first principal component is fixedly attachable to a rotating structure in use; the second principal component is rotatable in use with respect to the first principal component; and the second principal component has a centre of gravity offset from the axis of relative rotation, such that the action of gravity on the second principal component impedes rotation of the second principal component relative to the first principal component. The terms "impede", "impedes" and "impeded" as used herein should be interpreted broadly, to encompass both reducing rotation of the second principal component relative to the first principal component, and also (as may sometimes be possible) preventing rotation of the second principal component.

Thus, contrary to inertial generators, this invention uses gravitational acceleration to provide the counter-force, rather than inertia. This enables electricity to be generated from rotational motion of a rotating structure (e.g. a wheel or tyre), whilst requiring mechanical attachment of the first principal component to only a single point on the rotating structure. The point of attachment of the first principal component to the rotating structure can be at various positions on the rotating structure; it need not necessarily be on the axis of rotation (and, if it is not on the axis of rotation, it will be appreciated that no mechanical linkages or the like are required to couple the device to the axis of rotation). The invention does require that the axis of rotation have a horizontal component. The counter-torque may be provided by imbalancing the second principal component, for example by providing a discrete eccentric mass on the part that is not fixed to the rotating structure, or by virtue of the part that is not fixed to the rotating structure having an inherently non-uniform mass distribution about the axis of relative rotation.

By requiring attachment to only a single point on the rotating structure, embodiments of the present invention may be easier to install than generators which require attachment to two points. The generator may be used in applications in which attachment to two points would not be possible — such as being attached to a vehicle tyre, for example, or in situations in which the two potential attachment points are not in sufficiently close proximity. This also enables embodiments of the present invention to be relatively small, since they do not require attachment to two points. The relative rotational motion of the first and second principal components may be continuous, or it may be discontinuous (e.g. oscillatory motion of one component relative to the other).

Preferable yet optional features are defined in the dependent claims.

Thus, the first principal component may comprise one or more permanent magnets, and the second principal component may comprise one or more current-carrying coils, the magnet(s) and coϊϊ(s) arranged such that, in use, electricity is generated by the interaction between the magnet(s) and coil(s).

Alternatively, the first principal component may comprise one or more current- carrying coils, and the second principal component may comprise one or more permanent magnets, the magnet(s) and coil(s) arranged such that, in use, electricity is generated by the interaction between the magnet(s) and coil(s).

As another alternative, the first principal component may comprise one or more current-carrying coils, and the second principal component may comprise one or more current-carrying coils, the coils arranged such that, in use, electricity is generated by the interaction between the coil(s) of the first principal component and the coil(s) of the second principal component. In such an arrangement, the coil(s) of one of the principal components may be energised in the manner of an electromagnet, thus enabling it to function as a magnet, and enabling electricity to be generated in the coil(s) of the other principal component.

The first principal component may be arranged radially outwardly from the second principal component, relative to the axis of relative rotation.

Alternatively, the second principal component may be arranged radially outwardly from the first principal component, relative to the axis of relative rotation. The maximum power output from the generator will generally increase as the relative rotational velocity of the first and second principal components increases. For this reason, particularly when the rotational velocity of the source is low, it may often be of benefit to increase this velocity by providing a gearing arrangement between the rotating structure and first principal component. However, in the applications addressed by this invention it will not be possible to fixedly attach the housing of any gearbox to a stationary object or structure.

To overcome this difficulty a gearbox may be introduced between the rotating structure and the first principal component of the generator, with the input axle of the gearbox coupled to the rotating structure in use, and the output axle coupled to the first principal component, and the gearbox housing having a non-uniform mass distribution about its axis of relative rotation such that the centre of mass of the gearbox housing is offset from the axis of rotation, so that gravitational force effects a torque which inhibits rotation of the gearbox housing. Alternatively the gearbox housing may be fixedly attached to, or integral with, the second principal component of the generator, the gearbox housing and second principal component taken together having a non-uniform mass distribution.

As a further alternative, the gearbox housing may be fixedly attached to the rotating structure, with an eccentric mass coupled to its input axle so as to inhibit absolute rotation of this axle, and its output axle coupled to the first principal component of the generator.

In each of the above alternatives the relative rotational velocity of the first and second principal components can be increased with respect to the rotational velocity of the rotating structure as a result of the gearing factor of the gearbox, and therefore may provide an increased level of output power.

References to a "gearbox" herein should be interpreted broadly, to encompass any arrangement which provides a gearing effect. Such arrangements are by no means limited to interconnected cog wheels; those skilled in the art will appreciate that a gearing effect may be achieved in many other ways, for example using wheels of different diameters connected to one another by belts or chains, or opposing cones which frictionally engage one another, or an arrangement in which the input axle is not directly connected to the output axle, but drives the output axle by means of a viscous fluid.

According to a second aspect of the present invention there is provided a device for generating electrical power, the device comprising first and second principal components, and a gearbox having a gearbox housing and including first and second axles, the device being operable to generate electrical power as a result of relative rotational motion of the first and second principal components about an axis of relative rotation, wherein: the gearbox housing is fixedly attachable to a rotating structure in use; the first principal component is fixedly attached to the gearbox housing; the second principal component is rotatable in use with respect to the first principal component; the first axle of the gearbox is fixedly attached to a mass having a centre of gravity offset from the axis of relative rotation, such that the action of gravity on the mass impedes rotation of the first axle relative to the rotating structure in use; and the second principal component is fixedly attached to the second axle of the gearbox.

With both the first and second aspects of the invention, the device may further comprise a voltage regulation circuit arranged to regulate electricity generated by the device. The voltage regulation circuit may comprise a switched mode power supply, and may be arranged to do one or more of: converting AC electricity to DC electricity; converting electricity at a varying voltage to electricity at a more constant voltage; and limiting the current supplied to a load so as to prevent damage to the generator or load or for some other purpose.

The voltage regulation circuit may also act to maximise the power generated by the generator device, by presenting an optimal input electrical impedance to the device.

The dynamic control of input impedance of switch-mode power supplies is well known in the art. In embodiments of the invention, the first and second principal components of the generator may comprise a DC generator for which the output power is maximised at a specific rotational frequency if the input impedance of the load equals the armature resistance of the DC generator (this is known as impedance matching). In such embodiments the regulation circuit may be controlled to provide such impedance matching over a range of rotational frequencies.

Furthermore, the armature current may be monitored and controlled to prevent it from reaching a level at which the motor torque exceeds the maximum gravitational torque that can be provided to counteract it. Then, for rotational frequencies above this level, the input impedance of the regulation circuit may be adapted to maintain the motor torque at or close to the maximum level that does not exceed the gravitational torque limit.

The device may be attached to a rotating structure which rotates continuously in use. For example, the device may be attached to the interior or exterior of a vehicle wheel or tyre. In addition, or alternatively, the device may be arranged to provide power to a wireless sensor.

According to a third aspect of the invention there is provided a method of generating electrical power using a device in accordance with the first or second aspect of the invention.

Brief Description of the Drawings

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

Figure 1 illustrates an equivalent circuit of a DC generator;

Figure 2 is a cross-sectional schematic of a DC generator to which a mass has been attached, and illustrates the torque due to the generator and an offset mass, in accordance with an embodiment of the invention; Figure 3 illustrates a possible laboratory implementation of an embodiment of the invention;

Figure 4 illustrates an electrical generator as known in the art; Figure 5 illustrates a first embodiment of the invention; Figure 6 illustrates a second embodiment of the invention;

Figure 7 illustrates a generator according to an embodiment of the invention, electrically connected to a voltage regulation circuit; and Figures 8, 9 and 10 illustrate further embodiments of the invention incorporating gearboxes.

In Figures 4, 5, 6, 8, 9 and 10, like elements are indicated by like reference numerals.

Detailed Description of Preferred Embodiments

The present embodiments represent the best ways known to the applicants of putting the invention into practice. However, they are not the only ways in which this can be achieved.

DC (direct current) generators provide a convenient system with which to illustrate possible embodiments of the invention. A simple equivalent circuit of a DC generator is shown in Figure 1, with the generator modelled as a voltage source and a series winding resistance, along with an external load.

By applying Kirchhoff s Voltage Law around the circuit in Figure 1, we obtain:

where E_G is the generated voltage, R_A is the armature resistance of the motor, IA is the current through the circuit and Ri is the load resistor [3].

E_G varies with the shaft's angular velocity, ω as:

E_G = K_Eω (2)

Here K_E is a constant often called the motor constant. Hence, the electrical power, P_ei_ec dissipated in Ri is:

⁽³⁾ The maximum P_eι_ec occurs when R_L=R_A- The torque generated is T_gm = K_EI_A .

Figure 2 shows a cross-sectional schematic of a DC generator to which a mass has been attached, and Figure 3 illustrates a possible laboratory implementation of such an arrangement. A mass (designated by the label "Mass" in Figures 2 and 3) is fixed to the stator of the generator, while the rotor of the generator is fixed to a rotating structure which is to be used as the mechanical power source. In this case the axis of rotation is assumed to be horizontal. By attaching the mass to the generator's body, a gravitational torque is produced, of magnitude T_1n = mgLsin(θ) with m representing the mass, g the acceleration of gravity, L the distance from the centre of the mass to the axis of rotation of the generator, and θ the deflection angle. This leads to an expression for the net torque:

Net Torque = I_MΘ = K_EI_A -mgLsin(θ) (4)

I_M is the moment of inertia of the generator's stator, and θ is the second time derivative of deflection angle. If the device is operated such that the stator does not rotate in steady state, then this second derivative will be zero, and so the equilibrium deflection will be that which satisfies mgL s\n(θ) = K_EI_A . Since sin(θ) cannot exceed 1 , the limit of the armature current is therefore given by.

If IA is increased beyond this limit, then the electromagnetic torque will be greater than can be compensated by the gravitational torque, and the stator will not remain at an equilibrium deflection but will rotate continuously. The maximum power achievable without stator rotation is given by the corresponding maximum torque multiplied by the rate of rotation in radians per second, ω, so that:

P(max) = mgLω (6) Figures 4, 5 and 6 illustrate the differences between a conventional prior art generator and embodiments of the invention in more detail.

Figure 4 shows an electrical generator as known in the art. Generation is achieved by the rotation of a rotor 120 with respect to a stator 110, which are mechanically coupled via bearings 130. The axis of rotation of the rotor 120 is indicated by the line

150. The rotor 120 is typically coupled to a source of rotation 100 and the stator 110 is attached to a fixed body or surface 140. Various forms of electromagnetic coupling between the rotor 120 and stator 110 are employed to produce electrical power, as is well known in the art.

A first embodiment of the present invention is illustrated in Figure 5. Electrical generation is achieved by relative rotation of a rotor 120 with respect to a stator 110, which are mechanically coupled via bearings 130. The rotor 120 is mechanically coupled to a rotating structure 100. The stator 110 is not directly attached to any structure external to the device. The axis of rotation of the rotating structure 100 may be the same as the axis of rotation 150 of the rotor 120, or may be different. It is required that the axis of rotation of the rotating structure 100 is parallel to the axis of rotation 150 of the rotor 120, or has a geometric component which is parallel to the axis of rotation 150 of the rotor 120. It is also required that the axis of rotation of the rotating structure 100 and the axis of rotation 150 of the rotor 120 are horizontal or have geometric components which are horizontal. The rotor 120 will rotate as a result of its coupling to the rotating structure 100. The rotation of the stator 110 is impeded by gravity as a result of its centre of mass not lying on the axis of rotation 150. The deviation of the centre of mass of the stator 110 from the axis of rotation 150 may be inherent to its design, or may be effected or enhanced by the addition of a mass 160.

A second embodiment of the present invention is illustrated in Figure 6. As with the embodiment of Figure 5, electrical generation is achieved by relative rotation of a rotor 120 with respect to a stator 110, which are mechanically coupled via bearings 130. However, in the embodiment of Figure 6, the stator 110 is mechanically coupled to a rotating structure 100, and the rotor 120 is not directly attached to any structure external to the device. The axis of rotation of the rotating structure 100 may be the same as the axis of rotation 150 of the rotor 120, or may be different. It is required that the axis of rotation of the rotating structure 100 is parallel to the axis of rotation 150 of the rotor 120, or has a geometric component which is parallel to the axis of rotation 150 of the rotor 120. It is also required that the axis of rotation of the rotating structure 100 and the axis of rotation 150 of the rotor 120 are horizontal or have geometric components which are horizontal. The stator 110 will rotate as a result of its coupling to the rotating structure 100. The rotation of the rotor 120 is impeded by gravity as a result of its centre of mass not lying on the axis of rotation 150. The deviation of the centre of mass of the rotor 120 from the axis of rotation 150 may be inherent to its design, or may be effected or enhanced by the addition of a mass 160.

Use of the electrical power generated by the present invention may be effected by connection of an electrical load to the electrical output terminals of the generator. The electrical power so provided may in some instances be at a voltage which is dependent on the relative rotational speed of the rotor 120 and stator 110 when driven by an external rotation source, or on some other parameter. The electrical power may also be provided in alternating current (AC) form. It may be desirable to supply the electrical power to the load in direct current (DC) form at a fixed voltage at some predetermined level. Figure 7 illustrates an implementation of the invention which may satisfy this requirement. The generator 170 has electrical output connections 190 which are connected to the input terminals of a voltage regulation circuit 180. An electrical load may then be connected to the output terminals 195 of the voltage regulation circuit 180. Such voltage regulation circuits are known in the art and may for example be a circuit of the type known as switched mode power supplies. The functions of the voltage regulation circuit 180 may include one or more of: converting AC electricity to DC electricity; converting electricity at a varying voltage to electricity at a more constant voltage; limiting the current supplied to a load so as to prevent damage to the generator or load or for some other purpose; presenting an optimal input impedance to the generator device to maximise its output power; or limiting the motor torque to a value not more than the maximum gravitational torque available. A third embodiment of the present invention is illustrated in Figure 8. The arrangement is as illustrated in Figure 5 with the addition of a gearbox 200 between the rotor 120 and the rotating structure 100. The gearbox 200 is of a type such that the rotational velocity of its output axle 220 is greater than that of its input axle 210. Rotational motion of the housing of the gearbox 200 is impeded by a deviation of its centre of mass from its axis of rotation. This deviation of the centre of mass of the gearbox 200 from its axis of rotation may be inherent to its design, or may be effected or enhanced by the addition of a mass 260. As a variant on the arrangement of Figure 5, the gearbox housing may be affixed to the stator 110, such that rotation of bom the gearbox housing and the stator 110 is impeded by a single discrete eccentric mass or by a deviation of the centre of mass of the combined structure from its axis of rotation. As a result of the gearbox arrangement, the rotational velocity of the rotor 120 may be higher than that of the rotating source 100. It will be appreciated that a corresponding gearbox arrangement may also be applied between a rotating structure and a generator configured as illustrated in Figure 6.

A fourth embodiment of the present invention is illustrated in Figure 9. The arrangement is as illustrated in Figure 5 with the addition of a gearbox 200. The gearbox 200 is of a type such that the rotational velocity of its output axle 220 is greater than that of its input axle 210. The housing of the gearbox 200 is affixed to the rotating structure 100 such that the gearbox 200 rotates at the same velocity as the rotating structure 100. The rotation of the gearbox input axle 210 is impeded by an offset mass 260. The rotor 120 is connected to the gearbox output axle 220. As a result of the gearbox arrangement, the rotational velocity of the rotor 120 may be higher than that of the rotating structure 100. It will be appreciated that a corresponding gearbox arrangement may also be applied between a rotating structure and a generator configured as illustrated in Figure 6.

A fifth embodiment of the present invention is illustrated in Figure 10. The gearbox 200 is of a type such that the rotational velocity of its output axle 220 is greater than that of its input axle 210. The housing of the gearbox 200 is affixed to the rotating structure 100 such that the gearbox 200 rotates at the same velocity as the rotating structure 100. The rotation of the gearbox input axle 210 is impeded by an offset mass 260. The rotor 120 is connected to the gearbox output axle 220. The stator 110 is affixed to the housing of the gearbox 200 or to the rotating structure 100. As a result of the gearbox arrangement, the rotational velocity of the rotor 120 may be higher than that of the rotating source 100.

One possible implementation of an embodiment of the present invention would be to incorporate it in a micro-engineered power generation mechanism of the type described by Holmes et al. [4]. Other implementations are also possible.

REFERENCES

[1] Paul D. Mitcheson, et al., "Architectures for Vibration-Driven Micropower Generators", Journal of Microelectromechanical Systems, vol. 13, pp.429 - 440, 2004,

[2] Eric M. Yeatman, "Energy Scavenging from Motion Using Rotating and Gyroscopic Proof Mass", Journal of Mechanical Engineering Science, vol. 222, pp. 27-36, 2008.

[3] USA Electro-Craft Corporation, DC Motors, Speed Controls, Servo Systems. Oxford, New York, Toronto, Sydney, Paris, Frankfurt, Pergamon Press Ltd. 2-7 - 2- 96, 1977.

[4] Andrew S. Holmes, et al., "Axial-Flux Permanent Magnet Machines for Micropower Generation", Journal of Microelectromechanical Systems, vol. 14, pp. 54 - 62, 2005.

Claims

1. A device for generating electrical power, the device comprising first and second principal components, the device being operable to generate electrical power as a result of relative rotational motion of the first and second principal components about an axis of relative rotation, wherein: the first principal component is fixedly attachable to a rotating structure in use; the second principal component is rotatable in use with respect to the first principal component; and the second principal component has a centre of gravity offset from the axis of relative rotation, such that the action of gravity on the second principal component impedes rotation of the second principal component relative to the first principal component.

2. A device as claimed in Claim 1, wherein the first principal component comprises one or more permanent magnets, and the second principal component comprises one or more current-carrying coils, the magnet(s) and coil(s) arranged such that, in use, electricity is generated by the interaction between the magnet(s) and coil(s).

3. A device as claimed in Claim 1, wherein the first principal component comprises one or more current-carrying coils, and the second principal component comprises one or more permanent magnets, the magnet(s) and coil(s) arranged such that, in use, electricity is generated by the interaction between the magnet(s) and coil(s).

4. A device as claimed in Claim 1, wherein the first principal component comprises one or more current-carrying coils, and the second principal component comprises one or more current-carrying coils, the coils arranged such that, in use, electricity is generated by the interaction between the coil(s) of the first principal component and the coil(s) of the second principal component.

5. A device as claimed in any of Claims 1 to 4, wherein the first principal component is arranged radially outwardly from the second principal component, relative to the axis of relative rotation.

6. A device as claimed in any of Claims 1 to 4, wherein the second principal component is arranged radially outwardly from the first principal component, relative to the axis of relative rotation.

7. A device as claimed in any preceding claim, wherein the inherent structure of the second principal component has a non-uniform mass distribution about the axis of relative rotation, thereby offsetting the centre of gravity of the second principal component from the axis of relative rotation.

8. A device as claimed in any preceding claim, wherein the second principal component incorporates a discrete eccentric mass, thereby offsetting the centre of gravity of the second principal component from the axis of relative rotation.

9. A device as claimed in any preceding claim, further comprising a gearbox interposed in use between the rotating structure and the first principal component, the gearbox arranged to provide a relative rotational velocity between the first and second principal components which is greater than the absolute rotational velocity of the rotating structure in use.

10. A device as claimed in Claim 9, wherein: the gearbox includes input and output axles and a gearbox housing; the input axle is coupled to the rotating structure in use, and the output axle is coupled to the first principal component; and the gearbox housing has a non-uniform mass distribution about its axis of relative rotation, or the gearbox housing incorporates a discrete eccentric mass, thereby offsetting the centre of gravity of the gearbox housing from its axis of relative rotation.

11. A device as claimed in Claim 9, wherein: the gearbox includes input and output axles and a gearbox housing; the input axle is coupled to the rotating structure in use, and the output axle is coupled to the first principal component; the gearbox housing is fixedly attached to, or integral with, the second principal component; and the gearbox housing and second principal component taken together have a non-uniform mass distribution about the axis of relative rotation, or the gearbox housing and second principal component taken together incorporate a discrete eccentric mass, thereby offsetting the centre of gravity of the combined structure from its axis of relative rotation.

12. A device as claimed in Claim 9, wherein: the gearbox includes input and output axles and a gearbox housing; the gearbox housing is fixedly attached to the rotating structure in use; and the output axle of the gearbox is coupled to the first principal component, and the input axle of the gearbox is coupled to a discrete eccentric mass having a centre of gravity offset from the axis of relative rotation of the gearbox input axle.

13. A device for generating electrical power, the device comprising first and second principal components, and a gearbox having a gearbox housing and including first and second axles, the device being operable to generate electrical power as a result of relative rotational motion of the first and second principal components about an axis of relative rotation, wherein: the gearbox housing is fixedly attachable to a rotating structure in use; the first principal component is fixedly attached to the gearbox housing; the second principal component is rotatable in use with respect to the first principal component; the first axle of the gearbox is fixedly attached to a mass having a centre of gravity offset from the axis of relative rotation, such that the action of gravity on the mass impedes rotation of the first axle relative to the rotating structure in use; and the second principal component is fixedly attached to the second axle of the gearbox.

14. A device as claimed in any preceding claim, further comprising a voltage regulation circuit arranged to regulate electricity generated by the device.

15. A device as claimed in Claim 14, wherein the voltage regulation circuit comprises a switched mode power supply.

16. A device as claimed in Claim 14 or Claim 15, wherein the voltage regulation circuit is arranged to do one or more of: converting AC electricity to DC electricity; converting electricity at a varying voltage to electricity at a more constant voltage; limiting the current supplied to a load so as to prevent damage to the generator or load or for some other purpose; presenting an optimal input impedance to the device to maximise its output power; and limiting the motor torque to a value not more than the maximum gravitational torque available.

17. A device as claimed in any preceding claim, attached to a rotating structure.

18. A device as claimed in Claim 17, wherein the rotating structure rotates continuously in use.

19. A device as claimed in Claim 18, wherein the rotating structure is the interior or exterior of a vehicle wheel or tyre.

20. A device as claimed in any preceding claim, arranged to provide power to a wireless sensor.

21. A method of generating electrical power using a device as claimed in any preceding claim.

22. A method as claimed in Claim 21, for providing power to a wireless sensor.

23. A device substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.

24. A method of generating electrical power substantially as herein described with reference to and as illustrated in any combination of the accompanying drawings.