TITLE

MINIATURISED GENERATOR FOR THE PRODUCTION OF ELECTRICAL ENERGY

FROM VIBRATIONS DESCRIPTION Field of the invention

The present invention relates in general to the field of the energy production, more particularly through recovery from sources available in the environment. More specifically the invention relates to a device for converting the energy associated with vibrations into electrical power. Description of the state of the art

Among the various environmental sources from which energy, otherwise lost, can be recovered, vibrations have attracted broad interest due to their abundance and diffusion.

Most devices proposed to date which enable vibrational energy to be recovered use a mobile mass connected to an elastic member. Under the action of external vibrational field, the mass starts to oscillate and the oscillatory motion allows electrical power to be collected basically in two ways: through a piezoelectric generator (see for example patents FR2838885 and JP2005335692) or through induction via the magnetic coupling of a mobile part (magnet or coil) and a fixed part (coil or magnet).

For example, in US 7009315 the environmental vibrations are applied to a copper coil and are transmitted via a spring to a magnet, causing a relative movement between the copper coil and the magnet. Other examples of systems of this type are described in WO2005022726, GB231 1 171 and JP8065992.

Devices of this kind effectively produce electrical energy only when immersed in vibrational fields having a narrow spectrum which is regular in time, such as that shown in Figure Ia. Devices of this type would be insensitive to pulsed stimulations, that is to say characterised by low duty cycles and high intensity, such as those shown in the vibrational spectrum of Figure Ib.

The power spectrum of in a vibration field extends, in general, over a range of frequencies [f _m j _n , f _max ]. In order to be able to convert into electricity the maximum energy available, the recovery device (hereinafter also referred to as harvester) should be effective in a range of frequencies, included in [f _m j _n , f _max ], which is as wide as possible. For an electromechanical system with its own resonance frequency f _n , the energy conversion efficiency is maximum only for frequencies of the spectrum in the region of f _n given that

these vibrations are able to generate resonance phenomena with the mechanical structure. To widen the range of spectral frequencies which contribute significantly to the production of electrical energy, structures free from inherent resonances should be made. One device operating in this way is described, for example, in US 7009310. This system involves the sliding of mobile masses inside appropriate hollow guides, on whose surface the electrical windings are arranged, at the terminals of which the EMF (electromotive force) is collected.

The systems of this type, however, produce an EMF proportional to the speed of the mobile member. In fact the known law of Faraday-Lenz lays down that the EMF at the terminals of the winding is proportional to the derivative of the magnetic flux φ and to the number of coils, N, coupled to φ, according to the equation: f.e.m. = -N^- (1)

The variation in coupled flux is due to the movement of the magnet so that it increases with the speed of the latter.

Given a field of accelerations, of average value A, the maximum speed which can be reached is of the order of: AT, T being the average time during which the mass is exposed to the acceleration A.

Given that, as mentioned, the EMF is proportional to the speed of movement of the magnet, as a result it is possible to generate significant EMF only for long T, i.e. for sufficiently slow vibrational fields. As proof of this the typical application of the systems of the type described in patents such as US 7009310 is the extraction of energy from sea wave motion.

The applications of devices for the conversion of vibrational energy into electrical energy range from the supply of other energy sources such as rechargeable batteries, contributing in this way to extending their duration, to the elimination of wiring for electric devices installed far from sources of supply, to the power supply of mobile electronic instruments or wireless devices for communication and control.

In many cases the need to provide a independent electrical supply source to devices of this type is associated with the need to restrict as far as possible the dimensions of the energy generator. The possibility of miniaturising the energy generator requires high structural simplicity with the smallest possible number of moving parts and with extremely restricted movements. For example, in the case of the device according to the

aforementioned patent US 7009310, a significant variation in the magnetic induction flux can only be obtained with relatively high strokes of the mobile member. Objects and summary of the invention

The general object of the present invention is to provide a device for converting into electrical energy the mechanical energy associated with vibrations of all types, even of the pulsed type, that is to say which can be compared to an impact.

A particular object of the present invention is to provide a device of the type mentioned above which allows the conversion of mechanical energy associated with vibrations into electrical energy via a miniaturised system, that is to say having characteristic dimensions of the order of a centimetre and which can easily be scaled down to lower dimensions, up to typical dimensions of the order of a millimetre.

A further object of the present invention is to provide a miniaturised generator device for the conversion of vibrational energy into electrical energy having a simple and reliable constructional solution, in which there are no mobile electrical connections and rotating components.

These objects are achieved with the device for the conversion of mechanical energy associated with vibrations into electrical energy according to the present invention, comprising at least one ferromagnetic stator with a coaxial winding and a magnetic mass that is mobile in relation to the stator, the magnetic mass and the stator with the relative winding forming a magnetic circuit. The magnetic mass is suitable for moving along a path lying on a plane perpendicular to the axis of the winding in response to vibrations generated by a vibrating means with which the device is associated, so that the magnetic circuit has a correspondingly variable gap width, elastic means being provided at the end-strokes of the mobile magnetic mass, the winding being electrically connected to means for storing the energy produced via a conditioning circuit.

Brief description of the drawings

The features and advantages of the device for the conversion of mechanical energy associated with vibrations into electrical energy according to the present invention will be apparent by the following description of its embodiments, given by way of a non-limiting examples with reference to the accompany drawings, in which:

Figures Ia and Ib illustrate two different examples of intensity of vibration fields vs time;

Figure 2 shows the basic diagram of the device according to the invention;

Figure 3 shows the functional block diagram of the device according to the invention;

Figure 4 shows an embodiment of the device according to the invention wherein two permanent magnets are used, fixedly connected one to the other;

Figure 5 shows the diagram of a magnetic circuit with flux divider; Figure 6 schematically shows the device according to the invention with flux divider according to the diagram in Figure 5;

Figures 7a, 7b and 7c show possible embodiments of the structure of Figure 6 in which the variation in thickness of the gap is made possible by the deformation of the ferromagnetic structure; Figure 8 shows a variation of the diagram of Figure 6 in which two windings are used;

Figure 9 shows another embodiment of the invention with generally shaped polar expansions;

Figure 10 shows a further embodiment in which the magnet moves relatively to the stator, rolling on a suitable surface; Figures 1 Ia and 1 Ib show two possible diagrams of an impedance converter.

Detailed description of the invention

Referring to Figure 2, the device for generating electrical power according to the invention comprises schematically a box-like casing 10 wherein a mobile magnet 11 is placed, slidingly mounted on a low-friction guide 12, formed for example by a metal bar and by a slider in self-lubricating material such as bronze or polytetrafluoroethylene, extending at right angles from a wall 13 of the casing 10. The sliding of the mobile magnet 11 takes place between the wall 13 and a ferromagnetic stator 14 attached to the casing 10 and holding a winding in copper 15, whose axis X is perpendicular to the guide 12. Elastic end- of-stroke members 16 and 17 are provided on the wall 13 and on the stator 14. The elastic members 16, 17 can be formed by common helical springs or flat springs or functionally equivalent means such as, for example, bearings in an elastic polymeric material.

The casing 10 is intended to be connected to a vibrating means schematised in 18, so that, following sliding of the mobile magnet 11, due to the vibrational field generated by the vibrating means 18, an EMF is induced in the winding 15 which, via an electrical connection 19, is sent to energy storage means 20 via an optionally programmable aconditioning electronics 21.

This solution has the advantage of effectively collecting the available power, also using a miniaturised device, by exploiting the principle of the variable reluctance. When the

magnetic circuit is open, that is to say the mobile magnet 11 is far from the wound ferromagnetic stator 14, the overall reluctance of the magnetic circuit is high. When the magnetic circuit is closed, that is to say the mobile magnet 12 is in contact with the stator 14, the overall reluctance of the magnetic circuit is low. As a result the position of the mobile part influences the overall reluctance of the magnetic circuit and therefore the value of magnetic flux coupled to the winding.

The capacity for generating high magnetic forces on small dimensions of the device is at the basis of the greater efficacy of energy conversion of the device described here compared to those already known in the art. To reduce the constant reluctance of the magnetic circuit it is necessary to reduce the length of the magnet. For this purpose, according to the invention, use is made of the configuration shown in Figure 4. In this embodiment two permanent magnets 23a, 23b, placed at the end of a mobile mass 22 and integral thereto, have a total length, i.e. that obtained from the sum of the length of each of the two, lower than the length of the mobile mass. In this way the overall reluctance of the magnetic circuit is smaller than that relating to the circuit in Figure 2. Also in the configuration of Figure 4 elastic members (not shown) are provided for increasing the coefficient of elastic return following the end-of-stroke impacts.

A further implementation, which allows avoidance of the disadvantage represented by the high constant reluctance associated with permanent magnets, involves a magnetic flux divider according to what is illustrated in Figure 5 wherein a magnetic circuit with flux divider is schematised. Rn and R _f2 represent the reluctances of the sections in iron, while Rt represents the variable reluctance of the gap. The permanent magnet, fixed and integral with the frame in the diagram of Figure 5, is schematised as a flux generator φ ₀ = M ₀ /R _mag . The winding is placed around one of the two loops which make up the magnetic circuit. The flux produced by the magnet is divided between the two loops in accordance with the subsequent formula (2). As can be seen from (2), the flux in one branch does not depend on the reluctance of the permanent magnet (R _ma g). The use of the configuration with flux divided therefore allows, as previously mentioned, avoidance of the disadvantage represented by the high value of R _mag .

Figures 6 and 7a,b,c illustrate possible uses of the magnetic circuit of Figure 5. In Figure 6 the flux generated by the permanent magnet 11 is divided along two possible magnetic circuits formed in this embodiment by a U-shaped stator 25 and a bar 24

(mobile mass). The position of the mobile mass controls the division of the flux and its variation in time allows an EMF to be collected at the terminals of the winding 15. The motion of the mobile ferromagnetic mass 24 is guided by appropriate systems, not shown, such as a guide in a low friction material integral with the casing or with the stator 25. The casing, and the stator 25 integral thereto, is connected rigidly to the vibrating base.

Figures 7a, 7b and 7c show other possible embodiments of the variable reluctance device with magnetic flux divider. The frame of the structure is designed in such a way as to have an intrinsic elasticity, such that the inertial actions due to the vibrations, acting both on the frame and on the winding 15, cause deformation of the frame, which in turn is responsible for a variation in the width of the gap. To achieve an intrinsic yielding of the structure frame, advantageously according to the embodiments of Figures 7a, 7b and 7c, the section of the frame 27 affected by stresses with a flexural component is elongated. In these embodiments the mobile parts do not require guides. In Figures 7a, 7b and 7c the vibrating means is connected to the lower face of the device. Following the deformation produced by the inertia forces, the frame not connected to the base undergoes deformation as a result of which the width of the gap and hence the reluctance of the magnetic circuit vary. The performances of a system with flux divider can be assessed as follows. Denoting with the subscript 2 the branch of the magnetic circuit on which the winding is placed, the flux through this branch is obtained from:

Formula (2) shows that the flux varies between φ ₀ (when ^R fel ^{+ R} fe2 ^{<< R} t ) and approximately Vz φo (when R _t = 0). In other words the flux variation is of the order of O.5φo.

To obtain flux variations close to 100%, the embodiment of Figure 8 can be used or its variants shown in Figures 9 and 10. In the embodiment of the invention shown in Figure 8 two ferromagnetic stators 26a and 26b are shown with relative windings 15a and 15b, arranged symmetrically on opposite sides in relation to a mobile magnet 28 supported between two pairs of elastic members 16a, 17a and 16b, 17b having one of their ends attached to the magnet 27. When the mobile magnet 28 is close to the upper end of stroke, the magnetic flux is coupled almost completely to the upper stator 26a, while the flow linked to the lower stator 26b is minimal. Vice versa, when the mobile magnet 28 is close to the lower end of stroke, the magnetic flux is coupled almost completely to the lower stator 26b, while the flux coupled to the upper stator 26a is minimal. In this way the complete variation

of flow in the two wound cores 26a and 26b is obtained (from almost zero to the maximum flux allowed by magnetic saturation) with always limited strokes.

Summarising, in relation to the solution of Fig. 2, simpler from the constructional standpoint, the solution of Figure 8 enables a greater variation in magnetic flux to be achieved.

In all the above embodiments, the magnetic force on the magnet varies with its position in relation to the stators. In the embodiment of the invention shown in Figure 9 the magnetic force is controlled by acting on the geometry of the ferromagnetic parts. This allows an improvement in the performances of the harvester for certain acceleration profiles, for example in the case wherein it is appropriate to compensate possible constant accelerations. According to this embodiment the casing of the device is formed by two, substantially U-shaped and opposite ferromagnetic structures 31 and 32, wherein a magnet 33 is free to move along a guide 34, extending between two opposite bases 31a and 32a of the ferromagnetic structures. On their bases 31a and 32a respective windings 35 and 36 are wound, whose axes are therefore at right angles to the guide 34. The EMF produced is collected at the terminals of the windings 35 and 36. Elastic end-of-stroke members 37, placed at the ends of the guide 34, absorb and return part of the kinetic energy of the magnet at the motion reversal. The ferromagnetic structures 31 and 32 are traversed by magnetic fluxes which vary according to the position of the magnet 33 and exchange with it a non- zero force of attraction in the direction of motion of the magnet (magnetic force, F _mag ).

The possibility of properly shaping the polar expansions allows the best exploitation of the spectrum of power of the vibrational fields or to meet specific application needs.

A variation of the embodiment of the invention in Figure 9 is shown in Figure 10. In this figure the device according to the invention is shown in a cross section taken on a plane perpendicular to the direction of magnetisation of the magnet 44. The windings 45 and 46 are arranged so as to couple to the magnetic flux which traverses the respective magnetic structures 41 and 42. The elastic end-of-stroke members 47 and 48 are positioned so as not to dissipate kinetic energy during impact. The width of the gap between the ferromagnetic cores 41 and 42 and the magnet 44 is varied by means of a wall 49 with variable profile, in particular uniformly slanting due to an increasing thickness between one end of stroke and the other of the magnet, to obtain a magnetic force with a desired trend along the stroke of the magnet which is so shaped as to roll without dragging on the wall.

The advantage of this configuration compared to that shown in Figure 9 is the significant friction reduction, thanks to the magnet rolling motion which allows dissipation of energy through friction to be reduced to a minimum.

In all the above described devices either the mobile part or the fixed parts are provided with appropriate elastic members with the function of increasing the mechanical energy return coefficient mechanical energy associated with impact of the mobile mass at the ends of stroke. Given their construction, these elastic members do not connect the mobile part to the fixed part and therefore do not determine inherent frequencies of the system. While the spring-mass type systems have an intrinsic resonance frequency, the frequency at which the mobile mass of the device of the invention moves is a function of the kinetic energy absorbed from the vibrational fields.

Figure 2 shows that, in comparison to most similar devices present in the art, the device of the invention exhibits a considerable structural simplicity in all the above described embodiments. In fact, transmissions are not required for modifying the properties of motion. This positively reflects on the complexity of the production and assembly of the various components with significant saving in manufacturing costs. The guide of the mobile parts can be implemented through an appropriate mechanical system (e.g. linear sliding or rolling guides), through the use of ferromagnetic frames with controlled yielding (as in Figures 7a, 7b and 7c), or through the appropriate generation of a field of magnetic forces. In brief, the device according to the present invention offers the following advantages compared to other devices according to the known art.:

- possibility of miniaturisation: the magnetic induction flux variations are proportional to the position of the mobile mass and not to its speed, which allows a more compact device to be obtained, without reducing the EMF; - flexibility of use: thanks to its small size, the system can be used on board portable devices and, in general, can be mounted in all those systems which cannot be modified invasively; lack of specific resonance frequencies: the conditions of motion of the mobile mass adapt to the properties of the vibrational field, without significant frequency filterings; in particular, the device is able to absorb effectively the energy associated with pulsed accelerations;

- high constructional simplicity: low cost of manufacture, robustness and reduced maintenance requirements.

Figure 3 shows a functional block diagram of the device according to the invention.

The vibrational mechanical energy is converted into electrical energy in the conversion device I ₅ rectified in a synchronous rectifier 2 and stored in a capacitor 3. From the capacitor the electrical energy can be powered to a load 4 through a voltage regulator 5. Any other means of storage of the electrical energy can be provided as an alternative to the capacitor 3, for example a rechargeable battery.

Batteries have a far higher energy density than capacitors. For example rechargeable lithium ion batteries have an energy density of roughly 1000 J/cm ³ . Ceramic capacitors have an energy density of approximately 1-10 J/cm ³ . However most lithium ion batteries are limited to 500-1000 recharge cycles, have a limited shelf life with need for their replacement after 1-2 years of operation and perform better when a specific charge-up profile is followed. In particular, lithium ion batteries perform better when charged at constant current. This type of charge-up profile is simply not possible using a vibration generator unless sophisticated battery charging circuitry is used. Preferably, in order to monitor the amount of electrical energy stored in the storage device 3 and manage this energy when it exceeds a certain threshold value, the device is associated with a controller 6.

In order to extract the maximum quantity of energy from the source (with output impedance Zout) and transfer it as much as possible to the load, it is important to observe at all times the adaptation condition between source and load. Supposing for the sake of simplicity that the impedance of the diodes of the synchronous rectifier 2 is negligible, the adaptation condition is observed by placing a so-called impedance converter in between, i.e. a dual-port 6, whose input impedance Zn is worth Zout* and its output impedance Z22 is worth Zin*, Zin being the impedance of the buffer +DC/DC converter assembly. Since it has to be supposed that both Zout and Zin vary in time, the block 6 should in principle be composed of a control circuit composed of an estimator of Zout and Zin which reacts, dynamically changing Zi 1 and Z22. Probably an active dual port which strictly maintains the adaptation conditions would be too complex and would consume too much, annulling its same potential advantages. The practical embodiment consists of a simplified version which ensures in any case an increase in the overall energy efficiency. More particularly a buck- boost DC-DC converter used discontinuously with open loop (open-loop DCM) can approximately produce the condition of adaptation Zn=ZoUt* and effectively transfer energy from the scavenger 1 to the storage device 3.

A simplified diagram is shown in Figure 11. Two clocks in counterphase CKl and

CK2 connect the inductor Ll alternatively between source and mass (phase 1) and between mass and output (phase 2). The MOS transistors Ml and M3 create the connection of phase 1 ; the transistor M2 and the diode Dl that of phase 2. The energy accumulated in Ll during phase 1 is transferred to the output during phase 2. If the input voltage varies slowly in relation to the clock frequency and if during phase 2 the inductor can be "discharged" completely, it can be demonstrated that the input is loaded by an equivalent resistance R _eq = 2Lf _ck /D ² , where L is the value of the induction, / _c/c is the clock frequency and D is the duty cycle of the clock signal. The filter Rl-Cl dampens the oscillations at high frequency which occur when, during phase 2, the current of the inductor drops to zero and the diode breaks, converting into a parasitic capacitance. Both the clock frequency and its duty cycle should be chosen so that the Req is equal to the output impedance of the scavenger (virtually resistive). The capacitance C2 has the task of dampening the ripple which otherwise would originate in the input voltage and which would have the effect of making the transfer of energy less efficient.

Since this is a circuit operating in switching regime, it comprises a series of auxiliary blocks such as the clock generator, which absorb energy and have to be actuated only in the case of need. For this reason the block 6 also comprises a pulse detector which actuates the block only if the scavenger is supplying voltage. The energy efficiency of the impedance converter is conditioned by the losses due to the ohmic effect in the transistors M1-M3 and in the diode Dl: the more their number is reduced, the higher the efficiency. An alternative embodiment of the impedance converter is shown in Figure 1 Ib; the functioning is similar to the circuit of Figure l la, however phase 1 uses only the transistor Ml and phase 2 only the diode Dl. Although definitely more efficient, this circuit generates an output voltage with the sign inverted compared to the input one. As can be seen in Figure l ib, the negative of the output voltage does not coincide with that of the input voltage and therefore two different ground lines will be present, an input one and an output. It can be demonstrated that the input ground voltage becomes negative in relation to that of output and this greatly complicates the implementation in an integrated circuit.

The storage device 3 acts as a storage unit for energy between the vibration generator 1 and the load 4. The load will preferably be with intermittent functioning. For example it may be a control circuit which periodically detects and transmits data, alternating periods of

rest, in which the energy consumption is very low, with periods of activity, in which it dissipates much more energy than the vibration generator can produce. Therefore a drop in potential in the storage device is observed when the load is active and an increase when it is at rest. This leads to the need for the DC/DC converter 5 for converting the variable voltage in output from the storage device 3 into constant voltage for the load 4.

The key element in the design of the AC/DC converter 2 is represented by the fact that the input signal has voltage peaks depending on the specific kinetic energy profile of the different applications and for a specific application the peak can range from 1 to 4 V. This low output voltage makes it challenging to develop rectifier circuits that are efficient as many half wave or full wave diode rectifiers require non-zero turn-on voltages to operate. In order to achieve good power transfer efficiency a non-standard approach is necessary of a full-wave diode rectifier, but instead of a synchronous rectifier or current wave rectifier. This type of rectification allows good efficiency to be achieved in the range of 70-80 %. With a typical nMOS transistor threshold voltage of 0.4 V, there is a significant reduction in the output voltage of the rectifier and of the overall efficiency. A synchronous rectifier circuit constitutes a way of overcoming this limitation. This circuit considerably reduces the equivalent diode voltage drop, as a conducting MOSFET has a lower voltage drop than a conducting diode (even a Schottky diode).

Moreover account has to be taken of that fact that, using a scaled technology such as a 130nm or 90nm CMOS, a dual gate oxide device is necessary to handle a 4V signal amplitude and a triple well that allows insulating PWELL from the substrate to handle -4 V signal amplitude. In the 90nm CMOS process, for example, a different gate oxide provides from 1.2V to 3.3V as power supply with a maximum rating up to 5.2 V.

In the design of the capacitor it has to be taken into account that it must be large enough to source the necessary current to the load when it turns on, without dropping the input voltage to the DC/DC converter below an acceptable value, and that as much energy as possible can be stored. For this purpose the use of supercapacitors with capacity exceeding 1 F/cm ³ may be advantageous. Moreover account must also be taken of the restraints of overall dimensions of the entire system. As regards the DC/DC converter, usually, if the input voltage is always higher than the output voltage, the solution is a buck-converter topology, whereas if the input voltage is always lower than the output voltage, a boost configuration is required. Instead, in the case wherein the input voltage ranges above and below the output voltage, as in the case of the

present invention, either a buck or boost converter can be used, but it must be adapted to the non-standard input voltage range. These converters are known as switching regulators, in that fully on or off switching devices alternatively store or deliver energy to the load via inductors and capacitors. Viewed from a different perspective the LC components filter the inherent switch wave forms of the circuit and the duty cycle of these wave forms is in turn normally regulated by a PWM controller or by another switching scheme. The PWM technique affords high efficiency over a wide load range. Moreover, since the switching frequency is fixed, the noise spectrum is relatively narrow and this allows a considerable reduction in the peak-to-peak voltage ripple with simple low-pass filtering techniques. An aspect to take particularly into account is the generation of the clock. It is used for the start and for maintaining in operation the DC/DC converter and therefore has to be separate from the supply it generates, at least initially. The options include:

• a resonant passive network piloted by the scavenger, whose output would then be rectified and squared; • an oscillator with low consumption and high robustness, supplied by the rectifier output via, for example, a charge pump which ensures an adequate supply voltage even with very small rectified input voltages.

Given that the clock should be present even when the device does not generate energy and the system is supplied by the buffer, the clock generator would provide for automatic switching to the supply of the DC/DC converter once this has come into operation.

The use of the system for harvesting energy from vibrations is particularly advantageous when:

- the system to be supplied is normally subject to vibrations or impact (for example pedometers, cardiac frequency meters, portable digital music readers, portable telephones, PDAs, etc. );

- it would be impractical or difficult to replace or recharge normal batteries (for example, supply of electronic or electromechanical apparatuses integrated in sports equipment such as bicycle frames, tennis rackets, skis or running shoes; electromechanical systems mounted in points that are poorly accessible or inaccessible such as parts of frames of machines or endothermic motors, etc.);

- the powered system can have longer periods of inactivity than the discharge time of an electrochemical battery (for example electric torches, electronics on board missiles, etc.).

Variations and/or modifications may be made to the device for converting vibrational energy into electrical energy according to the present invention without departing from the scope of the invention as set forth in the following claims.