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Title:
POWER TAKE-OFF WITH INTEGRATED RESONATOR FOR ENERGY EXTRACTION FROM LINEAR MOTIONS
Document Type and Number:
WIPO Patent Application WO/2014/094778
Kind Code:
A1
Abstract:
The invention relates to a magnetic gear for converting linear motion into rotational motion and vice versa. The present invention converts slow linear irregular oscillating motion of wave energy devices into torque on a high speed shaft for powering a generator while making the wave energy device resonate with the waves. The invention relates to the field of energy-harvesting from energy sources, where the energy-harvesting requires the extraction of energy from slow and often irregular reciprocating motion of bodies. The present invention relates to a wave power apparatus for converting power of sea or ocean waves into useful energy, such as electricity. The invention relates to the control and operation of a magnetic gear based motor/generator system. The invention provides a high force density electric powered linear actuator.

Inventors:
HANSEN RICO HJERM (DK)
RASMUSSEN PETER OMAND (DK)
Application Number:
PCT/DK2013/050420
Publication Date:
June 26, 2014
Filing Date:
December 06, 2013
Export Citation:
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Assignee:
WAVE STAR AS (DK)
International Classes:
F03B13/20; F16H25/24; H02K7/06
Domestic Patent References:
WO2011161203A22011-12-29
Foreign References:
US20090251258A12009-10-08
Attorney, Agent or Firm:
DANFOSS A/S (L25Nordborgvej 81, Nordborg, DK)
Download PDF:
Claims:
CLAIMS

1 . A system for converting energy comprising an interactor to an environment, where the interactor is connected to a magnetic lead screw (1 ) comprising; a translator prevented from rotating and which supports a first helical arrangement of magnets with a given first pitch around a centerline; a rotor able to rotate around a centerline and which supports a second helical arrangement of magnets arranged with the a given second pitch around the center- line; where at least one of said units the translator and the rotor has a hollow with an internal diameter larger than the external diameter of the other unit such that the other unit may move at least partial within the unit with the hollow along the center- line giving an interaction of the magnetic fields of the said first and second helical arrangements of magnets such that a force acting to translate rotor and translator relative to each other causes said magnetic fields to exert a torque about the said centerline, forming a rotation of said rotor and translator relative to each other about said centerline.

2. A system as in claim 1 , characterized in that said first pitch and second pitch is designed such that the total amount of moment of inertia of rotor and generator about said centerline is such that when the oscillating bodies are caused to oscillate by the naturally occurring source of energy oscillating at a source frequency, then the system of oscillating bodies and said moment of inertia of rotor and generator will have a system resonance frequency equal to said source frequency, and where the natural frequency is controlled by placing at least one fly wheel on the rotor side of the magnetic lead screw, where the fly wheel has variable inertia, or by changing the number and / or sizes of the fly wheels.

3. A system according to claim 2, An where the fly wheels are engaged and / or disengaged at zero angular velocity of the rotor, this being each time the oscillating bodies caused to oscillate by a naturally occurring source is at the top or bottom of a wave (19).

4. A system according to claim 3, where the fly wheel displaces a mass (21 ) to change its moment of inertia.

5. A system as in one of claims 2-4, characterized in that if the source of energy causes oscillations of varying frequencies then the said pitch and the total moment of inertia of rotor and generator are designed to make the said system resonance frequency match the oscillation frequency of the source of energy having the highest contribution of energy over time.

6. A system as in claim 5, characterized in that the total amount of moment of inertia attached to the rotor is made adjustable such that the resulting system reso- nance frequency may adapt to the varying source frequency of the naturally occurring source of energy.

7. A system as in one of claims 2 to 6, characterized in that the control of the said adjustable moment inertia is controlled based on a controller reacting on measurements of the sea waves and/or the measurement of the motion of the oscillating bodies.

8. A system according to any of the preceding claims, characterized in that a motor drives a rotor making the magnetic lead screw (1 , 52) an actuator adapting the interactor to the naturally source of energy.

9. A system as in any of the preceding claims, characterized in that the helical pitch of the hollow unit of the rotor and translator is made longer than the other, such that the helical pitch of the other unit may be fully concealed within the helical pitch of the hollow unit.

10. A system according to of the preceding claims, wherein a generator is attach to the said rotor and the relative movement of said translator and rotor along the centerline is caused by the movement of the interactor caused to oscillate by a naturally occurring sources of mechanical energy where the oscillation of the said translator causes the said generator to produce electricity.

1 1 . A system according to claim 10, where the generator is used actively to perform a small amount of reactive control by fine tuning the natural frequency (1 7). 1 2. A system as in one of claims 1 0 or 1 1 , characterized in that an inverter is connected to the generator to provide torque control of said generator.

1 3. A system as in any of the preceding claims, characterized in that the space between rotor and translator are filled with a lubricating fluid such that the fluid be- tween translator and rotor function both as a linear guide of the translatory motion and supports the rotation of the rotor.

14. A system as in any of the preceding claims, characterized in that the rotor is attached to two or more bearings sliding on the internal sides of the translator.

15. A system according to claim 14, characterized in that the bearings sliding inside the translator (3) carries the rotor (2) though one or more rotational bearings such that the sliding bearings (76) are not forced rotate together with the rotor (2).

16. A system according to any of the preceding claims, where the system comprise two or more pair of translator and rotor in parallel such that all translators are translated simultaneously 17. A system according to any of the preceding claims, characterized in having a translator which supports two or more helical arrangement of magnets with a given first pitch around a centerline;

a rotor which supports a two or more helical arrangement of magnets arranged with a given second pitch around the centerline.

18. As system according to claim 18, where at least one of said units the translator and the rotor is one or more tubes with same centerline, where each tube both have an inner and outer helical arrangement of magnets such that the other unit have corresponding sets of helical arranged magnets about said centerline and may move at least partial within the unit along the centerline, such that the magnetic field of the sets of rotor and translator helical arrangements of magnets interacts such that a force acting to translate rotor and translator relative to each other causes said magnetic fields to exert a torque about the said centerline, forming a rotation of said rotor and translator relative to each other about said centerline.

Description:
Power take-off with integrated resonator for energy extraction from linear motions

The invention relates to a magnetic gear for converting linear motion into rotational motion and vice versa. The present invention converts slow linear irregular oscillat- ing motion of wave energy devices into torque on a high speed shaft for powering a generator while making the wave energy device resonate with the waves. The invention relates to the field of energy-harvesting from energy sources, where the energy-harvesting requires the extraction of energy from slow and often irregular reciprocating motion of bodies. The present invention relates to a wave power appa- ratus for converting power of sea or ocean waves into useful energy, such as electricity. The invention relates to the control and operation of a magnetic gear based motor/generator system. The invention provides a high force density electric powered linear actuator. BACKGROUND

Numerous technologies are under development for harvesting the energy of the ocean waves. These technologies are often referred to as Wave Energy Converters (WEC). A large number of embodiments of these WECs bases on first converting the waves into irregular oscillation of bodies, e.g. point absorbers, multiple absorb- ers, attenuators and terminators. The converter disclosed in WO2006/108421 is an example of a multiple absorber system, consisting of having multiple arms, each of which is rotationally supported at one end by a shaft, and wherein each arm carries a float at its other end. As the wave passes through, the floats will move up and down with the waves, thereby converting wave energy into a mechanical oscillation of bodies. The converter disclosed in WO00/17519 contains an example of an attenuator class WEC, where buoyant cylindrical body members are connected together at their ends to form an articulated chain-like structure, such that when the waves pass through the length of the converter, the bodies of the chain oscillate relative to each other. Common for all concepts is that the energy in the irregular mechanical oscillation of the bodies must be converted into useful energy or work, e.g. electricity. The technology converting the mechanical oscillation into useful energy is referred to as the Power Take-Off (PTO).

In order to increase the amount of harvested energy, the reaction or load force on the oscillating body needs to be controllable, see e.g. WO2005069824. Thus, if only a constant reaction force is applied to the body, the amount of harvested energy will be reduced. In general, the reaction force must be controlled such that the movement of the body is tuned to resonate with the varying wave frequency. This may require a PTO-system with four-quadrant operation, as energy is delivered to the body from the PTO system during an oscillation cycle in order to tune its motion.

So far, a reliable, efficient and cost-effective PTO-system for producing electricity has not been developed. Electricity producing PTO-systems can overall be divided into two groups: Directly driven linear generators or PTO-systems, comprising a transmission with a conventional generator, where the transmission converts the irregular oscillation of bodies into a high-speed rotational motion for powering electrical generators. Regarding the second group, different transmissions have been explored for driving conventional generators. In WO2010/078890, WO2009/003598, WO2008/128830 and WO2009/132762 PTO-systems based on a conventional hy- draulic transmission are suggested. The main problem with these fluid power based PTO system is that fluid power systems are characterised by not being able to maintain sufficient part load efficiency, which is inherently important within wave energy. At e.g. 50% load of components, the efficiency of the individual fluid power components can easily drop below 60%.

Different mechanical transmission has also been explored. However, due to the high gearing ratio required, conventional multi-stage gear-based transmissions re- sultantly have a very high inertia. As the wave motion is reciprocating, the torque requited for acceleration and de-acceleration of the large inertia of the mechanical transmissions renders them infeasible. Moreover, mechanical transmissions do not offer the required durability. In WO2009125429 a unidirectional gearbox is suggest- ed to convert alternating rotation of float systems into a continuous rotary output with no significant power loss. However, such a system offers reduced controllability of the load force, and may also lack sufficient durability. Regarding linear to rotational motion mechanical transmission, the transmission displayed in may be mentioned. These include rack and pinion based systems and ball screws. In WO9951877, a helical screw is suggested for driving a generator. However, only a low gearing ratio is possible due to the self-locking nature of a lead screw if the thread pitch becomes too low. In CN201517465 and WO20061 13855 a ball screw is used. This allows increased the gearing ratio, as the ball screw is not self locking at a low thread pitch. The ball screw in GB 2443101 B is suggested combined with a one way clutch to achieve a uni-directional drive of the generator, avoiding acceleration and de-acceleration of gear and generator inertia. The advantage of these direct mechanical transmissions from linear to rotational motion is their simplicity and enabling the use of more conventional generators. However, their main disadvantage is to offer the required durability, especially if a high gearing ratio is sought. Concerning direct driven generators, a lot of attention has been given to linear generators driven directly by the wave induced motion. Linear generators for point absorbers are suggested in WO9951877 and WO2010061 199. The main disadvantage of these systems is the poor force density delivered by a linear generator. In the paper by Spooner et.al., "Snappertm: an efficient and compact direct electric power take-off device for wave energy converters" (in IMarEST 4th MAREC Conf, March 2006) it is estimated that a conventional linear generator would have a power to weight ratio of only 4kW per tonne.

In EP1589643 a combination of a linear magnetic gear and linear generator is pro- posed. The system has a conventional linear generator in which a set of magnets mounted in a translator is moved up and down inside multiple coils of wire of an ar- mature. However, the system also incorporates a second set of magnets, functioning as a magnetic coupling, thereby preventing the translator magnet assembly from moving up and down smoothly together with the float. Instead, the translator is fixed, until the external force from float is able to overcome the magnetic coupling. This result in a series of fast movements of the translator, which is more suited for producing electricity, thereby yielding a better power to weight ratio compared to conventional linear generators.

To overcome the durability issues regarding using a ball screw, a magnetic helical screw is suggested for wave energy in US2009251258 and GB2443101 . The idea is based on the concept of a screw and a nut, where the thread on one or both parts is created as a magnetic field by permanent magnets placed in a helical screw pattern. Thus there is no physical contact between the rotor ( nut") and translator ( " screw"), but instead a frictionless power transfer trough the interaction of the magnetic fields of screw and nut. Thus the mechanical thread has been replaced with a thread shaped magnetic field created by permanent magnets. The magnetic helical screw disclosed in GB2443101 is shown with a magnetic tread on the "nut"- part, but where the "screw" is made of a screw shaped low-reluctance (low magnetic resistance) material. The idea is, that based on the principle, that the rotor will position itself to minimise the reluctance of the magnetic circuit, making the rotor behave as a screw. The force density will however be reduced compared to having permanent magnets on both parts.

In US2009251258 it is suggested to have magnets on both parts. The magnetic screw was disclosed for the first time in US237151 1 A. In US005984960A the magnetic screw is suggested as actuator for a heart pump. The screw is also mentioned in US5079458A and in US3483412A.

However, maximising energy extraction from e.g. wave energy requires that the movement of the oscillating body is tuned to resonate with the waves, whose frequency is time varying. To solve this, a PTO-system with four-quadrant operation is used, as energy is delivered to the body from the PTO system during an oscillation cycle in order to tune its motion as disclosed in WO2005069824. This is often referred to as reactive control, and requires reactive power to oscillate between PTO system and body. A method to do this control or generally to make the system res- onate with waves using the magnetic lead screw is not solved in US2009251258 and GB2443101 .

The object of the present invention in one embodiment is to utilize a magnetic lead screw to implement a passive resonator with integrated generator as PTO system for WECs based on oscillating bodies in order to maximize energy production, where the oscillating bodies operates as an interactor with the waves. In this and the following the term 'interactor' is understood as an part of a system interacting with an environment, where this environment may have a changing character such as waves on the water, wind, the surface forming the road for a vehicle etc. The object is to present a PTO system, which in turns of engaging and disengaging passive control means may adapt its dynamics behavior to match the time varying characteristics of naturally occurring source of mechanical energy. The purpose of the invention is a new energy-efficient method to optimize the load on the source of mechanical energy for maximizing energy production. The present invention is a compact and efficient PTO system unit which takes an irregular oscillating linear motion as input and outputs electric power. The object is to present an invention with high efficiency in both low, mid and high energy conditions of the source of energy. SUMMARY

The present invention relates in one embodiment to the conversion of energy from naturally occurring sources of mechanical energy, such as the mechanical energy present in ocean wave caused oscillation of bodies. The present invention resides in combining a magnetic lead screw and a system of electrical generator/motors. The magnetic lead screw system is powered by a natural source of mechanical en- ergy, where the natural occurring source of mechanical energy can by means of the mentioned magnetic lead screw be used for powering one or more generators. The present invention relates to utilizing the magnetic lead screw design to replace the conventional reactive control of PTO systems in wave energy. The present inven- tion relates to using a magnetic lead screw to implement a passive controllable resonator to optimise energy extracting.

DETAILED DESCRIPTION The present invention converts slow linear irregular oscillating motion of wave energy devices into torque on a high speed shaft for powering a generator while making the wave energy device resonate with the waves. The conversion from linear to rotational motion is performed by a magnetic lead screw (1 ), converting the linear motion of an oscillating wave energy device (4) into a higher speed rotational motion on a shaft (32). The magnetic lead screw is illustrated in Fig. 1 , being a magnetic implementation of a mechanical lead screw, where the thread is created by the interacting magnetic fields, generated by a helical arrangements of permanent magnets (43) and (47) about a centerline (48) on two units (2) and (3). Hence the idea is that waves by means of a buoyant body (4), translates the nut and screw of the magnetic lead screw relative to each other, resulting in a rotation of either the screw or nut, depending on the embodiment. The rotating part (2) will referred to as the rotor, and the translating (non-rotating) part as the translator (3). Figs. 2 and 3 illustrate an embodiment of the present invention utilized on the wave energy converter (in the following referred to as WEC), as disclosed in WO2006/1 08421 . The waves cause an irregular oscillation of the floating body (4) (being the interactor with the waves of the present embodiment) and its attached arm (5), where the arm translates the translator (3) of the magnetic lead screw (1 ). This causes the rotor (2) to rotate, where the rotor is directly attached to a generator (6). In this embodiment with a floating body on an arm, the linear velocity of the translator (3) and the re- quired stroke (8) is reduced, but the required force (7) and gearing ratio of the magnetic lead screw (1 ) is increased. Thus by changing the mounting of the magnetic lead screw the moment arm d a (1 0) is changed, leading to increase or decrease of stroke (8). The magnetic lead screw (1 ) may also be applied directly at the floating body (4). One embodiment of a direct mounting on a point absorber is shown in Fig. 4.

In the above embodiments the rotational velocity of the generator (6) will be bidirectional. Thus, every time the movement of the floating body changes direction, the generator velocity (9) reverses. Thus the moment of inertia Jo n of generator (6) and the rotor J r (2) has to be periodically accelerated and de-accelerated (the direc- tion of rotation will change two times per wave period). This is generally an unde- sired characteristic in motor/transmissions drive systems, as the energy required for performing this acceleration does not produce work on the system or load attached to the motor/transmissions and also reduces durability of drives. However, the key in the present invention is to use it as an advantage as follows: Wave energy con- verters like point absorbers are pre-dispositioned to have a too high resulting natural frequency of the floating body compared to the frequency of the incoming waves. Resultantly, the amount of energy extracted from waves may be increased by designing the transmission (1 ) such that the moment of inertia of the rotor (2) J r and generator (6) Jo n is reflected to the body (4), such the natural frequency ω Ν of the floating body is reduced, matching the frequency of the incoming waves.

Design of Magnetic Lead Screw PTO to make WECs Match Dominating Wave Peri- od

Analysing the system in Fig.3 shows that the moment of inertia of rotor and genera- tor lowers the resulting natural frequency of the body (4) as desired. The dynamics of the system consisting of floating body and arm as in Fig. 2 oscillating in the waves may be described by linear wave theory, which yields the following second order system: where: e arm is the angle of the arm (5)

ouarm is the angular velocity of the arm (5)

Jmech is the total moment of inertia of the floating body (4) and arm (5)

Jadd is the added moment of inertia (also known as added mass) due to displace- ment of nearby water

jbhyd is the hydro-dynamic damping coefficient due to the body (4) radiating waves /(res is the hydrostatic restoring coefficient, which is due to the combined effect of buoyancy and gravity on the body (4).

Text is the exciting wave torque, which is the resulting torque applied to the body and arm by the incoming waves.

Tpio is the torque applied by the PTO system to the body (4) and arm (5).

Expressed as a transfer function:

The natural frequency of the system is given as,

The PTO torque T pto is given as:

If the pitch of the magnetic lead screw thread is denoted / P i tC h (1 1 ), the relation of the angular velocity of the rotor OUMLS (9) and the relative linear velocity of translator and rotor V be found as, where D U ,MLS=-^T 1 [m/rad] is the displacement of the translator per rotation of the rotor in radians.

The force and torque relations of the magnetic lead screw are given as:

The gear ratio grfrom ) arm to OUMLS is given as:

Seen from the floating body (4) and arm (5), reflecting or transferring the moment of inertia contribution of the rotor and generator J r + J Gn to the arm (5) rotation yields,

T LS — ώ «(Λ · ιι 3 — <f»W«n» { Jr + JGn ) " T rm ~ QT~ J r - JGn ϊώ ¾ ;κ where it was used, that the torque TMLS may be expressed as a torque ½m acting about the pivot of arm (5) by the relation τ ΡΤΟ = ^ . Likewise the magnetic lead screw angular acceleration may be expressed as an arm angular acceleration

<^MLS = <¾-m 9 r - Thus, the reflected moment of inertia of the rotor (2) and generator (6) to the floating body and arm is the rotor and generator moment of inertia scaled with gi 2 (square of the gearing ratio).

The expression for the resulting natural frequency of the float and arm now be- comes: j Jadid 4 4 i Jv + c,x )gr a

Thus, if the moment of inertia of the rotor and generator are fixed, this will permanently change the system's natural frequency WN. The gearing ratio of the system may then be designed to obtain the desired natural frequency.

Using standard permanent magnets, a pitch down to e.g. 14 mm would be possible. For a body of 5 m in diameter in Fig. 2 the moment arm d arm would typically be about 2.36 m. This would result in a gearing ratio grot

M- ~. » · » ····: 2.3G rO 1 059 This means, that given the gear ratio of approximately 1000, the moment of inertia of the rotor is amplified with a factor of 10 6 when seen from floating body (4) and arm (5).

As an example of the embodiment of invention, a WEC as in Fig.2 with a floating body of a diameter of 5 m may be used. The floating body's natural frequency without the ma netic lead screw is given as,

where the following values have been inserted: J a dd =2.0- 10 6 kgm 2 , J meC h =2.5- 10 6 kgm 2 and k res =14- 10 6 Nm.

This corresponds to a natural oscillation period of 3.5 s. The stroke required of the magnetic lead screw in the example is 2 m, which is then the required rotor length / r in this embodiment. To give the necessary forces, the rotor diameter will be approximately 32 cm (r 0 =32/2 cm) with a rotor thickness of about 1 cm. The magnets and iron of the rotor forms a cylindrical tube with inner radius /i and outer radius r 0 . The moment of inertia of the rotor is,

i , ^ ^ .. „ < >

Jr ~ 7, "/¾ <'r — — a.7 kgm " where p r is the average density of iron and magnets set to 8000 kg/m 3 .

Hence, with a design with a gearing ratio of e.g. gr=800, the moment of inertia contribution to the arm and float is 2.4- 10 6 kgm 2 , which size wise is in the range of the total moment of inertia of float and arm J me ch-

The resulting natural frequency of the floating body and arm with the magnetic lead screw now becomes:

Statically, increasing the natural period to 4.4 s is an advantage for the system, as the given example of a 5 m diameter floating body would be designed to operate in wave conditions as inFig. 5. The entries with bold font indicate the wave periods and wave heights, where the primary energy production is performed. The natural period should approximately match the peak wave period T p . Thus increasing the natural period from 3.5 s to 4.68 s is overall an advantage. Note that the mean wave period T z is shown Fig. 5, with the relation Γ ρ ,= ~ 1 .17-T Z . However if the change of the natural frequency is still too large with a low gearing ratio, the same gearing ratio and force with reduced change of natural frequency may be obtained using two or more smaller magnetic lead screws in parallel instead of one. Two magnetic lead screws with smaller rotor diameters r 0 (12) will give a smaller reflected inertia contribution compared to one magnetic screw with equiva- lent force capability due to the moment of inertia scaling with r 0 to the fourth.

This is an even more advanced embodiment of the present invention having an implementation with Parallel Magnetic Lead Screws. By implementing the magnetic lead screw as a system of multiple screws, the total inertia may be reduced.

In the magnetic lead screw, the magnetic force FMLS between the rotor and translator is proportional to the surface area A s of the magnets facing each other on the rotor and translator, and therefore it is proportional to the rotor's length / r and radius r 0 . (If the rotor is the shorter than the translator):

Has \ r

An example of force as a function of diameter. To have the equivalent area /A s with n parallel magnetic lead screws, the diameter or radius is divided by n, such that the surface area A s ,\ of the th magnetic lead screw becomes, thus the total area A s of the n screws, and thereby the total force FMLSM is the same as for one big magnetic lead screw. The moment of inertia J r j of the Ah rotor now becomes:

thus the total moment of inertia J r , to t of the n screws is now almost a factor of n 3 smaller (depending on the values of w) than the moment of inertia J r for one big magnetic lead screw. The value w is the total thickness of magnets, yoke and housing. It here assumed constant, however if it was assumed to scale with n, the ratio J r ,fof JrWOuld be precisely equal to Mr?.

For the previous example with rotor length / r = 2.0, rotor diameter of 32 cm (r 0 =32/2 cm) and a rotor thickness w of about 1 cm, the inertia was 3.74kgm 2 . If this was designed as parallel screws, the moment of inertia as a function of n is shown in Fig. 17. Thus, shifting from one to two magnetic lead screws in parallel scales the inertia to 0.85kgm 2 (a factor for 4.4). And for n=3 the moment of inertia becomes 0.344 kgm 2 (a factor for 10.9). Thus a magnetic lead screw system with low inertia and high force may be constructed.

Illustration of different parallel magnetic lead screw design is illustrated in Fig. 18. The parallel magnetic lead screws (1 ) may be driven by separate generator/motors (6) as in implementation (58) or as in implementation (59), where one motor (6) is used, which may drive or be driven by the magnetic leas screws (1 ) though mechanism (49), which may be some sort of gear system. A magnetic lead screw with multiple threads may also be constructed. In Fig. 1 9 an embodiment is shown, where a double sided implementation of the magnetic lead screw is shown. The implementation (64) both has an inner (60) and outer thread (61 ), which is also illus- trated with a mechanical analog in (63). Using the double sided implementation (54) twice the magnetic surface A s may be obtain from the same diameter.

Passive Reactive Control

The energy transfer from wave to absorber (4) is said to be optimal when the wave period and the natural period of the absorbers (4) are equal. Resultantly, point absorbers (4) are prone to operate off-resonance and thereby non-optimal, as the wave period varies wave-to-wave, and the average or dominating wave period may shift with a factor of two within hours, e.g.. shifting between the states in Fig. 5.

To overcome this problem, it is well known that appropriate control of the applied PTO force may potentially increase the amount of energy extracted from sea waves. This is achieved by continuously using the PTO actively to shift the natural/resonance frequency of the floating body to match the incoming waves. However, this requires the PTO to transfer energy to the absorber (assisting its movement) in parts of an oscillation cycle. Due to the bi-directional energy transfer, this type of control has been termed reactive control and it requires a PTO capable of four- quadrant behavior and force control. The PTO system may be in some strategies being viewed as emulating extra inertia. A typical example of control to produce the PTO torque reference is,

T PTO,ref = < ^arm c \ + ) arm c 2 + c 3 arm

where the coefficients C , c∑ and C3 which are adjusted to maximize energy production from the current waves.

However, this is a very expensive form of control due to high amount reactive power being processed by the entire PTO system. The terms with C\ and C3 lead to reactive power. Thus, adding extra inertia using the magnetic lead screw is much more efficient, as the xx reactive" power is now provided by the rotor and generator inertia and not processed by the remaining PTO system. Thus a form of passive reactive control may be obtained, if the size of the rotor inertia is controllable.

Thus, the present invention implements passive reactive control, which makes the natural frequency controllable using adjustable inertia on the rotor side of magnetic lead screw. This may be implemented by placing engageable fly wheels on the rotor side of the magnetic lead screw as shown in Fig. 6. By having n different sized flywheels (13), 2 n different inertia adjustments are possible. Moreover, if the size of the moment of inertia of the fly-wheels are designed such that they follow a binary code, i.e. if the size of the smallest fly-wheel is J, then their sizes follow a series J, 2J,4J,8J,..., 2 n' Resultantly, n evenly distributed inertia contributions (16) may be created as illustrated in Fig. 6. As illustrated in Fig. 7 depending on the current wave period (18), the appropriate combination (16) of fly wheels may be selected to make the floating body's (4) natural frequency WN (17) match the wave period (18). An important feature of system is that the fly-wheels are engaged/disengaged at zero angular velocity of the rotor and fly wheels (19), minimizing the wear and requirements of the required clutch (14) for performing the said engaging. Thus, each time the float is at the top or bottom of a wave (19), the rotor (2) and fly wheels (13) have zero velocity, and the desired configuration (15) of fly wheels (13) may be chosen.

Instead of having discrete variation of the moment of inertia connected to the rotor (2), fly wheels with variable inertia (20) may be utilized. See Fig. 8 for such an embodiment of a fly wheel, where the fly wheel by some means may displace mass (21 ) to change its moment of inertia.

As the control of the natural frequency (17) is now controlled by fly-wheels, the generator (6) may be operated solemnly in generator mode, as the reactive power is delivered by the fly-wheels. The generator may still, however be used actively, to perform a small amount of reactive control, e.g. to fine tune the natural frequency (17). It is well known, that if a floating body's natural period (17) matches the wave period (18), the optimal control of PTO system is linear damping, meaning that the generator torque T GN (22) is in the form of:

T Gn= w MLS C Gn where the coefficient <¾ n is adjusted to maximize energy production from the current wave. Thus the generator (6) only needs to produce high torques (22) at high speeds (9), which is desirable for the resulting efficiency of the generator. To further increase its efficiency, it may e.g. be chosen to only let the generator produce torque or power at a generator speed (9) above e.g. 500 RPM, depending on the design.

Design Embodiments

Concerning the design of the magnetic lead screw (1 ) based PTO system, different embodiments may be chosen. The main challenge is that the rotor (2) is rotating at a high speed (9) while being displaced relative to the translator (3), see Fig.9. Thus, the rotation of the rotor (2) must be supported while allowing the rotor and translator (3) to translate frictionless relative to each other. Moreover, the rotor and translator are required to form or be supported by linear guide (24), such that the air gap (23) between rotor and translator is mechanically fixed. Otherwise the rotor will be pulled to one of the sides, as the translator and rotor magnetically attract each other. The linear guide (24) is also required to absorb the radial/side load forces (25) when the magnetic lead screw is under compression load, see Fig. 9. Otherwise, the assembly can in worst case buckle. Finally, a bearing (26) for absorbing the axial load (27) on the rotor (28) is required, otherwise, the generator (6) bearing would be required to absorb the axial load (28).

In Fig. 10 different embodiments of the system is shown. First of all, either the rotor (2) or translator (3) may be the longer part. Having the rotor being the shorter part reduces the moment of inertia of the rotor, which may be advantageous. In (28) it is shown that the rotor (2) is supported in both ends by bearings (34) (supporting the high speed rotation of the rotor and absorbs radial load ) which is mounted inside a sliding bearing (33) (functioning as a linear guide, supporting the translatory motion and absorbing side-way loads.). One of the points (31 ) is attached to the body (4) motion and the point (31 ) to the part of the WEC (35) delivering the reaction forces. At one of the points (31 ) also needs to deliver a radial reaction force to prevent the magnetic lead screw assemble (28) from rotating. In (28) the translator (3) slides on a through-going shaft (32) using sliding bearing (33), whereas the implementation in (30) incorporates sliding bearing (36), sliding on the inside of the translator (3). This may be obtained by manufacturing the translator with a hard and smooth inner surface. A protecting bellow (38) may be implemented to seal the varying distance between generator housing and translator.

In the embodiment (29) the rotor (2) is the longer part. Here the rotor (2) is supported in both ends by bearings (26), fixed to the same structure as the generator (6). Some sort of linear guide (26) supports the motion of the translator (3), i.e. maintaining air gap (23) and absorbing side load forces.

In the embodiments (28), (29) and (30) some sort of axial thrust bearing (26) for absorbing the axial load force on the rotor (2). To implement the linear guide (26) different approaches may be used. In Fig. 1 1 it is illustrated with a graphite bushing (42) as the sliding bearing, which is a "dry" bearing. The translator magnets (47) are covered by a smooth and hard non-magnetic shell (46) on which the graphite bushing slides. The sliding bearing should preferably be made of a non-magnetic and non-conducting material to reduce eddy-current losses. In Fig. 12 it is suggested to fill the air-gap (23) and translator (3) with a fluid (37) to obtain a hydrodynamic lubrication when the rotor (2) is rotating, in which the lubrication film (44) between rotor (2) and translator (3) comprises the linear guide (26) and may also comprise the rotational support of the rotor. E.g. the sliders (45) attached to the rotor rest on the film (44) and constitute both a linear guide and the bearing for supporting the rota- tion of the rotor. The challenge with the hydrodynamic lubrication is that velocity is zero at some points (19), risking that at these points the lubrication film (44) diminishes. However, at zero velocity the generator load is also zero, reducing the risk of failure. Otherwise, approaches with hydro-static lubrication may be used, or a combination of hydro-static and hydro-dynamic lubrication. The oil pressure required for hydro-static lubrication may e.g. be obtained by implementing a simple pump device for lubrication pressure integrated into the magnetic lead screw, which is driven by either the rotor or the translator. A key property of the mentioned embodiments is that the translator and rotor are not capable of mechanically meshing. Thus if the force/torque exerted to the translator and rotor exceeds the capability of the magnetic lead screw (1 ), the rotor will simple "thread" skip inside the translator. Hence the design has an inherit overload protection.

Figs. 1 3 to 1 5 illustrate a preferred embodiment of the present invention where the rotor (2) is shorter than the translator (3), thus reducing inertia and allowing the rotor (2) to be contained within the translator (3) in a manner where all magnets (46, 47) are internal to the construction, meaning all magnets of rotor (2) and translator (3) at all times will be within the inner (72) of the translator (3).

To maintain a gap between rotor (2) and translator (3) and ensure a linear guide the translator magnets (47) are covered by a smooth and hard non-magnetic shell (46) allowing bushings or bearings (76), sliding directly on the smooth inner surface of the translator (71 ). The sliding bearing (76) preferably are made of a non-magnetic and non-conducting material to reduce eddy-current losses. Rotational bearings (34) carrying the rotor then may be attached to the sliding bushings (71 ). Resultant- ly, the sliding bushing (71 ) now ideally experience only translational motion, and the rotational bearings experience radial loads only, except for the small axial load due to friction in the sliding bushings (71 ).

The rotor shaft (32) is attached to a motor/ generator (6) through an axial thrust bearing (26) to deliver the reaction force to the axial load force produced by the system. Otherwise, this load force must be absorbed by the motor/generator (6). A protecting bellow (38) may be implemented to seal the varying distance between generator housing and translator. At each point (31 ) the mounting is also required to deliver a reaction force to prevent the magnetic lead screw assemble (28) from rotating.

The sliding bushings (76) may be implemented for example as a graphite bushings (42), or by filling the assembly with fluid as shown in Fig. 15 , and implement the sliding bushings with low friction bearing bands (75) sliding on a fluid film. The rotor may be may be made with a bore (78), allowing fluid to pass through the rotor as it translates. The sliding bearing (76) may also be formed as needle bearings, where arc shaped needles or rollers, rolls on the smooth translator walls.

As illustrated in Fig. 13 the rotor (2) may divided into two or more pieces, such that bearings (34) can be placed in between for extra support.

Inverter/Converter Design

To control the torque (22) of the generator (6) at varying angular velocity (9), an inverter or a converter (41 ) may be utilised. The inverter controls the torque (22) of the generator (6), such that the torque (22) tracks the desired reference generated to maximise energy production, i.e. the energy output to the grid. The reference may be provided by a feedback law of the type,

T Gn,ref = < ^arm c \ + M arm c 2 + c 3^arm

which provides the option of performing reactive control of the generator. Otherwise if only damping (non-reactive) control is used of the generator (6), the torque refer- ence for torque (22) may be calculated based on the generator velocity (9) as, TGn,ref = sea state, wave period)

where the function /maps the current sea state information and measured wave period (18) or/and predicted wave period into a torque reference. One of the simplest version of is linear damping, the generator torque is controlled in a given sea state as, T Gn,ref = ω ΜΙΞ ' c

where c is some coefficient.

To further improve generator efficiency, the torque reference may be generator as,

such that the generator is only activated at velocities of (9) over some threshold value ω τηίη . Otherwise, torque (22) might have to be provided at zero or low velocities, where the efficiency of generators generally is poor.

The inverter or converter (41 ) may be integrated into the same unit as the generator (6) and magnetic lead screw (1 ) as design (39) in Fig.16. Thus, the unit (39) may be used as finished module for converting linear motion into e.g. a three phase power output at a fixed frequency and voltage. The inverter may also output a DC-current, if multiple units are operating together, where their outputs are connected at a common DC-bus, and then place a common inverter, converting the DC-voltage into a three phase power output at a fixed frequency and voltage

The inverter or converter may also be placed external as in design (40) in Fig.16, if e.g. reduced size of the module (40) is required.

Other applications

Though the main description of example has been related to extracting wave ener- gy, the present invention is not limited to this use, but may just as well be utilized in other systems as a generator or actuator, together with some interactor_As mentioned the magnetic lead screw allows four quadrant behaviors, i.e. all combination of velocity direction and force direction. Thus the magnetic lead screw actuator (1 ) with a motor/generator (6) may be used as a high performance in other systems. One example of use in an alternative system is illustrated in Fig. 20, where the adaptation to the naturally source is performed by letting the magnetic screws (52) work as generators adapting the source interactor to the present conditions of the source. Here the magnetic lead screw (52) is used as the actuator for pitching (55) the blades (53) of a wind turbine (54), where the blades (53) operate as the interac- tor. The magnetic lead screw actuator (52) is driven by a motor (51 ) mounted to the turbine, which may extend the magnetic lead screw (52) mounted at point (56), causing the blade (53) to pitch around point (57).

A parallel implementation as in Fig. 21 may be used, here illustrated with two parallel magnetic lead screws.

In both embodiments, the WEC illustrated in figs. 2 and 3, and the wind turbine (54) illustrated in Figs. 20 and 21 , the magnetic lead screws (1 , 52) works with the in- teractor (the floating body (4) and the blades (53)) to present conditions of the naturally source of energy (waves or wind).

Another application of the magnetic lead screw is shown in Fig. 22 for a hexapod, or steward platform, where the magnetic leads screw (66) is used to control the motion of the plate (7). Another robot example is illustrated in Fig. 23 the magnetic lead screw (66) may be used to control the motion of arms (67).

In Fig. 24 the magnetic lead screw (70) may be used as the shock-absorber (68) for a suspension, where the magnet lead screw also allow the generator to produce power from the shock absorption. In this embodiment e.g. the wheel may be seen as the interactor being in connection to a non-plane surface.