GB2523088A

GB2523088A - Magnetic power-split

Info

Publication number: GB2523088A
Application number: GB1402334.5A
Authority: GB
Inventors: Stuart Calverley; Jeff Birchall
Original assignee: Magnomatics Ltd
Current assignee: Magnomatics Ltd
Priority date: 2014-02-11
Filing date: 2014-02-11
Publication date: 2015-08-19
Also published as: GB201402334D0

Abstract

A magnetic power-split system (e.g. for use in an automotive powertrain in place of a dual mass flywheel (DMF)) comprises: an input member; an output member; a magnetic gear connecting the input member to the output member; and control means arranged to control the flow of power from the input member to the output member. The magnetic gear includes first and second sets of magnetic poles (e.g. a set of permanent magnets and a set of windings) and a set of pole pieces that modulate the magnetic field between the first and second sets of magnetic poles. The control means reduces the transmission of torque pulsation and/or oscillation from the input member to the output member. The number of magnetic poles in the first and second sets of magnetic poles, the number of pole pieces in the set of pole pieces, and the inertias of moveable elements in the magnetic gear may be selected to attenuate torque pulsation and/or oscillation in a bandwidth of frequencies. The control means may be used to control the flow of power from the input member to an energy sink means or from an energy supply means to the output member if the power required at the output member is different from a power at the input member.

Description

Magnetic power-split

Introduction

This disclosure covers three-rotor magnetic power-split and two-rotor magnetic power-split concepts, for use in, for instance, an automotive powertrain. In particular it covers how the characteristics of the magnetic gear elements can be used to delete a dual mass flywheel from the vehicle powertrain (with simplification and cost benefits) while meeting the required NVH (noise Vibration and Harshness) specifications. The system can be extended to allow for active cancellation/filtering of engine torque pulsations.

The Problem It is a requirement in all vehicles to match variable road load demands (speed and torque) to the output of the engine.

The engine operating profile has areas of greater and lower efficiency and a given power level (product of torque and speed) can be delivered with a number of different combinations of engine torque and speed that can give greater/lower efficiency and hence fuel consumption and emissions. This has historically been achieved in two ways, either with discrete number of gears, engaged manually by driver or automatically by a control mechanism or with a continuously variable system (CVT) employing mechanical elements such as cone drive systems. The latter does not have fixed steps in gearing.

One of the ways that electric hybrid vehicles aim to increase fuel efficiency and lower emissions is to use a combination of energy sources (i.e. battery and fuel tank) and a combination of electrical and mechanical drives to ensure that the primary engine is operated at the most appropriate torque/speed combination. Any surplus/deficit in power is then addressed with an electrical powertrain (energy storage, inverters and motors). One of the elegant ways this can be achieved is to use a "power-split" in what has become known as a "blended" electric hybrid as shown in Figure 1. The power-split element is a planetary gear combined with a motor/generator. The motor/generator is used to control the rotation of an inner sun gear to vary the gear ratio of the planetary gear, while the engine input and output to final drive are connected to the ring and planet carrier.

It should be well understood that a planetary gear has three elements. If any one of these elements is held static there is a fixed gear ratio between the other two elements and 3 different ratios can be achieved in any gear. By allowing the third element to rotate the ratio between the other two rotors will be varied.

By rotating and reacting torque on the sun gear the motor/generator imports or exports electrical power from the mechanical power train. For example, when there is a surplus of engine power above the required road load, power is extracted electrically and stored in the energy storage system. This energy may then be used at some later time either by the power-split motor returning power through the power split system, or via a secondary traction motor driving the wheels. Therefore a "variator" path has been provided that allows engine power to reach the wheels through different paths and engine power requirement is to some extent decoupled from required road power.

These power-split systems can effectively give infinitely variable control of gear ratio, and can de-clutch engine to a point where engine has finite speed and the wheels have zero speed. This function has traditionally been achieved by the use of a disengaging clutch which itself is coupled to an inertial flywheel (on the ICE crank) which is also used to filter out torque pulsations from reciprocating engine. However, the flywheel reduces vehicle dynamic performance due to the added inertia. This clutch and flywheel may now be removed from the system and the engine crank would now be directly coupled to the power train through the power split device.

However, elimination of the clutch and adoption of direct coupling between engine and drivetrain leads to a more direct transmission of engine torque pulsations which would normally be largely attenuated by the clutch (through slipping/micro-movement) and inertial flywheel. This has impact on NVH (Noise, Vibration and Harshness) and hence "driver-feel" and wear on driveline components. This issue has been addressed with the use of Dual Mass Flywheels (DMFs).

Dual mass flywheels (DMF5) are usually fitted to diesel engine vehicles as they eliminate excessive transmission noise, protect the gearbox from damage and reduce gear change/shift effort. In a DMF, the mass of the conventional flywheel is split into two. One part is added to the engine's moment of inertia, while the other part increases the moment of inertia of the transmission. The two decoupled masses are linked by a spring/damping system and the DMF acts as a damper between the crankshaft and the input shaft on the gearbox. It also has a set of springs inserted between two rotating masses; the slip is cushioned by a set of torsional springs that smooth out irregular torque pulses from the engine. The springs are sized to absorb resonant vibration from the engine under load conditions.

This device introduces added cost and complexity to the drive train and has a number of wearing parts. Replacement is complex and costly due to its location between the engine and transmission.

Further, the above mechanical system suffers from a fundamental limitation of mechanical gears. The planetary gear must follow the big-wheel/ small-wheel principle and the lowest torque, highest speed element is always the inner-most sun gear (while the ring gear and planet carrier are high-torque low-speed elements). It is advantageous to allow the mechanical power flow to be carried on the highest torque outer elements and drive the lowest torque highest speed element with the motor generator (as this will reduce the electrical machine size and increase its efficiency). However, this leads to a complex shaft arrangement as access to the inner most gear is required, within the confines of two coaxial drive shafts.

Description of power-split system using magnetic gearing A number of advantages can be obtained by replacing the mechanical planetary gear in the above system with a non-contacting magnetic gear as shown in Figure 2.

Advantages of the magnetic gear include: * No lubrication required (as no wearing parts, only bearing lubrication required) * High efficiency (as no frictional loss) * No wearing parts increasing reliability(reduced servicing time and lower MTBF) * As will be shown late the magnetic gear maybe inverted (as does not need to follow the big-wheel/small-wheel principle of the mechanical equivalent which allows for a high degree of flexibility. *

Additionally, a magnetic gear has inherent torsional compliance between input and output rotor/inertias (through the magnetic field coupling) which will be exploited in this application.

As with the mechanical equivalent, a fixed gear ratio is achieved between two of the rotors by holding the third rotor static. By allowing the third rotor to rotate the speed relationship between the rotors can be altered as discussed below.

For example, an input rotor (driven by the ICE crank) is the intermediate rotor with a number of ferromagnetic pole-pieces which modulate the magnetic field from an second output rotor ( connected to the final drive to the wheels) which typically would be a PM array. This creates a consequent modulated magnetic field, as shown in Figure 3 which has a pole-pair number equal to the number of pole-pieces minus the pole-pair number on the output rotor. In the example shown in the figures, there are 23 pole pairs on the inner PM array and 27 pole-pieces which creates a 4 pole consequent field radially outwards from the pole-pieces. As is consistent with magnetic gear operation, an externally created magnetic field with the same pole-pair number as this "consequent" field will interact/magnetically couple with this consequent field. For example, a 4 pole permanent magnet array is placed outside the modulation rotor as shown in Figure 4. By interacting the externally created field and the consequent field a torque can be imparted on both the input/pole-piece rotor and the output rotor.

If the externally generated field is held stationary (i.e. it has a series of spatially-disposed poles that are fixed in time and space)) then any rotation of the input shaft will cause a geared rotation of the output shaft, as is consistent with any magnetic geared system where the third field (or rotor) is held stationary. This is analogous to a mechanical planetary gear in which the inner sun gear rotates, with the outer ring gear held stationary which causes the planet carrier to rotate, through the planets. The ratio of torques and reciprocal ratio of speeds is fixed and dictated by the ratio of pole-numbers/pole-pieces on the input and output rotor as given in equations [1] and [2] below. [1] [2]

--Gr where, is the number of pole pieces on the input rotor, N0 is the number of pole pairs on the output rotor; and are the torque and speed of the input rotor respectively and T, and o, are the torque and speed of the output rotor respectively. Gr is the gear ratio.

In the above example, the external field (which interacts with the consequent field) is held static to achieve a fixed gear ratio between the input rotor and the output rotor. However, if the outer external field is also allowed to rotate, the speed ratio between the two primary input and output rotors then follows the relationship given in equation [3] where is the number of pole pairs on external source of field and o is its rotational speed. This field is referred to as the control field. Note, the control field poles are spatially disposed relative to each other as above and it is the entire array of poles rotates.

= -N0w0 [3] The speed of the output rotor is then a function of the control rotor given by: - [4]

N op

For example, if N=8, N,,,,=19, N0=11, and w11=l300rprn, then the output rotor is given by 19X1300-BXWCT [5] This demonstrates that a variable speed output con be achieved with a fixed input speed. For example, Figure 5 shows the range of speeds that can be achieved using the parameters given in equation [5], with a fixed input speed of 1300rpm while varying the control field from -200rpm to 4000rpm which demonstrates the ability to increase speed above the quiescent point of 2245rpm and reduce speed all the way to zero and beyond into reverse. (The quiescent point with (O = 0 is governed by equation [2] above as N/NO =19/11 = 1.727, and 1.727 X 1300rpm = 2245rpm). The input speed may also be varied (for example as the engine operating point shifts to higher efficiency operating points across the operating map), and as shown in Figure 6 a given output speed can be obtained with a continuous range of input and control element speeds.

The above demonstrates how the speed and hence speed gear ratio is controlled. However, the torque ratio remains fixed as proved below. In a lossless system the sum of the powers and torques on all the shafts must add to zero, i.e [6] Tcr+Tpp+TopO [7] Therefore) [8] As power = speed X torque, from [8] = + T0w0 [9] By inspection of [3] and [9] N OC Tcr [iDa] cc [lOb] cc [bc] Therefore, NT [ha]

-[hib]

which demonstrates that the ratio of the torques is fixed and is only dictated by the geometry. It is only the speed ratio that changes in response to the change in control field speed.

Note that from equation [3] if Ucr= 0 [12] = N0w0 Which is the inherent or quiescent gear ratio of the system wN [13]

-

Also from L81, if the control field is held static, the power associated with the control rotor is zero, therefore = -p0, [14] The above demonstrates that that speed ratio can be varied (by controlling the speed of the controlling field) but that the torque ratio is always defined by the ratios of number of pole pairs and pole-pieces and is fixed by the rotor geometries / pole combinations and is selected at the design stage. However, it is important to note that the torque ratio is independent of the rotor diameters. This is a major advantage over mechanical gears which must follow the "big-wheel/small-wheel" principle in which as the gear teeth must have an identical pitch in order to mesh, the only way a gear ratio can be achieved is by having larger and smaller gears/cogs to accommodate the different numbers of teeth.

The decoupling of gear ratio from gear diameter in a magnetic gear allows for higher degrees of freedom in design and the ability to ensure appropriate inertias can be employed.

Typical design choices would lead to one rotor having a high torque (i.e. output) and one rotor having a low torque (usually the input) to allow a useful level of gearing! mechanical advantage. The third rotor by virtue of equation [7] has a torque equal to the highest minus the lowest torque and is therefore also a high torque.

The above equations can also be used to prove that as N,,,, is largest number in the system (N1713=N011+N,), and therefore that this rotor bears the largest torque in system. i.e. the intermediate rotor will always be a high torque element preferably carrying engine or output torque. It is then possible to have the lowest torque rotor in either inner or outer position to this pole-piece rotor. It is advantageous to have through-power (mechanical drive line) using the PPR and the second highest torque rotor, i.e. the one with largest number of pole pairs. This leaves the outer element (which will be used to provide control) to be the lowest torque element. This is advantageous as this element is active with currents and high torques require high currents and hence losses leading to a reduction in efficiency.

This is consistent with the example given above where the consequent field has the lowest pole number (and therefore associated with the lowest torque element in the system) and is used to provide speed control of the two primary rotors, following the maths and graph above.

As with the mechanical power split system described in the introduction, as the control element produces the control field is reacting a torque and is rotating it develops power. If an electrical machine is used to apply the torque and speed control it will be acting as a generator or motor and will export or import power off the mechanical powertrain as electrical energy (through the motor/generators electromechanical energy conversion). This energy is then transiently stored in an energy store (i.e. battery) and/or supplied to consumer of electrical energy such as traction motors providing power to the wheels.

However, providing the external control field to interact with the consequent field and providing a method to control its rotation can be achieved in a more integrated way than in the mechanical prior art, as discussed in the two embodiments described below.

Embodiment 1: Three rotor device -Control element has a physical third rotor with an array of PMs interacting with the consequent field, driven by a concentric electrical machine The outer control field which the consequent field interacts with is generated by an array of PMs, attached to a rotor.

This rotor is rotated by an external means, preferably a stator/windings wrapped around / concentric with the gear rotors for packaging benefits as shown in Figure 7.

There are a number of options for this outer control motor. For example) A. PM inner and outer -each array has different pole numbers with back iron-allows independent optimisation of gear and motor/generator (as shown in Magnomatics prior-art mCVT1) as shown in Figure 8 B. PM inner and outer have same number of poles with back iron ( back iron is not necessarily required magnetically although provides support) C. B above but with the back-iron now removed. The magnetics then can be a single array, with inner surface facing the gear pole-pieces and the outer surface facing the stator. The magnets do not have any structural integrity themselves and are held on a preferably non-magnetic sleeve (i.e. composite of CF tube/sleeve). This has significant advantages in terms of achieving a very low inertia to achieve high dynamics andlow loss as removes sources of iron loss and eddy currents D. B above with buried/interior permanent magnet (1PM) rotor. The magnets are held within a preferably laminated structure. Flux focussing may be used.

Typically a PM rotor for a motor or generator is created by mounting magnets onto a laminated or solid steel structure in order to provide mechanical strength (as it carries the load torques) and to provide a return path for magnetic flux. If the magnetic pole number on the inner and outer surfaces are identical (i.e. the magnetic gear outer rotor and the outer control machine rotor have the same pole number) this "back-iron' is no longer required and can be omitted magnetically. Variant 1C in which the control rotor is a single array of permanent magnets supported on a non-magnetic structure (for example on a composite or CF tube) can have significant operation advantages in that the control rotor is now a low inertia element and allows the system to respond rapidly. An example of this is shown in Figure 9. However) as the permanent magnets themselves are brittle and have low structural strength they still require a support structure.

Figure 10 shows possible arrangements of low inertia control rotors. The supporting composite tube could be a manufactured from a glass fibre or carbon fibre tube. The examples in Figure 10 a) and b) might be a pre-wound / pre-formed CF tube onto which the magnets are bonded on the inner or outer surfaces. The example in Figure 10 c) may be manufactured for example by bonding magnets onto a pre-formed tube and then overwrapping (wet-winding) a layer of CF/CF with resin and curing. Alternatively a pre-wound tube may be pushed over the inner tube and magnet assembly. A further way to realise such a structure is to form the rotor using a resin transfer mould process (RTM) in which the magnets and layers of CF/CF matting are clamped and held in a mold cavity and low viscosity resin/epoxy is injected under pressure before curing with heat to produce a highly integral part. Alternatively) the support tube may be pre-formed sing RTM techniques and the magnets attached/inserted afterwards. Figure 9 also shows a further variant of the inner PM array in that the magnetic poles are created through circumferentially aligned magnets focussing flux into pole pieces.

Alternatively, the control rotor may be driven by a different machine technology such as: F. IM cage outer (control rotor would have an PM array on one side and a IM cage (wound field or solid bars) rotor F. Switched reluctance C. WF. Synch The above may have benefits in terms of reducing required permanent magnet material but will not have as high a torque density/efficiency.

Figure 11 show two possible arrangement of this embodiment of magnetic power split in a hybrid vehicle power train.

The IC engine in connected to either the inner PM rotor or the pole-piece rotor, while the other is connected to the final drive. The control is not connected to either shaft (but is mechanically supported on bearings to allow it to freely rotate.

Embodiment 2: Two rotor device -Physical control rotor is eliminated and consequent field interacts directly with a

field produced by an external stator

In this embodiment the control element is not a physical rotor, and is achieved through coupling of the consequent field with the field of the control stator. The power split element now consists of one permanent magnet array (typically the output rotor), and a modulating array of pole-pieces, usually the input rotor. The consequent field produced by the interaction the PM generated field with the pole pieces couples with a spatial field produced by a set of windings to produce torque and hence geared operation as shown in Figure 12. If the currents in the winding are fixed the gear would operate with a fixed gear ratio. By supplying the windings which are spatially distributed, typically a three phase degree displaced winding, with a temporally displaced current (typically a three phase) 120 degree displaced currents), the external field rotates. The gear ratio is then altered as explained above.

A possible way to explain this is to imagine the inner and outer arrays on eth control rotor above have the same pole number and are aligned. They then do not require back-iron to carry flux and this can be removed. We then gradually reduce the radial thickness of these arrays. They tend to zero and eventually disappear -but the consequent field has the same pole number and disposition as the field produced by the control stator and so will couple. Therefore the intermediate control rotor is not required.

The resulting two rotor device shown in Figure 13. Other variants of stator and PM rotor are also possible. For example, Figure 14 shows a variant with a different stator with distributed windings as opposed to the concentrated windings shown in Figure 13. Figure 14 shows a variant where the inner PM rotor is a buried magnet/interior magnet design with circumferentially aligned magnets focussing flux into ferromagnetic poles which divert the flux radially into the airgap.

This embodiment has a number of distinct advantages beyond the previous case. For example: * By eliminating the physical control rotor the control system has zero inertia and its response is determined by the much quicker electrical and magnetic time constants. This provides much high bandwidth of control and can improve drivetrain dynamics significantly.

* The torque capability of the gear is no longer fixed (as in the case above with two permanent magnet arrays) and torque is now proportional to current. In highly transient systems this allows the device size to be reduced as currents can be increased for short durations to deal with peak demands with the device thermally rated to deal with a lower mean torque requirement.

* The removal of the physical control rotor which is typically a high speed rotor eliminates its associated bearings, losses and reduces noise (through removal of a large physical rotating element).

* There is a significant reduction in the volume/mass and hence cost of permanent magnet material.

Figure 16 shows two possible arrangement of the two rotor system in a hybrid vehicle power train.

Magnetic gear characteristics allowing for flywheel-less transmission The magnetic gear introduces a degree of compliance into the drivetrain. In this respect the magnetic powersplit can be used to replace the functionality of both the powersplit hybrid architecture and DMF and hence the DMF can be deleted from the system. This can be achieved in two ways, passively using the torsional compliance of the gear alone or additionally actively controlling the gear as discussed below.

Passive cancellation through the magnetic gear compliance The coupling of rotors within the magnetic gear system, is achieved through magnetic fields. This introduces a degree of torsional compliance into the system which is akin to the coupling of a dual mass flywheel (DMF) in which inertias are coupled together through mechanical springs and dampers. Introducing a magnetic gear within a transmission can replace the requirement to have a DMF, and allows for a single degree of freedom tuning system to eliminate a range of frequencies from the drivetrain torque allowing a reduction in torque pulsations. As stated previously, to some extent the inertias are decoupled from the required gear ratio and the inertias can be tuned to achieve a required band of frequency attenuation. This can be considered as a passive system. The stiffness of the system is dictated by the pole-numbers within the magnetic gear, whilst the diameters can be chosen to achieve a given mass. Inherent damping is included through the electromagnetic losses in the system (eddy current losses in magnets, losses in windings etc) but this can be enhanced if necessary with the inclusion of damper winding or damper bars (which are conductive elements often short circuited that have currents induced within them due to changing fluxes from any asynchronous fields which occur during transient rotor oscillations.

By introducing a DMF function through the magnetic gear, any additional flywheel inertias may then be reduced, which allows for rapid vehicle transients while still maintaining a smooth engine output and reduction in NVH.

Although the above feature can be used to eliminate the DMF (with cost and simplification benefits), it is also possible that the system is used in conjunction with a DMF to realise a two degree of freedom (2-DOF) system, which is able to attenuate a wider band of frequencies (which may be employed in luxury class of vehicles for example).

A further advantage of employing a magnetic gear in that it introduces a torque limit. If any loads apply exceed the torque carrying capability of the airgap shear stress developed by the magnetic fields, the rotors disengage and harmlessly pole-slip. The gear will automatically re-engage once the overload torques have passed. This "torque fuse" operation can protect the engine and drivetrain mechanical system from sudden/rapid destructive transients in the drive train, during extreme shock load events such as kerb strikes.

Active cancellation through control of magnetic powersplit currents and torques The attenuation capabilities and hence NVH of a conventional powertrain, employing a DMF could be significantly enhanced by introducing an electrical control element into the transmission which can actively apply cancellation torques by actively controlling currents (in anti-phase to the torque pulsations) to reduce or filter out torque pulsations, as shown in Figure 18.

The primary torque transmission in both the 2 and 3 rotor systems is achieved by developing a reaction torque on the control element. By controlling the currents in either system the function of an active torsional vibration/oscillation cancellation system can be obtained using the magnetic powersplit as shown in Figure 19.

For the case of the 3 rotor system the control element is the control rotor which provides a rotating magnetic field into the magnetic gear system by virtue of the permanent magnet poles on the rotating element. The torque is developed by providing a "load angle" between the magnetic poles which is realised by a relative electrical position between rotors and their corresponding rotating magnetic fields. This implies that the load in the magnetic gear and the torque transmitted through the 3 rotor system can be entirely controlled by changing the load angle in the system which may be achieved by momentarily accelerating or decelerating the control rotor. In this manner the transient energy is stored and returned from the energy storage system and is not simply dissipated to cause loss. However, the VA rating of the inverter has to accommodate this peak power and there are associated conversion losses.

The 2 rotor system differs slightly inasmuch as the control element is the rotating magnetic field generated by the sequential current flow in the phase-displaced windings in the stator. In this case the load angle is always maintained at degrees to give the maximum torque per amp in the stator and achieved the highest efficiency (this control method is analogous to the control of a synchronous machine where the stator field is synchronised to the consequent magnetic field). In this case the primary torque transmitted through the magnetic gear is controlled by modulating the amplitude of the current in the stator windings (again analogous to the control of synchronous machines). This provides further ability to isolate the torque and speed oscillation/pulsations from the engine (or other prime source). For example, if the input rotor has a high level of vibration and torque oscillation the controller can be required to maintain a constant unchanging current magnitude. This implies that the torque transmitted to the output rotor (i.e. the vehicle drive train) will also be constant and the vibration will manifest itself as a speed oscillation of the input rotor (crankshaft and flywheel). The amplitude of the speed variation will be a function of the inertia but the vibration will not be transmitted to the output shaft.

This has a secondary advantage where, unlike other active electromagnetic vibration control systems that provide a reaction torque via a motor/generator system working in anti-phase which generate losses in response to the control action the 2 rotor system will not induce electrical losses or require an increase in rating of drive VA to provide vibration attenuation.

The two-rotor system therefore has significant advantages including: 1. The system has a very high bandwidth as the response is governed by the electrical and magnetic systems only which can have time constants an order of magnitude lower than the mechanical system and does not have to accelerate/decelerate a mechanical inertia.

2. The system provides true attenuation in that the stator will not provide a reaction torque for pulsations at a certain frequency and therefore the pulsations cannot manifest themselves in the powertrain in the first instance which would then have to be attenuated by this or secondary systems (as opposed to a vibration cancelation system).

Control systems for active cancellation systems The undesirable engine torque pulsations accelerate the crank shaft forward, which would reflect a torque through the entire drivetrain system if the shafts were rigidly or compliantly coupled (the amount that is let through depends on the compliance). However, the proposed active cancellation system addresses this in one of two ways depending on the magnetic power split used.

1. In the three rotor system, the control rotor that is not connected directly to the drive train is accelerated forward in response to a potential acceleration of the drive shafts through the interaction of the control rotor with the stator winding currents. This does not allow a load to develop in the magnetic gear and hence the crankshaft acceleration is not transmitted to the drive shaft. The third control rotor experiences the consequent acceleration rather than the drivetrain and hence it is advantageous that this has low inertia (such that it can be rapidly accelerated in response to engine pulsations). The controller demands current and hence torque to accelerate the control rotor potentially in response to an error signal generated when shaft speeds increase above an expected steady state torque.

2. The control system for the two-rotor system differs from above. In a normal speed controlled motor drive system a speed error (caused by an acceleration of the crankshaft input) would lead to a speed error, and hence a speed controller response and hence a torque response to limit the speed error. This is analogous to coupling the shafts together and limiting the differential speed between them. This also causes losses and hence a reduction in efficiency. In the two-rotor rotor system the proposed control would actively ignore the torque pulsations, and allow an acceleration of the consequent magnetic field and not control it i.e. it would not increase the magnitude of the torque in response to the speed error. As there is zero inertia, the torque is not reflected anywhere else in the system.

For either system, the input controller acts to control vibration by responding to slight position/velocity/accelerations detected on the drive shafts. This could be improved by using pre-knowledge of the incoming vibration (for example the crankshaft oscillation caused by pulsating piston torque can be largely determined using knowledge of the geometry and combustion parameters of the engine). Using model predictive control, the controller then applies the required action in a pre-emptive fashion rather than responding to an error signal. Crankshaft oscillations are a function of cylinder pressure acting on the piston, the piston imparts torque onto the crank shaft and is a predictable cycle depending on engine load and speed and can be mapped/or stored as a look-up table. Therefore, any controller can have prior knowledge of incoming loads based on driver inputs and/or vehicle performance. In the 3-rotor system, this knowledge can be employed so that the controller is not waiting for a speed error to develop, and instead pre-accelerates the rotor to overcome the system response/time constant i.e. time for torque to develop in response to a demanded increase due to inductance limiting rate of rise of current etc. In the 2-rotor system, as stated above the controller differs in that it is effectively ignoring disturbances in the system to maintain a constant torque. However, the system has to respond to required/demanded acceleration and must differentiate these from the undesirable torque pulsations.

Further optimisation of dynamic response through Cantilevered Open CUD Structure It is apparent that vibration control and attenuation is highly linked to the rotational inertias in the system and the ability to tune this inertia upwards and downwards can offer a range of design benefits. For example: * Increased engine transient response * Reduced flywheel requirements * Higher bandwidth active cancellation * Lower vibrational energy In the magnetic gear system the diameters of the components are not linked to the gear ratio and hence the inertia is not dictated by the gear ratio. This advantage can be enhanced by the use of low inertia rotating components. One method of achieving this is to use a cantilevered open-cup structure where the rotating element is only supported at one side as shown in Figure 21 as opposed to a more traditional approach as shown in Figure 20. This also has the added benefit of reducing the sub-assembly cost by virtue of the reduced part-count, simplifying assembly and increasing system efficiency by removing a bearing and element that may have magnetic losses induced in them due to

asynchronous leakage fields.

This open cup technique can be also used to tailor the torsional characteristics of the gear to further degrees of design freedom to enhance vibration attenuation and performance over a range of loads. Figure 22 shows a possible pole-piece rotor frame structure for holding the laminated pole-pieces. This structure must be non-magnetic to allow correct modulation by the pole-pieced and is preferably non-metallic to prevent eddy currents or "cage" currents which would lead to losses and reduced efficiency. It is typically manufactured from an engineering plastic or composite material. It may be machined or preferably moulded from a composite material for example by Resin Transfer Moulding as discussed above. The support structure may be pre-formed and pole-pieces inserted and held afterwards or the pole-pieces may be over-moulded into the structure. By manufacturing the pole-piece rotor support structure from suitable materials with particular modulus of elasticity, the pole piece rotor can allow a degree of twist as it is loaded (the load torque applied causing a torsional deformation which is progressive along the length away from the fixed end. This adds an additional compliance into the system providing a further degree to allow attenuation of torsional oscillations from manifesting on the drive train. The materials may also be selected for their damping properties to again reduce torque oscillations. It is also possible to include a degree of reverse skew (when unloaded) as shown in Figure 23 and allow load torque to untwist increasing torque capability (rather than diminish) as the pole-pieces are forced into correct position to maximise the magnetic modulation.

Claims

Claims 1. A system comprising an input member, an output member, a magnetic gear connecting the input member to the output member and control means arranged to control the flow of power from the input member to the output member wherein the magnetic gear comprises a first set of magnetic poles, a second set of magnetic poles) and a set of pole pieces arranged to modulate the magnetic field between the first set of magnetic poles and the second set of magnetic poles and wherein the system further comprises means for reducing the transmission of torque pulsation and/or oscillation from the input member to the output member.
2. A system comprising an input member, an output member, an energy supply means, an energy sink means and a magnetic gear connecting the input member to the output member) wherein the magnetic gear comprises a set of permanent magnets, a set of windings, and a set of pole pieces arranged to modulate the magnetic field between the set of permanent magnets and the set of windings; the system further comprising control means arranged to control the current in the windings, and thereby control the flow of power from the input member to the energy sink means or from the energy supply means to the output member if a required power at the output member is different from a power at the input member.
3. The system of claim 2 wherein the set of permanent magnets is mounted on a rotor associated with one of the input member and the output member and the set of pole pieces is mounted on a rotor associated with the other of the input member and the output member.
4. The system of claim 2 or claim 3 wherein the control means are configured to control the current in the windings such that the magnetic field produced by the windings interacts with a consequent magnetic field generated by modulation by the set of pole pieces of a magnetic field produced by the set of permanent magnets.
5. The system of claims 2 to 4 wherein the control means are configured to control the current in the windings such that the magnetic field produced by the windings rotates.
6. The system of any of claims 2 to 5 wherein the control means is configured to vary the amplitude of the current in the windings to produce a required torque.
7. The system of any of claims 2 to 6, wherein the system further comprises means for reducing the transmission of torque pulsation and/or oscillation from the input member to the output member.
8. The system of claim 1 or claim 7 wherein the means for reducing the transmission of torque pulsation and/or oscillation comprises damping means.
9. The system of claim 8 wherein the damping means comprises a damper bar.
10. The system of claim 8 wherein the damping means comprises a damper winding.
11. The system of claim 1 or claim 7 wherein the control means comprises the means for reducing the transmission of torque pulsation and/or oscillation, the control means being configured to apply cancellation torque.
12. The system of claim 11 wherein the cancellation torque is applied in response to measurements of position, velocity or acceleration of the input member.
13. The system of claim 11 wherein the cancellation torque is determined using predictions of input member torque pulsations.
14. The system of claim 13 wherein the predictions of input member torque pulsations are calculated using geometry and combustion parameters of an engine connected to the input member.
15. The system of claim 1 or claim 7 wherein the means for reducing the transmission of torque pulsation and/or oscillation comprises a cantilevered open-cup rotor on which the set of pole pieces is mounted.
16. The system of claim 15 wherein the cantilevered open-cup rotor is reverse skewed when unloaded.
17. The system of claim 16 wherein the cantilevered open-cup rotor is untwisted under load torque.
18. The system of claim 1 or claim 7 wherein the means for reducing the transmission of torque pulsation and/or oscillation comprises a pole piece support structure formed from compliant material.
19. The system of claim 18 wherein the compliant material is a composite or engineering plastic.
20. The system of claim 1 wherein the first set of magnetic poles comprises a first set of permanent magnets.
21. The system of claim 1 wherein the first set of magnetic poles is associated with a first moveable element.
22. The system of claim 1 wherein the set of pole pieces is associated with a second moveable element.
23. The system of claim 21 wherein the first moveable element is connected to the input member.
24. The system of claim 21 wherein the first moveable element is connected to the output member.
25. The system of claim 22 wherein the second moveable element is connected to the input member.
26. The system of claim 22 wherein the second moveable element is connected to the output member.
27. The system of claim 1 wherein the second set of magnetic poles is produced by a set of windings.
28. The system of claim 27 wherein the windings are mounted on a stator.
29. The system of claim 27 or claim 28 wherein the current in the windings is controlled by the control means.
30. The system of claim 1 wherein the second set of magnet poles comprises a second set of permanent magnets.
31. The system of claim 30 wherein the second set of permanent magnets is mounted on a third moveable element.
32. The system of claim 31 wherein the control means is arranged to control movement of the third moveable element.
33. The system of any preceding claim wherein the input member is connected to an engine.
34. The system of any preceding claim wherein the system forms part of a vehicle drivetrain.
35. The system of claim 1, or any of claims 8 to 34 when dependent on claim 1. further comprising an energy storage system, wherein the control means is configured to control the flow of power from the input member to the energy storage system or from the energy storage system to the output member if a required power at the output member is different from a power at the input member.
36. The system of claim 2 wherein the energy supply means and the energy sink means comprise an energy storage system.
37. The system of claim 36 wherein the energy storage system is a battery.
38. The system of claim 2 wherein the energy sink means comprise a consumer of electrical power.
39. The system of claim 2 wherein the energy sink means comprise a traction motor.
40. The system of claim 2 wherein the energy sink means comprise a hotel load of a vehicle.
41. A system comprising an input member, an output member, a magnetic gear connecting the input member to the output member and control means arranged to control the flow of power from the input member to the output member wherein the magnetic gear comprises a first set of magnetic poles, a second set of magnetic poles) and a set of pole pieces arranged to modulate the magnetic field between the first set of magnetic poles and the second set of magnetic poles and wherein a number of magnetic poles in the first and second sets of magnetic poles, a number of pole pieces in the set of pole pieces and inertias of moveable elements in the magnetic gear are selected to attenuate torque pulsation and/or oscillation in a bandwidth of frequencies.