WO2012042278A2

WO2012042278A2 - Electromechanical device

Info

Publication number: WO2012042278A2
Application number: PCT/GB2011/051864
Authority: WO
Inventors: Gerard Sauer; Marcus Allard; Stephen Grosvenor
Original assignee: Ets Design Ltd.
Priority date: 2010-10-01
Filing date: 2011-09-30
Publication date: 2012-04-05
Also published as: GB201016603D0; GB2484147A; WO2012042278A3

Abstract

An electromechanical device, comprising: a first member and a second member arranged to move relative to the first member and comprising first and second retardation portions; wherein the first member is arranged to generate a movable magnetic field and wherein the relative speed at which the magnetic field moves relative to the second member is adjustable, wherein at a first relative speed the interaction of the magnetic field with the second member results in the first retardation portion generating an electrical current and a retardation force opposing the movement of the second member being applied to the first retardation portion; and wherein at a second relative speed, different from the first relative speed, the interaction of the magnetic field with the second member results in a) the amount of current generated by the first retardation portion being less than the amount of current generated by the first retardation portion at the first relative speed and b) an amount of retardation force opposing the movement of the second member being applied to the second retardation portion, which is greater than an amount of retardation force applied to the second retardation portion at the first relative speed.

Description

Electromechanical device

Field of the invention

The present invention relates to an electromechanical device. Background of the Invention Commercial and passenger carrying vehicles have for many years been fitted with secondary braking systems know as retarders, and in some applications such devices are mandatory. The main benefits of retarders are enhanced safety as a result of prevention of overheating of the primary braking system, and extended life of the primary braking system components.

One of the principal types of retarder is the electromagnetic or eddy current retarder. An electromagnetic retarder has two main components, a stator and a rotor, and it is usually mounted in the driveline of the vehicle between the output of the gearbox and the input of the axle.

The stator, which can be fixed to either the gearbox, the axle or the chassis of the vehicle, consists of a circular array of coils positioned around a bearing housing. Each coil consists of wire wound around a core of magnetic material such as iron. The rotor consists of a central shaft and a metal disc at each end, usually iron. The discs are positioned such that there is a small air gap between each disc and the shoes which are fixed to the ends of the coil cores. The shoes themselves are provided to aid with the distribution of the magnetic field generated by the coils. The rotor is permanently fixed to the propeller (prop) shaft of the vehicle so that it rotates as the shaft rotates.

To operate the retarder, a DC voltage is applied to the coils, creating an electromagnetic field. This field generates eddy currents in the rotor discs as they rotate within the field. These eddy currents cause a braking torque to act upon the rotor discs which opposes their rotation, thus slowing the vehicle. Additionally, the eddy currents cause heat to be generated within the rotor discs, which is typically dissipated via cooling fins on the discs. In this way, the kinetic energy of the vehicle is converted to heat. The vehicle slows down and the heat is lost to the air surrounding the retarder. Once the braking operation has been completed, the DC voltage to the coils is simply switched off to allow the rotor discs, and hence the prop shaft, to rotate freely.

The general principle of regenerative braking in vehicles is also known. In regenerative braking, kinetic energy of a moving vehicle is converted into a useful form of energy during braking and stored for subsequent use, rather than simply being dissipated as heat.

One method of regenerative braking is used for vehicles powered, in whole or in part, by electric motors, and involves reversing the operation of the electric motor under braking conditions, such that the motor acts as an electrical generator. Operating the motor as a generator provides a resistive torque which acts to slow the vehicle, and as such the motor provides a retarding function which assists the primary braking system of the vehicle. Additionally, electricity generated by the reversely- operated motor may be stored in batteries or capacitors, and later supplied to the electric motor to assist in driving the vehicle.

However, the amount of braking torque provided by this type of system is governed by the amount of generated electrical current which the power control electronics of the system, which directs the generated current to the batteries or capacitors, is able to handle i.e. the electrical power rating of the power control electronics. The higher the power rating, the greater the cost of the power control electronics. Hence, in order to achieve high braking torques, power electronics with a high power capability are required, leading to increased costs for the system.

Additionally, the batteries or capacitors may become fully charged under a period of sustained or repeated braking, for example when the vehicle travels downhill. In these circumstances, it is no longer possible to channel further electrical energy to the batteries or capacitors, such that the motor/generator no longer provides a retarding function. To overcome this problem, further electrical energy generated during braking can be channelled via the power control electronics to a resistor to be "dumped" or lost as heat. However, this requires additional componentry, and increases the cost of the system.

Retarder-like devices are also used in other applications, for example in fitness equipment. Some types of fitness equipment, in particular rowing machines, exercise bicycles and cycle trainers use a resistive torque applied to a rotating part as a way of absorbing power generated by the user. This resistive torque may be applied in any of the following ways:

1. A fan rotating in air

2. A fluid being pumped through a variable orifice

3. A friction brake 4. A permanent magnet eddy current device

5. An electrical generator

The range of equipment runs from very simple machines with a crudely adjustable fan to complex and expensive virtual reality trainers incorporating a motor to simulate downhill sections of cycle races. Some machines will even feed electricity back into the grid.

However, on many machines the adjustment of resistance is basic, for example by adjusting the inlet vanes on a fan, or by physically moving the magnets on an eddy current device. Further, the machines may be noisy, for example where a fan is employed to provide the resistive torque, and may need to be powered by mains or battery power supplies.

The present invention seeks to address problems associated with the prior art.

Statement of the Invention

According to a first aspect of the present invention, there is provided an electromechanical device according to claim 1.

According to a second aspect of the present invention, there is provided a system according to claim 28.

According to a third aspect of the present invention, there is provided an electromechanical device according to claim 29.

According to a fourth aspect of the present invention, there is provided a machine according to claim 36.

Brief Description of the Drawings In order that the present invention may be more readily understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

FIGURE 1 shows a system provided with a regenerative retarder according to a first embodiment of the present invention attached to a chassis of a vehicle;

FIGURES 2 A to 2C show the regenerative retarder of Fig. 1 in more detail;

FIGURE 3 schematically illustrates electrical connections in the system of Fig. 1; FIGURE 4 schematically illustrates the three-phase winding of the regenerative retarder of Fig. 1;

FIGURE 5 is a graph illustrating torque applied by the regenerative retarder of Fig. 1 at different slip speeds; and

FIGURE 6 schematically illustrates a second embodiment of an electromechanical device according to the present invention in the form of a self energising retarder.

Detailed Description of the Embodiments In the following description, like and functionally similar parts carry the same reference numerals between different embodiments.

First Embodiment Figure 1 shows a regenerative retarder 1 according to a first embodiment of the present invention mounted to a chassis 2 of a vehicle in place of a conventional centre bearing for the prop shaft 3 of the vehicle. The prop shaft 3 transmits a driving force from a motor (not shown), which may for example comprise an internal combustion engine, to a driven axle (not shown). Also mounted to the chassis 2 is a supercapacitor unit 5 and a control system and power electronics module 6, all of which are electrically connected to the regenerative retarder 1.

The regenerative retarder 1 of the present embodiment is shown in greater detail in Figs. 2A to 2C, and comprises an outer casing 7 containing a central bearing housing 8, a stator 9 and a rotor shown generally at 10A and comprising a pair of rotor discs 10 (of which one is illustrated in Figs. 2B and 2C, the other disc corresponding thereto) which are fixedly fitted to a rotor shaft (not shown) such that the rotor discs 10 are located on respective sides of the stator 9 in the fully-assembled regenerative retarder 1 and rotate as one with the rotor shaft. The rotor shaft is rotatably supported within the casing 7 by bearings contained within the central bearing housing 8 and is connected between first and second shafts 3A, 3B of the prop shaft (see Fig. 1) such that it rotates as the prop shaft 3 rotates, and vice versa.

The stator 9 comprises a circular array of nine coils 12, each of which is wound onto a respective laminated coil core 13. The laminated cores 13 are bolted via respective pairs of coil mounting spacers 16 onto a pair of coil mounting rings 15 (of which one is illustrated in Fig. 2B, the other coil mounting ring corresponding thereto). The coil mounting rings 15 are bolted between a pair of end plates 14, which end plates 14 are in turn bolted to the outer casing 8 and also to the central bearing housing 8. Hence, the stator 9 is fixed to the outer casing 7. The circular array of coils 12 surrounds the central bearing housing 8 and thus also the rotor shaft in the assembled regenerative retarder 1.

As shown in Fig. 2C, each rotor disc 10 comprises a copper "wheel" 18 sandwiched between inner and outer rotor disc faces 19, 20, which are secured together by a plurality of bolts. The copper wheel 18 is formed from a plurality of copper bars 21 (numbering thirty-one in the present embodiment) which are slotted into radially-extending grooves 22 formed in the rotor disc faces 19, 20, and which are connected at their radially inner and outer ends by copper shorting rings 23, 24 thus giving the overall appearance of a spoked wheel. The rotor disc faces 19, 20 are formed of iron, and the outer disc face 20 is provided with a plurality of cooling fins 25 on its exterior side i.e. the side axially furthest from the stator 9.

As shown in Fig. 3, the supercapacitor unit 5 is electrically connected to the stator coils 12 via the control system and power electronics module 6, which comprises a variable frequency inverter 26 and a control module 27. The control system and power electronics module 6 is further connected to a rotor disc rotation speed detector 28 operable to detect the rotational speed of the rotor discs 10. The rotor disc rotation speed detector 28 may for example comprise a component of the vehicle's own control system, such as an ABS system, which produces data from which the rotational speed of the rotor discs 10 may be assessed, or a rotation speed detection device such as an encoder attached to the rotor discs 10 or other part of the rotor 10A, or a detector which is operable to analyse the current flow in the coils 12 so as to determine the rotational speed of the rotor discs 10.

The variable frequency inverter 26 converts a DC supply from the supercapacitor unit 5 to a three-phase AC supply which is used to power the array of coils 12 in such a manner that the array of coils 12 produces a rotating magnetic field.

The three-phase winding of the coils 12 is schematically illustrated in Fig. 4. The coils 12 are split into three groups (indicated as A, B and C in the Figure) with the coils 12 of each group being located alternately around the stator 9 and with each group being connected to a respective one of the three phases of the power supply. The current of each phase varies sinusoidally, and is out of phase by 120 degrees from the following phase, such that, taking one phase as a reference, the other two phases are delayed in time by one-third and two-thirds of a cycle, respectively. Thus the current in each of the three groups A, B, C of coils 12, and therefore the magnetic fields generated thereby, peaks at different times for each of the three groups A, B, C of coils 12. The sum of the magnetic field vectors produced by the coils 12 is a magnetic field vector which rotates around the stator 9. The coils 12 are wound around the coil cores 13 such that the overall magnetic field vector is directed axially i.e. parallel to the central axis of the circular stator 9 so as to permeate the rotor discs 10 located at either side of the stator 9.

The control module 27 of the control system and power electronics module 6 is operable to control the frequency of the AC power supply output by the variable frequency inverter 26, and hence to control the speed of rotation of the magnetic field generated by the stator 9. Additionally, the control system and power electronics module 6 is able to control the amount of current supplied to the stator coils 12. This current control is achieved via pulse width modulation in the present embodiment. The control system and power electronics module 6 is further operable to direct electrical current generated within the stator coils 12, when the regenerative retarder 1 is operated as a generator as discussed hereinafter, to the supercapacitor unit 5 for storage.

The regenerative retarder 1 according to the present embodiment may be operated as an electrical induction motor to drive or assist in driving the vehicle, and as a regenerative retarder which retains full retarding capability regardless of the state of charge of the supercapacitor unit 5. These modes of operation are effected by adjusting the slip speed of the regenerative retarder 1 i.e. the difference in rotational speed between the rotating magnetic field generated by the coils 12 and the speed at which the rotor discs 10 rotate. At low slip speeds the regenerative retarder 1 will efficiently convert kinetic energy of the vehicle into electrical energy, and vice versa. At high slip speeds the regenerative retarder 1 is very inefficient and kinetic energy of the vehicle is converted to heat in the rotor discs 10 rather than electrical energy in the coils 12. This enables the regenerative retarder 1 to retain its full retarding functionality, even when the supercapacitor unit 5 is fully charged.

In more detail, when a driving demand is made of the regenerative retarder 1 (which may for example be signalled by a driver of the vehicle depressing an acceleration pedal), the control system and power electronics module 6 provides a three-phase AC supply to the stator coils 12 by drawing a current from the supercapacitor unit 5. In particular, the control module 27 of the control system and power electronics module 6 notes the rotational speed of the rotor discs 10 as detected by the rotor disc rotational speed detector 28 and controls the variable frequency inverter 26 to output the AC supply at a frequency corresponding to a low slip speed between the rotational speed of the magnetic field and the detected rotational speed of the rotor discs 10. Specifically, the magnetic field is controlled to rotate at a similar, but slightly higher, rotational speed than the rotor discs 10. This relative rotational speed differential results in the magnetic field experienced by the copper bars 21 of each rotor disc 10 varying with time, resulting in a current being induced in the copper bars 21. This induced current in turn creates a magnetic field which interacts with the magnetic field originating from the stator coils 12, with the result that a driving torque is applied, acting in the same direction of rotation, to each of the two rotor discs 10. The control system and power electronics module 6 is able to control the amount of driving torque produced in this way by adjusting the amount of current flowing to the coils 12, using pulse width modulation. The driving torque is transmitted by the rotor discs 10 to the prop shaft 3, thus providing a driving force to move the vehicle. Hence, the regenerative retarder 1 acts as an axial induction motor in this mode.

In the present embodiment, the number of bars 21 has been selected to facilitate the smooth driving of the rotor discs 10 by the magnetic field generated by the stator coils 12. Specifically, the present embodiment utilises a prime number of bars 21 (namely, thirty-one) and as such the lowest common multiple of the number of bars 21 and the number of coils 12 is high. As a result, "cogging" effects (wherein the rotating bars 21 are oriented relative to the magnetic field such that the rotor discs 10 are jerked forward more strongly than at other orientations) are ameliorated. However, other numbers of bars 21 (prime or otherwise) may be employed, according to the operational requirements of a given application.

When a braking demand is made of the regenerative retarder 1 (which may for example be signalled by a driver of the vehicle depressing a brake pedal), the regenerative retarder 1 is operated in an inductive generator mode. In this mode, the control module 27 of the control system and power electronics module 6 again notes the rotational speed of the rotor discs 10 as detected by the rotor disc rotational speed detector 28 and draws a current from the supercapacitor unit 5, and controls the variable frequency inverter 26 to supply an AC current to the coils 12 to create a magnetic field which rotates at a similar but slightly slower speed than the rotor disc 10. As each rotor disc 10 is therefore again moving relative to the magnetic field, currents are induced in the copper bars 21 which in turn establishes a magnetic field around the bars 21. This induced magnetic field then induces currents in the stator coils 12, which currents are transmitted by the control system and power electronics module 6 to the supercapacitor unit 5 for storage. Hence, the regenerative retarder 1 acts as an axial induction generator in this mode, and the charge stored in the supercapacitor unit 5 can subsequently be re-used to power the regenerative retarder 1 when it is operated in its axial induction motor mode.

Additionally, when the regenerative retarder 1 is operated in its axial induction generator mode, a retarding torque results on the rotor discs 10, hence acting to slow the rotation of the prop shaft 3 and accordingly to assist in slowing the vehicle itself. The control system and power electronics module 6 is able to control the amount of retarding torque produced in this way by adjusting the amount of current flowing to the coils 12, using pulse width modulation. However, as noted previously, once the supercapacitor unit 5 is fully charged it will accept no further charge, such that the retarding force resultant from the regenerative retarder 1 being operated as a generator diminishes.

The present regenerative retarder 1 can however retain its full retarding capability in these circumstances, as once the supercapacitor unit 5 is fully charged, the control system and power electronics module 6 then operates to increase the slip speed between the magnetic field and the rotor discs 10. The efficiency of the regenerative retarder 1 as a generator reduces significantly as the slip speed increases, such that the kinetic energy of the vehicle is no longer efficiently converted into electrical energy in the coils 12. However, eddy currents generated in the iron rotor disc faces 19, 20 increase significantly as the slip speed increases, resulting in a significant retarding torque being applied thereto, as well as heat being generated in the rotor discs 10. That is, as the slip speed increases, the regenerative retarder acts decreasingly as an axial induction generator, and increasingly as a conventional electromagnetic retarder. Consequentially, the retarding capability of the regenerative retarder 1 may be retained, even though the supercapacitor unit 5 is fully charged. This is illustrated by the graph of Fig. 5, which shows that the torque acting on the copper bars 21 rapidly increases from zero at zero slip speed to a peak at a low slip speed of around 35 r.p.m., and thereafter rapidly decreases at increasing slip speeds. In contrast, the torque on the iron rotor disc faces 19, 20 increases steadily with increasing slip speed to a maximum at a much higher slip speed than the copper bars 21, and substantially plateaus at this peak torque level within the illustrated r.p.m. range.

Further, by controlling the slip speed it is also possible to convert a desired proportion of the vehicle's kinetic energy into electrical energy for diversion to the supercapacitor unit 5, and to "dump" a further proportion as heat in the rotors discs 10, by selecting a suitable slip speed and hence a corresponding effective efficiency of the axial induction generator function of the regenerative retarder 1.

It should also be noted that the amount of retardation which may be provided by the regenerative retarder 1 is not limited by the amount of current which may be channelled from the coils 12 to the supercapacitor unit 5 by the control system and power electronics module 6, unlike a conventional electric regenerative braking system, as by adjusting the slip speed the regenerative retarder 1 may convert significant amounts of the kinetic energy of the vehicle into heat in the rotor discs 10, in addition to or in place of converting kinetic energy of the vehicle into electrical energy. Indeed, if the frequency of the AC supply to the retarder coils 12 is reduced to zero by the control module 27, then it becomes a DC supply and the regenerative retarder will function in exactly the same way as a conventional electromagnetic retarder. In the extreme, therefore, the regenerative retarder 1 may in fact provide a braking torque without having to channel any current to the supercapacitor unit 5 (or to a resistive dump unit to be lost as heat, as in a conventional electric regenerative braking system).

Heat generated in the rotor discs 10 is dissipated therefrom, and in particular from the cooling fins 25, as in a conventional retarder. Advantageously, increased rotation speed of the rotor discs 10 results in a greater air cooling effect, thus helping to dissipate higher levels of eddy current heating which may be encountered such as when braking is initiated. Further, the rotor discs 10 of the present regenerative retarder 1 may be arranged to warp when heated in such a manner that the air gap between the rotor discs 10 and the coils 12 increases. This can act to protect the rotor discs 10 from unwanted heating levels, as the electromagnetic braking effect on the rotor discs 10 will decrease as the air gap increases, hence reducing eddy current heating in the rotor discs 10.

An additional advantage of the present embodiment is that a conventional electromagnetic retarder typically requires a vehicle to have an upgraded alternator to supply the necessary excitation current (i.e. the current required to generate the magnetic fields which result in a braking torque being applied to the rotor). This may however potentially be avoided according to the present embodiment, as currents generated by the regenerative retarder 1, when it is operated as an axial induction generator, may be used as the excitation current for the stator coils 12. Hence, upgrades to the main vehicle electrical system may potentially be avoided.

Alternative applications The first embodiment of a regenerative retarder 1 is described above in use in a vehicle braking system. However, the present invention is not limited to this application, and embodiments of the present invention may be employed in other applications. For example, regenerative retarders 1 according to embodiments of the invention may be employed to provide a braking and/or driving torque in applications such as dynamometers, winding gear, elevator, ski lifts and cable car systems.

In a further example, regenerative retarders 1 according to embodiments of the invention may be employed as the primary source of resistive torque in fitness equipment machines, such as rowing machines, exercise bicycles, cycle trainers and the like, with a number of potential advantages, in particular in terms of improvements in control and noise reduction. Furthermore, by operating the regenerative retarder 1 in its axial induction generator mode, power may be generated as the user exercises, which power may be used to power the coils 12 of the stator 9 to produce the rotating magnetic field discussed above, as well as to run control and display functions of the fitness equipment machine. As such, machines provided with a regenerative retarder 1 according to the present invention would not require an external power source such as a mains supply. Yet further, by operating the regenerative retarder 1 in its axial induction motor mode, downhill simulation on cycle trainers may for example be provided, whereby the actions of the user in turning the pedals of the machine are aided by a driving torque being applied to the rotor discs 10 of the regenerative retarder 1, in the manner described above.

In more detail, a regenerative retarder 1 according to an embodiment of the present invention may be employed in place of a fan rotating in air in a conventional rowing machine, potentially to provide a more compact, much quieter unit, with the possibility of programmable resistance settings which could be stored in memory to suit a particular user. It would also be possible for users of different weights to compete against each other by selecting the resistance according to weight. The different resistance settings may be achieved by the control system and power electronics module 6 providing a greater or lesser current to the coils 12. Alternatively, the control system and power electronics module 6 can adjust the slip speed, and hence the overall torque experienced by the rotor discs 10.

Yet further, a regenerative retarder 1 according to an embodiment of the present invention may be employed in place of a permanent magnet eddy current brake, such as may typically be found in a mid-priced cycle trainer. By being run in its axial induction generator mode, the regenerative retarder 1 could self-power the electromagnetic coils 12 of the stator 9, as well as providing stepless resistance control to a user - by adjusting the slip speed and/or the amount of current directed to the coils, the power electronics and control module 6 can finely adjust the resistive torque experienced by the rotor discs 10. The onboard control electronics for the cycle trainer may also be powered by the electricity generated by the regenerative retarder, such that mains power for the trainer would not be required.

In other examples, the regenerative retarder 1 could replace the friction pad typically used for resistance in club type cycle trainers, to allow for self-generated power to be produced. This power may for example be used to power the control system and power electronics module 6 of the regenerative retarder 1. Furthermore, as the regenerative retarder is controlled electronically (namely, by the control system and power electronics module 6), more accurate resistance control may potentially be provided than by a mechanically-operated friction pad. Furthermore, the machine would typically require lower maintenance, as unlike a friction pad, no wear is caused on the parts of the regenerative retarder 1 during its operation.

Alternative embodiments

In the above-described embodiment, the bars 21 and shorting rings 23, 24 of the wheel 18 are formed from copper, but other materials, for example non-ferrous materials such as aluminium or brass may be used. Further, although the bars 21 are connected by inner and outer shorting rings 23, 24 either or both of these rings may be omitted. Yet further, the wheel 18 may be incorporated into the rotor discs 10 in a variety of ways. For example, the wheel 18 may be (a) attached to the face of the rotor discs 10 facing the coil 12, or (b) embedded in the face of the rotor discs 10 facing the coil 12, so that the surface of the wheel 18 is flush with the surface of the iron of the rotor discs 10, or (c) the wheel 18 may be completely embedded within the rotor discs 10, or (d) the wheel 18 may be attached to the face of the rotor discs 10 that is furthest from the coils 12. However, arrangements in which the wheel 18 is embedded within the rotor discs 10 (such as in the embodiment of Fig. 2B) are generally preferred, as this reduces the degree to which the iron of the rotor discs 10 is shielded from the stator magnetic field by the copper wheel 18.

According to further embodiments, the rotor discs 10 may be manufactured in other ways. For example, the wheel 18 may be cast directly within the rotor discs 10, by pouring molten material for the wheel 18 (e.g. molten copper) into a suitably-formed hollow within the main body of the rotor disc 10. Where this approach is taken, the materials for the main body of the rotor disc 10 (e.g. iron) and the wheel 18 (e.g. copper) should be selected carefully to ensure that their relative melting points permit this method of formation. In a further example, powdered material for the main body of the rotor disc 10 (e.g. powdered iron) may be sintered around the wheel 18 to form the completed rotor disc 10.

The bars 21 are arranged radially on the rotor disc faces 19, 20 in the first embodiment. However, these bars 21 may instead be skewed slightly relative to this radial positioning, to reduce resonance effects which might otherwise occur in operation of the regenerative retarder 1.

In the above-described embodiment, the coil cores 13 are laminated to reduce the effect of eddy current heating during operation. However, other means for reducing heating may be employed. For example, the cores 13 may be made from soft magnetic composites.

A ring of nine coils 12 is employed in the first embodiment, but any other suitable number of coils 12 may be used. For example, three or six coils 12 may be used.

A supercapacitor unit 5 is used to store electrical energy in the above embodiment. However, electrical energy may instead be stored in one or more batteries, or in a combination of batteries and supercapacitors.

According to further embodiments of the present invention, it is possible to retrofit existing electromagnetic retarder devices i) by fitting bars 21 to the rotor, ii) modifying the control circuitry to provide a three-phase AC current, rather than a DC current, under the control of a control system and power electronics module 6 connected to a suitable rotor disc rotational speed detector 28, and iii) replacing existing coil cores with laminated or soft-magnetic coil cores to counter eddy-current heating effects associated with the AC -power supply, to provide an arrangement according to the present invention. Alternatively, a regenerative retarder 1 according to the present invention could be retrofitted to a vehicle either as a new installation or to replace an existing conventional retarder.

The regenerative retarder 1 of Fig. 1 is fitted in place of a conventional centre-bearing unit on the prop shaft 3. However, a vehicle may be provided with one or more of the regenerative retarders 1, fitted in other ways. For example, a regenerative retarder 1 may be fitted at the wheel hub of one or more wheels of the vehicle.

In the first embodiment described above, the regenerative retarder 1 is provided as an auxiliary braking unit, to supplement the main braking system of the vehicle. However, according to further embodiments, the regenerative retarder 1 and the main braking system may be combined into a single unit. In particular, in an alternative embodiment, one or both of the rotor discs 10 of the regenerative retarder 1 also serves as a friction brake disc, which is clamped by callipers as in a conventional braking system, in addition to being subjected to magnetic fields from the stator coils 12. Alternatively, the regenerative retarder 1 may be provided with a single rotor disc 10, to be braked by clamping callipers and by the stator magnetic field. The combined disc brake and regenerative retarder 1 could be hub mounted on a vehicle to provide a motor / generator at each wheel. This could be used in a pure electric application (i.e. in a vehicle driven purely by electrical means) or as part of a hybrid system when combined with another power source such as an internal combustion engine.

The first embodiment has a number of potential advantages. For example:

1. Low cost. The design of the regenerative retarder 1 makes it significantly cheaper to manufacture than a permanent magnet machine of similar power.

2. Full retarder capability may be maintained even when energy store (i.e. the supercapacitor unit 5) is full, as if the energy store is full, excess energy can be dissipated in the rotor discs 10 themselves as heat.

3. Maximum retarder power can be much higher than the maximum power which may be handled by the control system and power electronics module 6, by dissipating some or all of the kinetic energy of the vehicle into heat in the rotor discs 10 rather than simply relying on the regenerative retarder 1 being operated in its axial induction generator mode to provide a resistive torque to the rotor discs 10. As such, the power handling capability of the control system and power electronics module 6 can be optimised for cost and fuel saving at a fairly low power, but the system retains the capability of providing extremely high braking torque even at high vehicle speeds. Second Embodiment

A second embodiment of an electromechanical device according to the present invention, in the form of a self energising electromagnetic retarder R will now be described with reference to Fig. 6. The self energising electromagnetic retarder R comprises a stator 9 and a rotor 10A. The stator is fixed to the casing 7 of the self energising electromagnetic retarder R and comprises a plurality of DC-powered brake coils 29, which are supplied with a DC supply from a battery 5 under the control of a power control module 30, and a set of generator coils 33, which are electrically connected to the battery 5 and to the power control module 30 via a bridge rectifier/regulator module 34.

The rotor 10A comprises a pair of rotor discs connected by a rotor shaft 11. The rotor shaft 11 may for example be fixedly fitted between first and second portions of a drive shaft, as in the first embodiment, or may for example be driven by a belt (not shown) or other means. A small air gap is provided between the rotor discs 10 and the DC brake coils 29 of the stator 9.

Permanent magnets 32 are fitted to the exterior of the rotor shaft 11, and are spaced by a small air gap from the generator coils 33 of the stator 9.

In operation, the power control module 30 supplies a DC voltage from the battery 5 to the brake coils 29 of the stator 9, thus creating a magnetic field, and resulting in a resistive torque being applied to the rotor discs 10 as they rotate within this field owing to eddy-current effects. By varying the amount of current supplied to the brake coils 29, for example via pulse width modulation, the power control module 30 may adjust the amount of resistive torque experienced by the rotor 10A. In particular, the power control module 30 may be arranged to cause different retarder torque profiles to be effected. These could include (a) constant torque independent of speed (b) torque increasing in a linear fashion with respect to speed (c) torque increasing exponentially with respect to speed (d) zero torque until a certain speed threshold is reached and then a torque applied according to (a), (b), or (c) above.

At the same time, the movement of the permanent magnets 32 relative to the generator coils 33 as the rotor shaft 11 rotates results in an alternating current being induced in the generator coils 33. This AC voltage is then converted to DC by the bridge rectifier/regulator 34. This direct current may then be stored in the battery 5 for use in powering the brake coils 29.

Furthermore, this direct current may also be used to power the power control module 30.

The generated current may also be used for other purposes e.g. to power a performance monitoring system where the self energising electromagnetic retarder R is used in fitness equipment.

As such, the self energising electromagnetic retarder R does not require any external power supply to operate; rather, the self energising electromagnetic retarder R may be self-powered. This provides an additional safety feature, for example where the self energising electromagnetic retarder R is used in an application such as a vehicle braking system, as the self energising electromagnetic retarder R generates its own power and hence may potentially be used to decelerate the vehicle, independent of whether or not the main power system of the vehicle is functioning correctly. Alternative Embodiments

In the second embodiment of Fig. 6, a DC supply from the battery 5 is provided to the plurality of DC-powered brake coils 29 under the control of a power control module 30. However, in alternative embodiments, the battery 5 and/or the power control module 30 may be omitted, and the output from the generator coils 33, rectified by the bridge rectifier/regulator 34, may be supplied directly to the brake coils 29 to provide a retarding effect. In such an arrangement, the braking torque provided by the brake coils 29 would increase as the speed of rotation of the rotor 10A increases.

Claims

1. An electromechanical device, comprising:

a first member and a second member arranged to move relative to the first member and comprising first and second retardation portions;

wherein the first member is arranged to generate a movable magnetic field and wherein the relative speed at which the magnetic field moves relative to the second member is adjustable,

wherein at a first relative speed the interaction of the magnetic field with the second member results in the first retardation portion generating an electrical current, and a retardation force opposing the movement of the second member is applied to the first retardation portion; and

wherein at a second relative speed, different from the first relative speed, the amount of current generated by the first retardation portion is less than that generated by the first retardation portion at the first relative speed; and the interaction of the magnetic field with the second member results in an amount of retardation force opposing the movement of the second member being applied to the second retardation portion, which amount is greater than the amount of retardation force opposing the movement of the second member applied to the second retardation portion at the first relative speed.

2. The electromechanical device of claim 1, wherein at the second relative speed the amount of retardation force opposing the movement of the second member applied to the first retardation portion is less than that applied thereto at the first relative speed.

3. The electromechanical device of claim 1 or 2, wherein the first retardation portion comprises at least one elongate, electrically conductive element.

4. The electromechanical device of claim 3, wherein the first retardation portion comprises a prime number of said elements.

5. The electromechanical device of claim 3 or 4, wherein said at least one elongate, electrically conductive element comprises a non-ferrous material.

6. The electromechanical device of any one of claims 3 to 5, wherein the first retardation portion comprises a plurality of said at least one elongate, electrically conductive elements, respective first and/or second ends of which are electrically connected to each other.

7. The electromechanical device of any one of claims 3 to 6, wherein the second retardation portion comprises a disc.

8. The electromechanical device of claim 7, wherein the disc comprises a different material from a material of the at least one elongate, electrically conductive element.

9. The electromechanical device of claim 7 or 8, wherein the disc comprises a ferrous material.

10. The electromechanical device of any one of claims 7 to 9, wherein the at least one elongate element is located substantially radially relative to the disc.

11. The electromechanical device of any one of the preceding claims, wherein the first retardation portion is embedded within the second retardation portion.

12. The electromechanical device of claim 11, wherein the second retardation portion comprises two discs, and the first retardation portion is located between the two discs.

13. The electromechanical device of claim 11, wherein the second retardation portion comprises a disc having a face adjacent the first member, and the first retardation portion is embedded in the face such that an outer surface of the first retardation portion lies flush with the face.

14. The electromechanical device of claim 11, wherein the first retardation portion is cast within a hollow formed within the second retardation portion.

15. The electromechanical device of any one of the preceding claims, wherein the second retardation portion comprises a disc and the first retardation portion is attached to a face of the disc.

16. The electromechanical device of claim 15, wherein the disc comprises two faces, one of the two faces being located closer to the first member than the other face, and wherein the first retardation portion is attached to the face of the disc which is located closer to the first member than the other face.

17. The electromechanical device of claim 15, wherein the disc comprises two faces, one of the two faces being located closer to the first member than the other face, and wherein the first retardation portion is attached to the face of the disc which is located further from the first member than the other face.

18. The electromechanical device of any one of the preceding claims, wherein the first member comprises a stator, the second member comprises a rotor arranged to rotate relative to the stator, and the magnetic field comprises a rotatable magnetic field.

19. The electromechanical device of claim 18, wherein the first and second retardation portions are connected to a common, rotatable shaft.

20. The electromechanical device of claim 18 or 19, wherein the rotor comprises at least one rotor disc comprising the first and second retardation portions.

21. The electromechanical device of any one of claims 18 to 20, wherein the stator is arranged around a rotatable shaft, and wherein the magnetic field produced by the stator has a magnetic field vector aligned substantially parallel to the axis of the shaft.

22. The electromechanical device of any one of claims 18 to 21, wherein the stator comprises a plurality of coils which produce a rotating magnetic field when powered by a three-phase AC power supply.

23. The electromagnetic device of any one of the preceding claims, wherein the first retardation portion generates a current at the first relative speed by induction in at least one coil.

24. The electromechanical device of claim 23, wherein the first member comprises at least one coil to generate the movable magnetic field, and wherein the first retardation portion induces a current in the at least one coil of the first member.

25. The electromechanical device of any one of the preceding claims, wherein the second relative speed is higher than the first relative speed.

26. The electromechanical device of any one of the preceding claims, wherein the movement of the magnetic field relative to the second member may be adjusted to a relative speed at which the interaction of the magnetic field with the second member results in a driving force acting to accelerate the second member being applied to the first retardation portion.

27. The electromechanical device of any one of the preceding claims further comprising a friction brake having a portion arranged to contact the second member to oppose its movement.

28. A system comprising at least one electromechanical device of any one of the preceding claims, a controller for controlling the relative speed at which the magnetic field moves relative to the second member, and an electrical power store for storing the current generated by the first retardation member at the first relative speed.

29. An electromechanical device comprising:

a first member comprising a magnetic field generator; and

a second member which is arranged to move relative to the first member,

wherein the interaction of the magnetic field generated by the magnetic field generator with the second member acts to oppose the movement of the second member relative to the first member;

wherein the electromechanical device further comprises an electrical current generator arranged to generate an electrical current as a result of the second member moving relative to the first member.

30. The electromechanical device of claim 29, further comprising means for supplying the generated electrical current to the magnetic field generator to enable it to generate a magnetic field.

31. The electromechanical device of claim 29 or 30, wherein the electrical current generator comprises at least one magnet and at least one coil, one of the at least one magnet and the at least one coil being connected to the second member such that it moves as the second member moves, and wherein the second member moves relative to the other of the at least one magnet and the at least one coil as it moves relative to the first member, such that a current is generated in the at least one coil.

32. An electromechanical device according to any one of claims 29 to 31, wherein the first member comprises a stator and the second member comprises a rotor arranged to rotate relative to the stator.

33. An electromechanical device according to any one of claims 29 to 32, further comprising a controller for controlling the strength of the magnetic field generated by the magnetic field generator.

34. An electromechanical device according to claim 33, wherein the controller is powered by the generated electrical current.

35. An electromechanical device according to any one of claims 29 to 34, further comprising an electrical power store for storing the generated electrical current.

36. A machine provided with at least one electromechanical device according to any one of claims 1 to 27 or 29 to 35, or a system according to claim 28.

37. The machine of claim 36, wherein kinetic energy of the machine or a change in potential energy of the machine is converted into heat in the second retardation portion at the second relative speed.

38. The machine of claim 37, wherein the heat in the second retardation portion is generated by eddy currents which are induced in the second retardation portion as a result of the interaction of the magnetic field with the second member.

39. The machine of any one of claims 36 to 38, wherein the machine comprises one of a vehicle, a fitness equipment machine, a dynamometer, winding gear, elevator, a winch, a ski lift or cable car system.

40. An electromechanical device substantially as herein described with reference to Figs. 1 to 5.

41. An electromechanical device substantially as herein described with reference to Fig 6.