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Title:
ELECTRICAL POWER STEERING SYSTEM
Document Type and Number:
WIPO Patent Application WO/2012/056213
Kind Code:
A1
Abstract:
An electric servo assistance unit for use in a power steering system comprises a stator (10) comprising a plurality of magnets and a rotor (8) comprising a plurality of coil windings, a input shaft (1) and output shaft connected by a deflection member (3), at least two brushes (6) supported by a brush carrier which contact the commutator (7) and a brush deflection mechanism that physically connects the brush carrier to the steering shaft (1, 2) and which is arranged to extract the angular movement of the input shaft relative to the output shaft from the overall movement of the two shafts in use with zero torque applied to the deflection member the brush carrier remains stationary relative to the stator and further in use brush deflection the application of a torque across the deflection member causes the brush carrier to move relative to the stator.

Inventors:
FURMSTON, Paul, Timothy (18 Goldsmith Avenue, Warwick, Warwickshire CV34 6JB, GB)
STONE, Jay (35 Newton Road, Bromsgrove, Worcestershire B60 3EA, GB)
Application Number:
GB2011/051989
Publication Date:
May 03, 2012
Filing Date:
October 13, 2011
Export Citation:
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Assignee:
TRW LIMITED (Stratford Road, Solihull, West Midlands B90 4AX, GB)
FURMSTON, Paul, Timothy (18 Goldsmith Avenue, Warwick, Warwickshire CV34 6JB, GB)
STONE, Jay (35 Newton Road, Bromsgrove, Worcestershire B60 3EA, GB)
International Classes:
B62D5/04; B62D6/08; H02K23/18; H02K23/68; H02K26/00; H02K49/08
Domestic Patent References:
WO2008102112A1
WO2008102112A1
Foreign References:
US5719459A
DE102006038192A1
Attorney, Agent or Firm:
BARKER BRETTELL LLP (100 Hagley Road, EdgbastonBirmingham, West Midlands B16 8QQ, GB)
Download PDF:
Claims:
CLAIMS

1. An electric servo assistance unit for use in a power steering system comprising:

a stator, a rotor located concentrically within the stator for rotation about an axis , the stator comprising a plurality of magnets at spaced locations around the axis of rotation of the rotor, the rotor comprising a plurality of coil windings arranged at spaced locations around axis of rotation of the rotor,

an input shaft and an output shaft connected by a deflection member which permits relative rotation of the input and output shaft in response to application of a torsional load applied between the input and output shafts , the rotor being fixed to the output shaft;

a commutator comprising at least two conductive portions, each of which provides an electrical connection to a different coil; and

at least two brushes supported by a brush carrier which are each adapted to contact the commutator and to selectively connect at least one contact portion and its coil to an electrical supply according to the relative angular position of the brush and commutator; and

characterised by further comprising a brush deflection mechanism that physically connects the brush carrier to the steering shaft and which is arranged to extract the angular movement of the input shaft relative to the output shaft from the overall movement of the two shafts such that, in use, as the input shaft and output shaft rotate together with a substantially constant zero torque applied to the deflection member the brush carrier remains substantially stationary relative to the stator and selectively connects the power supply to one or more of the coils such that the forces created between the current carrying coils and the magnets are approximately in balance, and further in which the brush deflection mechanism is so arranged that, in use, the application of a torque across the deflection member causes the brush carrier to move relative to the stator to cause the motor to apply an assisting torque to the output shaft.

2. A servo system according to claim 1 in which, in the absence of torque carried by the deflection member the brushes are held substantially stationary by the brush deflection mechanism, and in the presence of torque across the deflection member the brush deflection mechanism causes the brushes to move relative to the stator of the motor. 3. A servo system according to claim 1 or claim 2 in which the brush deflection mechanism comprises a gear train.

4. A servo system according to claim 3 in which the gear train comprises: a first gearing stage having an output gear whose movement provides a measure of the common rotation of both the input shaft and the output shaft about the axis of the output shaft plus additional rotation of the input shaft caused by torque applied across the torsion bar, a second gearing stage having an output gear whose movement provides a measure of the absolute movement of either the input shaft or the output shaft about the axis of rotation of the shafts , and a third gearing stage which receives as an input the output of the first stage and as a further input the output of the second stage and which includes an output gear whose movement is dependent upon the relative rotation of the input shaft relative to the output shaft but is independent of the common rotation of the input and output shafts.

5. A servo system according to claim 3 or claim 4 in which the gear train includes one or more sun gears and one ore more planet gears .

6. A servo system according to claim 5 in which the output gear of the first stage of the gearing means comprises a sun gear which has a common axis with the output shaft and is free to rotate around the output shaft, the output sun gear being fixed to the input shaft either directly or through one or more additional gears. 7. A servo system according to claim 6 in which the additional gears comprise:

an input sun gear fixed against rotation to the input shaft and having an axis of rotation common with the axis of the input shaft, a planet gear meshed with the input sun gear, a further planet gear which shares an axis of rotation with and is fixed to the other planet gear and which meshes with the output sun gear, and a carrier which supports the planet gears which is fixed relative to the output shaft, the additional gears locking the output sun gear in position relative to the input sun gear unless a torque is applied across the deflection member whereupon the output sun gear moves rotates relative to the input sun gear.

8. A servo system according to any one of claims 3 to 7 in which the output gear of the second stage comprises a sun gear which is fixed to the output shaft and rotates about the axis of the output shaft together with the output shaft.

9. A servo system according to claim 8 when dependent on claim 7 in which the third stage comprises an annular ring gear which is free to rotate about an axis of rotation which is common with the axis of rotation of the output shaft, a first planet gear which meshes with the output sun gear of the first stage and the annular gear, and a second planet gear which meshes with the output of the second stage and also meshes with the annular gear, the first planet gear rotating about an axis which is fixed relative to the motor stator, and the second planet gear rotating about an axis which can undergo planetary movement relative to the axis of the output shaft.

10. An electric power assisted steering system for a vehicle including an electric servo unit according to any one of claims 1 to 9, a steering wheel operatively connected to the input shaft and at least one steered wheel connected to the output shaft, and a power supply.

Description:
ELECTRICAL POWER STEERING SYSTEM

This invention relates to improvements in electrical power steering systems , and in particular to low power steering systems.

In WO2008102112A1 there is disclosed a low cost, robust, electrical power steering system that includes a brushed permanent magnet electric motor that applies torque to the steering system as a function of how much twist is applied to a deflection member which is located between input and output sides of a steering column shaft. The system is low cost because it requires no electronic sensors to measure the torque demanded by a driver or to control the motor that provides the assistance. Instead, a brush deflection mechanism is provided which mechanically moves the brushes of the motor relative to the commutator which alters the amount of torque produced by the motor.

The system can be considered to operate in the same way as a conventional electrical power steering system: The twist of the deflection member of the system provides the required measure of the torque that the driver is demanding and as more torque is measured, more steering assistance is applied. In that document, the motor is in-line with the steering column and therefore does not have any geared advantage when applying torque assistance. The motor must therefore provide all the required driver assistance directly to the steering column.

The torque output by the motor is controlled by varying the position of the motor brushes on the commutator. It is assumed that at zero electrical degrees , zero torque is produced, and at 90 electrical degrees, maximum motor torque is produced. For a 2 pole motor the brushes must move through + /-90 degrees mechanical to achieve 90 degrees electrical change, and for a 4 pole motor they must move through + /-45 degrees. The apparatus disclosed in WO2008102112A1 discusses several different brush deflection mechanisms . However, the disclosed arrangements only work effectively where the magnets of the motor are fixed to the shaft and the windings of the motor are fixed to the stator.

According to a first aspect the invention provides an electric servo assistance unit for use in a power steering system comprising:

a stator, a rotor located concentrically within the stator for rotation about an axis, the stator comprising a plurality of magnets at spaced locations around the axis of rotation of the rotor, the rotor comprising a plurality of coil windings arranged at spaced locations around axis of rotation of the rotor,

an input shaft and an output shaft connected by a deflection member which permits relative rotation of the input and output shaft in response to application of a torsional load applied between the input and output shafts, the rotor fixed to the output shaft such that the rotor and output shaft rotate together;

a commutator comprising at least two conductive portions, each of which provides an electrical connection to a different coil; and

at least two brushes supported by a brush carrier which are each adapted to contact the commutator and to selectively connect at least one contact portion and its coil to an electrical supply according to the relative angular position of the brush and commutator; and

characterised by further comprising a brush deflection mechanism that physically connects the brush carrier to the steering shaft and which is arranged to extract the angular movement of the input shaft relative to the output shaft from the overall movement of the two shafts such that, in use, as the input shaft and output shaft rotate together with a substantially constant zero torque applied to the deflection member the brush carrier remains substantially stationary relative to the stator and selectively connects the power supply to one or more of the coils such that the forces created between the current carrying coils and the magnets are approximately in balance, and further in which the brush deflection mechanism is so arranged that, in use, the application of a torque across the deflection member causes the brush carrier to move relative to the stator to cause the motor to apply an assisting torque to the output shaft.

The system of the present invention provides the following functionality. The steering column must be able to rotate to provide the full range of steering control, typically 3-4 full rotations lock-to-lock. Whilst rotating, the system can control the torque delivered to the driver, independently of the column rotation. For instance, in the apparatus of the present in venton if no torque is present and the steering wheel moved is from lock to lock the column moves from lock to lock, but the motor brushes do not move, thus the motor provides zero torque. On the other hand, when torque is applied but the output shaft is fixed the column does not rotate but the motor brushes will move angularly according to the torque applied so that the motor produces assistance torque. Finally, if torque is applied while the column is rotating the column rotates as required and the Motor brushes angularly move according to the torque applied, but within the range of zero to 90 electrical degrees and independently of the column rotation. The motor provides assistance torque.

This means that the torque measurement mechanism is totally independent of the rotation of the steering column.

The torsion bar is used to "measure" the torque. The torsion bar connects the input shaft to the output shaft and this bar deflects according to the amount of torque incident on it. The torsion bar is therefore fixed to both the input shaft and the output shaft and the amount of deflection of the torsion bar gives an indication of the torque present. This deflection can be measured by measuring the angular difference between the input shaft and the output shaft. It is this angular difference that is transferred into an angular rotation of the motor brushes around the DC motor commutator by the brush deflection mechanism.

The brush deflection mechanism may comprise a gear train.

In one preferred arrangement the gear train may comprise: a first gearing stage having an output gear whose movement provides a measure of the common rotation of both the input shaft and the output shaft about the axis of the output shaft plus additional rotation of the input shaft relative to the output shaft caused by torque applied across the torsion bar, a second gearing stage having an output gear whose movement provides a measure of the absolute movement of either the input shaft or the output shaft about the axis of rotation of the shafts, and a third gearing stage which receives as an input the output of the first stage and as a further input the output of the second stage and which includes an output gear whose movement is dependent upon the relative rotation of the input shaft relative to the output shaft but is independent of the common rotation of the input and output shafts.

The first stage, second stage and third stage may comprise combinations of one or more of planetary gears, sun gears , gear carriers and annulus gears . The gears and carriers may be free to rotate about their respective axes , although the axis itself may be constrained by any carrier of the gear. The gear train may therefore include one or more epicyclic gears . One or more of the stages may include one or more sun gears concentrically arranged around the input shaft or output shaft, and which may be fixed to the input shaft or output shaft. Each stage may be independent of the other stage, apart from the inputs and outputs of stages connecting to the inputs or outputs of one or more of the other stages.

The output gear of each stage may mesh with one or more gears of the other stages .

The output gear of the first stage of the gearing means may comprise a sun gear which has a common axis with the output shaft and is free to rotate around the output shaft, the output sun gear being fixed to the input shaft either directly or through one or more additional gears.

The additional gears may comprise:

an input sun gear fixed against rotation to the input shaft and having an axis of rotation common with the axis of the input shaft, a planet gear meshed with the input sun gear, a further planet gear which shares an axis of rotation with and is fixed to the other planet gear and which meshes with the output sun gear, and a carrier which supports the planet gears which is fixed relative to the output shaft, the additional gears locking the output sun gear in position relative to the input sun gear unless a torque is applied across the deflection member whereupon the output sun gear moves rotates relative to the input sun gear.

The additional gears may be chosen such that an angular gain is achieved between the input sun gear and output sun gear, the output sun gear moving through a larger angle than the input sun gear for a given applied torque. The output gear of the second stage may comprise a sun gear, and this sun gear may be fixed to the output shaft and rotates about the axis of the output shaft together with the output shaft.

The third stage may comprise an annular ring gear which is free to rotate about an axis of rotation which is common with the axis of rotation of the output shaft, a first planet gear which meshes with the output sun gear of the first stage and the annular gear, and a second planet gear which meshes with the output of the second stage and also meshes with the annular gear, the first planet gear rotating about an axis which is fixed relative to the motor stator, and the second planet gear rotating about an axis which can undergo planetary movement relative to the axis of the output shaft.

The second planet gear may form the output gear for the third stage, albeit that it is the movement of the axis of the gear that is dependent on the torque as opposed to the rotation of that gear about its axis.

The second planet gear may be mounted on a carrier which is concentrically mounted to the output shaft such that it is free to move to rotate about the output shaft as the second planet gear P4 undergoes planetary motion relative to the output shaft. This carrier can therefore truly be considered to be the output of the brush deflection mechanism.

The carrier may carry the brushes of the motor. There may be two brushes for each electrical phase of the motor, one connected to a positive and the other a negative (or ground) supply. Alternatively, one or more additional gears may be provided between the output gear of the third gear assembly and the brush carrier. The additional gears may provide a gain between the movement of the output of the third gear assembly and the movement of the brush carrier. For instance, the additional gears may convert a given angular rotation of the carrier of the output gear of the third assembly to an angular rotation of the brush carrier that is at least twice as great, or three times as great. This can be used to provide a greater angular rotation of the brush carrier for a given rotation of the

The first gearing stage may comprise a first sun gear SI which is mounted concentrically to the input shaft and is fixed to the input shaft, two planet gears PI and P2 which share an axis that is mounted on a carrier that is mounted concentrically to the output shaft and is fixed to the output shaft, and a sun gear which forms the output of the first stage which is mounted concentrically to the output shaft but is free to rotate independently of the output shaft, such that P2 will only rotate about its axis when torque is applied across the torsional bar and P2 will undergo planetary rotation due to rotation of the input and output shaft, in turn to cause the sun gear S2 to rotate about its axis due to the sum of the common rotation and the rotation of P2 about its axis.

This gearing connection may provide some gearing so that the output sun gear rotates about the axis of the output shaft through an angle which is a multiple of the angle through which the input shaft rotates due to torque carried by the torsion bar.

The magnets may comprise permanent magnets or may comprise electromagnets. The magnets may be fixed relative to the stator. The electrical phase windings of the motor may be fixed relative to the output shaft so that they rotate with the output shaft. The commutator may be fixed to these windings so that it also rotates with the windings . The brushes may then contact this commutator.

There are preferably two brushes , and a pair of contact portions on the commutator, for each phase coil of the motor. All commutation may be connected together via the windings so that at any angular postion of the output shaft all coils are energised. The direction of current in each coil varies according to where the brushes contact the commutator, and this is what causes the change in torque in the motor.

The deflection member may comprise a quill shaft (sometimes called a torsion shaft or torsion bar) which is designed to twist through a known angle for a given torque applied across it. One end may connect to the input shaft and the other to the output shaft. The quill shaft may be arranged to permit a maximum twist of the input shaft relative to the output shaft of between + 1-2 degrees and + I A degrees and the brush deflection mechanism may magnify this to give a angular movement of the brush carrier of between + 1-20 degrees and + /-40 degrees . This corresponds to an electrical angle of between + /-AO degrees and + /-80 degrees . This is ideal for a design with 2 magnets that are diametrically opposed as peak torque is created with an energised coil in line with the magnets and zero torque for an energised coil at a point 90 degrees from that. The brush deflection mechanism may provide an overall ratio of movement of the brush carrier compared with angular twist of the deflection member of at least 2 : 1 , and most preferably at least 10 : 1 , or anywhere between these values . The brush may have sufficient width to contact up to two contact portions of the commutator at any given time. The input shaft may be connected to a steering wheel of a vehicle directly or through one or more shafts , and the output shaft may be connected directly or indirectly to one or more steered wheels of the vehicle, again through one or more shafts and steering gear.

The skilled reader will appreciate that were reference is made to a planet gear, in a practical arrangement there may be several instances of that planet gear spaced at different locations around a respective sun gear. For instance, each planet gear may comprise three discrete yet identical gears (in terms of the number of teeth) spaced 120 degrees apart around the sun gear.

Also, the skilled person will understand that the terms sun gear and planet gear in the context of this application simply refer to the location of some gears (the planet gears) at spaced locations around a central (sun) gear. There is no requirement for the planet gears to mesh with a ring gear, unless stated in this description, nor is there a requirement for the planet gears to undergo planetary motion around the axis of the sun gear, unless otherwise stated in this description.

According to a second aspect the invention provides an electric power assisted steering system for a vehicle including an electric servo unit according to the first aspect, a steering wheel operatively connected to the input shaft and at least one steered wheel connected to the output shaft, and a power supply.

There will now be described by way of example only two embodiments of the present invention with reference to and as illustrated in the accompanying drawings of which: Figure 1 is an isometric view of a first embodiment of a servo system according to the present invention, with some parts partially transparent to enable parts that would otherwise be obscured to be viewed,

Figure 2 is a similar view of the system of Figure 1 but from a slightly different angle,

Figure 3 is a still further alternative view of the system of Figure 1 in which the motor casing has been omitted;

Figure 4 is a still further alternative view of the system of Figure 1 in which a part of the motor housing has been omitted but the motor housing end caps are shown,

Figure 5 is a schematic diagram showing the relative location and interactions of the key parts of the system of Figure 1 using a gearing notation shown in Figures 11 (a) to (f) , Figure 6 is a schematic view of the input and output shafts and torsion bar of the system of Figure 1 , all other parts being omitted

Figure 7 is a schematic view corresponding to Figure 1 in which the initial parts of the gearing corresponding to a first gearing stage are shown, the remainder of the gearing being omitted,

Figure 8 is a schematic view similar to Figure 7 but also showing a second stage of the gearing assembly, Figure 9 is a schematic view similar to Figure 8 in which a third stage of the gearing assembly is shown in addition to the first and second stages, Figure 10 is a view similar to that of Figure 9 that includes an additional fourth gearing stage that provides additional rotational gain,

Figures 11(a) to (f) are illustrations of the notational system used to illustrate the various types of gears shown in Figures 4 to 10,

Figure 12 is a perspective view of the input shaft and output shaft and corresponds with the notational view of Figure 4,

Figure 13 is a perspective view corresponding to the notational view of Figure 5 ,

Figure 14 is a perspective view of the input shaft, output shaft and the gears of the second and third gearing stages, with the first gearing assembly omitted,

Figure 15 is a perspective view showing the location of the brush carrier relative to the commutator and the gearing assembly,

Figure 16 is a notational representation of a third embodiment of a servo system according to the first aspect of the present invention, and

Figure 17 is a view of an electric power steering system which may incorporate a servo system of the first, second or third embodiments and as such falls within the scope of the second aspect of the invention.

Several embodiments of a servo system which may be incorporated into an electric power assisted steering system are described hereinafter. Each one includes a brush deflection mechanism which includes epicyclic gears . To simplify the description of the manner in which the system operates a system of schematic diagrams as shown in Figures 5 to 11 and Figure 16 is used. A gear symbol syntax has been used to describe the various parts of the system, and this syntax is used as the basis of those figures. The syntax is as shown in Figures 11 (a) to (f) for the following gear types:

(a) is the notation for a single toothed gear;

(b) is the notation for a toothed gear on a central shaft

(c) in the notation for two gears meshing with both gears mounted on independent central shafts

(d) is the notation for a single gear mounted concentrically around a shaft but free to rotate relative to the shaft;

(e) is the notation for a single gear mounted concentrically around a shaft and fixed to that shaft; and (f) is the notation of a brush carrier and brush that is concentrically mounted around a shaft but is free to rotate independently around the shaft.

Note that for (d) to (f) the diagrams show the cross section of the gearing concentrically around a shaft, in the examples provided herein the shaft being either an input or output shaft.

A first embodiment of a servo system is shown in perspective views of Figures 1 to 4 of the accompanying drawings. At the heart of the system is a deflection member in the form of a torsion bar 3 , best seen in Figure 6, sometimes called a quill shaft that connects an input shaft to an output shaft. An input shaft 1 is connected to one end of the torsion bar 3 , and an output shaft 2 to the other end of the torsion bar. As the input shaft is turned and the output resists that turning force, the torsion bar will twist. This motion may also cause the output shaft to turn. The amount of twist of the torsion bar is proportional to the torque applied across it, with a maximum of about 3 degrees each way for a torque across it of + /- 15 NM during normal use in this example.

A rotor 8 is provided around the output shaft 2 and comprises an annular sleeve of laminated insulating material that is fixed to the output shaft 2. The insulator carries a set of coil windings 9 spaced around the sleeve. Each coil can have any number of coil windings and each end of each coil terminates with an electrical contact pad. Around the outside of the rotor 8 is a stator 10. This comprises a carrier that is fixed to an earth, in this embodiment a motor housing or casing 5 which can be secured to a fixed part of a vehicle such as a bulkhead. The carrier supports permanent magnets disposed on diametrically opposite sides of the axis of the output shaft. One is a North pole the other is a South pole facing the rotor. Each magnet comprises an elongate bar that extends in a plane that contains the axis of the quill shaft, and having a length similar to the length of the coils . The magnets are held as close to the coils as possible without contact so the ends of the magnets line up with the ends of the coils.

The output shaft 2 also carries a commutator 7 which is fixed relative to the output shaft 2 and rotor 8. The commutator 7 comprises a ring-like base of insulating material which is sleeved on to the output shaft 2 so it can only rotate if the output shaft rotates. The surface of the insulating material carries 45 commutation bars for 15 sets of windings . A brush carrier is also provided which carries a pair of brushes 6 each arranged 90 degrees mechanical (180 degrees electrical) apart from one another about the axis of the output shaft. One brush connects to a positive DC electrical supply such as a 12 volt battery. The other connects to an earth. As will be apparent, depending on the relative position of the brushes and the commutator an electrical path may be formed from the battery through at least one pair of contact portions on the commutator 7 and its corresponding coil winding. Since the coil windings lie within the magnetic field created by the two magnets this current in the coil may create forces between the rotor 10 and stator depending on the position of the coils relative to the stator.

The movement of the brush carrier and brushes 6 relative to the commutator 7 is controlled by a brush deflection mechanism. This comprises a gear train that directly connects the brush carrier to the input and output shafts 1 , 2 and extracts the relative movement of the input and output shafts due to torque applied across the torsion bar whilst isolating the brush carrier from any common rotation of the shafts due to the driver turning the wheel.

As shown in Figure 7, the first stage of the gear assembly comprises a first sun gear SI which is mounted concentrically to the input shaft and is fixed to the input shaft. Two planet gears PI and P2 share an axis, and that axis is mounted on a carrier CI that is mounted concentrically to the output shaft and is fixed to the output shaft. A second Sun gear, S2, which forms the output of this first stage is mounted concentrically to the output shaft but is free to rotate independently of the output shaft. A small gap exists between the inner annular face of the sun gear S2 and an outward face of the output shaft onto which the sun gear S2 is located. The measurement of torque is signified by rotational movement of PI and therefore P2 around their central axis . If there is no torque, PI , and hence P2, do not rotate about their axis . This movement is transferred to S2. The gears SI , PI and P2 provide an angular gain to the stage so that the sun gear S2 moves through a larger angle than the gear SI . If no gain is desired, the gear S2 could be connected directly to the input shaft.

In simple terms the rotation of the sun gear S2 will be equal to the sum of the overall rotation of the steering column AND the difference in angle between the input shaft and the output shaft multiplied by any gain achieved using the first stage gearing. Therefore, this gear will rotate as the column rotates , and also rotate a little more (or less) as torque is applied due to the additional rotation of PI turning the sun gear S2. The two cannot be distinguished from observing the rotation of S2 alone.

The second stage of the gearing system is shown in Figure 8. It comprises a further sun gear S3 which is fixed to the output shaft. It may be fixed by virtue of an interference fit between the sun gear S3 and the output shaft, or by locating screws that connect the sun gear S3 to the output shaft. As the output shaft rotates around its axis, the sun gear S3 will rotate so this gear, which forms the output of the second gearing stage, provides a measure of the angular rotation of the output shaft which is independent of the torque applied to the torsion bar. The sun gear S3 meshes with a planet gear P4 which is supported on a carrier that is free to rotate relative to the output shaft, input shaft and housing. The carrier may be concentrically mounted onto the output shaft with a small gear, or low friction engagement between them, or may be mounted such that it is supported by the inside face of the motor casing, again with a small gap, or low friction engagement, allowing relative movement of the carrier and the casing. Rotation of the sun gear S3 will cause P4 to rotate at the same rate but in the opposite direction.

The final, third, stage of the gear system is shown in Figure 9. It comprises a planetary gear P3 and an annular ring gear Al . The gear P3 rotates about an axis which is stationary with respect to the position of the motor stator, in this example by being fixed to the motor housing. The axis of the planet gear P3 is therefore grounded relative to a fixed datum, fixed relative to the motor stator. The gear P3 meshes with the annular gear Al and with the sun gear S2. The annular gear is free to rotate concentrically around the output shaft, and has an annular outer face that is a sliding fit within an inwardly facing wall of the motor casing. This wall can be lubricated as required, or low friction materials may be used for both the annular gear and bearing race (which may also be lubricated if desired) . The annular ring gear Al is also meshed to the planetary gear P4.

In use the ANNULUS GEAR Al transfers the rotation of the planet gear P3 onto the further planet gear, P4. P4 will therefore rotate at the same rate as the planet gear P3. Because P4 is also connected to the sun gear S3 , the axis of the gear P4 will remain in a fixed position, relative to the motor casing, unless the Annulus gear rotates. The annulus will only rotate if the planet gear P3 moves out of alignment with the gear P4, and when this happens it will move the axis of the gear P4 around the output shaft. This rotation of the annulus gear is controlled by the movement of the sun gear SI relative to sun gear S2, so that the relative rotation between S2 and S3 that results causes the planet gear P4 to undergo planetary motion. The carrier for the planet gear P4 forms the output of the whole brush deflection mechanism and, as shown in Figure 9, may be connected directly to the brush carrier which is mounted concentric to the output shaft. As shown in Figures 5 and 10 additional gears P5 and S5 may be provided between the carrier for P4 and the brush carrier if extra gain is needed.

In this system, if both shafts are rotating without torque applied then (assuming unity gear ratios)

PI will not rotate

P2 will therefore not rotate,

S2 will rotate at the same rate as the OUTPUT SHAFT

S3 will rotate at the same rate as the OUTPUT SHAFT

P3 will rotate at the same rate as the OUTPUT SHAFT, but in the opposing direction

P4 will rotate at the same rate as the OUTPUT SHAFT, but in the opposing direction

P4 will therefore rotate at exactly the same rate as S3 , but in the opposing direction. This means that the brush carrier will not rotate and will therefore remain stationary.

If torque is applied, then P3 will rotate at the same speed required to remain stationary PLUS the movement of its axis of rotation created by the torque measured. The interaction between the planet gear P4 and the sun gear S3 effectively subtracts the movement of the steering column. The result is planetary movement of P4 that is proportional to the movement created by the torque measurement i.e. PI .

The planet gear P4 is mounted to a carrier, and this carrier is connected to the brush carrier. The axis of the carriers is coincident with the axis of the output shaft, and so the brush carrier rotates around the output shaft as P4 undergoes planetary movement. In order for the gearing system shown in Figures 5 to 9 to work correctly, it is desirable that the following relationships are maintained: P3 = P4

S2 = S3

Perspective views of the servo system are shown in Figures 12 to 15 of the accompanying drawings, each of which shows either the whole or selected parts of the apparatus. As can be seen, each of the planet gears PI , P2, P3 and P4 in fact comprises three gears equi-spaced around the axis of the steering shaft. This provides for a self centering of the gears around the sun gears SI , S2 and S3 , and within the annulus gear. It would be possible to use less or more gears for each planet gear, but three is in many ways optimum for balancing wear and loading.

An alternative embodiment is illustrated in Figure 16 of the drawings. In this embodiment a different motor 20 is provided which is not concentric with the output shaft, and gears are used to take off the rotation of the carrier for the planet P4 to the brush carrier. The motor rotor is coupled back to the output shaft through further gears. This arrangement may allow for an axially more compact assembly and may offer better packaging in some applications. All other parts of the system remain the same as for the first embodiment and so the same reference numerals have been used to describe like parts .

The skilled reader will appreciate that there is a possibility within the scope of the invention to add gearing to the motor such that it one turn of the column is many turns of the motor. This should allow increased torque output for a given size of motor. Again this can improve packaging by reducing the motor size. In this example, the motor is offset to one side of the shaft but is working concentrically with the shaft. However, the gearing is such that the motor turns several times for every single turn of the steering shaft. The theory of brush movement is the same as the in-line version. This same concept could be expanded to place the motor in any orientation to the main shaft, such as perpendicular to the shaft.

The servo system is especially suited for use in an electric power steering system. A system of this type is shown in Figure 17 of the accompanying drawings . The input shaft 1 of a servo system 200 as described hereinbefore is connected to a steering wheel 100, and the output shaft 2 to the roadwheels of a vehicle such as a passenger car through a rack and pinion gearbox 300 or other steering gear. As the input shaft 1 is turned by a driver applying a torque, it will twist the torsion bar 3 which in turn causes the output shaft 2 to turn. The amount of twist of the torsion bar is proportional to the torque applied across it, with a maximum of about 3 degrees each way for a torque across it of + /- 15 Nm in this example. This moves the brush carrier which alters the torque produced by the motor.