WO2008102112A1

WO2008102112A1 - An electrical servo-assistance unit

Info

Publication number: WO2008102112A1
Application number: PCT/GB2008/000548
Authority: WO
Inventors: James Owen Patrick Farrelly; Jeffrey Ronald Coles
Original assignee: Trw Limited
Priority date: 2007-02-20
Filing date: 2008-02-19
Publication date: 2008-08-28
Also published as: GB0703215D0; EP2121416A1

Abstract

An electrical servo assistance unit for use in a power assisted steering system comprises a stator (10) comprising a plurality of coil windings arranged at spaced locations, a rotor (8) comprising a plurality of magnets at spaced locations around its axis, an input shaft (4) and an output shaft (6) connected by a deflection member (2) which permits relative rotation of the input and output shaft in response to application of a torsional load applied therebetween the rotor being fixed to the output shaft and sharing a common axis of rotation; a commutator (12) comprising at least two conductive portions, each of which provides an electrical connection to a different coil; and at least one brush (14a, 14b) which is 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.

Description

AN ELECTRICAL SERVO-ASSISTANCE UNIT

This invention relates to improvements in electric motors for use in power assisted steering systems.

Electric power assisted steering systems are well known and comprises an electric motor which applies an assistance torque to the steering in the same sense as the torque applied by a driver to steer the vehicle. In order to determine what torque should be applied by the motor, a torque sensor is usually provided which measures the driver applied torque. This measurement is then converted into a suitable drive signal for the motor.

A problem with such systems is the need for the precision torque sensor and the consequent need for complex control electronics for the motor. This can price the system out of certain low cost applications such as ride on lawn mowers or golf carts or the like.

According to a first aspect the invention provides an electrical servo assistance unit for use in a power assisted steering system comprising: a stator, a rotor located concentrically within the stator for rotation about an axis, one of the stator and rotor comprising a plurality of magnets at spaced locations around the axis of rotation of the rotor, the other of the stator and 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 and sharing a common axis of rotation; a commutator comprising at least two conductive portions, each of which provides an electrical connection to a different coil; and at least one brush which is 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; the unit being so constructed and arranged that as the input shaft and output shaft rotate together with a constant zero torque applied to the deflection member the commutator selectively connects the power supply to one or more of the coils such that at any given time the coil or coils which are energised lie within a defined range of angles centred about a point having a substantially fixed angular relationship to the magnets, that angular relationship being chosen such that the forces created between the current carrying coils and the magnets are approximately in balance, and in which that angular relationship is changed in a step-wise manner as an increasing torque is applied to the deflection member which torque causes additional relative movement between the brushes and commutator contact portions, the step-wise change in angular relationship causing a corresponding step wise increase in an imbalance of the forces between the current carrying coil or coils and the magnets.

The invention therefore provides a unit which is able to provide a servo assistance torque- created by imbalance in forces between a stator and rotor- in response to an input torque applied through an input shaft to a deflection member. This servo assistance changes in a step-wise manner due to deflection of the deflection member causing additional relative movement of the brush and commutator. In addition, the brushes and commutators maintain the assistance as the input/output shaft rotate by providing a more typical commutation of the coils as is similar to a DC servo motor. The brushes and commutator may be arranged such that at any one time at least one coil is always connected to the brush, and the angular relationship is preferably such that at low torques the forces between the magnets and coils are substantially balanced and when a high torque is applied the forces between the coils and magnets are unbalanced such that the rotor and stator together apply their own torque between the body of the unit and the output shaft in the same sense as the applied torque.

To achieve this at least one of the commutator and brushes may be adapted to rotate about the axis of rotation of the rotor and may be connected to either the rotor or the input shaft.

In fact several different arrangements of brush and commutator are envisaged within the scope of the invention:

1) The magnets may be on the stator and the coils on the rotor;

2) The coils may be on the stator and the magnets on the rotor;

3) In case 1) the commutator may be carried by the input shaft and the brush fixed relative to the stator; 4) In case 1) the commutator may be carried by the rotor and the brush connected to the input shaft through a brush deflection mechanism; 5) In case 2) the commutator may be carried by the stator and the brush by the input shaft.

The rotor or stator may carry N coils which are spaced around the circumference of the rotor as is known in the art DC servo motor design where N is an integer greater than 1. Correspondingly, the stator or rotor may comprise M magnets where M is also an integer number greater than 1. These may also be spaced evenly around the stator or rotor. Most preferably, two magnets are provided which are disposed diametrically opposite one another relative to the axis of rotation of the rotor. Three coils may be provided, each of which may comprise a plurality of windings of conductive cable. The coils may be spaced equally around the stator or rotor. This will place each of the two opposing arms of the coil at 60 degrees to its neighbours.

In this arrangement, with 2 magnets and 3 coils, the term "substantially fixed angular relationship" may mean that an energised coil will be in a fixed angular position relative to the magnets within a tolerance of approximately + /-30 degrees electrical as defined by the arc subtended by the brush. This may be slightly more or less depending on the design of the brush and contact portions of the commutator. This is in fact determined by the arc subtended by each commutator portion and also by the arc subtended by each brush. In a preferred arrangement the commutator portions subtend an arc of 60 degrees whilst the brushes subtend and arc of 30 degrees.

In an optimal design of this type, and with no torsional load applied to the input shaft, a coil or coils will only be energised if they lie within a range of + /-30 degrees from a point that together with the axis of rotation of the rotor defines a line that extends at right angles to a line connecting the two magnets . The centre point of this range will depend on the torque in the deflection member, and with increasing torque this relationship will be changed so that the range will remain + /-30 degrees but will no longer be centred on a point at 90 degrees electrical to that line. Instead at maximum torque they may be centred on a point that coincides with the line connecting the two magnets. This will produce peak torque. The steps occur as the brush moves from one contact portion on the rotor to the next. A DC source of constant current may be provided which is fed to the at least one brush. A switch may be provided which isolates the brush from the supply when the torque applied between the input and output shafts is below a limit, such as substantially zero torque. This saves passing current through a coil at zero torque, which has no effect on the operation of the unit.

The applicant has appreciated that in some arrangements there will exist a torque ripple as the input/output shaft rotate due to the energised coils being able to adopt a non-optimal position within the fixed range of possible positions. For example at zero torque in the input shaft in the preferred example they could be anywhere within + /-30 degrees of the ideal and other than being at the centre an unwanted torque will be produced as the forces are not balanced. This can be minimised through appropriate shaping of the magnets so that the forces between the rotor and stator under zero loading of the deflection member are more approximately in balance at all times. It can also be reduced by increasing the number of magnets and coils, say to 4 and 6 respectively or 6 and 8. The disadvantage is that the reduction in ripple is traded against cost and complexity.

The magnets may comprise permanent magnets or may comprise electromagnets .

There are preferably two brushes, and a pair of contact portions on the commutator for each coil.

The deflection member may comprise a quill shaft (sometimes called a torsion shaft) 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 90 degrees where two magnets are provided. 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 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.

A brush deflection mechanism may connect the input shaft to the output shaft, the mechanism being operable to cause relative deflection of a brush set and a commutator connected to one or other of the input and output shafts.

The brush deflection mechanism may include a rotating slider which converts relative rotational movement of the input and output shafts into axial movement of the slider. This may be connected to the brush set and hence create axial movement of the brush set.

The brush deflection mechanism may include an epicyclic gearbox which include a first sun gear connected to the input shaft (or output shaft) , a second sun gear connected to a brush set, and a planet gear that connects both sun gears and which is connected to the other of the input and output shaft. This converts relative angular movement of the input and output shafts into angular movement of the brush set.

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.

The vehicle may comprise a ride on lawn mower.

There will now be described, by way of example only, three embodiments of the present invention with reference to and as illustrated in the accompanying drawings of which:

Figure 1 is a general schematic of a first embodiment of a servo assistance unit according to a first aspect of the invention;

Figure 2 is an end view of the commutator and brushes of the unit of Figure 1;

Figure 3 (a) is a view of the relative location of the brushes and commutator under zero load and 3(b) is a corresponding view showing which coils carry current in this position;

Figure 4 (a) is a view of the relative location of the brushes and commutator under a low torque load and 4(b) is a corresponding view showing which coils carry current in this position

Figure 5 (a) is a view of the relative location of the brushes and commutator under a greater torque load than shown in Figure 4 (a) and 4(b) is a corresponding view showing which coils carry current in this position;

Figure 6 (a) is a view of the relative location of the brushes and commutator under maximum torque load and Figure 6(b) is a corresponding view showing which coils carry current in this position;

Figure 7 is a graph showing the increase in servo assistance torque with increasing angle of twist of the quill shaft , i.e. with increased torsional load;

Figures 8 to 10 show the location of current carrying coils under zero torsional load at different angular positions of the input/output shaft which leads to some torque ripple;

Figure 11 (a) is a schematic illustration of a second embodiment of a servo assistance unit in accordance with the first aspect of the invention which incorporates a brush deflection mechanism;

Figure ll(b) is a schematic illustration of a third embodiment of a servo assistance unit in accordance with the first aspect of the invention which incorporates a brush deflection mechanism;

Figure 12 is a schematic illustration of a fourth embodiment of a servo assistance unit in accordance with the first aspect of the invention ;and

Figure 13 shows an application of the unit to a ride on lawn mower which falls within the scope of the second aspect of the invention.

As shown in Figure 1 an electrical servo assistance unit 100 comprises an elongate quill shaft 2 that connects an input shaft 4 supported in bearings 5 to an output shaft 6 supported in bearings 7. The quill shaft 2 is chosen to permit a range of twist between the input shaft and output shaft that corresponds to 90 degrees for a maximum expected torsional loading between the input and output shaft. In other words, if the output shaft 6 is fixed solid and the maximum expected torque is applied to the input shaft 4 then the quill shaft 2 will twist through 90 degrees .

A rotor 8 is provided around the quill shaft 2 and comprises an annular sleeve of insulating material having a length approximately 2/3^rd the length of the quill shaft 2. It is fixed at one end to the output shaft 6. The insulator carries a set of three coil windings A, B, C spaced equally around the sleeve, giving a spacing of 60 degrees between (each coil having defining two sets of conductors each running axial along the sleeve from one end to the other and connected by end portions of the coils. Each coil can have any number of coil windings and each end of each coil terminates with an electrical contact pad that is fixed to the free end of the rotor (that end furthest from the output shaft) . The coils can be seen in Figure 3 for example.

Around the outside of the rotor 8 is a stator 10. This comprises a carrier that is fixed to an earth. The carrier supports two permanent magnets disposed on diametrically opposite sides of the axis of the quill shaft and rotor. One is a North pole the other is a South pole facing the rotor. This can best be seen in Figure 3 of the drawings. Each magnet comprises an elongate bar that extends in a plane that contains the axis of the quill shaft, and having a length equal 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 input shaft 4 also carries a commutator 12 which works with pair of brushes 14a, 14b fixed to the stator or to the same earth as the stator 10. The commutator 12 and brushes 14a, 14b are shown in more detail in Figure 2. The commutator 12 comprises a ring-like base of insulating material which is sleeved on to the input shaft. The surface of the insulating material carries three pairs of conductive pads. Each pair comprises two diametrically opposed pads, and the three pairs of contacts are spaced equally around the ring with a 60 degree spacing between each one. A flying lead 16 connects from one pad of each pair to the end of the nearest coil on the rotor and a similar lead connects the other pad of the each pair to the other end of that coil on the rotor. Thus, each pair corresponds to a single coil. The flying leads 16 each have a length sufficient to prevent the leads straining over a relative angle of twist between the input and output shaft of 90 degrees. To ensure it does not twist more than this a stop is provided on the input shaft that engages a corresponding stop on the output shaft when this angle of rotation is reached.

The pair of brushes 14a, 14b are each arranged diametrically one another about the axis of the quill 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 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 and stator depending on the position of the coils relative to the stator.

The operation of the unit of figure 1 can be understood by reference to Figures 3 to 6 of the accompanying drawings. In the explanation of Figure 3 to 6 it is assumed that the input and output shafts are not rotating but that the output shaft is fixed in position. The rest position of the unit is shown in Figures 3 (a) and 3(b). This corresponds to the situation where no torque is applied between the input and output shafts, and hence the quill shaft is not twisted. In this position, as shown in Figure 4(b) the brushes connect the electrical supply to one contact portion of the commutator which in turn means it connects to one coil winding A. This winding A is located midway between the two magnets. This can be seen in Figure 4 (a) .

Thus, whilst current flows in the winding there will be zero net force generated between the rotor and stator. Note that the energised coil is denoted by the arrowhead and the tail (with the arrow head indicating current flowing out of the page and the tail indicating current flowing into the page) .

Next, assume that a small but increasing torque is applied between the input and output shafts. Initially, this will cause the quill shaft to start to twist and in turn move the brushes relative to the commutator. Until sufficient torque is applied to twist the quill shaft through 30 degrees the brushes remain in contact with the same contact portion of the commutator. Once it reaches 30 degrees the brushes instead begin to also contact the adjacent contact portion of the commutator. Note that they remain in contact with the original contact portions too. This can be seen in Figure 5 (b) .

At this level of torque, two coils will be energised. This is shown in Figure 5 (a) . In addition to the original coil A and second coil B is now also energised. With two coils energised the current in the coils and the magnetic field are no longer balanced and the interaction between current and field causes a torsional force that tends to twist the rotor relative to the stator. This new force works in the same sense as the torque applied to the quill shaft to supplement the torque in the quill shaft. In effect this force provides some servo assistance to any torque applied to the input shaft, making the shaft easier to turn for a given torque at the output shaft.

As the torque increase still further the quill shaft continues to twist. When the angle of twist reaches 60 degrees the brush moves around the commutator far enough for the original coil to no longer be energised. This position of brush and commutator is shown in figure 6(b) and of coils in Figure 6 (a) . The torque generated between the rotor and stator increases again.

Finally, when the torque reaches its maximum level and the quill shaft has twisted through 90 degrees the brushes move to contact a third contact portion as well as the second. This passes current through coil B and also coil C and creates even more torque between the rotor and stator.

The servo torque generated by the unit therefore increases in steps each time a new coil is connected by the relative movement of the brushes and the commutator. This is shown in Figure 7.

It is notable that the stepped servo assistance effect is achieved if the torque is applied in the opposite sense because the arrangement is symmetrical.

In normal use the output shaft will not be fixed in position relative to the stator, but the whole shaft arrangement of input shaft, quill shaft and output shaft are free to rotate relative to the stator. Lets consider the case in which there is zero torque applied between the input and output shaft. Initially this will correspond to the position shown in Figure 3 (a) and 3(b) with current in coil A.

Next, as the input and output shafts rotate, the commutator will rotate relative to the brushes. In this case, rather than relative movement occurring between the commutator and brushes and also the commutator and rotor it occurs between the commutator and the brushes only, with the coils also rotating within the magnetic field. The operation is therefore similar to that of a conventional DC servo motor but with the coils arranged 90 degrees away from their usual alignment. Coil A, then

A plus B, then B, then B plus C will be energised in turn as the shaft rotates, but in each case the coils that are energised will generally remain at the same angle relative to the magnets and hence no torque will be produced between rotor and stator. This fixed angular relationship holds true for non-zero torques too.

It has been appreciated that an unwanted torque ripple may be produced during rotation. When the rotor is rotated 15 degrees for example with no torque applied to the input shaft the first coil A will have moved 15 degrees relative to the magnets yet the commutator will not have switched at that time to the next coil B. This produces a ripple torque that disappears once 30 degrees of rotation has occurred and coil B is energised. This ripple then reappears but with the opposite sign as the shaft rotates to 45 degrees (again with no torque applied to the input shaft) . This could, however, be minimised through careful selection of the magnet geometry. This is shown in Figure 8 to 10 of the accompanying drawings.

An alternative embodiment 200 of a servo unit is shown in Figure 11 (a) of the accompanying drawings. Where appropriate the same reference numerals have been used as for the embodiment of Figure 1 to denote like components. In this arrangement the flying leads are eliminated by fixing the commutator 12 to the rotor 8 and the brushes 14a, 14b to a brush deflection mechanism 18 (comprising a rotating slider) fitted to the input shaft 4 (rather than to the stator) . The purpose of the brush deflection mechanism 18 is to ensure that with a torsional load applied across the quill shaft the brushes move relative to the stator rather than remain in a fixed position, yet not cause the brushes to move in response to rotation of the input and output shaft (with the quill shaft remaining under a constant torsional load) .

In more detail, the operation of the embodiment of Figure 11 (a) is as follows. A rotating slider 18 in the form of an annular collar is provided which connects the input and output shafts. It is connected to the input shaft by a pair of diametrically opposed pins 18a, 18b which are keyed to the input shaft and which are located within respective elongate slots 19a in the slider. The slider 18 can therefore move axially relative to the input shaft by an amount determined by the free movement of the pins in the slots, but is prevented from moving rotationally relative to the input shaft. It is connected to the output shaft 6 in a similar manner, but this time the slots 19b are aligned at an angle to the axis of the output shaft (an angle of 45 degrees is shown but any any greater than about 10 degrees and less than about 80 will work just as well) . Thus, as the output shaft rotates relative to the input shaft the slider will be forced to move axially relative to the input/output shafts.

The slider 18 abuts a second collar 14 which is fixed to the housing by another pair of pins 14' , 14" in respective slots 19c. These pins ensure the second collar 14 can move axially but not rotationally relative to the housing. This second collar carries the brush set 14a, 14b which will contact the commutator 12 that is secured to the output shaft 6. As the rotating slider moves axially it in turn causes the second collar and brushes to move axially. Finally, the commutators 12 are arranged at an angle to the axis of the output shaft (an angle of about 45 degrees is shown) . The effect of the brushes 14a, 14b sliding axially is to provide the required additional relative movement of the brush and commutator. A spring (not shown) pushes the second collar onto the rotating slider 18.

A similar effect is achieved in the alternative embodiment 250 of Figure ll(b) of the accompanying drawings. In this arrangement the rotating slider is replaced with an epicyclic gearbox 110. The gearbox comprises a sun gear 111 fixed to the input shaft and a planet gear 112 fixed to the output shaft. A secondary sun gear 113 is fixed to the brush set 14. Both sun gears 111,113 rotate with the planet gear 112. The brush set is mounted to the housing in a bearing such that it can rotate about the axis of the output shaft. These components define a brush deflection mechanism 18. In use, the relative movement of the input and output shafts cause the relative positions of the two sun gear to vary, which therefore creates the desired additional movement of the brush set.

A still further alternative embodiment 250 is illustrated in Figure 12 of the accompanying drawings. In this arrangement, the commutator 12 is fixed to the earth or stator and does not move. The stator 10 now comprises the coils that were part of the rotor of the first two embodiments. The rotor 8 now carries the two magnets that were part of the stator of the first two embodiments. An insulating sleeve 20 is fixed to the input shaft which carries two annular tracks 22,24 forming slip rings. Supply brushes 26,28 connect one track to a positive supply and the other to an earth. Flying leads then connect the positive supply to a pair of brushes 14a, 14b fixed to the insulating sleeve which contact the commutator. In the event of a torque being applied to the quill shaft, relative movement between the rotor and stator is produced as for the first and second embodiments. As the input and output shafts rotate together the brushes also move across the commutator in the same way as for the first and second embodiments.

It will be understood that various modifications can be made to the embodiments that have been illustrated whilst remaining within the scope of the present invention. In particular the number of magnets and their type and the number of coils can be varied to suit each application. This may require a corresponding change to the number of contact regions of the commutator. As a general rule, an increase in the number of coils can yield an increasingly small set of step changes in assistance torque as a torque is applied by the driver. For example, with two pairs of magnets and two sets of three coils, it is possible for six step changes in servo torque to be achieved over a given 90 degrees of twist of the quill shaft. Alternatively, the same 3 steps could be achieved from only a 45 degree twist of the quill shaft.

The servo assistance unit is self contained apart from the need to connect to a power supply. This makes it low cost and robust and it is envisaged that it may find application in a number of areas of vehicle steering in which conventional electric power assisted steering would be prohibitively expensive or complex. One example is in steering for a ride on lawn mower 400. This is shown in Figure 13 in which it connects a steering wheel 410 to the front wheels 420 of the mower to help the driver turn the wheel.

Claims

1. An electrical servo assistance unit for use in a power assisted steering system comprising: a stator, a rotor located concentrically within the stator for rotation about an axis, one of the stator and rotor comprising a plurality of magnets at spaced locations around the axis of rotation of the rotor, the other of the stator and 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 and sharing a common axis of rotation; a commutator comprising at least two conductive portions, each of which provides an electrical connection to a different coil; and at least one brush which is 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; the unit being so constructed and arranged that as the input shaft and output shaft rotate together with a constant zero torque applied to the deflection member the commutator selectively connects the power supply to one or more of the coils such that at any given time the coil or coils which are energised lie within a defined range of angles centred about a point having a substantially fixed angular relationship to the magnets, that angular relationship being chosen such that the forces created between the current carrying coils and the magnets are approximately in balance, and in which that angular relationship is changed in a step-wise manner as an increasing torque is applied to the deflection member which torque causes additional relative movement between the brushes and commutator contact portions, the step-wise change in angular relationship causing a corresponding step wise increase in an imbalance of the forces between the current carrying coil or coils and the magnets.

2. An electrical servo assistance according to claim 1 in which the brushes and commutator are arranged such that at any one time at least one coil is always connected to the brush, and the angular relationship is such that at low torques the forces between the magnets and coils are substantially balanced and when a high torque is applied the forces between the coils and magnets are unbalanced such that the rotor and stator together apply their own torque between the body of the unit and the output shaft in the same sense as the applied torque.

3. An electrical servo assistance according to claim 1 or claim 2 in which at least one of the commutator and brushes is adapted to rotate about the axis of rotation of the rotor and is connected to either the rotor or the input shaft.

4. An electrical servo assistance according to any preceding claim in which the magnets are on the stator and the coils on the rotor.

5. An electrical servo assistance according to any one of claims 1 to 3 in which the coils are on the stator and the magnets on the rotor;

6. An electrical servo assistance according to claim 4 in which the commutator is carried by the input shaft and the brush fixed relative to the stator;

7. An electrical servo assistance according to claim 4 in which the commutator is carried by the rotor and the brush connected to the input shaft through a brush deflection mechanism.

8. An electrical servo assistance according to claim 5 in which the commutator is carried by the stator and the brush by the input shaft.

9. An electrical servo assistance according to any preceding claim in which the rotor or stator carries N coils which are spaced around the circumference of the rotor where N is an integer greater than 1.

10. An electrical servo assistance according to claim 9 in which two magnets are provided which are disposed diametrically opposite one another relative to the axis of rotation of the rotor.

11. An electrical servo assistance according to claim 10 in which three coils are provided, each of which comprising a plurality of windings with the coils spaced equally around the stator or rotor.

12. An electrical servo assistance according to any preceding claim in which the term substantially fixed angular relationship means that an energised coil will be in a fixed angular position relative to the magnets within + /-30 degrees electrical as defined by the arc subtended by the brush

13. An electrical servo assistance according to any preceding claim in which with no torsional load applied to the input shaft, a coil or coils will only be energised if they lie within a range of + /-30 degrees from a point that together with the axis of rotation of the rotor defines a line that extends at right angles to a line connecting the two magnets and in which with increasing torque the centre point of this range will be changed so that the range will remain + /-30 degrees but will no longer be centred on a point at 90 degrees electrical to that line.

14. An electrical servo assistance according to any preceding claim which further includes a DC source of constant current which is fed to the at least one brush and a switch which isolates the brush from the supply when the torque applied between the input and output shafts is below a limit, such as substantially zero torque.

15 An electrical servo assistance according to any preceding claim in which the magnets comprise permanent magnets.

16. An electrical servo assistance according to any preceding claim in which the deflection member comprises a quill shaft, one end of which connects to the input shaft and the other to the output shaft.

17. An electrical servo assistance according to claim 16 in which the quill shaft permits a maximum twist of the input shaft relative to the output shaft of 90 degrees where two magnets are provided.

18. An electrical servo assistance according to any preceding claim in which the brush has sufficient width to contact up to two contact portions of the commutator at any given time.

19. An electrical servo assistance according to any preceding claim in which the input shaft is connected to a steering wheel of a vehicle directly or through one or more shafts, and the output shaft is connected directly or indirectly to one or more steered wheels of the vehicle.

20. An electrical servo assistance according to any preceding claim when dependent from claim 7 in which the brush deflection mechanism connects the input shaft to the output shaft, the mechanism being operable to cause relative deflection of a brush set and a commutator connected to one or other of the input and output shafts .

21. An electrical servo assistance according to claim 20 in which the brush deflection mechanism includes a rotating slider which converts relative rotational movement of the input and output shafts into axial movement of the slider, the slider in turn being connected to the brush set and hence creating axial movement of the brush set.

22. An electrical servo assistance according to claim 20 in which the brush deflection mechanism includes an epicyclic gearbox which include a first sun gear connected to the input shaft (or output shaft) , a second sun gear connected to a brush set, and a planet gear that connects both sun gears and which is connected to the other of the input and output shaft, the gearbox converting relative angular movement of the input and output shafts into angular movement of the brush set.

23. 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.

24. An electric power assisted steering system according to claim 23 in which the vehicle comprises a ride on lawn mower.

25. An electrical servo assistance substantially as described herein with reference to and as illustrated in Figures 1 to 12 of the accompanying drawings.