WO2007137348A1

WO2007137348A1 - Pawl actuator

Info

Publication number: WO2007137348A1
Application number: PCT/AU2007/000742
Authority: WO
Inventors: Nui Wang
Original assignee: Bosch Chassis Systems Australia Pty Ltd
Priority date: 2006-05-30
Filing date: 2007-05-30
Publication date: 2007-12-06
Also published as: CA2653775A1; EP2029908A1; JP2009538766A; AU2007266327A1

Abstract

A pawl actuator for use with a toothed wheel, including: a pawl member having a mounting section; and a tooth engagement section, the pawl member being mountable to a support structure at the mounting section and the tooth engagement section being movable between a tooth engaged position and a tooth disengaged position, the pawl actuator including a biasing arrangement to bias the tooth engagement section in a first direction toward the tooth engaged position when the tooth engagement section is in a first section of travel between the tooth engaged position and the tooth disengaged position and in a second direction toward the tooth disengaged position when the tooth engaged section is in a second section of travel between the tooth engaged position and the tooth disengaged position, the pawl actuator being further arranged to alter the direction in which the tooth engagement section is biased at a region intermediate the first and second sections of travel, the pawl actuator including an actuating arrangement arranged to act on the pawl member for displacing the tooth engagement section against the bias of the biasing arrangement from the tooth disengaged position through the second section of travel to the intermediate region, to a change of bias position within the intermediate region at which the direction of bias changes so that the biasing arrangement thereafter biases the tooth engagement section towards the tooth engaged position.

Description

PAWL ACTUATOR

The present invention relates to a pawl actuator. It will be convenient to describe the invention in relation to its use in a vehicle equipped with an electric parking brake, although it is to be appreciated that the invention could be employed in other devices.

Electric parking brakes employ an electric motor to apply the brakes. While it is feasible to maintain power to the electric motor to apply the brakes for short periods, such as for hill starts during driving, it is not feasible to continue to power the electric motor for long periods, such as parking overnight, when the engine of the vehicle has been turned off. This is because battery power supply in a vehicle is limited and would be quickly exhausted if the electric motor was required to be powered for lengthy periods. Accordingly, mechanical locking of the parking brakes is preferred. However, mechanical locking devices can suffer from various undesirable performance characteristics, such as performance variation due to rubbing interfaces between parts having inconsistent friction losses and those interfaces being subject to wear over time, through contamination by ingress of debris over time.

It is an object of the present invention to provide a pawl actuator that overcomes or at least alleviates one or more disadvantages associated with the prior art and preferably which provides improved operation over prior art pawl actuators.

According to the present invention there is provided a pawl actuator for use with a toothed wheel, including: a pawl member having a mounting section; and a tooth engagement section, the pawl member being mountable to a support structure at the mounting section and the tooth engagement section being movable between a tooth engaged position and a tooth disengaged position, the pawl actuator including a biasing arrangement to bias the tooth engagement section in a first direction toward the tooth engaged position when the tooth engagement section is in a first section of travel between the tooth engaged position and the tooth disengaged position and in a second direction toward the tooth disengaged position when the tooth engaged section is in a second section of travel between the tooth engaged position and the tooth disengaged position, the pawl actuator being further arranged to alter the direction in which the tooth engagement section is biased at a region intermediate the first and second sections of travel, the pawl actuator including an actuating arrangement arranged to act on the pawl member for displacing the tooth engagement section against the bias of the biasing arrangement from the tooth disengaged position through the second section of travel to the intermediate region, to a change of bias position within the intermediate region at which the direction of bias changes so that the biasing arrangement thereafter biases the tooth engagement section towards the tooth engaged position.

The pawl member described above is a bi-stable member, meaning that it has two stable positions, one on either side of an unstable position. The intermediate position is unstable, while the tooth engaged and disengaged positions are stable. Thus, when the tooth engagement section of the pawl member in the intermediate region, there is instability and the tendency is for the tooth engagement section to move to one of the available stable positions. The pawl actuator is operable when the tooth engagement section is in the intermediate region, to cause the tooth engagement section to move under the influence of the biasing arrangement to one of the stable positions and the selection of the stable position is dependent on which section of travel the tooth engagement section has travelled through to arrive at the intermediate region. For example, if the tooth engagement section has travelled through the first section of travel, then the biasing arrangement will bias the tooth engagement section to travel through the second section of travel.

The present invention, as will be readily understood, is for use with a toothed or splined wheel or shaft. The pawl actuator is provided to engage a tooth or a plurality of teeth of the wheel or shaft for the purpose of preventing the wheel or shaft from rotating in one direction. In an electric parking brake, the pawl actuator is usually operable to prevent rotation of a toothed wheel in a direction that would release the parking brakes. For this, the pawl actuator engages the face or flank of the tooth and will prevent further rotation in the direction which would enable brake release. When used in an electric parking brake, the pawl actuator can be employed to lock a wheel or shaft against rotation in a direction that will release the parking brakes. For example, an electric parking brake might apply the brakes by rotating a ball screw to shift a nut axially against a disc brake pad, to shift the pad to engage a rotor. In that arrangement, the pawl actuator is employed to prevent reverse rotation of the ball screw when the disc brake pad has engaged the rotor, to maintain the brakes applied. The pawl actuator can engage a toothed profile that is applied to the ball screw, or a toothed wheel that is connected to the ball screw. Alternatively, the toothed wheel may form part of a gear train between the electric motor and the ball screw. Still alternatively, the toothed wheel can be part of the rotor of the electric motor that drives the electric brakes, as well as being part of a position sensor system that senses an aspect of the electric motor, such as the rotational position of the rotor, to determine the extent of application of the brakes.

The pawl actuator of the invention can alter the direction of bias of the tooth engagement section in any suitable manner. In one arrangement, the actuator includes electromagnetism to shift the tooth engagement section from the tooth disengaged position to the tooth engaged position. In one arrangement of this kind, the pawl member can include a ferrous component or can be manufactured substantially or completely from a ferrous material. In this arrangement, magnetism can be employed to act on the pawl member to cause it to shift through one or each of the first and second sections of travel. Thus, the pawl actuator can be operable to shift the tooth engagement section from the tooth disengaged position to the tooth engaged position under a magnetic influence, preferably an attractive magnetic influence.

The tooth engagement section can be shifted alternatively by use of one or more thermal bi-metallic strips, that are heated when movement of the tooth engagement section is required. A single strip can be employed, although in the preferred arrangement, a pair of thermal bi-metallic strips is employed. In this arrangement, the pair of bi-metallic strips are in spaced apart facing relationship and they are constructed and arranged, so that when the pair of strips experience an equal change in temperature, such as if a vehicle is driven from a cool climate to a warm climate, or if the brake temperature increases as a result of braking then the strips tend towards displacement in equal and opposite directions. Thus, the position of the bi-metallic strips within the pawl actuator remains unchanged. However, the arrangement can be such that when one of the pair of bi-metallic strips experiences a change in temperature so that its temperature is different to the other of the bi-metallic strips, the temperature differential causes displacement of one of the strips so that the position of the pair of bi-metallic strips changes and that change in position is employed to shift the pawl member from the tooth disengaged position to the intermediate region. In this arrangement, the pawl actuator or the actuating arrangement can further include a heat facility for the application of heat when desired, to one of the pair of bi-metallic strips to cause the required displacement.

To shift the tooth engagement section in the opposite direction from the tooth engaged position to the tooth disengaged position, a displacement mechanism can be employed and in one form, that can include the use of a repellent magnetic influence. However in the preferred arrangement, the tooth engagement section can be shifted by engagement of the pawl member with a tooth which is adjacent to the tooth or teeth that is/are engaged by the tooth engagement section in the tooth engaged position. In this arrangement, rotation of the toothed wheel in a direction to release the tooth engagement section from engagement with the tooth face can be such as to cause an adjacent tooth to ride along a facing surface of the pawl member and to shift the tooth engagement section through the first section of travel against a biasing force, to the intermediate region between the first and second sections of movement. At that position, the direction of bias is altered and the tooth engagement section is thereafter biased towards the tooth disengaged position. Thus, displacement of the tooth engagement section from the tooth engaged position is complete. The adjacent tooth can be immediately adjacent the engaged tooth or teeth, or it can be a tooth which is close to, but not immediately adjacent the engaged tooth or teeth. Also, the surface of the adjacent tooth which rides along the surface of the pawl member can be any suitable tooth surface, and may change as the relative positions between the tooth and the pawl member change. For example, the initial contact surface could be at an edge between the top land and the adjacent face of the tooth. However, as the pawl member is shifted and the tooth wheel rotates, the contact surface might shift to the top land only. The teeth may be specially shaped for suitable contact with the pawl member.

In an alternative arrangement, the tooth engagement section can be arranged for at least partial receipt of a tooth of the toothed wheel in the tooth engaged position. In this arrangement, portions of the tooth engagement section are in facing relationship with opposite faces of the tooth and at least a first of those portions engages a first face of the tooth so that the toothed wheel is prevented from rotating in a first direction. To release the pawl member from the tooth engaged position, the toothed wheel is rotated in a second and opposite direction such that a second of the portions of the tooth engagement section which faces an opposite face of the tooth, slides against and relative to the second face causing displacement of the tooth engagement section from the tooth engaged position, against the bias of the biasing arrangement to the intermediate region. Sliding contact with the second face may not displace the tooth engagement section fully to the intermediate region, as there may be some final contact with the top of the tooth, but it is preferred that a substantial amount of the displacement occurs through sliding engagement between the second portion of the tooth engagement section and the second tooth face.

The biasing arrangement can take any suitable form. In one form, the pawl member can be constructed to have a biasing characteristic to fulfil the requirements of the biasing arrangement. In one arrangement, the pawl member is formed from a plate or sheet material, such as a metal plate and has an opening formed in the mounting section thereof for receipt of a portion of a support structure to which the pawl member is mounted. One pair of opposed side edges of the opening is arranged for engagement of the support structure portion, while the sides which are transverse or lateral to the pair of side edges include a sprung section.

To apply the pawl member to the support structure portion, the sprung sides may be stretched to allow the pair of opposed side edges to be shifted apart to accept the support structure portion and thereafter be released to engage the support structure portion. In this arrangement it is preferable that a first side edge of the pair of opposed side edges engages the support structure portion on a side of that portion which faces the teeth to be engaged. That side edge can be separated from connection with the pair of sides, so that the first side edge forms the leading edge of a tongue which can move separately from the sides. The tongue can be arranged to pivot relative to the support structure portion about the leading edge and that pivoting can facilitate travel of the tooth engagement section through the first and second sections of travel The separation can be by slits which are cut into the pawl member and which extend rearwardly from the first side edge in a direction toward the teeth to be engaged In this arrangement, the pawl member tends to adopt a position on the support member in which the sprung sides have minimum tension therein or in other words, are in their most relaxed state. Thus, by careful selection of the dimensions of the pawl member, in particular the spacing between the opposed side edges of the opening which engage the support structure portion and of the dimensions of the sides, the pawl member can be biased toward either of the tooth engaged or tooth disengaged positions by the sprung sides tending to shift to positions of reduced tension. In this arrangement, the sprung sides experience maximum tension at the region intermediate the tooth engaged and tooth disengaged positions and therefore bias the tooth engagement section to one of those positions depending on which side of the region the tooth engagement section is located.

The sprung sides therefore tend to cause the pawl member to rotate about the first side edge or leading edge of the tongue, to reduce the length of the sprung sides, and by that rotation, the tooth engagement section of the pawl member can be shifted. In the preferred arrangement, the sprung sides can facilitate biased movement of the tooth engagement section to each of the tooth engaged position and the tooth disengaged position about the position at which the sprung sides are in maximum tension.

In the above arrangement, the sprung sides can have any suitable form. For example, each sprung side can include a coil spring section extending between facing non-spring sections. In another form, the sides can have a wave form. In either form the length of the sides can be increased by stretching the sides for mounting the pawl member on the support structure portion, and the sides act as a spring, which can expand and contract. In this arrangement, maximum spring extension occurs at the region intermediate the first and second sections of travel and on either side of that maximum extension the pawl member is biased to either the tooth engaged position or the tooth disengaged position.

The wave form of the sprung sides can include any number of wave segments, where a segment is measured between adjacent wave troughs. In one arrangement, the pawl member includes two wave segments, although a single wave segment, or three or more wave segments could be provided. To minimise the length of the pawl member the sides thereof may be sprung from at or adjacent the end of the pawl member which is remote from the tooth engagement section. Thus, the sides of the pawl member adjacent the support structure portion may have a wave form.

In the above described embodiment of the invention, in which the sides of the pawl member are sprung, the pawl member is preferably formed from a spring metal, such as a phosphorus bronze, stainless steel or spring steel or engineering plastics. The pawl member could alternatively be formed as a composite, in which the sprung sides are of a spring metal and other parts of the pawl member are of different metal or material. As discussed earlier, the pawl member includes a ferrous portion and that portion may be formed as part of the pawl member or as an attachment to the pawl member. The pawl member could be made for example, completely from a ferrous material if desired.

The above discussion relates to the formation of the sides of the pawl member in a sprung form. An alternative to forming the sides as sprung members, is to form the tongue described above as having a sprung component and in that arrangement, the sides of the pawl member could have no sprung component. A further alternative is to have a second tongue formed opposite to the tongue described above, or if the tongue described above is not provided, to form a different tongue in the pawl member which extends from the second side edge of the pair of side edges. In either case, the tongue will have a leading edge which engages the support structure portion on a side of that portion opposite to the side which faces the teeth to be engaged. In this alternative, the second or different tongue described above can have a sprung component.

In a still further alternative form, the pawl member is formed to include a pivot edge that is arranged to engage a surface of the support structure and to be pivotable on that edge relative to the support structure. In this arrangement, the biasing arrangement could be provided by a spring arrangement which extends from engagement with the pawl member to an anchor provided by the support structure. In this arrangement, the spring arrangement would act in the same manner as the sprung sections discussed above, such that tension in the spring arrangement is at a maximum when the tooth engagement section is in the intermediate region and is reduced when the tooth engagement section is disposed at either side of the intermediate region, toward either of the tooth engaged or tooth disengaged positions. As with the earlier embodiments, pivoting of the pawl member about the pivot edge facilitates travel of the tooth engagement section between the tooth engaged and tooth disengaged positions.

In the above arrangement, the spring arrangement can include a coil spring, or a pair of coil springs which are spaced-apart on opposite sides of the pawl member. Alternatively, the spring arrangement could include an elongate, elastic member, such as a rubber cord.

For secure location of the pawl member discussed above, the pivot edge can form an inside edge of a locating recess with the recess being defined by the pivot edge and a pair of side portions which, in use extend on opposite sides of the support structure. Thus, the recess accepts the portion of the support structure at which the pawl member is mounted and securely locates the pawl member relative to the support structure.

In a further embodiment of the invention, the support structure can include a sprung section which cooperates with the pawl member to facilitate travel of the tooth engagement section between the tooth engaged and tooth disengaged positions. Thus, the support structure can include a sprung section in which the tension in the sprung section is at a maximum when the tooth engagement section is in the intermediate region and is reduced when the tooth engagement section is on either side of the intermediate region toward either of the tooth engaged or tooth disengaged positions. This arrangement can be embodied in a pawl member which includes an opening for receipt of the support structure, and which opening includes a pair of opposed edges that engage a surface or surfaces of the support structure. The support structure could include a pair of legs, each of which is engaged by a respective one of the edges of the opening and one or each of the legs could be arranged for resilient flexing to form a sprung section.

In each of the above embodiments, electromagnetic actuation of the pawl member to displace the pawl member from the tooth disengaged position to the intermediate region, can be employed. Thus, an electromagnetic coil can energise a magnetic pad, such as a piece of ferrous metal, to apply an attractive charge to a similar metallic component of the pawl member. The same electromagnetic actuator can be employed to repel the pawl member from the tooth engaged position, although the mechanisms discussed above relating to sliding engagement of the tooth engagement section to displace the pawl member are preferred. In an alternative embodiment, thermal actuation can be employed, such as a thermal bi-metallic actuator. In that arrangement, the application of heat to the actuator can cause it to shift, and that movement can be employed to act on the pawl member in the manner required. A heat source may take the form of an electric coil that heats upon application of an electric current.

Alternatively, a piston or plunger (hereinafter a plunger) can be employed to physically engage the pawl member, either directly or indirectly. The plunger can operate to shift the tooth engagement section in each direction between the tooth engaged and disengaged positions, although in the preferred arrangement, the plunger is operable to shift the tooth engagement section only in one direction and that is from the tooth disengaged position to the intermediate region. A solenoid or other electro-magnetic actuation can be employed so that the position of the plunger is controlled electrically.

The plunger can shift the pawl member is any suitable manner, but the preference is for the pawl member to be rotated by the plunger. Thus the pawl member can be mounted for rotation and the plunger positioned to cause rotation upon displacement of the plunger at least in one direction.

In the above arrangement, the biasing arrangement can be operable in tension or compression. In either case, the maximum tension or compression will occur in the intermediate region, and the biasing arrangement will bias the pawl member in one of two directions depending on the position of the pawl member.

In the above arrangement, the biasing arrangement preferably is a wire spring. In a preferred arrangement, the wire spring is a C or U shaped spring, which has one of the free ends thereof attached to the pawl member and the other free end attached to a relatively stationary or grounded part of the pawl actuator or the support structure.

In this arrangement, the load required of the plunger can be very low, given that the plunger is only required to rotate the pawl member against the biasing influence of the coil spring or other biasing arrangement, and against any frictional and gravitational resistance to rotation. Frictional resistance can be arranged to be very low. The plunger may be arranged for electro-magnetic displacement only in one direction, to rotate the pawl member from the tooth disengaged position to the intermediate region. Return movement of the plunger can take place by the pawl member pushing against the plunger as it is displaced from the tooth engaged position to the intermediate region. Thus, whatever mechanism is used to cause the pawl member to travel from the tooth engaged position to the tooth disengaged position, can also be employed by engagement between the pawl member and the plunger to shift the plunger in the reverse direction. The load required to shift the plunger can be very low.

In an alternative arrangement in which a plunger is employed, the plunger can shift the pawl member in each of its two directions of rotation.

A pawl actuator according to any one of the embodiments discussed above, can include an abutment or stop that is engaged by the pawl member in the tooth disengaged position. By this arrangement, the pawl member will be in engagement with a suitable abutment in each of the tooth disengaged and tooth engaged positions, whereby in the tooth engaged position, engagement is with a tooth or teeth of a toothed wheel.

The toothed wheel for which the pawl actuator can be employed, can form part of an electric braking assembly, such as an electric parking brake assembly, and the toothed wheel can form part of a gear train of such an assembly, or as part of a ball screw of a ball screw actuator, or as part of the rotor of a motor, or as part of a position sensor wheel of an arrangement employed to sense the rotational position of the motor rotor.

The intermediate region between the first and second sections of travel can be a region which is generally central between the tooth engaged and the tooth disengaged positions, or it can be closer to one of those positions than the other. The actual position within the intermediate region at which the bias applied to the tooth engagement section changes direction, typically will be slightly different depending on whether the tooth engagement section is moving towards the intermediate region from the tooth engaged position or the tooth disengaged position. Within the intermediate region, there will be a neutral and unstable position at which the biasing arrangement exerts no biasing influence in either direction on the tooth engagement section, but on either side of that position, or just past it, the biasing arrangement applies a biasing influence. Thus, the actuating arrangement of the pawl actuator must act on the pawl member to displace the tooth engagement section to the intermediate region and just past the neutral position, so that the direction of bias changes and the tooth engagement section is thereafter biased from the intermediate region toward the tooth engaged position. Additionally, when the tooth engagement section is displaced through the first section of travel from the tooth engaged position, the displacement mechanism employed for that purpose must displace the tooth engagement section to the intermediate region and just beyond the neutral position, so that the direction of bias changes and the tooth engagement section is thereafter biased from the intermediate region toward the tooth disengaged position.

The pawl actuator according to the invention can be employed in motor driven drum brakes or in motor driven disc brake calipers, particularly those which employ electric parking brake facilities. Thus the present invention could be employed in a disc brake assembly of the kind disclosed in co-pending International application PCT/AU2005/000854, which has been published under the number WO 2005/124180. The full disclosure of the International application is included herein by cross reference.

A significant advantage of the invention is that the biasing arrangement can act to snap the tooth engagement section into either of the tooth engaged or tooth disengaged positions, so that actuation of the pawl actuator can occur rapidly. That movement can be a form of toggle movement.

For a better understanding of the invention and to show how it may be performed, embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings.

Figures 1 and 1 a are perspective views of pawl members according to two embodiments of the invention.

Figure 2 is a side view of the pawl member of Figure 1 shown in a tooth engaged position in a pawl actuator according to the invention.

Figure 3 is a side view of the pawl member of Figure 1 shown in a tooth disengaged position in a pawl actuator according to the invention.

Figure 4 is a plan cross-sectional view of an alternative form of pawl actuator according to the invention. Figures 5 and 6 are side views of the pawl actuator of Figure 4 shown respectively in tooth engaged and disengaged positions.

Figure 7 is a plan view of a pawl actuator according to a further aspect of the invention.

Figures 8 and 9 are side views of the pawl actuator of Figure 7 shown respectively in tooth engaged and disengaged positions.

Figure 10 is a cross-sectional view of a further embodiment of the invention.

Figure 1 1 is a side view of still a further embodiment of the invention.

Figure 1 1 a is a cross-sectional view of one section of Figure 1 1.

Figure 12 is a cross-section view of a disc brake caliper which employs an electric brake operating arrangement, of the kind in which a pawl actuator according to the invention can be employed.

Figures 1 and 1 a are perspective views of generally rectangular pawl members according to two aspects of the present invention. Referring to Figure 1 first, the pawl member 10 is constructed from sheet metal or other suitable material such as an engineering plastic, and is punched to produce an opening 1 1 . The opening 1 1 is sized to accept a support structure on which the pawl member 10 is mounted and opposing side edges 12, 13 of the opening 1 1 are arranged for engagement with the support structure.

A plurality of slots open into the opening 1 1 . Slots 14, 15 extend rearwardly from the side edge 12, while slots 16, 17 extend rearwardly from the side 13. The slots 16, 17 may be omitted if desired. The slots 14, 15 have a significantly greater extent than the slots 16, 17. The slots 14, 15 separate side portions 18, 19 from a central tongue 20. The tongue 20 defines the side edge 12 at one end and is formed integrally connected with other parts of the pawl member 10 at the other end 21 thereof. As will become apparent from later discussion, the tongue 20 can be displaced out of the general plane of the pawl member 10 by pivoting about its end 21. The pawl member 10 includes a tooth engagement section 22, which is arranged for engagement of a toothed wheel, shaft or spline 40 (see Figures 2 and 3). The pawl member 10 further includes sprung sides 25, 26 which can be formed by or included in the side portions 18, 19. With reference to Figure 2, the sprung side 26 is shown in side view, while the sprung side 25 is obscured. From this figure it can clearly be seen that the sprung side 26 is formed in a wave. The sprung side 25 is identical in form to the sprung side 26. It will be appreciated that with appropriate selection of material the sprung sides 25, 26 can be resiliently stretched or bent to allow pivoting movement of the pawl member 10.

Figure 2 shows the pawl member 10 mounted to a support 27. The support 27 has a head 28 and a neck 29. The neck 29 includes a pair of oppositely positioned, preferably convex shaped surfaces 30 against which the side edges 12, 13 of the opening 1 1 abut and roll or pivot. That abutment secures the pawl member 10 to the support 27. Also, the convex shape of the surfaces 30 facilitates rolling contact with the side edges 12, 13 when the pawl member 10 rotates relative to the support 27, as will be described later herein.

The spacing between the side edges 12, 13 in a rest condition of the pawl member 10, i.e. when it is not fixed to the support 27, is less than when it is fixed to the support 27 and therefore it is necessary to increase the spacing in order to mount the pawl member 10 to the support 27. This can be done in several ways. Firstly, the sprung sides 25, 26 can be stretched. Alternatively, the sprung sides 25, 26 can be bent. Still alternatively, the tongue 20 can be resiliently flexed about the end 21 . The increase in spacing obtained by anyone of these methods is required to be sufficient for the opening 1 1 to clear the head

28 of the support 27 and then for the side edges 12, 13 to nest within the neck

29 and to abut against the surfaces 30. The resilience of the stretching, bending or flexing should be such as to cause the side edges 12, 13 to be biased into firm engagement with the surfaces 30 to firmly hold the pawl member 10 in position on the support 27. The neck 29 may be formed square, so that inside edges of the side portions 18, 19 of the pawl member 10 also abut and locate against the surface of the neck. Other arrangements could also be employed.

Figure 2 shows the pawl member 10 in position on the support 27, with the side edges 12, 13 in abutting engagement with the surfaces 30. Where the tongue 10 would be obscured by the sprung sides 25, 26 it is shown in broken outline. Figure 2 also shows a plurality of teeth 31 a, 31 b and 31 c of a toothed wheel 40 and an abutment or stop 32 formed as part of the structure from which the support 27 extends.

Attached to the pawl member 10 by any suitable arrangement, is a magnetic pad 33. The pad 33 preferably is a ferrous pad. The pad 33 can be fixed to the pawl member 10 by any suitable arrangement, such as by riveting or brazing.

In Figure 2, the section of fixing is indicated by reference numeral 34, and occurs at one end of the pad 33. The pad 33 extends inboard of the section of fixing 34, toward the support 27. The pad 33 is also shown in Figure 1 in broken outline and rivets 35 can be seen as the mechanism of attachment to the pawl member 10.

Figure 2 further illustrates an electrical coil 36 from which a pair of arms 36a extend at either end thereof (only one is visible in the side view of Figure 2). The coil 36 is connected by the arms 36a to a ferrous pad 37 which is fixed against movement. The arrangement is such that by energising the coil 36, the pad 37 can be temporarily magnetised and in that state, the pad 37 can be employed to attract the magnetic pad 33. In an alternative arrangement, the pad 33 can be a permanent magnet and the coil 36 can be energised so that the pad 37 either attracts or repels the pad 33.

Figure 2 illustrates the pawl member 10 in a first position in which the tooth engagement section 22 is engaged with a face 38 of the tooth 31 a. This is the "tooth engaged position". The tooth engagement section 22 is further in line engagement with the edge 39 of the tooth 31 b. In that first position, the pad 33 has been attracted by the magnet 37 and the respective pad 33 and magnet 37 are closely spaced apart. Figure 3 illustrates a second position of the pawl member 10, in which the pawl member 10 is spaced from engagement with the teeth 31 and is in abutting engagement with the stop 32. This is the "tooth disengaged position".

The dimensions of the pawl member 10, are such that when it is attached to the support 27 as shown in Figure 2 or 3, it causes the sprung sides 25, 26 to flex and to bias the tooth engagement section 22 to either the tooth engaged position (as shown in Figure 2), or the tooth disengaged position (as shown in

Figure 3). The position of the pawl member 10 in each of Figures 2 and 3 is a stable position, whereas at the region intermediate those positions, the pawl member is unstable.

Movement of the pawl member 10 from the tooth engaged position of Figure 2, to the tooth disengaged position of Figure 3, occurs in the following manner. The toothed wheel 40 is rotated in an anti-clockwise direction to disengage the tooth engagement section 22 from the face 38 of the tooth 31 a. By that rotation, the line of engagement between the tooth edge 39 and the section 22 moves towards the distal end 41 of the section 22, which lifts (in the orientation of the pawl member 10 of Figure 2) the section 22 out of the valley 42. The lifting movement shifts the pawl member 10 towards the tooth disengaged position of Figure 3. That movement causes the sprung sides 25, 26 to stretch resiliently. As discussed earlier, the tension in the sprung sides 25, 26 increases as the sides 25, 26 stretch. The pawl member 10 reaches an intermediate region while the tooth engagement section is still in contact with the tooth edge 39. The tension in the sprung sides 25, 26 at the intermediate region is at a maximum. As discussed earlier, the maximum tension may not be exactly centred between the tooth engaged and disengaged positions, but rather, in the illustrated embodiment, the maximum tension occurs when each of the side edges 12, 13 and the distal end 41 are in planar alignment. Moreover, because the movement of the pawl member 10 is pivoting movement substantially about the side edge 12 of the tongue 20, at that position, the pawl member 10 is unstable.

The sprung sides 25, 26 act to bias the pawl member 10 toward the tooth engaged position until such time when the pawl member 10 is displaced just past the unstable position. At that position, the direction of the bias changes, and the sprung sides 25, 26 act to bias the pawl member 10 toward the tooth disengaged position, so that the pawl member 10 engages the stop 32. The tension in the sprung sides 25, 26 reduces as the pawl member 10 moves from the unstable position to the tooth disengaged position. However, the arrangement is such as to maintain tension in the sprung sides 25, 26 in each of the tooth engaged and disengaged conditions, in order to maintain a biasing influence on the pawl member to retain it in the tooth engaged or tooth disengaged condition.

The magnetic attraction that is employed in this embodiment, is provided to displace the pawl member 10 from the tooth disengaged to the tooth engaged condition. Thus, the attractive magnetic force must be sufficient to overcome the biasing influence of the sprung sides 25, 26, resisting travel of the pawl member 10 towards the tooth engaged position. The attractive force must be applied at least until the pawl member 10 has shifted to just beyond the unstable position, whereafter, the sprung sides 25, 26 will bias the pawl member 10 to the tooth engaged position.

An advantage of the embodiment of the invention illustrated in Figures 1 to 3, is that the pawl member 10 can be arranged to snap into and out of tooth engagement. The pawl member 10 acts in a toggle-like manner. A further and significant advantage is that the arrangement includes no rubbing parts, which cause wear and friction losses. This is important in automotive applications, because the life of the component is required to be maximised and in particular, wear of the component can compromise brake operation.

The coil 36 is shown schematically only and can take any suitable form.

The pawl member 10 of Figures 1 to 3 includes sprung sides 25, 26. In an alternative arrangement, the tongue 20 of the pawl member 10 can be sprung so that the sides 12, 13 include no sprung characteristic. Figure 1 a illustrates that arrangement, by showing a pawl member 10^'. Parts which are common between the pawl members 10 and 10^', have the same reference numeral plus ^'. It will be readily apparent that in relation to Figure 1 a, the operation of the pawl member 10^' is substantially the same as that of the pawl member 10, however it is the tongue 20^' which biases the tooth engagement section 22^' between the tooth engaged and tooth disengaged positions. Otherwise, the construction and operation of the pawl member 10^' can be substantially the same as that of the pawl member 10.

Figures 4 to 6 illustrate a second embodiment of the invention in which a pawl member 50 is shown. The pawl member 50 includes a body section 51 , which includes a tooth engagement section 52, and locating sides 53. The locating sides 53 are disposed in use, on either side of a support 54, which is shown in cross-section in Figure 4 and in side view in Figures 5 and 6. The pawl member 50 further includes a pair of lateral lugs 55 extending from opposite sides of the body section 51.

The locating sides 53 define a channel within which the support 54 is accommodated and an inner edge 56 of the channel engages the support 54 within a neck 57 of the support 54. The support 54 also has a head 58 and is constructed generally to the same form as the support structure of the embodiment of Figures 1 to 3. Thus, the support structure of the embodiment of Figures 4 to 5 includes the support 54 and a stop 59 spaced from the support 54.

The lugs 55 form anchor points for one end of a pair of springs 61 , the other ends of which are fixed to anchors 69. The springs 61 are coil springs, but they could be an elastic material, such as a rubber cord. The springs 61 are shown as a pair, but they could be a single spring which extends about an anchor point.

The side views of Figures 5 and 6 show a magnetic pad 62, preferably a ferrous magnetic pad, fixed to the underside of the body section 51 of the pawl member 50. The pad 62 can be fixed to the pawl member 50 in any suitable manner, such as in the same manner as the pad 33 is fixed to the pawl member 10.

Figures 5 and 6 also show a toothed wheel 63, which has teeth 64a and 64b. These figures show the pawl member 50 at tooth engaged (Figure 5) and tooth disengaged (Figures 6) positions. The movement of the pawl member 10 is the same as that described earlier, such that in the tooth engaged position, the distal end 66 of the body section 51 engages the face 65 of the tooth 64a. There is also engagement by the pawl member 50 of the edge 67 of the tooth 64b and in that position, the tooth engagement section 52 extends into the valley 68 between the teeth 64a and 64b.

Not illustrated in Figures 4 to 6 is a coil arrangement like that of Figures 2 and 3. The omission of such a coil arrangement is for clarity purposes only and therefore it should be assumed that a like coil arrangement would be employed in the pawl actuators of Figures 5 and 6. Thus, movement of the pawl member 50 from the tooth disengaged position of Figure 6, to tooth engaged position of Figure 5, requires energisation of a coil to magnetise a magnetic pad which is positioned to attract the pad 62. With sufficient attraction, the pawl member 50 is shifted towards the tooth engaged position, with the pawl member 50 pivoting about the edge 56 and the springs 61 pivoting about the anchors 69. The springs 61 are therefore stretched, so that the spring tension increases.

The spring tension tends to bias the pawl member 50 into one of the tooth engaged or tooth disengaged positions and at all times the springs 61 are tensioned, so that regardless of whichever position the pawl member 50 is in, the biasing influence of the springs 61 is to maintain that position in the absence of a larger, opposite force.

Movement of the pawl member 50 between the tooth engaged and disengaged positions occurs substantially in the same manner as that of the pawl member 10 described earlier. Thus, magnetic attraction is employed to shift the pawl member 50 against the biasing influence of the springs 61 from its position abutting the stop 59 to the intermediate region, and to a position in that region just beyond a position of maximum tension in the springs 61 . At that position, the direction of bias of the springs 61 changes and thereafter, the springs 61 bias the pawl member 50 into engagement with the toothed wheel 63. Thus, movement of the pawl member 50 is partly dependent on the magnetic bias and partly dependent on the spring bias and the magnetic bias is not required to be maintained when the direction of the spring bias is acting in the same direction as the magnetic bias, although it may be maintained without an adverse effect. Return movement of the pawl member 50 from the tooth engaged position to the tooth disengaged position also occurs in the same manner as that of the pawl member 10. Thus, the toothed wheel 63 is rotated anti-clockwise to disengage the distal end 66 from the tooth face 65, and the tooth edge 67 commences lifting of the tooth engagement section 52 out of the valley 68. That movement shifts the pawl member 50 toward the tooth disengaged position and as the movement occurs, the tension in the springs 61 increases. Spring tension continues to increase until the position of maximum tension is reached, and whereby just beyond that position, the direction of the spring bias changes and the springs 61 then bias the pawl member 50 to the tooth disengaged position. Thus, the return movement which is initiated by the tooth 64b is only required to shift the pawl member 50 until the pawl member 50 reaches a position just beyond the position at which the springs 61 are at maximum tension.

A further embodiment of the present invention is illustrated in Figures 7 to 9. In this embodiment, a pawl member 80 is shown mounted on a support structure 81 which has a pair of support legs 82, 83. The pawl member 80 is illustrated in Figure 7 and has a similar construction as the pawl member 10 of Figures 1 to 3. Thus, the pawl member 80 includes an opening 85 and slots 86 and 87, which open into the opening 85. Slots 86 and 87 separate side portions 90 and 91 from a tongue 92. The tongue 92 has a leading edge 93 which faces a further edge 94 disposed between slots 88 and 89 (which can be omitted if desired), and the edges 93 and 94 engage within recesses 95 and 96 respectively provided on the support legs 82 and 83. The pawl member 80 further includes a tooth engagement section 97 and a distal edge 98.

It is to be noted that in the embodiment of Figures 7 to 9, that the side portions 90, 91 are not formed in the same manner as the side portions 18, 19 of the pawl member 10, so that they are not sprung sides. Accordingly, the pawl member 80 does not have facility for increasing the separation between the edges 93 and 94 in the same manner as that of the pawl 10. Instead, in the arrangement of Figures 7 to 9, one or each of the support legs 82, 83 are resiliently flexible or pivotable to enable the pawl member 80 to operate in the same manner as the pawl member 10. Thus, either the support leg 82 or the support leg 83 can be arranged to flex in the direction shown by the arrows A, or indeed both of the support legs can be arranged for that flexing movement. However, the preference is for the leg 83 to be flexible and for the leg 82 to be rigid, given that the leg 82 reacts the tooth load (the load the tooth face 102 imparts to the distal end 101 of the pawl member 80) and rigidity of the leg 82 provides for a more precise positional control of the toothed wheel by the pawl member 80.

To illustrate the operation of the arrangement shown in Figures 7 to 9, an example will be described in relation to a construction of the support structure 81 , in which the support leg 83 has a resilient flexibility.

The pawl member 80 is displaceable between a tooth engaged position which is illustrated in Figure 8, and a tooth disengaged position which is illustrated in Figure 9. The pawl member 80 has a ferrous magnetic pad 99 fixed to an undersurface thereof, and movement between the tooth engaged and disengaged positions occurs in the same manner as described for the earlier embodiments of Figures 1 to 6. Thus, the toothed wheel 100 is rotated in an anti-clockwise direction, to disengage the distal end 101 from the tooth face 102 and the tooth edge 103 commences lifting of the tooth engagement section 97 from the valley 104. By that movement, the pawl member 80 is shifted in a direction towards the stop 105 provided as part of the support structure 81 .

With the movement of the pawl member 80 described above, in the previous embodiments, the sprung sides 25, 26, or the springs 61 , facilitated the movement. In the embodiment of Figures 7 to 9, movement is facilitated by resilient flexing of the leg 83. Thus, the leg 83 shifts inwardly towards the leg 82 as the pawl member 80 lifts away from the toothed wheel 100. The leg 83 will continue to move inwardly, until it reaches a position of maximum inward movement. Up until that position, the pawl member 80 is biased in a direction towards tooth engagement and against the direction in which it is being moved by the tooth edge 103. However, at the position of the leg 83, just beyond where it has reached its maximum inward movement, the bias applied to the pawl member 80 by the leg 83 changes and then is biased to it move to a position at which it engages the stop 105.

Movement in the opposite direction, from the tooth disengaged position to the tooth engaged position, occurs by energisation of a coil (not shown), so as to magnetically attract the pad 99. The operation of the coil is as described earlier for the alternative embodiments.

It should be appreciated that in the embodiment of Figures 7 to 9, that either of the legs 82 or 83, or both of them, can have a resilient flexibility. Resilience in these legs can also be provided by or in conjunction with external springs.

Figure 10 illustrates a further embodiment of the invention, in which the pawl actuator 130 includes a rotatable pawl member 131 . The pawl member 131 is mounted on an axle 132 for rotation about that axle. The pawl member 131 is acted on by both a compression spring 133 and a push rod 134. In the position shown, the spring 133 is at maximum compression.

The pawl member 131 includes a tooth engagement section 135 which is movable into and out of engagement with teeth of a toothed wheel (not shown) by rotation about the axle 132. The tooth engagement section 135 includes an opening 136 which is shaped complementary to a tooth of the toothed wheel and the opening 136 can therefore have any suitable shape.

The push rod 134 is shown acting against an abutment surface 137 and in the position shown, the push rod is close to, but not quite yet at one end of an actuating stroke. Thus, the push rod 134 has caused the pawl member 131 to rotate in an anti-clockwise direction against the direction of bias of the spring 133 in a manner causing the spring 133 to increase in compression load. When the spring 133 has reached its maximum compression load, only a slight further movement of the push rod 134 to rotate the pawl member 131 is necessary to change the direction of bias of the spring 133 whereafter the spring 133 will itself be operable to continue the rotational movement of the pawl member 131 in an anti-clockwise direction. The pawl actuator 130 includes a ferrous sleeve 138 which is fixed to an inside surface 138a of an opening in the housing 145. The push rod 134 has a neck portion 139 and a magnetic head member 143 attached to one end of the neck portion 139. A coil 141 is operable to displace the push rod 134 by an electro- magnetic load acting on the sleeve 138 and the head member 143, and energisation of the coil 141 can be such as to shift the head member 143 towards the sleeve 138, so as to shift the push rod 134 in a direction to cause the pawl member 131 to rotate anti-clockwise. This extends the push rod 134 and whereby withdrawal of the push rod 134 in the preferred arrangement is achieved by clockwise movement of the pawl member 131 during disengagement of the tooth engagement section 135 from a tooth. Such clockwise movement causes the surface 137 to push on the push rod 134 in a withdrawal direction. By this arrangement, operation of the pawl actuator 130 is simplified because the coil 141 is not required to shift the pawl member in both directions.

The push rod 134 and the spring 133 combine to rotate the pawl member 131 fully from a position of tooth disengagement to a position of tooth engagement. The push rod 134 causes the rotation from the tooth disengaged position to the intermediate region against the influence of the spring 133, while the spring 133 causes the rotation from the intermediate region to the tooth engaged position. The mechanism to cause reverse rotation of the pawl member 131 and therefore to cause displacement of the tooth engagement section 135 from the tooth engaged position to the tooth disengaged position, can be the same as previously disclosed in the earlier embodiments. Thus the toothed wheel is rotated in a reverse direction and interaction between the tooth engagement section 135 of the pawl member 131 and teeth of the toothed wheel cause the pawl member 131 to rotate in a clockwise direction against the bias of the spring 133 until the spring 133 reaches a point just past where it has experienced maximum compression and thereafter the direction of the spring 133 changes and biases the pawl member 131 to continue rotation in a clockwise direction to the tooth disengaged position. The compression spring 133 can take any suitable form, but preferably it has a C or U shaped form, in which one of the free ends 146 of the spring is fixed or secured to the pawl member 131 and in the pawl actuator 130, location is by the spring end 146 being located within a recess 147. The other end 148 of the spring 133, is secured to a spring anchor 149.

Figure 1 1 illustrates a further pawl actuator 150 according to the invention. The pawl actuator 150 includes a form of actuation which employs a bi-metallic strip actuator according to the following description.

The pawl actuator 150 includes a pawl member 151 which could take either of the forms of the pawl members 10 or 10^' previously disclosed. Additionally, the pawl actuator 150 includes a support structure 152 which is of generally the same construction as the support structure illustrated in Figures 2 and 3. Thus, the support structure 152 includes a support 153 having a head 154 and a neck 155. The pawl member 151 includes a tongue 156 which engages against a surface of the neck 155 and about which the pawl member 151 is pivotable. The tongue 156 has a wave form providing a biasing mechanism for the pawl member 151 . In the arrangement shown, the tooth engagement section 157 is engaged against a face 158 of a tooth 159 of a toothed wheel 160. Thus the position of the pawl member 151 in Figure 1 1 is a "tooth engaged position".

The pawl actuator 150 further includes a pair of bi-metallic strips 161 and 162. As shown in Figure 1 1 a, each bi-metallic strip consists of two materials 165 and 166, one overlying the other, and the respective materials 165 and 166 have different rates of thermal expansion. The materials 166 are in facing relationship. The strip 161 is acted on by a suitable heating arrangement, such as a heating coil 163. The provision of two strips 161 , 162 arranged in the manner shown in Figure 1 1 a provides a balance in the event that a non- operational ambient temperature change occurs about the pawl actuator 150 which is not intended to induce a change in the position of the pawl member 151 . By the arrangement shown, an increasing ambient temperature will cause the strips 161 , 162 to tend to move in equal and opposite directions, i.e. either away from each other or towards each other. Thus, there is a zero actual displacement. However, when actuation of the pawl actuator 150 is required, a temperature differential can be created by the application of heat to one of the strips 161 , 162 and in the embodiment illustrated, the coil 163 is applied to the strip 161 and that will cause the strip 161 to move separately to the strip 162. By suitable arrangement, the movement can be such as to bring the actuating member 164 to which the strips 161 , 162 are connected into engagement with the tooth engagement section 157 (as shown in broken outline), so as to shift the pawl member 151 from a tooth disengaged position to the tooth engaged position of Figure 1 1 . As with each of the previous embodiments, the actuating member 164 is only required to cause movement of the tooth engagement section 157 from the tooth disengaged position to a position within the intermediate region at which the biasing influence of the tongue 156 continues movement of the tooth engagement section 157 to the tooth engaged position.

Movement of the tooth engagement section from the tooth engaged position to the tooth disengaged position is the same as the movements described for the earlier embodiments, whereby rotation of the toothed wheel 160 in and anticlockwise direction will cause disengagement of the tooth engagement section 157 from the face 158 and will further cause lifting of the tooth engagement section in a direction towards the tooth disengaged position.

It will be appreciated that heat applied to the strip 161 will also be conducted to the strip 162 given the close proximity of the strips 161 , 162 to each other.

Indeed the actuating member 164 will automatically retract shortly after current to the heating coil 163 is turned off, given that temperature equalisation between the strips 161 and 162 will occur through conduction. However, the initial differential in temperature between the strips 161 , 162 is sufficient for actuation of the pawl member 151 to occur.

It will be appreciated that the coil 163 which is illustrated in Figure 1 1 is just one of many heat application devices that could be employed in this embodiment of the invention.

Figure 12 is taken from applicant's co-pending International application PCT/AU2005/00854 although the numbering has been changed. That figure shows a disc brake caliper 1 10 of the kind which could employ a pawl actuator according to the present invention. The disc brake caliper 1 10 is described in detail in International application PCT/A LJ 2005/00854 but briefly, the caliper 1 10 includes a nut 1 18 and a ball screw 140 and the ball screw is rotated upon rotation of a rotor 122 through a gear mechanism 142. In the present invention, a toothed wheel 165 can be fixed to the rotor 122 adjacent the bearing 123 and the pawl actuator can act on the teeth of that toothed wheel.

The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the spirit and scope of the above description.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1 . A pawl actuator for use with a toothed wheel, including: a pawl member having a mounting section; and a tooth engagement section, the pawl member being mountable to a support structure at the mounting section and the tooth engagement section being movable between a tooth engaged position and a tooth disengaged position, the pawl actuator including a biasing arrangement to bias the tooth engagement section in a first direction toward the tooth engaged position when the tooth engagement section is in a first section of travel between the tooth engaged position and the tooth disengaged position and in a second direction toward the tooth disengaged position when the tooth engaged section is in a second section of travel between the tooth engaged position and the tooth disengaged position, the pawl actuator being further arranged to alter the direction in which the tooth engagement section is biased at a region intermediate the first and second sections of travel, the pawl actuator including an actuating arrangement arranged to act on the pawl member for displacing the tooth engagement section against the bias of the biasing arrangement from the tooth disengaged position through the second section of travel to the intermediate region, to a change of bias position within the intermediate region at which the direction of bias changes so that the biasing arrangement thereafter biases the tooth engagement section towards the tooth engaged position.

2. A pawl actuator according to claim 1 , wherein the pawl member is elongate and includes the mounting section toward one end thereof and the tooth engagement section toward the other end thereof.

3. A pawl actuator according to claim 1 or 2, wherein the pawl member includes a magnetically attractive part, the attraction of which is employed by the actuating arrangement to move the tooth engagement section through the second section of travel against the bias applied to the tooth engagement section, to the change of bias position within the intermediate region.

4. A pawl actuator according to claim 3, wherein the magnetically attractive part is a ferrous metal attachment attached to the pawl member.

5. A pawl actuator according to claim 3, wherein the magnetically attractive part is an integral portion of the pawl member.

6. A pawl actuator according to claim 1 or 2, wherein the pawl member is formed of a magnetically attractive material the attraction of which is employed to move the tooth engagement section through the second section of movement against the bias applied to the tooth engagement section, to the change of bias position within the intermediate region.

7. A pawl actuator according to any one of claims 1 to 6, wherein the biasing arrangement forms part of the pawl member.

8. A pawl actuator according to claim 7, wherein the biasing arrangement is provided by at least one section of the pawl member which is formed as a spring.

9. A pawl actuator according to claim 8, wherein the mounting section of the pawl member includes an opening for receipt of a portion of a support structure, the opening being defined partly by a pair of opposed edges that are arranged to engage opposite sides of the support structure portion, one of the edges forming an end edge of a tongue which extends away from the opening and about which the tongue can pivot relative to the support structure portion, pivoting of the tongue facilitating travel of the tooth engagement section between the tooth engaged and the tooth disengaged positions, the tongue further having opposite side edges, the opening further being defined partly by a pair of side portions that extend in generally the same direction on opposite sides of the support structure portion and on opposite sides of the tongue, spaced from the side edges thereof, the biasing arrangement being provided by a sprung section in one or each of the side portions and wherein tension in the or each sprung section is at a maximum when the tooth engagement section is at the change of bias position within the intermediate region and is reduced when the tooth engagement section is on either side of the that position toward either of the tooth engaged or tooth disengaged positions.

10. A pawl actuator according to claim 8, wherein the mounting section of the pawl member includes an opening for receipt of a portion of a support structure, the opening being defined partly by a pair of opposed edges that are arranged to engage opposite sides of the support structure portion, one of the edges forming an end edge of a tongue which extends away from the opening and about which the tongue can pivot relative to the support structure portion, pivoting of the tongue facilitating travel of the tooth engagement section between the tooth engaged and the tooth disengaged positions, the tongue further having opposite side edges, the opening further being defined partly by a pair of side portions that extend in generally the same direction on opposite sides of the support structure portion and on opposite sides of the tongue, spaced from the side edges thereof, the biasing arrangement being provided by a sprung section formed in the tongue, wherein tension in the sprung section is at a maximum when the tooth engagement section is at the change of bias position within the intermediate region and is reduced when the tooth engagement section is on either side of that position toward either of the tooth engaged or tooth disengaged positions.

1 1 . A pawl actuator according to any one of claims 1 to 6, wherein the pawl member includes a pivot edge that is arranged to engage a surface of a support structure and to be pivotable on that edge relative to the support structure, the biasing arrangement being provided by a spring arrangement which extends in use from engagement with the pawl member to an anchor provided by the support structure, wherein the tension in the spring arrangement is at a maximum when the tooth engagement section is at the change of bias position within the intermediate region and is reduced when the tooth engagement section is on either side of that position toward either of the tooth engaged or tooth disengaged positions, and whereby pivoting of the pawl member about the pivot edge facilitates travel of the tooth engagement section between the tooth engaged and the tooth disengaged positions.

12. A pawl actuator according to claim 1 1 wherein the spring arrangement includes a coil spring.

13. A pawl actuator according to claim 12, wherein the spring arrangement includes a pair of coil springs, spaced-apart on opposite sides of the pawl member.

14. A pawl actuator according to claim 1 1 , wherein the spring arrangement includes an elongate, elastic member.

15. A pawl actuator according to any one of claims 1 1 to 14, wherein the pivot edge forms an inside edge of a locating recess of the pawl member, the recess being defined by the pivot edge and a pair of side portions which extend on opposite sides of the support structure to locate the pawl member relative to the support structure.

16. An assembly including:

a pawl actuator for use with a toothed wheel, and

a support structure,

the pawl actuator including a pawl member having a tooth engagement section

the pawl member having an opening for receipt of a portion of the support structure for mounting the pawl member to the support structure, the opening being defined partly by a pair of opposed edges that engage generally opposite surfaces of the support structure portion, one of the edges forming an end edge of a tongue which extends away from the opening and about which the tongue can pivot relative to the support structure portion, whereby pivoting of the tongue facilitates travel of the tooth engagement section between a tooth engaged position and a tooth disengaged position, the tongue further having opposite side edges, the opening further being defined partly by a pair of side portions that extend in generally the same direction on opposite sides of the support structure portion, and on opposite sides of the tongue, spaced from the side edges thereof, the assembly including a biasing arrangement to bias the tooth engagement section in a first direction toward the tooth engaged position when the tooth engagement section is in a first section of travel between the tooth engaged position and the tooth disengaged position and in a second direction toward the tooth disengaged position when the tooth engaged section is in a second section of travel between the tooth engaged position and the tooth disengaged position, the biasing arrangement being provided by a sprung section in the support structure portion,

the assembly being further arranged to alter the direction in which the tooth engagement section is biased at a change of bias position within a region intermediate the first and second sections of travel, tension in the sprung section being at a maximum when the tooth engagement section is at the change of bias position and is reduced when the tooth engagement section is on either side of that position toward either of the tooth engaged or tooth disengaged positions,

the pawl actuator including an actuating arrangement arranged to act on the pawl member for displacing the tooth engagement section against the bias of the biasing arrangement from the tooth disengaged position through the second section of travel to the change of bias position within the intermediate region at which the direction of bias changes so that the biasing arrangement thereafter biases the tooth engagement section towards the tooth engaged position.

17. An assembly according to claim 16, wherein the support structure portion includes a pair of legs, each of which is engaged by a respective one of the opposed edges, one of the legs being resiliently flexible to form the sprung section.

18. An assembly according to claim 17, wherein each of the legs is resiliently flexible to form a respective sprung section.

19. A pawl actuator according to any one of claims 1 to 15, the actuating arrangement further including an electromagnetic actuator arranged to exert an attractive force on the pawl member to cause travel of the tooth engagement section from the tooth disengaged condition to the change of bias position of the intermediate region.

20. A pawl actuator according to claim 1 , wherein the pawl member is rotatably mounted to the mounting section and the actuating arrangement is operable to rotatably displace the pawl member from the tooth disengaged position through the second section of travel to the change of bias position of the intermediate region.

21 . A pawl actuator according to claim 20, wherein the biasing arrangement is a spring which acts in compression and wherein maximum compression occurs in the intermediate region.

22. A pawl actuator according to claim 21 , wherein the spring is a coil spring which is formed in a U shape and wherein one end of the spring is attached to the pawl member and the other end is anchored at an anchor point.

23. A pawl actuator according to any one of claims 20 to 22, wherein the actuating arrangement includes a push rod that engages a surface of the pawl member.

24. A pawl actuator according to claim 23, wherein the push rod is electro- magnetically actuated at least in a first actuating direction to displace the pawl member through the second section of travel from the tooth disengaged position to the change of bias position of the intermediate region.

25. A pawl actuator according to claim 1 or 2, wherein the actuating arrangement includes a thermal bi-metallic strip which is readable to temperature change by displacement, that displacement being employed to shift the pawl member from the tooth disengaged position to the intermediate region.

26. A pawl actuator according to claim 1 or 2, wherein the actuating arrangement includes a pair of thermal bi-metallic strips each of which is readable to temperature change by displacement, the pair of thermal bi-metallic strips being in spaced apart facing relationship and being arranged so that when the pair of bi-metallic strips each experience an equal change in temperature, the strips tend towards displacement in equal and opposite directions so that the position of the bi-metallic strips is relatively unchanged, and when one of the pair of bi-metallic strips experiences a change in temperature that causes the strips to have different temperatures, displacement is induced in one of the strips that causes the position of the bi-metallic strips to change, with the change in position being employed to shift the pawl member from the tooth disengaged position to the change of bias position of the intermediate region.

27. A pawl actuator according to claim 25, wherein the actuating arrangement further includes a heat facility for the application of heat to one of the pair of bi-metallic strips.

28. A pawl actuator according to any one of claims 1 to 27, further including a toothed wheel having teeth for engagement by the tooth engagement section in the tooth engaged position, in that position the tooth engagement section engages a face of a first tooth of the toothed wheel and rests in contact with a top edge of an adjacent second tooth, whereby the pawl member is operable to prevent rotation of the toothed wheel in a first direction by the engagement with the first tooth and whereby movement of the tooth engagement section from the tooth engaged position to the tooth disengaged position is initiated by rotation of the toothed wheel in a second direction, opposite the first direction, such that the tooth engagement section is disengaged from contact with the face of the first tooth and sliding contact between the tooth engagement section and the second tooth causes displacement of the tooth engagement section from the tooth engaged position against the bias of the biasing arrangement to the change of bias position of the intermediate region, whereafter the biasing arrangement biases the tooth engagement section to the tooth disengaged position.

29. A pawl actuator according to any one of claims 1 to 28, further including a toothed wheel having teeth for engagement by the tooth engagement section in the tooth engaged position, the tooth engagement section being arranged for at least partial receipt of a tooth of the toothed wheel in the tooth engaged position, so that portions of the tooth engagement section are in facing relationship with opposite faces of the tooth and at least a first of those portions engages a first face of the tooth, whereby the pawl member is operable to prevent rotation of the toothed wheel in a first direction by the engagement with the tooth and whereby movement of the tooth engagement section from the tooth engaged position to the tooth disengaged position is initiated by rotation of the toothed wheel in a second direction, opposite the first direction, such that a second of the portions of the tooth engagement section which faces a second tooth face opposite to that faces by the first portion, slides against and relative to the second face causing displacement of the tooth engagement section from the tooth engaged position against the bias of the biasing arrangement to the change of bias position of the intermediate region, whereafter the biasing arrangement biases the tooth engagement section to the tooth disengaged position.

30. A pawl actuator according to claim 28 or 29, wherein the toothed wheel is a gear wheel of a vehicle parking brake arrangement.

31 . A pawl actuator according to claim 30, wherein the tooth wheel is fixed to a ball screw actuator of a vehicle parking brake arrangement.

32. A pawl actuator according to any one of claims 1 to 31 , wherein the support structure includes an abutment that is engaged by the pawl member in the tooth disengaged position.

33. A vehicle parking brake arrangement, including a pawl actuator according to any one of claims 1 to 31 .