Patent Publication Number: US-2012024642-A1

Title: Brake Actuator &amp; Control Valve Assembly

Description:
The present invention relates to a brake actuator and control valve assembly, particularly, but not exclusively, to a pneumatically operated brake actuator for use in the braking system of a heavy goods vehicle. 
     Braking of a vehicle is normally required for two reasons—to decelerate the vehicle when the vehicle is in motion (generally referred to as service braking), or to ensure that the vehicle does not move when it is parked. Service braking is usually effected by the driver operating a foot pedal, with a separate lever, usually manually operable, being provided to actuate and hold the brake or brakes on whilst the vehicle is parked. In both cases, however, each vehicle brake is typically moved to the applied position by means of a fluid pressure operable brake actuator. 
     Two sorts of fluid pressure operable brake actuators are known—a lock actuator, and a spring brake actuator. In the both of these, service braking is achieved by the movement of a piston or diaphragm which divides a service brake housing into first and second chambers. The piston or diaphragm carries an actuating rod, which extends from the first chamber and through an aperture in the service brake housing, and which is mechanically connected to a brake. In order to apply the brake, a fluid pressure (typically pneumatic) braking operating signal is supplied to the second chamber, and this causes the piston or diaphragm to move so that the actuating rod is pushed out of the housing to an extended position. A return spring is provided in the first chamber to return the piston or diaphragm and actuating rod to the retracted position when the fluid pressure is exhausted from the second chamber. 
     In the case of a lock actuator, operation of the parking brake causes a locking device to operate to mechanically lock the actuating rod (and possibly also the piston/diaphragm) in the extended position. The brake will thus remain applied even if fluid pressure is exhausted from the second chamber, until the parking brake, and hence the lock is released. The present invention relates to an improved control valve assembly for a lock actuator. 
     According to a first aspect of the invention we provide a brake actuator and control valve assembly, the brake actuator having a housing in which is provided a movable assembly including a brake actuating element which is adapted, in use to be connected to a vehicle brake, the movable assembly being movable from a brake apply position which, in use, results in the application of the brake, and a brake release position which, in use, results in the release of the brake, the brake actuator further including a lock which is operable to change from a locked configuration in which movement of the movable assembly from the brake apply position to the brake release position is prevented, and a released configuration in which movement of the movable assembly from the brake apply position to the brake release position is permitted, wherein the lock includes a fluid pressure operated actuator, supply of pressurised fluid to which results in the lock changing from either the locked configuration to the release configuration or vice versa, the control valve assembly including an electrically operable lock control valve by means of which supply of pressurised fluid to the lock actuator is controlled, the lock control valve having an inlet port which is connected to a source of pressurised fluid, a delivery port which is connected to the lock actuator and a valve member which is movable between a first position in which the inlet port is substantially closed, and a second position in which the inlet port is connected to the delivery outlet, the valve member being movable between the first and second positions only on the supply of an electrical current to the lock valve. 
     The lock valve preferably also includes an exhaust port which is connected to a low pressure region, preferably the atmosphere, the delivery port being connected to the exhaust port when the valve member is in the first position. 
     The lock control valve preferably includes a solenoid, passage of an electrical current to which causes the valve member to move to the second position if the valve member is in the first position, or to the first position if the valve member is in the second position. 
     The lock actuator preferably comprises a piston or diaphragm which, with the actuator housing, encloses a lock control chamber, the chamber having a port to which the delivery port of the lock control valve is connected and being sealed such that flow of fluid into or out of the chamber other than via the port is substantially prevented. 
     A resilient biasing means may be provided in the lock control chamber, the resilient biasing means deforming when the lock actuator moves the lock to the locked configuration, and exerting a biasing force on the lock tending to return it to the release configuration. 
     Preferably the movable assembly and housing together enclose a service braking chamber of variable volume, the service braking chamber being provided with a port and being sealed such that flow of fluid into or out of the chamber other than via the port is substantially prevented. In this case the control valve assembly preferably further includes a modulator which has a supply inlet connected to the source of pressurised fluid, an exhaust outlet which is vented to a low pressure region, and a delivery port, the modulator being operable to move between a build position in which the supply inlet is connected to the delivery port and the exhaust outlet substantially closed, an exhaust position in which the delivery port is connected to the exhaust outlet and the supply inlet is substantially closed. 
     Where the lock actuator includes a piston or diaphragm which, with the housing enclose a lock control chamber, the piston or diaphragm of the lock actuator preferably separates the lock control chamber and the service braking chamber such that fluid pressure in the service braking chamber exerts a force on the piston or diaphragm which acts to reduce the volume of the lock control chamber if the fluid pressure in the lock control chamber is not sufficient to prevent this. 
     The movable assembly may further include first and second movable members which are movable relative to one another and which together enclose a compliance chamber of variable volume, the compliance chamber having a port and being sealed such that flow of fluid into or out of the chamber other than via the port is substantially prevented. In this case, preferably the port into the compliance chamber is connected to the delivery port of the lock control valve. 
    
    
     
       An embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings of which: 
         FIG. 1  shows cross-section through a braking assembly including a brake actuator suitable for use in a brake actuator and control valve assembly according to the invention, 
         FIG. 2  shows a schematic illustration of a cross-section through the brake actuator shown in  FIG. 1 , the brake actuator being in a passive state in which no braking force is applied, 
         FIG. 3  shows a schematic illustration of a cross-section through the brake actuator shown in  FIG. 1 , the brake actuator being an active state as adopted during service braking, 
         FIG. 4  shows a schematic illustration of a cross-section through the brake actuator shown in  FIG. 1 , the brake actuator being in an active state as initially adopted during application of the parking brake, 
         FIG. 5  shows a schematic illustration of a cross-section through the brake actuator shown in  FIG. 1 , the brake actuator being in an active state as adopted during application of the parking brake but after cooling of the brakes, 
         FIG. 6  shows a schematic illustration of a brake actuator and control valve assembly according to the invention. 
     
    
    
     Referring now to  FIGS. 1 to 5 , there is shown a brake actuator  10  having a housing  12  which is divided into a first chamber  14 , hereinafter referred to as the return spring chamber  14 , and a second chamber by means of a first movable member, which in this example is a piston hereinafter referred to as the spring support piston  18 . Whilst use of a piston is described in this embodiment, it will be appreciated that a diaphragm or rolling diaphragm could be used instead. On the spring support piston  18  is mounted a brake actuating rod  20  which extends from the piston into the return spring chamber  14  and out through an aperture  22  in an end face  12   a  of the housing  12 . The rod  20  extends with its longitudinal axis generally perpendicular to the plane of the spring support piston  18 , such that movement of the spring support piston  18  in the housing  12  causes the rod  20  to move in a direction generally parallel to its longitudinal axis. The brake actuating rod  20  is, in use, mechanically connected to a vehicle disc brake (not shown) such that this axial movement of the rod  20  out of the housing  12  causes the brake to apply a braking force to a vehicle wheel, the braking force being removed by axial movement of the rod  20  back into the housing  12 . The extended position of the rod  20  in which a braking force is applied will hereinafter be referred to as the brake apply position, and the retracted position of the rod  20  as the brake release position. 
     A resilient biasing element, in this example a helical spring hereinafter referred to as the return spring  24 , surrounds the actuating rod  20 , and extends between a first end face  12   a  of the housing  12  and the spring support piston  18 , movement of the spring support piston  18  and rod  20  from the brake release position to the brake apply position causing the return spring  24  to be compressed from its equilibrium state. The return spring  24  therefore exerts a biasing force on the spring support piston  18  when the rod  20  is in the brake apply position, the biasing force tending to return the rod  20  to the brake release position. It should be appreciated that the return spring  24  is relatively weak, and the bulk of the return force acting on the spring support piston  18  is provided by the resilience of the braking mechanism. The return spring  24  is mainly effective towards the end of the travel of the spring support piston  18  to its retracted position when the brakes are disengaged, and is provided to ensure that the spring support piston  18  completes its movement back to the fully retracted position. 
     Whilst not absolutely necessary for the functioning of the actuator  10 , a seal may be provided to substantially prevent flow of fluid from the return spring chamber  14  through the aperture  22  to the exterior of the housing  12 . The seal may extend between the housing  12  surrounding the aperture  22  and the rod  20 , and provide a substantially fluid tight seal whilst allowing axial movement of the rod  20 . More preferably, however, the seal is provided by means of a flexible sleeve  26 , known as a boot, which extends between the rod  20  and the return spring  24  from the housing  12  surrounding the aperture  22  to the spring support piston  18 , the boot  26  being compressed in a concertina fashion as the rod  20  moves from the brake release position to the brake apply position. 
     In use, the end face  12   a  of the brake actuator housing  12  is typically engaged with a housing of a brake calliper (not shown), a substantially fluid tight seal being provided between the brake actuator housing  12  and the brake calliper housing. The provision of the boot  26  means that fluid from the return spring chamber  14  is prevented from entering the brake calliper. This is particularly important if the return spring chamber  14  is open to the atmosphere, and therefore contains water vapour or salt from the road surface, as such contaminants could cause corrosion of and serious damage to the brake calliper. 
     In this example, the mechanical connection between the actuating rod  20  and the vehicle brake typically requires a small degree of variation in the angular position of the rod  20  relative to the direction of movement of the piston  18  (of the order of ±4° from the direction of movement of the spring support piston  18 . This is accommodated by securing the rod  20  to the spring support piston  18  by means of a generally flexible disc  28  (shown only in  FIG. 1  for clarity). A first end  20   a  of the rod  20  extends through the aperture  22  in the housing  12 , whilst a second opposite end  20   b  of the rod  20 , which is provided with a convex curved end surface, engages with a corresponding concave curved central portion  18   a  of the spring support piston  18 . The curved central portion  18   a  of the spring support piston  18  thus acts as a bearing for the second end  20   b  of the rod  20 , and the disc  28  is sufficiently flexible to allow the rod  20  to pivot about its second end  20   b  through an angle of around 8°. 
     The brake actuator is also provided with a second movable member, in this example a second piston hereinafter referred to as the service braking piston  30 . Again, a diaphragm or rolling diaphragm could equally be used in the place of a piston. This divides the second chamber into two further chambers—a spring chamber  16   a  and a service braking control chamber  16   b,  the spring chamber  16   a  being between the service braking piston  30  and the spring support piston  18 , and the service braking control chamber  16   b  being between a second end face  12   b  of the housing  12  and the service braking piston  30 . A plurality of breathing apertures  19  are provided in the spring support piston  18  which connect the return spring chamber  14  to the spring chamber  16   a.    
     A second resilient biasing means, in this example a helical spring hereinafter referred to as the compliance spring  32 , is provided in the spring chamber  16   a , and extends between and is secured at each end to the spring support piston  18  and the service braking piston  30 . 
     An aperture, hereinafter referred to as the service braking chamber port  17  is provided in the second end face  12   b  of the housing  12 , and this is connected to the delivery port of a modulator (not shown). The modulator is provided with an inlet port, a delivery port and an exhaust port, the inlet port being connected to a source of pressurised fluid, typically compressed air, and the exhaust port being connected to a low pressure volume, typically vented to atmosphere. The modulator is controllable, typically by means of at least one solenoid operated valve, to move between a build position in which the inlet port is connected to the delivery port (and hence the service braking chamber  16   b ) and the exhaust port closed, an exhaust position in which the delivery port (and hence the service braking chamber  16   b ) is connected to the exhaust port, and the inlet port closed, and a hold position in which all three ports are closed. The details of suitable configurations of modulator are given in our co-pending patent application GB0902989.3, or application GB2407131. 
     The brake actuator  10  may therefore be used to effect service braking by operating the modulator so that it adopts the build position, thus connecting the service braking chamber  16   b  to the supply of pressurised fluid via the service braking chamber port  17 . The increasing pressure in the service braking chamber  16   b  pushes the service braking piston  30  towards the spring support piston  18 . The compliance spring  32  does not deform under the pressure of fluid in the service braking chamber  16   b  until this exceeds a predetermined amount, in this example around 5.9 bar, and therefore, initially at least, both the service braking piston  30  and the spring support piston  18  move together, as if connected by a rigid connection, under the influence of the increasing fluid pressure in the service braking chamber  16   b  so as to decrease the volume of the return spring chamber  14 . This, of course, has the effect of moving the spring support piston  18  and rod  20  from the brake release position to the brake apply position, as illustrated in  FIG. 3 . 
     Whilst it would be theoretically possible to hold the applied braking force during parking of the vehicle by moving the modulator to the hold configuration in order to retain the fluid pressure in the service braking chamber  16   b,  this is not practically viable as the seals in the system, whilst good enough to retain the fluid pressure during for the durations typically required for service braking, will not retain the high fluid pressure required to applied the desired braking force for significant periods of time. Thus, leakage of pressurised fluid from the system would cause the brakes to be slowly released if the vehicle were parked for any significant period of time. This would clearly be unacceptable, and, consequently, it is a legal requirement that the parking brake is held by a purely mechanical device. A locking means is therefore provided to mechanically lock the spring support piston  18  and the rod  20  in the brake apply position. 
     Whilst it will be appreciated that the exact configuration of locking means is not critical, and other locking mechanisms, for example of the type used on caulking hand guns, may be used. In this example, however, the locking means is fluid pressure operated, and relies on frictional forces resulting from the engagement of a plurality of ball bearings, or rollers, with a portion of the service braking piston preventing movement of the service braking piston  30 . 
     This embodiment of locking means comprises a locking piston  34  which is mounted around a generally cylindrical locking tube  36  which extends from generally the centre of the service braking piston  30  into the service braking chamber  16   b.  A first tubular extension portion  37  extends from the second end face  12   b  of the housing  12  into the service braking chamber  16   b,  and the locking tube  36  extends into the cylindrical space enclosed by the first extension portion  37 . Sealing means, in this example an O-ring, is provided on the locking tube  36  to provide a substantially fluid tight seal between the locking tube  36  and the first extension portion  37  whilst permitting the locking tube  36  to slide inside the extension portion  37  as the service braking piston moves under the influence of fluid pressure in the service brake control chamber  16   b.    
     A generally cylindrical inner surface  34   a  of the locking piston  34  engages with an outer surface of the first extension portion  37 , whilst a generally cylindrical outer surface  34   b  of the locking piston  34  engages with a second generally cylindrical tubular extension portion  38  which is located around and spaced from the first extension portion  37  and also extends from the second end face  12   b  of the housing  12  into the service braking chamber  16   b.  A generally annular chamber, hereinafter referred to as the locking control chamber  40 , is therefore formed between the locking piston  34  and the second end face  12   b  of the housing  12 . 
     Sealing means, in this example O-rings, are provided in a groove in each of the inner  34   a  and outer  34   b  surfaces of the locking piston  34  to provide a substantially fluid tight seal between the locking piston  34  and the extension portions  37 ,  38 . A second aperture, hereinafter referred to as the locking chamber port  40   a,  is provided in the second end face  12   b  of the housing  12  and extends from the exterior of the housing  12  into the locking control chamber  40 . The locking chamber port  40   a  is connected to a source of pressurised fluid. 
     Resilient biasing means, in this example a helical spring hereinafter referred to as the locking return spring  42 , is provided in the locking control chamber  40 , extending between the second end face  12   b  of the housing  12  and the locking piston  34 . It will therefore be appreciated the movement of the locking piston  34  against the biasing force of the locking return spring  42  to decrease the volume of the locking control chamber  40  may be achieved by the exhausting of pressurised fluid from the locking control chamber  42  via the locking chamber port  40   a,  whilst retaining fluid pressure in the service braking chamber  16     b .    
     A locking ring  48  is mounted at the free end of the second extension portion  38 , the locking ring  48  having a generally cylindrical outer surface which engages with an inner surface of the second extension portion  38  and an inner surface with a generally circular transverse cross-section which tapers from a larger diameter to a smaller diameter moving from a first end of the locking ring  48  towards the second end face  12   b  of the housing  12 . The angle of the taper is around 5° from the longitudinal axis of the locking ring  48 . The locking piston  34  is located between the locking ring  48  and the second end face  12   b  of the housing  12 . 
     The locking means is also provided with a generally cylindrical tubular carrier part  44  which extends from the locking piston  34  into the service braking chamber  16   b  around the locking tube  36 . The carrier part  44  is located in the space between the tapered inner surface of the locking ring  48  and the locking tube  34 , and provides a race in which a plurality of ball bearings  46  are supported. The ball bearings  46  protrude from inner and outer surfaces of the carrier part  44  so that an inner portion of each ball bearing  46  engages with the locking tube  36  whilst an outer portion of each ball bearing  46  may, by movement of the locking piston  34  and carrier part  44  towards the second end face  12   b  of the housing  12 , be brought into engagement with the inner, tapered surface of the locking ring  48 . 
     The angle of the taper of the inner surface of the locking ring  48  is selected according to the coefficient of friction between the ball bearing and the locking tube  36 , which in this example is the coefficient of friction of greased steel on greased steel. When the ball bearings  46  are engaged with the locking ring  48  and an attempt is made to move the locking piston  34  to reduce the volume of the service braking chamber  16   b,  the ball bearings  46  are pushed by the locking ring  48  against the locking tube  36 , the direction of the force between each ball bearing  46  and the locking tube  36  depending on the angle of taper of the inner surface of the locking ring  48 . This is selected so that the magnitude of the component of the force between each ball bearing  46  and the locking tube  36  normal to the outer surface of the locking tube  36  is such that the frictional force resisting sliding movement of the ball bearings  46  relative to the locking tube  36  is always greater than the force applied to move the service braking piston  30 . Engagement of the ball bearings  46  with the locking tube  36  and locking ring  48  therefore acts to prevent movement of the service braking piston  30  to reduce the volume of the service braking chamber  16   b.    
     Whilst in this example, the locking is achieved by engaging a plurality of ball bearings  46  between the locking tube  36  and the locking ring  48 , this need not be the case. The locking tube  36  and the locking ring  48  need not have generally circular transverse cross-sections. For example the exterior surface of the locking tube  36  and the interior surface of the locking ring  48  may be hexagonal or octagonal (or in the shape of any polyhedra) and, in this case, the ball bearings  46  may be replaced by a plurality of generally cylindrical rollers each of which is arranged with its longitudinal axis generally perpendicular to the longitudinal axis of the locking tube  36 . When in the locked position each roller engages with one of the external faces of the locking tube  36 , and one of the internal faces of the locking ring  48 . 
     The use of ball bearings  46  and a cylindrical locking tube  36  and locking ring  48  as described above can be advantageous in the arrangement described above where the service braking piston  30  is a piston (as opposed to the diaphragm), because, if operation of the actuator  10  generates internal forces which tend to cause the service braking piston  30  to rotate in the housing  12 , such rotation can be accommodated, and has no effect on the operation of the actuator  10 . If the service braking piston  30  is replaced with a diaphragm which is secured by its periphery to the housing  12 , the use of rollers and a polyhedral locking tube might be preferred, as this would act against any such internal forces to substantially prevent any rotation of the service braking diaphragm which otherwise could cause damage to or effect movement of the diaphragm in the housing  12 . 
     When parking the vehicle, the brake actuator may therefore be operated as follows. The brakes are applied by moving the spring support piston  18  and rod  20  from the brake release position to the brake apply position as described in relation to service braking above. During this process, as pressurised fluid is supplied to the service braking chamber  16   b,  pressurised fluid is also supplied to the locking control chamber  40  through the locking chamber port  40   a,  and this ensures that the locking return spring  42  remains uncompressed, and the ball bearings  46  retained spaced from the locking ring  48 . 
     When the desired braking pressure is achieved, the locking means is actuated by exhausting fluid pressure from the locking control chamber  40  so that the fluid pressure in the service braking chamber  16   b  pushes the locking piston  34  against the biasing force of the locking return spring  42  to reduce the volume of the locking control chamber  40 . This brings the ball bearings  46  into engagement with the locking ring  48 . The modulator can then be operated to exhaust the fluid pressure from the service braking chamber  16   b,  engagement of the ball bearings  46  with the locking ring  48  and locking tube  36  preventing the service braking piston  30  from moving under the action of the biasing force of the return spring  24  to reduce the volume of the service braking chamber  16   b  as the fluid pressure in the service braking chamber  16   b  is reduced. In other words, the locking means locks the brakes in the applied position. 
     To release the parking brake, pressurised fluid is supplied to the locking control chamber  40 , and the service braking chamber  16   b.  This allows the service piston  30 , spring support piston  18  and rod  20  to move under the action of the return spring  24  to the brake release position. 
     When the pressure in the service braking chamber  16   b  is relatively low, the force exerted on the locking piston  34  by the locking return spring  42  is not sufficient to overcome the force exerted by the brake mechanism on the actuating rod  20  which acts to maintain the ball bearings  46  in engagement with the locking ring  48 , i.e. to maintain the parking brake lock. To overcome this force, it is necessary for the fluid pressure in the service braking chamber  16   b  to be increased to around the same level it was at when the locking means was actuated to apply the lock. The fluid pressure in the service braking chamber  16   b  then counteracts the force exerted by the brake mechanism, and effectively “unloads” the lock, thus enabling the locking piston  34  to move under the action of the locking return spring  42  to increase the volume of the locking control chamber  40  and disengage the ball bearings from the locking ring  48 . 
     In order to avoid the problems associated with conventional locking actuators, when applying the parking brake the fluid pressure in the service braking chamber  16   b  is increased sufficiently to compress the compliance spring  32 , before the locking means is actuated. This is illustrated in  FIG. 4 . 
     The provision of the compliance spring  32 , and the compression of this spring  32  during application of the parking brake means that the braking force is retained at a generally constant value even after cooling of the brakes. If cooling of the brakes would cause the braking force to increase if the brake actuating rod  20  were locked in position, in the arrangement described above, cooling of the brakes will further compress the compliance spring  32  whilst maintaining the braking force at a generally constant level. Alternatively, if cooling of the brakes would cause the braking force to decrease if the brake actuating rod  20  were locked in position, in the arrangement described above, cooling of the brakes will cause the compliance spring  32  to expand whilst maintaining the braking force at a generally constant level. This is illustrated in  FIG. 5 . 
     Whilst a brake actuator including all the features described above would work as described above, with the compliance spring  32  providing the advantage discussed in the preceding paragraph, the brake actuator  10  in this example has been further improved by the inclusion of a further control chamber, hereinafter referred to as the compliance control chamber  50 , between the spring support piston  18  and the service braking piston  30 . 
     The spring support piston  18  is provided with a divider wall  52  which encloses a generally cylindrical space and which extends from the spring support piston  18  into the spring chamber  16   a,  the compliance spring  32  being located in the generally annular space between the divider wall  52  and the actuator housing  12 . Whilst the breathing apertures  19  could be omitted from the spring support piston  18  and a substantially fluid tight seal provided between the spring support piston  18  and the housing, so that the entire space between the spring support piston  18  and the service braking piston  30  forms the compliance control chamber  50 , in this example, the pistons  18 ,  30  are configured so that the compliance control chamber  50  occupies only a fraction of this space. 
     In order to achieve this, in this example, rather than being a generally planar disc, the service braking piston  30  has a top-hat shaped cross-section, and includes an annular outer part  30   a,  a generally circular centre disc  30   b,  the inside edge of the annular outer part  30   a  being connected to the edge of the centre disc  30   b  by a tubular connection part  30   c.  The outer edge of the annular outer part  30   a  engages with the actuator housing  12  to provide a substantially fluid-tight seal between the housing  12  and the piston  30  whilst permitting sliding movement of the piston  30  in the housing  12 . In this example, the outer edge is provided with a groove in which is located an O-ring  54 , or other suitable sealing element. The connection part  30   c  extends towards the spring support piston  18 , and an end portion of the outer surface of the connection part  30   c  is surrounded by and engages with the inner surface of the divider wall  52  to provide a substantially fluid tight seal between the spring support piston  18  and the service braking piston  30 . In this example, the outer surface of the connection part  30   c  is provided with a groove in which a further O-ring  56  or other suitable sealing part is provided. 
     The compliance control chamber  50  is therefore formed in the space between the spring support piston  18  and the centre disc  30   b  of the service braking piston  30  and is enclosed by the divider wall  52 . 
     Flow of fluid into and out of the compliance control chamber  50  is provided for by means of an aperture provided in the centre disc  30   b  of the service braking piston  30 . A compliance control tube  58 , having an axially extending central bore  58   a  and a transverse bore  58   b  connecting the central bore  58   a  to the axially extending outer surfaces of the tube  58 , extends from the spring support piston  18  through this aperture into the cylindrical space enclosed by the locking tube  36 . A restrictor  59  (shown in  FIG. 1  only for clarity) is provided in the central bore  58   a  of the compliance control tube  58 , and this acts as a choke which restricts the rate of flow of fluid into the compliance chamber  50  but does not impede flow of fluid out of the compliance control chamber  50 . 
     In this example, the compliance control tube  58  also provides a stop which limits the separation of the spring support piston  18  and the service braking piston  30 . In this case the stop is a step  58   c  provided in the outer circumference of the compliance control tube  58  separating a smaller diameter portion of the tube  58  from a larger diameter portion of the tube  58 , the larger diameter portion of the tube  58  being at the other end to the spring support piston  18 . A corresponding step is provided in the wall of the centre disc  30     b   surrounding the aperture in the service braking piston  30  through which the compliance control tube  58  extends. When the actuator  10  is in the passive state as illustrated in  FIG. 1 , the two steps engage, and set the maximum separation of the spring support piston  18  from the service braking piston. 
     It will be appreciated that the provision of such a stop is advantageous as it means that the compliance spring  32  can be pre-charged, i.e. is compressed from its equilibrium state at all times. In other words, the compliance spring  32  is compressed even when the two steps are engaged and the separation of the two pistons  18 ,  30  is maximum. It will be appreciated, of course, that using engagement of the service braking piston  30  with the compliance control tube  58  is only one way of achieving this, and other ways, such as connecting the two pistons  18 ,  30  with an easily compressible yet substantially inextensible element, could equally be used. 
     A further aperture is provided in the second end face  12   b  of the housing  12  and the locking tube  36  extends through this aperture, the space enclosed by the locking tube  36  communicating with the locking control chamber port  40   a.    
     Thus, during service braking, when pressurised fluid is supplied to the locking control chamber  40 , the compliance control chamber  50  is also pressurised. This means that, even if the pressure of fluid in the service braking chamber  16   b  would, in itself, be sufficient to overcome the biasing force of the compliance spring, if the force exerted by the pressurised fluid in the service braking chamber  16   b  is not greater than force required to compress the pressurised fluid in the compliance control chamber  50 , there will be no compression of the compliance spring  32 . The service braking piston  30 , spring support piston  18  and rod  20  will therefore move together as if connected by a rigid rod up to higher pressures than if the compliance control chamber  50  were not provided. 
     Without the compliance control chamber  50 , the compliance spring  32  would start to compress at high service brake pressures. This is undesirable as this not only reduces the longevity of the compliance spring  32  but also increases the “active volume” for service braking. Specifically, during operation of an anti-lock braking system (ABS) it is desirable to reduce the applied braking force as quickly as possible, as any appreciable delay could increase the depth of the skid and reduce wheel control. If there is compression of the compliance spring  32  during routine service braking, during operation of the ABS, the initial reduction in pressure in the service braking chamber  16   b  would simply result in expansion of the compliance spring  32 , and there could be a significant delay before any reduction in the braking force at the brakes is seen. This would have a detrimental effect on the performance of the ABS. With the compliance control chamber  50 , higher service braking forces can therefore be attained without the compliance spring  32  compressing. 
     During application of the parking brake, both the locking control chamber  40  and the compliance control chamber  50  are vented to atmosphere, so compression of the compliance spring  32  is permitted as described above. The reduction in volume of the compliance control chamber  50  occurring during the initial application of the parking brake can be seen in  FIG. 4 . 
     To minimise the space occupied by the brake actuator  10  and modulator, the modulator is, in this example, located in a modulator housing  60  mounted on the second end face  12   b  of the actuator housing  12 . Specifically, the modulator is located in the portion  60   a  of the modulator housing  60  directly adjacent the service braking chamber port  17 , which facilitates a direct connection between the service braking chamber port  17  and the delivery port of the modulator. 
     The interior of the modulator housing  60  is vented to the atmosphere via an exhaust port  62  and a passage ventilation passage  64  provided in the actuator housing  12  extends from an exhaust volume  60   b  within the modulator housing  60  adjacent the exhaust port  62  to the return spring chamber  14 . The modulator is arranged so that fluid from the modulator exhaust port is expelled into the exhaust volume  60   b  of the modulator housing  60 . 
     During service braking, when the brakes are released, compressed air is expelled from the service braking chamber  16   b  and out of the modulator exhaust port. At the same time, the service braking piston  30  and spring support piston  18  move to reduce the volume of the service braking chamber  16   b,  and the volume of the return spring chamber  14  increases. By virtue of the provision of the ventilation passage  64 , the fluid exhausted from the service braking chamber  16   b,  which is clean air from a compressed air reservoir, is drawn into the return spring chamber  14 . Moreover, as the spring chamber  16   a  is connected to the return spring chamber  14  by the breathing apertures  18   a  in the spring support piston  18 , fluid entering the spring chamber  16   a  during expansion of the compliance spring  32  is also clean air drawn from the return spring chamber  14 . 
     This is advantageous compared to drawing atmospheric air into the return spring chamber  14 , as the introduction of contaminants such as water and/or salt into the return spring chamber  14  and spring chamber  16   a  can be minimised or avoided altogether. Thus, the risk of corrosion of the return spring  24  and, more importantly the compliance spring  32 , can be minimised. It will be appreciated that, in view of this arrangement, the boot  26  need not be provided to avoid contamination of the brake calliper. 
     An advantageous arrangement of valves which may be used to control operation of the brake actuator  10  as described above will now be described. Referring now to  FIG. 6 , there is shown a schematic illustration of the brake actuator  10  described above, a modulator  70  having a supply inlet  72 , a delivery port  74 , an exhaust outlet  76  and a control port  78 . The modulator  70  includes an arrangement of pistons or diaphragms, movement of which is controlled by flow of pressurised fluid through the control port  78 . The pistons or diaphragms can be controlled so that the modulator adopts one of three working states—a build configuration in which the supply inlet  72  is connected to the delivery port  74  and flow of fluid through the exhaust port  76  is substantially prevented, an exhaust configuration in which the delivery port  74  is connected to the exhaust port  76  and flow of fluid through the supply inlet  72  is substantially prevented, and a hold or lapped configuration in which flow of fluid through all three of the supply inlet  72 , delivery port  74  and exhaust outlet  76  is substantially prevented. Various possible arrangements for achieving this are well known, and therefore are not described in detail here. Examples are described in our co-pending patent application GB0902989.3, or application GB2407131, for examples. 
     The exhaust outlet  76  vents to a low pressure region, in this example, to atmosphere via a water exclusion valve  77  which includes a valve member  77   a  which is biased to a closed position in which flow of fluid from the atmosphere, which may include water and/or salt, into the modulator via the exhaust outlet  76  is substantially prevented, but moves to open the exhaust outlet  76  when fluid pressure at the exhaust outlet builds to a minimal level, thus allowing fluid to be exhausted from the modulator  70 . The water exclusion valve  77  may be as described in our co-pending UK patent application GB0902990.1. 
     The delivery port  74  of the modulator  70  is connected to the service braking chamber  16   b  of the actuator  10 , whilst the supply inlet  72  is connected to a supply of pressurised fluid via a supply line  80 . The supply of pressurised fluid comprises first  82   a  and second  82   b  pressurised fluid reservoirs and a double check valve  84 . The double check valve  84  has a first inlet port  84   a  which is connected to the first compressed air reservoir, and an outlet port  84   b  which is connected to the supply line  80 . The second pressurised fluid reservoir  82   b  is connected to a brake pedal valve assembly  86  comprising a brake pedal  88 , an electrical braking demand signal generator  90  and a fluid pressure braking demand signal generator  92  which is supplied with pressurised fluid from the second reservoir  82   b.  Typically, the pressurised fluid used is compressed air. 
     The electrical braking demand signal generator  90  is configured so that on operation of the brake pedal by a driver of the vehicle in which the system is fitted causes the generation of an electrical braking demand signal which is generally proportional to the degree of deflection of the brake pedal, and hence indicative of the level of braking required by the driver. This signal is transmitted to a braking electronic control unit (not shown). 
     Similarly, the pneumatic braking demand signal generator  92  is configured so that on operation of the brake pedal  88  by a driver of the vehicle in which the system is fitted causes the generation of a fluid pressure braking demand signal (using fluid from the second reservoir  82   b ) the pressure of which is generally proportional to the degree of deflection of the brake pedal, and hence indicative of the level of braking required by the driver. This signal is transmitted via a fluid flow line to a second inlet port  84   c  of the double check valve  84 , and also to a first inlet port  94   a  of a three port, two position valve hereinafter referred to as the redundancy valve  94 . 
     The double check valve  84  is provided with a valve member which, if the pressure at the first inlet port  84   a  exceeds the pressure at the second inlet port  84   c,  moves to a first position to close the second inlet port  84   c  and connect the first inlet port  84   a  to the outlet port  84   b,  and if the pressure at the second inlet port  84   c  exceeds the pressure at the first inlet port  84   a,  moves to a second position to close the first inlet port  84   a  and connect the second inlet port  84   c  and the outlet port  84   b.    
     The redundancy valve  94  is, in this example, a solenoid operated valve which, in addition to the first inlet port  94   a,  has a second inlet port  94   b  which is connected to the first pressurised fluid reservoir  82   a,  an outlet port  94   c,  and a valve member which is biased using a resilient biasing means such as a helical spring into a first position in which the first inlet port  94   a  communicates with the outlet port  94   c  and the second inlet port  94   b  is closed. In this example, a solenoid is provided, passage of an electrical current through the solenoid causing the valve member to move from the first position to a second position in which the first inlet port  94   a  is closed and the second inlet port  94   b  communicates with the outlet port  94   c.    
     The outlet port  94   c  of the redundancy valve  94  is connected to an inlet port  96   a  of a two port, two position valve, hereinafter referred to as the build valve  96 . The build valve  96  is, in this example, a solenoid operated valve which, in addition to the inlet port  96   a,  has an outlet port  96   b,  and a valve member which is biased using a resilient biasing means such as a helical spring into a first position in which fluid flow from the inlet port  96   a  to the outlet port  96   b  is permitted. In this example, a solenoid is provided, passage of an electrical current through the solenoid causing the valve member to move from the first position to a second position in which fluid flow from the inlet port  96   a  to the outlet port  96   b  is substantially prevented. 
     The outlet port  96   b  of the build valve  96  is connected to the control port  78  of the modulator  70  and to the inlet port  98   a  of a second two port, two position valve, hereinafter referred to as the exhaust valve  98 . The exhaust valve  98  is, in this example, a solenoid operated valve which, in addition to the inlet port  98   a,  has an outlet port  98   b,  and a valve member which is biased using a resilient biasing means such as a helical spring into a first position in which fluid flow from the inlet port  98   a  to the outlet port  98   b  is substantially prevented. In this example, a solenoid is provided, passage of an electrical current through the solenoid causing the valve member to move from the first position to a second position in which fluid flow from the inlet port  98   a  to the outlet port  96   b  is permitted. The outlet port  98   b  of the exhaust valve  98  could simply be vented to atmosphere, but in this example is connected to the exhaust port  76  of the modulator  70  so that it vents to atmosphere via the water exclusion valve  77  which acts to prevent ingress of atmospheric fluids into the exhaust valve  98  in addition to the modulator  70 . 
     Using an arrangement of modulator  70 , brake pedal assembly  86 , reservoirs  82   a,    82   b,  valves,  84 ,  94 ,  96 ,  98  is known from prior art braking systems, and one of the novel features of this system resides in the provision of the lock control valve  100 , which in this example is a further three port, two position valve. The lock control valve  100  has an inlet port  100   a  which is connected to the supply line  80 , a delivery port  100   b  which is connected to the locking control chamber  40  (via the locking chamber port  40   a ) and the compliance control chamber  50  of the brake actuator  10 , and an exhaust port  100   c.  The restrictor  59  is illustrated in the schematic shown in  FIG. 6  as the line connecting the delivery port  100   b  to the compliance control chamber  50  splitting into two parallel lines, one including a choke  102 , and the other a non-return valve  104  which is oriented to permit flow of fluid from the compliance control chamber  50  to the lock control valve  100  whilst preventing fluid flow in the opposite direction. 
     The exhaust port  100   c  may vent directly to atmosphere, or any other low pressure region, but as with the exhaust valve  98 , in this example it is connected to the exhaust port  76  of the modulator  70  so that it vents to atmosphere via the water exclusion valve  77  which acts to prevent ingress of atmospheric fluids into the lock control valve  100 . The lock control valve  100  also includes a valve member which is movable between a first position in which flow of fluid from the inlet port  100   a  to the outlet port  100   b  is permitted whilst the exhaust port  100   c  is closed, and a second position in which flow of fluid between the outlet port  100   b  and the exhaust port  100   c  is permitted whilst the inlet port  100   a  is closed. This valve  100  is provided with a solenoid, but, in this example, does not include resilient biasing means, and the valve member moves between the first and second positions only when an electrical current is passed through the solenoid. Such a valve is generally known as a bi-stable solenoid valve. 
     Other configurations of bi-stable solenoid valve may, of course, be used. For example, the valve  100  may include a magnet which holds the valve member in one of the first or second positions, a spring which holds the valve member in the other of the first or second positions, and a solenoid passage of an electrical current through which one way causes the valve member to move against the biasing force of the spring, and the other way causes the valve member to move away from the magnet. If such a valve were used, it would preferably be oriented such that the spring holds the valve member in the first position, whilst the magnet holds the valve member in the second position. Equally, a purely mechanical mechanism for latching the valve member in each of the two positions may be employed, providing passage of an electrical current to the valve causes the valve member to move from the position it is latched in, to the other position. 
     Advantageously, in order to achieve a compact braking system, the ECU and redundancy  94 , build  96 , exhaust  98  and locking control  100  valves are located in the modulator housing  60 , for example within the exhaust volume  60   b.    
     The system is operated as follows. During normal driving of the vehicle, when there is no demand for braking, the system adopts the configuration illustrated in  FIG. 6 . There is no pneumatic braking control signal, so the valve member of the double check valve  84  is pushed by the pressure of fluid in the first reservoir  82   a  to the first position so that the supply line  80  is connected to the first reservoir  82   a.  Pressurised fluid at reservoir pressure is therefore supplied to the inlets of the modulator  72  and the locking control valve  100   a . The locking control valve  100  is in its first position, so that its inlet port  100   a  is connected to its delivery port  100   b,  and hence pressurised fluid at reservoir pressure is also supplied to the locking control chamber  40  and the compliance control chamber  50 . 
     No electrical power is supplied to the redundancy valve  94 , the build valve  96  or the exhaust valve  98 , so in each case, the valve member moves to its rest position, i.e. the position into which it is biased by the resilient biasing means. As such, whilst pressurised fluid is supplied to the second inlet port  94   b  of the redundancy valve  94 , the second inlet port  94   b  is closed, and the first inlet  94   a  connected to the outlet port  94   c,  which is in turn connected to the control inlet  78  of the modulator  70  via the build valve  96 . There is, however, no fluid pressure braking demand signal, so no pressurised fluid is supplied to the control inlet  78  of the modulator  70 . This causes the modulator  70  to adopt the exhaust configuration, in which the delivery port  74  is connected to the exhaust port  76  and therefore vents to atmosphere. As a result, the service braking chamber  16   b  of the actuator  10  is also vented to atmosphere, and the spring support piston  18  and rod  20  are in the brake release position. 
     When service braking is required, the brake pedal  88  is actuated which causes the electrical braking demand signal generator  90  to generate an electrical braking demand signal and to transmit this to the braking ECU. The braking ECU is connected to the solenoids of the redundancy, build, exhaust and locking control valves  94 ,  96 ,  98 ,  100 , and an electrical current is applied to the solenoid of the redundancy valve  94  which causes the valve member to move to the second position in which the first inlet  94   b  is connected to the outlet port  94   c.  Pressurised fluid from the first reservoir  82   a  is therefore supplied to the control inlet  78  of the modulator  70  via the build valve  96 . This causes the modulator  70  to adopt the build configuration in which the supply inlet  72  is connected to the delivery port  74  whilst the exhaust outlet  76  is closed. Pressurised fluid is therefore supplied to the service braking chamber  16   b  of the brake actuator  10 , which causes the service braking piston  32  to move to increase the volume of the service braking chamber  16   b  and to push the spring support piston  18  and rod  20  to the brake apply position, and therefore to actuate the vehicle brakes. No electrical power is supplied to the locking control valve  100 , so supply of pressurised fluid to the locking control chamber  40  and compliance control chamber  50  is maintained. There is, therefore, no compression of the compliance spring  32  during this operation. 
     A pressure sensor is provided to monitor the pressure in the service braking chamber  16   b,  and this transmits an electrical pressure signal to the ECU. When the ECU determines that the pressure in the service braking chamber  16   b  is at the level demanded by the braking demand signal, electrical signals are transmitted to the solenoids of the build valve  96  so that the valve member of the build valve  96  moves to the second position, and closes the inlet port  96   a.  The modulator  70  then moves to the hold configuration in which the delivery port  74  is effectively closed, and therefore the pressure in the service braking chamber  16   b,  and hence the braking force is maintained at the desired level. 
     When the driver demand for braking pressure is no longer present, and the braking demand signal falls to zero, the ECU sends an electrical current to the exhaust valve  98  so that the control inlet  78  of the modulator  70  vents to atmosphere until the pressure at the control inlet  78  is reduced to atmospheric pressure, and the modulator  70  moves to the exhaust position. At this point, the delivery port  74  of the modulator  70  becomes connected to the exhaust port  76 , and the fluid pressure in the service braking chamber  16   b  of the actuator falls to atmospheric pressure too. The service braking piston  32 , spring support piston  18  and rod  20  therefore move under the influence of the return spring back to the brake release position, and the braking force is removed. 
     Again, the position of the valve member of the locking control valve  100  is not changed during this process, so the locking control chamber  40  and compliance control chamber  50  are still pressurised. 
     By virtue of this arrangement of control valves, the service braking chamber  16   b  and the compliance control chamber  50  are connected to the same source of pressurised fluid, and are therefore at the same pressure. If, as suggested above, the entire space between the spring support piston  18  and the service braking piston  30  were used as the compliance control chamber  50 , the force restricting the compression of the compliance spring  30  would be extremely high—far higher than required. Given that movement of the service braking piston  30  towards the spring support piston  18  is already resisted by the compliance spring  32 , the additional force generated by the compliance chamber  50  is only required to supplement the force provided by the spring  32  and to bring the total force up to the maximum service braking force. 
     It is therefore preferred that the compliance control chamber  50  is provided in a relatively small, central portion of the space between the spring support piston  18  and the service braking piston  30 . Despite the fact that this reduces the force resisting compression of the compliance control chamber  50  provided by the fluid in the chamber  50 , the ratio of the surface area of the service braking piston  30  over which fluid pressure in the compliance chamber  50  acts to the surface area of the service braking piston  40  over which fluid pressure in the service braking chamber  16   b  acts is calculated to be high enough to prevent compression of the compliance spring  32  even at high service braking pressures. 
     This is how service braking is normally operated, however, it is a legal requirement to provide a back-up system which enables service braking in the event of a complete electrical power failure or accidental loss of pressure in the first reservoir  82   a  through a fractured line or the like. As mentioned previously, actuation of the brake pedal  88  causes a fluid pressure braking signal demand signal to be transmitted to the double check valve  84  and to the first inlet port  94   a  of the redundancy valve  94 . 
     In the event of electrical power failure, the redundancy, build and exhaust valves  94 ,  96 ,  98  are in their rest positions, and, as such, the first inlet port  94   a  of the redundancy valve  94  is connected to the control inlet  78  of the modulator  70  via the build valve  96 , and the exhaust valve  98  is closed. As such, the fluid pressure braking demand signal from the second reservoir  82   b  causes the modulator  70  to move to the build configuration, and allows passage of fluid from the first reservoir  82   a  from the supply inlet  72  of the modulator  70  to the brake actuator  10  to operate the brakes. 
     In the event of loss of pressure in the first reservoir  82   a,  the valve member of the double check valve  84  will move automatically to allow passage of the fluid pressure braking demand signal to the supply line  80 , whilst closing the connection between the supply line  80  and the first reservoir  82   a.  The fluid pressure braking demand signal will therefore pass via the supply line to the supply inlet  72  of the modulator  70  and the inlet port  100   a  of the locking control valve  100 , and also to the control inlet  78  of the modulator  70  via the redundancy valve  94  and build valve  96 . 
     The pressure of the fluid pressure braking demand signal at the control inlet  78  of the modulator  70  causes the modulator  70  to adopt the build position, so that the fluid pressure braking demand signal at the supply inlet  72  passes via the delivery outlet  74  to the service braking chamber  16   b  of the actuator  10  which causes the service braking piston  32  to move to increase the volume of the service braking chamber  16   b  and to push the spring support piston  18  and rod  20  to the brake apply position, and therefore to actuate the vehicle brakes as before. The supply of fluid for brake actuation is therefore supplied by the second reservoir  82   b.    
     The system also includes a parking brake lever, button or the like, which is actuated if parking braking is required. This results in an electrical parking brake signal being transmitted to the ECU which in turn acts exactly as described in relation to service braking to apply a braking force, but also sends an electrical signal to the locking control valve  100  to move the valve member to the second position. The locking control chamber  40  and compliance chamber  50  are therefore exhausted to atmosphere. The pressure in the service braking chamber  16   b  is brought to a sufficiently high pressure to compress the compliance spring  32  to the required degree, and the pressure in the service braking chamber  16   b  is then released by the supply of the an electrical current to the exhaust valve  98 . As the locking control chamber  40  is no longer pressurised, the locking means is actuated and acts as described above to prevent the service braking piston  30  from retracting to reduce the volume of the service braking chamber  16   b,  i.e. to lock the brakes in the applied position. The spring support piston  18  and rod  20  therefore remain in the brake apply position, and the braking force is maintained even if electrical power to the braking system is shut off or fails. 
     It should be noted that if the brake pedal  88  is actuated whilst the vehicle is parked, the resulting fluid pressure braking demand signal would be transmitted to the control inlet  78  of the modulator  70  via the redundancy valve  94  and the build valve  96 , and this could cause the modulator  70  to move to the build position to direct pressurised fluid to the service braking chamber  16   b  of the actuator  10 . But, because the locking control valve  100  remains in the second position, the locking control chamber  40  and compliance control chamber  50  remain vented to atmosphere. As such, if the pressure of the fluid pressure braking demand signal were sufficiently high it would cause further compression of the compliance spring  32  before applying additional force to the brakes. As such, the potential for damaging the brakes is limited, and can be avoided by setting a maximum limit to the fluid pressure braking demand signal which of the order of or is less than that required to achieve maximum compression of the compliance spring  32 . 
     Electrical release of the parking brake is achieved by the ECU applying an electrical current to the locking control valve  100  to move it back to the first position, and to the redundancy valve  94  to move it to the second position, thus building up the pressure in the service braking chamber  16   b,  locking control chamber  40  and compliance chamber  50  in the same way as if service braking were required. When the pressure in the service chamber  16     b   is sufficiently large to “unload” the lock, the locking piston  34  moves under the actuator of the locking return spring  42  to release the lock as described above. 
     Before the pressure in the service braking chamber  16   b  is sufficiently high to release the lock, the pressure building in the compliance chamber  50  may push the spring support piston  18  and rod  20  to reduce further the volume of the first chamber  14 , thus increasing the force applied to the vehicle brakes. If the fluid pressure in the compliance chamber  50  were to increase at the same rate as the pressure in the locking chamber  40  there is a possibility that, before the locking means is released, the pressure in the compliance chamber  50  rises to a sufficiently high level to cause damage to the brakes. It is therefore advantageous to provide the restrictor  59  in the connection between the outlet port  100   b  of the locking control valve  100  as this ensures that there is a delay in the increase in fluid pressure in the compliance control chamber  50  compared to the locking control chamber  40  (without delaying exhausting of fluid from the compliance chamber  50 ). Such damage is therefore avoided. 
     It will be appreciated that release of the parking brake as described above requires a pressurised fluid and electrical power supply to the braking system. It is a legal requirement also to provide an additional, self-contained means of releasing the parking brake, and this is achieved using a release bolt  110 , which for clarity is illustrated in  FIG. 1  only. The release bolt  110  has a head  110   a  and a threaded shaft  110   b  which extends through a bolt hole in the modulator housing  60  into the cylindrical space enclosed by the locking tube  36 . The head  110   a  of the bolt  110  is outside the modulator housing  60 , and a hexagonal nut  112  is threaded on the shaft  110   a  of the bolt  110  so that the modulator housing wall lies between the head  110   a  and the nut  112 . The nut  112  is held captive so that rotation of the nut  112  with the bolt  110  is substantially prevented. As such, if a tool such as a spanner or Allen key is engaged with the head  110   a  of the bolt  110  and the bolt  110  turned in the bolt hole, the nut  112  will be driven to move longitudinally down the shaft  110   b  (away from the head  110   a ) until it engages with the locking tube  36 . Further downward movement of the nut  112  un-loads the lock, thus allowing the locking piston  34  to move under the force of the locking return spring  42  to bring the ball bearings  46  out of engagement with the locking ring  48 . The lock is thus released, and the spring support piston  18  and rod  20  can return to the brake release position. 
     When used in this specification and claims, the terms “comprises” and “comprising” and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. 
     The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.