Abstract:
A method of applying a parking brake mechanism for a foundation brake includes several steps. The parking brake mechanism incorporates an electric actuator, an extensible device drivably connected to the electric actuator, and a resilient device arranged to act on the extensible device and maintain a desired level of force to be applied by the parking brake mechanism in the event of contraction of components of the foundation brake. The method includes the steps of signaling application of a service brake actuator to apply the brake, signaling driving of a first electric actuator to cause the extensible device to be able to retain the foundation brake in the brake applied position achieved by the service brake actuator, signaling the release of the service brake actuator, signaling driving of a second electric actuator to further compress the resilient element, monitoring a characteristic of the brake to determine if a desired force has been applied by the parking brake mechanism, and signaling driving of the electric motor to stop once the desired force has been reached.

Description:
REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to Great Britain Patent Application No. GB 0817229.8 filed Sep. 19, 2008. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a control system and a method for a parking brake mechanism. More particularly, the present invention relates to a control system and method for an electrically actuated parking brake mechanism for disc brakes or drum brakes having an air actuated service brake. 
         [0003]    Various proposals have been put forward for utilizing an electric motor to apply parking brakes, both on light passenger vehicles utilizing hydraulic brake systems and heavy commercial vehicles that use air actuated service brakes. 
         [0004]    Electric parking brakes have gone into commercial production for certain models of passenger cars, in which they essentially replace a cable linkage between a handbrake lever located in the passenger compartment and a disc or drum brake mounted in proximity to rear wheels of a vehicle. 
         [0005]    By contrast, despite various proposals being put forward for heavy vehicle brakes that are intended to replace a conventional spring brake on commercial vehicles, to the knowledge of the applicants, no electric parking brake has yet entered volume production for commercial vehicles. Conventional parking brake cylinders include a spring acting in a brake-on direction connected to a piston and a push rod that is normally held in a parking brake-off position by pressurized air, but in which the air is vented to apply the parking brake. One disadvantage of spring parking brakes is their size. A second disadvantage is their inability to finely control a parking brake clamp force that they apply. Additionally, a failure in an air supply may cause the parking brake cylinders to become applied with no way for this to be controlled by the driver. 
         [0006]    A number of hurdles need to be overcome to provide a practical electric parking brake that is specific to commercial vehicles. It is believed these have prevented adoption of this technology to date. One problem is that disc brakes used on commercial vehicles have significantly thicker discs and pads compared to light passenger vehicles to enable the brakes to have a suitably long service life despite the increased energy that is dissipated during braking due to their increased vehicle weight. As a result, when a heavy commercial vehicle is parked when the brakes are hot, an appreciable shrinkage of those brake components, in particular the brake disc and the brake pads, will occur. If this is not accounted for in some way by a parking brake mechanism, the clamp load applied by the parking brake will reduce as the brake components cool and contract, and there is a reduced clamp load exerted by the brake pads on the brake disc that may cause the vehicle to roll away. 
         [0007]    If used in conjunction with a drum brake on the other hand, the drum brake may contract as it cools and the reduction of the drum diameter may damage components within the brake due to a lack of the compliance of such mechanisms 
         [0008]    Such a problem does not arise with conventional spring parking brake cylinders because the spring can extend by a certain amount with only a slight drop in clamp load. 
         [0009]    However, parking brakes such as those disclosed in U.S. Pat. No. 6,851,761 (Knorr-Bremse) that are electrically powered are not provided with a similar resilient, extensible component, and it is therefore necessary either to apply an initial excess parking brake force to account for this shrinkage or to re-apply the parking brake once a certain amount of time has lapsed to bring the clamp load back up to the amount required. A control sequence for the latter approach is discussed in U.S. Pat. No. 6,851,761. Neither of these solutions is particularly satisfactory, since in the former case an excess stress is placed on the brake components that may shorten their life and in the latter scenario, there is a danger that if electrical power is not available to drive the parking brake motor once the vehicle has been parked, a re-application of the parking brake will not be achieved and there is a risk that the vehicle will roll away. 
         [0010]    A further problem with known electric parking brakes relates to their speed of application. In order to produce a parking brake having a sufficiently compact size, it is usual to propose the use of a relatively small electric motor and a reduction gear arrangement that results in a relatively low speed of application for the parking brake. In U.S. Pat. No. 6,851,761, a two-speed application arrangement is proposed, in order to attempt to overcome this problem. However, such arrangements are relatively complex. 
         [0011]    The present invention seeks to overcome, or at least mitigate, the problems of the prior art. 
       SUMMARY OF THE INVENTION 
       [0012]    A first aspect of the present invention relates to a method of applying a parking brake mechanism for a foundation brake. The parking brake mechanism incorporates an electric actuator, an extensible device drivably connected to the electric actuator, and a resilient device arranged to act on the extensible device and maintain a desired level of force to be applied by the parking brake mechanism in the event of contraction of components of the foundation brake. 
         [0013]    The method includes the steps of signaling application of a service brake actuator to apply a brake, signaling driving of a first electric actuator to cause the extensible device to be able to retain the foundation brake in the brake applied position achieved by the service brake actuator, signaling the release of the service brake actuator, signaling driving of a second electric actuator to compress the resilient element, monitoring a characteristic of brake mechanism to determine if a desired force has been applied by the parking brake mechanism, and signaling driving of the electric motor to stop once the desired force has been reached. 
         [0014]    A second aspect of the present invention provides a control system for a parking brake mechanism for a foundation brake. The parking brake mechanism incorporates an electric actuator, an extensible device drivably connected to the actuator, and a resilient device arranged to act on the extensible device and maintain a desired level of force to be applied by the parking brake mechanism in the event of contraction of components of an associated foundation brake. The control system is configured to signal the following sequence of operations to apply the brake: 
         [0015]    a) application of an associated service brake actuator to apply the brake, 
         [0016]    b) driving of a first electric actuator to bring the extensible device up to a point at which it retains the parking brake in the braked position achieved by the service brake actuator, 
         [0017]    c) the release of the service brake actuator, 
         [0018]    d) the driving of a second electric actuator to compress the resilient device, 
         [0019]    e) monitoring a characteristic of the brake to determine if a desired force has been applied by the parking brake, and 
         [0020]    f) driving of the electric actuator to stop once a desired force has been reached. 
         [0021]    A third aspect of the present invention relates to a method of disengaging a parking brake mechanism for a foundation brake from a parking brake applied state. The parking brake mechanism incorporates an electric actuator, an extensible device drivably connected to the actuator, and a resilient device arranged to act on the extensible device and maintain a desired level of force to be applied by the parking brake mechanism in the event of contraction of components of the foundation brake. The method includes the steps of signaling application of a service brake actuator to at least partially remove the load on the parking brake mechanism, signaling drive of the electric actuator such that the extensible device is no longer able to retain the foundation brake in the brake applied position, and signaling release of the service brake actuator. 
         [0022]    A fourth aspect of the present invention provides a control system for a parking brake mechanism configured to signal the disengaging of a parking brake mechanism in accordance with the method of the above paragraph. 
         [0023]    A fifth aspect of the present invention provides a method of applying a parking brake mechanism for a foundation brake in the event of failure of an associated service brake actuator. The parking brake mechanism incorporates an electric actuator, an extensible device drivably connected to the actuator, and a resilient device arranged to act on the extensible device and maintain a desired level of force to be applied by the parking brake mechanism in the event of contraction of components of the foundation brake. The method includes the steps of detecting a failure of an associated service brake actuator, signaling driving of an electric actuator to cause the extensible device to be able to retain the foundation brake in the brake applied position and to compress the resilient element, monitoring a characteristic of the brake to determine if a desired force has been applied by the parking brake mechanism, and signaling driving of an electric motor to stop once the desired force has been reached. 
         [0024]    A sixth aspect of the present invention provides a control system for a parking brake mechanism configured to signal application of a parking brake mechanism in accordance with the method of the preceding paragraph. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Embodiments of the present invention will be described, by way of example only, with reference to the accompanying drawings, in which: 
           [0026]      FIG. 1  is an isometric cross-sectional view through a brake actuator along an axial centerline thereof incorporating an electric parking brake mechanism; 
           [0027]      FIG. 2  is an isometric perspective view of a sub-assembly of the electric parking mechanism of  FIG. 1 ; 
           [0028]      FIGS. 3 to 7  are cross-sectional views through the brake actuator of  FIG. 1  along an axial centerline thereof illustrating various successive stages in the application of the parking brake according to a method of the present invention, when a compressed air supply to the brake actuator is available; 
           [0029]      FIGS. 8 ,  9  and  10  illustrate various successive stages in the application of the parking brake utilizing electrical actuation only; 
           [0030]      FIGS. 11 ,  12  and  13  illustrate the sequence of releasing the parking brake after it has been applied with the brake components hot; 
           [0031]      FIGS. 14 ,  15  and  16  illustrate the release of the parking brake after it has been applied with the brake components at a cold temperature; 
           [0032]      FIG. 17  is a cross-section through a brake actuator along an axial centreline thereof and incorporating a further electric parking brake mechanism; 
           [0033]      FIG. 18  is an isometric view of a sub-assembly of the parking brake mechanism of  FIG. 17 ; 
           [0034]      FIG. 19  is a brake actuator attached to a brake caliper, the brake actuator incorporating a further electric parking brake mechanism; 
           [0035]      FIG. 20  is a flowchart illustrating a parking brake application sequence according to a method of the present invention for an electric parking brake of  FIG. 1  or  FIG. 17  when compressed air is used; 
           [0036]      FIG. 21  is a flowchart illustrating a parking brake application sequence for a parking brake of  FIG. 1  or  FIG. 17  when no air is available; 
           [0037]      FIG. 22  is a flowchart illustrating a release sequence according to a method of the present invention for a parking brake of  FIG. 1  or  FIG. 17  when compressed air is available; 
           [0038]      FIG. 23  is a flow chart illustrating an application sequence according to an alternative method of the present invention for a parking brake of  FIG. 19  when compressed air is available; and 
           [0039]      FIG. 24  is a schematic diagram of a control system according to a further embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0040]    With reference to  FIG. 1 , one half of a substantially cylindrical brake actuator  10  is shown in longitudinal cross-section about a central axis of the brake actuator  10 . The brake actuator  10  includes a housing, which in this embodiment includes a first shell  12  that forms a major part of a service brake chamber  14  and a second shell  16  that forms a minor part of the service brake chamber  14 , and largely houses a parking brake mechanism  18 . 
         [0041]    The term “inboard” as used below denotes a direction towards a centerline of a vehicle to which the brake is fitted, whereas “outboard” refers to a direction away from the centerline. 
         [0042]    The first shell  12  and the second shell  16  are held together by a clamp band arrangement  20  that engages corresponding lips on the shells  12  and  16 , as is well known. In this embodiment, the clamp band arrangement  20  also acts to sandwich a flexible diaphragm (not shown) between the lips and which is also connected to a service brake push rod  22  so as to split the service brake chamber  14  into a non-pressurized region at an outboard side of the chamber (a side incorporating a free end of the push rod) as illustrated in  FIG. 1  and a pressurized region  26  in the inboard portion of the service brake chamber  14 , the other side of the diaphragm, as is well known in the art. The first shell  12  further includes two studs  28  (one visible in  FIG. 1 ) to mount the brake actuator  10  to an inboard face of a known brake caliper  208 , e.g., of the type disclosed in the applicant&#39;s earlier patent EP1000263 (see the third embodiment of  FIG. 19 ). 
         [0043]    The service brake push rod  22  terminates at its inboard end with a pressure distribution disc  30  and a central recess  32  to releasably accommodate a parking brake push rod  34  of the parking brake mechanism  18 , as discussed in more detail below. 
         [0044]    The parking brake mechanism  18  includes a stepped piston  36  that is sealed in relation to the second shell  16  at its axially outboard end and may slide axially relative thereto. At its inboard end, it is also sealed relative to a circular lip  38  that projects from an inboard end wall  40  of the second shell  16  in an outboard direction. The stepped piston  36  therefore also separates the parking brake mechanism  18  into a pressurized region  42  that is contiguous with the pressurized region  26  of the service brake chamber, and an unpressurized toroidal region  44 . The stepped piston  36  is prevented from sliding outboard beyond a predetermined position by stops  37  (see  FIG. 3 ) provided on the second shell  16 . 
         [0045]    The unpressurized region  44  houses a resilient device in the form of a helical spring  46  that is supported at its inboard end by the inboard end wall  40  and its outboard end by the stepped piston  36 . The helical spring  46  is designed such that it is preloaded by a predetermined amount when resting against the stops  37 . 
         [0046]    An electric motor  48  is provided in a separate housing  50  to the side of the second shell  16 . The electric motor  48  is connected to a drive shaft that extends down the center of the parking brake mechanism  18  via reduction gears  54   a,    54   b  and  54   c.  A cover plate  55  is provided on the inboard end of the second shell  16  and the electric motor housing  50  to protect the reduction gears  54   a,    54   b  and  54   c.  The outboard end of the drive shaft  52  drives an extensible device and is splined such that it is rotationally fixed to an inner bayonet member  56  of the extensible device, but enables the inner bayonet member  56  to slide axially with respect to the drive shaft  52 . 
         [0047]    As can be seen more clearly from the exploded isometric view of  FIG. 2 , the inner bayonet member  56  includes a central shaft portion  58 , an inboard enlarged head portion  60  including three bayonet lugs  62  equally angularly spaced around its circumference, and an outboard threaded portion  64  at the opposite end of the central shaft portion  58  to the enlarged head portion  60 . 
         [0048]    An outer bayonet sleeve  66  is provided with three axially extending channels  68  (only one visible in the cut-away of  FIG. 2 ) that are as wide or wider than the bayonet lugs  62 , such that when the bayonet lugs  62  are aligned with the channels, and the inner bayonet member  56  is able to slide axially with respect to the outer bayonet sleeve  66  without any restriction. Circumferentially intermediate each of the axially extending channels  68  are an array of axially spaced projections  70  that are arranged in three axially aligned rows. If the inner bayonet member  56  is rotated through  60  degrees from its position in which the bayonet lugs  62  are aligned with the axially extending channels  68 , the bayonet lugs  62  engage or latch instead between a pair of projections, thus preventing the inner bayonet member  56  from moving axially with respect to the outer bayonet sleeve  66 . A wall  71  at the inboard end of the outer bayonet sleeve  66  prevents the inner bayonet member  56  from sliding inboard beyond the outer bayonet sleeve  66 . 
         [0049]    Referring back to  FIG. 1 , it should be noted that the outer bayonet sleeve  66  is sized to locate within the stepped piston  36  and has a peripheral inboard lip  72  that is in contact with a circular leaf spring  74  mounted to an outboard face of the stepped piston  36 . A needle roller thrust bearing (not shown) is located between the outboard face of the stepped piston  36  and the peripheral inboard lip  72  so the two components move axially together with only restricted relative axial movement between the stepped piston  36  and the outer bayonet sleeve  66  (e.g., due to vibration). 
         [0050]    The outboard threaded portion  64  of the inner bayonet member  56  locates within an internally threaded bore  76  of the parking brake push rod  34 . Consequently, rotation of the drive shaft  52  and the inner bayonet member  56  results in the extension and retraction of the parking brake push rod  34  with respect to the remainder of the parking brake mechanism  18 . A pair of axially extending slots  78  are provided at opposed locations on the outer face of the parking brake push rod  34  and are arranged to be engaged by a corresponding pair of prongs (not shown) on the circular leaf spring  74  such that the parking brake push rod  34  may move axially, but not rotate, with respect to the remainder of the brake actuator  10 . 
         [0051]    The parking brake push rod  34  terminates at its outboard end in a spherical ball-shaped head  80  that is dimensioned to fit within the central recess  32  of the pressure distribution disc  30  of the service brake push rod  22 . A spring loaded ball-bearing  82  is mounted within the spherical ball-shaped head  80  and sits within a circumferentially extending depression  84  within the recess  32 . As a result, a pre-determined force is required to cause the ball-bearing  82  to retract and for the parking brake push rod  34  to separate from the service brake push rod  22 . 
         [0052]    With reference to  FIG. 3 , a cross-section along a slightly different axial plane to  FIG. 1  is shown, and it can be seen that an air inlet portion  86  is provided in the second shell  16  through which pressurized air may be introduced into the pressurized region of the service brake in order to apply the service brake. 
         [0053]      FIG. 3  illustrates the brake actuator  10  in a condition in which air has been introduced into the pressured region  26  up to a pre-determined pressure. It can be seen that this pressure has caused the service brake push rod  22  to move outboard and to also pull the parking brake push rod  34  with it, because the inner bayonet member  56  is not engaged with the projections on the outer bayonet sleeve  66 . In addition, the stepped piston  36  has moved inboard by comparison with  FIG. 1 , such that it is in contact with the inboard end wall  40  of the second shell  16 . 
         [0054]    In normal service brake operations, the compressed air is allowed to vent, and a return spring (not shown) causes the service brake push rod  22  to return to the rest position of  FIG. 1 , and the brake to cease being applied. 
         [0055]    Referring to  FIG. 24 , a simplified schematic diagram illustrates an electric brake control system incorporating a parking brake control system  350  according to an embodiment of the present invention. The parking brake control system  350  is configured to control parking brake function of four foundation air disc brakes that include a brake caliper  208  and a brake disc  310 . 
         [0056]    In this embodiment, a parking brake hand control  320  located in a vehicle cab or a passenger compartment incorporates an ECU  325  (typically a microprocessor controller) programmed to interpret the demand signal, along with other vehicle parameters, such as its weight, angle of slope and brake status and send appropriate control signals to EBS modules  330  provided for each brake or an EBS twin module  330  for two brakes on the same axle and directly to the parking brake mechanism(s)  18 . In this embodiment, communication takes place via a controller area network (CAN) bus  324  (dotted lines), but any other suitable communication method may be used in other embodiments. Each EBS module  330  also receives a compressed air supply  326  via air lines  328  (solid lines) and includes appropriate electronics and solenoid valves to control the functioning of its associated service brake mechanism(s)  14 . 
         [0057]    The electric brake control system also includes an electric brake system electronic control unit (EBS-ECU)  322 . This controls the service brake function of the braking system. 
         [0058]    With reference to  FIGS. 20 and 24 , when a vehicle user wishes to apply the parking brake, they activate the parking brake hand control  320  to indicate parking brake demand at step  410 . In normal operation, the first step of the parking process is for the ECU  325  to signal the parking brake demand to each parking brake mechanism  18  and to the EBS modules  330  in accordance with a number of pre-programmed steps. The EBS module  330  opens a valve to supply air to apply the service brake at steps  412  and  414  of  FIG. 20  and as shown in  FIG. 3 . 
         [0059]    It is then necessary for the ECU  325  to signal the electric motor  48  to drive at step  416  via the reduction gears  54   a,    54   b,  and  54   c  for the next stage of parking brake application, as shown in  FIG. 4 . Driving the electric motor  48  causes the inner bayonet member  56  and the outer bayonet sleeve  66  to rotate relative to the parking brake push rod  34  until the bayonet lugs  62  are engaged in alignment with a gap between the axially spaced projections  70  of the outer bayonet sleeve at step  418  (note spacing X 1  between the inner bayonet member  56  and the end of the threaded bore of the parking brake push rod  34 ). Once alignment occurs, friction from the leaf spring  74  stops the outer bayonet sleeve  66  rotating, and the inner bayonet member  56  is able to rotate with respect to the outer bayonet sleeve  66  so it is latched between axially spaced projections  70 , and axial movement of the inner bayonet member  56  with respect to the outer bayonet sleeve  66  is now prevented. The electrical current through the electric motor  48  can be monitored by the ECU  325  or internally within the parking brake mechanism to determine when this occurs, as the load on the motor reduces once alignment occurs. 
         [0060]    With reference to  FIGS. 5 and 20 , the ECU  325  signals the EBS module  330  to release the compressed air within the pressurized region  26  at step  420 . This enables the helical spring  46  to relax slightly so that the stepped piston  36  slides slightly outboard with respect to the second shell  16 . Then, at step  422 , in order to fully compress the helical spring  46 , the ECU  325  signals the electric motor  48  to drive forward (note increase in spacing X 2  compared to X 1 ). Advantageously, by monitoring the current through the electric motor  48  and correlating this to the load on the electric motor  48 , it is possible to apply a pre-determined parking brake clamp load. At step  424 , the ECU  325  signals ceasing of the electric motor  48  drive once this load has been achieved. 
         [0061]    It will thus be appreciated with reference to  FIG. 6 , that the clamp load acts from the compressed helical spring  46 , via the stepped piston  36 , which is in engagement with the outer bayonet sleeve  66 , the inner bayonet member  56 , the parking brake push rod  34 , and then to the service brake push rod  22 , which is acting on the operating shaft  206  in the brake caliper  208  of the brake to cause the brake pads to clamp the brake disc. 
         [0062]    As is often the case, the parking brake is applied when the brake disc  310  and other brake components are hot, due to energy dissipated as heat by previous service brake applications as the heavy vehicle is operated. As the brake disc and other brake components cool back to ambient temperature while the heavy vehicle is parked, it is inevitable that the brake disc and the brake pads contract. In order to prevent the heavy vehicle from rolling away if parked on a slope, it is necessary to maintain a certain level of clamp load despite this contraction. 
         [0063]    Referring to  FIG. 7  and step  426  of  FIG. 20 , it can be seen that in such a situation, the helical spring  46  relaxes, thus causing the parking brake mechanism  18  to shift outboard by a significant amount. However, the preload on the helical spring  46  means that despite this relaxation, it is able to continue to apply a high force through the parking brake mechanism  18  such that a necessary clamp load is applied by the parking brake even after the brake disc  310  and other brake components have cooled to ambient temperatures. 
         [0064]    Alternatively, if the brake disc and other brake components are cold (i.e., at ambient temperature) when the parking brake is applied, then at step  428  the helical spring  46  remains compressed. 
         [0065]      FIGS. 8 ,  9  and  10 , in conjunction with the flowchart of  FIG. 21 , illustrate an application of the parking brake using the electric motor  48  alone. This may be necessary if there is a failure in the air supply of the vehicle. In  FIG. 8 , following a demand signal  430 , the ECU  325  signals the electric motor  48  at step  432  to drive the parking brake push rod outboard with respect to the inner bayonet member  56  to the point at which it has equaled the pre-load on the spring at step  434  (note increased spacing X 3 ). The inner bayonet member  56  is restrained in its most extreme inboard position with respect to the outer bayonet sleeve  66  so that the parking brake push rod  34  and the inner bayonet member  56  are in compression and the outer bayonet sleeve is in tension, abutting against the stepped piston  36 . 
         [0066]    In  FIG. 9  and step  436 , the electric motor  48  is continued to be driven forward to fully compress the spring (it can be seen that the inboard end of the stepped piston  36  abuts the outboard face of the inboard end wall  40  to prevent further compression of the spring and the spacing X 4  then further increases with respect to X 3 ). At this point, in this embodiment, the spring is loaded. The electric motor  48  current can again be monitored to determine when this loading has been achieved, and at step  438 , the ECU  325  signals drive to cease. 
         [0067]    With reference to  FIG. 10  and step  440 , it can be seen that the brake has again cooled, but that the helical spring  46  has relaxed and shifted the parking brake mechanism  18  outboard so that a sufficient clamp load continues to be transmitted via the service brake push rod  22  to the brake. Alternatively, at step  442 , if the parking brake is applied cold, the helical spring  46  remains compressed. 
         [0068]      FIGS. 11 ,  12  and  13  in conjunction with the flowchart of  FIG. 22  illustrate a parking brake release operation after a standard parking brake application when the brake was hot. Following an indication of release demand at step  442 , the ECU  325  signals the EBS module  330  to supply air to the service brake chamber  14  at step  444 , and this introduces compressed air at step  446  into the pressurized region  26 . This in effect unloads the parking brake mechanism  18  inboard and causes the spring loading of the ball-bearing  82  to be overcome such that the parking brake push rod  34  separates from the service brake push rod  22 . 
         [0069]    In  FIG. 12  and step  448 , the ECU  325  signals the electric motor  48  to drive in reverse to unlock the bayonet lugs  62  from the axially spaced projections  70  at step  450 . The inner bayonet member  56  may now retract inboard with respect to the outer bayonet sleeve  66 . In  FIG. 13  and step  452 , the ECU  325  signals the EBS module to release the air pressure causing the parking brake push rod  34  to re-engage with the service brake push rod  34 , the inboard retraction of the inner bayonet member  56  to occur, and the stepped piston  36  to return back to its rest position under the influence of helical spring  46 . Simultaneously, at step  454 , the electric motor  48  is driven backwards to retract the parking brake push rod back to its rest position with respect to the inner bayonet member  56 . 
         [0070]      FIGS. 14 ,  15  and  16  illustrate a similar release process to  FIGS. 11 ,  12  and  13 , except that the parking brake was originally applied with the brake components cold (i.e., at ambient temperature), and parking was achieved without compressed air. Thus, no contraction of the components has occurred while the vehicle is parked and compressed air is introduced into the pressurized region  26 , and no separation of the parking and service brake push rods occurs. Because the release follows a motor applied parking operation, a greater amount of reverse drive is needed from the electric motor  48  to return the parking brake push rod  34  to its inboard rest position with respect to the inner bayonet member  56 . 
         [0071]    Therefore, it will be appreciated that the use of the helical spring  46  means that contraction of the brake disc and other brake components may be compensated for while the vehicle is standing with the parking brake applied. As a result, on the one hand the risk of the vehicle rolling away due to a reduced clamp load is minimized, while at the same time excess loadings do not need to be applied to the brake to account for such contractions and therefore fatigue on components may be reduced. Additionally, the use of the bayonet components enables a rapid application and release of the parking brake in normal circumstances when compressed air is available while enabling a relatively small, low power parking brake motor to be used, and still having a back-up of solely electrical parking in the event of failure of the air supply. Finally, by locating the motor external the main body of the cylinder, it may be orientated at any desired angle with respect to the brake caliper to ensure that its packaging can be optimized for wide variety of vehicle configurations. 
         [0072]      FIGS. 17 and 18  illustrate a second embodiment of the present invention in which like parts have been denoted by like numerals but with the addition of the prefix “1.” Only differences with respect to the actuator of the first embodiment are discussed in detail below. 
         [0073]    The actuator  110  of  FIG. 17  functions using similar principles to the actuator shown in  FIG. 1  and the various operation sequences shown in  FIGS. 3 to 16  and  20  to  22  are similar. However, the parking brake mechanism  118  of the second embodiment is located entirely within the unpressurized region of the parking brake  142 , and the electric motor  148  is mounted concentrically between the helical spring  146  and the outer bayonet sleeve  166 . The electric motor  148  drives an internally splined drive sleeve  152  via reduction gearing instead of the drive shaft  52  of the first embodiment. The inner bayonet member  156  has external splines along a major portion of its length such that it may axially slide with respect to the splined drive sleeve  152 . The inner face of the inner bayonet member  156  is threaded and receives a complementary threaded inboard portion of the parking brake push rod. The outboard end of the inner bayonet member  156  however includes a plurality of the bayonet lugs  162 , which, with reference to  FIG. 18 , are able to be positioned with respect to the outer bayonet sleeve  166  as shown in  FIG. 18  to permit axial movement of the inner bayonet member  156  with respect to the outer, or be rotated through approximately  90  degrees by the electric motor  148  so as to engage between the projections  170  and axially latch the inner bayonet member  156  to the outer bayonet sleeve  166 . 
         [0074]    The ball-shaped head  80  at the outboard end of the parking brake push rod  34  of the first embodiment is replaced by a load spreading plate  180  that is magnetized such that it is normally held in contact with the pressure distribution disc  130  of the service brake push rod. In this embodiment, a flexible diaphragm  131  is shown extending between the pressure distribution disc  130  and the clamp band arrangement  120 . 
         [0075]    The parking brake push rod  134  extends through the piston  136 , and a sealing arrangement is provided between the piston  136  and the parking brake push rod  134 . Furthermore, the outboard end of the parking brake push rod  134  is provided with a non-circular profile to prevent rotation of the push rod with respect to piston  136 . In this embodiment, the profile is a tri-lobed profile. In other embodiments, alternative profiles such as ovals, etc. may be used. 
         [0076]    A thrust bearing arrangement  167  is provided between the outer bayonet sleeve  166  and a spring seat  169  that connects the helical spring  146  to the piston  136 , such that the outer bayonet sleeve  166  is able to rotate freely with respect to the piston  136  but, nevertheless, transmit axial loads to the spring. 
         [0077]    In operation, the parking brake mechanism  118  functions in a similar manner to that of the first embodiment. Compressed air is introduced via air inlet port  186  to shift the service brake push rod  122  outboard to apply the brake, and simultaneously, the parking brake push rod  134  is shifted outboard under the influence of the magnetic connection between the pressure distribution disc  130  and load spreading head  180 . The electric motor  148  is then driven so as to engage the bayonet lugs  162  between appropriate projections  170  of the outer bayonet sleeve  166  so as to latch the two components together. Further driving of the motor causes the additional loading of the spring since the inner bayonet member  156  rotates relative to the parking brake push rod  134  and the two components are threaded together. 
         [0078]    Once the required parking brake load has been achieved, the air can be released via an inlet port  186 , and the parking brake load from the helical spring  146  is transmitted via the piston  136 , the outer bayonet sleeve  166 , the inner bayonet sleeve  156 , the parking brake push rod  134 , and the load spreading head  180  to the service brake push rod  122  to thereby maintain the parking brake clamp load and also account for any contraction of the brake disc by enabling this to be accommodated by relaxation of the helical spring  146  as required. 
         [0079]    Furthermore, in the event of failure of the air supply, the parking brake can be applied by the electric motor  148  alone, via the rotation of the inner bayonet member  156  relative to the parking brake push rod  134 , albeit more slowly than if air is available. 
         [0080]      FIG. 19  illustrates a further embodiment of the parking brake mechanism  218  in which like parts are denoted by like numerals, but with the addition of the prefix “2.” This embodiment provides a simplified arrangement that dispenses with the bayonet-type mechanism. 
         [0081]    The brake actuator  210  is shown connected to a caliper housing  208  having an operating shaft  206  located therein, which is pivoted by movement of the service brake push rod  222 . 
         [0082]    In addition, in this embodiment, the electric motor  248  is mounted within the helical spring  246 , but is off-set from the parking brake push rod  234 , rather than being arranged concentrically around it. The electric motor  248  drives the parking brake push rod via an epicyclic reduction gear arrangement  254  that outputs its drive to an internally threaded sleeve portion  266  of a lead screw assembly, which additionally includes a parking brake push rod  234  having a complementary external thread and a splined central shaft  258  that is rotationally fixed such that drive from the outer sleeve causes the push rod to extend or retract. 
         [0083]    A fixed wall  288  is provided between the service brake chamber and the parking brake mechanism  218 , and a guide bore  290  extends inboard from the fixed wall  288  to support the parking brake push rod  234 . A seal  292  is provided in the bore guide  290  such that the entire parking brake mechanism is in an unpressurized portion of the brake actuator  210 . 
         [0084]    The electric motor  248 , the reduction gear arrangement  254  and the splined central shaft  258  are all mounted with respect to a moving casing  294 , with a thrust bearing  296  supporting the reduction gear arrangement  254 . The helical spring  246  is mounted between the second shell  216  and the moving casing  294  such that the extension of the parking brake push rod  234  may not only cause the brake to be applied by shifting the service brake push rod  222  outboard, but may also cause the moving casing to move inboard with respect to the second shell  216 , and the helical spring  246  to thereby be compressed. Thus, when the parking brake is applied while the brake is in a hot condition, the spring may relax and enable a suitably high brake force to be applied to the op-shaft, despite the contraction of the hot brake components 
         [0085]    As in the previous embodiment, a pre-determined amount of pre-load is applied to the helical spring  246  when in the rest position shown in  FIG. 19 , in which the moving casing  294  abuts the fixed wall  288  such that a high load can be applied to the op-shaft even as the casing approaches the fixed wall  288 . Thus, this embodiment benefits from the same advantages as regards ensuring a sufficiently high clamp load even during cooling and contraction of brake components as the parking brake mechanisms of the first two embodiments. However, since the brake does not include an equivalent of the bayonet latching mechanism, it may take longer for the parking brake push rod to extend and retract by comparison with the parking brake mechanisms of the first two embodiments. 
         [0086]    The shell  298  illustrates the usual position of a conventional spring parking braking shell so the overall reduction in size of the parking brake of the present invention can be seen by comparison. 
         [0087]      FIG. 23  illustrates the normal application sequence for the electric parking brake of  FIG. 19 . The sequence uses similar labeling to that of  FIG. 20  except that the prefix ‘5’ replaces the prefix ‘4.’ The only differences in the sequence by comparison with  FIG. 20  are that in step  514 , the service brake application does not compress the helical spring  246 , and in step  518  it is necessary for the motor to be driven forward to extend the lead screw assembly until the parking brake push rod  234  contacts the pressure distribution plate of the service brake push rod  222 . At this point, the ECU  325  detects the change in motor current and signals air pressure release, but the motor continues to drive until the required clamp load is achieved. 
         [0088]    The release sequence for the parking brake of  FIG. 19  is similar to the steps outlined in  FIG. 22 , except that the motor is signaled to drive the lead screw assembly in reverse until it is fully retracted before it signals release of air pressure. 
         [0089]    It should be appreciated that terms such as inner and outer, inboard and outboard, upper and lower should not be regarded as limiting and that the position of components may be adjusted as required. In particular, the actuator may be angled with respect to the caliper housing such that it is not strictly positioned in the inboard-outboard direction of a vehicle to which it is fitted. 
         [0090]    It should be appreciated that numerous changes may be made within the scope of the present invention, the control system and method of the present invention may be used in conjunction with other parking brake mechanisms that include a resilient/compliant device in the transmission path to account for contraction of brake components due to cooling. For example, the system and method may be adapted for parking brakes in which the reduction gear arrangement may be replaced by suitable alternative types of reduction gearing, the helical spring may be replaced by other resilient components such as a stack of Belville washers, and the bayonet arrangement may be replaced by alternative latching mechanisms, such as clamping devices or collet arrangements similar to those of our earlier patent application, EP1596090. A magnetic parking brake push rod to service brake push rod connection may be used in the first embodiment and the spring loaded connection of the first embodiment used in the second embodiment. The control system may be adapted for use with an electrically actuated service brake. The control system may also be used in conjunction with drum brakes as well as disc brakes. Operation of the parking brake may be controlled by any other suitable control unit, such as the central EBS-ECU, the EBS modules or a microprocessor within each parking brake mechanism itself rather than the dedicated ECU module for the parking brake. Control may also be distributed between these components. The EBS module may be connected to load sensors to directly measure loads applied by the parking brake rather than deriving loads from motor current and the like. The deflection of the spring or components connected thereto such as the piston may alternatively be measured to determine the load. In alternative embodiments, the entire parking brake mechanism may be within the pressurized area, or the mechanism of the first embodiment may be entirely within the unpressurized area. The bayonet arrangement may have lugs and projections with a curved or helical form to assist with engagement thereof during latching. 
         [0091]    The foregoing description is only exemplary of the principles of the invention. Many modifications and variations are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims, the invention may be practiced otherwise than using the example embodiments which have been specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.