Abstract:
An electrical park latch actuator for the automatic transmission selector lever of a road vehicle, the actuator including a support member, first and second electromagnet windings carried by a common electromagnet core of the support member, and first and second plungers moveable relative to the support member, and responsive to energization of the first and second electromagnet windings respectively, said first and second plungers being positioned with their movement axes parallel and part of said second plunger being slidably received within said first plunger.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to an electrical park latch actuator for the automatic transmission selector lever of a road vehicle. 
     It is usual for the selector lever of a vehicle automatic transmission system to incorporate a manually operable release button or the like which must be operated by the vehicle driver in order to release a latch mechanism permitting the selector lever to be moved between certain operating positions. For example, it is usual for the lever to be latched in the “Park” position so that the release button must be operated to permit the lever to be moved from the park position. 
     For safety reasons it is usual for there to be a requirement that both the ignition switch of the vehicle is in the “on” position and the brake pedal of the vehicle is depressed before the release button can be operated to release the latch mechanism freeing the selector lever for movement from the park position. 
     Conventionally, as illustrated in, for example, U.S. Pat. No. 5,489,246, the ability to operate the release button, to free the selector level for movement from its park position, is controlled by a first solenoid responsive to energization of the ignition circuits of the vehicle, and a second solenoid responsive to the position of the brake pedal of the vehicle. The first and second solenoids include plungers which cooperate independently of one another with respective cam/abutment regions of a pushrod moving with the release button. 
     It is an object of the present invention to provide an electric park latch actuator which can be responsive to the energization of the ignition circuit of the vehicle and the depression of the brake pedal of the vehicle in a simple and convenient form. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention there is provided an electrical park latch actuator including support member, first and second electromagnet windings carried by the support member, and first and second plungers moveable relative to the support member and responsive to energization of the first and second electromagnet windings respectively, said first and second plungers being positioned with their movement axes parallel, and part of said second plunger being slidably received within said first plunger. 
     Preferably said first and second plungers have their movement axes coextensive. 
     Desirably said first and second plungers are urged by respective return springs into the path of movement of an element moveable with the release button of the associated transmission selector lever in use, and said first plunger includes a cam surface cooperable with said element. 
     Preferably said support member includes an electromagnet core common to both electromagnet windings. 
     Conveniently said first plunger is hollow, and said support member is received therein. 
     Conveniently said first plunger is hollow, and said second plunger is received therein. 
     Desirably said first plunger includes a manually operable interlock selectively engageable with said second plunger, whereby movement of said first plunger against the action of its return spring carries the second plunger with it against the action of its return spring. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     One example of the invention is illustrated in the accompanying drawings wherein: 
     FIG. 1 is a diagrammatic cross-sectional view of an electrical park latch actuator; and 
     FIG. 2 is a view similar to FIG. 1 of a modification. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1 of the accompanying drawings reference numeral  11  identifies an axially moveable pushed forming part of the selector lever assembly of the automatic transmission of a road vehicle. The pushed  11  is associated with a park latch release button of the selector lever and is moveable vertically downward from the position illustrated in FIG. 1, against the action of a light return spring (not shown), by depression of the release button to release the latching mechanism which locks the selector lever in its “Park” position. It will be understood that if the downward axial movement of the rod  11  is prevented then the latch mechanism of the selector lever cannot be released, and the selector lever cannot be moved from its park position. 
     The pushrod  11  cooperates with the electric park latch actuator  12  so that the actuator controls when the rod  11  can be moved to permit release of the selector lever from its park position. 
     The park latch actuator  12  includes a support member which, in use, is fixed and provides a mounting arrangement (not shown) whereby the actuator is secured within the vehicle adjacent the transmission selector lever in use. The support member includes an electromagnet core  13  and an outer, hollow, frame  15 . The core  13  is generally cylindrical, being of circular cross-section, and includes an integral, radially outwardly extending, peripheral flange  14  adjacent one axial end thereof. The core  13  and frame  15  are formed from ferromagnetic material, and the frame  15 , which conveniently is also of circular cross-section, has the core  13  mounted coaxially therein by means of the flange  14 . The outer, face of the flange  14  engages the inner surface of the frame  15  partway along the length of the frame  15 , and one axial end of the frame  15  is partially closed by a radially inwardly extending, integral, peripheral flange  16 . 
     Housed within the annular volume defined between the outer surface of the core  13  and the inner surface of the frame  15  are first and second annular, moulded synthetic resin coil formers  17 ,  18 . The coil formers  17 ,  18  are positioned with their axes coextensive with the axes of the core  13  and frame  15  and are axially spaced from one another by the flange  14 . Each of the formers  17 ,  18  supports a respective electromagnet winding  19 ,  21  the windings thus having their axes coextensive with the axis of the core  13 . 
     The actuator  12  further includes a first, hollow, plunger  22 . As will become apparent is it not necessary for the plunger  22  to be formed from a ferromagnetic material, and the plunger  22  can therefore be a synthetic resin moulding, or a zinc alloy die-casting or the like. The support member assembly  13 ,  15  carrying the windings  19 ,  21  is coaxially received within the bore  23  of the plunger  22 . Axially extending slots (not shown) are formed in the wall of the plunger  22  and mounting tabs (not shown) extending radially outwardly from the support member  13 ,  15  project through the slots to permit the actuator to be fixed in the vehicle body in use. It will be recognised therefore that in use the support member  13 ,  15  together with the windings  19 ,  21  are fixed and the plunger  22  can move axially relative thereto. 
     The plunger  22  has a closed end  24  presented to the flange  16  of the frame  15 . The opposite end of the frame  15  and the end of the core  13  remote from the flange  14  define parts of common, planar surface at right angles to the axis of the core  13  and frame  15  and a planar, ferromagnetic disc  25 , attached at its periphery to the plunger  22 , can abut the coplanar ends of the core  13  and frame  15 . A helically wound compression spring  26  of relatively light rating, acts between the flange  16  and the closed end  24  of the plunger  22  to urge the plunger  22  to a rest position relative to the support member  13 ,  15 , in which the disc  25  abuts the core  13  and frame  15 . 
     The former  17  carrying the electromagnet winding  19  has a hollow, integral, cylindrical extension  27  of circular cross-section which extends through the central aperture of the flange  16  towards the closed end  24  of the first plunger  22 . Slidable axially within the extension  27  is a second plunger  28 , the plunger  28  being of circular cross-section, and being formed from a ferromagnetic material. The plunger  28  includes an axial extension  29  which protrudes into a corresponding through bore  31  in the closed end wall  24  of the first plunger  22 . 
     The extension  27  is divided, by a plurality of axially extended slots, to define a plurality of axially extending fingers  32  which extend through respective apertures in a radially outwardly extending, integral, flange  33  of the plunger  23 . At their free ends the fingers  32  are bent inwardly to engage an end face of the second plunger  28  to define an abutment against which the plunger  28  is urged by a helically wound compression spring  34  also of a relatively light rating. The spring  34  extends between the flange  16  of the frame  15  and the flange  33  of the second plunger  28 , and the position of the abutment surfaces defined by the fingers  32  is such that when the first and second plungers  22 ,  28  occupy their rest positions, under the action of their respective return springs  26 ,  34 , then the free end of the extension  29  of the plunger  28  is substantially flush with the outer surface of the end wall  24  of the plunger  22 . 
     The end surface  24  of the plunger  22  is cut away to define an inclined cam surface  35  with which the free end of the pushrod  11  can cooperate. 
     The operation of the actuator is as follows. 
     The electromagnet windings  19  and  21  are both energized by movement of the vehicle ignition switch to an “on” position so that the ignition circuits of the vehicle are energized. However, the winding  21  is associated with the brake pedal of the vehicle in use such that when the brake pedal is depressed then the winding  21  will be de-energized irrespective of the ignition switch being in its “on” position. 
     To understand the operation of the actuator, assume firstly that the selector lever is in its park position, and the end of the pushrod  11  is engaged with the cam  35  of the plunger  22 . Assume also that both windings  19 ,  21  are energized because the ignition switch of the vehicle is closed, but the brake pedal has not been depressed by the driver of the vehicle. In this situation the energization of the winding  21  attracts the disc  25  firmly against the axial end faces of the core  13  and frame  15  so that the plunger  22  is held against movement to the right against the action of its return spring  26 . The pushrod  11  will thus be abutting the cam  35 , and will be held by the cam  35  against the vertical downward movement which is necessary in order to release the park latch of the selector lever, the electromagnetic attraction of the disc  25  to the core  13  and frame  15  being too strong to be overcome by the force which the driver is able to apply to the pushrod  11  by way of the release button of the selector lever. It will be recognised that by virtue of energization of the winding  19  the plunger  28  will have been attracted towards the core  13  so that a frusto-conical end region  36  of the plunger  28  is received within a frusto-conical recess  37  in the face of the core  13  remote from the disc  25 . However, this movement of the plunger  28  is irrelevant to movement of the pushrod  11  since the plunger  22  and cam surface  35  cannot move. 
     It will be recognised that the actuator in effect contains first and second electromagnets having respective plungers moveable in opposite directions, and sharing a common core. The flux path of the winding  21  will be through the core  13 , the flange  14 , and part of the cylindrical frame  15 , the frame  15  and core  13  being bridged by the disc  25 . The flux path of the winding  19  includes the flange  14 , a small of part of the core  13 , the left-hand end of the frame  15 , the flange  16  and the plunger  28 . Thus energization of the winding  19  tries to close the air gap between the face  36  of the plunger and the face  37  of the core  13  while energization of the winding  21  tries to close the air gap between the disc  25  and the coplanar ends of the core  13  and frame  15 . 
     If the operator now depresses the brake pedal of the vehicle he will cause the winding  21  to be de-energized while the winding  19  remains energized. Immediately the winding  21  is de-energized then the only force preventing movement of the plunger  22  to the right as shown in the drawing, is the return spring  27  which, as mentioned above, is of relatively light rating. Thus the driver can readily apply sufficient force to the release button of the selector lever to depress the pushrod  11  relative to the cam  35  so that the plunger  22  is driven to the right against the action of the return spring  26 . The driver can therefore release the park latch freeing the selector lever for movement from its park position. 
     In the event that the driver attempts to release the latch mechanism while the ignition switch in an “off” position then again the only resistance to movement of the plunger  22  will be the return spring  26  which can be overriden by the action of the pushrod  11  on the cam  35 . However, in this situation the plunger  28  has not been withdrawn towards the core  13  since the winding  19  is not energized. The plunger  28  and extension  29  thus remain in their rest position under the action of the second plunger return spring  34  and the extension  29  remains in the path of movement of the pushrod  11  notwithstanding the fact that the pushrod  11  has displaced the plunger  22  to the right. 
     The distance through which the pushrod  11  moves to displace the plunger  22  to the right and to abut the extension  29 , is insufficient to release the latch mechanism locking the selector lever in its park position. Thus in a situation in which the ignition circuits of the vehicle are not energized then the extension  29  of the plunger  28  prevents latch release movement of the pushrod  11 . 
     It will be recognised that there may be a need to move the selector lever from its park position when the ignition circuit of the vehicle is not energized, for example in a failure situation where the vehicle needs to be towed. To facilitate such operation the inner face of the wall  24  of the plunger  22  is provided with a manually deflectable, integral leg  38  which is accessible through an aperture (not shown) in the wall of the plunger  22 . An operator inserts a tool, such as a small screwdriver, through the aperture in the wall of the plunger  22  to deflect the leg  38  to abut the flange  33  of the plunger  28 , while at the same time applying pressure to the release button of the selector lever to depress the pushrod  11  against the cam  35 . In this situation, the cam  35  will move against the action of the spring  26  as described above, but by virtue of the deflected leg  38  the plunger  28  will move with the plunger  22 , and thus both plungers will move together against the action of their springs  26 ,  34 . As both plungers are moving together then the extension  29  does not remain in place impeding the movement of the pushrod  11  and the latch mechanism locking the selector lever in the park position can be released. 
     In the arrangement illustrated in FIG. 2 the operating principle is extremely similar, but the fixed, support member, is defined by the frame  15  and flange  6 , and the flange  14  of the core  13  is replaced by a flange  44  extending radially inwardly from the frame  15 . The core  13  has the disc  25  integral therewith and thus the core and disc move as one relative to the frame  15 . The return spring of the plunger  22  is replaced by a bowed leaf spring, or dished spring disc  45  acting between the outer face of the disc  25  and a fixed abutment  47  to which the frame  15  is anchored. Similarly the return spring of the plunger  28  is replaced by a dished spring washer  46  acting between the flange  16  and the plunger  28 . 
     In some selector mechanisms the release button will move a bell crank lever one limb of which cooperates with the cam  35  in place of the pushrod  11 . The operation of the actuator  12  is unchanged.