Abstract:
A park-brake actuator includes a cam including first serrations and an open-ended slot, a piston including second serrations displaceable relative to the cam in response to variable pressure applied to the piston, a spring, and a rod including a pin urged by the spring toward the slot and contact with the first and second serrations, the pin moved by piston displacement from the slot, onto one of the first serrations to a flat-tow position.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to actuation of a motor vehicle park brake that produces engagement, disengagement and flat-tow operation of a park brake operated by a shift-by wire system. 
     2. Description of the Prior Art 
     Parking mechanisms or assemblies are used in automotive vehicles to allow the vehicle to enter into and maintain a “parked” condition or state. Parking assemblies typically include a park rod, which selectively actuates a parking pawl that operatively engages the vehicle&#39;s park gear, thereby maintaining the vehicle in a “parked” position or state. 
     The park rod is typically mechanically linked to the vehicle&#39;s shifter and moves in response to shifter movement. When the shifter is moved to the “park” position, a contact member, which is disposed on the park rod, engages the parking pawl, which pivots or rotates into a position where it is disposed between a pair of teeth on the park gear, thereby substantially preventing the park gear and the output shaft from further rotating. 
     The park rod includes a spring which is sometimes compressed during the engagement with the parking pawl. Particularly, the spring within the park rod is compressed only during certain shifts into park. For instance, when the park rod contact member engages the parking pawl and causes it to pivot into the park gear, the parking pawl is sometimes pivoted into and forcibly abuts a tooth of the park gear. During these types of shifts, the pivotal movement of the parking pawl is temporarily limited or stopped, thereby preventing further movement of the contact member until the park gear is rotated and causing the spring within the park rod to be compressed. When the park gear rotates, the parking pawl slides along the tooth which it abuts until it “drops into” or is forced into a space between teeth of the park gear. During other shifts into the park position, the parking pawl pivots directly into a space between gear teeth. During these types of shifts, the path of travel of the park rod is substantially unobstructed and the spring within the park rod is not compressed. The resulting inconsistency of spring compression causes the force required to shift the vehicle into park to vary from shift to shift. Because the shifter is mechanically coupled to the park rod in these prior systems, the user or driver of the vehicle undesirably “feels” or experiences inconsistent or different forces at the shifter when shifting the vehicle into park. Furthermore, when shifting a vehicle out of the park position while the vehicle is disposed on an incline, the loading of the park pawl will vary with the loading of the vehicle. This causes the park “pull out” load to vary based upon the vehicle weight and incline. This load variation is likewise undesirably experienced by an operator of the vehicle. 
     Another drawback with these prior mechanical systems occurs when a driver inadvertently shifts into park while the vehicle is still moving. Particularly, if the vehicle is moving above a certain speed, the parking pawl may not fully engage the park gear, but rather “ratchets” against the gear, thereby creating an unaesthetic sound and vibration and potentially damaging the parking pawl and park gear. Moreover, when residual torque remains within the output shaft during a parking event (e.g., when the vehicle is parked on an inclined or declined surface), the park gear imparts a torsional force or load on the parking pawl which may be transmitted through the park rod and may result in undesirable vibrations which can be “felt” or experienced at the vehicle&#39;s shifter. 
     Vehicles which utilize a “shift by wire” system, where the shifter is mechanically coupled to and actuates the park rod by use of a wire or cable, suffer from other drawbacks. By way of example and without limitation, the cable may act as a noise and/or vibration path or medium which generates undesirable noise and/or vibration that can be experienced within the passenger compartment. 
     SUMMARY OF THE INVENTION 
     A park-brake actuator includes a cam including first serrations and an open-ended slot, a piston including second serrations displaceable relative to the cam in response to variable pressure applied to the piston, a spring, and a rod including a pin urged by the spring toward the slot and contact with the first and second serrations, the pin moved by piston displacement from the slot, onto one of the first serrations to a flat-tow position. 
     By using a cam follower that limits travel of the park-apply rod, a single hydraulic control can be used for both normal Park and no-Park operation, and to latch and unlatch the actuator for engine-off, flat-tow operation. 
     The Park-apply rod is loaded through two springs in series. The first spring is of the rate needed for the Park-state and no-Park state. The second spring has a higher rate requiring more pressure to compress it, thereby allowing the two states to be separated enough that one oil source at two different pressures can be used to actuate both functions: normal park operation and flat tow/neutral operation. 
     The springs are separated by a fluid damper to prevent severe ratcheting forces from accidentally actuating the flat tow/neutral state. 
     The actuator uses the same control servo oil source to activate the neutral latch, and requires no additional valves, just the cam follower system. 
     The scope of applicability of the preferred embodiment will become apparent from the following detailed description, claims and drawings. It should be understood, that the description and specific examples, although indicating preferred embodiments of the invention, are given by way of illustration only. Various changes and modifications to the described embodiments and examples will become apparent to those skilled in the art. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be more readily understood by reference to the following description, taken with the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of a parking assembly; 
         FIG. 2  is cross sectional side view of the actuator in  FIG. 1 ; 
         FIGS. 3-9  illustrate progressive positions of a cross pin on the cam and piston during a full cycle of flat-tow/neutral engagement and disengagement. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The parking assembly  10  shown in  FIG. 1  is adapted for use in combination with a vehicle having a park gear  12  and a shifter  14 , by which the operating range of an automatic transmission is selected manually. 
     Park gear  12  is secured to the vehicle&#39;s transmission output shaft  13  and rotates with the output shaft  13 . Park gear  12  includes several peripherally disposed and substantially identical teeth or projections  16 , each tooth  16  being separated from an adjacent tooth by a recess portion  18 . When the parking pawl  20  engages the park gear  12  by entering one of the recesses, thereby substantially preventing the park gear  12  and output shaft  13  from rotating. 
     The shifter  14  is movable among several positions, such as a “park” position, “reverse” position, “neutral” position, and “drive” position (P R N D), which respectively correspond to various operating states of the transmission. The operating states may be selected by moving a shifter lever or by a push-button. 
     Parking assembly  10  includes a selectively and pivotally movable parking pawl  20 , a park rod  22 , and an actuator  24 , which selectively produces longitudinal displacement of park rod  22 . Actuator  24  is operatively and communicatively coupled to an electronically controlled valve assembly  26  (e.g., a solenoid valve) through a conduit  36 . Valve  26  is communicatively coupled to a source of pressurized fluid  28  (e.g., a conventional fluid reservoir and pump assembly) by a conduit  38 . Pressurized fluid received through conduit  36  controls the operation of actuator  24 . The actuator  24  may be actuated hydraulically, pneumatically, or electromagnetically, such by an electric motor. 
     Valve assembly  26  is further communicatively coupled to an electronic controller  30  by a bus  40 . Controller  30  controls the operation of valve  26  based upon data (e.g., vehicle speed data, gear selected and/or time data) received from vehicle sensors  32  and transmission range sensor  34 , which are respectively and communicatively coupled to controller  30  by use of busses  42 ,  44 . The sensors  32  are vehicle attribute sensors, which are effective to measure or sense certain vehicle attributes (i.e., vehicle speed data) and to communicate signals to controller  30  representing those measured attributes. Transmission range sensor  34  is further communicatively coupled to shifter  14  by bus  46  and is effective to sense or detect the position of shifter  14 . In other alternate embodiments, transmission range sensor  34  may be mechanically coupled to the shifter  14 . 
     Parking pawl  20  pivots on a park pawl shaft  50 , which is secured to a fixed component, such as a transmission housing. A torsion spring  52  continually applies a force to pawl  20  tending to disengage the pawl from the park gear  12  and to hold a rod-engaging surface  54  in contact with a contact member  56  on the park rod assembly  22 . Pawl  20  is formed with a tooth projection  58 , which selectively enters a recess  18  between successive teeth  16  on the park gear  12 , thereby preventing rotation of the park gear  12  and the vehicle&#39;s transmission output shaft  13 . 
     Park rod assembly  22  includes a park rod  60 ; a contact member  56 , which slides on park rod  60 ; and a spring member  62 , which is operatively disposed around the park rod and abuts contact member  56 . Park rod  60  is pivotally coupled to a park apply rod  64  by a ball and socket joint  66 . In this manner, park rod  60  is able to flex or move with respect to park apply rod  64  in the directions of arrow  68  when contact member  56  engages the surface  54  of pawl  20 . Contact member  56  slides on park rod  60  and is movable relative to park-apply rod  64  in the directions of arrows  70 ,  72 . One end of spring  62  abuts contact member  56  and the opposite end of spring  62  abuts a washer  74 , which is secured to park rod  60 . 
       FIG. 2  shows that actuator  24  includes a cam  80 , which secured to a housing  82  by a roll pin  86 , thereby preventing its rotation about a central, longitudinal axis  88 . The cam  80  is formed with a bore  90  containing a hydraulic piston  92 , which is connected by a key  93  against rotation relative to the cam yet allows axial displacement along axis  88 . Piston  92  is displaced leftward along axis  88  preferably by hydraulic pressure supplied through conduit  36  to the bore  90  of cam  80 . Piston  92  is displaced rightward by forces produced by a park apply spring  96  and a flat-tow spring  98  or by any suitable means, such as an electric motor, solenoid, or servo of any sort, etc. 
     Piston  92  moves among three states. A position sensor  94  is used to verify that the desired state is produced. 
     Piston movement from a park-state to a no-park-state requires constant application of leftward force to the piston  92 ; otherwise, the piston will return to the park-state, its default position or state due to the force of a park apply springs  96  and a flat-tow spring  98 . The spring rate of spring  96  is lower than the spring rate of spring  98 . When park-apply spring  96  is fully compressed, the load applied to park rod  60  per unit displacement of the park apply rod  64  increases due to the higher spring rate of the flat tow spring  98 , thereby allowing for some leeway in the actuation force required to reach the no-park state. 
     The actuator  24  is further enhanced by a damper orifice  100  formed in a piston  102 , which is secured to the park apply rod  64  and located axially between the park-apply spring  96  and the flat-tow spring  98 . The chamber  104  extends on opposite sides of piston  102  and contains a fluid, preferably hydraulic fluid. As the Park-apply rod  64  is displaced along axis  88 , fluid in chamber  104  flows through damper orifice  100 , thereby dissipating energy and stiffening the actuator  24  against very rapid and violent forces experienced during ratchet events of a park system that has been actuated while the park gear  12  is rotating. This stiffness prevents actuation of the flat-tow spring  98  inadvertently. Piston  102  functions similarly to a shock absorber, and is tuned preferably to prevent rapid, axial displacement and to control displacement of the park apply rod  64 . 
     The park apply rod  64  is connected preferably by a joint to the park rod  60 , to which contact member  56  and spring  62  are secured (see  FIG. 1 ). Although joint  66  is shown as a ball joint, other types of joints can be used as well. 
     Preferably a snap ring  106  secures the actuator  24  to the housing  82 , although other fastening techniques may be used. 
     In operation, when the shifter  14  is in the Park position and the vehicle is in a parked condition or state as determined by controller  30  in response to signals produced by sensors  32 ,  34 , valve assembly  26  is closed such that no hydraulic pressure is supplied through conduit  36  to chamber  108 . Then spring  96  expands, causing contact member  56  to engage parking pawl  20 , forcing the pawl to engage park gear  12 , and holding the vehicle driveline against rotation. 
     When shifter  14  is moved out of the Park position, sensor  34  produces a signal to controller  30 , which, determines whether a shift out of park is allowable. If so, valve assembly  26  opens, allows fluid pressure to enter chamber  108 , and causes piston  92  to move leftward, disengages pawl  20  from park gear  12 , and releases the park brake with the force of spring  52 . 
     A cross pin  110 , secured to park-apply rod  64  and extending diametrically through the park-apply rod, can rotate about axis  88  relative to the cam  80 . In  FIG. 2 , pin  110  is shown in both its vertical and horizontal positions. Springs  96 ,  98  continually apply rightward force to park-apply rod  64  through a washer  112 . 
     As shown in  FIGS. 6-9 , the left-hand end of piston  92  is formed with a series of serrations having peaks  114 ,  115 ,  116  and valleys  118 ,  119 . Similarly, the left-hand end of cam  80  is formed with a series of inclined surfaces  120 ,  122 , and an axial slot  124  sized to receive and allow pin  110  to move axially in the slot. These features of the piston  92  and cam are not shown in  FIG. 2  for clarity in showing the overall actuator assembly. 
     Movement of actuator  24  to and from the flat-tow/neutral position of  FIG. 6  is described next with reference to  FIGS. 3-9 , in view of  FIGS. 1 and 2 .  FIG. 3  shows piston  92  having pushed pin  110  to the edge of slot  124  on the serrated surface  126  of the piston in response to hydraulic pressure in chamber  108 . Additional leftward displacement of piston  92  frees pin  110  from slot  124 . 
       FIG. 4  shows piston  92  having freed pin  110  from slot  124  of cam  80 . Spring force F on park-apply rod  64  has urged pin  110  to rotate to the bottom of a recess between peaks  114 ,  115  of the serrations on piston  92 , since the pin  110  is no longer prevented by cam  80  from rotating. 
       FIG. 5  shows piston  92  having retracted rightward due to the spring forces F and reduced pressure in chamber  108  until pin  110  moves out of contact with the piston. Pin  110  contacts the serrated inclined surfaces  122  on cam  80 . The force F of park-apply spring  96  urges pin  110  to rotate further while contacting serrated surface  122 . 
       FIG. 6  shows piston  92  fully retracted due to pressure in chamber  108  being reduced substantially to zero, and pin  110  having rotated to its vertical, flat-tow position against inclined surface  122  of cam  80  due to the force F of the park-apply spring  96 . No source of power such as pressure in chamber  108  or electric power is required to maintain actuator  24  in its flat-tow position. 
       FIG. 7  shows piston  92  being stroked leftward from the fully retracted position of  FIG. 6  by increased hydraulic pressure in chamber  108  until pin  110  is separated from cam  80 . Then pin  110  is urged to one of the piston&#39;s valleys, causing pin  110  to rotate until contacts the inclined surface  120  of cam  80 . 
       FIG. 8  shows piston  92  fully retracted rightward, pin  110  urged to travel along inclined surface  120  into the Park, no-Park slot  124 .  FIG. 8  shows the actuator having completed a full cycle and the pin having rotated 180 degrees from the position of  FIG. 3 . 
     No matter how many times the actuator  24  is cycled between the park-state and no-park state, the park apply rod  64  does not index its location in the cam  80 , but remains in the same cam slot moving only axially. 
     When it is desired to obtain the flat tow/neutral state, park-apply rod  64  must be actuated to travel further so as to move the pin  110  in the park-apply rod out of the slot  124  in the cam  80 , and allow the pin  110  to fall to the valley in the piston  92 . Upon retraction, i.e., leftward movement, of piston  92 , pin  110  in the park apply rod  64  will come to rest in the flat-tow position of the cam  80 . This prevents the conventional park systems ratchet cam from touching the park pawl  20 . The force required to reach this state is greater than that required to actuate the Park state and the no-Park state. This allows one control force/pressure to obtain all three states of the system. 
     To return to the Park-state from the flat tow/neutral state, the piston  92  is simply stroked with the same pressure used to reach flat tow and the pin  110  is lifted from the cam valley and slides back into the next available slot  124  ready for Park engagement. 
     The actuator  24  can be cycled through this range of states any number of times to calibrate the sensor  94 , to verify its functionality, or for any other reason. 
     In accordance with the provisions of the patent statutes, the preferred embodiment has been described. However, it should be noted that the alternate embodiments can be practiced otherwise than as specifically illustrated and described.