Abstract:
A brake mechanism includes a pedal arm, a beam and a cam. The pedal arm is pivotally connected to the vehicle, while the beam is interposed between the pedal arm and the input rod for transmitting force from the pedal arm to the input rod. The beam is pivotally connected to the pedal arm and rotatable relative thereto. The cam defines a cam profile, and the beam contacts the cam and follows the cam profile as the pedal arm is activated. The cam profile is shaped to adjust the position of the beam relative to the pedal arm as the pedal arm swings relative to the vehicle. In this way, a variable force ratio is provided to maintain brake pedal feel while achieving an acceptable force ratio for failed power situations. The mechanism can be designed such that the force ratio does not drop off further into the pedal travel.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to brake pedal mechanisms for motor vehicles, and more particularly relates to braking mechanisms providing a variable force ratio. 
   BACKGROUND OF THE INVENTION 
   A brake pedal mechanism is usually employed to effectuate braking of a motor vehicle through its braking system. The braking system typically includes a brake booster which supplements the braking force provided by the vehicle operator, which in turn operates a hydraulic master cylinder for pressurizing fluid and the brake lines and applying a braking force to the wheels of the vehicle via individual wheel brakes. The brake pedal mechanism is typically a simple lever, wherein a pedal arm has the brake pedal at one end while the opposing end is pivotally connected to the vehicle frame. An input rod leading to the brake booster is connected to the pedal arm, and based on the position of its connection to the pedal arm, the lever action of the pedal arm increases the output force generated by the input force on the brake pedal, i.e. forms a force ratio of the output force divided by the input force. 
   One common problem with these typical braking mechanisms is that performance of the brake system often ends in a compromise between having enough force capability to stop the vehicle under mandated failure mode conditions and not having too much pedal travel to maintain good pedal feel. That is because to increase the force ratio the distance the pedal must travel must be increased. If a vehicle is marginal in meeting a deceleration requirement with a given brake pedal input force, a typical step is to increase the brake pedal ratio, resulting in greater braking force and vehicle deceleration. Unfortunately, increasing the force ratio increases the pedal input travel required to meet the same rate of deceleration. 
   Some designs have attempted to address the problem by providing a multiple link brake pedal which lowers the pedal travel initially, when the pedal is at low stroke and relatively low force values. The concept is that an increase in the force required in this range is acceptable since initial forces are low. Unfortunately, the nature of the linkages results in an overshoot of the ratio required at failed power conditions. This also results in some of the pedal stroke gained initially being lost during the remainder of the pedal travel. That is, the force ratio will steadily increase with pedal travel, but then quickly drops off as pedal travel increases further towards a fully extended position. Thus, the brake system parameters become extremely critical since it must be assured that the failure conditions occur only in the pedal travel zone which provide an adequate force ratio. 
   Accordingly, there exists a need to provide a simple brake mechanism for use with the braking system in a motor vehicle which does not have too much pedal travel to maintain good pedal feel, but yet provides a sufficient force ratio under failure mode conditions to provide sufficient force capability to stop the vehicle. Ideally, such a brake mechanism also eliminates the unwanted drop off in force ratio as the pedal travel increases towards a fully extended position. 
   BRIEF SUMMARY OF THE INVENTION 
   One embodiment of the present invention provides a brake mechanism for a braking system in a motor vehicle. The braking system includes an input rod for effecting actuation of vehicle brakes. The brake mechanism includes a pedal arm, a beam and a cam. The pedal arm is pivotally connected to the vehicle, while the beam is interposed between the pedal arm and the input rod for transmitting force from the pedal arm to the input rod. The beam is pivotally connected to the pedal arm and rotatable relative thereto. The cam defines a cam profile, and the beam contacts the cam and follows the cam profile as the pedal arm is activated. The cam profile is shaped to adjust the position of the beam relative to the pedal arm as the pedal arm swings relative to the vehicle. In this way, a variable force ratio is provided to maintain brake pedal feel while achieving an acceptable force ratio for failed power situations. Further, the mechanism can be designed such that the force ratio does not drop off further into the pedal travel. 
   According to more detailed aspects, the position of the beam relative to the pedal arm determines the force ratio of the brake mechanism. The beam is pivotally connected to the input rod, and the beam rotates relative to the input rod as the pedal arm swings relative to the vehicle. The pedal arm travels between at least a neutral position and an extended position, and the force ratio quickly increases as the pedal arm travels beyond a predetermined point past the neutral position to the extended position. The force ratio at the extended position is sufficient for vehicle braking in a failed power situation. In one embodiment, the beam is generally perpendicular to the input rod when the pedal arm is in the neutral position, and the beam is aligned with the input rod when the pedal arm is in the extended position. The cam profile includes a first portion generally perpendicular to the input rod and a second portion generally parallel to the input rod. The cam profile preferably includes a third portion connecting the first and second portions, the third portion being curved in shape. 
   Another embodiment of the present invention provides a brake mechanism for a braking system in a motor vehicle, the braking system including an input rod for transmitting force to a master cylinder for pressurizing braking fluid in brake lines leading to wheel brakes. The brake mechanism includes a pedal arm pivotally connected to the vehicle, the pedal arm receiving an input force from an operator of the vehicle. A beam is pivotally connected to the pedal arm at a first point along the beam. The beam is connected to the input rod at a second point along the beam. The beam transmits force from the pedal arm to the input rod. A cam has a surface defining a cam profile. The beam contacts the cam at a third point along the beam and follows the cam profile. The beam pivots relative to the pedal arm as the beam follows the cam profile. Again, a variable force ratio is provided to maintain brake pedal feel while achieving an acceptable force ratio for failed power situations, without unwanted drop-offs in the force ratio. 
   According to more detailed aspects, the position of the beam relative to the pedal arm is determined by the shape of the cam profile. Likewise, the position of the second point relative to the first point is determined by the shape of the cam profile. The position of the second point relative to the first point determines the force ratio of the braking mechanism. The second point moves from a position vertically below the first point to a position substantially horizontally aligned with the first point. The pedal arm travels between a non-braked position and a braked position, and wherein the second point moves vertically upward as the pedal arm travels from the non-braked position to the braked position. The cam profile preferably includes a substantially vertical surface transitioning into a substantially horizontal surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: 
       FIG. 1  is a schematic view of the brake mechanism constructed in accordance with the teachings of the present invention; 
       FIG. 2  is a detailed front view of a brake mechanism constructed in accordance with the teachings of the present invention; 
       FIGS. 3-6  are detailed view of specific components forming the brake mechanism depicted in  FIG. 2 ; 
       FIGS. 7-11  are a series of side views schematically depicting the operation of the brake mechanism depicted in  FIG. 1  through the range of pedal travel; and 
       FIG. 12  is a graph showing the force ratio in relation to travel of the input rod for the brake mechanism depicted in  FIGS. 1 and 2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Turning now to the figures,  FIG. 1  depicts a schematic illustration of a brake mechanism  20  constructed in accordance with the teachings of the present invention. The brake mechanism  20  comprises a portion of the braking system in a motor vehicle, the braking system including an input rod  32  for effecting actuation of the vehicle brakes, typically through a brake booster  16  and a master cylinder  18  pressurizing fluid in the brake lines leading to the individual wheel brakes. 
   The brake mechanism  20  generally includes a pedal arm  22  having a brake pedal  24  at a first end of the pedal arm  22  which is depressed by. A pivotal connection  26  is made at the opposing end of the pedal arm  22  for rotatably connecting the pedal arm  22  to the frame  28  of the vehicle. The operator of the vehicle provides an input force  30  to the brake pedal  24  which cause the pedal arm  22  to rotate about its pivotal connection  26 . The input force  30  is transmitted to the input rod  32  for providing an output force  34  and effecting actuation of the vehicle brakes. 
   An idler beam  36  is interposed between the pedal arm  22  and input rod  32 . The beam  36  is pivotally connected to the pedal arm  22  at pivotal connection  38 . A first end of the beam  36  makes a pivotal connection  40  with the input rod  32 . The opposing end of the beam  36  includes a roller  44  which engages a cam  42  for regulating the position of the beam  36  and hence the transmission of force from the pedal arm  22  to the input rod  32 , as will be described in more detail herein. In the neutral un-braked position shown in  FIG. 1 , the pressure in the master cylinder  18  and booster  16  biases the input rod  32  to the left in the figure, thereby pressing the roller  44  against the cam  42  in the neutral position. 
   The cam  42  provides a cam surface  46  on which the roller  44  rides, thereby controlling the position of the beam  36  as the pedal arm  22  rotates through its travel path. It will be recognized that the cam  42  could provide a track which the beam  36  would engage and follow through the pedal travel. Other engagement mechanisms allowing relative movement will also be readily envisioned by those skilled in the art. 
   Turning now to  FIG. 2 , a detailed front view of an embodiment of the brake mechanism  20  is depicted. It can be seen that the pedal arm  22  is pivotally connected to the frame  28  at pivotal connection  26 . The frame  28  is also utilized to allow the cam  42  to be positioned relative to the rest of the mechanism  20  using a channel  50  or other supporting structure which is attached to the frame  28  in any known manner. Details of the cam  42  can be found in  FIG. 3 . The cam  42  includes a flange  51  for connecting the cam  42  to the support channel  50 . An end  45  of the cam  42  defines the cam surface  46  which includes an upper portion  60  which is substantially vertical, a second portion  62  which is substantially horizontal, and a third portion  64  which is curved and links the first portion  60  to the second portion  62 . 
   The idler beam  36  is comprised of two primary pieces, namely a crank  54  and an offset pin  56 . Details of the offset pin  56  can be found in  FIG. 4 . Details of the crank  54  can be found in  FIGS. 5 and 6 . The offset pin  56  includes a first portion  66  which is fit through a pedal arm  22  to provide the pivotal connection  38  thereto about axis  55 . A second portion  68  of the offset pin  56  is separated from the first portion by a flange  70 , and the second portion  68  is structured to connect to the crank  54 . The crank  54  includes a first bore  72  which is sized and positioned to receive the roller  44  ( FIG. 2 ) for engaging the cam surface  46 . A second bore  74  is formed in the crank  54  for receiving the second portion  68  of the offset pin  56 . 
   A support arm  52  is connected to the pedal arm  22  for providing additional support to the idler beam  36 . A pin portion  78  of the crank  54  is structured and positioned to extend through the support arm  52 , also providing a pivotal connection therewith about an axis denoted by line  55  in  FIG. 2 . Accordingly, the pivotal connection  38  is formed by both the offset pin  56  (via first portion  66 ) and the crank  54  (via pin portion  78 ), which are pivotally attached to the pedal arm  22  and support arm  52 , respectively. Finally, the crank  54  includes a notch  76  (best seen in  FIG. 6 ) which is sized to receive the cam  42 , as well as to expose the bore  72  and more particularly the roller  44  to the cam surface  46  for engagement therewith. 
   Accordingly, as best seen in  FIG. 2 , the idler beam  36  comprises the offset pin  56  attached to the crank  54 , the combination of which is pivotally mounted to the pedal arm  22  via pin portions  66  and  78 . The second portion  58  of the offset pin  56  forms the pivotal connection  40  with the input rod  32  via the brake switch  80 . The brake switch  80  is directly connected to the input rod  32  and provides a signal of braking (i.e. for the taillights), and transfers force from the idler beam  36  to the input rod  32 . The brake switch  80  is fitted over the second portion  68  of the offset pin, thereby forming the pivotal connection  40  on a second rotational axis  57  that is offset from the rotational axis  55  of the first pivotal connection  38 . 
   Operation of the brake mechanism  20  will now be described with reference to  FIGS. 7-11 , which depicts a sequence of illustrations showing the travel of the pedal arm  22  (and thus travel of the input rod  32 ).  FIG. 7  depicts the pedal arm  22  and brake pedal  24  in an unbraked or neutral position. In this position, the pedal  24  is located a distance A from the pivotal connection  28 , while pivotal connection  40  (i.e. the point of force transmission in input rod  32 ) is located a distance B 1 , from pivotal connection  28 . This gives a force ratio of A/B 1 , for the neutral or initial un-braked position. As previously described, the input rod  32  is biased towards the pedal arm  22  resulting in the idler beam  36  taking the position shown in  FIG. 7 , i.e. having its roller  44  engaging the cam surface  46 . 
   As the vehicle operator places an input force on the brake pedal  24 , the pedal arm  22  rotates towards the input rod  32 . The force is transmitted through the beam  36  to the input rod  32 , which moves to the right in  FIG. 8 . It can be seen that the beam  36  follows with the roller  44  engaging the cam surface, causing the beam  36  to rotate counterclockwise. Since the beam  36  has rotated, it will also be recognized that the pivotal connection  40  has rotated relative to the point  38 . Thus, the pivotal connection  40  is now closer to pivotal connection  28 , giving a shorter distance B 2  and a greater force ratio equal to A/B 2 . 
   Moving to  FIG. 9 , the pedal arm  22  has been rotated further into its path of travel, with the force continuing to be transmitted to the input rod  32  causing the rod to move to the right in the figure. Again, the idler beam  36  also rotates as it is forced to the right in the figure by the pedal arm  22 , while the engagement of the roller  44  with the first vertical portion  60  of the cam surface  46  determines the amount of rotation of the beam  36 . Likewise, the pivotal connection  40  has again rotated relative to the pivotal connection  38 , and specifically the connection point  40  continues to move upwardly or vertically relative to the pivotal connection  38  and closer to pivotal connection  28  (distance denoted by B 3 ). Thus a greater force ratio A/B 3  is generated. 
   In  FIG. 10 , the pedal arm  22  and input rod  32  continue to move to the right through the pedal travel, while the beam  36  continues to rotate as dictated by the cam surface  46 . Here, the roller  44  has reached the curved surface  64  of the cam while the pivot point  40  continues to move vertically relative to the pivot point  38  as the beam  36  rotates counterclockwise. The distance B 4  is likewise shorter, giving a greater force ration of A/B 4 . Finally, in  FIG. 11 , the brake pedal  24  and pedal arm  22  have reached an extended position where the roller  44  has reached the substantially horizontal surface  62  of the cam profile  46 , resulting in the beam  36  being fully rotated and substantially parallel to and aligned with the input rod  32 . It will also be recognized that the pivot point  40  has been rotated to a position substantially vertically equal to the pivot point  38 , resulting in the highest force ratio for the brake mechanism  20 . That is, the distance B 5  is at its shortest, and remains relatively unchanged with further pedal travel to give a force ratio A/B 5  at its greatest value. 
   The effect of the idler beam  36  and the shape of the cam  42  on the force ratio of brake mechanism  20  has been depicted in the graph of  FIG. 12 . The input rod travel  32  is depicted on the X-axis as a percentage of total travel. The force ratio (force out divided by force in) is shown in the Y-axis, and the line  82  represents the force ratio throughout the input rod travel. As the input rod  32  travels from 0 to 10%, it can be seen that the force ratio only slightly increases. At a first transition point  84 , the force ratio begins to quickly increase during the input rod travel of about 10% to about 22%. At the second transition point  86 , the beam  36  has fully rotated (as shown in  FIG. 11 ), and the force ratio is set at its highest value of about 4. The extended horizontal surface  62  of the cam profile  46  maintains this force ratio throughout the remainder of the input rod travel. 
   Based on the foregoing, it will be recognized by those skilled in the art that by interposing an idler beam  36  between the pedal arm  22  and input rod  32 , the force ratio of the brake mechanism  20  may be adjusted based on the relative rotational position of the beam  36 . The rotation of the beam  36  through the pedal travel or input rod travel is determined by the cam  42  and its cam profile  46 . Accordingly, the cam profile  46  may be designed and selected to achieve any desired characteristics of the brake mechanism  20 , but the preferred embodiment has been selected to provide a rapid increase in force ratio until a certain point in pedal travel wherein the force ratio is maintained at a constant value that meets failed power requirements. Further, this decreases the initial pedal travel providing increased pedal feel to the vehicle operator. It will also be recognized that the particular point along the idler beam  36  at which the pedal arm  22  and input rod  32  are connected may be adjusted to achieve certain results or certain curvatures in the force ratio graph. 
   The foregoing description of various embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Numerous modifications or variations are possible in light of the above teachings. The embodiments discussed were chosen and described to provide the best illustration of the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.