Abstract:
A method for controlling brakes of an automatic transmission including a first brake arranged in parallel with an overrunning brake between a planet pinion carrier and a housing, includes using the overrunning brake to provide brake torque equal to or less than a reference brake torque under positive torque conditions, increasing a torque capacity of the first brake when brake torque is greater than the reference brake torque, and fully engaging the first brake.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a planetary gearing of an automatic transmission for a motor vehicle, and, more particularly to controlling a friction brake arranged in parallel with an overrunning brake. 
     2. Description of the Prior Art 
     Automatic transmissions typically include a one-way brake (OWB), sometimes referred to as overrunning brake, as a reaction element in first gear to produce non-synchronous 1-2 upshifts and 2-1 downshifts between first and second gears. The reaction element for first gear has a high torque ratio to input torque and therefore requires a large, heavy, costly one-way brake. 
     An increase is the number of transmission speed ratios increases the torque ratio on the reaction one-way brake. The torque ratios on the first gear reaction member for transmission having a range of speed ratios is a s follows: 4 speed˜1.8, 5-speed˜2.1, 6-speed 2.4, 8-speed˜2.7. The increased torque ratio requires significantly larger, heavier, and more costly one way brakes. 
     The first gear reaction one-way brake is usually arranged in parallel with a hydraulically-actuated friction brake, which is engaged in reverse gear and to provide engine braking in manually selected first gear operation. Although this brake may have sufficient torque capacity to carry first gear torque, it must be sized for its required torque ratio in reverse gear for the number of speed ratios produced by the transmission: 4-speed˜3.32, 5-speed˜3.8, 6-speed˜4.6, 8-speed˜5.9. 
     SUMMARY OF THE INVENTION 
     A method for controlling brakes of an automatic transmission including a first brake arranged in parallel with an overrunning brake between a planet pinion carrier and a housing, includes using the overrunning brake to provide brake torque equal to or less than a reference brake torque under positive torque conditions, increasing a torque capacity of the first brake when brake torque is greater than the reference brake torque, and fully engaging the first brake. 
     The brake control strategy improves the durability of an overrunning or one-way control transmission device, such as a rocker one-way brake in a planetary gear unit and produces high quality, non-synchronous 1-2 and 2-1 shift events. 
     The brake control permits the first or hydraulic brake and the overrunning brake to be used in first gear during high torque events without affecting non-synchronous 1-2 and 2-1 shift events. 
     Because the maximum torque transmitted by the one-way brake is reduced due to the effect of the hydraulic brake, the cost, weight, and package size of the one-way brake are reduced. 
     One-way brakes typically react at only one or two points causing unusual carrier deflections and higher gear tooth loads, whereas hydraulic brakes typically react 360 degrees around the planetary carrier causing a normal deflection pattern in the planetary carrier and improved gear tooth loads. Planetary carrier deflection is reduced and gear robustness improved. 
     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 cross section of a portion of the kinematic assembly for an automatic transmission that includes a Ravigneaux gear set; and 
         FIG. 2  illustrates a logic flow diagram of a strategy for controlling the friction brake shown in  FIG. 1 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a planetary gear assembly  10  of the Ravigneaux type located in a transmission housing  12  includes a first and second sun gears (not shown); a planet pinion carrier  14 , which is supported for rotation about an axis  16 ; long pinion shafts  18 , each spaced angularly about axis  16  and supported on carrier  14 ; a long planet pinions  20 , each spaced angularly about axis  16 , supported for rotation on one of the long pinion shafts  18  and meshing with one of the first sun gears; short pinion shafts  22 , spaced angularly about axis  16  and supported on carrier  14 ; a set of short planet pinions  24  supported for rotation on the short pinion shafts  22  and meshing with the second sun gears and the long pinions  20 ; and a ring gear  26  engaged with the long pinions  20  and arranged coaxially with the sun gears and pinions  20 ,  24 . An integral ring gear assembly comprising ring gear  26 , disc  27  and output shaft  28 , is supported for rotation about axis  16 . 
     The carrier  14  includes a first disc  30 ; a second disc  32 , spaced axially from the first disc; and a third disc  34 , located between discs  30 ,  32 . The carrier discs  30 ,  32 ,  34  support the pinion shafts  18 ,  22 , on which pinions  20 ,  24  are supported for rotation, respectively. 
     The transmission housing  12  is formed at an inner surface with axially directed spline teeth  40 , which are engaged with external spline teeth formed on the circumferential periphery of the outer ring  42  of a rocker one-way or overrunning brake  44 . An annular pilot recess on the inner ring  46  of brake  44  is engaged by carrier disc  32 . U.S. Pat. No. 7,383,930 discloses the structure and operation of a rocker one-way clutch similar to brake  44 . 
     A low/reverse hydraulically-actuated friction brake  50  includes discs  52 , splined at their outer periphery to housing  12 , and plates  54 , interleaved with discs  52  and splined at their inner periphery to the intermediate disc  34  of carrier  14 . A servo piston  55  moves axially leftward to force the discs  52  and plates  54  into mutual friction contact. The force of the actuating piston is reacted on the housing  12  due to the outer ring  42  being secured against axial displacement. 
     An overrunning brake alternately engages to secure a component of the kinematic arrangement against rotation in one direction relative the housing  12 , and disengages to release the component to rotate freely in the opposite direction. A overrunning clutch alternately engages to secure two components of the kinematic arrangement against relative rotation when the speed of a first component is greater than that of a second component in one direction and disengages to release the components to rotate relative to one another when the speed of the second component is greater than that of the first component. 
     Referring now to  FIG. 2 , which illustrates a control strategy that utilizes the hydraulic low/reverse brake  50  in first gear during high torque events to maintain torque load on the one-way brake  44  at an acceptable magnitude and to reduce deflections of the planetary carrier  14 . 
     At step  60  a test is made to determine whether the transmission is operating in first gear. If the result of test  60  is logically false, at step  62  the parallel assist control is terminated. If the result of test  60  is logically true, at step  64  the OWB  44  is in the powerflow, and parallel brake assist control is activated. 
     At step  66  a test is made to determine whether engine braking is desired in first gear, i.e., whether first gear was manually selected ( 1 M) or automatically selected ( 1 A). 
     If the result of test  66  is true, indicating that engine braking is desired and the transmission is in gear  1 M, at step  76  keep brake  50  stroked at all times, maintaining sufficient torque capacity in brake  50  to handle all negative torque conditions without allowing the brake to slip. For positive torque conditions, maintain pressure in brake  50  just above stoke pressure until the brake reaction torque carried by one-way brake  44  exceeds the brake torque at which parallel brake assist is desired, referred to as the reference brake reaction torque. 
     Under positive torque conditions, torque is transmitted from the engine through the output shaft  28  to the vehicle wheels. Under negative torque conditions, torque is transmitted from the vehicle wheels through the output shaft  28  to the engine, such as when the vehicle is coasting down hill. The one-way brake  44  overruns during negative torque conditions. 
     Brake  50  is alternately engaged and disengaged in response to the magnitude of pressure applied to a servo cylinder that actuates servo piston  55 . Stroke pressure is the magnitude of pressure applied to that servo such that clearances among the servo piston  55 , discs  52  and plates  54  are removed by moving them into mutual friction contact, but without transmitting torque through the brake. The low/reverse brake  50  has substantially no torque transmitting capacity when stroke pressure is present in the servo cylinder that actuates piston  55 . 
     At step  70  a test is made to determine whether a transition from a negative torque condition to a positive torque condition is occurring based on an engine torque signal. 
     If the result of test  70  is false, at step  72  actuating pressure in brake  50  is maintained at a low magnitude to prevent engine braking. 
     If the result of test  70  is true, at step  74  monitor the transmission gear ratio for overrunning, i.e., a ratio greater than the first gear ratio. Limit the rate of change of pressure in the low/reverse brake  50  until the first gear ratio is produced. This is necessary to prevent a possible bump or torque disturbance from the low reverse brake  50  that would increase engine speed faster than the transition from negative torque to positive torque is increasing engine speed via engine combustion torque. 
     If the result of test  66  is true, indicating that engine braking is desired and the transmission is in gear  1 M, at step  76  keep brake  50  stroked at all times, maintaining sufficient torque capacity in brake  50  to handle all negative torque conditions without allowing the brake to slip. For positive torque conditions, maintain pressure in brake  50  just above stoke pressure until the brake reaction torque carried by brake one-way  44  exceeds the brake torque at which parallel brake assist is desired, referred to as the reference brake reaction torque. 
     The reference brake reaction torque is determined with reference to the endurance limit, required service life, strength and performance characteristics of the one-way rocker brake  44  in addition to in-service deflections of the carrier  14  and NVH effects. 
     At step  78  a test is made to determine whether disengagement of low/reverse brake  50  is required such as when an upshift from first gear is being commanded. If the result of test  78  is true and regardless of whether the transmission is in operating in gear  1 M or gear  1 A, at step  80  pressure actuating the low/reverse brake  50  is reduced to a magnitude at which the brake is released and fully disengaged. 
     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.