Hydraulic control system of automatic transmission

In an automatic transmission of a motor vehicle, in which a hydraulic pressure is supplied to a piston of a friction element is maintained at a first level that is lower than a given level at which the friction element is engaged, while the vehicle that is in a forward-drive range is being stopped, when the range of the automatic transmission is changed from a neutral range to the forward-drive range the hydraulic pressure having a second level that is higher than the first level is supplied to the piston of the friction element until a completion of the stroke of the piston is detected. The stroke completion of the piston is judged based on a predetermined difference between a turbine speed of the automatic transmission detected upon the change of the range and a current turbine speed.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a hydraulic control system of an automatic
 transmission of a motor vehicle.
 2. Description of the Prior Art
 A known example of hydraulic control system of an automatic transmission is
 disclosed in Japanese laid-open Patent Publication No. 62-18336. In this
 hydraulic control system, a creep preventive device controls a solenoid
 for neutral control when the position of the select lever is changed from
 a neutral range to one of forward-drive ranges, so that the line pressure
 is supplied as it is to a start clutch (forward-drive friction engaging
 element) until a piston stroke is completed. After the piston stroke is
 completed, the clutch engaging pressure supplied to the start clutch is
 reduced or the supply of the pressure is temporarily stopped, so that the
 pressure of the start clutch is controlled to a low level at which the
 clutch does not have an engaging capacity.
 With this arrangement, the vehicle is prevented from creeping while it is
 in one of drive ranges, and the supply of the line pressure can always be
 finished at a point of time when the start clutch is placed in the same
 engaging condition, thereby preventing an engaging shock and racing of the
 engine.
 In the known hydraulic control device of the automatic transmission,
 however, the completion of the piston stroke is determined when a
 difference between the engine rotating speed and the turbine rotating
 speed detected at a certain moment becomes larger than a predetermined
 value after the range is changed. If the engine speed (Ne) changes due to
 an increase in the idling speed (idle up) during the piston stroke,
 therefore, it is possible that the piston stroke of the forward-drive
 friction engaging element (FWD/C) is judged by mistake as being finished
 at a point of time when the difference between the engine speed and the
 turbine speed (Nt) is increased, as shown in FIG. 4, even though the real
 piston stroke is not finished yet. In this case, the supply of the
 pressure to the start clutch is stopped or reduced. If the driver tries to
 start the vehicle at the time when the judgement is made by such mistake,
 an engaging shock or racing of the engine occurs due to a delay in
 engagement of the forward-drive friction engaging element, and the vehicle
 may not be smoothly or readily started.
 SUMMARY OF THE INVENTION
 The present invention was developed so as to solve the above-described
 problem, by relating detection of the piston stroke, only with the turbine
 speed. Namely, the present invention provides a hydraulic control system
 of an automatic transmission of a motor vehicle, which includes a friction
 element (15) that is engaged when the vehicle is in a forward-drive range,
 and detecting means (forward-drive friction element piston stroke
 detecting means) (22) for detecting a stroke of a piston of the friction
 element (15) from a first position for releasing the friction element, to
 a second position for engaging the friction element, when the range of the
 automatic transmission is changed from a neutral range to the
 forward-drive range. The hydraulic control system further includes control
 means (forward-drive friction element control means (18)) that controls a
 hydraulic pressure supplied to the piston of the friction element (15), to
 a first level lower than a given level at which the friction element (15)
 has an engaging capacity, while the vehicle in the forward-drive range is
 being stopped. Further, the detecting means (22) for detecting the stroke
 of the piston of the friction element (15) generates a stroke completion
 signal (S) to the control means (18) when a difference between a turbine
 speed (TbnREV0) of the automatic transmission detected upon a change of
 the range, and a current turbine speed (TbnREV), is larger than a
 predetermined value (TbnREV1). The control means (18) supplies a hydraulic
 pressure having a second level that is higher than the first level, to the
 piston of the friction element (15), until the control means (18) receives
 the stroke completion signal (S).
 The reference numerals in parentheses are those of corresponding elements
 in one embodiment of the present invention as described later.
 If the idling speed of the engine is increased (idle up) during the stroke
 of the piston of the friction element from the releasing position to the
 engaging position when the vehicle is changed from the neutral range to
 the forward drive range, the turbine rotating speed is temporarily
 increased with a delay, following an increase of the engine speed, but the
 turbine speed is gradually lowered if the piston is in the middle of the
 stroke.
 By judging completion of the stroke only based on the turbine speed,
 therefore, a misjudgement on completion of the piston stroke based on the
 increase of the engine speed due to idle up can be prevented, and the
 completion of the piston stroke can be more accurately detected. Thus, the
 hydraulic control system of the present invention can surely avoid
 engaging shocks and racing of the engine while preventing the vehicle from
 creeping when the position of the select lever is changed from the neutral
 range to the forward-drive range.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 FIG. 1 shows one preferred embodiment of the present invention. An engine
 speed sensor 12 measures a rotating speed NE of an engine (ENG) 10 of the
 vehicle, and a turbine speed sensor NT measures a turbine rotating speed
 NT of an automatic transmission (AT) 14. Forward-drive friction element
 control means 18 receives signals EngREV and TbnREV that represent the
 engine speed and the turbine speed, respectively. An inhibitor switch 20
 serves to detect a currently selected range of the automatic transmission
 14, and a signal indicative of the detected range is received by the
 forward-drive friction element control means 18 and forward-drive friction
 element piston stroke detecting means 22. The forward-drive friction
 element piston stroke detecting means 22 also receives the above-indicated
 signal TbnREV from the turbine speed sensor 16.
 As described later, the forward-drive friction element piston stroke
 detecting means 22 detects completion of a stroke of a piston of a
 forward-drive friction element (FWD/C: friction element) 15 of the
 automatic transmission 14, and generates a detection signal S to the
 forward-drive friction element control means 18. Based on these input
 signals, the forward-drive friction element control means 18 generates a
 control signal Duty% to a duty solenoid for neutral control(DUTY SOL) 24.
 The duty solenoid for neutral control 24 is provided independently from a
 base solenoid(not shown) which controls the line pressure in accordance
 with driving conditions. The duty solenoid for neutral control 24 then
 controls the level of the line pressure to be supplied to the piston of
 the forward-drive friction element 15 during neutral control, in
 accordance with the control signal Duty%.
 The forward-drive friction element control means 18 also receives a vehicle
 speed Vsp detected by a vehicle speed sensor installed on the vehicle, and
 a throttle opening Tvo detected by a throttle opening sensor.
 The operation of the present invention will be now described, with
 reference to the flow diagram of FIG. 2.
 Initially, step 50 is executed to check if the currently selected range
 detected by the inhibitor switch 20 (FIG. 1) is one of forward-drive
 ranges (D, 2, and 1 in this embodiment). If a negative decision (NO) is
 obtained in step 50, namely, if any of N, P and R ranges is being
 selected, step 76 is executed to set a select N-D flag to "0" for
 inhibiting neutral control. In next step 78, the duty % of the duty
 solenoid 24 is controlled to 0%, and the control flow goes to step 74 as
 described later. If an affirmative decision (YES) is obtained in step 50,
 step 52 is executed to check if the range stored in the previous cycle is
 N range or not.
 If an affirmative decision (YES) is obtained in step 52, step 54 is
 executed to set the select N-D flag to "1", and store the turbine speed
 TbnREV0 detected by the turbine speed sensor 16 immediately after
 selecting one of the forward-drive ranges. If a negative decision (NO) is
 obtained in step 52, the control flow goes to step 56 (judgement on the
 vehicle speed).
 In step 56, it is determined whether the vehicle speed Vsp detected by the
 vehicle sensor is smaller than a preset vehicle speed Vspl for neutral
 control. If a negative decision (NO) is obtained in step 56, the neutral
 control is inhibited, and the control flow goes to the above-described
 step 76 to set the select N-D flag to "0", and the base solenoid for
 controlling the line pressure is controlled in accordance with driving
 conditions. If an affirmative decision (YES) is obtained in step 56, a
 suitable vehicle speed condition is established, and the control flow goes
 to step S58 to make a judgement on the throttle opening. Step 58 checks if
 the throttle opening Tvo detected by the throttle opening sensor is
 smaller than a preset throttle opening Tvol for neutral control.
 If a negative decision (NO) is obtained in step 58, the above step 76
 (setting the select N-D flag to "0") and following steps are executed so
 as to inhibit the neutral control. If an affirmative decision (YES) is
 obtained in step 58, suitable vehicle speed condition and throttle opening
 condition are established, and step 60 is executed to check if the select
 N-D flag is set to "1".
 If an affirmative decision (YES) is obtained in step 60, step 62 is
 executed to calculate a turbine speed TbnREV1 {=f1(EngREV)} used for
 detecting the piston stroke, based on the current engine speed EngREV
 measured by the engine speed sensor 12. Here, "f1" is a predetermined
 first coefficient. If a negative decision (NO) is obtained in step 60, the
 control flow goes to step 66 to set the select N-D flag as described
 later.
 After execution of step 62, step 64 is executed to check if a difference
 between the turbine speed TbnREV0 detected immediately after selecting the
 forward-drive range and the current turbine speed TbnREV is larger than
 the above-indicated turbine speed TbnREV1 for detecting the piston stroke.
 If a negative decision (NO) is obtained in step 64, the piston is supposed
 to be on the way of stroke, thus the neutral control is not adopted, and
 the control flow goes to the above-indicated step 78 to set the duty % to
 0.
 If an affirmative decision (YES) is obtained in step 64, the forward-drive
 friction element piston stroke detecting means 22 determines that the
 stroke of the piston of the forward-drive friction element 15 has been
 completed, and generates a completion signal S to the forward-drive
 friction element control means 18, and step 66 is then executed to set the
 select N-D flag to "0" to start the neutral control through the following
 steps.
 In the next step 68, the target engine speed Target for neutral control is
 calculated from the current engine speed EngREV {f2(EngREV)}, where "f2"
 is a predetermined second coefficient.
 Step 70 is then executed to obtain a deviation Err of the target engine
 speed Target for neutral control, with respect to a difference between the
 current engine speed EngREV and the current turbine speed TbnREV. Then,
 step 72 is executed to calculate the duty % {=f3(Err)} of the duty
 solenoid for neutral control 24 so that this deviation Err becomes equal
 to "0", and the forward-drive friction element control means 18 controls
 the duty solenoid 24 based on the duty % thus calculated. Here, "f3" is a
 predetermined third coefficient.
 Finally, step 74 is executed to store the range detected in the current
 cycle as "range in the previous cycle", and the current control cycle is
 terminated.
 In the neutral control as described above, the duty% of the duty solenoid
 24 is controlled so that the hydraulic pressure applied to the
 forward-drive friction element 15 is kept at a low level just below the
 level at which the friction element 15 has an engaging capacity, namely,
 the deviation Err is made equal to "0". This is because, if the hydraulic
 pressure of the forward-drive friction element 15 is controlled to be
 zero, it takes a lot of time until the forward-drive friction element 15
 has an engaging capacity, which results in racing of the engine 10 as
 shown in FIG. 4. The engine racing may be avoided by suitably controlling
 the engine, as disclosed in laid-open Japanese Patent Publication No.
 3-82638. This method, however, does not solve the problem of engine racing
 when the engine is in a condition where its output cannot be controlled.
 The neutral control is inhibited until the stroke of the piston of the
 forward-drive friction element 15 is completed, because it takes a
 relatively long time to charge the friction element 15 and the completion
 of the piston stroke is delayed if the duty% is set to a small value to
 apply a low hydraulic pressure to the friction element 15 during the
 piston stroke. Also, racing of the engine 10 may occur if the accelerator
 pedal is depressed during the piston stroke.
 By performing the control as described above, the pressure applied to the
 forward-drive friction element 15 changes in quick response to duty-ratio
 control of a solenoid (SOL) for idle control as shown in FIG. 3, and the
 turbine speed (Nt) of the torque converter also changes smoothly, thus
 avoiding a sudden change in the rotating speed (Ne) of the engine 10.
 Thus, racing of the engine 10 can be prevented.
 According to the present invention as explained above, neutral control is
 executed to prevent creeping of the vehicle when the select lever is
 changed from the neutral range to one of drive ranges, in which control
 the hydraulic pressure supplied to the friction element which is engaged
 in a forward-drive range is controlled at a low level just below the level
 at which the friction element has an engaging capacity, and wherein
 completion of the piston stroke of the friction element is judged when the
 difference between the turbine speed of the transmission detected upon the
 change of the range and the current turbine speed reaches the
 predetermined value to start the neutral control. Thus, even if the idle
 up of the engine occurs during the piston stroke, the start timing of the
 neutral control is not misjudged, so that the engine is prevented from
 racing and the automatic transmission is prevented from engaging shock of
 the friction element. And also, since a higher hydraulic pressure is
 supplied to the friction element up to the start of neutral control as
 compared with during the neutral control, the piston stroke can be
 promptly completed.