A known hydraulic pressure value controller for an automatic transmission directly controls a hydraulic pressure value from a hydraulic pressure source by an electromagnetic valve and the hydraulic pressure value provided to friction engaging elements (friction clutch and friction brake), then each friction engaging element becomes engaged or disengaged. In Japanese Patent Laid-Open Publication No. 2002-295529, a clutch torque point leaning method for preferably controlling the electromagnetic valve to be opened and closed has been disclosed. Specifically, according to such clutch torque point learning method, a duty ratio of a duty pulse is learned by a electronic control unit (hereinbelow referred to as ECU) as a torque transmitting start point (torque point) for controlling the clutch to be moved rapidly within an allowance range from the point where the clutch starts to be moved to the point where the clutch becomes in engaging condition, and for further controlling the clutch to be moved slowly by shifting the connection speed once the clutch becomes in engaging condition.
As shown in FIG. 8, in the allowance range of a first half of a piston stroke of a clutch piston (hereinbelow referred to as a piston), the friction engaging element is quickly filled with fluid (pre-charge) for increasing a piston speed. After a predetermined time of the pre-charge, the piston speed is decelerated to almost “zero” on the verge of that the friction engaging element becomes in engaging condition. Then, the piston speed needs to be maintained at a low speed corresponding to the low hydraulic pressure value (stand-by hydraulic pressure value) being equivalent to a force of a return spring as a reaction force element of the friction engaging element for keeping the piston at stand-by position. In this way, the automatic transmission needs to be more improved in terms of responsiveness and followability. Thus, the operation speed of transmission may be improved, at the same time, a shift shock may be prevented.
Although, an actual pressure may differ from an instruction pressure due to a difference of the reaction force between each return spring on each product. In addition, such phenomena result from an influence from each element excepting a control valve's performance, for example, an influence of the output flow amount from an oil pump when a stroke of the piston is controlled by the hydraulic amount control, not by the hydraulic pressure control. Thus, a means for precisely detecting and setting the return spring equivalent pressure (stand-by hydraulic pressure value) needs to be presented. Specifically, if the return spring equivalent pressure can be set to a proper value, especially at a factory initial setup, for preferably maintaining the piston at the stand-by position thereof, manufacturing variations of the automatic transmission, an engine and the electromagnetic valve on each product which may cause an individual difference between each product can be absorbed. Thus, the quality of the automatic transmission will be secured.
A known means for setting the return spring equivalent pressure (stand-by hydraulic pressure) in consideration of absorbing the individual difference between each product is disclosed in Japanese Patent Laid-Open Publication No. 8-338519. In this reference, an engagement controlling device of the friction element is disclosed, which is actuated by hydraulic pressure for monitoring an input shaft, detecting a rotating number change, and adjusting the pressure at critical value before engaging force is generated. In addition, a known controlling device of the automatic transmission is also disclosed in Japanese Patent Laid-Open Publication No. 8-303568, which can adjust an initial engaging pressure (stand-by hydraulic pressure) at a preferable value based on an engagement starting pressure being required when the clutch starts engaging movement corresponding to a preferable neutral control.
The controlling device in Japanese Patent Laid-Open Publication No. 8-338519, however, may misjudge the rotation number difference of the input shaft due to fluctuation on engine rotating number or noise, in addition, a problem may be occurred that the rotating number of the input shaft may not be changed if an inappropriate pre-charge control has been done. Thus, the known means for setting the return spring equivalent pressure (stand-by hydraulic pressure) in consideration of absorbing the individual difference between each product as disclosed in Japanese Patent Laid-Open Publication No. 8-338519 is not applicable for initial setting even if it is applicable for a long-term learning.
In such controlling device of the automatic transmission being adjustable for the initial engaging pressure (stand-by hydraulic pressure) at the preferable value as disclosed in Japanese Patent Laid-Open Publication No. 8-303568, a threshold for detecting the engaging starting pressure on each automatic transmission needs to be changed in consideration of the manufacturing variation on each automatic transmission, in addition, an influence from temperature change (oil temperature change) needs to be considered. In addition, such controlling device is based on the preferable neutral control, so that such controlling device is applicable for detecting the stand-by hydraulic pressure of the friction engaging element requiring a small input torque such as a C1 clutch for changing from N to D and having the rotation change equal to or less than the rotation change of the input shaft, however, such controlling device is not applicable for detecting the stand-by hydraulic pressure of another friction engaging element because the detecting accuracy may be decreased due to significant rotation change.
In consideration of aforementioned problems, the present invention seeks to provide a stand-by hydraulic pressure value setting means being high accuracy and applicable regardless of various conditions, such as individual difference between each product or temperature change, and a automatic transmission including such stand-by hydraulic pressure value setting means.