Control system for continuously variable transmission

Details Control system for continuously variable transmission Control system for continuously variable transmission

1998-07-16

2001-04-17

Wright, Dirk (Department: 3681)

Endless belt power transmission systems or components

Pulley with belt-receiving groove formed by drive faces on...

Fluid pressure actuator for adjustment of member

C477S049000, C477S045000, C477S018000, C477S013000, C474S018000, C474S013000

Reexamination Certificate

active

06217469

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to improvements in a control system of a continuously variable transmission (CVT) for a vehicle, and more particularly to a control system of a belt type CVT.
A Japanese Patent Provisional Publication No. 8-74959 discloses a line pressure control system for a belt type CVT. The conventional line pressure control system is arranged to control a duty valve by outputting a duty ratio indicative signal as a command value to the duty valve in order to control a belt holding force of the pulley of the CVT. The command value to the duty valve is limited within a range except dead zones so as to ensure the responsibility of the line pressure applied to the CVT.
SUMMARY OF THE INVENTION
However, if a limit value of the command value is simply set at a boundary value of the dead zone, the output pressure of the duty valve tends to fluctuate though the responsibility of the line pressure control is maintained. On the other hand, higher the rotation speed of the pulleys of the belt type CVT becomes, higher a centrifugal pressure of the working fluid in the pulleys becomes. When the rotation speed of the pulley becomes greater than a predetermined value, the belt is held by the centrifugal pressure through the pulley regardless the line pressure so as to prevent a shift control and a line pressure control from being accurately executed in such a centrifugal pressure dominant range. Therefore, it is necessary to omit the centrifugal pressure dominant range from a transmission ratio control range. Further, it is necessary to extend the centrifugal pressure dominant range upon taking account of the dispersion caused by the fluctuation of the output pressure. As a result, the transmission ratio control range is decreased by the margin for the dispersion due to the characteristic of the duty valve.
It is therefore an object of the present invention to provide a CVT control system which extends the controllable range of a CVT transmission ratio without degrading the responsibility of a line pressure for controlling the CVT.
A control system according to the present invention comprises a continuously variable transmission (CVT), a CVT pressure control valve, a centrifugal pressure detecting means, a centrifugal pressure deciding means, and a limit value switching means. The CVT comprises a driver pulley, a follower pulley and a belt intercoupling the pulleys. The CVT varies a transmission ratio by changing effective diameters of the pulleys. The CVT pressure control valve controls a pressure of working fluid to be supplied to the CVT according to a command signal. The centrifugal pressure detecting means detects a centrifugal pressure of the working fluid in the pulley. The centrifugal pressure deciding means decides that the centrifugal pressure is in a centrifugal pressure dominant range in which motive power is transmitted only by the centrifugal pressure. The limit value switching means switches a limit value of the command signal from a high response value to a high stability value when the centrifugal pressure detecting means detects the centrifugal pressure dominant range.


REFERENCES:
patent: 4857034 (1989-08-01), Kouno et al.
patent: 5157992 (1992-10-01), Hayashi et al.
patent: 5203233 (1993-04-01), Hattori et al.
patent: 5337628 (1994-08-01), Hendriks et al.
patent: 5458540 (1995-10-01), Moan et al.
patent: 5514047 (1996-05-01), Tibbles et al.
patent: 5697866 (1997-12-01), Okahara
patent: 8-74959 (1996-03-01), None
patent: 8-200461 (1996-08-01), None

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