Hydraulic control system for a continuously variable transmission

• US 20010023217A1
• Filed: 03/14/2001
• Published: 09/20/2001
• Est. Priority Date: 03/17/2000
• Status: Active Grant

• Current Assignee: Transtechnology Ltd.
• Original Assignee: Transtechnology Ltd.
• Inventors: Sakakibara, Satoshi, Miyagawa, Yoshikazu
• Granted Patent: US 6,547,694 B2
• Time in Patent Office: Days
• US Class Current: 477/45
• CPC Class Codes: F16H 61/66259 using electrical or electronical sensing or control means

Abstract

A transmission mechanism of the continuously variable transmission comprises a set of primary and secondary pulleys between which a belt is provided. The CVT control unit is operable to supply line pressure directly to a secondary pulley and also to a primary pulley via a shift control valve associated with a step motor in such a manner as to control a transmission ratio. The CVT control unit calculates a standard step position of the step motor corresponding to the target transmission ratio and provides a command signal representative of a difference between the standard step position and the current position of the step motor. The shift control valve actuated by the step motor controls the pressure to be supplied to the primary pulley. The actual transmission ratio obtained on the basis of the rotation speeds of the pulleys is compared with the target transmission ratio. If the shift is not accomplished, the CVT control unit commands additional step number. Unless the shift to the HI side is accomplished at the time when the additional step number reaches the threshold value, the line pressure is increased so as to achieve the shift. Thus, the transmission normally operates with a low line pressure sufficiently to reduce a frictional loss at the contact areas of the belt and the pulleys, thereby resulting in an enhanced fuel economy.

Claims

1. A hydraulic control system for a continuously variable transmission comprising a transmission mechanism which includes a set of adjustable first and second pulleys having first and second cylinder chambers, respectively, width of respective pulleys being variable according to hydraulic pressure applied to said cylinder chambers, and a belt provided between the pulleys so as to transmit torque therebetween;

means for producing a line pressure and continuously supplying the line pressure to said second chamber;

a shift control valve adapted to be operated under the action of an actuator associated therewith to supply a pressure which is reduced from said line pressure to the first cylinder chamber; and

shift control means for generating a shift command to the actuator depending on a driving condition and modifying the line pressure,

characterized in that said shift control means includes means for determining whether or not said shift is accomplished, said shift control means being operable to raise the line pressure from a first level to a second level when said determining means determines that the shift to the HI side is not accomplished.

2. A hydraulic control system for a continuously variable transmission according to

claim 1

wherein said actuator comprises a step motor which is adapted to be driven to an expected step position corresponding to a target transmission ratio to be established, said shift control means being operable to drive the step motor additionally in increments of a predetermined number of step and to raise the line pressure to the second level unless the shift to the HI side is accomplished at the time when the additional step number reaches a predetermined value.

3. A hydraulic control system for a continuously variable transmission according to

claim 1

wherein said actuator comprises a step motor which is adapted to be driven to an expected step position corresponding to a target transmission ratio to be established, said shift control means being operable to drive said step motor additionally in increments of a predetermined number of step and to raise the line pressure to the second level unless the shift to the HI side is accomplished at the time when said step motor is actuated to a position corresponding to the maximum speed ratio.

4. A hydraulic control system for a continuously variable transmission according to

claim 1

,

2

or 3 wherein said determining means determines whether or not the shift is accomplished by comparing the speed ratio of the set of the pulleys with the target transmission ratio.

5. A hydraulic control system for a continuously variable transmission according to any of

claims 1

to

5

wherein said shift control means is operable to reduce the raised line pressure to the first level once it is determined that the shift to the HI side is accomplished after raising the line pressure.