Creep-control method for an automatic transmission of a vehicle

A creep-control method for an automatic transmission of a vehicle comprising:

• creep-entering, in which a hydraulic circuit of the automatic transmission is controlled to form hydraulic pressure supply lines of a second shift-speed when a predetermined creep-entrance condition is satisfied;
• creep-maintaining, in which the formed hydraulic pressure supply lines of the second shift-speed are maintained and a hydraulic supply pressure of the formed hydraulic pressure supply lines is maintained at a predetermined creep duty ratio;
• and
• creep-escaping, in which one of controlling the hydraulic circuit to engage a second shift-speed shift mechanism of the automatic transmission and controlling the hydraulic circuit to engage a first shift-speed shift mechanism of the automatic transmission is selectively performed based on a vehicle speed and throttle valve open-angle when a predetermined creep-escape condition is satisfied, wherein the creep-escape condition is calculated based on a logical sum of the shift lever of the automatic transmission being disposed at a position of low “L”, park “P”, reverse “R”, and neutral “N” ranges, the vehicle speed being higher than a second predetermined speed, the throttle valve open-angle being greater than a second predetermined open-angle, and a shift-speed of the automatic transmission being required to be maintained at the second shift-speed and in the creep-escaping, the controlling the hydraulic circuit to engage the first shift-speed shift mechanism of the automatic transmission is performed under the condition of the throttle valve open-angle being greater than the second predetermined open-angle, wherein the controlling the hydraulic circuit to engage the first shift-speed shift mechanism of the automatic transmission comprises:
• duty ratio adjusting, in which a duty ratio for the hydraulic supply pressure of the hydraulic pressure supply lines of the second shift-speed is suddenly lowered from the creep duty ratio by a predetermined adjust value;
• first ramp-controlling, in which the duty ratio for the hydraulic supply pressure is gradually lowered until a turbine-rpm is increased by a predetermined rpm;
• duty ratio holding, in which the duty ratio for the hydraulic supply pressure is suddenly increased by a predetermined ratio and the increased duty ratio is held for a predetermined duration if the turbine-rpm is increased by the predetermined rpm in the first ramp-controlling;
• second ramp-controlling in which the duty ratio for the hydraulic supply pressure is gradually lowered until the turbine-rpm becomes within a predetermined range from a target rpm of the first shift-speed;
• and
• transforming the hydraulic pressure supply lines to be hydraulic pressure supply lines of the first shift-speed.