Method for controlling an automatic gearbox for a motor vehicle

1. A method for controlling an automatic gearbox for a motor vehicle having at least two different drivetrain states, the method comprising:

determining a minimum deceleration force constraint that needs to be produced by the drivetrain state as a function of speed of the vehicle, longitudinal acceleration, and resistive forces on the vehicle;

then taking a decision to authorize or prohibit the drivetrain state for which the minimum deceleration force constraint has been calculated as a function of a deceleration force target, a current drivetrain state, and a minimum force that can be produced by the drivetrain state in question.

2. The method as claimed in claim 1 , wherein the determining the minimum deceleration force constraint that the drivetrain state must produce comprises:

determining required deceleration of the vehicle with a foot off an accelerator of the vehicle by a first map as a function of a program of the vehicle and a current speed of the vehicle;

then determining a differential force as a difference between theoretical resisting force on a road of zero slope, with predefined mass and without wind, and instantaneously estimated resisting force taking account of current rolling conditions;

then determining an offset corrective parameter by a second map as a function of the differential force;

then determining a required deceleration of the vehicle taking into account the differential forces resulting from a sum of the required deceleration of the vehicle with the foot off the accelerator and the offset corrective parameter;

then determining overall deceleration force as a function of the required deceleration and mass of the vehicle;

then determining the force that the drivetrain state must produce as a sum of the resisting forces and the overall deceleration force;

then summing a force offset value and the deceleration constraint to determine the deceleration force with the foot off the accelerator with or without braking that a drivetrain state must comply with;

then determining which is greater of the deceleration force with the foot off the accelerator with or without braking and the force target at a level of wheels of the vehicle required by the driver;

then saturating a maximum value determined so that it is negative or zero, the saturated value corresponding to the minimum deceleration force constraint that the drivetrain state in question must achieve.

3. The method as claimed in claim 2 , further comprising determining if the drivetrain state for which the minimum deceleration force constraint is determined is the current state,

if so, a first alternative map is used that is less constraining than the first map used if the drivetrain state for which the minimum deceleration force constraint is determined is not the current state.

4. The method as claimed in claim 2 , wherein, to determine a force offset value, further comprising:

determining if depression of a brake pedal of the vehicle is maintained for a minimum duration and if the longitudinal acceleration of the vehicle is less than 0;

if so, the vehicle acceleration during braking is made equal to the longitudinal acceleration value, if not, the vehicle acceleration during braking is set at 0;

determining an acceleration offset value by maps as a function of the vehicle acceleration during braking and the vehicle speed, the map used depending on the program of the vehicle;

multiplying the acceleration offset value by the vehicle mass in order to obtain the force offset value.

5. The method as claimed in claim 1 , wherein, to determine whether to authorize or to prohibit a drivetrain state, further comprising:

authorizing the drivetrain state if, simultaneously, the drivetrain state is the current state and the minimum force available therein is less than or equal to the deceleration force target determined for the current state;

also authorizing the drivetrain state if, simultaneously, the drivetrain state is not the current state and the minimum force available therein is less than or equal to the deceleration force target determined for the drivetrain states that are not the current state;

otherwise prohibiting the drivetrain state.