Control apparatus and control method for automatic transmission

1. A control apparatus for an automatic transmission, the automatic transmission including a plurality of speeds, the automatic transmission being configured such that any one of the plurality of speeds is selectively established, the automatic transmission being mounted on a vehicle, the control apparatus comprising:

an electronic control unit configured to:

(i) store a plurality of shift lines, the plurality of shift lines including an N−1 speed downshift line, an N−2 speed downshift line, an N−1 speed upshift line, and a hysteresis line, and N is a natural number of 3 or higher, the N−1 speed downshift line being a shift line used to determine whether to change from an N speed to an N−1 speed, the N−2 speed downshift line being a shift line used to determine whether to change from the N−1 speed to an N−2 speed, the N−1 speed upshift line being a shift line used to determine whether to change from the N−2 speed to the N−1 speed, and the hysteresis line being a line that is a predetermined distance above the N−1 speed upshift line, and the hysteresis line being below the N−2 speed downshift line;

(ii) control the speed of the automatic transmission based on a quantity of state of the vehicle and the plurality of stored shift lines; and

(iii) execute a first downshift that changes a speed from the N speed to the N−2 speed, when the N speed is established and the quantity of state of the vehicle is in a region above the hysteresis line after the quantity of state of the vehicle has risen above the N−1 speed downshift line.

2. The control apparatus according to claim 1 , wherein

the electronic control unit is configured to, when the N speed is established, execute the first downshift when the quantity of state of the vehicle is in the region above the hysteresis line, and a difference between an estimated value of vehicle driving force when the speed is changed to the N−2 speed and an estimated value of vehicle driving force when the speed is changed to the N−1 speed is less than a predetermined value, after the quantity of state of the vehicle has risen above the N−1 speed downshift line.

3. The control apparatus according to claim 1 , wherein

the automatic transmission is connected to an engine via an input shaft of the automatic transmission, and

the electronic control unit is configured to, when the N speed is established, execute the first downshift when the quantity of state of the vehicle is in a first region that is above the hysteresis line, and below a maximum N−1 speed downshift line, after the quantity of state of the vehicle has risen above the N−1 speed downshift line, and

the maximum N−1 speed downshift line being a line in which the N−1 speed downshift line is expanded upward to the maximum extent possible so as to maintain a current N speed until a user required output reaches a value corresponding to maximum output of the engine.

4. The control apparatus according to claim 3 , wherein

the electronic control unit is configured to, when the N speed is established, execute the first downshift when the quantity of state of the vehicle is in the first region, and a difference between an estimated value of vehicle driving force when the speed is changed to the N−2 speed and an estimated value of vehicle driving force when the speed is changed to the N−1 speed is less than a predetermined value, after the quantity of state of the vehicle has risen above the N−1 speed downshift line.

5. The control apparatus according to claim 3 , wherein

the electronic control unit is configured to maintain the current N speed until a period of time for which the quantity of state of the vehicle is maintained in a second region exceeds a predetermined period of time when the N speed is established and the quantity of state of the vehicle is in the second region after the quantity of state of the vehicle has risen above the N−1 speed downshift line, the second region is a region that is below the hysteresis line and below the maximum N−1 speed downshift line, and

the electronic control unit is configured to execute a second downshift that changes the speed from the N speed to the N−1 speed when the period of time for which the quantity of state of the vehicle is maintained in the second region exceeds the predetermined period of time.

6. The control apparatus according to claim 3 , wherein

the electronic control unit is configured to, when the N speed is established and the quantity of state of the vehicle is in a second region that is below the hysteresis line and below the maximum N−1 speed downshift line, after the quantity of state of the vehicle has risen above the N−1 speed downshift line, maintain the current N speed when an increase rate of an accelerator operation amount is smaller than a predetermined rate, and

the electronic control unit is configured to execute the first downshift when the increase rate of the accelerator operation amount is greater than the predetermined rate.

7. The control apparatus according to claim 6 , wherein

the electronic control unit is configured to, when the N speed is established and the quantity of state of the vehicle is in the second region, after the quantity of state of the vehicle has risen above the N−1 speed downshift line, maintain the current N speed when a period of time for which the quantity of state of the vehicle is maintained in the second region has not exceeded the predetermined period of time and the increase rate of the accelerator operation amount is smaller than the predetermined rate,

the electronic control unit is configured to execute the first downshift when the period of time for which the quantity of state of the vehicle is maintained in the second region has not exceeded the predetermined period of time and the increase rate of the accelerator operation amount is greater than the predetermined rate, and

the electronic control unit is configured to execute a second downshift that changes the speed from the N speed to the N−1 speed when the period of time for which the quantity of state of the vehicle is maintained in the second region exceeds the predetermined period of time.

8. A control method for an automatic transmission, the automatic transmission including a plurality of speeds, the automatic transmission being configured such that any one of the plurality of speeds is selectively established, the automatic transmission being mounted on a vehicle, the control method comprising:

storing a plurality of shift lines, the plurality of shift lines including an N−1 speed downshift line, an N−2 speed downshift line, an N−1 speed upshift line, and a hysteresis line, and N is a natural number of 3 or higher, the N−1 speed downshift line being a shift line used to determine whether to change from an N speed to an N−1 speed, the N−2 speed downshift line being a shift line used to determine whether to change from the N−1 speed to an N−2 speed, the N−1 speed upshift line being a shift line used to determine whether to change from the N−2 speed to the N−1 speed, and the hysteresis line being a line that is a predetermined distance above the N−1 speed upshift line, and the hysteresis line being below the N−2 speed downshift line;

controlling the speed of the automatic transmission based on a quantity of state of the vehicle, and the plurality of stored shift lines; and

executing a first downshift that changes a speed from the N speed to the N−2 speed, when the N speed is established and the quantity of state of the vehicle is in a region above the hysteresis line after the quantity of state of the vehicle has risen above the N−1 speed downshift line.