Shift control method for hybrid electric vehicle

1. A shift control method for a hybrid electric vehicle comprising the steps of:

step a) of when a shift demand for a power-on down shift is detected, a controller starting a release control of a clutch and determining whether slip of a release element is generated from a rotation speed of a transmission input shaft, wherein the clutch is the release element in a transmission;

step b) of when generation of the slip of the release element is determined, the controller starting a feedback control of the release element in the transmission based on information, and simultaneously, controlling a speed of a vehicle driving source to increase a speed of the transmission input shaft, wherein the information is based on a torque of a transmission output shaft or a rotation acceleration of the transmission output shaft;

step c) of when the speed of the transmission input shaft reaches ‘synchronous speed+predetermined value’, the controller:

controlling the speed of the vehicle driving source, wherein the speed of the transmission input shaft follows ‘synchronous speed+predetermined value’; and

simultaneously increasing a rotation acceleration of the transmission output shaft, wherein the controller performs a feedback control of a connection element in the transmission based on the rotation acceleration of the transmission output shaft;

step d) of preparing a synchronization, wherein a transmission input torque from the vehicle driving source is maintained and the clutch torque of the connection element in the transmission is increased; and

step e) of completing a shift after determining the synchronization.

2. The shift control method of claim 1 , wherein in step b), the controller performs the feedback control of the release element, wherein the rotation acceleration of the transmission output shaft becomes a value of ‘a rotation acceleration of the transmission output shaft before shifting and before slip generation×k B1 (herein, k B1 <1)’.

3. The shift control method of claim 1 , wherein the controller calculates the rotation acceleration of the transmission output shaft of a target shift stage based on a gear ratio of a shift stage before shifting, a gear ratio of the target shift stage, and a rotation acceleration of the transmission output shaft before shifting and before slip generation; and

wherein in step c), the controller performs the feedback control of the connection element in the transmission, wherein the rotation acceleration of the transmission output shaft is increased to the rotation acceleration of the transmission output shaft of the target shift stage by increasing the clutch torque of the connection element.

4. The shift control method of claim 3 , wherein the rotation acceleration of the transmission output shaft of the target shift stage is calculated based on formula:

( dN 0 ) t =( i t /i c )×( dN 0 ) c ,

wherein, (dN 0 ) t is the rotation acceleration of the transmission output shaft of the target shift stage, i c is the gear ratio of the shift stage before shifting, i t is the gear ratio of the target shift stage, and (dN 0 ) c is the rotation acceleration of the transmission output shaft before shifting and before slip generation.

5. The shift control method of claim 3 , wherein in step c), the controller:

divides a predetermined time T B2 into a plurality of time sections, wherein the predetermined time T B2 is the time that a speed of the transmission input shaft reaches ‘synchronous speed+predetermined value’;

sets a target value of the rotation acceleration of the transmission output shaft in each time section of the plurality of the time sections; and

performs the feedback control of the connection element in the transmission, wherein the rotation acceleration of the transmission output shaft is increased to a corresponding target value in each time section of the plurality of the time sections.

6. The shift control method of claim 5 , wherein the target value of the rotation acceleration of the transmission output shaft, is set to be gradually increased, wherein the target value of the rotation acceleration of the transmission output shaft is set in each time section of the plurality of the time sections.

7. The shift control method of claim 6 , wherein the target value of the rotation acceleration of the transmission output shaft is calculated based on the formula:

( dN 0 ) c +k B1 {( dN 0 ) t −( dN 0 ) c },

wherein, (dN 0 ) t is the rotation acceleration of the transmission output shaft of a target shift stage, (dN 0 ) c is the rotation acceleration of the transmission output shaft before shifting and before slip generation, and k Bj (j is an index for dividing the time sections, j=1, 2, 3, . . . , n.

8. The shift control method of claim 5 , wherein step d) starts as the predetermined time T B2 passes after entering into step c).

9. The shift control method of claim 1 , wherein the controller performs the feedback control of the release element such that a clutch torque of the release element is decreased at a constant slope, wherein the constant slope is maintained as the clutch torque of the release element become 0 during step c) until entering into step d).

10. The shift control method of claim 1 , wherein in step e), the controller applies a maximum clutch torque to the connection element in the transmission and performs the feedback control of the release element, wherein the rotation acceleration of the transmission output shaft of the target shift stage is maintained based on the rotation acceleration of the transmission output shaft.

11. The shift control method of claim 10 , wherein the rotation acceleration of the transmission output shaft of the target shift stage is calculated based on the formula:

( dN 0 ) t =( i t /i c )×( dN 0 ) c ,

wherein, (dN 0 ) t is the rotation acceleration of the transmission output shaft of the target shift stage, i c is the gear ratio of the shift stage before shifting, i t is the gear ratio of the target shift stage, and (dN 0 ) c is the rotation acceleration of the transmission output shaft before shifting and before slip generation.