Transient reduction ratio control during shift between two mode operations

Abstract: According to a control for a continuously variable transmission for a motor vehicle, in order to avoid a rapid increase in engine revolution speed upon selecting a power mode from an ordinary mode, the reduction ratio of the continuously variable transmission is varied along a ramp which connects an ordinary shift schedule for the ordinary mode with a power shift schedule for the power mode. The ramp is expressed as a function of time elapsed from the instant when the power mode has been selected.

Description: The invention relates to a control apparatus and a control method as indicated in the precharacterizing parts of claims 1 and 6. Such a prior art control apparatus and method (FR-A-26 05 562) controls a continuously variable transmission being shiftable in reduction ratio in accordance with a first predetermined schedule when use of the first predetermined schedule is demanded and in accordance with a second predetermined schedule when use of the second predetermined schedule is demanded. When a change is made from demand for use of the first pedetermined schedule to demand for use of the second predetermined schedule by an actuating lever of the transmission, the mechanical part of the control means will react according to a predetermined function of time until a predetermined condition is satisfied even if the target engine speed value jumps instantaneously to the new value. Said control means being operated to control the reduction ratio in accordance with the second predetermined schedule after said predetermined condition is satisfied when use of the second predetermined schedule is demanded. Another conventional continuously variable transmission (EP-A-180 209) is shiftable in reduction ratio in accordance with one of a plurality of patterns or schedules. Upon mixing a change from one to another schedule among the plurality of schedules, a target value of a predetermined control variable such as a target engine revolution speed is subject to a step-like change. This causes the transmission to shift quickly in such a direction as to bring an actual value of the predetermined control variable into agreement with the target value. Thus, the engine revolution speed is subject to a rapid change. When a change is made from one to another schedules in order to produce more power for acceleration, the continuously variable transmission shifts down rapidly to force the engine to increase its speed rapidly. The inertia torque induced by this rapid engine speed increase absorbs torque produced by the engine, resulting in a temporal reduction in driving power for accelerating the vehicle, thus degrading ride feel for acceleration. An object of the present invention is to control a continuously variable transmission such that the above-mentioned power interruption is eliminated to give an improved ride feel for acceleration. Said object is solved by the features as claimed in the characterizing parts of claims 1 and 6. The control apparatus includes: Means for generating a time indicative signal which assumes a first level when use of the first predetermined schedule is demanded and a second level when use of the second predetermined schedule is demanded; means for generating a time indicative signal indicative of time elapsed from the instant when said state indicative signal is subject to a change from said first level thereof to said second level thereof; and means for controlling a reduction ratio of the continuously variable transmission in accordance with the first predetermined schedule when said state indicative signal assumes said first level thereof; said controlling means being operative to determine a ramp connecting the first predetermined schedule to the second predetermined schedule as a predetermined function of said time indicative signal and vary the reduction ratio in accordance with said ramp until a predetermined condition is satisfied after said change has been made; said controlling means being operative to control the reduction ratio in accordance with the second predetermined schedule after said predetermined condition is satisfied when said state indicative signal assumes said second level thereof. According to another aspect of the present invention, there is provided a control method for a continuously variable transmission for a motor vehicle including an engine, the continuously variable transmission being shiftable in reduction ratio in accordance with one of a plurality of predetermined schedules including a first predetermined schedule and a second predetermined schedule, the control method comprising the steps of: controlling a reduction ratio of the continuously variable transmission in accordance with the first predetermined schedule when use of the first predetermined schedule is demanded; varying the reduction ratio in accordance with a predetermined function of time elapsed from the instant when a change is made from demand for use of the first schedule to demand for use of the second predetermined schedule until a predetermined condition is satisfied; and controlling the reduction ratio in accordance with the second predetermined schedule after said predetermined condition is satisfied when use of the second predetermined schedule is demanded. According to a further aspect of the present invention, there is provided a control method for a continuously variable transmission for a motor vehicle including an engine, the continuously variable transmission being shiftable in reduction ratio in accordance with one of a plurality of predetermined schedules including a first predetermined schedule and a second predetermined schedule, the control method comprising the steps of: generating a first state indicative signal which assumes a first level thereof when use of the first predetermined schedule is demanded and a second level thereof when use of the second predetermined schedule is demanded; generating a time indicative signal indicative of time elapsed from the instant when said state indicative signal is subject to a change from said first level thereof to said second level thereof: and controlling a reduction ratio of the continuously variable transmission in accordance with the first predetermined schedule when said state indicative signal assumes said first level thereof; determining a ramp connecting the first predetermined schedule to the second predetermined schedule as a predetermined function of said time indicative signal; varying the reduction ratio in accordance with said ramp until a predetermined condition is satisfied after said change has been made; and controlling the reduction ratio in accordance with the second predetermined schedule after said predetermined condition is satisfied when said second state indicative signal is generated. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a power train of a motor vehicle showing a continuously variable transmission drivingly connected to an engine and a differential; Fig. 2 is a hydraulic circuit diagram showing a hydraulic control system for the continuously variable transmission shown in Fig. 1; Fig. 3 is a block diagram showing a control unit for cotrolling a shift motor in the form of a stepper motor 110 and a solenoid 224 of an electromagnetic valve 118 shown in Fig. 2; and Figs. 4A and 4B are flowcharts. DETAILED DESCRIPTION OF THE INVENTION Referring to Fig. 1, a power train comprises an engine 10 with an output shaft 10a which is coupled with a hydrodynamic torque transmitting unit in the form of a fluid coupling 12. The fluid coupling 12 is of the lock-up type and followed by a forward/reverse drive change-over mechanism 15 which is in turn followed by a driver pulley 16 of a continuously variable transmission 29. The driver pulley 16 is drivingly connected by a V-belt 24 to a follower pulley 26. The driver pulley 26 is drivingly connected to a differential 56 via a gear train including gears 46, 48, and 54. Referring to Fig. 2, a hydraulic control system comprises a shift motor in the form of a stepper motor 110 of a shift operation mechanism 112, and a solenoid 224 of an electromagnetic valve 118. The stepper motor 110 and the solenoid 224 are controlled by a control unit 300 as shown in Fig. 3. Referring to Figs. 4A and 4B, there is described a control according to the present invention. The continuously variable transmission which the present invention is applied to is shiftable inreduction ratio in accordance with a first predetermined schedule when an ordinary economy mode is selected, while it is shiftable in reduction ratio in accordance with a second predetermined schedule when a power mode is selected. Each of the first and second predetermined schedules is a retrievable data map containing target values in engine revolution speed versus varying sets of throttle opening degree and vehicle speed. The characteristic difference between these shift schedules is that, with the same throttle opening degree and vehicle speed, the second schedule used at the power mode gives a target engine revolution speed value higher than that given by the first schedule used at the ordinary economy mode. When the state of the continuously variable transmission shifts from the economy mode to the power mode, a change is made from demand for use of the first schedule to demand for use of the second schedule. This causes a step-like increase in target engine revolution speed at the instant when the change is made. In Fig. 4A, at a step 702, it is determined whether the power mode is selected or not. When the power mode is selected, a state indicative signal assumes a digital one level, while when the power mode is not selected and thus the economy mode is selected, the state indicative signal assumes a digital zero level. In the case the economy mode is selected, the content of timer is cleared to zero at a step 704, and then the first schedule is retrieved using a set of current throttle opening degree end vehicle speed given to set a result as a target engine speed E at the economy mode, see step 706. In the case the power mode is selected, the second schedule is retreived to set a result as a target engine speed P at the power mode, see step 708. The content of the timer is incremented regularly by executing the flowchart shown in Fig. 4B. Since the content of the timer is cleared at the step 704, the content of the timer is indicative of time elapsed from the instant when a change, in demand, is made from the economy mode to the power mode. After executing the step 706 or 708, a ramp A which connects the first schedule at the economy mode level with the second schedule at the power mode level is given as a predetermined function, see step 710. This function is expressed as: A = E + T x K₁ + K₂ where: T: the content of timer; K₁: constant; K₂: constant. After executing the step 710, the ramp A is compared with the target engine speed P at a step 512. At this step 710, it is determined whether a predetermined condition is satisfied or not, that is, whether A is greater than and equal to P or not. When it is determined that A is greater than or equal to P, the content of P is set as a final target engine speed at a step 716, while when A is still less than P and thus the predetermined condition is not yet satisfied, the ramp A is set as the final target engine speed at a step 714. A closed loop control to bring an actual engine speed into agreement with the final target engine which is equal to A or P at block 718. The detailed description as to the closed loop control is omitted. When a change from the ecomony mode to the power mode is made, the ramp A which increases as the function of time T elapsed from the instant when the change is made is used as the final target engine speed (see steps 710, 712 and 714), and the continuously variable transmission shifts down to bring the actual engine speed into agreement with this ramp A until the predetermined condition (A≧P) is satisfied. Thus, the actual engine speed gradually increases in accordance with the ramp A to the second shift schedule at the power mode level. Owing to this controlled gradual increase in engine speed during translation from the economy mode level to the power mode level, the temporal torque interruption has been avoided and improved ride feel for acceleration is given. In the preceding example, the engine speed is used as the predetermined control variable. A driver pulley revolution speed (= turbine revolution speed) may be used as the predetermined control variable.

Claims: A control apparatus (300) for a continuously variable transmission (29) for a motor vehicle including an engine (10), the continuously variable transmission (29) being shiftable in reduction ratio in accordance with one of a plurality of predetermined schedules including a first predetermined schedule and a second predetermined schedule, characterized in that said control apparatus (300) comprising: means for generating a state indicative signal which assumes a first level when use of the first predetermined schedule is demanded and a second level when use of the second predetermined schedule is demanded; means for generating a time indicative signal indicative of time (T) elapsed from the instant when said state indicative signal is subject to a change from said first level thereof to said second level thereof; and means for controlling a reduction ratio of the continuously variable transmission (29) in accordance with the first predetermined schedule when said state indicative signal assumes said first level thereof; said controlling means being operative to determine a ramp (A) connecting the first predetermined schedule to the second predetermined schedule as a predetermined function of said time indicative signal and vary the reduction ration in accordance with said ramp (A) until a predetermined condition is satisfied after said change has been made; said controlling means being operative to control the reduction ratio in accordance with the second predetermined schedule after said predetermined condition is satisfied when said state indicative signal assumes said second level thereof. A control apparatus as claimed in claim 1, wherein said predetermined function involves, as a variable, a target value (P) of a predetermined control variable which is given in accordance with the firt predetermined schedule upon making said change. A control apparatus as claimed in claim 2, wherein said predetermined condition is satisfied when a target value (P) of said predetermined control variable given by said ramp (A) becomes substantially equal to a target value (P) of said predetermined control variable given in accordance with the second predetermined schedule. A control apparatus as claimed in claim 3, wherein said predetermined control variable is engine revolution speed of the engine. A control apparatus as claimed in claim 3, wherein said predetermined control variable is driver pulley revolution speed of a driver pulley of the continuously variable transmission (29). A control method for a continuously variable transmission for a motor vehicle including an engine (10), the continuously variable transmission (29) being shiftable in reduction ratio in accordance with one of a plurality of predetermined schedules including first predetermined schedule and a second predetermined schedule, characterized by the steps of: generating a first state indicative signal which assumes a first level thereof when use of the first predetermined schedule is demanded and a second level thereof when use of the second predetermined schedule is demanded; generating a time indicative signal indicative of time (T) elapsed from the instant when said state indicative signal is subject to a change from said first level thereof to said second level thereof; and controlling a reduction ratio of the continuously variable transmission (29) in accordance with the first predetermined schedule when said state indicative signal assumes said first level thereof; determining a ramp (A) connecting the first predetermined schedule to the second predetermined schedule as a predetermined function of said time indicative signal, varying the reduction ration in accordance with said ramp (A) until a predetermined condition is satisfied after said change has been made; and controlling the reduction ratio in accordance with the second predetermined schedule after said predetermined condition is satisfied when said second state indicative signal is generated. A control method as claimed in claim 6, wherein said predetermined function involves, as a variable, a target value (P) of a predetermined control variable which is given in accordance with the first predetermined schedule upon making said change. A control method as claimed in claim 7, wherein said predetermined condition is satisfied when a target value (P) of said predetermined control variable given by said ramp (A) becomes substantially equal to a target value (P) of said predetermined control variable given in accordance with the second predetermined schedule.

Inventors: SUZUKI YUTAKA, SUZUKI, YUTAKA
Applicants: NISSAN MOTOR, NISSAN MOTOR CO., LTD.
Application year: 1989
Main classification: B60K41
IPC codes: F16H59, F16H61, F16H9, B60W10, B60W30