Method and apparatus to control a continuously variable transmission
A powertrain system including an internal combustion engine rotatably coupled to a variator of a continuously variable transmission (CVT) is described. A method for controlling the CVT includes determining a first speed ratio change rate based upon a desired speed ratio and a commanded speed ratio, and determining a commanded speed ratio trajectory based upon the first speed ratio change rate, an actual speed ratio and a commanded speed ratio. A ratio change coefficient and a force ratio factor are determined based upon the commanded speed ratio trajectory. A shift force is determined based upon the ratio change coefficient and a total speed ratio change rate. A primary pulley pressure and a secondary pulley pressure for the CVT are controlled based upon the shift force and the force ratio factor.
1. A method for controlling a continuously variable transmission (CVT), the method comprising:
determining a first speed ratio change rate based upon a desired speed ratio and a commanded speed ratio;
determining a commanded speed ratio trajectory based upon the first speed ratio change rate, an actual speed ratio and a commanded speed ratio;
determining a ratio change coefficient and a force ratio factor based upon the commanded speed ratio trajectory;
determining a shift force based upon the ratio change coefficient and a total speed ratio change rate; and
controlling, via a controller, a primary pulley force and a secondary pulley force for the CVT based upon the shift force and the force ratio factor.
2. The method of claim 1 , further comprising controlling the primary pulley pressure and the secondary pulley pressure for the CVT based upon the shift force, the force ratio factor, a minimum pulley clamping force, a centrifugal force and a spring force for the CVT.
3. The method of claim 1 , wherein determining the first speed ratio change rate based upon the desired speed ratio and the commanded speed ratio comprises determining the first speed ratio change rate based upon a difference between the desired speed ratio and the commanded speed ratio multiplied by a feed-forward gain factor when a change in the desired speed ratio is a ramp rate change.
4. The method of claim 1 , wherein determining the first speed ratio change rate based upon the desired speed ratio and the commanded speed ratio comprises determining the first speed ratio change rate in accordance with:
Rate=−( No *AccelProf)/ Ni 2
wherein:
Rate is the first speed ratio change rate,
Ni is a CVT input speed,
No is a CVT output speed, and
AccelProf is a commanded acceleration profile of an input member of the CVT when a change in the desired speed ratio is a step change.
5. The method of claim 4 , wherein the desired speed ratio is a step change in response to a tap-up/tap-down input to a transmission gear selector.
6. The method of claim 1 , wherein determining the first speed ratio change rate based upon the desired speed ratio and the commanded speed ratio comprises determining the first speed ratio change rate as a maximum achievable time-rate change in the speed ratio limited only by a fluidic pressure capability of a hydraulic pump supplying pressurized hydraulic fluid to the CVT.
7. The method of claim 1 , wherein determining the commanded speed ratio trajectory comprises integrating the first speed ratio change rate employing a variable coefficient that is based upon a difference between the actual speed ratio and the commanded speed ratio.
8. The method of claim 1 , wherein the total speed ratio change rate is determined based upon the actual speed ratio, the desired speed ratio and the commanded speed ratio, comprising:
determining a feedback ratio rate based upon a difference between the commanded speed ratio and the actual speed ratio;
determining a ratio change rate based upon a difference between the commanded speed ratio and the desired speed ratio; and
combining the feedback ratio rate and the ratio change rate.
9. The method of claim 1 , wherein determining the ratio change coefficient based upon the commanded speed ratio trajectory comprises analyzing empirical data to develop a relationship for a variator ratio derivative {dot over (r)} in accordance with the following equations:
{dot over (r)}=k ( VSR,ω p )* F shift and
F shift =F prim −KpKs ( VSR,TCR ) F sec
wherein:
k is the ratio change coefficient,
VSR is the desired speed ratio,
F prim is a force associated with a primary pulley of a variator of the VSR,
F sec is a force associated with a secondary pulley of the variator of the VSR,
F shift is the shift force,
ω p is CVT input speed,
KpKs is the force ratio factor, and
TCR is a torque capacity ratio for the CVT; and
wherein the variator ratio derivative {dot over (r)} corresponds to the commanded speed ratio trajectory.
10. The method of claim 1 , wherein determining a force ratio factor based upon the commanded speed ratio trajectory comprises analyzing empirical data to develop a relationship for a variator ratio derivative {dot over (r)} in accordance with the following equation:
{dot over (r)}=k ( VSR,ω p )*[ F prim −kpks ( VSR,TCR ) F sec ]
wherein:
VSR is the desired speed ratio,
F prim is a force associated with a primary pulley of a variator of the VSR,
F sec is a force associated with a secondary pulley of the variator of the VSR,
ω p is CVT input speed,
k is the ratio change coefficient determined based upon a ratio of F prim and F sec
kpks is the force ratio factor, and
TCR is a torque capacity ratio for the CVT.
11. The method of claim 1 , wherein controlling the primary pulley pressure for the CVT based upon the shift force and the force ratio factor comprises controlling a primary pressure command to drive an actuator of a moveable sheave of the primary pulley of a variator of the CVT in response to the shift force and the force ratio factor.
12. The method of claim 1 , wherein controlling the secondary pulley pressure for the CVT based upon the shift force and the force ratio factor comprises controlling a secondary pressure command to drive an actuator of a moveable sheave of the secondary pulley of a variator of the CVT in response to the shift force and the force ratio factor.
13. The method of claim 1 , wherein the actual speed ratio comprises a measured value determined based upon a ratio of a CVT input speed and a CVT output speed.
14. The method of claim 1 , wherein the desired speed ratio comprises speed ratio for operating the CVT that is responsive to the operator torque and vehicle speed.
15. The method of claim 1 , wherein the commanded speed ratio comprises a speed ratio that is achievable with a zero steady-state tracking error.
16. A continuously variable transmission (CVT), comprising:
a variator including a first pulley and a second pulley, the first and second pulleys rotatably couple by a flexible continuous rotatable device, wherein the first pulley rotatably couples to an input member and the second pulley rotatably couples to an output member;
the first pulley including a moveable sheave that translates along a first axis relative to a stationary sheave in response to urging of a first actuator;
the second pulley including a moveable sheave that translates along a second axis relative to a stationary sheave in response to urging of a second actuator;
a controller, in communication with a first sensor configured to monitor a first speed associated with the first pulley and a second sensor configured to monitor a second speed associated with the second pulley, and operatively connected to the first and second actuators;
the controller including an instruction set, the instruction set executable to:
determine a first speed ratio change rate based upon a desired speed ratio and a commanded speed ratio,
determine a commanded speed ratio trajectory based upon the first speed ratio change rate, an actual speed ratio and a commanded speed ratio,
determine a ratio change coefficient and a force ratio factor based upon the commanded speed ratio trajectory,
determine a shift force based upon the ratio change coefficient and a total speed ratio change rate, and
control a primary pulley pressure and a secondary pulley pressure for the CVT based upon the shift force, a minimum pulley clamping force, and the force ratio factor.
17. The CVT of claim 16 , wherein determining the first speed ratio change rate based upon the desired speed ratio and the commanded speed ratio comprises determining the first speed ratio change rate based upon a difference between the desired speed ratio and the commanded speed ratio multiplied by a feed-forward gain factor when a change in the desired speed ratio is a ramp rate change.
18. The CVT of claim 16 , wherein determining the first speed ratio change rate based upon the desired speed ratio and the commanded speed ratio comprises determining the first speed ratio change rate in accordance with:
Rate=−( No *AccelProf)/ Ni 2
wherein:
Rate is the first speed ratio change rate,
Ni is a CVT input speed,
No is a CVT output speed, and
AccelProf is a commanded acceleration profile of an input member of the CVT when a change in the desired speed ratio is a step change.
19. The CVT of claim 18 , wherein the desired speed ratio is a step change in response to a tap-up/tap-down input to a transmission gear selector.
20. The CVT of claim 18 , wherein the desired speed ratio is a step change in response to a step-out maneuver to an accelerator pedal.