Control device and control method of continuously variable transmission

Disclosed is lateral pressure control of a continuously variable transmission (CVT) with a transmission ratio changed by changing groove widths of a drive pulley and a driven pulley, a drive force from a drive source being transmitted to a wheel. A control part controlling respective lateral pressures of the drive pulley and the driven pulley is provided. The control part sets an increase correction amount of the lateral pressure of the drive pulley to a first lateral pressure increase correction amount if the CVT is not in an in-gear state or a shift position is consistent with a traveling direction of the vehicle, and, sets said amount to a second lateral pressure increase correction amount smaller than the first lateral pressure increase correction amount if the CVT is in the in-gear state and the shift position is not consistent with the traveling direction of the vehicle.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Japan application serial no. 2021-072212 filed on Apr. 21, 2021. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND

Technical Field

The invention relates to a control device and a control method which controls a lateral pressure of a pulley in a continuously variable transmission including a configuration in which a belt is wound on a drive (primary) pulley and a driven (secondary) pulley.

Description of Related Art

In a vehicle in which a belt-type continuously variable transmission (CVT) is mounted, various solutions have been proposed to prevent the durability from deteriorating due to belt slippage. For example, in the power transmission device disclosed in Patent Document 1, belt slippage is prevented by adjusting a clutch transmission torque so that a clutch of a forward/backward switching mechanism provided between a drive source and the CVT slips before the belt of the CVT slips, that is, belt slippage is prevented by functioning as a torque fuse. In the system disclosed in Patent Document 2, in the case where the vehicle moves in a direction opposite to the shift position at Drive (D) (moving forward) or Reverse (R) (moving backward), the engine output is limited or a hydraulic pressure of the primary pulley is increased to avoid belt slippage.

Patent Document 2 indicates that in the belt-type CVT, when a vehicle goes backward in a state in which neither the accelerator pedal nor the brake pedal is stepped on while the shift lever is kept at the D (Drive) range during uphill, the hydraulic pressure balance between the primary pressure and the secondary pressure is lost. As a result, the primary pressure drops, and the torque capacity (the maximum torque transmittable without causing belt slippage) decreases, and there is a possibility that a belt slip may occur. In Patent Document 2, in order to prevent such belt slippage, a means of limiting engine output or increasing the hydraulic pressure of the primary pulley is increased.

PRIOR ART DOCUMENTS

Patent Documents

However, in the case where the line pressure for supplementing the dropped hydraulic pressure of the primary pulley is maximized as in Patent Document 2, even though belt slippage can be prevented, due to the loss of the hydraulic pressure balance between the primary pulley and the secondary pulley, a desired transmission ratio cannot be maintained. In the following, an example of the case where a vehicle travels reversely during uphill is described.

As shown inFIG.1, when a vehicle10in which a belt-type CVT is mounted goes uphill, if there is a vehicle in the opposite direction, for example, it is necessary to stop ((A) ofFIG.1), retreat ((B) ofFIG.1), stop at a retreated location ((C) ofFIG.1), and then restart ((D) ofFIG.1). In such case, in a normal switch-back operation, that is, an operation in which the shift position is set to N/R to retreat, the brake is stepped on at the retreated location to stop, and the shift position is switched to D from N/R to restart, the CVT ratio is kept at LOW and no problem occurs.

Differing from such switch-back operation, in the case where the brake is released while the shift position is at D (Drive) and the vehicle retreats ((B) ofFIG.1), stepping on the accelerator pedal, instead of the brake can also stop the vehicle ((C) ofFIG.1). In this case, if it is possible to restart while stepping on the accelerator pedal ((D) ofFIG.1), the change of the shift position as well as the brake operation can be omitted, and the pedal operation can be simplified, making such operation more favorable.

However, as recognized in Patent Document 2, the hydraulic pressure balance between the primary pressure and the secondary pressure is lost, the primary pressure drops, and belt slippage occurs when the vehicle retreats in a state in which the shift position is kept at D and the brake is not stepped on ((B) ofFIG.1), and the control for increasing the lateral pressure of the primary pulley is exerted to prevent such belt slippage. Therefore, when the accelerator pedal is stepped on to stop the retreating vehicle, and the accelerator pedal keeps being stepped on to restart, the transmission ratio of the CVT is deviated from LOW, and the vehicle cannot move smoothly.

When the vehicle retreats while the shift position remains at D (Drive), the hydraulic pressure balance is lost, and the primary pressure drops. Therefore, in order to prevent such belt slippage, it is necessary to increase the lateral pressure of the primary pulley. However, to stop the retreating vehicle by stepping on the accelerator pedal, instead of the brake and keep stepping on the accelerator pedal to restart, the control of increasing the lateral pressure of the primary pulley is exerted, which deviates the transmission ratio of the CVT from LOW, obstructs the vehicle from moving smoothly, and deteriorates the traveling performance.

SUMMARY

An aspect of the invention provides a control device of a continuously variable transmission in a vehicle in which the continuously variable transmission is mounted. The continuously variable transmission includes a drive pulley, a driven pulley, and a belt wound on the drive pulley and the driven pulley, a transmission ratio is changed by changing groove widths of the drive pulley and the driven pulley, and a drive force from a drive source is transmitted to a wheel. The control device has: a first lateral pressure generation circuit, generating a lateral pressure of the drive pulley; a second lateral pressure generation circuit, generating a lateral pressure of the driven pulley; and a control part controlling the first lateral pressure generation circuit and the second lateral pressure generation circuit and controlling the respective lateral pressures of the drive pulley and the driven pulley. The control part is configured to: in a case where the continuously variable transmission is not in the in-gear state and the shift position is not consistent with the traveling direction of the vehicle, set the increase correction amount of the lateral pressure of the drive pulley to a first lateral pressure increase correction amount; and in a case where the continuously variable transmission is in the in-gear state and the shift position is not consistent with the traveling direction of the vehicle, set the increase correction amount of the lateral pressure of the drive pulley to a second lateral pressure increase correction amount. The second lateral pressure increase correction amount is smaller than the first lateral pressure increase correction amount.

Another aspect of the invention provides a control method of a continuously variable transmission in a vehicle in which the continuously variable transmission is mounted. The continuously variable transmission includes a drive pulley, a driven pulley, and a belt wound on the drive pulley and the driven pulley, a transmission ratio is changed by changing groove widths of the drive pulley and the driven pulley, and a drive force from a drive source is transmitted to a wheel. The control method includes: by a control part controlling respective lateral pressures of the drive pulley and the driven pulley, detecting an in-gear state of the continuously variable transmission, a shift position, and a traveling direction of the vehicle; in a case where the continuously variable transmission is not in the in-gear state and the shift position is not consistent with the traveling direction of the vehicle, setting the increase correction amount of the lateral pressure of the drive pulley to a first lateral pressure increase correction amount; and in a case where the continuously variable transmission is in the in-gear state and the shift position is not consistent with the traveling direction of the vehicle, setting the increase correction amount of the lateral pressure of the drive pulley to a second lateral pressure increase correction amount, the second lateral pressure increase correction amount being smaller than the first lateral pressure increase correction amount.

DESCRIPTION OF THE EMBODIMENTS

The invention provides a control device and a control method of a continuously variable transmission capable of reliably preventing belt slippage to protect the belt and preventing traveling performance from deteriorating.

According to the invention, in the case where the continuously variable transmission is not in the in-gear state and the shift position is not consistent with the traveling direction of the vehicle, the control part sets the increase correction amount of the lateral pressure of the drive pulley to the first lateral pressure increase correction amount, and in the case where the continuously variable transmission is in the in-gear state and the shift position is not consistent with the traveling direction of the vehicle, the control part sets the increase correction amount of the lateral pressure of the drive pulley to the second lateral pressure increase correction amount. The second lateral pressure increase correction amount is set to be smaller than the first lateral pressure increase correction amount. Accordingly, the pressure difference between the lateral pressure of the drive pulley and the lateral pressure of the driven pulley can be increased, the transmission ratio of the continuously variable transmission can stay at LOW, and the traveling performance at the time of restart can be prevented from being deteriorated.

The vehicle further includes a torque converter provided between an output shaft of the drive source and an input shaft of the continuously variable transmission. The control part is configured to, in the case where the continuously variable transmission is in the in-gear state and the shift position is not consistent with the traveling direction of the vehicle, calculate a rotation difference between an input side and an output side of the torque converter, and generate the second lateral pressure increase correction amount in accordance with the rotation difference. The greater the rotation difference, the greater the second lateral pressure increase correction amount can be. Accordingly, the second lateral pressure increase correction amount can decrease within the range where the foot torque variation is small, and the pressure difference between the lateral pressure of the drive pulley and the lateral pressure of the driven pulley can be efficiently increased.

The vehicle may further include a forward/backward switching mechanism provided between the torque converter and the input shaft of the continuously variable transmission, and the in-gear state can be realized in the state in which the clutch of the forward/backward switching mechanism is engaged.

According to the invention, belt slippage can be reliably prevented and traveling performance can be prevented from deteriorating.

In the following, the embodiments of the invention will be described in detail with reference to the drawings. However, the components described in the following embodiments are merely examples, and the technical scope of the invention shall not be construed as being limited thereto.

1. Power Transmission System

InFIG.2, an engine10as a drive source is mounted in a continuously variable transmission vehicle14including a drive wheel12. A throttle valve (not shown) provided in the intake system of the engine10is connected to a drive by wire (DBW) mechanism16including an actuator, such as an electric motor, and is opened and closed by the DBW mechanism16.

The intake air modulated by the throttle valve flows through an intake manifold, mixes with fuel injected from an injector20(INJ) in the vicinity of the intake port of each cylinder to form mixed gas, and flows into the combustion chamber of the cylinder when the intake valve opens. In the combustion chamber, the mixed gas is ignited by the spark plug and combusted, drives the piston to rotate a crankshaft22, becomes exhaust gas, and is discharged out of the engine10.

The rotation of the crankshaft22is input to a CVT26via a torque converter24and a forward/backward switching mechanism28. That is, the crankshaft22is connected to a pump impeller24aof the torque converter24, while a turbine runner24bprovided opposite thereto and receiving a fluid (hydraulic oil) is connected to a main shaft (input shaft) MS.

The CVT26which forms a continuously variable transmission includes the main shaft MS, more specifically a drive pulley26aprovided on an outer circumferential shaft thereof, a counter shaft (output shaft) CS parallel to the main shaft MS, more specifically a driven pulley26bprovided on an outer circumferential shaft thereof, and a power transmission element including an endless flexible member hung therebetween, such as a belt26cmade of metal.

The drive pulley26aincludes a fixed pulley half body26a1, which is provided to be not movable in the axial direction and relatively not rotatable on the outer circumferential shaft of the main shaft MS, and a movable pulley half body26a2not relatively rotatable on the outer circumferential shaft of the main shaft MS and relatively movable in the axial direction with respect to the fixed pulley half body26a1.

The driven pulley26bincludes a fixed pulley half body26b1, which is provided to be not relatively rotatable on the outer circumferential shaft of the counter shaft CS and not movable in the axial direction, and a movable pulley half body26b2not relatively rotatable on the counter shaft CS and relatively movable in the axial direction with respect to the fixed pulley half body26b1.

The forward/backward switching mechanism28includes a forward clutch28aallowing the vehicle14to travel in the forward direction, a backward brake28ballowing traveling in the backward direction, and a planetary gear mechanism28cprovided therebetween. The CVT26is connected to the engine10via the forward clutch28a.

The forward clutch28aand the backward brake28b, more specially mainly the forward clutch28afunctions as a so-called torque fuse.

In the planetary gear mechanism28c, a sun gear28c1is fixed to the main shaft MS, a ring gear28c2is fixed to the fixed pulley half body26a1of the drive pulley26avia the forward clutch28a.

A pinion28c3is provided between the sun gear28c1and the ring gear28c2. The pinion28c3is connected to the sun gear28c1by using a carrier28c4. When the backward brake28bis operated, the carrier28c4is fixed (locked) accordingly.

The rotation of the counter shaft CS is transmitted from a secondary shaft (intermediate shaft) SS to the drive wheel12via a gear. That is, the rotation of the counter shaft CS is transmitted to the secondary shaft SS via gears30aand30b, and the rotation thereof is transmitted via a gear30cfrom a differential32to the drive wheels12on the left and right (only the one on the right is shown) via the drive shaft34.

Disc brakes36are provided in the vicinity of the four wheels including drive wheels (front wheels) and driven wheels (rear wheels, not shown), and a brake pedal40is provided on a vehicle driver seat floor.

The switching between the forward clutch28aand the backward brake28bin the forward/backward switching mechanism28is performed by the driver operating a range selector44provided at the vehicle driver seat to select one of the ranges (shift positions) such as P, R, N, D, etc. The range selection performed by the driver's operation of the range selector44is transmitted to a manual valve of a hydraulic pressure supply mechanism46to be described afterwards.

When the shift positions, such as D, S, L, are selected via the range selector44, the spool of the manual valve moves correspondingly, and hydraulic oil (hydraulic pressure) is discharged from the piston chamber of the backward brake28b, whereas the hydraulic pressure is supplied to the piston chamber of the forward clutch28ato engage the forward clutch28a.

When the forward clutch28ais engaged, all the gears rotate integrally with the main shaft MS, and the drive pulley26ais driven in the same direction (forward direction) with the main shaft MS. Accordingly, the vehicle14travels in the forward direction.

When the shift position of R is selected, hydraulic oil is discharged from the piston chamber of the forward clutch28a, while the hydraulic pressure is supplied to the piston chamber of the backward brake28bto operate the backward brake28b. Accordingly, the carrier28c4is fixed and the ring gear28c2is driven in a direction opposite to the sun gear28c1, the drive pulley26ais driven in a direction (backward direction) opposite to the main shaft MS, and the vehicle14travels in the backward direction. In the following, a state in which either the forward clutch28aor the backward brake28bof the switching mechanism28is engaged is referred to as an in-gear state.

When the shift position of P or N is selected, hydraulic oil is discharged from the piston chambers of both the forward clutch28aand the backward brake28b, and the forward clutch28aand the backward brake28bare open, the power transmission via the forward/backward switching mechanism28is disconnected, and the power transmission between the engine10and the drive pulley26aof the CVT26is cut off.

2. Hydraulic Pressure Supply Mechanism

As shown inFIG.3, a hydraulic pressure pump46ais provided in the hydraulic pressure supply mechanism46. The hydraulic pressure pump46aincludes a gear pump, is driven by the engine (E)10, and pumps up the hydraulic oil stored in a reservoir46band to a PH control valve (PH REG VLV)46c.

The output (PH pressure (line pressure)) of the PH control valve46c, on the one hand, is connected to a piston chamber (DR)26a21of the movable pulley half body26a2of the drive pulley26aand a piston chamber (DN)26b21of the movable pulley half body26b2of the driven pulley26bof the CVT26from an oil passage46dvia first and second regulator valves (DR REG VLV, DN REG VLV)46eand46f, and, on the other hand, is connected to a CR valve (CR VLV)46hvia an oil passage46g.

The CR valve46hreduces the PH pressure to generate a CR pressure (control pressure), and supplies the CR pressure to first, second, and third (electromagnetic) linear solenoid valves46j,46k, and46l(LS-DR, LS-DN, LS-CPC).

The first and second linear solenoid valves46jand46kapply output pressures determined in accordance with the excitation of the solenoids thereof to the first and second regulator valves46eand46f, and supply the hydraulic oil of the PH pressure transmitted from the oil passage46dthe piston chambers26a21and26b21of the movable pulley half bodies26a2and26b2accordingly, and thereby correspondingly generating pulley lateral pressures. Therefore, the first linear solenoid valve46j, the first regulator valve46e, the piston chamber26a21, and the hydraulic pressure system thereof serve as a first lateral pressure generation circuit which generates a lateral pressure of the drive pulley26a, and the second linear solenoid valve46k, the second regulator valve46f, the piston chamber26b21, and the hydraulic pressure system thereof serve as a second lateral pressure generation circuit which generates a lateral pressure of the driven pulley26b.

Accordingly, the pulley lateral pressures moving the movable pulley half bodies26a2and26b2in the axial direction are generated, the pulley widths of the drive pulley26aand the driven pulley26bchange, and the winding radius of the belt26cchanges. Accordingly, by adjusting the pulley lateral pressures, a ratio (transmission ratio) of transmitting the output of the engine10to the drive wheel12can be changed steplessly.

The output (CR pressure) of the CR valve46his adjusted in accordance with the excitation of the solenoid of the third linear solenoid valve46l, and is transmitted to the manual valve (MAN VLV)46ovia an oil passage46m, and from there, the output is connected to a piston chamber (FWD)28a1of the forward clutch28aand a piston chamber (RVS)28b1of the backward brake28bof the forward/backward switching mechanism28.

The manual valve46o, as described above, connects the output of the CR valve46hto either of the piston chambers28a1and28b1of the forward clutch28aand the rearward brake28bin accordance with the position of the range selector44operated (selected) by the driver.

In addition, the output of the PH control valve46cis transmitted to a TC regulator valve (TC REG VLV)46qvia an oil passage46p, and the output of the TC regulator valve46qis connected to an LC shift valve (LC SFT VLV)46svia an LC control valve (LC CTL VLV)46r.//

The output of the LC shift valve46s, on the one hand, is connected to a piston chamber24c1of a lockup clutch24cof the torque converter24and, on the other hand, is connected to a chamber24c2on a back side thereof.

When the hydraulic oil is supplied to the piston chamber24c1via the LC shift valve46sand discharged from the chamber24c2on the back side, the lockup clutch24cis engaged (ON), and when the hydraulic oil is supplied to the chamber24c2on the back side and discharged from the piston chamber24c1, the lockup clutch24cis released (OFF). The slip amount of the lockup clutch24cis determined by the amount of the hydraulic oil supplied to the piston chamber24c1and the chamber24c2on the back side.

The output of the CR valve46his connected to the LC control valve46rand the LC shift valve46svia an oil passage46t, and a fourth linear solenoid valve (LS-LC)46uis inserted on the oil passage46t. The slip amount of the lockup clutch24cis adjusted (controlled) by the excitation/non-excitation of the solenoid of the fourth solenoid valve46u.

In addition, an electric oil pump (EOP)46wconnected to an electric motor46vis connected to a position equivalent to downstream of the hydraulic pressure pump46aand upstream of the PH control valve46cvia a check valve46x.

Like the hydraulic pressure pump46a, the EOP46walso includes a gear pump, is driven by the electric motor46v, and pumps up the hydraulic oil stored in the reservoir46band to the PH control valve (PH REG VLV)46c.

The power transmission system including the torque converter24, the CVT26, and the forward/backward switching mechanism28can also be construed as a continuously variable transmission having the torque converter24and the forward/backward switching mechanism28.

Referring toFIG.2again, a crank angle sensor50is provided at a suitable position in the vicinity of the cam shaft (not shown) of the engine10, and outputs a signal indicating a rotation speed NE at each predetermined crank angle of the piston. An absolute pressure sensor52is provided at a suitable position downstream of the throttle valve in the intake system, and outputs a signal proportional to an absolute pressure (engine load) PBA in the intake pipe.

A throttle opening degree sensor54is provided in the actuator of the DBW mechanism16, and outputs a signal proportional to a throttle valve opening degree TH through the rotation amount of the actuator.

In addition, an accelerator opening degree sensor56ais provided in the vicinity of the accelerator pedal56and outputs a signal proportional to an accelerator opening degree AP equivalent to the accelerator pedal operation amount of the driver, and a brake switch40ais provided in the vicinity of the brake pedal40and outputs an ON signal in accordance with the operation of the driver on the brake pedal40.

In addition, a water temperature sensor (not shown) is provided in the vicinity of a cooling water passage (not shown) of the engine10, and generates an output in accordance with an engine cooling water temperature TW, that is, the temperature of the engine10.

The outputs of the crank angle sensor50, etc., are transmitted to an engine controller66. The engine controller66includes a microcomputer having a CPU, a ROM, a RAM, an I/O, etc., and, based on these sensor outputs, determines a target throttle opening degree to control the operation of the DBW mechanism16and determines the fuel injection amount to drive the injector20.

An NT sensor (rotation speed sensor)70is provided on the main shaft MS, and outputs a pulse signal indicating the rotation speed of the turbine runner24b, specifically the rotation speed NT of the main shaft MS, and more specifically the transmission input shaft rotation speed (and the input shaft rotation speed of the forward clutch28a).

An NDR sensor (rotation speed sensor)72is provided at a suitable position in the vicinity of the drive pulley26aof the CVT26, and outputs a pulse signal in accordance with a rotation speed NDR of the drive pulley26a, in other words the rotation speed of the output shaft of the forward clutch28a.

An NDN sensor (rotation speed sensor)74is provided at a suitable position in the vicinity of the driven pulley26b, and outputs a pulse signal indicating a rotation speed NDN of the driven pulley26b, specifically the rotation speed of the counter shaft CS, and more specifically the rotation speed of the output shaft of the transmission.

In addition, a V sensor (rotation speed sensor)76is provided in the vicinity of the gear30bof the secondary shaft SS, and outputs a pulse signal indicating the rotation speed and the rotation direction of the secondary shaft SS (specifically a signal indicating a vehicle speed V and a signal indicating the traveling direction). Wheel speed sensors80are respectively provided in the vicinity of the four wheels including the drive wheels12and the driven wheels (not shown), and output pulse signals proportional to the wheel speeds indicating the rotation speeds of the wheels.

A range selector switch44ais provided in the vicinity of the range selector44, and outputs a signal in accordance with the range, such as R, N, D, selected by the driver.

As shown inFIG.3, a hydraulic pressure sensor82is provided on the oil passage to the driven pulley26bof the CVT in the hydraulic pressure supply mechanism46, and outputs a signal in accordance with the hydraulic pressure supplied to the piston chamber26b21of the movable pulley half body26b2of the driven pulley26b. An oil temperature sensor84is provided in the reservoir46band outputs a signal in accordance with the oil temperature (temperature TATF of hydraulic oil ATF).

The outputs of the NT sensor70, etc., including the outputs of other sensors not shown herein, are transmitted to the shift controller90. The shift controller90includes a microcomputer having a CPU, a ROM, a RAM, an JO, etc., and is configured to be able to communicate with the engine controller66.

The shift controller90excites/not excites the electromagnetic solenoids including the linear solenoid valves46jand46k(LS-DR, LS-DN) of the hydraulic pressure supply mechanism46based on the detected values to control the operations of the forward/backward switching mechanism28, the CVT26, and the torque converter24, and supplies power to the electric motor46vof the hydraulic pressure supply mechanism46to control the EOP46w.

3. Control for Lateral Pressure Increase

As shown inFIG.4, the lateral pressure control method (control method) of the CVT26according to the embodiment may be applied in the shift controller90. The respective functions of a pulley lateral pressure control part91, a lateral pressure increase correction amount calculation part92, and a hydraulic pressure command value calculation part93to be described in the following can be realized by executing a program stored in a storage device not shown herein on a CPU.

InFIG.4, the pulley lateral pressure control part91inputs a signal of the shift position selected by the range selector44, an in-gear determination signal of whether the shift position D or R is selected and the clutch of the forward/backward switching mechanism28is in the in-gear state, a traveling direction determination signal indicating whether the rotation direction of the output shaft of the CVT26is the forward direction as detected from the V sensor, an engine rotation speed NE and an input rotation speed Ni of the CVT input to the torque converter24. The pulley lateral pressure control part91calculates a slip rate ETR of the torque converter24to be described afterwards, and inputs a lateral pressure increase correction amount Δ in accordance with the properties of the torque converter24relating to the slip rate ETR from the lateral pressure increase correction amount calculation part92. The lateral pressure increase correction amount Δ calculated by the lateral pressure increase correction amount calculation part92will be described afterwards.

The pulley lateral pressure control part91calculates the slip rate of the torque converter24from the engine rotation speed NE (the rotation speed of the pump impeller24aof the torque converter24) detected by the crank angle sensor50, the rotation speed NT (the rotation speed of the turbine runner24bof the torque converter24) of the main shaft MS detected by the NT sensor70or the rotation speed NDR of the drive pulley26detected by the NDR sensor72. For example, the slip rate ETR of the torque converter can be obtained from the following equation: ETR (%)=(NDR/NE)*100. In other words, the slip rate of the torque converter24is an example of an index indicating the rotation difference between the pump impeller24aon the input side and the turbine runner24bon the output side, whereas the absolute value of the rotation difference or the rotation speed may also be adopted. In the following, when the turbine runner24bof the torque converter24rotates in the same direction with the pump impeller24a, the torque converter slip rate ETR is set to be a positive value, and when the turbine runner24bof the torque converter24rotates in the opposite direction, the value is set to be negative. When the turbine runner24brotates in the opposite direction with respect to the pump impeller24a, the engine rotation speed NE is in a positive rotation direction, and the rotation speed NDR of the drive pulley26ais in a negative rotation direction. Therefore, the smaller the negative value of the torque converter slip rate, that is, the closer the negative value to −1, the greater the absolute value of the rotation difference. In the following, the negative torque converter slip rate ETR is appropriately referred to as a reverse torque converter slip rate ETR. The reverse rotation of the torque converter24is detected as below.

The pulley lateral pressure control part91can determine whether the turbine runner24bof the torque converter24is in positive rotation or reverse rotation by using the in-gear determination signal, the shift position, and the traveling direction determination signal. Referring toFIG.2, in the case of the in-gear state in which the clutch of the forward/backward switching mechanism28is engaged, the rotation of the drive wheel12is transmitted to the main shaft MS and the turbine runner24bvia the CVT26and the forward/backward switching mechanism28. Therefore, if the vehicle retreats in the in-gear state in which the shift position of Drive (D) is selected, the turbine runner24bof the torque converter24rotates reversely with respect to the pump impeller24aindicating the rotation of the engine10. Thus, the in-gear determination signal, the shift position and the traveling direction determination signal are monitored, and when the shift position is inconsistent with the traveling direction are in the in-gear state, it can be determined that the torque converter24is reversed (see Japanese Laid-open No. 2010-078024).

When receiving the lateral pressure increase correction amount Δ from the lateral pressure increase correction amount calculation part92, the pulley lateral pressure control part91performs lateral pressure increase control which adds the lateral pressure increase correction amount Δ, so as to warrant a lateral pressure capable of coping with foot variation torque in addition to the belt slip warranty pressure during normal traveling as described in the following, and outputs the result to the hydraulic pressure command value calculation part93. The hydraulic pressure command value calculation part93calculates a hydraulic pressure command value from the lateral pressure increase result input from the pulley lateral pressure control part91, and outputs the hydraulic pressure command value to the linear solenoid valves46jand46k(LS-DR, LS-DN) of the hydraulic pressure supply mechanism46, respectively.

InFIG.5, the pulley lateral pressure control part91inputs the shift position of the range selector44, the traveling direction of the vehicle (the rotation direction detection signal of the V sensor76), the in-gear determination signal indicating whether the forward/backward switching mechanism28is engaged, the engine rotation speed NE, the rotation speed NT of the main shaft MS, and the rotation speed NDR of the drive pulley26a(Operation101).

Then, the pulley lateral pressure control part91determines whether the shift position is consistent with the traveling direction (Operation102). For example, if the vehicle travels forward at the shift position of Drive (D), the shift position is consistent with the traveling direction (YES), if the vehicle travels backward at the shift position of Drive (D), the shift position is not consistent with the traveling direction (NO), if the vehicle travels backward at the shift position of Reverse (R), the shift position is consistent with the traveling direction (YES), and if the vehicle travels forward at the shift position of Reverse (R), the shift position is not consistent with the traveling direction (NO). If the shift position is not consistent with the traveling direction (NO in Operation102), whether the in-gear state is present is determined (Operation103). Here, the in-gear state refers to a state in which one of the forward clutch or the backward brake of the forward/backward switching mechanism28is engaged. If the shift position is consistent with the traveling direction (Yes in Operation102), the lateral pressure increase control ends.

If the in-gear state is not present (NO in Operation103), the pulley lateral pressure control part91determines whether a switch-back operation is performed (Operation104). If the switch-back operation is performed (YES in Operation104), a lateral pressure increase correction amount Δ1(first lateral pressure increase correction amount) corresponding to the clutch transmission torque is calculated (Operation105). If the switch-back operation is not performed (NO in Operation104), the lateral pressure increase control ends.

If the in-gear state is present (YES in Operation103), the pulley lateral pressure control part91detects the reverse rotation of the torque converter24, and calculates the torque converter slip rate ETR (Operation106). The lateral pressure increase correction amount calculation part92uses the calculated torque converter slip rate ETR and the reverse properties of the torque converter24to calculate a lateral pressure increase correction amount Δ2(second lateral pressure increase correction amount) (Operation107). The lateral pressure increase correction amount Δ2, as will be described in the following, is set to a value smaller than the lateral pressure increase correction amount Δ1of the clutch transmission torque according to the properties when the torque converter24is reversed. This is because, in the in-gear state, unlike the switch-back, there is no sudden increase in the inertial force when the forward/backward switching mechanism28is engaged (in-gear), and thus a lateral pressure capable of coping with the foot torque variation as described in the following is sufficient.

When the lateral pressure increase correction amount Δ of the drive pulley26aand the driven pulley26bis so calculated, the pulley lateral pressure control part91controls the linear solenoid valves46jand46k(LS-DR, LS-DN) of the hydraulic pressure supply mechanism46by the hydraulic pressure command value calculation part93, and executes the lateral pressure increase control of the drive pulley26aand the driven pulley26b(Operation108).

For example, if the vehicle travels backward in the in-gear state even if the shift position is at Drive (D) (NO in Operation102and YES in Operation103), it is known that the vehicle is in the state of traveling reversely during uphill. If the vehicle is in the state of traveling reversely during uphill, the lateral pressure increase correction amount Δ is set to a value smaller than the clutch transmission torque according to the reverse property of the torque converter24. Accordingly, the lateral pressure of the drive pulley26aof the CVT26is less than the clutch transmission torque, the pressure difference in the case where the lateral pressure of the driven pulley26bis increased to a predetermined value can be sufficiently increased, and the CVT26can maintain the transmission ratio at LOW. In the following, the process of calculating the lateral pressure increase correction amount Δ2is described.

<Lateral Pressure Increase Correction Amount Δ2>

As shown inFIG.6, the lateral pressure increase correction amount calculation part92decreases the size of the lateral pressure increase correction amount Δ2as the reverse torque converter slip rate ETR increases within a predetermined range R (−70% to 0% in this example) of the reverse torque converter slip rate ETR. In this example, if the reverse torque converter slip rate ETR is less than equal to a predetermined value ETR(1)=−70%, the lateral pressure increase correction amount Δ2is fixed to a highest value Δ2(H). The lateral pressure increase correction amount Δ2decreases in accordance with the increase from ETR(1)=−70%, and decreases until being fixed to a lowest value Δ2(L) with a predetermined value ETR(2). The lateral pressure increase correction amount calculation part92has a function of returning the lateral pressure increase correction amount Δ2corresponding to the reverse torque converter slip rate ETR applied from the pulley lateral pressure control part91to the pulley lateral pressure control part91, and may maintain the relation shown inFIG.6as a table or as a formula.

As will be described in the following, the lateral pressure increase correction amount Δ2changes so that the predetermined value ETR(2) and the lowest value Δ2(L) are set in accordance with a point at which the variation of the transmission torque (foot torque variation) starts increasing in the reverse properties of the torque converter24, and the predetermined value ETR(1) and the highest value Δ2(H) are set in accordance with the highest amplitude of the foot torque variation, and a lateral pressure corresponding to the foot variation torque is warranted within the predetermined range R. In this way, according to the embodiment, in the case where the shift position is not consistent with the traveling direction in the in-gear state (e.g., the case where the torque converter slip rate ETR is negative at the time of traveling reversely during uphill), the lateral pressure increase correction amount Δ2is changed as shown inFIG.6within the predetermined range R of the reverse torque converter slip rate ETR. In the following, the predetermined reverse torque converter slip rate ETR(1) is set to be −70% to describe the setting of the lateral pressure increase correction amount Δ2.

As shown inFIG.7, the reverse torque converter properties in the case where the pump impeller24aand the turbine runner24bof the torque converter24rotate in opposite directions exhibit a negative slope region in which the decrease rate of the torque ratio increases as the absolute value of the reverse torque converter slip rate ETR increases. On the contrary, in the case where the pump impeller24aand the turbine runner24bof the torque converter24rotate in the same direction, there is a positive slope region in which the decrease rate of the torque ratio decreases as the absolute value of the torque converter slip rate ETR increases. That is, the torque converter exhibits a property in which, regardless of the torque converter slip rate ETR being positive or negative, the smaller the absolute value of the torque converter slip rate ETR, the greater the amplification rate of the torque of the torque converter24.

For example, when the vehicle travels reversely during uphill until entering the negative slope region, the foot torque variation due to torque converter judder starts occurring, as shown inFIG.8. Since the turbine of the torque converter24rotates in the opposite direction to the pump when the vehicle travels reversely during uphill, the rotation difference increases when the vehicle speed increases, and the rotation difference decreases when the vehicle speed decreases. Therefore, the foot torque variation shows a tendency of increasing as the vehicle speed increases and decreasing as the vehicle speed decreases. Here, the foot torque variation starts increasing when the reverse torque converter slip rate ETR becomes less than −70%. However, in the predetermined range R greater than −70%, the transmission torque transmitted from the torque converter24is smaller, and the torque variation is also smaller. Therefore, in the predetermined range R greater than −70%, as shown inFIG.6, the lateral pressure increase correction amount Δ2decreases in correspondence with the foot torque variation. Thus, in the in-gear state, since there is no sudden increase in the inertial force when the clutch of the forward/backward mechanism28is engaged, it suffices as long as a lateral pressure capable of coping with the foot torque variation is generated. Accordingly, as described in the following, even if the lateral pressure of the drive pulley26ais relatively small, belt slippage is sufficiently warranted, and the transmission ratio of the CVT26can be kept LOW.

4. Operation Example

Referring toFIG.9, the operation of the drive system according to the embodiment is described by using the case (FIG.1) in which the shift position is maintained at D while the vehicle restarts from traveling reversely during uphill as an example.

It is set that, when going uphill at the shift position of D (Drive), the brake is stepped on, and the vehicle14in which the CVT26is mounted stops. Then, the brake is released while the shift position stays at D (Drive), and at a time point t1, the vehicle14starts going backward ((a) and (b) ofFIG.9). At this time, the pulley ratio of the CVT26is LOW ((e) ofFIG.9). Since the vehicle14goes backward in a reverse direction with respect to the shift position of D in the in-gear state, the pulley lateral pressure control part91determines that the torque converter24is reversed, and performs control for increasing the lateral pressure of the driven pulley26b, and performs control of increasing the lateral pressure applied to the drive pulley26aby using the reverse torque converter slip rate ETR and the lateral pressure increase correction amount Δ2calculated from the relation shown inFIG.6.

The lateral pressure increase correction amount Δ2is a value smaller than the lateral pressure increase correction amount Δ1at the time of switch-back as described above. As shown in (f) ofFIG.9, the difference (DN−DR pressure difference) between the hydraulic pressure (DN pressure) of the driven pulley26band the hydraulic pressure (DR pressure) of the drive pulley26acan be greater than the conventional DN−DR pressure difference.

At the time point t1at which the brake is released, the vehicle speed of the vehicle14gradually increases, and the vehicle14travels backward ((b) ofFIG.9). At this time, if the accelerator pedal is stepped on at a time point t2((c) ofFIG.9), and the rotation speed NE of the engine10increases ((d) ofFIG.9), due to the in-gear state at the shift position of D, the drive force in the forward direction increases. Accordingly, after stopping (time point t3), the vehicle14starts moving forward (uphill). However, since the lateral pressure of the drive pulley26aat this time is added with the relatively small lateral pressure increase correction amount Δ2, even if the lateral pressure of the driven pulley26bis increased to the maximum pressure, the hydraulic pressure difference between the drive pulley26aand the driven pulley26bcan still be greater than the conventional hydraulic pressure difference ((f) ofFIG.9). Therefore, the transmission ratio of the CVT26can be maintained to be near to LOW ((e) ofFIG.9), and a favorable uphill acceleration is obtained ((b) ofFIG.9).

As described above, according to the embodiment, in the case where the vehicle travels (reversely) in a direction opposite to the shift position in the in-gear state, the lateral pressure difference between the drive pulley26aand the driven pulley26bcan be optimized, and the belt slip as well as the deviation of the transmission ratio from LOW can be prevented. In the following, the effect of the embodiment is described with reference toFIG.10.

As shown inFIG.10, since the switch-back operation in the conventional example is not in the in-gear state, by applying, to the drive pulley, a lateral pressure provided in the sudden increase of the inertial force when the clutch is engaged, belt slippage during reverse traveling is warranted. Therefore, in the case where the lateral pressure of the driven pulley is increased to the upper limit, when a high lateral pressure is applied to the drive pulley, it is possible that the hydraulic pressure difference (DN−DR pressure difference) between the driven pulley and the driven pulley is not sufficient. In the case where the backward-traveling vehicle stops and restarts by the accelerator operation while the shift position stays at Drive (D) in the in-gear state, as described inFIG.1, the transmission ratio of the CVT cannot be kept at LOW. As a result, the uphill climbing performance is poor during re-acceleration from the reverse traveling during uphill.

Regarding this, according to the embodiment, when reverse traveling is detected in the in-gear state, the lateral pressure of the drive pulley26ais increased by the lateral pressure increase correction amount Δ2taking into consideration the foot torque variation, and the lateral pressure increase correction amount Δ2is set to be smaller than the lateral pressure increase correction amount Δ1provided under the assumption of switch-back. This is because, in the in-gear state, unlike the switch-back, there is no sudden increase in the inertial force when the clutch of the forward/backward switching mechanism28is engaged, and a lateral pressure capable of coping with the foot torque variation suffices. That is, with respect to the foot torque variation occurring during reverse traveling, belt slippage can be warranted with the lateral pressure increase correction amount Δ2smaller than the lateral pressure increase correction amount Δ1. Accordingly, as shown inFIG.9, in the case where the lateral pressure of the driven pulley26bis increased to the upper limit, the hydraulic pressure difference (DN−DR pressure difference) between the drive pulley26aand the driven pulley26bcan be increased. Therefore, even in the case where the backward-traveling vehicle stops and restarts by the accelerator operation while the shift position stays at Drive (D) in the in-gear state, as described inFIG.1, belt slippage can be warranted, and the transmission ratio of the CVT26can be kept at LOW. That is, the belt26cof the CVT26can be reliably protected without deteriorating traveling performance, and the driver's discomfort can be eliminated.

According to the embodiment, the lateral pressure increase correction amount Δ2at the time during reverse traveling in the in-gear state is calculated based on the reverse torque converter properties of the torque converter24. For example, the foot torque variation decreases within the predetermined range R where the torque converter slip rate ETR of the reverse torque properties is greater than the predetermined value (−70%), and the foot torque variation increases within the range smaller than the predetermined value (−70%). By using this phenomenon, the greater the reverse slip rate ETR within the predetermined range R, the smaller the lateral pressure increase correction amount Δ2can be than the lateral pressure increase correction amount Δ1at the time of switch-back. Accordingly, the hydraulic pressure difference (DN−DR pressure difference) between the drive pulley26aand the driven pulley26bis increased, and, as described above, even in the case of restarting from reverse traveling in the in-gear state, it is possible to warrant belt slippage and keep the transmission ratio of the CVT26at LOW.