EP2379393A1

EP2379393A1 - System and method for controlling an infinitely variable transmission during a start under low grip conditions

Info

Publication number: EP2379393A1
Application number: EP09803827A
Authority: EP
Inventors: Julien Maynard
Original assignee: Renault SAS
Current assignee: Renault SAS
Priority date: 2009-01-19
Filing date: 2009-12-10
Publication date: 2011-10-26
Also published as: FR2941268A1; FR2941268B1; KR20110113749A; JP2012515313A; WO2010081948A1

Abstract

Method for controlling a continuously variable transmission mounted between an internal combustion engine (5) of an automotive vehicle and the driven wheels (11), the continuously variable transmission being able to operate in at least one manual mode and an assistance mode. The control method involves the steps during which: - a first torque ratio corresponding to a CVT assistance mode is estimated, - a second torque ratio between the input torque and the output torque of the CVT is calculated at each moment, - the first and second ratios are compared, - from this, a limiting torque of the internal combustion engine is deduced.

Description

System and method for controlling infinitely variable transmission during low adhesion starting

The field of the invention is the control of transmissions for motor vehicles, more specifically, the control of continuously variable transmissions (called "CVT" in the present description).

Starting a motor vehicle on a low traction coefficient surface without drive wheel slip involves lowering the torque to the wheel. To obtain such conditions, the drivers of vehicles equipped with a manual gearbox or automatic gearbox with several gears are generally placed on the second or third gear in order to obtain a reduction of the torque to the wheel.

In the case of certain continuously variable transmissions, it is not possible to modify the ratio when the vehicle is stationary. Indeed, the variable speed ratio device is stopped when the vehicle is stopped. It is therefore not possible, without assistance, to start with torque characteristics at the wheel and rotational speed of the drive members comparable to a start in second or third gear of a manual or automatic gearbox.

The US patent application US 2006-0014609 describes a method of controlling a continuously variable gearbox, also applicable to a manual or automatic gearbox, to reduce the torque to the wheel on slippery ground. For this, a mode of operation operated by the driver, reduces the engine torque. The torque at the wheel is reduced accordingly. The torque reduction is achieved by means of a calibration, reducing the engine torque at low rotational speed of the drive member and modifying little torque at high rotational speed. Several different calibrations are mentioned. However, it should be noted that the development of such calibrations is expensive and that their use is not adapted to the various conditions that may be encountered by a driver.

US patent application US5586953 discloses a control method for improving engine braking by limiting the ability of the transmission to return to short reports.

JP 1344109 discloses a method of limiting the torque ratio so that the vehicle stops at a torque ratio corresponding to the second gear. Thus, the vehicle restarts on a second report, promoting adhesion. It should be noted that these methods limit the ratio of the transmission before stopping the vehicle in order to avoid slippage. These methods have no effect if they are activated when the vehicle is stationary.

An object of the invention is a system and method for controlling a continuously variable transmission on slippery ground in which the engine torque is reduced in order to simulate for the driver a second or third gear ratio.

Another object of the invention is a system and method for controlling a continuously variable transmission which retains the perception of the driver of discrete ratios.

Another object of the invention is a system and method of controlling a continuously variable transmission allowing the driver to better dose the torque to the wheel to facilitate starting on slippery ground. According to one embodiment, a method for controlling a continuously variable transmission (CVT), mounted between an internal combustion engine of a motor vehicle, and the driving wheels, the continuously variable transmission being able to operate in at least one manual mode and a mode of assistance. The control method comprises the steps in which:

it is estimated a first torque ratio corresponding to a mode of assistance of the CVT,

a second torque ratio between the input torque and the output torque of the CVT is calculated at each instant, the first and the second ratios are compared, from which is deduced a limit torque of the internal combustion engine. For a continuously variable transmission comprising among different modes of assistance at least one snow mode of different ratio of the first gearbox report, it is possible to activate a takeoff phase or a snow mode re-acceleration phase according to the torque request. of the driver and the difference between the rotational speed of the motor and the input rotational speed of the continuously variable transmission with respect to a stored threshold. The calculation of the limit torque during a re-acceleration phase can be obtained from the comparison of the ratio of the transmission, the ratio resulting from a variogram in manual mode for the report considered, the ratio resulting from a variogram of the mode assistance and torque of the combustion engine. You can activate a second report snow mode when, at the same time:

- the driver requires the passage of a higher gear,

the vehicle speed is lower than a first stored speed, and the active operating mode is the manual mode.

You can activate a third report snow mode when, simultaneously:

- the passage of a higher gear is required, and

- the active operating mode is the second report snow mode.

It is possible to memorize, when switching from an assistance mode to a manual mode, the previously active assistance mode in order to carry out a subsequent reactivation of said assistance mode when the ratio conditions are satisfied. According to another embodiment, a system for controlling a continuously variable transmission, mounted between an internal combustion engine of a motor vehicle, and the driving wheels, the continuously variable transmission being able to operate, is defined. in at least one manual mode and a mode of assistance. The system includes:

multiplication means capable of determining at each instant a first torque ratio between the input torque and the output torque of the CVT,

multiplication means capable of estimating a second torque ratio corresponding to a mode of assistance of the CVT,

a calculation means capable of estimating a correction parameter as a function of the ratio between the first and second ratios, a determining means able to determine the limit torque of the internal combustion engine as a function of the correction parameter.

The control system may comprise at least one mapping of the ratio of the rotation speed of the internal combustion engine according to the speed of the vehicle for each report and for each mode. The control system can then be able to determine the ratio and the limit torque as a function of at least one mapping and operating parameters of the vehicle.

The control system may comprise a memory and the continuously variable transmission may comprise among different modes of assistance at least one snow mode of ratio different from the first gear ratio. The control system can then be able to memorize the report corresponding to the last active snow mode during a switch to manual mode. Other objects, features and advantages of the invention will become apparent on reading the following description, given solely by way of nonlimiting example and with reference to the appended drawing in which:

FIG. 1 illustrates a method for controlling a continuously variable transmission on low adhesion,

FIG. 2 illustrates the main elements included in and connected to a continuously variable transmission, FIG. 3a illustrates the main steps of the method for determining the conditions of activation or deactivation of the different modes of the control method,

FIG. 3b illustrates the main storage steps of the control method.

FIG. 4 illustrates the main steps of the method of changing the snow mode of the control method,

FIG. 5 illustrates the main steps of the method of calculating the ratio of the control method; FIG. 6 illustrates the main steps of the method of limiting the torque at the wheel of the control method;

FIG. 7 illustrates the main steps of the method of limiting the torque to the wheel in the case of a re-acceleration, and

FIG. 8 illustrates the main elements of a control system.

Figure 1 illustrates the main steps of the control method 1 of a continuously variable transmission on slippery floor. The control method comprises a method 2 for determining the conditions for activating or deactivating the various modes, a method 3 for calculating the ratio and a method 4 for limiting the torque to the wheel.

Figure 2 illustrates the main elements included in and connected to a continuously variable transmission. A motor 5 is connected at the input of a continuously variable transmission. At least one drive wheel 1 1 is connected at the output of the continuously variable transmission. The continuously variable transmission comprises a hydraulic torque converter 6, a device 7 for changing the direction of travel, a primary pulley 8, a secondary pulley 9 and a distribution device 10. The hydraulic torque converter 6 comprises an impeller 6a, a stator 6b and a turbine 6c. A switch 6d said lock-up is also present in order to be able to secure the turbine 6c to the impeller 6a in order to obtain a direct link between the primary shaft and the motor shaft. Several quantities make it possible to characterize some of the organs described above. We can mention the Veh_Speed vehicle speed, Turbine_Torque turbine output torque, Engine_rev engine rotation speed, CVT_Oil_Temp continuously variable transmission oil temperature, Sec_Pulley_tq secondary pulley output torque and the rotational speed of turbine Turbine_rev.

In addition, the variable Ratio is defined as the ratio between the speed of rotation of the primary pulley 8 and the speed of rotation of the secondary pulley 9.

We also define the Lockup variable that reports the state of the so-called lockup switch. This variable can take two values depending on whether the switch is engaged or disengaged.

FIG. 3a illustrates the main steps included in the method for determining the activation or deactivation conditions of the different modes of the control method.

The control method includes a manual mode, a second report snow mode and a third report snow mode. The manual mode does not include take - off assistance and corresponds to a mode in which all the gear ratios are accessible according to conditions dependent on the speed of the vehicle.

In particular, when stopped, only the first report can be activated. The second-report snow mode corresponds to an assisted mode in which the behavior of a vehicle for which the second gear is active is simulated, thanks to a limitation of the engine torque by the continuously variable transmission. The driver thus perceives a behavior of the vehicle similar to the behavior that the vehicle would have if the second report of the manual mode was activated. Similarly, the third report snow mode corresponds to an assisted mode in which the third report is simulated. Switching from one mode to another can be triggered by a request from the driver.

In addition, the snow modes each include a snow take-off mode and a snow re-acceleration mode, mutually exclusive. The mode of activation of these modes will be detailed later.

The normal manual mode includes, for each report, a map of the Engine_rev engine rotation speed as a function of the Veh_speed vehicle speed.

Limit mapping is also present in order to define the speed of rotation below which the primary pulley must not descend. Such mapping therefore limits the engine_rev engine rotation speed. The set of mappings related to the manual mode is called variogram.

The control method begins in step 12 with the vehicle in the manual mode. The stopping or crawling conditions of the vehicle are realized. A vehicle is said in a ramping situation, if its speed is less than a threshold speed V l, determined by calibration.

If the vehicle speed is lower than the value V l, and the driver requests a shift for a higher gear, and the condition Accel_off is satisfied, then the process continues in step 13, the active mode being then the snow mode 2. The Accel_off condition is true if the acceleration pedal is released. More particularly, step 13 corresponds to the so-called second takeoff snow mode. If the difference between the rotational speed of the Engine_Rev engine and the rotational speed of the turbine Turbine_Rev is less than a threshold Thdl, and the condition

Accel_off is checked, then the process continues in step 14. Step 14 corresponds to the so-called second-rate re-acceleration snow mode. Indeed, the calculation of the limiting torque must be made differently in the cases of takeoff and cases of re-acceleration. If the difference between the rotation speed of the motor

Engine_Rev and the turbine rotation speed Turbine_Rev is greater than a threshold Thd2, and the condition Accel_off is satisfied, then the process continues in step 13. Similarly, if the vehicle speed Veh_Speed is below the threshold Thd , the process continues In step 13. Following step 13 and step 14, the method can proceed to step 17 if the Ratio variable is less than or equal to the R2_exit value.

Following step 13 or step 14, if the vehicle speed is lower than the value V 1, and the driver requests a gearshift for a higher gear, and the condition Accel_off is verified, then the method continues in step 15, the active mode then being the snow mode 3. More particularly, step 15 corresponds to the so-called snow-lift mode of the third gear. If the difference between the rotation speed of the motor

Engine_Rev and the turbine rotation speed Turbine_Rev is lower than a Thdl threshold, and that the Accel_off condition is verified, then the process continues in step 16. Step 16 corresponds to the so-called re-acceleration snow mode. third report. Indeed, the calculation of the limiting torque must be performed differently in the case of takeoff and cases of re-acceleration. If the difference between the engine rotation speed Engine_Rev and the turbine rotation speed Turbine_Rev is greater than a threshold Thd2, and the condition Accel_off is satisfied, then the process continues in step 15. Similarly, if the vehicle speed Veh_Speed is below the threshold Thd, the process continues in step 15. Following step 15 and step 16, the process can continue in step 17, if the variable Ratio is less than or equal to the R3_exit value. Following step 15 or step 16, if the driver makes a downshift request, the process proceeds to step 13 if the

Ratio is greater than R2_exit. If the Ratio value is less than or equal to the R2_exit value, the process continues in step 17.

Step 17 corresponds to a standard manual mode without assistance. If the vehicle stops, the process proceeds to the previously defined step 12. If a value corresponding to the second report in snow mode is stored, and if the ratio is greater than or equal to the value R2_act then the process continues in step 14. If a value corresponding to the third report in snow mode is stored, and if the Ratio is greater than or equal to the value R3_act then the process continues in step 16.

FIG. 3b illustrates the rules for storing and erasing the storage of the various snow modes applied by the control method.

The storage method of Figure 3b is performed in parallel with the control method of Figure 3a.

The storage method starts with step 18 in which no storage is present. As soon as the snow mode 2 is activated, for example when going from step 12 to step 13, the storage method continues in step 19, by storing the activation of the snow mode 2. of a transition to the snow mode 3, the storage method continues in step 20, by storing the activation of the snow mode 3. Conversely, if the snow mode 2 is reactivated, the storage method continues to step 19, by storing the activation of the snow mode 2.

When the storage method is in steps 19 or 20, if the vehicle speed Veh_Speed is greater than or equal to a value V2, the storage method continues in step 18 by erasing the storage.

Likewise, when the storage method is in steps 19 or 20, if a gearshift request is made while the vehicle speed is greater than or equal to the value V 1, the storage process continues at the same time. step 18 by erasing the storage.

In other words, a gearshift request when the vehicle is stopped or in ramp and in the manual mode, causes the passage in the snow mode 2. A subsequent request to change gear uphill causes the passage in snow mode 3.

In contrast, a downshift request for a gear while the vehicle is in snow mode 3 causes the switch to snow mode 2. A downshift gear change request while the vehicle is in snow mode 2 causes the switch to normal manual mode.

When a mode change is required, a value corresponding to the last activated snow mode can be memorized. Thus, a vehicle whose ratio falls below the ratio corresponding to the snow mode 2, keeps in the form of a memorized value that the last active snow mode was the snow mode 2. Similarly, a vehicle which exceeds the mode ratio snow 3, keeps in the form of a stored value that the last active snow mode was snow mode 3.

This memorization of the last activated snow mode can be erased if the vehicle speed deviates above a calibrated limit speed V2. The memory can also be cleared if the driver actuates the control lever, for example to engage a higher gear, a lower gear or the driving mode, whether the snow mode is active or not. Driving mode ("drive" mode) is generally present in automatic or continuously variable gearboxes.

In the case where a snow mode value is stored, an additional mode change mechanism is possible, illustrated in Figure 3a.

If the storage of the snow mode 2 is active and the value Ratio is greater than a stored value R2_act, the control method activates the snow mode 2. Similarly, if the storage of the snow mode 3 is active and the value Ratio higher at a stored value R3_act, the control method activates the snow mode 3.

On the other hand, if the snow mode 2 is active and the Ratio value is lower than a stored value R2_exit, the control method deactivates the snow mode 2 in favor of the standard manual mode. If the snow mode 3 is active and the Ratio value is lower than an R3_exit stored value, the control method disables the snow mode 3 in favor of the standard manual mode. In the latter two cases, the variable Ratio corresponds to the engaged ratio, it is therefore not necessary to limit the torque to the wheel. Figure 4 describes the main steps of the method of changing the snow mode of the control method. FIG. 4 illustrates the case of the snow mode 3. The case of the snow mode 2 can be illustrated in an identical way by substituting the references relating to the snow mode 3 and the corresponding references relating to the snow mode 2. FIG. 3 of Figure 1.

The mode change process begins with step 21 in which it is determined whether the snow mode 3 is activated or stored. If the snow mode 3 is activated or stored, the method continues in step 22, otherwise the standard manual mode is activated in step 25. In step 22, it is determined whether the vehicle speed Veh_speed is greater than or equal to the limit speed V3. If this is the case, the method continues in step 23, otherwise the first report ratio setpoint is issued in step 26. In step 23, it is determined whether the active ratio variable is greater than or equal to the limit value R3_exit.

If this is the case, the method continues in step 24, otherwise the ratio setpoint is determined according to the mapping of the standard manual mode in step 27. In step 24, it is determined whether the vehicle is in a take-off phase, ie if a snow mode is activated and the re-acceleration snow mode is not activated. If this is the case, the method continues in step 29, otherwise the method continues in step 28 during which the ratio setpoint is determined according to the velocity dependent limit map. In step 29, the ratio setpoint is determined according to the mapping of the third gear manual mode.

After steps 25 to 29, the process resumes in step 21. FIG. 5 illustrates the main elements of a limit torque calculation method. The elements present in this figure correspond to block 4 of FIG. 1. The torque limitation described in this figure corresponds to an operation of the vehicle while the snow mode is activated. This calculation is done when the second take-off snow mode or the third gear takeoff snow mode is activated. The calculation method begins with step 30 during which the target ratio of the transmission as a function of the vehicle speed Veh_speed is determined using the manual mode second gear mapping. The target ratio is the ratio setpoint, which is the ratio that we want transmission to adopt.

During step 31, the target ratio of the transmission as a function of the vehicle speed Veh_speed is determined using the manual mode of third gear mapping. During step 32, it is determined which ratio calculated during steps 30 and 31 should be used according to the active snow mode at the time of calculation. The ratio thus used is represented by the variable Rneige. The ratio Rneige represents the ratio that the driver would like to have and that will be simulated since the physical system can not actually adopt this ratio guideline. In step 33, the rotational speed of the turbine is determined in snow Turbine_rev_Rneige mode by producing the product of the ratio Rneige by the speed of rotation of the secondary pulley Sec_pulley_rev. In step 34, the torque at the outlet of the turbine is determined in Snow Turbine_Torque_Rneige mode as a function of the engine rotation speed Engine_rev, the speed of rotation of the turbine in snow mode Turbine_rev_Rneige, the state of the Lockup, CVT_Oil_Temp continuously variable transmission oil temperature and Engine_Torque_Driver driver torque request. In step 35, the torque at the output of the secondary pulley in snow mode Sec_Pulley_tq_neige is determined by multiplying the output torque of the turbine in snow mode Turbine_Torque_Rneige by the ratio in snowfall mode Rneige determined in step 32. step 36, the torque ratio in snow mode Rcouple_neige is determined by dividing the output torque of the secondary pulley in snow mode Sec_Pulley_tq_neige of step 35 by the motor torque request of the driver Engine_Torque_Driver.

In step 39, the torque at the outlet of the turbine is determined

Turbine_Torque depending on the rotation speed of the motor

Engine_rev, turbine rotation speed Turbine_rev, Lockup condition, transmission oil temperature continuously variable CVT_Oil_Temp, and engine torque Engine_Torque. In step 40, the output torque of the secondary pulley Sec_Pulley_tq is determined by multiplying the torque at the output of the turbine Turbine_Torque by the variable Ratio. In step 41, the torque ratio Rcouple is determined by dividing the output torque of the secondary pulley Sec_Pulley_tq of step 40 by the motor torque Engine_torque.

In step 37, the torque limiting factor Tq_limit_factor is determined by dividing the torque ratio in snowfall mode of step 36 by the torque ratio Rcouple of step 41. In step 38, the the Raw_Engine_Tq_limit motor torque limiting raw value by realizing the torque limiting factor product Tq_limit_factor by the engine torque request of the driver Engine_Torque_Driver. In other words, the control method comprises a method for determining the ratio in snow mode, a method for determining the torque ratio in snow mode and a method for determining the current torque ratio.

The method for determining the ratio in snow mode comprises the steps 30, 31 and 32. It determines the target ratio of torque as a function of the speed of the vehicle and according to the chosen snow mode ratio. It has the advantage of simultaneously determining the second and third ratio ratios. The determining process during step 32 which of the two ratios to choose, according to the report in active snow mode.

The method for determining the current torque ratio comprises steps 39 to 41. The torque ratio of the continuously variable transmission is calculated at each instant.

The torque ratio Rcouple is defined as the ratio between the torque at the output of the secondary pulley of the continuously variable transmission Sec_pulley_tq and the Engine_Torque torque of the motor at the input of the continuously variable transmission.

_. , Dry pulley tq

Rcouple = - -

Engine_Torque The output torque of the secondary pulley of the continuously variable transmission Sec_pulley_tq is defined as the product of the torque at the output of the turbine Turbine_Torque by the variable Ratio which corresponds to the ratio of the speed of rotation of the primary pulley by the speed of the pulley secondary.

_,. Pri Pulley Rev

Ratio = - -

Sec_pulley_Rev

When the Lockup switch is closed, we have:

Turbine_Torque = Engine_Torque

It is thus possible to determine the torque ratio Rcouple. However, when the Lockup switch is open, the previous calculation is no longer possible, the value of the output torque of the turbine Turbine_Torque not being directly accessible. In order to be able to determine the torque ratio, the output torque of the turbine Turbine_Torque is then estimated as a function of the characteristics of the converter, the turbine rotation speed Turbine_rev, the engine rotation speed Engine_rev, and the temperature of the continuously variable transmission oil Temp_CVT.

The output torque of the Turbine_Torque turbine is determined by the following calculation:

Turbine_Torque = Z (i, CVT_Oil_Temp) • Engine_rev ² • K (i, CVT_Oil_Temp)

With i = Turbine_rev / Engine_rev,

K: the torque gain of the converter and Z: the TAU factor of the converter

The method for determining the torque ratio in snow mode comprises the steps 33 to 36. The torque ratio of the continuously variable transmission corresponding to the chosen snow mode ratio is calculated at each instant. The steps of the method are similar to steps 39 to 41, considering the snow mode variables corresponding to the common variables used in these steps.

Since the snow mode corresponds to an abstraction which is not representative of the current state of the vehicle, the speed of rotation of the turbine in snow mode Turbine_Rev_neige is not accessible by the measurement. It is determined indirectly as follows:

Turbine_Rev_neige = Sec_pulley_rev • Rneige

The output torque of the turbine in Turbine_Torque_Rneige snow mode is determined by performing a calculation similar to that used to determine the output torque of the turbine Turbine_Torque, using the speed of rotation of the turbine in snow mode Turbine_rev_Rneige instead of the rotational speed of turbine Turbine_rev.

The output torque of the turbine in Turbine_Torque_Rneige snow mode is then defined as follows:

Turbine _ Torq ⁴ ue - Rneig ^ë e = • • •

• • • Z (i_neige, CVT_Oil_Temp) • Engine_rev • K (i_neige, CVT_Oil_Temp)

The i_neige variable is defined as:

Turbine_rev_Rneige i_neige = -

In ¹ gOine rev

At the end of these two methods for determining torque ratios, the control method then has the value of the torque ratio Rcouple and the value of the torque ratio in snow mode Rcouple_neige. It is then possible to determine the factor resulting from the ratio between these two values, then to determine the torque limit of the engine Raw_Engine_Tq_limit by multiplying said factor by the torque request of the driver. Figure 6 illustrates an alternative method of determining the Rneige ratio taking into account the speed of the vehicle Veh_Speed and the depression of the accelerator pedal TVO. The alternative method of determining the Rneige ratio begins with a step 42 in which a fine determination of a correction variable called Delta_Ratio is made as a function of the vehicle speed Veh_Speed and the depression of the accelerator pedal TVO . The Delta_Ratio variable is an adjustment variable that allows you to precisely control the Snow variable depending on the speed of the vehicle and depending on the depression of the accelerator pedal through certain settings. Moreover, a step 43 makes it possible to determine the rotation speed of the primary pulley in manual mode Npri_manual as a function of the vehicle speed Veh_Speed. In step 44, the manual ratio Ratio_Manual is determined by dividing the speed of rotation of the primary pulley into manual mode.

Npri_manual by the speed of rotation of the secondary pulley Sec_Pulley_Rev. In step 45, the ratio Rneige is determined by adding the Ratio value of step 42 and the manual Ratio_manuel ratio of step 44. The Rneige value is then substituted for the value calculated by the control method at step 44. from step 32 of FIG.

Figure 7 illustrates the main elements of the wheel torque limiting method in the case where the third gear re-acceleration snow mode is active. The selected ratio is the third report, but the method can be adapted to the second report by replacing the third report variables with the second report variables.

The method starts at step 46 by subtracting the Ratio value of the snow mode reference ratio for the third ratio Cxx_snw_trq_lim_3. The Cxx_snw_trq_lim_3 Cxx_snw_trq_lim_3 snow mode reference ratio is a calibration value to determine the maximum torque limitation for the third gear re-acceleration snow mode. Step 47 determines the difference between the reference ratio in snow mode for the third report Cxx_snw_trq_lim_3 and the ratio of third report Cxx_man_3_gear. The third gear ratio Cxx_man_3_gear is a calibration value representing the third gear target ratio. In step 48, the deviation of the current ratio and the third ratio ratio is determined by dividing the result of step 46 by the result of step 47. The deviation represents the difference between the current ratio and the target ratio. In step 49, this deviation is limited between the values 0 and 1. In other words, any value greater than the value 1 is replaced by the value 1. Similarly, any value less than the value

0 is replaced by the value 0. The values between 0 and 1 are kept as they are.

In step 50, the torque limitation is determined in the case of a snow mode in re-acceleration Rtq_limit_reac according to a mapping depending on the deviation Rdeviation.

During step 51, the limited torque is determined in the case of a Raw_Engine_Tq_Limit re-acceleration snow mode by multiplying the engine_Torque_driver driver torque demand by the torque limitation in the case of a snow mode in the same mode. - acceleration Rtq_limit_reac.

When changing from the snow mode, in which the driver's torque request is limited, to the manual mode, in which the torque request of the driver is not limited, it is important that the torque request does not change sharply. For this, a gradient can be applied to gradually increase the torque request acceptable by the system so as to limit jolts.

Figure 8 illustrates the main elements included in a control system according to the invention. Sensors 64 are connected to maps 52 and 53. Mapping 52 corresponds to the second report snow mode, mapping 53 corresponds to the third report snow mode. The outputs of the maps 52 and 53 are connected to at least one input of the switching means 54. The means 54 also receives from the sensors 64 a two-state logic signal corresponding to the second or third report snow mode. The output of the switching means 54 is connected to a means 55 for multiplication. The multiplying means 55 is connected by its other input to the sensors 64. The output of the multiplication means 55 is connected to the means 56 for determining the torque of the turbine in snow mode.

The determining means 56 is input-connected to the sensors 64. The computation means 57 of the torque of the secondary pulley is connected at input to the determination means 56 of the torque of the turbine in snow mode and to the switching means 54, and is connected to the output by means of multiplication 58. The multiplication means 58 is furthermore connected to the sensors 64 at the input, and at the output to the calculation means 59.

The determining means 61 is input-connected to the sensors 64. The computation means 62 of the current torque of the secondary pulley is input-connected to the turbine torque determination means 61 and to the sensors 64, and is connected to the output multiplication means 63. The multiplication means 63 is furthermore connected to the input sensors 64 and output to the calculation means 59.

The calculation means 59 is connected by its output to the limit torque determining means 60, itself connected to the internal combustion engine 5 by a control means 65.

The control method and the control system of a continuously variable transmission make it possible to limit the slippage of the driving wheels of a motor vehicle when starting on slippery ground. The torque and driver ratio requests are taken into account in order to adapt the engine torque according to the actual ratio of the transmission to simulate different transmission ratios of the actual ratio. Such a system makes it possible to maintain the sensation of the gear ratios while allowing the driver to exercise optimum control of the torque at the wheel, allowing easy starting and without slipping.

Claims

A method of controlling a continuously variable transmission, mounted between an internal combustion engine (5) of a motor vehicle, and the drive wheels (1 1), the continuously variable transmission being operable in at least one manual mode and a mode of assistance, characterized in that it comprises the steps in which:

it is estimated a first torque ratio corresponding to a mode of assistance of the continuously variable transmission,

a second torque ratio between the input torque and the output torque of the continuously variable transmission is calculated at each instant,

the first and the second ratios are compared, from which is deduced a limit torque of the internal combustion engine.

2. Control method according to claim 1, for a transmission comprising among different modes of assistance at least one snow mode of different ratio of the first speed gear ratio, in which a take-off phase or a recovery phase is activated. acceleration of the snow mode according to the torque request of the driver and the difference between the rotation speed of the motor and the rotation speed at the input of the continuously variable transmission with respect to a stored threshold.

3. Control method according to one of the preceding claims, wherein the calculation of the limit torque during a re-acceleration phase is derived from the comparison of the ratio of the transmission, the ratio from a variogram in manual mode. for the ratio considered, the ratio resulting from a variogram of the assistance mode and the torque of the combustion engine.

4. Control method according to one of claims 2 to 3, wherein activates a second-rate snow mode when, simultaneously:

- the driver requires the passage of a higher gear, the speed of the vehicle is lower than a first stored speed, and

- the active mode of operation is the manual mode.

5. The control method as claimed in claim 4, in which a third report snow mode is activated when, simultaneously:

- the passage of a higher gear is required, and

- the active operating mode is the second report snow mode.

6. Control method according to one of the preceding claims, wherein is memorized when switching from a mode of assistance to a manual mode, the assistance mode previously active in order to proceed to a subsequent reactivation of said mode of operation. assistance when ratio conditions are met.

7. A system for controlling a continuously variable transmission, mounted between an internal combustion engine (5) of a motor vehicle, and the drive wheels (1 1), the continuously variable transmission being operable in at least one manual mode and a mode of assistance, characterized by the fact that it comprises:

multiplication means (58) capable of determining at each moment a first torque ratio between the input torque and the output torque of the CVT,

multiplication means (63) capable of estimating a second torque ratio corresponding to a mode of assistance of the CVT,

calculation means (59) capable of estimating a correction parameter as a function of the ratio between the first and second ratios,

a determination means (60) capable of determining the limit torque of the internal combustion engine as a function of the correction parameter.

8. Control system according to claim 7, comprising at least a map of the ratio of the rotational speed of the internal combustion engine (5) as a function of the speed of the vehicle for each report and for each mode, the control system being capable of determining the ratio and the limit torque as a function of at least one cartography and operating parameters of the vehicle.

9. Control system according to one of claims 7 or 8 for a continuously variable transmission comprising among different modes of assistance at least one snow mode of different gear ratio of the first speed gear ratio, the control system comprising a memory, the system command is able to memorize the report corresponding to the last active snow mode during a switch to manual mode.