METHOD AND CONTROL SYSTEM FOR CONTROLLING A CLUTCH OF A CONTINUOUSLY VARIABLE AUTOMATIC TRANSMISSION AND EXECUTION OF THE METHOD OF CONTINUOUSLY VARIABLE AUTOMATIC TRANSMISSION

Abstract

Method for controlling a clutch (38) of a continuously variable transmission (CVT) (14) including a variator arrangement (44) for a motor vehicle, the method comprising: Performing a variety of clutch slip tests, each clutch slip test comprehensive: reducing the pressure supplied to the clutch (38) until clutch slip occurs at a clutch slip data point (97) which contains a clutch slip pressure value and a corresponding clutch slip torque value, wherein the clutch slip is determined by a plurality of sensors; storing the clutch slip data point (97), which contains the clutch slip pressure value and the corresponding clutch slip torque value, in a memory (66); and increasing the pressure supplied to the clutch (38) after the clutch slip data point (97) has been stored in the memory (66), wherein the step of performing the plurality of clutch slip tests results in a plurality of clutch slip data points (97) stored in the memory (66), including at least one first clutch slip data point (97) with a clutch slip torque value in a first range and at least one second clutch slip data point (97) with a clutch slip torque value in a second range; characterized by Determining a gain and an offset based on the plurality of clutch slip data points (97), wherein the gain is an increase in clutch pressure relative to the clutch torque and the offset is a clutch pressure offset; and Supplying a driving clutch pressure to the clutch (38) that is lower than a pulley clamping pressure of the variator assembly (44) and higher than a clutch pressure determined based on the gain and offset.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method and a control system for controlling a clutch of a continuously variable automatic transmission, as well as to a continuously variable automatic transmission for implementing the method. A generic method is essentially disclosed in DE 103 04 436 A1. INTRODUCTION

[0002] A continuously variable automatic transmission (CVT) is a type of powertrain capable of varying the input / output speed ratio between a minimum (understeer) and a maximum (oversteer) ratio, allowing an infinitely variable range of engine operation that can achieve a preferred balance of fuel consumption and engine power in response to an output torque demand. Unlike conventionally driven transmissions, which use one or more planetary gear sets and multiple rotating and braking friction clutches to achieve a discrete gear state, a CVT uses a variable-diameter pulley system to achieve the infinitely variable range of gear ratios.

[0003] The pulley system, commonly referred to as a variator assembly, can transition continuously within a calibrated range of gear ratios. A typical belt or chain variator assembly includes two variator pulleys connected to each other by an endless rotatable drive member such as a chain or belt. The endless rotatable drive member runs within a variable-width gap defined by tapered pulley surfaces. One of the variator pulleys receives engine torque via a crankshaft, torque converter, and input pinion gear set, thus acting as a driving / primary / first pulley. The other pulley is connected to an output shaft of the CVT via additional gear sets, thus acting as a driven / secondary pulley. One or more planetary gear sets may be used on the drive side or output side of the variator assembly.For example, a planetary gear set can be used on the input side with forward and reverse clutches to change direction depending on the configuration.

[0004] To change a CVT gear ratio and transfer torque to the driveline, a clamping force (applied by hydraulic pressure) can be applied to the variator pulleys via one or both pulley actuators. The clamping force effectively clamps the pulley halves together to change the width of the gap between the pulley faces. Varying the gap size, or effective radius, causes the rotating drive element to run higher or lower within the gap. This, in turn, changes the effective diameters of the variator pulleys and can change the CVT gear ratio. A clamping force can also be used to transfer a desired amount of torque from one pulley to another through the passing parts, where the effort of the applied clamping force is intended to prevent the passing part from slipping off the pulleys.

[0005] A spike or disturbance in output torque can cause the endless rotating element to slip in the pulleys, potentially damaging the pulleys and resulting in poor performance. Accordingly, a CVT control system can apply maximum clamping pressure to the CVT pulleys when a torque increase is detected to prevent the continuously variable element from slipping. However, such maximum clamping pressure negatively impacts fuel economy. SUMMARY

[0006] The present disclosure provides a method, control system, and CVT that adjusts the pressure to an applied clutch to allow the clutch to slip first during a transient event to prevent the pulley system from slipping. The control system and method are configured to learn a desired operating pressure applied to a clutch that is lower than a pulley clamp pressure and higher than a pressure at which the clutch would slip (a critical clutch pressure) so that the clutch can act as a backup to prevent pulley slippage. Clutch slip tests are performed, which include decreasing the pressure applied to the clutch until clutch slippage occurs.Clutch slip data points in two or more regions are collected at the point of slip detection and used to determine a gain and offset, which is used to determine the critical clutch pressure. The gain is an increase in clutch pressure versus clutch torque, and the offset is a clutch pressure offset.

[0007] In a form that can be combined with or separated from other forms described herein, a method is provided for controlling a clutch of a continuously variable automatic transmission (CVT) having a variator arrangement for a motor vehicle. The method includes a step of performing a plurality of clutch slip tests. Each clutch slip test includes decreasing pressure applied to the clutch until clutch slip occurs at a clutch slip data point including a clutch slip pressure value and a corresponding clutch slip torque value, storing the clutch slip data point including the clutch slip value and the corresponding clutch slip torque value in a memory, and increasing pressure applied to the clutch after storing the clutch slip data point in the memory.Clutch slip is determined by a plurality of sensors. The step of performing the plurality of clutch slip tests results in a plurality of clutch slip data points stored in memory, including at least one first clutch slip data point having a clutch slip torque value in a first range and at least one second clutch slip data point having a clutch slip torque value in a second range. Additional ranges may also be used / required, such as the third and fourth ranges. The method further includes a step of determining a gain and an offset based on the plurality of clutch slip data points, wherein the gain is an increase in clutch pressure versus clutch torque and the offset is a clutch pressure offset.The method includes a step of supplying a drive clutch pressure to the clutch that is lower than a pulley clamping pressure of the variator assembly and higher than a clutch pressure determined based on the slip gain and the slip offset.

[0008] In another form, which may be combined with or separate from the other forms disclosed herein, a hydraulic control system is provided for controlling a clutch of a CVT. The control system has a memory and an instruction set. The instruction set is executable to perform a variety of clutch slip tests.For each clutch slip test, the control system is configured to: decrease the pressure supplied to the clutch until clutch slip occurs at a clutch slip data point including a clutch slip pressure value and a corresponding clutch slip torque value, wherein the clutch slip is determined by a plurality of sensors; store the clutch slip data point in the memory including the clutch slip pressure value and the corresponding clutch slip torque value; and increase the pressure supplied to the clutch after storing the clutch slip data point in the memory.The control system is configured to perform a plurality of clutch slip tests and store in memory a plurality of clutch slip data points including at least one first clutch slip data point with a clutch slip torque value in a first range and at least one second clutch slip data point with a clutch slip torque value in a second range. Additional ranges may also be used / required, such as the third and fourth ranges.The instruction set is further operable to determine a gain and an offset based on the plurality of clutch slip data points, the gain being an increase in clutch pressure over clutch torque and the offset being a clutch pressure offset, and causing a hydraulic system to supply a travel clutch pressure to the clutch that is lower than a pulley clamping pressure of the variator assembly and higher than a clutch pressure determined based on the gain and the offset.

[0009] In yet another form, which may be combined with or separate from the other forms disclosed herein, a continuously variable automatic transmission (CVT) for a motor vehicle is provided. The CVT includes a variator assembly having a first pulley and a second pulley, the first and second pulleys rotatably coupled by a continuously variable rotatable device. The second pulley is rotatably connected to an output member. The first pulley includes a movable side pulley that is laterally displaced along a first axis relative to a first stationary side pulley in response to pressure from a first actuator, and the second pulley includes a second movable side pulley that is laterally displaced along a second axis relative to a fixed second side pulley in response to pressure from a second actuator.A clutch is provided that is selectively engageable to couple the first pulley to an input member. A hydraulic system is configured to supply pressure to the variator assembly and the clutch. Additionally, the system also includes a control system having at least one controller, a memory, and a plurality of sensors in communication with the controller. The control system is configured to perform a plurality of clutch slip tests.For each clutch slip test, the control system is configured to: decrease the pressure supplied to the clutch by the hydraulic system until clutch slip occurs at a clutch slip data point including a clutch slip pressure value and a corresponding clutch slip torque value, wherein the clutch slip is determined by at least a portion of the plurality of sensors; store the clutch slip data point in the memory including the clutch slip pressure value and the corresponding clutch slip torque value; and increase the pressure supplied to the clutch by the hydraulic system after storing the clutch slip data point in the memory.The control system is configured to perform a plurality of clutch slip tests and store in memory a plurality of clutch slip data points including at least one first clutch slip data point with a clutch slip torque value in a first range and at least one second clutch slip data point with a clutch slip torque value in a second range. Additional ranges may also be used / required, such as the third and fourth ranges.The control system is further configured to determine a gain and an offset based on the plurality of clutch slip data points, wherein the gain is an increase in clutch pressure versus clutch torque and the offset is a clutch pressure offset, and cause the hydraulic system to supply a drive clutch pressure to the clutch that is lower than a pulley clamping pressure of the variator assembly and higher than a clutch pressure determined based on the gain and the offset.

[0010] Additional features may be provided, including, but not limited to, the following: the method and control system are configured to interpolate a best-fit line (99) through the plurality of clutch slip data points; wherein the first range is a medium to low range; wherein the second range is a medium to high range; further including at least one third clutch slip data point having a clutch slip torque value in a low to low range and at least one fourth clutch slip data point having a clutch slip torque value in a high to high range; wherein the control system and method is configured to perform the plurality of clutch slip tests for a duration of vehicle driving conditions within a normal driving range;wherein the low-to-low range, the mid-to-low range, the mid-to-high range, and the high-to-high range are greater than a zero low-torque range and less than a zero high-torque range; wherein the low-to-low range, the mid-to-low range, the mid-to-high range, and the high-to-high range do not overlap; wherein the method and control system are configured to perform each clutch slip test with a predetermined delay period between performing each clutch slip test.

[0011] Additionally, the method and control system may be configured to determine whether a plurality of predetermined conditions are met before performing the step of performing the plurality of clutch slip tests.The predetermined conditions may include one or more of the following: the vehicle is operating in steady-state conditions; the turbine acceleration is within a predetermined turbine acceleration range; the engine acceleration is within a predetermined engine acceleration range; the vehicle acceleration is within a predetermined vehicle acceleration range; a throttle position exceeds a predetermined threshold; a torque converter clutch is engaged; clutch slippage is not already detected; the transmission fluid temperature is within a predetermined temperature range; powertrain diagnostics and the plurality of sensors are clear of a default mode; CVT ratio change is within a predetermined CVT ratio change range; clutch pressure scheduling arbitration allows a clutch slippage check to be performed; and stability timer conditions are met.

[0012] Additional features may include the following: During a clutch slip test, the method and control system are configured to first rapidly reduce the pressure to a discharge pressure during a dip period, and second, reduce the pressure from the discharge pressure until clutch slip occurs during a slip period where the slip period is longer than the dip period. The pressure may first be rapidly reduced to the discharge pressure to bring the clutch pressure as close as possible to the critical clutch pressure without actually slipping, in order to shorten the time required for the test.

[0013] Further additional features may include: the plurality of sensors including a first speed sensor attached to a torque converter turbine and a second speed sensor attached to a primary pulley of the variator assembly; wherein clutch slip is determined by sensing a differential between the first and second speed sensors; wherein the method and control system are further configured to apply the driving clutch pressure to the clutch prior to performing each clutch slip test; wherein the driving clutch pressure is equal to a critical clutch pressure plus a safety factor; wherein the method and control system are configured to increase the pressure applied to the clutch after the clutch slip data point prior to the actual clutch slip is stored in memory;wherein the method and control system are configured to update the best-fit line (99) each time a clutch slip data point is stored; wherein the method and control system are further configured to perform a rate limiting function such that a change in the best-fit line (99) is limited to a predetermined maximum change for each stored clutch slip data point;wherein the output pressure is a first output pressure, the method and control system further configured to determine whether clutch slippage has occurred during the dip period, and if clutch slippage has occurred a predetermined number of times during the dip period (e.g., five or six times), clearing the clutch slip data points stored in memory and resetting the first output pressure to a second output pressure higher than the first output pressure by increasing the stored clutch pressure offset by a calibratable scale of the first output pressure to arrive at the second output pressure; and the clutch is a forward CVT clutch;

[0014] Further objects, advantages, and areas of application will become apparent from the description presented herein. It should be noted that the description and specific examples are for illustrative purposes only. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The drawings described herein are for illustrative purposes only. Fig. 1 is a schematic representation of the propulsion system of a motor vehicle including an internal combustion engine rotatably connected to a continuously variable transmission (CVT) in accordance with the principles of the present disclosure; Fig. 2 is a schematic representation of the Fig. 1, including a control system for controlling aspects of the vehicle propulsion system according to the principles of the present disclosure; Fig. Figure 3 shows a schematic block diagram of an exemplary method that can be used to connect a clutch of the drive system of Fig. 1-2 according to the principles of the present disclosure; Fig. 4 shows a graphical representation of exemplary turbine speed, primary pulley speed, and secondary pulley speed under clutch slip conditions according to the principles of the present disclosure; Fig. Figure 5 shows a graphical representation of the control strategy and procedure applied to the drive system and procedure described in the Fig. 1-3 according to the principles of the present disclosure; and Fig. Figure 6 shows a graphical representation and a bin system of the control system and method used in the Fig. 2-3 and in Fig. 5 according to the principles of the present disclosure. DETAILED DESCRIPTION

[0016] Several examples of the disclosure are described in detail below, as illustrated in the accompanying drawings. Where possible, the same or similar reference numerals are used throughout the drawings and description to refer to the same or similar parts or steps. The drawings are simplified and not drawn to exact scale. For clarity and understanding, directional terms such as top, bottom, left, right, upward, above, below, below, back, and front are used with reference to the drawings.

[0017] With reference to the drawings, wherein like reference numerals correspond to like or similar components throughout the figures, Fig. 1 and Fig. 2 schematically illustrates elements of a motor vehicle propulsion system 10 including an engine 12, such as an internal combustion engine, rotatably coupled to a continuously variable transmission (CVT) 14 via a torque converter 16 and a forward / reverse shifting mechanism 18. The motor vehicle propulsion system 10 is connected to a set of motor vehicle wheels 22 by means of a transmission 20 to provide traction when used in a vehicle. A transmission (not shown) may also be included upstream or downstream of the CVT 14 for additional transmission options. The operation of the motor vehicle propulsion system 10 may be monitored and controlled by a control system 60 (see Fig. 2) controlled in response to driver commands and other vehicle operating factors. The vehicle propulsion system 10 may be part of a device that may be a vehicle, a bicycle, a robot, agricultural equipment, sports equipment, or any other transportation device.

[0018] The engine 12 may be any suitable engine, such as an internal combustion engine, capable of converting hydrocarbon-based fuels into mechanical power to generate torque in response to control commands from the control system 60. The engine 12 may also or alternatively include an electric motor (not shown). The torque converter 16 may be a device that establishes a fluid connection between its input and output elements for transmitting torque. In alternative examples, the torque converter 16 could be omitted, and the clutches become the starting device.

[0019] The output member 24 of the torque converter 16 is rotatably connected to the forward-reverse shifting mechanism 18 and serves as an input to the CVT 14. The forward-reverse shifting mechanism 18 is provided because the engine 12 is operated in a predetermined single direction. In the specific example of Fig. 1, the forward-reverse shifting mechanism 18 includes a single planetary gear set 26 having a sun gear member 28, a ring gear member 30 disposed coaxially around the sun gear member 28, and a carrier member 32 supporting a plurality of pinion gears 34 that mesh with the sun gear member 28 and the ring gear member 30. In other variations, a double-pinion planetary gear set could be used, having one set of pinion gears meshing with a second set of pinion gears, the first set of pinion gears meshing with the sun gear member 28 and the second set of pinion gears meshing with the ring gear member 30. The output member 24 of the torque converter 16 is permanently connected to the ring gear member 30 in this example. An input member 36 on the CVT 14 is permanently connected to the sun gear member 28 in this example.

[0020] The forward-reverse shift mechanism 18 further includes a forward clutch 38 and a reverse brake 40. The forward clutch 38 is selectively engageable to connect the sun gear member 28 and the CVT input member 36 to the ring gear member 30 and the torque converter output member 24 so that these members rotate together as a single unit. Accordingly, the engine 12 can then drive the CVT 14 in a forward direction. The reverse brake 40 is selectively engageable to connect the carrier member 32 to a stationary member, such as the transmission case 42, so that the direction of input rotation would then be reversed when applied to the CVT input member 36.However, it should be understood that the torque converter output member 24 and the CVT input member 36, as well as the reverse brake 40 and the forward clutch 38, may be interconnected in other ways and still achieve forward-to-reverse shifting. For example, other power flows could be used to transition between forward and reverse operation, such as alternative configurations utilizing two or three clutches and one, two, or more gear sets. The forward clutch 38 and the reverse brake 40 may each be controlled by an actuator, such as a hydraulically controlled actuator that provides fluid pressure to the clutch 38 or the brake 40.

[0021] In this example, the CVT 14 is a belt or chain-type CVT, which may advantageously be controlled by the control system 60. The CVT 14 includes a variator assembly 44 that transmits torque between the CVT input member 36 and the CVT output member 46. The variator assembly 44 includes a first drive or primary pulley 48, a second driven or secondary pulley 50, and a continuously variable rotatable device 52, such as a belt or chain, or other flexible continuously variable device, that rotatably connects the first and second pulleys 48, 50 to transmit torque therebetween. The first pulley 48 and the input member 36 rotate about a first axis A, and the second pulley 50 and the output member 46 rotate about a second axis B.One of the first and second pulleys 48, 50 can serve as a ratio pulley to establish a speed ratio, and the other of the first and second pulleys 48, 50 can act as a clamp pulley to generate sufficient clamping force to transmit torque. As used herein, the term 'gear ratio' refers to a variator gear ratio, i.e., a ratio between the CVT output speed and the CVT input speed. Thus, the distance between the first pulley halves 48a, 48b can be varied (by moving one or more of the pulley halves 48a, 48b along axis A) to move the continuously variable element 52 higher or lower within the groove defined between the two pulley halves 48a, 48b.Similarly, the second pulley halves 50a, 50b can also be moved relative to each other along axis B to change the ratio or torque-carrying capacity of the CVT 14. One or both pulley halves 48a, 48b, 50a, 50b of each pulley 48, 50 can be moved with an actuator, such as a hydraulically controlled actuator, that varies the fluid pressure supplied to the pulleys 48, 50.

[0022] The vehicle driveline system 10 preferably includes one or more sensors or sensor devices, such as Hall-effect sensors, for monitoring speeds of various devices, including, for example, a torque converter turbine speed sensor 56, a CVT input speed sensor 58, an engine speed sensor (not shown), a CVT output speed sensor (not shown), and one or more wheel speed sensors (not shown). Each of the sensors communicates with the control system 60.

[0023] With reference to Fig. 2, the control system 60 preferably includes a controller 62 and possibly a user interface 64. A single controller 62 is depicted for ease of illustration. The controller 62 may include a plurality of control devices, with each controller 62 associated with monitoring and controlling a single system. This may include an engine control module (ECM) for controlling the engine 12 and a transmission control module (TCM) for controlling the CVT 14 and monitoring and controlling a single subsystem, such as a torque converter clutch and the forward / reverse shift mechanism 18.

[0024] The controller 62 preferably includes at least one processor and at least one memory device 66 (or other non-transitory, physical, and computer-readable storage medium) storing the recorded instructions for implementing command sets for controlling the CVT 14 and / or the forward clutch 38, and a buffer 68. The memory 66 can store controller-executable instruction sets, and the processor can execute the controller-executable instruction set stored on the memory 66.

[0025] The user interface 64 communicates with and monitors operator input devices, such as an accelerator pedal 70 and a brake pedal 72. The user interface 64 determines an operator's torque request based on the aforementioned operator inputs. The controller 62 also receives inputs from various sensors, including the torque converter turbine speed sensor 56 and a CVT variator input speed sensor 58 mounted on the primary pulley 48.

[0026] The tax system 60 from Fig. 2 can be used to carry out the steps of a method 100 according to Fig. 3 and as explained in more detail later with reference to the Fig. 4-6 will be discussed. With reference now to Fig. 3 depicts a flowchart of a variation of a method 100 stored on an instruction set and executable by the controller 62 of the control system 60. The method 100 is, for example, a method for controlling a continuously variable automatic transmission (CVT) including a variator assembly for a motor vehicle.

[0027] The method 100 may begin with a starting block 101 and then continue with a step or block 102, where the method 100 includes a step 102 for performing a plurality of clutch slip tests. Each clutch slip test includes substeps, including a substep 102A of decreasing pressure applied to the clutch (such as clutch 38) until clutch slip occurs. Clutch slip may be determined by a variety of sensors. For example, a first speed sensor 56 may be attached to the torque converter turbine of the torque converter 16, and a second speed sensor 58 may be attached to the primary pulley 48 of the variator assembly of the CVT 14. Clutch slip may be determined by sensing a differential between the first and second speed sensors 56, 58.For example, during steady-state, slip-free operation, the speeds of the torque converter turbine and primary pulley 48 should correlate. However, when slip occurs or is just beginning to occur, a differential may begin between the speed sensors 56, 58 of the torque converter 16 turbine and the primary pulley 48. This is illustrated in FIG. Fig. 4 shown.

[0028] Fig. Figure 4 shows a speed profile of the secondary pulley 59, a speed profile of the primary pulley 61, and a turbine speed profile 63. If there is no slippage of the forward clutch 38 or the pulleys 48, 50, then each of the speed profiles 59, 61, 63 should correlate in that each exhibits the same acceleration or deceleration, and the turbine speed 63 and the speed of the primary pulley 61 should be nearly identical, while the speed of the secondary pulley 59 may be increased or decreased by a ratio factor, as shown. In some forms, due to the architecture of the switching mechanism 18, the turbine speed profile 63 may also be offset from the primary pulley speed profile 61 by a factor. Fig. 4, slippage of the forward clutch 38 occurs, so that the turbine speed 63 does not correlate with or equal to the speed of the primary pulley 61 in section 65 in the graph. However, the speed of the secondary pulley 63 is still correlated with the speed of the primary pulley 61, indicating that the pulleys 48, 50 are not slipping against each other. Accordingly, the forward clutch 38 acts as a fuse, slipping between the turbine of the torque converter 16 and the primary pulley 48 before the pulleys 48, 50 slip against each other. After an early rated clutch slip is detected due to the differential between the speed sensors 56, 58, a greater slip is imminent between the primary pulley 48 and the turbine of the torque converter 16. Accordingly, it is desirable to increase the pressure supplied to the clutch 38 immediately after the early rated slip is detected.

[0029] A clutch slip data point at which clutch slip occurs includes a clutch slip pressure value with a corresponding clutch slip torque value. The clutch slip checking step 102 further includes a substep 102B for storing the clutch slip data point, which includes the clutch slip pressure value and the corresponding clutch slip torque value, in a memory. The clutch slip checking step 102 then includes a substep 102C for increasing the pressure supplied to the clutch 38 after storing the clutch slip data point in the memory.

[0030] The clutch slip test can be carried out with reference to Fig. 5, which shows a variety of pressure profiles as a function of time. The clutch torque is represented by profile line 76, and the pressure applied to the clutch 38 is represented by profile line 78. A critical clutch pressure is shown at profile line 80; the critical clutch pressure is a calibrated or learned pressure at which the clutch 38 is learned or assumed to begin slipping. The critical clutch pressure is learned adaptively, as explained below. Before the critical clutch pressure is learned for a particular vehicle, the critical clutch pressure is calibrated with a safety margin. Each of the pressure profiles 76, 78, 80 is shown with a pressure p on a y-axis and a time t along an x-axis. A clutch slip profile 82 is also shown in Fig. 5, which has a different scale than the pressure scale of the pressure profile lines 76, 78, 80. The clutch slip profile 82 is shown with speed sensor differential s on a y-axis and time t on an x-axis, where t is the same time for all profiles 76, 78, 80, 82 shown.

[0031] At a time before a clutch slip test is performed, such as time T1, the clutch 38 is operating at a drive clutch pressure 78 having a value of 78A and a corresponding torque having a value of 76A. The critical clutch pressure 80 has a value of 80A at time T1. If the clutch operating pressure value 78A is higher than the critical clutch pressure value 80A, the clutch 38 has zero or negligible slip, and thus the speed sensor differential (indicating early slip) is zero or approximately zero at point 82A at time T1.

[0032] The control system 60 and / or the method 100 begins a clutch slip test at time T2. Accordingly, the pressure 78 applied to the clutch 38 is decreased starting at time T2. As explained above with respect to step 102A, the clutch pressure 78 continues to decrease until clutch slip is detected. Thus, after time T2, the clutch slip detection profile 82 begins to increase until a clutch slip threshold V (a speed sensor differential with a difference of a predetermined magnitude V) is exceeded at time T3B, where the speed sensor differential is no longer zero but has a positive value of θ2S that equals or exceeds the threshold V. At time T3B, the applied clutch pressure 78 has a value of 78S, which corresponds to a clutch torque value of 76S.It should be noted that the critical clutch pressure at time T3B has a value of 80S, which is still lower than the applied clutch pressure 78S, even if clutch slippage has been detected. This is because the critical clutch pressure 80, as learned, calculated, or calibrated, is slightly offset from an actual pressure at which the clutch 38 would slip (due to changes in the system, an error or safety factor in the calibration, or any number of reasons).

[0033] As mentioned above, at time T3B, when clutch slip is detected, method 100 stores the clutch slip data point, including the applied pressure value 78S and the corresponding torque value 76S, in memory 66. The applied clutch pressure 78 is then immediately increased to a value 78H higher than the clutch slip pressure value 78S to prevent any significant perceptible slip from occurring. The speed sensor differential 82 typically continues to increase after time T3B, as shown, and may be reset at any time before performing another clutch slip test.

[0034] In some forms, the step 102B of reducing the applied clutch pressure 78 delivered to the clutch 38 may include first reducing the applied pressure 78 to an output pressure 78D before further reducing the applied pressure 78 to the clutch slip pressure 78S. Accordingly, during the clutch slip test, the applied pressure 78 may be rapidly reduced to the output pressure 78D in an output pressure phase or period D, from time T2 to time T3A, and then more slowly reduced in a ramp phase or slip period R from time T3A to time T3B. The pressure may first be quickly reduced to the lowest output pressure to bring the clutch pressure as close as possible to the critical clutch pressure without actually slipping, in order to shorten the amount of time required for the test.The slip period R may be longer than the decrease period D, but not necessarily; if the learned critical clutch pressure is too high (higher than the actual pressure that allows slip), then the slip period R will be longer. However, if the learned critical clutch pressure is too low (lower than the actual pressure that allows slip), the slip period R will be very short. Thus, the applied pressure 78 is quickly reduced to the output pressure 78D and then decreased more slowly to the last learned or calibrated critical clutch pressure 80. As can be seen in the slip period R, the applied clutch pressure 78 is decreased proportionally to the critical clutch pressure 80, following the shape of the critical clutch pressure profile 80 until slip is detected.Accordingly, the applied clutch pressure 78 is prevented from crossing the critical clutch pressure profile 80 too early due to acceleration changes and changes in the critical clutch pressure 80 due to varying inputs that would not provide a reliable learned clutch slip data point.

[0035] After the pressure 78 supplied to the clutch 38 is increased to the higher pressure 78H at time T3B, the supplied pressure 78 may gradually decrease to a learned profile line L that includes an offset determined from a new learned critical clutch pressure. The supplied pressure 78 may then be increased along the learned offset profile line L in a mitigated slip period or phase M. At time T4, after the slip is mitigated, the pressure 78 supplied to the clutch 38 is increased to a cruising clutch pressure having a value 78N based on a newly learned critical clutch pressure 80 plus a safety factor. The critical clutch pressure profile 80 is based on the newly learned critical clutch pressure 80 after time T3B when the new clutch slip data point is collected. However, it should be clear that the pressure reduction from point 78H can be calibrated in many different ways.Additionally, the pressure does not necessarily need to be increased as high as point 78H; rather, ramp L can simply be followed to increase the pressure. Ramp L can also be a fixed-rate ramp or a ramp based on the pressure difference between the current commanded pressure and the adjusted critical clutch pressure 80.

[0036] With further reference to Fig. 3, method step 102 includes performing a plurality of clutch slip tests. In some examples, the clutch slip tests are performed until a sufficient number of clutch slip data points are collected to learn a critical clutch pressure 80 of the system. The plurality of clutch slip tests may be performed for a duration of vehicle driving conditions within a normal driving range. The normal driving range may be determined by a set of conditions determined as safe conditions under which a clutch slip test is performed.For example, in some variations, a clutch slip test is only performed when one, more, or all of the following conditions are met: the vehicle is operating under steady-state conditions; the turbine acceleration is within a predetermined turbine acceleration range (above a low threshold and below a high threshold); the engine acceleration is within a predetermined engine acceleration range (above a low threshold and below a high threshold); the vehicle acceleration is within a predetermined vehicle acceleration range (above a low threshold and below a high threshold, not at idle); the throttle position exceeds a predetermined threshold (e.g., not at idle); the torque converter clutch is engaged; clutch slip is not detected (i.e.no reliable data would be obtained from performing a clutch slip test when slippage is already occurring); the transmission fluid temperature is within a predetermined temperature range; driveline diagnostics and the plurality of sensors are free from a default mode; CVT ratio change is within a predetermined CVT ratio change range; clutch pressure scheduling arbitration allows a clutch slip test to be performed; and stability timer conditions are met. Additional or alternative conditions may be required by the system.

[0037] In some forms, the clutch slip tests may be initiated each time a key cycle of the vehicle is executed. Additionally, each clutch slip test may be performed with a predetermined delay period between the performance of each clutch slip test, for example, to ensure that the clutch slip tests are not performed so close together that the clutch slip is not mitigated before a new test is initiated.

[0038] In some variations, a sufficient number of clutch slip tests to determine the critical clutch pressure requires the memory 66 to collect clutch slip data points in multiple torque ranges. For example, the method 100 and the control system 60 may perform clutch slip tests until at least one or more clutch slip data points with a clutch slip torque value in a first range and one or more clutch slip data points with a clutch slip torque value in a second range are collected. In some forms, more than two ranges may be used.

[0039] To further illustrate several torque ranges and with reference to Fig. 6, a bin system is illustrated, with four bins 84, 86, 88, 90 representing four torque ranges 84T, 86T, 88T, 90T used by memory 66 to collect clutch slip data points. The software bins 84, 86, 88, 90 may each be configured to hold a finite number of clutch slip data points, and in particular, the clutch torque values ​​of the clutch slip data points. For example, each bin 84, 86, 88, 90 may contain eight slots 92 for storing clutch slip torque values ​​corresponding to a clutch slip pressure at which clutch slip was detected (at time T3B in Fig. 5). The first bin 84 may be a low to low range for low torque values, such as torque values ​​in the range of approximately 50-80 Nm. The first bin 86 may be a medium to low range for medium torque values, such as torque values ​​in the range of approximately 80-110 Nm. The third bin 88 may be a medium to high range for medium to high torque values, such as torque values ​​in the range of approximately 110-140 Nm. The fourth bin 90 may be an upper high range for high torque values, such as torque values ​​in the range of approximately 140-200 Nm. It should be understood that other torque ranges could be used, more or fewer than four bins could be used, and other variations could be adopted.

[0040] In some variations, each torque range 84T, 86T, 88T, 90T is mutually exclusive and does not overlap with adjacent torque ranges 84T, 86T, 88T, 90T. Thus, in such a variation, it should be clear that only a single torque range 84T, 86T, 88T, 90T and bin 84, 86, 88, 90T includes a specific torque value, such as a torque value at the upper edge of a particular range 84T, 86T, 88T and at the lower edge of the adjacent higher torque range 86T, 88T, 90T, respectively. Accordingly, the adjacent upper torque range 86T, 88T, 90T would actually start with a torque value slightly higher than the highest torque range of the adjacent lower torque range 84T, 86T, 88T, respectively.

[0041] As in Fig. 6, a graphical representation 95 of clutch slip data points corresponds to torque bins 84, 86, 88, 90 and torque ranges 84T, 86T, 88T, 90T. Torque 94 in Nm is plotted along an x-axis, showing torque values ​​of 0, 50, 80, 110, 140, and 200. Pressure 96 in kPa is plotted along a y-axis, showing pressure values ​​of 250, 350, 450, 550, 650, 750, and 850. Each clutch slip data point 97 collected by memory 66 is plotted on graph 95.

[0042] Referring to Fig. 3 and Fig. 6, after performing at least a provisionally sufficient number of clutch slip tests (to obtain data points in one or more of the torque bins 84, 86, 88, 90), the method 100 includes a step 104 of determining a gain and an offset based on the plurality of clutch slip data points, where the gain is an increase in clutch pressure versus clutch torque and the offset is a clutch pressure offset. The gain and offset may be determined, for example, by interpolating a best-fit line 99 through the plurality of clutch slip data points 97. As each clutch slip test is performed in the iterative step 102 of the method 100, a new clutch slip data point 97 is recorded and fitted into one of the slots 92 of one of the torque bins 84, 86, 88, 90.When each slot 92 of the relevant torque bin 84, 86, 88, 90 is full, one of the clutch slip data points 97 (such as the oldest data point 97 in the particular bin 84, 86, 88, 90) can be replaced by the new clutch slip data point 97 in a slot 92 of the respective bin 84, 86, 88, 90. The best-fit line 99 can be updated each time a clutch slip data point is saved.

[0043] In some variations, the method 100 further includes performing a rate limiting function to update the best-fit line 99 such that a change in the best-fit line 99 is limited to a predetermined maximum change for each stored clutch slip data point 97. For example, if a new clutch slip data point 97 would cause a large change in the best-fit line 99, the method 100 and the control system 60 may only allow a smaller, incremental change in the best-fit line 99 each time a new clutch slip data point 97 is stored in the memory 66.

[0044] The required number of clutch slip data points 97 needed to determine the gain and offset, as well as the best-fit line 99, can vary based on design choices. For example, in some variations, a best-fit line 99 can be formed from only two clutch slip data points 97. The control system 60 could require each of the two clutch slip data points 97 to come from different bins 84, 86, 88, 90 for a more accurate determination of the gain and offset, if desired. In some variations, one clutch slip data point 97 in each bin 84, 86, 88, 90 might be required to determine the best-fit line 99 and / or the gain and offset, or a plurality of clutch slip data points 97 in each bin 84, 86, 88, 90 might be required. The use of a plurality of bins 84, 86, 88, 90, such as four bins with a finite number of slots 92, such aseight slots 92 in each bin 84, 86, 88, 90, allows fresh data to be cycled through with a reduced risk of data that is only in a certain torque range, such as a midpoint.

[0045] Some data points might be considered so unreliable that they are not placed in a bin 84, 86, 88, 90. For example, a zero-low torque range may exist that is lower than any of the torque ranges 84T, 86T, 88T, 90T of the four bins 84, 86, 88, 90, and any data point located in the zero-low torque range (e.g., less than 50 Nm) could be discarded. Similarly, a zero-high torque range may exist that is higher than any of the torque ranges 84T, 86T, 88T, 90T of the four bins 84, 86, 88, 90, and any data point located in the zero-high torque range (e.g., greater than 200 Nm) could be discarded.

[0046] The method 100 further includes a step 106 of supplying a drive clutch pressure 78 to the clutch 38 that is lower than a pulley clamping pressure of the variator assembly and higher than a critical clutch pressure 80 determined based on the gain and the offset, where, as described above, the gain is an increase in clutch pressure over the clutch torque, and the offset is a clutch pressure offset. For example, referring to Fig. 5, when a new critical clutch pressure 80 is determined at time T3B and the mitigation phase M ends, the pressure 78 supplied to the clutch 38 is supplied at a pressure value above the critical clutch pressure 80 and below a pulley clamp pressure (not shown). The supplied pressure 78 is based on the critical clutch pressure 80, a safety factor, and another desired offset.

[0047] For each iteration of the clutch slip test, the method 100 may include applying the drive clutch pressure 78 to the clutch 38 prior to performing each clutch slip test, where the drive clutch pressure 78 is equal to a critical clutch pressure plus a safety factor plus any other desired offsets.

[0048] Referring to Fig. 5, the method 100 and the system 60 may have further built-in protection whereby, if clutch slip is detected one or more times (such as five or six times) during the period of lowering D, all clutch slip data points 97 (in Fig. 6) may be deleted and method 100 may begin again to determine new clutch slip data points 97 since the current critical clutch pressure 80 is clearly set too low and lower than the actual pressure at which clutch 38 slips. First output pressure 78D may be reset to a second output pressure higher than first output pressure 78D to arrive at an output pressure that is not in a slip range. The first output pressure may be changed to the second output pressure by increasing the stored clutch pressure offset by a calibratable scale of the first output pressure to arrive at the second output pressure. In some variations, output pressure 78D is only reset after multiple clutch slips are detected in the decrement range D.If the discharge pressure 78D needs to be reset, the system 60 could also increase the pulley clamp pressure to prevent pulley slippage.

[0049] The control system 60 may be configured to control each of the Fig. 3. Thus, the entire description of method 100 can be applied to control system 60 to effect method 100. Furthermore, controller 62 can include or be a controller including a number of control logics configured to perform the steps of method 100 discussed above.

[0050] The controller 62 of the control system 60 may include a computer-readable medium (also referred to as a processor-readable medium), including any non-transitory (e.g., tangible) media involved in providing data (e.g., instructions) that could be read by a computer (e.g., by the processor of a computer).

[0051] As explained above, the forward clutch 38 acts as a backup, slipping between the turbine of the torque converter 16 and the primary pulley 48 before the pulleys 48, 50 slip against each other. However, with the adaptive learning method 100 disclosed herein, the critical clutch pressure 80 is learned so that the clutch 38 can fuse without a constant commanded microslip. For example, the system can be executed without attempting to maintain a slip reference greater than zero and still be capable of passively fusing.

Claims

[1] A method for controlling a clutch (38) of a continuously variable automatic transmission (CVT) (14) including a variator arrangement (44) for a motor vehicle, the method comprising: Perform a variety of clutch slip tests, each clutch slip test including: reducing the pressure supplied to the clutch (38) until clutch slip occurs at a clutch slip data point (97) including a clutch slip pressure value and a corresponding clutch slip torque value, the clutch slip being determined by a plurality of sensors; storing the clutch slip data point (97) containing the clutch slip pressure value and the corresponding clutch slip torque value in a memory (66); and increasing the pressure supplied to the clutch (38) after storing the clutch slip data point (97) in the memory (66), wherein the step of performing the plurality of clutch slip tests results in a plurality of clutch slip data points (97) stored in the memory (66), including at least one first clutch slip data point (97) having a clutch slip torque value in a first range and at least one second clutch slip data point (97) having a clutch slip torque value in a second range; characterized by determining a gain and an offset based on the plurality of clutch slip data points (97), wherein the gain is an increase in clutch pressure versus clutch torque and the offset is a clutch pressure offset; and Supplying a drive clutch pressure to the clutch (38) that is lower than a pulley clamping pressure of the variator assembly (44) and higher than a clutch pressure determined based on the gain and the offset. [2] The method of claim 1, wherein the step of determining the gain and the offset based on the plurality of clutch slip data points (97) further comprises interpolating a best-fit line (99) through the plurality of clutch slip data points (97), the first range being a medium to low range and the second range being a medium to high range, wherein the step of performing the plurality of clutch slip tests results in a plurality of clutch slip data points (97) being stored in the memory (66) further including at least a third clutch slip data point (97) having a clutch slip torque value in a low to low range and at least a fourth clutch slip data point (97) having a clutch slip torque value in a high to high range. [3] The method of claim 2, wherein the step of performing the plurality of clutch slip tests is performed for a duration of vehicle driving conditions within a normal driving range, the normal driving range being defined by a plurality of safe conditions in which to perform a clutch slip test, the step of performing the plurality of clutch slip tests comprising performing each clutch slip test of the plurality of clutch slip tests with a predetermined delay period between performing each clutch slip test, each of the low to low range, mid to low range, mid to high range, and high to high range being greater than a zero low torque range, each of the low to low range, mid to low range, mid to high range, and high to high range being less than a zero high torque range,and wherein the low to low range, the medium to low range, the medium to high range, and the high to high range do not overlap; the method further comprising determining whether a plurality of predetermined conditions are met before performing the step of performing the plurality of clutch slip tests,wherein the plurality of predetermined conditions include at least a plurality of the following conditions: the vehicle is operating in steady-state conditions; the turbine acceleration is within a predetermined turbine acceleration range; the engine acceleration is within a predetermined engine acceleration range; the vehicle acceleration is within a predetermined vehicle acceleration range; a throttle position exceeds a predetermined threshold; a torque converter clutch is engaged; clutch slip is not detected; the transmission fluid temperature is within a predetermined temperature range; the CVT ratio change is within a predetermined CVT ratio change range; clutch pressure scheduling arbitration allows a clutch slip check to be performed; and stability timer conditions are met, wherein the step of reducing the pressure supplied to the clutch until clutch slip occurs includes first reducing the pressure to an output pressure in a period of reduction and secondly reducing the pressure from the output pressure until the occurrence of clutch slip in a slip period, wherein the pressure is reduced more rapidly in the period of reduction than in the slip period, wherein the plurality of sensors includes a first speed sensor (56) attached to a torque converter turbine and a second speed sensor (58) attached to a primary pulley (48) of the variator assembly (44), the method further comprising determining clutch slip by sensing a differential between the first and second speed sensors, the method further comprising applying the drive clutch pressure to the clutch (38) prior to performing each clutch slip test, the drive clutch pressure being at least equal to the critical clutch pressure plus a safety factor, the method further comprising updating the best fit line (99) when a clutch slip data point (97) is stored, the method further comprising performing a rate limiting function,such that a change in the best-fit line (99) is limited to a predetermined maximum change for each stored clutch slip data point (97)., [4] The method of claim 3, wherein the output pressure is a first output pressure, the method further comprising: determining whether clutch slip has occurred during the period of reduction; and if clutch slip has occurred during the period of reduction, clearing the clutch slip data points (97) stored in the memory (66) and resetting the first output pressure to a second output pressure higher than the first output pressure. [5] A control system (60) for controlling a clutch (38) of a continuously variable automatic transmission (CVT) (14) having a variator arrangement (44), the control system (60) comprising a memory (66) and an instruction set executable for: Performing a plurality of clutch slip tests, wherein the control system (60) is configured to, for each clutch slip test: Reducing the pressure supplied to the clutch (38) until clutch slip occurs at a clutch slip data point (97) including a clutch slip pressure value and a corresponding clutch slip torque value, the clutch slip being determined by a plurality of sensors; storing the clutch slip data point (97) containing the clutch slip pressure value and the corresponding clutch slip torque value in the memory (66); and Increasing the pressure supplied to the clutch (38) after storing the clutch slip data point (97) in the memory (66), wherein the control system (60) is configured to store in the memory (66) at least one first clutch slip data point (97) having a clutch slip torque value in a first range and at least one second clutch slip data point (97) having a clutch slip torque value in a second range; determining a gain and an offset based on the plurality of clutch slip data points (97), wherein the gain is the increase in clutch pressure versus clutch torque and the offset is a clutch pressure offset; and Causing a hydraulic system to supply a drive clutch pressure to the clutch (38) that is lower than a pulley clamping pressure of the variator assembly (44) and higher than a clutch pressure determined based on the gain and the offset. [6] The control system of claim 5, wherein the control system (60) is integrated into a motor vehicle, the control system (60) being configured to perform the plurality of clutch slip tests with a predetermined delay period between each clutch slip test, the control system (60) being configured to perform the plurality of clutch slip tests for a duration of vehicle driving conditions within a normal driving range defined by a plurality of safe conditions, the control system (60) being configured to determine whether a plurality of predetermined conditions are met before performing a clutch slip test of the plurality of clutch slip tests, the plurality of predetermined conditions comprising at least a plurality of the following: the vehicle is operating in steady-state conditions; the turbine acceleration is within a predetermined turbine acceleration range;the engine acceleration is within a predetermined engine acceleration range; the vehicle acceleration is within a predetermined vehicle acceleration range; a throttle position exceeds a predetermined threshold; a torque converter clutch is engaged; clutch slippage is not detected; the transmission fluid temperature is within a predetermined temperature range; the CVT ratio change is within a predetermined CVT ratio change range; clutch pressure scheduling arbitration allows a clutch slippage check to be performed; and stability timer conditions are met.; [7] The control system (60) of claim 6, wherein the first range is a medium to low range and the second range is a medium to high range; wherein the plurality of clutch slip data points (97) further includes at least a third clutch slip data point (97) having a clutch slip torque value in a low to low range and at least a fourth clutch slip data point (97) having a clutch slip torque value in a high to high range, wherein the control system (60) is configured to interpolate a best-fit line (99) through the plurality of clutch slip data points (97), wherein the low to low range, the medium to low range, the medium to high range, and the high to high range are greater than a zero low torque range, wherein each of the low to low range, the medium to low range,the mid-to-high range and the upper-to-high range are smaller than a zero high-torque range, and wherein the low-to-low range, the mid-to-low range, the mid-to-high range, and the upper-to-high range do not overlap, wherein to perform a clutch slip test, the control system (60) is configured to first reduce the pressure to an output pressure in a decrease period and second reduce the pressure from the output pressure until clutch slip occurs in a slip period, wherein the pressure is reduced more quickly in the decrease period than in the slip period, wherein the control system (60) is configured to update the best-fit line (99) when a clutch slip data point (97) is stored, wherein the control system (60) is configured toto perform a rate limiting function such that a change in the best-fit line (99) for each stored clutch slip data point (97) is limited to a predetermined maximum change, wherein the control system (60) is further configured to determine whether clutch slip has occurred during the output period; and if clutch slip has occurred during the output period, clearing the clutch slip data points (97) stored in the memory (66) and resetting the first output pressure to a second output pressure that is higher than the first output pressure, wherein the control system (60) is configured to determine the clutch slip by sensing a differential between first and second speed sensors (56, 58) of the plurality of sensors, wherein the first speed sensor (56) is attached to a torque converter turbine,wherein the second speed sensor (58) is mounted on a primary pulley (48) of the variator assembly (44), wherein the control system (60) is configured to cause the hydraulic system to apply the drive clutch pressure to the clutch (38) before the control system performs each clutch slip test, the drive clutch pressure being at least equal to the critical clutch pressure plus a safety factor. [8] Continuously variable automatic transmission (CVT) (14) for a motor vehicle, comprising: a variator assembly (44) having a first pulley (48) and a second pulley (50), the first and second pulleys (48, 50) being rotatably connected by a continuously rotatable device, the second pulley (50) being rotatably connected to an output member (46); the first pulley (48) including a first movable sheave (48a) moving along a first axis relative to a first fixed sheave (48b) in response to movement of a first actuator; the second pulley (50) including a second movable pulley (50a) moving along a second axis relative to a fixed pulley (50b) in response to movement of a second actuator; a clutch (38) selectively engageable to connect the first pulley (48) to an input member (24) to position it in a forward operating mode to the CVT (14); a hydraulic system configured to supply pressure to the variator assembly (44) and the clutch (38); and a control system (60) having at least one controller, a memory (66) and a plurality of sensors in communication with the controller, the control system (60) being configured to: Performing a plurality of clutch slip tests, wherein the control system (60) is configured to, for each clutch slip test: Reducing the pressure supplied to the clutch (38) by the hydraulic system until clutch slip occurs at a clutch slip data point (97) including a clutch slip pressure value and a corresponding clutch slip torque value, the clutch slip being determined by at least a portion of the plurality of sensors; storing the clutch slip data point (97) containing the clutch slip pressure value and the corresponding clutch slip torque value in the memory (66); and Increasing the pressure supplied to the clutch (38) by the hydraulic system after storing the clutch slip data point (97) in the memory (66), wherein the control system (60) is configured to store in the memory (66) at least one first clutch slip data point (97) having a clutch slip torque value in a first range and at least one second clutch slip data point (97) having a clutch slip torque value in a second range; determining a gain and an offset based on the plurality of clutch slip data points (97), wherein the gain is the increase in clutch pressure versus clutch torque and the offset is a clutch pressure offset; and Causing the hydraulic system to supply a drive clutch pressure to the clutch (38) that is lower than a pulley clamping pressure of the variator assembly (44) and higher than a critical clutch pressure determined based on the gain and the offset. [9] The continuously variable automatic transmission of claim 8, wherein the first range is a medium to low range and the second range is a medium to high range, wherein the plurality of clutch slip data points (97) further includes at least a third clutch slip data point (97) having a clutch slip torque value in a low to low range and at least a fourth clutch slip data point (97) having a clutch slip torque value in a high to high range, wherein the control system (60) is configured to interpolate a best-fit line (99) through the plurality of clutch slip data points (97), wherein the low to low range, the medium to low range, the medium to high range, and the high to high range are greater than a zero low torque range, wherein each of the low to low range, the medium to low range,the mid-to-high range and the upper-to-high range are smaller than a zero high-torque range, and wherein the low-to-low range, the mid-to-low range, the mid-to-high range, and the upper-to-high range do not overlap, wherein the control system (60) is configured to perform a plurality of clutch slip tests with a predetermined delay time, wherein the control system (60) is configured to perform the plurality of clutch slip tests for a duration of vehicle driving conditions within a driving range defined by a plurality of safe conditions, wherein the control system (60) is configured to determine whether a plurality of predetermined conditions are met before performing a clutch slip test of the plurality of clutch slip tests,wherein the plurality of predetermined conditions include at least a plurality of the following: the vehicle is operating in steady-state conditions; the turbine acceleration is within a predetermined turbine acceleration range; the engine acceleration is within a predetermined engine acceleration range; the vehicle acceleration is within a predetermined vehicle acceleration range; a throttle position exceeds a predetermined threshold; a torque converter clutch is engaged; clutch slip is not detected; the transmission fluid temperature is within a predetermined temperature range; the CVT ratio change is within a predetermined CVT ratio change range; clutch pressure scheduling arbitration allows a clutch slip check to be performed; and stability timer conditions are met. [10] A continuously variable automatic transmission (14) according to claim 9, wherein, to perform a clutch slip test, the control system (60) is configured to cause the hydraulic system to first reduce the pressure supplied to the clutch to a first output pressure in a reduction period and secondly reduce the pressure supplied to the clutch (38) from the first output pressure until clutch slip occurs in a slip period, wherein the pressure is reduced more rapidly in the reduction period than in the slip period, wherein the control system (60) is configured to update the best-fit line (99) when a clutch slip data point (97) is stored, wherein the control system (60) is configured to perform a rate limiting function such that a change in the best-fit line (99) for each stored clutch slip data point (97) is limited to a predetermined maximum change,wherein the control system (60) is configured to determine clutch slip by sensing a differential between first and second speed sensors (56, 58) of the plurality of sensors, wherein the first speed sensor (56) is mounted on a torque converter turbine and wherein the second speed sensor (58) is mounted on the first pulley (48) of the variator assembly (44), wherein the control system (60) is configured to cause the hydraulic system to apply the drive clutch pressure to the clutch (38) before the control system (60) performs each clutch slip test, wherein the drive clutch pressure is at least equal to the critical clutch pressure plus a safety factor.

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