CVT AND METHOD TO MITIGATE GROSS VARIATOR SLIP OF THE SAME

Abstract

Continuously variable transmission (CVT) (14) comprising: a drive element (13), an output element (15); a variator assembly with a primary variator pulley (18) for receiving an input torque via the drive element (13), with a secondary variator pulley (20) for transmitting an output torque via the output element (15), and with an endless rotatable drive element (22) in frictional engagement with the primary (18) and secondary (20) variator pulleys; first (Sp) and second (Ss) speed sensors for measuring a respective rotational speed (ΔP, ΔS) of the primary (18) and secondary (20) variator pulleys; and a programmed controller (50) for detecting gross slip of the endless rotatable drive element (22) with respect to the primary (18) and secondary (20) variator pulleys using the measured rotational speeds (ΔP, ΔS).S), and in response to the detected coarse slip to request a reduction of the input torque by a calculated value over a calibrated time period until the detected coarse slip level again reaches a calibrated acceptable slip level, characterized in that the controller (50) is capable of temporarily switching off a feedback-based ratio control of the CVT (14) until the coarse slip again reaches the calibrated acceptable slip level.

Description

TECHNICAL FIELDThe present disclosure relates to a continuously variable transmission (CVT) and a method for alleviating the coarse variator slip thereof.BACKGROUNDA continuously variable transmission (CVT) is one type of power transmission that can achieve infinite variability in a calibrated range of speed ratios. Unlike conventional transmission technology which uses one or more planetary gear sets and multiple rotating and braking friction clutches to achieve a discrete gear condition, a CVT uses a variable diameter pulley system. The pulley system, commonly referred to as a variator assembly, may transition anywhere within the calibrated range of speed ratios.A typical variator assembly includes two variator pulleys connected together by an endless rotatable drive member such as a drive chain or belt. The endless rotatable drive member travels within a variable width space defined by conical surfaces of the conical discs. One of the variator pulleys receives engine torque via a crankshaft, torque converter and the input gear set and thus acts as a drive / primary pulley. The other pulley is connected via additional gear sets to an output shaft of the CVT and thus acts as a driven / secondary pulley. One or more planetary gear sets may be used on the input or output side of the variator assembly depending on the configuration.To vary the CVT speed ratio, a clamping force is applied to the variator pulleys via one or more pulley actuators. The clamping force effectively squeezes the pulley halves together, thereby changing the width of the space between the pulley surfaces. The variation of the clearance size, i.e. the effective radius, causes the rotatable drive member to run higher or lower in the clearance. This in turn changes the effective diameter of the variator pulleys and the speed ratio of the CVT.The document DE 101 00 778 A1 discloses a continuously variable transmission according to the preamble of claim 1 and a method for alleviating the effect of coarse slip in a continuously variable transmission according to the preamble of claim 6.In the document US 2012 / 0 108 390 A1, a control and a control method for a continuously variable transmission are disclosed, in which, in the case of steady-state conditions such as a small change in the transmitted torque capacity and a trusted estimated accuracy of an input torque, a belt slip control with reduced hydraulic pressure at a secondary pulley of the continuously variable transmission is switched over.The document US 2004 / 0 209 719 A1 discloses a control of a continuously variable transmission in which a control unit establishes the occurrence of belt slippage on the basis of a difference between an actual rotational speed ratio and a setpoint rotational speed ratio and sums up a total slip time.The object of the invention is to mitigate the effects of the occurrence of coarse slip in a continuously variable transmission.This object is achieved by the subject matters of the independent claims. Advantageous further developments of the invention are the subject matter of the dependent claims.SUMMARYA system having a continuously variable transmission (CVT) and a programmed controller to mitigate the effects of gross drags of the CVT is disclosed herein. The CVT includes an endless rotatable drive member, such as a drive chain or belt, and a variator assembly of the type set forth in the opening paragraph. However, temporarily the endless rotatable driving member may slip with respect to the variator pulleys. The variator surfaces are machined to provide a friction surface which engages the endless rotatable drive member. The controller is programmed to automatically execute the steps of an associated control method to mitigate the effects of coarse variator slippage and thereby prevent wear on the contacting surfaces of the endless rotatable drive member and the variator pulleys.In one embodiment, the CVT includes an input member, an output member, a variator assembly, the first and second speed sensors, and a controller. The variator assembly includes a primary variator pulley operable to receive input torque from, for example, an engine via the input member, a secondary variator pulley operable to transmit output torque via the output member, and an endless rotatable input member in frictional engagement with the primary and secondary variator pulleys. The speed sensors are operable to measure a corresponding speed of the primary and secondary variator pulleys.The controller, in this embodiment, is programmed to determine the coarse slip of the endless rotatable drive member relative to the primary and secondary variator pulleys from the measured speeds and, in response to the detected slip, request a decrease in input torque by a calculated value over a calibrated duration until a level of the detected coarse slip reaches a calibrated slip level.According to the invention, the controller temporarily disables feedback-based ratio control of the CVT to enable these actions to be implemented.At the beginning of the coarse slip event, the controller reduces the torque-capacity ratio by increasing a clamping force on the pulleys by a calibrated value, such as by an increase in clamping torque offset. The torque-capacity ratio is thereafter smoothly returned at a predefined rate to a calibrated level.Additionally, a look-up table may be updated for a calibrated number of events of coarse variator slip at a particular speed ratio. This measure is intended to increase the clamping torque offset into a band range of variator speed ratios around the region where the rough slip has occurred. The clamping torque offset is then applied in normal CVT operation when the variator speed ratio is again in the affected range. After a predetermined number of coarse slip events at a particular speed ratio, the controller may take such speed ratios into account. An error code may be detected in the controller's memory and indicate that the determined speed ratio is not available or request maintenance of the CVT.The above and other features and advantages of the present disclosure will become apparent from the following detailed description when considered in connection with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a schematic illustration of an example vehicle having a continuously variable transmission (CVT) with a variator assembly and a programmed controller to mitigate gross slip events of the variator assembly. FIG. 2 is a time chart of varying amplitudes of the various vehicle parameters illustrating the coarse variator slip condition and control actions performed by the control shown in FIG. 1. FIG. 3 is a flow chart describing one embodiment of a method for mitigating rough slip of the variator assembly shown in FIG. 1.DETAILED DESCRIPTIONReferring now to the drawings, wherein like reference numerals correspond to like or similar components throughout the several figures, a vehicle 10 is schematically illustrated in FIG. 1. The vehicle 10 includes a torque-generating device 12, such as the illustrated internal combustion engine (E), but may also be embodied as an electric motor or other device for generating output torque. For consistency in illustration, the torque producing device 12 is described herein below as a motor 12 without being limited to this configuration.The vehicle 10 also includes a continuously variable transmission (CVT) 14 and a controller (C) 50. As detailed below with reference to FIGS. 2 and 3, the controller 50 is programmed to calculate an actual speed ratio of the CVT 14, detect a coarse slip event of the CVT 14, and mitigate the effects of such a coarse slip event of the CVT 14 through control actions. The CVT 14 is embodied as or includes a variator assembly and has primary and secondary variator pulleys 18 and 20. the pulleys 18 and 20 have respective primary and secondary pulley speeds ω P and ω S, which are measurable by speed sensors S P and S S the variator pulleys 18 and 20 being responsive to clamping pressure (arrow Pc) signals transmitted from the controller 50. Alternatively, sensors may be disposed in other positions and configurations as long as the rotational speeds of the pulleys 18 and 20 can be derived, for example, wheel speeds can be measured and used in S instead of the secondary speed sensor S. As part of the method 100, the controller 50 continuously or periodically calculates the current CVT speed ratio (SR) wherein and uses the calculated CVT speed ratio to detect a coarse slip event of the CVT 14. Those skilled in the art will recognize that the speed ratio could be inverse, i.e., or the torque ratio could be used within the scope of the present disclosure.With respect to the example vehicle 10 of FIG. 1, the engine 12 includes a crankshaft 13, and the crankshaft 13 is connected to a drive member 13 of the CVT 14, e.g., via an input clutch or hydrodynamic torque converter (not shown). An output member 15 of the CVT 14 ultimately delivers the output torque (arrow T O) to a set of drive wheels 16. the CVT 14 includes the primary variator pulley 18 connected to and driven by the crankshaft 13, the secondary variator pulley 20 connected to the output member 15, and a continuously rotating drive member 22, that is, any closed / endless loop of rubber and / or metal such as a chain or belt suitable for torque transmission from the primary variator pulley 18 to the secondary variator pulley 20. The variator pulleys 18 and 20 form, together with the input member 22, a variator assembly of the CVT 14. As used herein, the terms "rough slip event" and "rough variator slip" refer to slip of the input member 22 with respect to the pulleys 18 and / or 20.The primary and secondary variator pulleys 18 and 20 have respective mating halves 19 and 21 with respective conical surfaces 23 and 25 defining a variable width space 26. The driving member 22 positioned within the variable width clearance 26 rides on the conical surfaces 23 and 25 when the motor 12 drives the primary disc 18 at the motor speed (N E) which therefore acts as the input / primary speed (ω P) of the primary disc 18. The secondary disk 20 rotates at the secondary rotational speed (ω S). As mentioned above, the speeds of both pulleys (ω P, ω S) may be measured via sensors S P and S S and reported to controller 50, for example, as signals over a control network or other suitable channels.The width of the variable width space 26 may be varied via movement of the mating halves 19 and / or 21 to change the actual speed ratio of the CVT 14. Therefore, the vehicle 10 includes respective first and second variator actuators 28 and 30, shown schematically in FIG. 1, each responsive to primary and secondary clamp pressures (arrows P p and P S respectively) measurable via respective pulley pressure sensors (S PP, S PS) to change the position of the respective primary and secondary pulleys 18 and 20. A spring (not shown) may be inserted within each variator pulley 18 and 20. An example of an embodiment of the first and second actuators 28 and 30 includes a hydraulic piston / cylinder system, although other linear actuators such as electro-mechanical devices or pneumatic pistons may also be used.The first variator actuator 28 acts on a movable one of the mating halves 19 of the primary variator pulley 18 in response to the application of the primary clamping pressure (arrow P P). Also, the second variator actuator 30 acts on a movable one of the mating halves 21 of the secondary variator pulley 20 in response to the secondary clamping pressure (arrow P S). Line pressure (arrow P L) may be provided to the CVT 14 via a liquid pump 32, where the liquid pump 32 draws liquid 33, such as oil, from a sump 34 and circulates the liquid 33 to the CVT 14 via conduits, hoses, fittings, and other suitable conduits (not shown). In one possible embodiment, the primary and secondary clamping pressures (arrows P P and P S, respectively) are independent of the line pressure (arrow P L). However, in other embodiments, the line pressure (arrow P L) may be equal to the secondary clamping pressure (P S).The controller 50 may be configured as one or more computing devices having memory (M). The controller 50 may also include hardware elements such as a processor (P) and logic circuits including, but not limited to, a timer, an analog-to-digital (A / D) circuit, a digital-to-analog (D / A) circuit, proportional-integral-differential control logic, a digital signal processor and the necessary input / output (I / O) units, as well as other signal conditioning and / or buffer circuits. The memory (M) may include both tangible, permanent storage space and read-only memory, for example magnetic, solid-state and / or optical storage, as well as a sufficient amount of random access memory, read-only memory EEPROM, and the like. The controller 50 may provide feedback-based ratio control of the CVT 14 under normal operating conditions, e.g., with proportional-integral or proportional-integral-differential control logic.As discussed below, the controller 50 may selectively deactivate the feedback-based ratio control at a coarse slip event of the CVT 14 until the coarse slip reaches a calibrated acceptable level again. Steps for executing the method 100 may be included in the memory (M) and executed by the processor (P) in the overall control of the vehicle 10.The controller 50 may be programmed to record a number of events of coarse variator slip at a given CVT speed ratio and execute a control action with respect to the CVT 14 when the number of slip events exceeds a calibrated threshold number of events.The controller 50, which is in communication with the first and second variator actuators 28 and 30, receives a set of control input signals (arrow CC I) as part of the method 100. The control input signals (arrow CC I) may include, among other things, the vehicle speed N 10, as reported, calculated, or measured by one or more wheels or speed sensors S 10 from the transmission output, including pulley speeds (ω P, Ω S) and driver demanded torque (arrow T REQ). As is known in the art, the driver demanded torque (arrow T REQ) is typically determined by driver actions of the vehicle 10, such as via throttle demand, brake levels, gear states, and the like. As part of method 100, controller 50 may request a decrease in output torque from engine 12, e.g., by transmitting an engine torque request signal (arrow 11) to an engine control unit (ECM) 150. The ECM 150 may respond by transmitting the engine control signals (arrow 111) to the engine 12 to achieve the requested reduction in output torque. Similarly, engine speed management may be used and may regulate engine speed to achieve the proper speed ratio between pulleys 18 and 20.Referring to FIGS. 2 and 3, the method 100 is executed by the controller 50 to mitigate the effects of slip on the endless rotatable input member 22 against the variator pulleys 18 and 20, including commanding a particular torque response from the engine 12 and a particular speed ratio of the CVT 14, thereby reducing or preventing wear of the friction surfaces for proper operation of the CVT 14. The individual steps of the method 100 are shown in FIG. 3, while all said parameters or curves are shown in the time diagram of FIG. 2, wherein FIG. 2 shows the amplitude (A) on the vertical axis and the time (t) on the horizontal axis.After initialization, the method 100 begins at step S 102. For example, the controller 50 of FIG. 1 detects a coarse variator slip event of the CVT 14. The controller 50 receives the primary and secondary pulley speeds (arrows ω P, ω S) and calculates a speed difference between actual and target speeds at the primary and secondary pulley speeds (arrows ω P and ω S) and compares the calculated speed difference to a calibrated threshold and determines therefrom whether coarse variator slip is present. Similarly, errors between commanded and actual speed ratios may be used for slip determination. If rough variator slip is detected, a bit flag may be activated as shown between t1 and t2 of FIG. 2 by trace F GS. If rough variator slip is detected, the controller 50 proceeds to step S104.At step S 104 of FIG. 3, the controller 50 activates a control logic state in which the controller 50 of FIG. 1 is permitted to execute the various control actions described herein to mitigate slip. Step S 104 may include setting a status in the logic of the controller 50 indicative of this permission, such as a corresponding bit flag. The controller 50 may temporarily disable feedback-based ratio control of the CVT 14 until completion of the method 100 when the rough slip eventually reaches a calibrated acceptable level again, as indicated below. Method 100 proceeds to step S 106 when slip control is active.In step S 106, in managed communication, the controller 50 requests a reduced amount of output torque from the engine 12 to the ECM 150 of FIG. 1, as well as transmission of an engine control signal (arrow 111). This limits the amount of torque provided to the variator assembly. The lower level of engine torque may be obtained for a calibrated minimum amount of time until the rough variator slip is sufficiently under control. This measure is shown as trace T E in FIG. 2, decreasing at t 1 (double arrows A) and maintained for a calibrated time duration (double arrow C) until after t 2 once the rough slip condition is removed. Thus, at t 3, the engine torque (curve T E) may again increase to its initial value over a duration (double arrow D) at a calibrated rate, i.e., A / D, which is reached at time t 4. Then, the method 100 may proceed to step S 108.Step S 108 includes increasing a clamping force of the primary and secondary variator pulleys by a calibrated amount of clamping torque offset. That is, at the beginning of a coarse slip event at t1 in FIG. 2, the controller 50 decreases the torque-capacity ratio (trace ASR) of the CVT 14 as indicated by the double arrow B, i.e., the required clamping force divided by the currently controlled clamping force. The torque-capacity ratio (trace ASR) may be lowered by increasing the clamping force according to arrow P C of FIG. 1 by a calibrated amount, i.e., the clamping torque offset, as indicated by plot P C of FIG. 2, and may hold the new clamping force (plot P C) for a calibrated amount of time (double arrow E) after the slip event until at point (t5) the torque-capacity ratio (plot ASR) smoothly returns to its normal calibrated level less a calibrated torque offset (double arrow G) over a calibrated amount of time t5to t6(double arrow F). The intention of step S108 is to reduce the probability of slip re-occurring at the same CVT torque ratio. The method 100 then proceeds to step S 110.At step S 110, the controller 50 may temporarily disable the selection of the current CVT speed ratio when the rough slip state is not removed by steps S 106 and S 108. The method 100 then proceeds to step S 112.At step S 112, the controller 50 may then increase a slip counter in the commanded CVT speed ratio memory (M). That is, the controller 50 tracks the number of occurrences of coarse slip events at each speed ratio or range of speed ratios of the CVT 14, e.g., in a look-up table indexed by CVT speed ratios, such that for each CVT speed ratio, the controller 50 can quickly determine how many times the endless rotatable input member 22 has already exhibited slip more than a minimum allowable threshold. The method 100 then proceeds to step S 114.Step S 114 includes determining whether the slip counter of step S 112 has reached a threshold value. If so, then the method 100 proceeds to step S 116. Otherwise, the method 100 is ended and restarts with step S 102.Step S 116 includes updating a clamping torque offset table over multiple firing cycles. The result of step S116 is an increase in clamping torque offset within a range of variator speed ratios proportional to the number of coarse variator slip events. That is, the goal of step S116 is that with each occurrence of slip at a particular CVT speed ratio, the clamping torque offset to be applied is increased by a calibrated value. The new clamping torque offset is then applied in normal operation from the updated table when the torque ratio is regulated, i.e., a calibrated additional amount of clamping torque is applied. The method 100 continues iteratively or adaptively over time. With each subsequent occurrence of coarse slip at a torque ratio, the clamping torque offset is increased. That is, at a second occurrence of slip at a particular torque ratio or speed ratio, the controller 50 uses a greater offset than at the first occurrence, at a third occurrence a greater offset than at the second occurrence, etc. The method 100 is ended and restarts at step S 102.While the best modes for carrying out the disclosure have been described in detail, those familiar with the art to which this disclosure relates will recognize various alternative embodiments and embodiments with which the invention may be practiced within the scope of the claims set forth below.

Claims

A continuously variable transmission (CVT) (14) comprising: a drive member (13), an output member (15); a variator assembly having a primary variator pulley (18) for receiving an input torque via the drive member (13), a secondary variator pulley (20) for transmitting an output torque via the output member (15), and an endless rotatable drive member (22) in frictional engagement with the primary (18) and secondary (20) variator pulleys; first (Sp) and second (Ss) speed sensors for measuring a respective speed (ω P, ω S) of the primary (18) and secondary (20) variator pulleys; A programmed controller (50) for detecting coarse slip of the endless rotatable drive member (22) with respect to the primary (18) and secondary (20) variator pulleys using the measured speeds (ω P, ω S), and responsive to the detected coarse slip, for requesting a decrease in input torque by a calculated value over a calibrated period of time until a level of the detected coarse slip reaches a calibrated acceptable slip level again, characterised in that the controller (50) is capable of temporarily disabling feedback-based ratio control of the CVT (14) until the coarse slip reaches the calibrated acceptable slip level again.The CVT (14) of claim 1, wherein the controller (50) is programmed to request a decrease in a torque-capacity ratio of the CVT (14) at the beginning of the sensed coarse slip.The CVT (14) of claim 1, wherein the controller (50) includes a counter and is programmed to track a number of coarse variator slip events at a particular speed ratio and to perform a control action with respect to the CVT (14) when the number of events exceeds a calibrated threshold number of events.The CVT (14) of claim 3, wherein the control action includes preventing selection of a given speed ratio.The CVT (14) of claim 1, wherein the controller (50) is capable of increasing a torque offset in a range of variator speed ratios around the speed ratio where the slip event occurs.A method (100) for alleviating the effects of coarse slip in a continuously variable transmission (CVT) (14), the method (100) comprising: detecting coarse slip of an endless rotatable input member (22) of the CVT (14) relative to the primary and secondary variator pulleys of the CVT (14); requesting, via a controller (50), a reduction in drive torque on the CVT (14) by a calculated value over a calibrated period of time; and maintaining the reduction in drive torque until a level of the detected coarse slip reaches a calibrated acceptable slip level; characterized in that the method further comprises temporarily disabling feedback-based ratio control of the CVT (14) until the coarse slip reaches the calibrated acceptable slip level.The method (100) of claim 6, further comprising, at the beginning of the detected coarse slip, the command to decrease a torque-capacity ratio of the CVT (14) via the controller (50) including increasing a clamping force on the primary (18) and secondary (20) variator pulleys.The method (100) of claim 6, wherein the controller (50) includes a counter and further comprises tracking a number of events of coarse variator slip at a predefined speed ratio via the counter, and wherein a control action is performed with respect to the CVT (14) via the controller (50) when the number of events exceeds a calibrated threshold number of events.

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