Control for a continuously variable transmission in a vehicle drive system

Abstract

A controller (12) for a continuously variable transmission (140) in a vehicle drive system (100), the controller (12) including a set of instructions, the set of instructions being executable to: Measuring a speed ratio (304, 404) of the continuously variable transmission (140); Determining whether the measured speed ratio (304, 404) drifts away from a desired speed ratio (302, 402); and adjusting the target speed ratio (302, 402) to a value that differs from a desired speed ratio (406) when the measured speed ratio (304, 404) drifts away from the target speed ratio (302, 402); and wherein adjusting the target speed ratio (302, 402) to the value that differs from the desired speed ratio (406) when the measured speed ratio (304, 404) drifts away from the target speed ratio (302, 402) comprises a target speed ratio (302, 402) that is higher than the desired speed ratio (406) when the measured speed ratio (304, 404) drifts upward.

Description

The present description relates to a continuously variable transmission control for a vehicle propulsion system.A continuously variable transmission (CVT) is a type of power transmission that can achieve infinite variability in a calibrated range of speed ratios. Unlike conventional driven transmissions which employ one or more planetary gear sets and a plurality of rotating and brake friction clutches to achieve a discrete gear condition, a CVT employs a variable diameter pulley system. The pulley system, commonly referred to as a variator assembly, may continuously transition 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 widely variable space defined by tapered pulley surfaces. One of the variator pulleys receives engine torque via a crankshaft, a torque converter and a drive 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 change a CVT speed ratio, a clamping force is applied to the variator pulleys via one or more pulley actuators. The clamping force effectively clamps the pulley halves together to change the width of the gap between the pulley surfaces. The variation of the gap dimension, i.e. the effective radius, causes the rotatable drive element to run higher or lower within the gap. This in turn changes the effective diameters of the variator pulleys and changes the speed ratio of the CVT. The tension forces for the variator pulleys are provided with hydraulic actuators, the tension force produced by these hydraulic actuators depending on the hydraulic pressure of the fluid supplied to the actuators. Thus, the pressure of the fluid supplied to each actuator is used to control the speed ratio of the CVT.It will be appreciated that in order to maintain a speed ratio, sufficient hydraulic pressures must be available for each pulley. This pressure may vary and each pulley may require a different pressure to maintain a particular speed ratio. However, under certain conditions, such as cold temperatures, wheel slip, engine dropouts, a reduction in power available for pumping hydraulic fluid, and / or a pressure capacity of a variator pulley may be lower than may be required to maintain a speed ratio. These conditions may negatively affect the ability of the CVT to maintain a desired and / or desired speed ratio. As a result, the speed ratio may drift higher or lower than intended. This may result in loss of drive, decreased starting power, decreased capacity of the CVT to transmit torque, chain or belt slippage, decreased durability, reliability, fuel consumption, efficiency, power, and / or the like.DE 10 2016 120 912 A1 describes a drive system that includes a continuously variable transmission (CVT). A method of controlling the CVT includes determining, by a first controller, a first desired speed ratio in response to an output torque request. The first desired speed ratio is evaluated to determine whether it is proper and achievable. A second control unit is used to determine a second target speed ratio, and a final target speed ratio is determined based on the first and second target speed ratios. The final desired speed ratio is evaluated and the CVT is controlled based on the final desired speed ratio.It can be considered an object to specify a control which makes it possible to reduce and / or prevent drifting away of the rotational speed ratio.According to the invention, a continuously variable transmission controller in a vehicle propulsion system includes measuring a speed ratio of the continuously variable transmission, determining whether the measured speed ratio drifts away from a desired speed ratio, and adjusting the desired speed ratio to a value different from a desired speed ratio when the measured speed ratio drifts away from the desired speed ratio.Setting the target speed ratio to a value other than a desired speed ratio in a ratio drift situation may enable speed ratio control and the possibility of returning to a state in which the ratio drift may no longer be a potential problem. In this way, any further drift in speed ratio may be reduced and / or eliminated, improving starting performance, improving torque capacity, improving drivability and control, and improving reliability, durability, fuel consumption, efficiency, performance, and the like.In one embodiment, adjusting the desired speed ratio to a value different than a desired speed ratio when the measured speed ratio drifts away from the desired speed ratio includes adjusting the desired speed ratio to the measured speed ratio.According to the invention, adjusting the desired speed ratio to the value different from the desired speed ratio when the measured speed ratio drifts away from the desired speed ratio includes a desired speed ratio that is higher than the desired speed ratio when the measured speed ratio drifts upward.In another embodiment, adjusting the desired speed ratio to the value different from the desired speed ratio when the measured speed ratio drifts away from the desired speed ratio includes a desired speed ratio that is lower than the desired speed ratio when the measured speed ratio drifts downward.In another embodiment, determining whether the measured speed ratio drifts away from the desired speed ratio includes determining whether a difference between a measured speed ratio and a desired speed ratio exceeds a predetermined threshold.In another embodiment, determining whether the measured speed ratio drifts away from the desired speed ratio includes determining whether a pulley pressure is limited or an error between a desired pulley pressure and a measured pulley pressure exceeds a predetermined threshold.In another embodiment, determining whether the measured speed ratio drifts away from the desired speed ratio includes determining whether the torque capacity of the continuously variable transmission is higher than a desired torque capacity by a predetermined amount or whether the engine torque is at a maximum limit.In another embodiment, the controller further includes determining whether a head range for two pulleys in the continuously variable transmission exceeds a predetermined amount, and setting the target speed ratio to a constant when the head range for both pulleys in the continuously variable transmission exceeds the predetermined amount.In another embodiment, the controller further includes determining whether a head range for two pulleys in the continuously variable transmission exceeds a predetermined amount, and setting the target speed ratio to a measured ratio when the head range for both pulleys in the continuously variable transmission exceeds the predetermined amount.In another embodiment, the controller further includes determining whether a desired speed ratio has changed and setting the target speed ratio to a desired speed ratio when the desired speed ratio has changed.The present description will become more fully understood from the detailed description and the accompanying drawings, in which: FIG. 1 schematically illustrates a vehicle propulsion system having an internal combustion engine rotatably connected to a continuously variable transmission (CVT) via a torque converter and a gearbox within the scope of the description; FIG. 2 is a schematic cross section of a variator of a chain-type CVT; FIG. 3 is a graph of various signals from a CVT having ratio drift; FIG. 4 is a graph illustrating various signals from an exemplary control system and method according to the present description; and FIG. 5 is a flow chart illustrating an exemplary method according to the present description.In the drawings, the same reference numerals are used for similar and / or identical elements.Referring now to the drawings, wherein the showings are for the purpose of illustrating certain exemplary embodiments and not for the purpose of limiting the same, FIG. 1 schematically illustrates elements of a vehicle propulsion system 100 having an internal combustion engine (engine) 110 rotatably connected to a continuously variable transmission (CVT) 140 via a torque converter 120 and a gearbox 130. The vehicle propulsion system 100 is connected to a vehicle wheel 160 by a driveline 150 to generate tractive force when used in a vehicle. Operation of the vehicle propulsion system 100 is monitored and controlled by a control system 10 in response to driver commands and other factors.The engine 110 may be any suitable internal combustion engine capable of converting hydrocarbon-based fuels into mechanical power to generate torque in response to the control commands from the control system 10. Torque converter 120 is a device having a fluid coupling between its input and output members for transmitting torque, and preferably includes a pump 122 connected to engine 110, and a turbine 124 connected via the output member to transmission 130 and a torque converter clutch 126 that locks the rotation of pump 122 and turbine 124 and that is controlled by control system 10. The output member of the torque converter 120 is rotatably coupled to the gearbox 130, which includes meshing gears or other suitable transmission mechanisms that produce reductions between the torque converter 120 and the CVT 140. Alternatively, the gearbox 130 may take another suitable configuration for establishing a speed ratio between the engine 110, the torque converter 120, and the CVT 140, including, by way of non-limiting examples, a chain drive or planetary gear configuration. In alternative embodiments, the torque converter 120 and the gearbox 130 may be omitted.The gear box 130 includes an output member rotatably connected to the CVT 140 via a drive member 51. An exemplary embodiment of the CTV 140 is described with reference to FIG. 2. An output member 61 of the CVT 140 is rotatably connected to the powertrain 150, which is rotatably connected to the vehicle wheels 160 via an axle, half shaft, or other suitable torque transfer member. The powertrain 150 may include a differential gear, a chain transmission, or other suitable transmission arrangement for transmitting torque to one or more vehicle wheels 160.Vehicle propulsion system 100 preferably includes one or more sensing devices for monitoring speeds of various devices, including, for example, an engine speed sensor 112, a turbine speed sensor of torque converter 125, a CVT variator input speed sensor 32, a CVT variator output speed sensor 34, and a wheel speed sensor 162 over which vehicle speed is monitored. Each of the aforementioned speed sensors may be any suitable rotational position / speed sensing device, such as a Hall effect sensor. Each of said speed sensors is in communication with the control system 10.The control system 10 preferably includes a controller 12 and a user interface 14. the controller 12 may include a plurality of control devices, each device associated with monitoring and controlling a single system. This may include an engine control module (ECM) for controlling the engine 110, a transmission controller (TCM) for controlling the CVT 140, and for monitoring and controlling a single subsystem, such as the torque converter clutch 126. The controller 12 preferably includes a non-volatile storage device 11 that includes sets of executable instructions and a cache 13. The user interface 14 is in communication with user input devices including, for example, an accelerator pedal 15, a brake pedal 16, and a gear selector 17 for determining an output torque request. In some embodiments, the gear selector 17 has a tip-up / drop-down function that allows the driver to manually select a speed ratio and transition the automatic system of the CVT 140. An upshift command generates an instruction to the CVT 140 to decrease its speed ratio by increasing the variator speed ratio. A downshift command generates an instruction to the CVT 140 to increase its speed ratio by decreasing the variator speed ratio.FIG. 2 schematically illustrates elements of a variator 30 of an exemplary embodiment of the CVT 140 that may be controlled by the TCM. The arithmetic transmission 30 transfers torque between the first rotary element 51 and the second rotary element 61, and the first rotary element 51 is referred to as the input member 51 and the second rotary element 61 is referred to as the output member 61.The variator 30 includes a first or primary impeller 36, a second or secondary impeller 38, and a flexible continuous rotatable device 40 rotatably connected to the first and second impellers 36, 38 to transmit torque. The first pulley 36 is rotatably connected to the input member 51 and the second pulley 38 is rotatably connected to the output member 61, and the rotatable device 40 is adapted to transmit the torque between the first and second pulleys 36, 38 and thus between the input and output members 51, 61. The first pulley 36 and the drive link 51 rotate about a first axis 48 and the second pulley 38 and the output link 61 rotate about a second axis 46. the continuous rotatable device 40 may be a belt, chain, or other suitable flexible continuous device. The input speed sensor 32 may be mounted close to the input member 51 to produce a CVT input speed 33 that depends on the speed of the first input pulley 36, and the output speed sensor 34 may be mounted close to the output member 61 and produces a CVT output speed 35 that depends on the speed of the second output pulley 38. One of the first and second pulleys 36, 38 acts as a ratio pulley to establish a speed ratio and the other of the first and second pulleys 36, 38 acts as a clamping pulley to generate a clamping force sufficient to transmit torque. As used herein, the term "speed ratio" refers to the variator speed ratio, which is a ratio of the speed of the output member 61 relative to the speed of the input member 51. The speed of the input member 51 may be determined from signal inputs from the engine speed sensor 112, the torque converter turbine speed sensor 125, or the input speed sensor 32 described herein, or other suitable speed / position sensors. The rotational speed of the output member 61 can be determined on the basis of signal inputs of the output rotational speed sensor 34 or of the wheel rotational speed sensor 162 described here, or else by other suitable rotational speed / position sensors. Regardless of the measurement system(s) used, the speed ratio parameters depend on the CVT input speed and the CVT output speed. Alternatively, the speed ratio may also be dependent on the ratio of the impellers on each side or on the drive torque and the output torque.The first impeller 36 is divided perpendicular to the first rotation axis 48 to form a first annular groove 50 between a first movable side plate 52 and a first fixed side plate 54. Movable side plate half 52 moves laterally along first axis 48 relative to fixed side plate 54; for example, first movable side plate 52 may be splined to a drive member 51, thereby permitting axial movement of first movable side plate 52 along first axis 48. The first fixed side plate 54 is disposed opposite to the first movable plate 52. The first fixed side plate 54 is axially fixed to the driving member 51 along the first axis 48. Thus, the first fixed side plate 54 does not move in the axial direction of the first axis 48. the first movable side plate 52 and the first fixed side plate 54 each include a first grooved surface 56. the first groove surfaces 56 of the movable plate 52 and the first fixed plate 54 are arranged opposite to each other to define the first annular groove 50 therebetween. The opposing first grooved surfaces 56 are preferably inversely frustoconical such that movement of the first movable side plate 52 toward the first fixed side plate 54 increases the outer impeller diameter of the annular first groove 50. An actuator 55 is disposed with the first idler 36 to control the axial position of the first movable side plate 52 in response to a control signal 53, including pushing the first movable side plate 52 toward the first fixed side plate 54. In certain embodiments, the first actuator 55 is a hydraulically controlled device and the control signal 53 is a hydraulic pressure signal. The hydraulic pressure may be monitored by a sensor in the first actuator 55, or elsewhere in a hydraulic circuit supplying pressurized hydraulic fluid to the first actuator 55.The second pulley 38 is divided perpendicular to the second axis 46 to form a second annular groove 62 therebetween. The annular second groove 62 is disposed perpendicular to the second axis 46. The second idler 38 includes a second movable side plate 64 and a second fixed side plate 66. the second movable side plate 64 moves axially laterally along the second axis 46 relative to the fixed side plate 66. for example, the second movable side plate 64 may be splined to the output member 61, thereby permitting axial movement of the movable second side plate 64 along the second axis 46. The second fixed side plate 66 is disposed opposite to the second movable side plate 64. The second fixed side plate 66 is axially fixed to the output member 61 along the second axis 46. Thus, the second fixed side plate 66 does not move in the direction of the second axis 46. the second movable side plate 64 and the second fixed side plate 66 each have a second grooved surface 68. The second groove surface 68 of the second movable plate 64 and the second fixed plate 66 are oppositely disposed and the second annular groove 62 is located therebetween. the opposing second grooved surfaces 68 are preferably inversely frustoconical such that movement of the second movable side plate 64 toward the second fixed side plate 66 increases the outer impeller diameter of the annular second groove 62. A second actuator 65 is positioned with respect to the second impeller 38 to control the axial position of the second movable side plate 64 in response to a control signal 63 including urging the second movable side plate 64 toward the second fixed side plate 66. In certain embodiments, the second actuator 65 is a hydraulically controlled device and the control signal 63 is a hydraulic pressure signal. The hydraulic pressure may be monitored by a sensor in the second actuator 65, or elsewhere in a hydraulic circuit supplying pressurized hydraulic fluid to the first actuator 65. A ratio between the outer diameter of the first impeller 36 and the outer diameter of the second impeller 38 defines a transmission torque ratio. Other elements, such as clutch assemblies in the form of selectable one-way clutches and the like, may be employed between the variator 30 and other power transmission and powertrain components and systems.The speed ratio can be described in terms of an actual and a desired speed ratio. An actual speed ratio is a present measured value for the speed ratio and can be determined based on a ratio of the input speed signal 33 and the output speed signal 35. A desired speed ratio indicates a future desired value for the speed ratio that may be determined based on monitored and estimated operating conditions relating to an output power command, vehicle speed, and engine torque. The TCM uses control routines to control the CVT 140 to regulate the desired speed ratio by pressure regulation at one or both of the primary and secondary pulley units 36 and 38 of the CVT 140. The pressure regulation of one or both of the primary pulley 36 and the secondary pulley 38 of the CVT 140 can be achieved by controlling the input and output signals 53, 63 to apply the pressure required for the measured speed ratio to the first and second actuators 55, 65 without affecting it. The required pressures are preferably provided in the form of a primary pressure and a secondary pressure command.As mentioned above, under certain conditions, a CVT may not be able to maintain a desired speed ratio. For example, colder temperatures may require higher closing forces, increasing the need for hydraulic pressure beyond that available for division between the variator pulleys. Generally, a variator pulley may have a lower pressure capacity than is required to achieve and / or maintain a speed ratio under various conditions. Referring to FIG. 3, a diagram 300 of various sensor signals from a CVT with undesirable ratio drift is illustrated. FIG. 3 illustrates a desired speed ratio signal 302 and a measured speed ratio signal 304, where the measured speed ratio signal 304 initially follows the desired speed ratio signal 302, but begins to drift away. In this case, the measured speed ratio signal 304 drifts higher than the target speed ratio signal 302, however, it is understood that the measured speed ratio signal 304 may also drift lower than the target speed ratio signal 302. The present description is applicable to both situations without limitations.The graph 300 of FIG. 3 also includes a primary pulley desired pressure signal 306, a primary pulley measurement pressure signal 308, a secondary pulley desired pressure signal 310, and a secondary pulley measurement pressure signal 312. Generally, each of the measured pressure signals 308 and 312 closely tracks the respective desired pressure signals 306 and 310. The primary pulley pressure signals 306 and 308 both jump upward. This is probably due to the engaging clamping torque and the primary pulley acting as the clamping pulley and the secondary pulley acting as the transmission roller, whereby the pressure of the secondary pulley required to prevent an increase in the speed ratio may increase, but the secondary pulley may not have an additional capacity for setting the target additional pressure. The pressure of the secondary pulley may already be at a maximum pressure for the secondary pulley. The control system requests (instructs) an increase in the primary pulley pressure to prevent the variator from slipping. Because the primary pulley acts as a clamping disc, it may not have the desired effect on the speed ratio. The result is that the measured speed ratio 304 begins to drift away from the desired speed ratio 302. A conventional CVT control system continues to attempt to reduce the "error" between the desired speed ratio 302 and the measured speed ratio 304 by continuing to request / command pulley pressures. However, since the pressure capacity of one or both variator pulleys may be exceeded, the measured pulley pressures are unable to achieve the desired pressures.Referring to FIG. 4, the graph 400 illustrates various signals of an exemplary control system and method according to the present description that serves to reduce and / or eliminate speed ratio drift in a CVT. Similar to FIG. 3, FIG. 4 includes a desired speed ratio signal 402 and a measured speed ratio signal 404. The plot 400 of FIG. 4 also illustrates a desired speed ratio 406. The desired speed ratio 406 may depend on multiple conditions and inputs, such as a driver demand for torque. FIG. 4 also includes a primary pulley desired pressure signal 408, a measured primary pulley pressure signal 410, a secondary pulley desired pressure signal 412, and a measured secondary pulley pressure signal 414.First, the desired speed ratio 402 follows the desired speed ratio 406 as was conventional. Both the target primary pulley pressure signal 408 and the target secondary pulley pressure signal 412 increase gradually, in response to which the measured primary pulley pressure 410 follows the target primary pulley pressure 408 and the measured secondary pulley pressure 414 follows the target secondary pulley pressure 412. However, FIG. 4 also illustrates the maximum pressure capacity for the secondary pulley as line 416 as well as the fact that the target secondary pulley pressure 412 abuts but is limited to the maximum pressure capacity for the secondary pulley 416. Therefore, there is a determinable risk that the desired pulley pressures 408 and 412 are insufficient to prevent the measured speed ratio 404 from drifting away from the desired speed ratio 402. In response, the exemplary control system and method of the present description may set the desired speed ratio 402 to the measured speed ratio 404 and have the desired speed ratio 402 follow the measured speed ratio 404 at "A.". This sets each "error" between the desired speed ratio 402 and the measured speed ratio 404 to zero and at least temporarily eliminates any influence of feedback control on the adjustment of the desired pulley pressures 408 and 412 to minimize and / or eliminate this error.Subsequently, it can be seen on the graph 400 between "A" and "B" that the pulley pressures 408, 410, 412 and 414 tend to gradually decrease and, in particular, the secondary pulley pressures 412 and 414 decrease and increase their distance between the maximum capacity limit 416. At point "B", the secondary pulley pressures 412 and 414 increase so much "margin" below the maximum pressure capacity limit 416 that the measured speed ratio 404 can be reliably controlled to closely follow the desired speed ratio 402. In an exemplary embodiment, the control system and method may determine that the difference between the secondary roller pressures 412 and / or 414 and the maximum pressure limit of the secondary pulley 416 exceeds a predetermined threshold. In response to this determination, the example control system and method may then set the desired speed ratio 402 to a constant value, which may optionally correspond to the measured speed ratio 404 at point "B", and the system and method may then be operated to control the CVT such that the measured speed ratio 404 follows the constant desired speed ratio 402.FIG. 4 illustrates that the measured speed ratio 404 is further controlled to closely follow the constant desired speed ratio 402 until point "C". At point "C", another condition may be identified that justifies a return to conventional control methods and systems. For example, in the case shown in FIG. 4, the desired speed ratio 406 increases from a value below the target speed ratio 402 to a value that is above the target speed ratio 402. In this situation, it is known that increasing the speed ratio corresponds to a reduced need for pulley pressures to maintain the speed ratio. Therefore, it is likely that the risk of possible speed ratio drift may be significantly reduced and / or eliminated. Thus, in this exemplary embodiment of the control system and method, the desired speed ratio 402 may then be conventionally controlled to follow the desired speed ratio 406.In general, multiple different exit conditions that may be relied upon may be identified, either immediately or gradually, to exit the exemplary control system and method and return to a conventional control system and method without limitation. Additional alternative and exemplary exit conditions are discussed below, however, it is to be understood that the present invention is not limited to any particular exit condition.FIG. 5 illustrates a flow diagram 500 of an exemplary method according to the present description. The method begins at step 502 and continues to step 504. In step 504, control determines whether the actual speed ratio drifts away from the desired speed ratio. For example, the controller may determine whether the measured speed ratio drifts away from the desired speed ratio by determining: 1) whether the actual / measured speed ratio differs from the desired speed ratio by an amount that exceeds a predetermined threshold; 2) the pressure of at least one of the pulleys is limited or whether there is a sufficient error between a desired pulley pressure and a measured pulley pressure; and / or 3) the actual torque capacity of the CVT is higher than the desired torque capacity by a predetermined threshold or that the engine torque is at a maximum amount. In an exemplary embodiment, the controller determines that the speed ratio drifts when all three of the foregoing conditions are met. If control determines in step 504 that the measured speed ratio is drifting away from the desired speed ratio, the method continues to step 506. In step 506, control sets the desired speed ratio to correspond to the measured speed ratio. Preferably, the controller sets the target speed ratio to follow the measured speed ratio. In this way, the pressure adjustments to the CVT that would have occurred to reduce the error between the desired and measured speed ratios are inactive and the overall system tends to abort, where the pressures may gradually decrease by themselves and the ratio drift may be significantly reduced and / or eliminated. The method then continues to step 508.In step 508, the method determines whether the measured speed ratio exceeds a predetermined maximum drift value. The controller executes this step to avoid having an extreme condition and the present description may not be useful. In this way, the controller determines whether a condition exists under which the method according to the present description is to be ended. In other words, the controller determines whether an exit condition exists. If control determines in step 508 that the measured speed ratio exceeds the predetermined maximum drift value, the method jumps to step 518, leaving the method. Alternatively, if control determines in step 508 that the measured speed ratio does not exceed the predetermined maximum drift value, the method continues to step 510.In step 510, the method determines whether there is greater margin in the CVT system. In other words, the controller determines whether both pulley pressures differ from their respective maximum limit pressures by large predetermined thresholds. Preferably, these large predetermined threshold values correspond to amounts that ensure high certainty that the pressure margin for both pulleys is sufficient to reliably control the measured speed ratio to closely follow the target speed ratio. If the method determines in step 510 that a large portion of the margin is present in the system, the system jumps to step 518 ending the method. Alternatively, if the method determines in step 510 that there is no large margin in the system, the method continues to step 512.In step 512, the method determines whether the margin in the system is greater than another predetermined threshold that is less than the predetermined threshold of step 510. Preferably, the controller determines whether the margin in the system is greater than the predetermined threshold by determining whether the difference in measured pulley pressures is each sufficiently below their respective maximum pressure limits by an amount that exceeds the predetermined threshold. This predetermined threshold may be selected so that reliable control may control the measured speed ratio to closely follow the desired speed ratio. If the method determines in step 512 that the margin in the system is greater than the predetermined threshold, the method continues to step 514, otherwise the method returns to step 506.In step 514, the method sets the desired speed ratio to a constant value. In an exemplary embodiment, the controller may set the desired speed ratio to a value corresponding to the measured speed ratio when the controller reaches method step 514. The method then continues to step 516. In step 516, the method determines whether a predetermined amount of time has elapsed since execution of step 514. In an exemplary embodiment, the predetermined period of time may ensure that the method of the present description is not applied when the system is faulty for any reason not treated by the method of the present description. If control determines in step 516 that the timeout has not expired, the method returns to step 514. However, if control determines in step 516 that the predetermined period of time has elapsed, the method continues to step 518 ending the method.Optionally, and possibly preferably, the method may further include a step wherein the controller determines whether the desired speed ratio has changed, and if the controller determines that the desired speed ratio has changed, the controller may also end the method. For example, the controller may determine whether the desired speed ratio exceeds the desired speed ratio, and if the controller determines that the desired speed ratio exceeds the desired speed ratio, then the controller may then end the method of the present description. The desired speed ratio may change, for example, when a driver inputs an input indicative of a change in desired torque. Alternatively, the controller may further perform the method of the present description.Moreover, it should be understood that while the method described with reference to FIG. 5 includes specifically identified exit criteria / conditions, the method of the present description is not limited to particular exit criteria and / or conditions alone or in combination. For example, in some CVT systems, the pressure may be increased with respect to stopping the vehicle, so that a rapid downshift may be performed. In this case, it may be desirable not to apply / execute the method described in the present specification. Other conditions may be detectable, such as a speed sensor fault, a pulley pressure fault, which may optionally be used as a deactivation reason for not using / executing the described method. Further, the thresholds described above may be determined according to methods known to those skilled in the art, such as by known calibration techniques and methods.While the present detailed description describes an example condition where a speed ratio may drift upward, it should be understood that the control system and method of the present description is also applicable to a condition where the speed ratio is drifted downward. Adaptations of the control system and method for handling this other condition will be understood by persons of ordinary skill in the art with reference to the present description.

Claims

A controller (12) for a continuously variable transmission (140) in a vehicle propulsion system (100), the controller (12) including an instruction set, the instruction set executable to: measure a speed ratio (304, 404) of the continuously variable transmission (140); determine whether the measured speed ratio (304, 404) drifts away from a desired speed ratio (302, 402); and adjust the desired speed ratio (302, 402) to a value different from a desired speed ratio (406) when the measured speed ratio (304, 404) drifts away from the desired speed ratio (302, 402); and wherein adjusting the desired speed ratio (302, 402) to the value different from the desired speed ratio (406) when the measured speed ratio (304, 404) drifts away from the desired speed ratio (302, 402) comprises a desired speed ratio (302, 402) that is higher than the desired speed ratio (406) when the measured speed ratio (304, 404) drifts upward.The controller (12) of claim 1, wherein adjusting the desired speed ratio (302, 402) to the value different from the desired speed ratio (406) when the measured speed ratio (304, 404) drifts away from the desired speed ratio (302, 402) comprises adjusting the desired speed ratio (302, 402) to the measured speed ratio (304, 404).The controller (12) of claim 1, wherein adjusting the desired speed ratio (302, 402) to the value different from the desired speed ratio (406) when the measured speed ratio (304, 404) drifts away from the desired speed ratio (302, 402) comprises a desired speed ratio (302, 402) that is lower than the desired speed ratio (406) when the measured speed ratio (304, 404) drifts downward.The controller (12) of claim 1, wherein the command set that determines whether the measured speed ratio (304, 404) drifts away from a target speed ratio (302, 402) comprises a command set that determines whether a difference between a measured speed ratio (304, 404) and a target speed ratio (302, 402) exceeds a predetermined threshold.The controller (12) of claim 1, wherein the command set that determines whether the measured speed ratio (304, 404) drifts away from the target speed ratio (302, 402) comprises a command set that determines whether a pulley pressure is limited or an error between a target pulley pressure (408) and a measured pulley pressure (410) exceeds a predetermined threshold.The controller (12) of claim 1, wherein the command set that determines whether the measured speed ratio (304, 404) drifts away from the desired speed ratio (302, 402) comprises a command set that determines whether the torque capacity of the continuously variable transmission (140) is a predetermined amount higher than a desired torque capacity or whether the engine torque is at a maximum limit.The controller (12) of claim 1, wherein the set of instructions further includes instructions executable to: determine whether a pressure margin for two pulleys (36, 38) in the continuously variable transmission (140) exceeds a predetermined amount; and set the desired speed ratio (302, 402) to a constant when the pressure margin for both pulleys (36, 38) in the continuously variable transmission (140) exceeds the predetermined amount.The controller (12) of claim 1, wherein the set of instructions further includes instructions executable to: determine whether a pressure margin for two pulleys (36, 38) in the continuously variable transmission (140) exceeds a predetermined amount; and set the desired speed ratio (302, 402) to a desired speed ratio (406) when the pressure margin for both pulleys (36, 38) in the continuously variable transmission (140) exceeds the predetermined amount.The controller (12) of claim 1, wherein the instruction set further includes instructions executable to: determine whether the desired speed ratio (406) has changed; and set the desired speed ratio (302, 402) to the desired speed ratio (406) when the desired speed ratio (406) has changed.

Images