An adaptive clutch control system for a continuously variable transmission

Abstract: An adaptive control system controls the slip across a start clutch (12) located up-stream of a continuously variable transmission (10) to allow the clutch to be use as a torsional isolator in a continuously variable transmission. The system includes a torsional vibration detector (58) for determining the actual level of torsional vibrations transmitted by the clutch. The actual level of vibrations is compared with an acceptable level and the slip across the clutch is regulated in order to maintain the transmission of vibrations to the acceptable level.

Description: This invention relates to a clutch control system for a continuously variable transmission (CVT). A continuously variable transmission system utilizes a pair of adjustable pulleys, including a primary pulley and a secondary pulley. Each pulley has at least one sheave that is axially fixed and another sheave that is axially movable relative to the first. A flexible belt of metal or elastomeric material interconnects the pulleys. The inner faces of the sheaves of the pulley are bevelled or chamfered so that as the axially displaceable sheave moves relative to the fixed sheave, the distance between the sheaves and, thus, the effective pulley diameter may be adjusted. The primary pulley is driven by an engine and the output of the secondary pulley drives the drive train of the vehicle. The drive train is typically connected to the secondary pulley through a clutch. The displaceable sheave of each pulley includes a fluid constraining chamber for receiving fluid to move the sheave and thus change the effective pulley diameter. Increasing the fluid in the chamber increases the effective diameter of the pulley. As fluid is exhausted from the chamber, the pulley diameter decreases. Generally the effective diameter of the primary pulley is moved in one direction as the effective diameter of the secondary pulley is moved in the other. The movement of the sheave of the primary pulley servo regulates the transmission ratio across the CVT. The hydraulic pressure of the sheave of the secondary pulley servo varies the clamping force on the belt connecting the primary and secondary pulleys. US-A-4433594, entitled "Variable Pulley Transmission", provides further information in relation to CVTs and is mentioned in terms of background to the present invention. Engines that drive continuously variable transmissions, especially reciprocating engines, tend to produce torque fluctuations that lead to torsional vibrations on the input shaft of the belt and sheave system. It is important to the smooth operation of the continuously variable transmission that such torsional vibrations are kept within acceptable limits. The manner in which vibrations are reduced or dampened is dependent, in part, on the location of the different elements of the CVT. In a system such as that described in the aforementioned US-A-4433594 which includes a slippable start clutch downstream of the belt and sheave system, a long travel damper is generally located up-stream of the belt and sheave system to damp start-up, shut-down and drive mode vibrations, thereby preventing transmission of such vibrations to the belt and sheave system. In a continuously variable transmission having a start clutch positioned up-stream of the belt and sheave system, the larger torsional vibrations experienced during start-up and shut-down are not transmitted as a result of the slip between the clutch plates. This is to some extent a by-product of clutch control primarily aimed at ensuring that torque transmitted to the belt and sheave system increases or decreases smoothly during start-up and shut-down, respectively. Once the transmitted torque has attained the required level, the clutch is locked. A short travel mechanical damper is generally located upstream of the belt and sheave system to damp the vibrations experienced during clutch lock-up. As a further example of the prior art, attention is drawn to US-A-4466311 in which a torque transmitting system for transmitting torque from a drive shaft of an engine to an output shaft without transmitting torque variation comprises a friction clutch unit interposed between these shafts, a fluid pressure-operated actuating unit for engaging and disengaging the friction clutch unit, and vibration detecting means detecting vibration occurring in the engine or the torque transmitting system. In the system, means are provided for controlling the pressure of actuating fluid supplied to the fluid pressure-operated actuating unit on the basis of the output signal from the vibration detecting means thereby adjusting the difference between the rotation speed of the drive shaft and that of the output shaft, whereby the torque can be transmitted from the drive shaft to the output shaft without any substantial variation. Another example of the prior art is featured in FR-A-2561181 which generally discloses a clutch control system for a continuously variable transmission having a torque input means, a torque output means, a belt and sheave system and a slippable start clutch means for transmitting torque to said belt and sheave system, said slippable start clutch means being operatively located between said torque input means and said belt sheave system (according to the preamble of claim 1). More particularly, FR-A-2561181 describes an alternative configuration of a CVT which includes a wet start clutch positioned upstream of the belt and sheave system. A control system regulates the hydraulic circuit that controls slip across the start clutch. During the drive mode, the clutch is substantially locked to allow selected slip in accordance with signals from the control unit. Empirically collected data is used to provide information on the level of vibrations likely to be experienced by the continuously variable transmission for a range of engine conditions defined in terms of a set of engine parameters. These parameters include the degree of opening of the throttle valve, the engine torque and the engine speed. For a given set of measured engine parameters the probable level of vibration experienced by the continuously variable transmission is determined and a requisite amount of slip to remove that level of vibration is calculated. The clutch slippage is regulated by operation of a hydraulic fluid circuit. In accordance with the present invention as claimed, a clutch control system as generally disclosed in the aforementioned FR-A-2561181 is characterized in that the clutch control system comprises means providing a first value representative of a limiting value of torsional vibrations transmitted to said belt and sheave system; means providing a first signal representative of actual torsional vibrations transmitted to said belt and sheave system, said means comprising a torsional vibration sensor; means comparing said first signal with said first value; and means modifying slip across said slippable start clutch means adaptively in incremental amounts, said modifying means being responsive to said comparing means to reduce torsional vibrations. An advantage of the invention is that it avoids the need for collecting empirical data by providing an adaptive system to control the slip in the start clutch. More specifically, the present invention provides a simplified and effective control system for a slippable start clutch in a continuously variable transmission system. The system adapts to the requirements of the engine by avoiding slip below a specified level of vibration and by providing varying levels of slip when the vibrations are greater than an acceptable level. The system avoids the need for empirically derived data other than that data necessary to establish the acceptable level of torsional vibration. None of the memory storage available in the control system needs to be utilized to store data for achievement of active damping. A feedback loop providing information relating to actual clutch slip can be employed in the clutch control system of the present invention to ensure that the clutch is slipping by the required amount. The feedback loop includes means to measure the rotational speed of the clutch input shaft and the rotational speed of the clutch output shaft. The difference in rotational speed across the clutch determines the amount of clutch slip. Sufficient slip is ensured by increasing the clutch slippage in increments until the torsional vibrations are reduced to the acceptable level. The clutch may be locked up when the torsional vibrations are sufficiently reduced for more efficient operation of the transmission. The adaptive nature of the system of this invention allows the engine to damp vibrations that occur as a result of circumstances that may not be taken into account in a system that relies upon empirically collected data. It also allows the system to adapt to an increase in the level of vibrations that may occur on account of aging of the vehicle and resultant deterioration in the engine and transmission systems. In contrast, with a non-adaptive control system, the efficiency of the system is liable to deteriorate with time and wear, or, alternatively, the empirical data must be updated. An adaptive system is also preferable in some applications because the empirical data required for operation of such a control system will be obtained from a test group and the operational characteristics of some individual vehicles will inevitably deviate from those of the group. In addition, with the adaptive control system of this invention, not only is there no necessity for collection of data, the same control system can be fitted in exactly the same manner into a range of vehicles without requirement for the collection of additional empirical data. In order that the present invention may be well understood there will now be described an embodiment thereof, given by way of example, reference being made to the accompanying drawing, in which: Figure 1 is a schematic representation of a continuously variable transmission and associated clutch control system embodying this invention; and, Figure 2 is a schematic representation of the same clutch control system. Referring first to Figure 1, a continuously variable transmission (CVT) generally designated at 10 can be seen having a slippable start clutch 12 located upstream of a belt and sheave system 14. The continuously variable transmission has an input shaft 16 and an output shaft 18. A first variable pulley 20 having a first movable sheave 22 is disposed on the input shaft 16. A second variable pulley 24 having a second movable sheave 26 is disposed on the output shaft 18. The first and second pulleys 20 and 24 are coupled by a flexible belt 28 of metal or elastomeric material. A first servo chamber 30 provides fluid to control movement of the first movable sheave 22 and a second servo chamber 32 provides fluid to control movement of the second movable sheave 26. The operating ratio across the belt and clamping force on the belt of the continuously variable transmission 10 is determined by the pressure of fluid supplied to the first and second servo chambers 30, 32. The pressure in the first servo chamber 30 moves the first movable sheave 22 to regulate the ratio between sheaves and the resulting ratio between the input shaft 16 and the output shaft 18. The pressure supplied to the second servo chamber 32 varies the hydraulic pressure on the second movable sheave 26 and thereby varies the clamping force on the belt 28 to prevent belt slippage. The slippable start clutch 12 is located on the transmission input shaft 16. The clutch 12 includes a housing 36 fixed for rotation with an engine output shaft 38 and a movable clutch plate 40 fixed for rotation with the transmission input shaft 16. The degree of slip between the engine output shaft 38 and the transmission input shaft 16 is regulated by the pressure of the fluid in a clutch servo chamber 42. Regulation of the clutch slip may be used during engine start up to regulate the increase in torque delivered to the transmission. Generally, after completion of start-up, the start clutch is locked up permanently and damping is achieved through one or more mechanical dampers. In this invention, however, selective slippage of the clutch is permitted during drive in order to reduce the torsional vibrations experienced at the transmission input shaft. The pressure of the fluid in the clutch servo chamber 42 is controlled by a hydraulic circuit 44 that provides fluid to the clutch servo chamber 42. The hydraulic circuit is regulated by a control system 46 which is shown in more detail in Figure 2. The hydraulic circuit 44 includes a spool valve 48 that regulates the fluid pressure in the clutch servo chamber 42, a hydraulic solenoid 50 that regulates the pressure of fluid allowed through the spool valve 48, and a electronic control unit (ECU) 52 that provides the duty cycle for control of the hydraulic solenoid 50 in response to varying engine conditions. The actual slip of the start clutch is monitored using a first transducer 56 located on the engine output shaft 38 for measuring the clutch input speed and a second transducer 57 located on the transmission input shaft 16 for measuring the clutch output speed. The torsional vibrations on the transmission input shaft are monitored by a torsho graph 58. This device provides a signal to the ECU 52 representative of the level of torsional vibration on the transmission input shaft. The torsho graph 58 could be replaced by any commercially available torsional vibration measuring device. Torsional vibration measuring devices generally operate by creating a carrier frequency that represents a constant rotation speed of a shaft or other rotating object. Any deviation from the carrier frequency is representative of the torsional vibrations of the shaft and can be measured or displayed as desired. By using such a device on the input shaft 16, the torsional vibrations on a shaft can be nearly instantaneously determined and apparatus can be used to eliminate the excess vibrations if they exceed a predetermined threshold value set within the control system 34. The ECU 52 receives a signal from the torsho graph 58 on a line 60. The signal is sampled at predetermined intervals and delivered to a block 62 where an acceptable torsion level calculation is performed. The actual torsion level is compared with an upper limit for acceptable vibrations stored within the ECU 52 at each sampling. If the actual level of torsional vibration exceeds the upper acceptable limit, slip control is requested at block 64. A signal is delivered to block 72, where the duty cycle that controls the hydraulic solenoid 50 is generated. When slip control is requested at block 64, the duty cycle generated provides for a decrease in clutch pressure that corresponds to an incremental increase in clutch slip. Such an incremental increase can be on the order of, for example, 10 rpm. The period between successive samplings is preferably greater than the time lapse between generation of the sampled signal and the corresponding modification in clutch slip. The ECU 52 also receives signals from the first and second transducers 54, 56 that provide signals representative of the clutch input and the clutch output speeds on lines 66 and 68, respectively. The actual clutch slip is calculated at block 70 and is used in feedback control of the clutch pressure. The control system could be operated without the feedback loop. It is, however, preferable to include a feedback control to guard against variations in response by the clutch mechanism. If after a single incremental increase in clutch slip, the vibrations are still above the acceptable level in the clutch output shaft, the vibrations registered by the torsho graph 58 at the next sampling will also still exceed the acceptable level. Slip control will be requested once more at block 64 and the duty cycle regulated to provide a further incremental increase in clutch slip. This series of steps is repeated and the clutch slip is incrementally increased, until the torsional vibration level sampled is below the set acceptable value. At this time, slip control is no longer requested at block 64 with a corresponding absence of further increase in clutch slip. It is important to the efficient operation of the transmission that the clutch is locked up as soon as possible after the drive condition of excessive torsional vibration is passed. This can be achieved in a number of ways including, for example, a time lapse arrangement which can periodically lock up the clutch to determine whether the vibrations are still in evidence. If the clutch is locked-up and torsional vibrations still exist at unacceptable levels, a signal will immediately be developed by block 64 initiating slip control. The clutch will again slip by increasing amounts until the vibrations are reduced to the acceptable level. After the predetermined time lapse, the clutch can again be locked up to redetermine whether or not the vibrations are at or below the set acceptable level. An alternative arrangement to ensure the clutch is locked-up whenever possible can include a second torsion vibration detector positioned on the input shaft to the clutch to determine when vibrations on the input shaft have reduced by a significant amount or perhaps to a level below that at which slip control was first requested. In order to ensure that the engine always operates at or above a threshold level of efficiency, a further limitation can be imposed on the control system. This limitation would prevent the slip across the clutch being increased beyond a threshold value. If the actual slip exceeds a predetermined value stored within the ECU, the slip control request signal from block 64 will be cancelled and no further slippage permitted. The duty cycle will then maintain the clutch slip at its maximum permitted level. The limiting value may be a difference in rate of revolution between the clutch input shaft and the clutch output shaft of around 100 rpm. The adaptive nature of the control system of this invention allows damping of torsional vibrations from either internal or external sources. Instead of requiring extensive testing of a prototype vehicle in order to derive the operating characteristics during operation, the torsional vibrations of an engine are monitored at all times and corrective action taken through clutch slippage to maintain vibrations at a desirable level. One advantage of this adaptive system is its ability to remove vibrations that originate from areas unforeseen during the collection of empirical data. Preprogrammed systems provide a limited number of parameters that are monitored and can therefore be incorporated into a control system. As a result, only vibrations that correspond to specific values of the measured parameters or interpolations therebetween can be counteracted by the control system. Any peculiarities of a particular engine that result in vibrations in addition to those experienced in the test group will therefore not be counteracted. Vibrations that increase due to the age of a transmission will also not be damped. By providing an adaptive system that controls the slippage of a start clutch to reduce torsional vibrations in response to the actual vibrations experienced on the input shaft of the belt and sheave system, torsional vibrations caused as a result of many different conditions can be damped. By introducing an increment in slip in response to vibrations, only a limited amount of stored data is necessary to determine the amount of clutch slip necessary to reduce the torsional vibrations to a desired level. Storage space is therefore reserved for data necessary to the control of other parts of the hydraulic control system. The adaptive control system of this invention thereby provides for an efficient and effective way of using a slippable start clutch as a torsional isolator.

Claims: A clutch control system for a continuously variable transmission (10) having a torque input means (16), a torque output means (18), a belt and sheave system (14) and a slippable start clutch means (12) for transmitting torque to said belt and sheave system (14), said slippable start clutch means (12) being operatively located between said torque input means (16) and said belt and sheave system (14), characterised in that said clutch control system comprises: means (62) providing a first value representative of a limiting value of torsional vibrations transmitted to said belt and sheave system (14); means (58) providing a first signal representative of actual torsional vibrations transmitted to said belt and sheave system (14), said means comprising a torsional vibration sensor (58); means (52) comparing said first signal with said first value; and, means (64) modifying slip across said slippable start clutch means (12) adaptively in incremental amounts, said modifying means (64) being responsive to said comparing means (52) to reduce torsional vibrations. A clutch control system as claimed in claim 1, wherein said torsional vibration sensor (58) is located on an output shaft (16) of said slippable start clutch means (12). A clutch control system as claimed in claim 1, wherein said torsional vibration sensor (58) providing said first signal makes discrete time interval sampling of the level of torsional vibrations on an output shaft (16) of said slippable start clutch means (12). A clutch control system as claimed in claim 3, wherein a first time period lapses between a first sampling of the level of torsional vibrations on said clutch output shaft (16) and the corresponding activation of said modifying means (64) for modification of the slip across said slippable clutch means (12). A clutch control system as claimed in claim 4, wherein a second time period lapses between said first sampling and a second sampling of the level of torsional vibrations, said first time period being of greater length than said second time period. A clutch control system as claimed in any of the preceding claims, wherein said means (64) for modifying the slip across said slippable start clutch means (12) includes means (64) for developing a clutch control signal in response to a torsional vibration level in excess of said first limiting value. A clutch control system as claimed in claim 6, wherein said means (64) for modifying the slip across said slippable start clutch means (12) further comprises means for increasing clutch slip by a predetermined incremental amount in response to said clutch control signal. A clutch control system as claimed in claim 7, wherein said predetermined incremental amount corresponds to an increase in slip across the start clutch of around 10 rpm. A clutch control system as claimed in any of the preceding claims, further comprising means (56, 57) providing a slip signal representative of the actual slip across said slippable start clutch means (12). A clutch control system as claimed in claim 9, wherein said slippable start clutch means (12) includes a clutch input shaft (38) and a clutch output shaft (16) and wherein said slip signal providing means (56, 57) comprises means (56) determining the rotational speed of said clutch input shaft (38) and means (57) for determining the rotational speed of said clutch output shaft (16). A clutch control system as claimed in any of claims 1 to 5, wherein said means (64) for modifying the slip across said slippable start clutch means (12) includes means developing a second signal in response to said comparing means (52) determination that said first signal value exceeds said first value; and means responsive to said second signal to modify the slip across said slippable start clutch means (12) to increase clutch slip by incremental amounts.

Inventors: SCHNEIDER KARL F, SCHNEIDER, KARL F.
Applicants: BORG WARNER AUTOMOTIVE, BORG-WARNER AUTOMOTIVE, INC.
Application year: 1991
Main classification: B60K41
IPC codes: F16H63, F16H59, F16H9, F16D48, F16H61, B60W10