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Title:
CONTROL APPARATUS FOR A POWER TRAIN AND METHOD FOR CONTROLLING A POWER TRAIN AS WELL AS A POWER TRAIN INCLUDING THE CONTROL APPARATUS.
Document Type and Number:
WIPO Patent Application WO/2019/129860
Kind Code:
A1
Abstract:
A control method for controlling a torque converter with lock-up clutch unit (TC/LUC) comprises providing a TC/LUC control signal (P set ) to the TC/LUC to set the TC/LUC (20) into a desired operational mode. The method comprising a calibration procedure with the following steps: - with a disabled feedback control causing a gradual disengaging of the TC/LUC from an engagement state preceding the calibration procedure; - subsequent to detecting that a monitored slip rate of the TC/LUC exceeds a predetermined threshold value, enabling feedback control to determine a representative value of a feedback control signal which in combination with a reference open loop control signal results in an operation of the TC/LUC (20) at a reduced slip rate, - disabling feedback control and modifying the reference open loop control signal with a modification value based on the representative value.

Inventors:
LAHEIJ, Thierry Matheus Hendrikus Kornelis (Poort Sint-Truiden, 3800 Sint-Truiden, 3800, BE)
RICE, Kevin Strandby (Poort Sint-Truiden, 3800 Sint-Truiden, 3800, BE)
Application Number:
EP2018/097094
Publication Date:
July 04, 2019
Filing Date:
December 28, 2018
Export Citation:
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Assignee:
PUNCH POWERTRAIN N.V. (Ondernemerslaan 5429, Poort Sint-Truiden, 3800 Sint-Truiden, 3800, BE)
International Classes:
F16D48/06
Foreign References:
US5627750A1997-05-06
EP1201483A22002-05-02
US20030150683A12003-08-14
Attorney, Agent or Firm:
JANSEN, C.M. (V.O, P.O. Box, 2508 DH Den Haag, 2508 DH, NL)
Download PDF:
Claims:
CLAIMS

1. A control apparatus (80, 90, 100) for a power train including a variator (40) and a torque converter with lock-up clutch (TC/LUC) (20) that is arranged in series with the continuously variable transmission, the control apparatus comprising a TC/LUC controller (C20) for providing a TC/LUC control signal (Pset) to control an engagement state of the lock-up clutch of the torque converter, and comprising a calibration facility for calibration of the TC/LUC controller, wherein the TC/LUC controller comprises an open loop control section (OLC) to determine a nominal control signal value (Pf) of the TC/LUC control signal (Pset) estimated to achieve a predetermined engagement state , and wherein the TC/LUC controller further comprises a closed loop control section (CLC) causing the TC/LUC controller to issue the TC/LUC control signal (Pset) with a value to minimize deviations between an actual engagement state and the predetermined engagement state in an activated state of the closed loop control section, wherein the control apparatus is at least operable in a calibration mode comprising the following calibration stages:

a first calibration stage wherein the feedback control section (CLC) is disabled and wherein the open loop control section gradually modifies the value of the TC/LUC control signal from an initial open loop control signal value, being the value of the TC/LUC control signal (Pset) immediately before entering the calibration mode until the TC/LUC control signal assumes a stop value (TstoP) for which it is detected that the TC/LUC assumes a slipping operational mode,

a second calibration stage entered subsequent to said detection, wherein the feedback control section (CLC) is enabled to adapt the value of the TC/LUC control signal (Pset) from said stop value (Pstop) to in order to determined an intermediate value (Pc,iock) at which the TC/LUC achieves the predetermined engagement state,

a third calibration stage being entered subsequent to detection of the predetermined engagement state, wherein the feedback control section is disabled, and wherein the open loop control section is calibrated in accordance with a difference between the intermediate value (Pc,iock) and the nominal control signal value (Pf).

2. The control apparatus according to claim 1, wherein the predetermined engagement state is a state of the TC/LUC with a minimal degree of engagement required to maintain a slip free operation at a current value of a torque transferred by the TC/LUC.

3. The control apparatus according to claim 1 or 2, wherein the calibration facility in its third calibration stage calibrates the open loop control section to provide a TC/LUC control signal (Pset) that maintains the TC/LUC in a stronger engagement state than the predetermined

engagement state.

4. The control apparatus according to claim 3, wherein the calibration facility in its third calibration stage has a transitional phase wherein it enables the open loop control section to cause a gradually change of the engagement state of the clutch from the predetermined engagement state to the stronger engagement state, and a stationary phase wherein it causes the open loop control section to maintain the clutch in the stronger engagement state.

5. The control apparatus according to one of the previous claims, wherein the closed loop control section comprise an integral action control component and wherein the intermediate control signal value is determined with said integral action control component.

6. A control method for controlling a torque converter with a lock-up clutch (TC/LUC) arranged in series with a variator (CVT) in a power train, the control method comprising providing a TC/LUC control signal (Pset) to the TC/LUC to control an engagement state of the TC/LUC, wherein the control signal is obtained by closed loop control, open loop control or by a combination of open loop control and closed loop control, the method comprising a calibration procedure with the following steps:

with a disabled feedback control providing an open loop based control signal to the TC/LUC that causes a gradual disengaging of the TC/LUC from an engagement state preceding the calibration procedure until it is detected that the TC/LUC assumes a slipping operational mode,;

subsequent to detecting the slipping operational mode, enabling feedback control to determine an intermediate value (Pc.iock) of the TC/LUC control signal (Pset) for which it is detected that the TC/LUC (20) assumes a predetermined engagement state,

disabling feedback control and enabling open loop control to control an engagement state of the TC/LUC (20) based on a difference between the determined intermediate value (Pc,iock) and a predetermined nominal value (Pf) expected to achieve the predetermined engagement state. 7. The control method according to claim 6, wherein the

predetermined engagement state is a minimal degree of engagement of the TC/LUC required to maintain a slip free operation at a current value of a torque transferred by the TC/LUC.

8. The control method according to claim 6 or 7, wherein the calibration procedure is initiated upon detection of an enable condition, which at least requires that the TC/LUC is in a lock-up mode.

9. The control method according to claim 8, wherein the enabling condition includes the expiry of a predetermined time interval (g) lapsed since completion of a previous calibration procedure.

10. The control method according to claim 6, wherein occurrence of slip is detected as a sliprate (ns) being a function of a rotational speed (ne) at an input and a rotational speed ( ) at an output of the TC/LUC.

11. The control method according to claim 10, wherein the sliprate (ns) is the value obtained for the rotational speed (ne) at the input of the

TC/LUC minus the rotational speed ( ) at the output of the TC/LUC.

12. The control method according to claim 10, wherein the sliprate (ns) is the value obtained for the difference between the rotational speed (ne) at the input of the TC/LUC divided by the rotational speed ( ) at the output of the TC/LUC.

13. The control method according to one of the claims 6-12, wherein an integral action control component is used to determine the closed loop control signal and wherein the intermediate control signal value is determined with said integral action control component.

14. A power train in a continuously variable transmission system including a torque converter / lock-up clutch (TC/LUC), a drive-neutral- reverse clutch (DNR) and a variator, comprising a control apparatus according to either one of claims 1 to 5.

Description:
Control apparatus for a power train and method for controlling a power train as well as a power train including the control

apparatus.

BACKGROUND

The present invention pertains to a control apparatus for a power train.

The present invention further pertains to a method for controlling a power train.

The present invention still further pertains to a power train including the control apparatus.

A power train in a continuously variable transmission typically includes a torque converter / lock-up clutch (TC/LUC), a drive-neutral- reverse clutch (DNR) and a variator. The variator is typically provided as a transmission belt that mechanically couples two pulleys. In normal driving stages all elements of the power train preferably operate in a on-slipping mode as a slipping operation would entail energy losses and therewith negatively affect fuel economy. A slipping mode of the variator should particularly be avoided as this results in wear of the transmission belt and/or the pulleys. This can be achieved by high clamping levels. On the other hand clamping levels, in particular a clamping level of the

transmission belt should not be set too high because this would imply an unnecessarily high drive current is supplied by the charging system to maintain this high clamping level, which also is unfavorable for fuel economy. Additionally increasing the clamping level above a level necessary for slip free operation of the variator tends to increase transmission losses of the variator and an increased wear of the variator due to an increased friction. Furthermore, it should be taken into account that the driving conditions may change suddenly, for example by damages of the road or by a rapid braking of the vehicle. To avoid a slipping of the variator in such situations a LUC torque capacity should be set to a value that is lower than a torque capacity of the variator. Therewith it is achieved that in case of an unexpectedly high torque to be transferred, the TC/LUC serves as a fuse that absorbs the unexpected torque by slipping, therewith avoiding a slipping of the variator. The LUC, usually designed as a wet clutch, is capable to operate in a continuous slipping mode without damage.

US2003150683 discloses a control method for a power train including a continuously variable transmission and a clutch arranged in series therewith. The control method includes a procedure in which an engaging pressure of the clutch is first reduced until a slip occurs, and is then increased after detection of the slip so as to re-engage the clutch, and an engaging pressure of the clutch to be established is calculated by giving an excess pressure to the engaging pressure at which the clutch is re-engaged, such that an excess amount of the transmitted torque of the clutch is set smaller than that of the continuously variable transmission. The control method involves variations in the settings of the clutch which may be sensed by the driver or the passengers in the car and therewith reduce driving comfort.

SUMMARY

It is a first object to provide a control apparatus for a power train wherein the risk of discomfort by variating clutch settings is reduced.

It is a second object to provide a control method that is arranged to control a power train with a reduced risk of discomfort by variating clutch settings.

It is a third object to provide a power train including the improved control apparatus.

In accordance with said first object a control apparatus is provided as claimed in claim 1. The control apparatus as claimed comprises a TC/LUC controller for providing a TC/LUC control signal to control an engagement state of the lock-up clutch of the torque converter. TC/LUC controller comprises an open loop control section and a closed loop control section. The TC/LUC control signal may for example be provided to a driver that generates electric drive signals for a hydraulic control unit provided to actuate the TC/LUC.

Alternatively the TC/LUC may be actuated without an intervening

hydraulic control unit, for example by one or electromagnetic actuators directly coupled to a TC/LUC element. The control apparatus comprises a calibration facility to calibrate the TC/LUC controller. Therewith it can be achieved that the operation of the TC/LUC controller is adapted to changes in behavior over time due for example to wear and temperature variations. The open loop control section is configured to determine a nominal control signal value, which is an originally estimated value of the control signal that is expected to achieve a predetermined engagement state of the

TC/LUC.

The TC/LUC controller further comprises a closed loop control section. This control section is arranged to adapt the TC/LUC control signal to a value that minimizes a deviation between an actual engagement state and the predetermined engagement state when the closed loop control section is enabled. Such deviation may for example be detectable as a slip rate, e.g. a ratio between an input rotational speed and an output rotational speed, or a difference between an input rotational speed and an output rotational speed.

The control apparatus is at least operable in a calibration mode comprising a first, a second and a third calibration stage.

In the first calibration stage the feedback control section is disabled and the open loop control section gradually modifies the value of the

TC/LUC control signal from an initial control signal value to a stop control signal value. The initial TC/LUC control signal value is the value of the TC/LUC control signal immediately before entering the calibration mode. Hence, if before entering the calibration mode the feedback control section is disabled, the initial TC/LUC control signal value is equal to the immediately preceding open loop control signal value. The latter may comprise a predetermined component which does not change in time and a calibration component determined in the calibration mode. Alternatively the open loop control signal value may be provided as a single calibratable. The stop control signal value is the value of the TC/LUC control signal where it is detected that the TC/LUC assumes a slipping operational mode. e.g. the value for which a slipping of the TC/LUC becomes detectable, a value for which it is detected that the output rotational speed starts to differ from the input rotational speed. Alternatively, the predetermined slip rate may be a slip rate define by a predetermined ratio between the output rotational speed and the input rotational speed.

Subsequent to detecting the slipping operational mode, a second calibration stage is entered. Therein the feedback control section is enabled to adapt the value of the TC/LUC control signal from the stop control signal value to an intermediate control signal value for which it is detected that the TC/LUC achieves the predetermined engagement state. This

predetermined engagement state typically is a state of operation of the TC/LUC at its slip limit, i.e. with a minimal degree of engagement required to maintain a slip free operation at a current value of a torque transferred by the TC/LUC. Alternatively the predetermined engagement state may be another engagement state which is taken as a reference, e.g. a state wherein the TC/LUC has a particular slip rate at a current value of a transferred torque. Preferably however, the predetermined engagement state to be achieved with the feedback control section is the state of operation of the TC/LUC at its slip limit, as this makes it possible to minimize slipping of the TC/LUC during the calibration procedure to the maximum extent possible. Then a third calibration stage is entered wherein the feedback control section is disabled again and wherein the calibration value is set to a value based on a difference between the intermediate control signal value and the nominal control signal value.

In the control apparatus as claimed the activation of the feedback control section in the second calibration stage achieves a smooth but rapid transition between the first and the third calibration stage. This contributes to driving comfort.

This advantage is likewise achieved with the method of claim 6.

Furthermore an improved power train as claimed in claim 14 is provided. The improved power train comprising a continuously variable transmission system including a torque converter / lock-up clutch (TC/LUC), a drive-neutral-reverse clutch (DNR) and a variator, further comprises the claimed control apparatus.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects are described in more detail with reference to the drawings. Therein:

FIG. 1 schematically shows a power train in a vehicle;

FIG. 2 shows in more detail a part of a controller for the power train;

FIG. 3A-C illustrates various signals and state indicators during operation;

FIG. 4A-4C illustrate the controller in various operational states;

FIG. 5 illustrates a control method;

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 schematically shows a power train in a vehicle to transfer power from a power source 10, such as a combustion engine or an electric motor, to wheels 70 of the vehicle. The power train as shown in FIG. 1 comprises a torque converter/lock-up clutch (TC/LUC) 20, a drive- neutral- reverse clutch (DNR), a variator 40, a fixed gear 50, and a differential 60. The TC/LUC 20 couples an output axis of the power source 10 to the DNR 30, with a controllable sliprate and torque ratio correlated therewith, i.e. the ratio between the transmitted torque at its output and the torque received at its input from the power source 10. The DNR clutch 30 is provided to couple the TC/LUC 20 to the variator 40. The DNR clutch 30 is controllable to assume one of a drive mode D corresponding to driving the vehicle in a forward direction, a reverse mode R, wherein the vehicle is driven backward and a neutral mode wherein it keeps the variator 40 decoupled from the TC/LUC 20. The variator 40 transmits the power delivered from the power source 10, through the TC/LUC and the DNR clutch 30 via the fixed gear 50 and the differential 60 to the wheels 70, at a gear rate that can be selected from a continuous range.

In the embodiment shown a setting or operational mode of the torque TC/LUC 20, the DNR 30, and the variator 40 is determined by hydraulic signals, i.e a pressure of an hydraulic fluid. The hydraulic signals are supplied by a hydraulic control unit (HCU) 80, which is supplied with a supply flow Peo by a pump 85. In the embodiment shown the state of the TC/LUC 20 is controlled by hydraulic pressure P20, the state of the DNR clutch 30 is controlled by hydraulic pressure P32 and the state of the variator is set by hydraulic pressures P41 and P42. To that end the hydraulic control unit 80 on its turn is controlled by transmission control unit (TCU) 100. Alternatively the state of the various power train elements may be controlled by electric signals, for example using electro-magnetic actuation elements. The TCU 100 is further coupled, e.g. via a bus, here a CAN-bus 95, to an engine control unit 90. The TCU is further configured to receive input signals from various inputs, such as a turbine speed signal (the output rotational speed of the TC/LUC), a primary pulley rotational speed, corresponding to the DNR output speed, a secondary pulley rotational speed at the output of the variator 40, a secondary pulley pressure and an oil reservoir temperature. Other input signals, for example from a throttle pedal, a brake pedal (not shown) and sensor elements, e.g. speed sensors, temperature sensors, torque sensors and the like (not shown) may be received and monitored by the engine control unit 90 and passed on to the TCU 100 via the CAN bus 95.

FIG. 2 shows in more detail a TC/LUC controller C20 for controlling a state of the TC/LUC 20. In this drawing, component 115 indicates the elements that translate a control signal P set into a pressure P20 to be provided to the TC/LUC 20 to achieve a desired setting thereof. The TC/LUC controller C20 comprises an open loop control section OLC and closed loop control or feedback control section CLC. The operation thereof is controlled by a main control unit 110.

Dependent on a general operational mode the main controller 110 may configure the TC/LUC controller C20 in various ways. The general operational mode may be determined by the operational state of the vehicle, e.g. start-up, standstill, accelerating from standstill, stationary driving, braking, and may further be determined by a power setting e.g. selected in a range from a low power mode, to a high power mode.

In the embodiment shown, the main controller 110 determines operation of the TC/LUC controller C20. The main controller 110 typically also controls other parts of the transmission system, such as the DNR 30 and the variator 40, as schematically indicated by the block arrows S10, S30, S40 representing issued control signals and received state signals. The main controller 110 on its turn interacts with the driver 5, for example receiving driver input signals, such as a throttle pedal pressure, and a brake signal pressure, a transmission mode selection R/N/D and the like. Also the main controller may issue status signals to the driver via a user interface. The main controller 110 may also request certain functionality from the ECU 90. In exceptional cases, e.g. upon detection of an error such as a defect in the transmission the main controller may become dominant and for example force the vehicle to drive only at low speed (to get to the side of the road, etc.)

An embodiment of the TC/LUC controller C20 is now described in more detail. As mentioned above it comprises an open loop control section OLC and closed loop control or feedback control section CLC. In the embodiment shown the open loop control section has a nominal control signal generator 120 to generate the nominal control signal Pf indicating a nominal control value estimated to achieve a predetermined engagement state. The predetermined engagement state is a state of the TC/LUC wherein it should be capable to transmit a reference torque M ref specified by main controller 110 at a slip rate n ref , also specified by main controller 110. Typically the predetermined engagement state is the state wherein the TC/LUC operates at its slip limit.In reality the actual behavior of the TC/LUC will be different from these reference characteristics, due to wear of the TC/LUC and due to temperature variations. A calibration signal generator is provided that is to generate a calibration signal P cai indicating a calibration value. The open loop controller OLC is configured to provide an open loop control signal Pole having a value based on the nominal control value and on the calibration value to control the TC/LUC in open loop mode. In the embodiment shown this is achieved by summation of these values in adder 122. The TC/LUC controller further comprises a closed loop control section CLC that is arranged to provide a correction signal P c which indicates a correction value to change the value indicated by the open loop control signal Pole so as to correct for deviations between actual engagement state and the predetermined engagement state in an activated state of the closed loop control section.

In the embodiment shown, the closed loop control section CLC includes a comparator 111 to issue an error signal e indicative for a difference between a slip-rate n s ,ref as specified by the main controller 110 and a measured slip-rate n s , as determined by a slip rate monitor 116. The slip speed monitor may for example determine the slip rate n s as the difference n e -nt, i.e. the difference between the rotational speed n e at the input of the TC/LUC and the rotational speed at the output of the LUC. An adaptation controller, such as a PI controller 112 is provided that issues the correction signal Pc which can be added to the open loop control signal P ole to obtain the control signal P set . The control signal P set may be converted to a control current that on its turn controls the hydraulic control unit HCU 80, which in response thereto generates the requited hydraulic pressure signal P20 to the TC/LUC 20. In FIG. 2, the combined functionality of converting to a control current and generating the pressure signal P20 is schematically represented by module 115.

The main controller 110 is configured to selectively enable the feedback control loop as is schematically indicated by the switching element 113 controlled by switching signal S113. FIG. 2 schematically illustrates various computational steps as a separate element. For example adders 122, 123 and 114 are shown to combine various control signals. It is however not necessary that the controller is actually implemented in this manner. For example various functionalities in the controller may be performed in various ways.

It is important that open loop control section OLC is capable to keep the TC/LUC properly engaged in a locked mode. That is to say that during normal driving conditions slipping of the TC/LUC should be avoided, whereas the TC/LUC is capable of serving as a fuse in the transmission system so as to avoid that slippage occurs in the variator if an uexpectedly high torque occurs. The behavior of the TC/LUC however varies over time due to wear and temperature variations.

To maintain a reliable open-loop control the control apparatus is operable in a calibration mode which is described in more detail with reference to FIG. 3. FIG. 3A - 3C illustrates various signals and state indicators during operation of the control apparatus. In FIG. 3A the solid line M 21 shows an actual torque capacity Mi uc (Torque [Nm]) that is transferrable by the TC/LUC 20 without slip and the longer dashed line M 22 shows a nominal value M ref of the torque that the LUCU should be capable of to transmit without slip in the current circumstances, taking into account a safety margin value. The shorter dashed line M 23 , slightly above the longer dashed line indicates a value corresponding to a desired operation around the slip limit.

As becomes apparent from the example in FIG. 3, in particular as shown in FIG. 3A, at point in time to, the actual torque capacity Mi uc indicated by curve M 21 is substantially higher than what is desired given the current circumstances. In order to avoid the risk of variator slipping, this implies that also the variator transmission belt must be kept at an unnecessary high tension. If this situation pertains an unnecessarily high drive current is supplied by the charging system to maintain this high clamping level which would be at the cost of an additional fuel consumption. Also the variator may suffer from increased wear and dissipate more power than strictly necessary.

In FIG. 3B the curve P 21 shows a value of a TC/LUC control signal P set as a function of time. In this case the value is represented as a pressure value (Pressure [bar]) applied to a hydraulically controlled TC/LUC. In this example, the degree of engagement generally increases with an increasing value of the control signal. In other embodiments, the degree of engagement may generally decrease with an increasing value of the control signal.

Alternatively the value of the TC/LUC control signal may be expressed as a voltage or a current, for example the value of the control voltage or control current used to control a pressure value of a hydraulic pressure to control a hydraulically controlled TC/LUC or a control voltage or control current used to control an electromagnetically operated TC/LUC. The dashed line P22 extending over the full width in FIG. 3B is a value of the TC/LUC control signal that is originally expected to achieve an engagement state of the TC/LUC wherein the latter is capable, while operating at its slip limit to transmit the nominal value (M ref ) of the transmitted torque. The shorter dashed line P23, slightly above the longer dashed line, indicates the value of the TC/LUC control signal P set required to actually achieve that the TC/LUC can transfer that torque while operating at its slip limit.

FIG. 3C shows indicators for a state of the TC/LUC. The straight line S24 (SW state) indicates the state of the TC/LUC 20 at a higher level of control. In particular it indicates that in the time interval represented by this graph the TC/LUC is desired to be in a locked mode at the higher level of control. The piecewise linear curve S25 schematically shows the actual, physical state of the TC/LUC 20.

An exemplary control method for controlling a TC/LUC is now described with reference to FIG. 3A-3C introduced above. As can be seen in FIGs. 3A-3C during a first time interval extending from to to ti, a control signal P set is applied, which is the sum of a first feedforward component P f expected to be required to achieve a nominal clamping required to transfer a torque M ref while operating at its slip limit, and a calibration component P cai which is provided as a second feedforward component to allow the TC/LUC to transfer a torque up to a predetermined threshold level Mi uc without slip. The lock-up clutch controller in this operational state is shown in FIG. 4A.

As indicated above, in the circumstances depicted in this example the torque capacity Mi uc is set too high. Starting from this operational state, the TC/LUC being locked, it is possible to initiate a calibration procedure. Other requirements may be checked to determine whether or not a calibration procedure is initiated, such as whether or not a predetermined period of time lapsed since a previous execution of the calibration procedure. If a lapse of a predetermined period of time is one of the conditions an initiation of the calibration procedure may take place before the predetermined laps in time if a malfunction, such as a high fuel consumption or unexpected occurrences of discontinuities in the torque transmission or slippage rate are detected.

At point in time ti, and as shown in FIG. 4B, the calibration

procedure is initiated. In this first stage of the calibration procedure a modification component P mod is added to the nominal control value. In FIG.

2 this is schematically shown as the contribution P mod provided by

rampsignal generator 124 in the open loop control section OLC. Starting from the original value, equal to the value of the calibration signal P cai , the value of the modification component P mod is gradually changed, as indicated by inclination“a” in FIG. 3, so as to cause a gradual disengaging of the TC/LUC. It is presumed here that the extent of engagement of the TC/LUC is positively correlated with the signal P set . Alternatively the correlation may be negative, in which case a positive inclination is to be applied for modification component. At point in time t 2 , the control signal value P set is decreased to a value where the TC/LUC operates at its slip limit. At a further point in time t3, with t3-t 2 = f, the control signal has achieved a stop value P stop for which slip actually is detected.

Upon this detection at point in time t3, a second calibration stage is entered wherein the feedback control mode is re-enabled as shown in FIG. 4C. In FIG. 2, this would imply that the main controller 110 causes a closure of the switching element 113 with control signal S 113 . The feedback control section CLC gradually adapts the value of the control signal P set from the value P f +P mod (t3) to the value necessary to set the TC/LUC in its

predetermined engagement state, typically an engagement state wherein the TC/LUC has the minimal degree of engagement required to maintain a slip free operation at a current value of a torque transferred by the TC/LUC. Then the control signal P set stabilizes when the feedback control signal P c has achieved an intermediate value P c,iock . The feedback component may be considered sufficiently stabilized if variations therein are less than a predetermined threshold value, e.g. based on an estimated noise level for example a slip rate corresponding to zero slip or to a predetermined minimum amount of slip. The intermediate value may for example be an average or a median value of the control signal P se t during a time interval wherein the feedback component is stabilized. Alternatively, the feedback component may be deemed sufficiently stabilized upon expiry of a

predetermined time interval. This predetermined time interval may be related to a time-constant of the feedback loop, for example a time interval having a duration of 2 or 3 times that time constant. Although the feedback component may still show variations exceeding the noise level the

intermediate value may be calculated by extrapolation of the feedback control signal P c based on the value at t3 and the value of the feedback component at a predetermined time interval after t 4 . As can be seen in the time interval t3-t 4 , the feedback controller rapidly restores the TC/LUC 20 towards a slip-free operational mode, while providing for a smooth

transition from low slip to the slip free operation near the end of the second stage.

Then a third calibration stage is entered wherein the feedback control is disabled again, as shown in FIG. 4A, and wherein an updated calibration value for the calibration signal P cai is set. The updated calibration value is based on the difference between the intermediate value P c ,iock and the nominal control value Pf. In one embodiment the updated calibration value may be equal to this difference. In another embodiment the updated calibration value is set to the sum of this difference and an additional value “b” as illustrated in FIG. 3B. The difference P c ,iock - Pf may be calculated from the difference of the value of the control signal Pset at point in time t 4 or may be the correction signal P c at point in time t 4 that is needed to modify the signal Pf to achieved the predetermined engagement state. As noted above, in the embodiment shown, the feedback control section includes a PI controller 112, i.e. the feedback control section comprise an integral action control component. In an embodiment the output of the integral action control component may be used to determine the intermediate signal P c ,iock. This is advantageous in that this signal is already free from noise due to the integrating action of this component.

This aspect is schematically illustrated by update element 125, which records the intermediate value of the feedback component Pc, with which the predetermined engagement state was achieved. Based on this signal the update element 125 updates the calibration signal Peal to be provided by element 121.

As can best be seen in FIG. 3B, at point in time ts, in stage 3 of the calibration procedure the updated calibration value is based on the intermediate value in that a further component denoted as“b” is added as part of the feedforward signal. In this way a safety margin is provided for. Therewith it is achieved that normal torque variations, due to minor road surface level variations do not immediately cause the TC/LUC to slip. As further shown in the middle graph, in a first phase of the third calibration stage from point in time t 4 to t 4a , the further component b is gradually increased from 0 to its final value, according to a ramp denoted as c. At point in time t 4a a stationary phase is achieved wherein the calibration signal is maintained at the excess modification value. Therewith

discontinuities in the transmission behavior of the TC/LUC are avoided.

At point in time ts a new calibration cycle is initiated, wherein stages of the calibration procedure at points in time ts to t g respectively correspond to the stages ti to t 4 .

It is noted that the calibration procedure is performed basically in a locked mode of the TC/LUC. Although the TC/LUC is temporarily caused to slip, this is in a controlled manner. Accordingly, during the calibration procedure, nominal control signal generator 120 generates the nominal control signal Pf with the presumption that the specified torque M re f is transferred with zero slip, even though a minimal amount of slip occurs during the calibration procedure. In other operational states the nominal control signal generator 120 may calculate the nominal control signal P f by also taking into account a specified slip rate n s,ref other than 0.

It is noted that the graphs are not drawn to scale. For example the time interval f may be substantially smaller than the time interval suggested by the drawing. For example, the length f of the time interval for detection of slip, may be in the order of magnitude of some tens of ms, e.g.

20 ms as it is mainly determined by valve hysteresis and slip detection debounce time. The timer for initiating a new calibration cycle may for example be set at a value g in the order of a few seconds to tens of seconds. The timer is provided to leave a time frame wherein the TC/LUC is enabled to accommodate a new operating point as specified by external drive control signals.

FIG. 5 schematically shows a calibration procedure in a control method for controlling a TC/LUC 20 arranged in series with a variator 40 in a continuously variable transmission system. The control method provides a TC/LUC control signal P set to set the TC/LUC 20 into a desired operational mode. The control signal is obtained by closed loop control, open loop control or by a combination of open loop control and closed loop control.

In step SO it is determined whether the calibration procedure should be initiated. For example it can be determined whether the TC/LUC is currently operating in a locked mode. If this is not the case an intermediate step may be performed wherein the TC/LUC is controlled to enter a locked operational mode. It may further be determined whether the transmission system is operating at a relatively low clamping level, such as a level denoted as MIN or ECO. If the transmission system is operating at a relatively high clamping level an intermediate step may be performed wherein the clamping level is reduced to a relatively low clamping level. It may also be verified if a predetermined time interval has lapsed since completion of a previous calibration procedure. In some cases this

requirement may be absent, or be overruled in case a system malfunctioning is detected.

In case it is decided in step SO to initiate a calibration procedure, a first calibration stage is entered in step SlA wherein an open loop based control signal is provided to the TC/LUC that causes a gradual disengaging of the TC/LUC from an engagement state preceding the calibration

procedure, while monitoring a slip rate (n s ) during said gradual disengaging. This process of gradually disengaging continues until it is detected in step SIB that slip occurs, i.e. a value of n s deviating from 0. In practice this may imply a slip rate exceeding the detection accuracy, i.e. the accuracy with which the slip rate is measured.

In step S2A the second calibration stage is entered wherein feedback control is enabled to determine a representative value of a feedback control signal which in combination with the open loop control signal results in an operation of the TC/LUC at a reduced slip rate. This is typically an

engagement state wherein the TC/LUC operates slip free. If it is determined in step S2B that this state is achieved (Selection No), control flow proceeds with the third calibration stage.

In the third calibration stage in step S3 feedback control is disabled. Instead the feed forward control signal is modified with a modification value based on the representative value determined in the second calibration stage. The modification value may gradually increase from an initial modification value equal to the representative value to an excess

modification value, which is slightly higher than the initial modification value, so as to avoid that the TC/LUC slips as a result of normal torque value variations.

In step S4 a timer is activated to postpone a subsequent calibration procedure until after a predetermined laps of time. Optionally a timer may be absent and the calibration procedure may be initiated upon detection of a start condition, indicative for unproper calibration of the transmission system. Also an operation of the timer may be overruled upon detection of such condition.

It is noted that the calibration procedure as described herewith is particularly suitable to be applied in a low power operational mode, wherein the TC/LUC operates relatively close to its slip limit.

In low power operational modes low clamping levels are set for the TC/LUC and the variator. This also implies that the torque capacity of the variator should not be set significantly higher than that of the TC/LUC, but just sufficiently higher to enable the TC/LUC to function as a torque fuse. In such operational modes minor deviations in the actual state of these transmission components, for example due to production variations, wear and temperature dependencies may easily become dominant so that the highest torque capacity is inadvertently with the TC/LUC and not with the variator. Accordingly, the calibration procedure is particularly relevant for low power operational modes. If the transmission system is operative in a higher power mode, a transitional stage may be provided, wherein the extent of engagement of the TC/LUC is gradually decreased from the relatively high extent in the higher power mode to the relatively modest extent in the low power mode. This is schematically illustrated in FIG. 6. In higher power operational modes a larger margin may be applied between the torque capacity of the variator and the torque capacity of the TC/LUC, so that the above mentioned uncertainties in the actual state can in practice be ignored. For these modes op operation a manual calibration is sufficient.

It is noted that exemplary embodiments may be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. Whereas by way of example specific functions may be performed by respective dedicated functional elements, it is alternatively possible to perform various functions by a same element at different points in time. Example embodiments may be implemented using a computer program product, e.g., a computer program tangibly embodied in an information carrier, e.g., in a machine-readable medium for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple computers. In an example embodiment, the machine-readable medium may be a non-transitory machine- or computer-readable storage medium.