A transmission of this type is known, for example, from European Patent EP-A-0 232 979 and the later German Patent Application DE-A 19 631 216 and comprises a continuously variable transmission of the known drive belt-and-pulley type (known as the drive belt CVT) and a fixed transmission with a fixed transmission ratio which can be shifted in parallel in a mechanical sense in order to transmit power between the input shaft and the output shaft of the transmission, the drive belt CVT, the fixed transmission and one of the said shafts of the transmission each being separately coupled to one shaft of a three-shaft epicyclic gearwheel set.

This transmission has the particular and advantageous property that the rotational speed and the direction of rotation of the output shaft with an input shaft rotating within a defined range can be controlled by varying the transmission ratio of the drive belt CVT.

At a certain transmission ratio of the drive belt CVT, known as the geared neutral ratio, the rotational speed of the output shaft itself is equal to zero. This makes it possible, inter alia, to avoid the use of a moving- off clutch for stopping the wheels or accelerating the wheels from a standstill when the engine is running.

In a known control method for the infinitely variable transmission, the transmission ratio of the drive belt CVT is controlled as a function of the difference

between a desired rotation, i. e. a desired rotational speed and associated direction of rotation, and an actual rotation of the output shaft of the transmission by controlling this difference towards zero. Although this control method in theory functions appropriately, it has the significant inherent drawback that for a desired rotation of the output shaft which is equal to zero, the actual rotation of this shaft will always vary slightly around the zero mark. It will be clear that, especially if the transmission is used in a motor vehicle for passenger transport, this constant rotation of the output shaft and therefore also the driven wheels forwards and backwards is extremely undesirable.

In a known improvement to the control method for the infinitely variable transmission, in which rotational oscillations of this type do not occur or at least occur to a significantly reduced extent, the transmission ratio of the drive belt CVT is controlled as a function of the difference between a desired torque on the output shaft of the transmission, i. e. a desired absolute level and a desired direction of rotation for this torque, and an actual torque by controlling this difference towards zero. However, this improved control method has the drawback that there are no available sensors which can detect the actual torque on the said shaft with such an accuracy and robustness, or at least not without incurring relatively high costs.

It is an object of the present invention to provide an alternative control method for the infinitely variable transmission, which makes it possible to avoid the abovementioned shortcomings of the known control methods. According to the invention, this object is achieved by the control method according to Claim 1.

In the control method according to the invention, the torque of the output shaft on the transmission is

controlled by adjusting the slip across a frictional element in the transmission, such as a clutch or a drive belt CVT, having an input component and an output component, the slip being defined as the difference between the input speed of the frictional element and an output speed of the frictional element of the frictional element, to a desired value and by controlling the normal force in the frictional contact.

In this context, the invention exploits the insight that there is a known relationship between the abovementioned slip, a normal force which is applied in frictional contact between the output component and the input component, and a torque on the output component, and this relationship is in turn related to the torque on the output shaft of the transmission.

To elucidate the invention, the control method will be explained in more detail on the basis of an exemplary embodiment and with reference to the figures, in which: Figure 1 diagrammatically depicts the known infinitely variable transmission; Figure 2 shows a simplified illustration of a typical traction curve of a drive belt CVT having a drive belt in accordance with the VDT design; Figure 3 shows a block diagram illustrating the control method according to the invention; Figure 4 shows a simplified illustration of a typical traction curve of a standard wet disc clutch.

The infinitely variable transmission 10 shown in Figure 1 comprises an input shaft 2, which is coupled to an engine 1, and an output shaft 3, which is coupled to driven wheels 4, between which a drive belt CVT 11 with a variable transmission ratio, which is provided with an input pulley 111 and an output component 112,

in this case an output pulley 112 surrounded by an input component 113, in this case a drive belt 113, and a fixed transmission 12 with a fixed transmission ratio, which is provided with an input gearwheel 121 and an output gearwheel 122 surrounded by a chain 123, are accommodated, the transmissions 11 and 12 being connected in parallel in a mechanical sense and the drive belt CVT 11, the fixed transmission 12 and one of the said shafts 3 of the transmission 10 each being separately coupled to one shaft 131,132, 133 of a three-shaft epicyclic gearwheel set 13. A gearwheel set 13 of this type is generally known and comprises, for example, a sun gear 134, which is positioned centrally and is coupled to a first shaft 131, and a hollow ring gear 135, which is coupled to a second shaft, between which there is a number of planetary gears 137 which can rotatable freely about their own axis and are secured to planet carrier 136 which is coupled to a third shaft 133. In this exemplary embodiment, an output pulley 112 of the drive belt CVT 11 is coupled to the first shaft 131 of the gearwheel set 13, the output gearwheel 132 of the fixed transmission 12 is coupled to the third shaft 133 of the gearwheel set 13, and the output shaft 3 of the transmission 10 is connected to the second shaft 132 of the gearwheel set 13.

The transmission 10 also comprises two clutches 14 and 15, of which, at least in an infinitely variable transmission mode, the first clutch 14 transmits at least some of the engine power and, of which, the second clutch 15 is open. Moreover, Figure 1 shows that a final reduction gearwheel set 5 is accommodated between the output shaft 3 of the transmission 10 and the driven wheels 4.

The drive belt CVT 11 is of a type which is generally known per se. In this type of transmissions, which are based on friction, each pulley 111, 112 requires a

clamping force, by which they clamp in the drive belt 113 in order to transmit a rotation between the pulleys 111,112. For each pulley 111,112, this clamping force is determined as a function of frictional force required between the pulley 111,112 and the drive belt 113 and by realizing a desired transmission ratio icvt between the rotational speed of the input pulley Mm and that of the output pulley cocvT-ouT. It should be noted that the clamping forces are often generated with the aid of a hydraulic pressure.

As has been stated, at a specific transmission ratio of the drive belt CVT known as the geared neutral ratio iCvT-GN, the rotational speed of the output shaft (Dour is equal to zero, irrespective of the rotational speed of the input shaft OIN. A very accurate and stable control of drive belt CVT 11 at least in the geared neutral ratio iCvT-GN is extremely desirable, but is also difficult to achieve with the aid of the known control methods. However, according to the invention it is possible to indirectly control the torque Tour on the output shaft 3 of the transmission 10 by controlling the output torque TcvToT which is actually output by the drive belt CVT 11 to the epicyclic gearwheel set 13 towards a desired value Tcvr-Dv for this output torque.

In this case, the relationship between the said output torque TcvT-ouT and the torque TouT on the output shaft 3 of the transmission 10 itself is substantially advantageously linear, and the output torque Tcvr-our of the drive belt CVT 11 can in principle be calculated by dividing the torque Tour on the output shaft 3 of the transmission 10 by the ratio of the geometric radius of the hollow ring gear 135 and that of the sun gear 134.

In the control method according to the invention, a first clamping force Fcl as a function of the desired value for the output torque TcvT-DV of the drive belt CVT 11, i. e. in such a manner that the friction between pulley 111,112 and drive belt is sufficient to

transmit the desired value for the output torque Tcvr-Dv- To do this, it is possible, for example, to make use of the following equation: Fc = {TcvT-DVcos (#) }/{2#ROUT#µ (s) } (1) where k is an angle between the contact surfaces of the drive belt 113 and a pulley 111,112, Row is a radius with which the drive belt 113 runs over the output pulley 112 and R (ds) is a coefficient of friction between the drive belt 113 and the output pulley 112, which is defined as a function of the difference in speed between them, known as the drive belt slip. The said coefficient of friction R (ds), for a fixed value of the clamping force Fc and the running radius Row, can be defined in what is known as a traction curve, in which the output torque Tcvr-Dv is plotted against the drive belt slip ds, which drive belt slip ds can be defined as follows: ds = f (icvT/icvT-O)-l) (2) where icvr is the actual instantaneous transmission ratio of the drive belt CVT 11 and iCvT=o is the transmission ratio of the drive belt CVT 11 if the output torque Tcvr-Dv of the drive belt CVT 11 were to be equal to zero.

Figure 2 diagrammatically reproduces a traction curve for a metal drive belt 113 in accordance with the Van Doorne's Transmissie (VDT) design in oil-lubricated contact with metal pulleys 111,112. A drive belt of the VDT design is described in EP-A-0 014 013 and comprises at least one continuous carrier in ribbon form, over which a large number of so-called transverse elements in plate form can move, the carrier being accommodated in a recess in the transverse elements. It has been found that at least this type of drive belt 113 can be subjected to a certain degree of long-term

drive belt slip ds without suffering fatal damage, i. e. without adversely affecting the functioning of the transmission 10. In Figure 2, the line MA represents the limit beneath which the drive belt slip ds is still acceptable according to this criterion. The line MI represents the limit beneath which the drive belt slip ds has a considerable influence on the transmitted output torque Tcvr-Dv.

Allowing the drive belt 113 to slip in a controlled manner up to a defined level of the drive belt slip ds between the two lines MI and MA in the traction curve presented in Figure 2 therefore allows the output torque Tcvr-Dv of the drive belt CVT 11 to be controlled relatively accurately irrespective of minor variations in the drive belt slip Ads. In the control method according to the invention, this factor is exploited by determining a second clamping force Fc2 as a function of the difference between a desired value for the drive belt slip dSDv and the actual value of the drive belt derv, in such a manner that this second clamping force Fc2 is proportional to the said difference.

It should be noted that it is not readily possible to measure the actual value of the drive belt slip derv. To do this, the transmission 10 has to be provided with additional measurement means, for example means which measure the longitudinal speed of the drive belt 113 and compare it with the rotational speed (DiM, cOcvT-our of a pulley 111,112 or which measure the actual running radius of the drive belt 113 on a pulley 111,112 and compare it with a value for this radius which has been calculated from the pulley rotational speeds OIN, OCVT- our In the control method according to the invention, a pulley 111, 112 then exerts the first clamping force Fcl on the drive belt 113, and another pulley 111,112 exerts the sum of the two abovementioned clamping

forces Fcl and Fc2 on the drive belt 113. As a result of the control method selected, the drive belt slip ds is therefore in fact controlled by using the second clamping force Fc2 to adjust the transmission ratio of the drive belt CVT icvrf while the first clamping force Fcl ensures a sufficiently high frictional force between the pulleys 111,112 and the drive belt 113 to transmit a desired torque through the transmission. The transmission ratio of the drive belt CVT icvr has to be increased or reduced depending on the sign of the said difference between the desired value for the drive belt slip dSDv and the actual value dsRv. This means that the sum of the said clamping forces Fcl and Fc2 relates to the input pulley 111 if the transmission ratio icvry which in this context is defined as McvT-our/OiN has to increase, and on the output pulley 112 in the opposite scenario, while the first clamping force Fcl in each case relates to the respective other pulley 111,112.

The control method according to the invention is summarized once again in the block diagram presented in Figure 3. In block I, the output torque TCvT-DV for the drive belt CVT is calculated from the desired torque TouT for the output shaft 2. Then, in block II, the first clamping force Fcl required for this purpose is calculated, for example in accordance with equation 1.

In block III, the sign of the desired drive belt slip dsDv is defined as a function of the sign of the desired output torque Tcvr-DVf and then in block IV the difference Ads between the desired value and sign dsov for the drive belt slip dsrv and the measured value for this slip is determined. In block V, the required second clamping force Fc2 is calculated on the basis of the absolute value of the latter difference lads, for example using a proportional amplifier P. In block VI, the said clamping forces Fcl and Fc2 are added.

Finally, in block VII, the sign of the latter difference Ads is determined, and on the basis of this block VIII determines to which of the two pulleys 111,

112 the first clamping force Fcl applies and to which of the two pulleys 111,112 the added clamping forces Fcl and Fc2 apply.

It should be noted that the desired value for the torque TouT can be determined, for example, as a function of the extent to which an accelerator pedal is depressed. Alternatively, it is possible for the said extent to which an accelerator pedal is depressed to be representative of a speed or acceleration of the vehicle, from which the desired value for the torque TouT can then be determined.

It should also be noted that in practice it may be difficult or even impossible to adjust the desired a drive belt slip dSDv given a low desired value for the torque Tour (for example a torque level equal to zero) may be difficult. In such a situation, it is possible, for example, to define a minimum achievable lower limit for this torque TOUT-MIN, at which, for example, the brakes of the vehicle are activated in order to dissipate the power which is then supplied, i. e. to allow the wheels to stop despite the presence of the said torque TOUT-MIN on the output shaft 3 of the transmission 10.

It should furthermore be noted that in the geared neutral ratio iCvT-GN of the transmission 10 no power is released to the output shaft 3, so that the engine 1 is subject to load only from power losses in the drive train 1,11, 12,13, 14,15 to the output shaft 3, such as frictional losses in the drive belt CVT 11. If the engine power is then increased, for example by more fuel being supplied to the engine as a result of the accelerator pedal being depressed, the torque can easily exceed a critical value. According to the invention, it may therefore be important to provide independent control of the engine speed and/or the rotational speed Mm of the input shaft 2 of the

transmission 10 in the control method according to the invention.

The two clutches 14 and 15 shown in Figure 1 are, incidentally, not required for the infinitely variable transmission 10 to function, but they do enable the transmission 10 also to be used as a conventional drive belt CVT 11 with a continuously variable transmission ratio and an associated generally known control method.

It should be noted that it is also possible in this case, in which the transmission 10 is used as a conventional drive belt CVT 11, for it to be controlled using the control method according to the invention.

Opening the first clutch 14 means that the fixed transmission 12 no longer transmits any mechanical power, while closing the second clutch 15 brings about a direct one-on-one connection between the drive belt CVT 11 and the output shaft 3. All this has the advantage that part of the range of transmission ratios of the drive belt CVT 11 can be passed through again after the first clutch 14 has been opened and/or the second clutch 15 has been closed, on account of the fact that the transmission ratio of the drive belt CVT 11 during the closure of the second clutch 15 has to be reduced in order to synchronize the rotational speed oc-ou of the output pulley 112 with the rotational speed coour of the output shaft 3.

The use of the first clutch 14 in this case allows an alternative control method for the infinitely variable transmission. This is because, according to the invention, there is a fixed and substantially advantageous linear relationship between the torque TCL output to the fixed transmission by the first clutch 14, as well as between the torque Tour on the output shaft 3 and the torque Tcw ow which is output by the drive belt CVT to the epicyclic gearwheel set. The torque TCL is in this case adjusted to the desired level

by corresponding activation of the clutch 14, for example, in the case of a standard wet disc clutch, by pressing the friction discs, which in this case form the output and input components according to the invention, onto one another with the aid of a hydraulically applied pressure PCL, where: TcL = fJLdyn * C * PCL (3) where C is a constant which takes account of the clutch geometry and pds is the coefficient of friction in a dynamic contact between the friction discs.

At the same time, a clutch slip SCL-RV across the first clutch 14, i. e. a difference between the rotational speed coin of the input pulley 111 of the transmission 10 and the rotational speed OVO-IN of the input gearwheel 121 the fixed transmission 12, is controlled to a defined desired value for this slip SCL-DV by controlling the transmission ratio of the drive belt CVT 11 as a function of the difference between this desired value ScL-DV and the actual value of the clutch slip SOL RU across the first clutch 14. In this case, of course, the abovementioned drive belt slip ds is not required.

As illustrated in Figure 4, which presents a typical traction curve of a standard wet disc clutch, the said desired value for the clutch slip SCL-DV across the first clutch 14 should not be selected to be too small, in order to limit the possible error TERROR in the torque TCL output to the fixed transmission 12 as a result of variations in the clutch slip ASCL. However, this desired value SCL-DV should also not be selected to be too high, so that the power dissipated by the clutch 14 remains below a maximum acceptable limit.

In addition to what has been described above, the invention also relates to all the details in the figures and everything which is described in the claims which follow.