NL1022242C2 - Operating method for continuously variable transmission, interrupting normal transmission operation by adaptation phase in which axial clamping force is reduced to level at which movement between pulleys and drive belt occurs - Google Patents

Abstract

The normal operation of the transmission is interrupted at any time by an adaptation phase in which the axial clamping force is reduced to a level at which a given degree of movement, e.g. drive belt slip, between one of the pulleys (1,2) and the drive belt (3) occurs and in which a value for an operating parameter of the transmission is relate to the level. An independent claim is also included for a continuously variable transmission.

Description

METHOD FOR OPERATING A CONTINUALLY VARIABLE TRANSMISSION

The present invention relates to a method for operating a continuously variable transmission as shown in the preamble of claim 1.

Such a transmission is generally known, for example from EP-A-1,167,839 in the name of the Applicant. With this type of transmissions, which are based on friction and clamping force, a minimum force is required with which the discs clamp the drive belt in order to cause it to transfer a rotation from one pulley to the other. In this case, a minimally required axial clamping force Fax-min, which is applied to the drive belt by a pulley and which is necessary to pass a offered torque at least practically without slip between the belt and the pulley, is in practice often calculated from the offered torque T times the cosine 15 of half an angle φ defined between the tapered pulley sheaves and divided by the product of 2 times a running radius R of the drive belt between the sheaves of a pulley and an empirically determined torque transfer coefficient τ:

Fax-min = (T * cos (1 / ^)) / (2 * R * x) (1) 20

Herein, for the torque transfer coefficient x, the friction coefficient μ is often substituted in the mutual contact between the drive belt and the pulley. The equation (1) is a good approximation.

The value thus determined for the minimum clamping force Fax-min 25 required in principle applies to both pulleys, but to maintain a constant transmission ratio it may be necessary for the clamping force on one of the two pulleys to be selected higher. In order for the transmission to switch, other clamping forces may be required, at least a different ratio between the clamping forces of the respective pulleys. The said minimum required clamping force Fax-min, however, always applies to at least one of the two pulleys, whereby a higher clamping force is then applied to the drive belt on the other pulley in order to achieve the desired transmission ratio or a desired speed of a change thereof.

The clamping force Fax to be applied is in turn generally defined as 1 022242 H as the minimum required axial clamping force Fax-min times a safety factor Sf: I Fax = Sf · Fax-min (2)

With the safety factor Sf, for example, an inaccuracy in the calculation of the minimum required clamping force Fax-min is taken into account.

Such an operating method is generally used and is known, for example, from the patent publication EP-A-1,218,654 in the name of the Applicant.

The general aim here is to apply the lowest possible clamping force to the drive belt in relation to the torque offered, that is to say, the lowest possible safety factor. The reason for this is that the lower the clamping force, the better the efficiency of the transmission. For example, the frictional losses will be smaller, and the absorbed for the generation of the clamping forces will be lower.

A lower clamping force is also better for the life of the belt. However, with clamping forces that only rise just above the minimum force required, i.e. just above the clamping force where slip between pulley and belt can occur, there is a relatively great risk of drive belt slip, for example as a result of an unexpected change in the load absorbed by the load. couple or the said inaccuracy in the calculation of the minimum required clamping force Fax-min.

According to the state of the art, however, drive belt slip must in principle be avoided in order to guarantee the robustness and optimum efficiency of the transmission, because drive slip by definition results in a power loss and, moreover, can lead to excessive wear.

In the known transmission, therefore, a value of 1.3 or higher I is used for the safety factor. As an alternative, additional measures are known with which the occurrence of drive belt slip can also be avoided even with a relatively small safety factor. From the European patent EP-A-1,069,331 it is known, for example, to energize a clutch in a drive line in which the transmission is included such that it already slips at a lower torque level than the drive belt. After all, once the clutch slips, the torque offered can hardly increase, or hardly increases, and excess power is dissipated by the clutch. Another alternative solution is provided by the first-mentioned prior art document in which the clamping force is selectively lowered, that is to say the safety factor is dependent on the I0. 2 7 a 2 3 conditions is selectively reduced. Such a reduction then takes place from the point of view that changes in torque to be transmitted, either initiated by the combustion engine or due to load conditions, become relatively smaller with increasing torque levels.

Thus, it is an object of the present invention to provide a method for operating the continuously variable transmission with which the efficiency or robustness of the transmission can be improved, in particular by the use of a relatively small safety factor, which method is advantageous. can be applied independently of the known additional measures. In accordance with a new insight on which the invention is based, a drive belt can be subjected repeatedly, even under permanent circumstances, to a relatively considerable slip without thereby incurring fatal damage, that is without affecting the normal operation of the transmission. This is in contrast to the technical insights up to now, according to which a complete slipping of the belt relative to the pulleys should in principle be limited as far as possible or even completely avoided.

According to the invention, a slip characteristic can advantageously be used in which, for the transmission ratios of the transmission and as a function of the axial clamping force and the mutual tangential slip speed between the pulley and the drive belt, it is determined whether non-fatal conditions are already present under these specific circumstances. damage to the belt or pulley occurs, with a so-called damage line indicating the transition or border between them. Such a slip characteristic shows that under all circumstances the transmission can withstand to some extent the intended drive belt slip and that in particular with a relatively small clamping force or a relatively large transmission ratio (which is defined as the ratio between the output speed and the input speed of the transmission) even a considerable slip speed can be allowed.

On the basis of this slip characteristic and the new insight that results therefrom, the invention proposes a new method based on the known operating method of the clamping force, wherein advantageously use is made of said mutual slipping of the drive belt and a pulley. The method according to the invention is given in claim 1 and is characterized by an adaptive approach, wherein at least one parameter of the operation of the transmission during its normal operation is updated. This method has the advantage that the operation of the transmission over time can be optimal 1022242 H with regard to the aspects efficiency and robustness. For example, the influence of wear on the maximum transmission efficiency can be significantly reduced.

In a further elaboration of the method according to the invention, the torque transfer coefficient τ is taken from the equation (1) for the said operating parameter. Updating this parameter to its actual value implies that the relationship between the applied torque T and the minimum required clamping force Fax-min is always accurately known, so that under certain circumstances a relatively small safety factor Sf can be used in the calculation of the clamping force to be applied Fax in accordance with equation (2). According to the invention, the safety factor Sf can for instance have a value between 1.3 and 1. As already noted, a small safety factor Sf benefits the efficiency of the transmission. The method according to the invention furthermore has the advantage that inaccuracies, or systematic errors that can occur during the determination of the running radius R, the actual application of the desired clamping force Fax and the applied torque T are discounted in the torque transfer coefficient τ.

The actual torque transfer coefficient τ is determined according to the invention by lowering the clamping force Fax in the adaptation phase until the aforementioned degree of drive belt slip S is detected. With the equation (1), or in a comparable manner, from the I clamping force level Fax-slip prevailing under those circumstances, the actual, that is, the actual value of the H torque transfer coefficient τ can be accurately determined: τ = (r- cos (X)) / (2-R'Fax slip) (3)

Here, the level of the axial clamping force Fax slip, at which drive belt slip occurs, is of course slightly smaller than the minimum level of the axial clamping force at which no slip occurs. However, with a sufficiently large safety factor 30 Sf, for example with a value greater than 1.1, this difference falls within the safety margin and the two clamping force levels can, however, be approximated to each other as approximations. Alternatively, it is possible to increase the determined level of Fax-slip clamping force level or the torque transfer coefficient τ determined by the equation (3) by a certain factor.

Z J / A> 5

In yet a further elaboration of the method according to the invention, the said adaptation phase is carried out while the transmission ratio of the transmission has its largest value, either the Overdrive ratio, or its smallest value, or the Low ratio. Namely, it is customary in these transmission ratios that at least one of the axially displaceable discs of the pulleys is positioned against a stop, so that the geometric transmission ratio GR of the transmission and therefore also the running radius R from the equation (3) have a fixed and known value has. Any difference between this geometric gear ratio GR and the actual measured gear ratio couit / g> in i e. the quotient of the speeds ωυιτ, ω! Ν of the two pulleys, then indicates a certain value for the drive belt slip S. According to the invention, the drive belt slip S can be defined as follows: S = (1 - ((cou, T / co , N) / GR) 100% (4) 15

It is also possible to use the absolute value of the drive belt slip in meters per second.

Since there is inevitably some slip occurring in the frictional contact between drive belt and pulley, the so-called micro-slip, said difference between the geometric and the actual transmission ratio must exceed a certain value before drive belt slip, or macro- slip. The definition according to equation (4) refers to macro slip if the drive belt slip S is in the order of percentages.

Incidentally, in the case of drive belt slip, the measured transmission ratio is always smaller than the geometric transmission ratio, so that in the Low ratio the occurrence of drive belt slip can be assumed with certainty if the measured transmission ratio assumes a value smaller than the smallest possible geometric transmission ratio, or the Low ratio.

In any other geometric transmission ratio of the transmission, i.e. a transmission ratio between the Low ratio and the Overdrive ratio, it cannot be determined without additional detection means. Nevertheless, according to the invention, an elaboration of the method for operating the transmission is also possible in which the transmission is provided with detection means for determining the geometric transmission ratio. To this end, various suitable detection means are known in the prior art, such as a drive belt speedometer for determining the circumference speed of the drive belt from which the running radius thereof can be calculated, a pulley disc position sensor for determining the distance between the discs of a pulley from which again the running radius of the drive belt can be calculated, or a driving belt position sensor for directly determining the running radius of the driving belt.

The invention will now be further explained by way of example with reference to a drawing in which:

Figure 1 is a schematic representation of a portion of a continuously variable transmission with drive belt and pulleys; Figure 2 is a representation of the insight according to the invention in the form of a so-called slip characteristic determined for the present transmission; and

Figure 3 gives an example of a so-called traction curve for the present transmission.

Figure 1 shows the central parts a continuously variable transmission as it is used in the drive of, for example, passenger vehicles. The transmission is generally known per se and comprises a first and a second, two pulley sheaves 4, 5 comprising pulleys 1 and 2 with a drive belt 3 between them. The pulley sheaves 4, 5 are conically shaped and at least one disk 4 of a pulley 1 12 is axially displaceable along a respective axis 6, 7 on which the discs 4, 5 are mounted. Also included in the transmission are I activation means, not shown in the figure, which are generally electronically controllable and hydraulically acting, which can impose an axial force Fax I on said one disc 4, such that the drive belt 3 between the respective discs 4, 5 is clamped and the driving force of a shaft 6,7, or an applied torque T, I by friction in the conical contact surface between the discs 4, 5 and the drive belt 3, is transferred between the pulleys 1,2. The axial I clamping force Fax-min required for this purpose is determined in accordance with equation (1) by the torque T applied, a contact angle of the said contact surface with the radial direction, or half of an angle I 30 φ defined between the tapered pulley sheaves, a running radius R of the drive belt 3 and a torque transfer coefficient τ.

However, the value determined by means of the equation (1) for the minimum axial clamping force Fax-min required is I to some extent inaccurate due to possible deviations between the value determined for the clamping force determination I for the torque T applied and for the torque T running radius R and the actual value of those parameters. Moreover, in practice there is often an otherwise unknown difference between the desired value for the axial clamping force Fax to be applied by the activating means and the actually prevailing clamping force. Said inaccuracies and uncertainties result in that the desired value for the axial clamping force Fax-min is chosen to be slightly higher than the minimum required clamping force Fax-min, in particular by multiplying it in accordance with equation (2) with a safety factor Sf.

The drive belt 3 shown in Figure 1 comprises a pair of an endless metal support elements 31, each consisting of a set of nested thin metal rings, which form a support for a series of metal transverse elements 32, which between the disks 4, 5 of a pulley 1 2 take up clamping forces exerted and which, by rotation of a driving pulley 1, are mutually moved mutually over the carrier elements 31 to a driven pulley 2. Such a type of drive belt is also known as the Van Doorne push belt and is further described in, for example, the European patent EP-A-0.626.526.

According to the insight underlying the present invention, this type of drive belt 3 can withstand at least some degree of mutual slipping of the pulley 1, 2 and the belt 3, or drive belt slip S. In this connection, a so-called slip characteristic can form a handle for the pursuit of optimization of robustness and efficiency of the continuously variable transmission underlying the invention. Figure 2 shows an example of such a slip characteristic in which for the geometric transmission ratios of the transmission and in dependence on the axial clamping force Fax and the absolute degree of drive belt slip S, it has been laid down whether non-fatal damage to it is already provided under these particular circumstances. the drive belt 3 or the pulley 1, 2 occurs. The curves or the damage lines A, B and C in Figure 2 here represent for a certain geometric transmission ratio the maximum permissible value of the drive belt slip S including, which is indicated by the arrows I, not fatal damage and above which is indicated by the arrows II, it does occur. According to an idea underlying the invention, adhesive wear due to slip occurs if a certain combination of a force Fax applied to the drive belt by a pulley sheave and a mutual slipping speed, or degree of drive belt slip S, is exceeded.

However, the slip characteristic shows that the transmission is resistant to the drive belt slip S at least to some extent. Figure 2 further shows that the maximum degree A of drive belt slip S is reached faster in the Low ratio than that maximum. "H value C in the Overdrive ratio, that is, with the same degree of drive belt slip S1 in the Low ratio, there is already a lower level of the axial clamping force

Fax-a damage to the drive belt or pulley occurs, for example in comparison with the critical level of the clamping force Fax-b in the Overdrive ratio, or that in comparison 5 with the Overdrive ratio in the Low ratio at an equal level of the axial clamping force

Fax a smaller degree of drive belt slip S can be considered permissible. The H claims line B concerns a transmission ratio between the Low ratio and the I Overdrive ratio.

Based on the discovery of this phenomenon and the detailed quantification in its slip characteristic, a new operating method for the continuously variable transmission has been developed, starting from any known operating method for determining the axial clamping force Fax and adding a so-called adaptation phase in which at least the current I-level Fax-slip of the axial clamping force is determined, with macro-slip I occurring between drive belt 3 and pulley 1, 2 at a given transmission ratio I GR and torque T and the current value of an operating parameter at this level I Fax slip relates. According to the invention, the said macro slip preferably has an value in an absolute sense in a range between 0.5 and 2 m / s, in particular between 1 and 1.5 m / s.

For clarification, Figure 3 illustrates the difference between micro-slip and macro-slip according to the invention on the basis of a so-called traction curve. In this figure, with a given transmission ratio, in this case I Overdrive ratio, and a given axial clamping force Fax, the drive belt slip S is shown as a function v of the torque T applied. Figure 3 shows that in principle the drive belt slip S increases with the torque level T up to a maximum torque level I Tmax. In accordance with the invention, this type of drive belt slip S is defined as micro-slip.

Although the drive belt slip S increases further above the maximum torque level Tmax, the transmitted torque T remains substantially constant or even I decreases slightly. In accordance with the invention, this type of drive belt slip S is defined as macro slip.

According to a preferred embodiment of the invention, during normal operation and at least with a constant transmission ratio, the required clamping force Fax is calculated on the basis of the equation (1) and in I the adaptation phase on the basis of the equation (3), the current value of the I torque transfer coefficient τ determined by reducing the clamping force until I drive belt slip occurs.

/ 9

According to yet another preferred embodiment of the invention, during normal operation 6, the axial clamping force Fax to be applied to the drive belt 3 by a pulley 1 or 2 is determined by multiplying the minimum required level thereof Fax-min by a safety factor Sf whose value is related to the permissible value of the drive belt slip S according to the slip characteristic at the instantaneous transmission ratio of the transmission and the said axial clamping force. That is, a greater safety factor is applied under circumstances in which the slip characteristic indicates that the transmission is less resistant to the drive belt slip S, such as in the low ratio 10 with a high torque T or a large axial clamping force Fax to be applied, and vice versa.

According to the invention, the adaptation phase can be cyclically, for instance every 10 minutes during the operation of the transmission, cyclically, for example each time when it is put back into operation, or on the basis of a combination of both. In the latter case, for example, the adaptation phase can be performed every time the Overdrive ratio is reached and then run every 10 minutes. A multitude of possible strategies can of course be envisaged in this context.

Finally, according to the invention, it is advantageous to relate the transmission parameter determined during the adaptation phase 20, such as, for example, the actual torque transfer coefficient t, to a predetermined nominal value or range of values. This procedure provides numerous additional control options for the transmission. For example, if the actual torque transfer coefficient τ falls outside the determined nominal range, the method for operating the transmission during normal operation is at least temporarily replaced by a safe method in which, for example, the torque offered is limited or in which a relatively high safety factor is applied. In such a case, the need for a service such as an oil change can also be signaled to the user of the transmission. 1 n.

Claims (9)

2. Method as claimed in claim 1, characterized in that the operating parameter is a torque transfer coefficient τ of the transmission, which is representative I for the relationship between the applied torque T and the level of the axial clamping force Fax that is minimally required to pass that offered torque T almost without I drive belt slip S through the transmission during normal operation. 3. Method according to claim 2, characterized in that the torque transfer coefficient τ is given by the equation: I τ = (T * cos (1/4 <|>)) / (2 * R * Fax slip), I wherein φ is an angle defined between the tapered pulley sheaves (4,5) and R is a running radius of the drive belt (3) between the sheaves (4,5) of a pulley (1,2). Method according to claim 3, characterized in that the application of an I axial clamping force Fax to be applied to the drive belt (3) during normal operation by a pulley (1,2) is given by the equation: I 1022242 Fax = Sf · (T * cos (1/2 <|))) / (2 * R »x), where Sf is a safety factor greater than or equal to 1. Method according to claim 4, characterized in that the safety factor Sf has a value in a range between 1.3 and 1, preferably greater than 1.1. 6. Method as claimed in any of the foregoing claims, characterized in that during normal operation axial clamping force Fax to be provided by a pulley (1,2) on the drive belt (3) is determined by the level of the axial clamping force Fax which is minimally necessary is to pass that offered torque T quasi without drive belt slip S through the transmission by a safety factor Sf whose value is related to the according to the slip characteristic at the current transmission ratio of the transmission and the axial clamping force Fax 15 permissible value of the drive belt slip S. 7. Method as claimed in any of the foregoing claims, characterized in that the adaptation is carried out while the transmission ratio of the transmission has assumed a smallest value, or Low ratio, or a largest value, or Overdrive ratio. A method according to any one of the preceding claims, characterized in that the degree of drive belt slip S is determined in the adaptation phase depending on the difference or the ratio between a geometric transmission ratio GR 25 of the transmission and a ratio ωυιτ / ωιΝ between the speeds of the respective pulleys (1,2). 9. A method according to any one of the preceding claims, characterized in that said degree of drive belt slip S has an absolute value in a range between 0.5 and 2 m / s, preferably between 1 and 1.5 m / s. A method according to any one of the preceding claims, characterized in that the transmission is further provided with detection means for determining the geometric transmission ratio GR of the transmission. ft 0 -10, * V i .. / / A,. "