Control method for a shift clutch in the drivetrain of a vehicle to improve driving stability

Abstract

Control method for a shift clutch (13, 33) arranged in a drive train (12, 32) of a motor vehicle (1) between a drive unit (16, 36) and a vehicle wheel (W1, W2 / W3, W4) driven by the drive unit to increase or restore the driving stability of the motor vehicle (1), wherein the motor vehicle (1) further comprises a mechanically decelerating wheel brake (B1, B2 / B3, B4) for the individual vehicle wheel (W1, W2 / W3, W4), characterized in that the control method comprises the following steps: - Monitoring a trigger condition for a vehicle stability control intervention, wherein the trigger condition includes, ▪ that an impermissible slippage or loss of traction between the vehicle wheel (W1, W2 / W3, W4) and the surface (U) has occurred or is imminent; AND / OR ▪ that a target deceleration power at the vehicle wheel (W1, W2 / W3, W4) is greater than a maximum deceleration power that can be provided at present by the control of the drive system (16, 36); - Upon occurrence of the trigger condition: Opening of the switching clutch (13, 33), wherein the opening of the switching clutch (13, 33) takes place in a time window (To) that is shorter than a time window (Tb) that is necessary to build up the brake pressure required for a brake intervention at the wheel brake (B1, B2 / B3, B4).

Description

[0001] The invention lies in the field of vehicle stability systems, in particular systems for maintaining or restoring wheel grip against the road surface.

[0002] In practice, various systems are known to improve driving stability. These include the anti-lock braking system (ABS) and the electronic stability program (ESP). The anti-lock braking system detects a situation in which at least one wheel of the vehicle loses traction during braking, or that such a situation is imminent, and initiates alternating increases and decreases, or activation and deactivation, of the braking force on the wheel to improve the vehicle's steering. ABS systems most often activate during hard braking while driving forward, specifically on wet or icy roads.

[0003] The Electronic Stability Program (ESP) has a control system that monitors certain actual parameters of the vehicle's movement and compares them with corresponding target parameters that should result from sufficient driving stability. If the actual parameters deviate from the target parameters, particularly if the vehicle's actual yaw rate differs from the target yaw rate during cornering, an unstable vehicle condition can be detected. An ESP system can also detect that a vehicle wheel is losing traction or that such a condition is imminent, even during driving with longitudinal and / or lateral acceleration. The ESP system can generate target values ​​for wheel-specific braking, which can be independent of or superimposed on a driver-defined target value for the vehicle's braking.Depending on their specific configuration, the target values ​​for braking can therefore result in targeted braking at one or more wheels of the vehicle. Furthermore, the ESP system can reduce drive power to certain wheels to facilitate or achieve a return to stable driving conditions.

[0004] The ABS and ESP systems can act in opposing ways in certain driving situations. While the ABS system temporarily reduces braking force, the ESP system often aims to increase braking force.

[0005] Today, electric drive systems are frequently used to provide power to individual or all vehicle wheels, either temporarily or continuously. These electric drive systems can be used independently or in conjunction with other drive systems, such as internal combustion engines. Because electric drive systems can generate torque from a standstill and cover a very wide speed range, often exceeding that of internal combustion engines, axles with an electric drive system often no longer require a starting clutch—that is, a typically friction-based, disengageable clutch like those used in vehicles with internal combustion engines and manual transmissions. A starting clutch transmits torque via a frictional connection when engaged.

[0006] In some vehicle configurations, an electric drive is only used intermittently, specifically when there is a high torque demand. In driving situations with essentially constant speed and low torque demand, however, it is disconnected from the vehicle wheels via a clutch to increase energy efficiency. A clutch transmits torque in the engaged state via a positive connection between a torque input and a torque output. Clutches are generally smaller and less expensive than starting clutches of a comparable torque class.

[0007] The vehicle configurations known so far therefore use switching clutches to be able to completely interrupt the operation of a drive system for reasons of increasing range or improving energy efficiency.

[0008] Future emissions guidelines will not only consider exhaust gases and energy efficiency, but also the emission of other substances such as brake dust.

[0009] It has been shown that the use of multiple drive systems, and in particular the supplementary use of electric drive systems, has a negative impact on brake wear and thus on brake dust generation. Specifically, it was found that the control of drive systems does not react quickly enough during highly dynamic changes in driving conditions to utilize the deceleration potential generated by the drive system itself.

[0010] Thus, in the event of an abrupt change from a state of wheel grip to a state of loss of grip, or an abrupt change from an acceleration demand to a braking demand, the drive system often cannot provide any support measures. As a result, it can happen that, in the event of an abrupt loss of wheel grip, the drive system makes it difficult to regain grip. On the other hand, the wheel brakes often have to provide not only the energy to decelerate the vehicle wheels and the vehicle itself, but also to decelerate the rotating masses of the drivetrain and the drive system itself.

[0011] Various designs of motor vehicles with one or more drive units and at least one clutch, as well as associated control methods for their operation, are known from EP 1 506 890 B1, DE 10 2026 212 132 B3, DE 10 2018 214 082 A1, DE 10 2021 127 055 A1, DE 10 2005 053 852 A1, US 5 362 287 A, DE 10 2017 201 483 A1, DE 101 57 506 A1 or KR 10 1 521 388 B1.

[0012] The control methods known so far for clutch systems in motor vehicles are not optimally designed.

[0013] The object of the present invention is to demonstrate an improved control method for a shift clutch that is arranged in a drive train of a motor vehicle between a drive unit and a vehicle wheel driven by the drive unit, wherein the control of this shift clutch contributes to the driving stability of the motor vehicle.

[0014] The invention solves this problem through the features of claim 1.

[0015] The control method according to the present disclosure is designed and configured for controlling a clutch, wherein the clutch is arranged in the drivetrain of a motor vehicle between a drive unit and a vehicle wheel driven by the drive unit. The control method is further designed to increase or restore the vehicle's driving stability.

[0016] The motor vehicle includes a mechanically decelerating wheel brake for each individual vehicle wheel.

[0017] The tax procedure includes at least the following steps: Monitoring a trigger condition for a vehicle stability control intervention, wherein the trigger condition includes that • an impermissible slippage or loss of traction between the vehicle wheel and the surface has occurred or is imminent; and / or • the target deceleration power at the vehicle wheel is greater than the maximum deceleration power that can currently be provided by the control of the drive system;

[0018] When the trigger conditions are met: the clutch opens.

[0019] That the required deceleration power of the vehicle is greater than the maximum deceleration power currently available through the control of the drive system is to be expected in an electric drive system with a currently common control system, especially if a switch from motor operation with a power output of at least 50% of the maximum power to generator operation has to take place within less than 500 ms, and in particular within less than 200 ms.

[0020] Opening the clutch disconnects the drive connection between the traction motor and the vehicle wheel. This changes the state of the vehicle wheel from "driven wheel" to "free-rotating wheel." In this case, the vehicle wheel is only connected to a very small portion of the rotating masses of the drivetrain, particularly to the associated wheel brake. This results in two immediate effects: Firstly, the power of the wheel brake, if and as soon as it engages, can be used entirely to reduce the rotational speed of the associated vehicle wheel and, when wheel traction is (re)established, to decelerate the vehicle. Secondly, the wheel brake does not need to be used to decelerate the rotating masses of those drivetrain components that are decoupled from the vehicle wheel by the opening of the clutch.

[0021] On the other hand, in the event of reduced or lost wheel traction, minimizing the moment of inertia of those parts still in drive connection with the vehicle wheel allows for a very rapid adjustment of the vehicle wheel's circumferential speed to the ground speed. In other words, any excessive speed of the vehicle wheel (slippage) is eliminated by the remaining transmissible forces between the vehicle wheel and the ground, without requiring any additional controlled intervention. The vehicle wheel, which essentially assumes the state of a "freely rotating wheel," is thus brought back into stable contact with the ground as quickly as possible.

[0022] The two aforementioned effects can occur individually or in combination. They both lead to an increase or restoration of the vehicle's driving stability because they enable the restoration of wheel traction in the shortest possible time and allow the use of maximum braking power for deceleration.

[0023] Further advantageous embodiments of the invention are specified in the dependent claims and the following description.

[0024] The invention is illustrated schematically and by way of example in the figures. These show: Fig. 1 to 3: A schematic top view of a motor vehicle (1) with two drive trains and one clutch per drive train, to illustrate a first effect to increase driving stability; Fig. 4A and Fig.4B: Possible arrangement positions for a shift clutch in the drive train and a resulting change in the moment of inertia reduced to the vehicle wheel when the shift clutch is open; Fig. 5: A schematic functional representation of a switching clutch; Fig. 6A and Fig. 6B: A schematic representation analogous to Fig. 1 to 3 to explain the achievable effect in a different driving condition; Fig. 7A and Fig. 7B: Explanatory diagrams for a torque flow in the drive train during loss of wheel traction and after opening the shift clutch; Fig. 8A and Fig. 8B: Representations analogous to Fig. 7A and Fig. 7B for a highly dynamic transition from an acceleration phase to a phase with a high target deceleration performance.

[0025] Fig.Figure 1 shows a schematic top view of a motor vehicle (1), which in this illustration is a four-wheeled vehicle, in particular a passenger car or a motorized transport vehicle. The vehicle has at least two driven wheels, preferably four driven wheels (W1, W2, W3, W4). Preferably, each wheel is assigned a wheel brake (B1, B2, B3, B4), which more preferably can cause a deceleration of the rotational speed (o_w) of the wheel by means of a mechanical engagement.

[0026] A preferred embodiment provides that one or two vehicle wheels (W1, W2 / W3, W4) are arranged on a common drive axle (11, 31). Furthermore, preferably, the one or more drive axles (11, 33) are each connected via a drive train (12, 32) to a common or separate drive unit (16, 36). In the example of Fig.1. The vehicle wheels (W1, W2) are arranged on a front axle (11) and connected via a front drive train (12) to a first or front drive unit (16). Two further vehicle wheels (W3, W4) are arranged on a rear axle (31) and connected via a rear drive train (32) to a second drive unit (36).

[0027] The one or more drive trains (12, 32) can be of the same or different design. In particular, they can include a transmission gear (15, 35) and / or a wheel differential (14, 34) between the drive unit (16, 36) and the associated vehicle wheels (W1, W2 / W3, W4).

[0028] The shift clutch (13,33) can be located at any point in the drive train (12,32). Fig.Figures 1 to 3 show an arrangement of the shift clutch (13,33) in the direction of power transmission from the drive unit (16,36) to the vehicle wheel (W1, W2 / W3, W4) downstream to a transmission gearbox (15,35) and upstream to a wheel differential (14,34). Fig. 4A shows alternative arrangement positions. Arrangement position (A) provides an arrangement in the direction of power transmission directly upstream of a wheel brake (B1, B2 / B3, B4). Position (B) provides an arrangement directly upstream or downstream of a wheel differential (14, 34).

[0029] According to position C, the arrangement of the shift clutch (13, 33) is provided within a transmission (15), in particular between a first transmission stage (15a) and a further transmission stage (15b). Finally, position D relates to an arrangement of the shift clutch (13) directly downstream of the drive (16) and upstream of the transmission (15).

[0030] The Fig. Figure 4B shows comparative representations of a reduced moment of inertia (I) of the vehicle wheel (W1 / W2), resulting from those parts of the drivetrain (12) that are still in drive connection with the vehicle wheel (W1,W2) when the clutch is open, according to positions A to D. The bar graph labeled E shows, in comparison, that the vehicle wheel (W1 / W2) also has a reduced moment of inertia in the case where no clutch is provided in the drivetrain.

[0031] The torque required for a rotational acceleration of a body, i.e., for a change in rotational speed per unit time, is directly proportional to the moment of inertia (I). From the representations of the Fig. 4A and Fig.4B thus shows that a change in the rotational speed (o_w) of a vehicle wheel (W1, W2 / W3, W4) can be achieved more smoothly or dynamically the fewer mass-bearing components of the drive train (12) are in a drive connection with the vehicle wheel. Accordingly, by placing a shift clutch at one of the positions A or B, a reduction of, for example, 1% to 10% of the value that occurs in the reference case without a shift clutch (identification letter E) can be achieved. Therefore, for a given desired change in wheel rotational speed, a torque that is 90% to 99% lower is required.

[0032] With an arrangement according to position C or D, the moment of inertia is reduced to approximately 17.5% or 27.5% in the example.

[0033] The drive system(s) (16, 36) can have any configuration. At least one of the drive systems (16, 36) can be an electric drive system, in particular an electric drive system with a single assignment to a vehicle wheel (individual wheel drive) or with a single assignment to only one axle (11, 31) (single axle drive) or with a single assignment to only some of the driven axles (axle group drive). Alternatively, only a single drive system can be provided, in particular an electric drive system that is assigned to all driven axles (11, 31) jointly.

[0034] For passenger vehicles, it has proven advantageous to provide at least one electric drive with a singular assignment to a driven axle, which is generally intended as the main drive or primary drive, and at least one further electric drive, which has a singular assignment to a single additional vehicle wheel or to a further axle and is only used temporarily as a secondary drive, in particular to provide maximum system power during phases with high acceleration requirements or to provide maximum recuperation power during phases with longer deceleration.

[0035] Each of the electric drive units can operate exclusively in motor mode, i.e., delivering torque, or also temporarily in generator mode, i.e., receiving torque. Sub-combinations are also possible, so that, for example, a first electric drive unit operates in either motor or generator mode as needed, while a second electric drive unit operates exclusively in motor mode as needed.

[0036] If two or more electric drive units are provided, they may be of the same or different design. Preferred designs for an electric drive unit are: • Permanent magnet synchronous machine (PSM) • Externally excited synchronous machine (EESM) • Induction machine / asynchronous motor (ASM) • Other type of electric motor, for example, reluctance motor (FSRM)

[0037] Particularly preferred is a first electric drive, which is to be used as the primary drive, designed as a permanent magnet synchronous machine (PSM) and a second electric drive, which is to be used as the secondary drive, designed as an asynchronous machine (ASM) or as a permanent magnet synchronous machine (PSM).

[0038] It has been shown that the electrical control of an electric drive exhibits a dynamic response, particularly during abrupt transitions between different target speeds and during abrupt changes between motor and generator operation, which can result in an unacceptable delay compared to the force changes in the drivetrain (12, 32). In particular, switching from a state in which the electric drive is to draw high electrical power from an energy storage device (2) and deliver it as torque, to a state in which the motor is to draw torque and feed it back into an energy storage device, is only possible with considerable delays. This is especially true when alternating torques are present in the drivetrain, which can lead to abrupt changes in phase.Accordingly, the deceleration power theoretically available from the electric drive system is often not available, or not available quickly enough, during highly dynamic transitions between driving states. In other words, the electric drive system cannot be used, or not quickly enough, in certain driving situations to effect or assist a deceleration of the wheel rotational speed (o_w). It has been observed, in particular, that when transitioning from maximum torque output to maximum torque input, a time interval of >500 ms, and especially >1000 ms, can elapse before the actual torque substantially matches the requested target torque. In other words, this means that during a dynamic transition from full acceleration to full deceleration within a period of >500 ms or >1000 ms, the application of a wheel brake is insufficient to achieve the desired deceleration. Fig.The torque curve in the drivetrain (12) shown in Figure 8A must be supported. On the one hand, the wheel brake supports the torque that must be supported by the vehicle wheel (W1) as a result of the deceleration of the vehicle (1). On the other hand, the drive unit supports a torque that is necessary for reducing the rotational speed (o_m) of the drive unit. Even if the drive unit (16) no longer delivers any additional torque, it is not yet in recuperation mode at first, which is why it does not contribute to the deceleration of the rotating masses of the drivetrain. In other words, in the example as shown in Figure 8A, the torque curve in the drivetrain (12) is supported. Fig.8A The wheel brake (B1) not only supports the torque necessary to decelerate the vehicle's (1) motion, but also supports the entire torque required to decelerate the rotating masses of the drivetrain. Quantitatively, this second torque drive is the full amount of the Fig. Column 4B shown under identifier E.

[0039] If, according to the present disclosure in the tax proceedings, the trigger condition is met, namely that a target deceleration power at the vehicle wheel (W1, W2 / W3, W4) is greater than a maximum deceleration power currently available through the control of the drive system (16, 36), then the following applies instead of the one in Fig. 8A example shown in Fig. The moment flow shown in 8B is shown. While in the example of Fig.8A the shift clutch (13) is closed and consequently the rotational speed (o_m) of the drive (16) is in a proportional relationship to the rotational speed (o_w) of the vehicle wheel according to the gear ratio in the drive train (12), is shown in the illustration according to Fig. 8B the shift clutch (13) is open. Consequently, the rotational speed (o_w) of the vehicle wheel can change independently of the rotational speed (o_m) of the drive system during the opening period of the clutch (13). It is therefore possible that, with the shift clutch (13) open, the wheel brake (B1) can only apply a significantly reduced proportion of the moment of inertia. Fig.4B has to support less and is therefore subjected to less stress, or can provide a larger portion of its power capacity for supporting the torque that must be applied to the vehicle wheel (W1) for deceleration of the vehicle (1). In other words, brake wear is reduced, thus reducing brake dust emissions, and driving stability is increased because a larger proportion of the braking power of the wheel brake (B1) can be made available for the actual deceleration of the vehicle (1).

[0040] It is preferably provided that the opening of the shift clutch (13, 33) occurs within a time window (To) that is shorter than the time window (Tb) required to build up the brake pressure necessary for brake engagement at the wheel brake (B1, B2 / B3, B4). Tests on known brake systems with mechanical brake engagement have shown that a time window (Tb) for building up the brake pressure is, for example, 200 milliseconds. Therefore, if the opening of the shift clutch (13, 33) occurs within a time window (To) of less than 200 milliseconds, the wheel brake (B1) can remain essentially unloaded from the torque required to decelerate the rotating masses of the drivetrain (12, 32) upstream of the shift clutch (13, 33).

[0041] According to a preferred embodiment, the control method as disclosed herein provides that, after the opening of the clutch (13) due to the occurrence of one of the aforementioned triggering conditions, further steps are carried out. These may include, in particular: Monitoring the instantaneous rotational speed(s) (o_w) of the vehicle wheel(s) (W1, W2 / W3, W4) connected to a torque output (13b) of the open clutch (13, 33); Determining the presence or restoration of wheel adhesion (optional); Controls of the drive system (16, 36) to a rotational speed (o_m) which corresponds to the instantaneous rotational speed (o_w) of the vehicle wheel or wheels (W1, W2 / W3, W4) such that a closing condition of the clutch (13, 33) is met.

[0042] The closing condition of a switching clutch (13, 33) can depend on its design. The switching clutch can certainly be opened or closed in a state in which the rotational speed of a torque input (13a) of the switching clutch matches the rotational speed of the torque output (13b) of the switching clutch, or even in which there is a corresponding phase relationship between the torque input and torque output.

[0043] According to a preferred embodiment, a switching clutch is provided which also allows, within certain limits, differences between the rotational speeds and / or the rotational positions of the torque input and the torque output.

[0044] In other words, a closing condition of the switching clutch preferably provides that a difference between an instantaneous rotational speed of the torque input (13a) and the instantaneous rotational speed of the torque output (13b) of the switching clutch (13, 33) is less than or equal to a predetermined speed difference limit.

[0045] Alternatively or additionally, a closing condition of the switching clutch provides that a difference between the instantaneous rotational position of the torque input (13a) and the instantaneous rotational position of the torque output (13b) of the switching clutch (13,33) is less than or equal to a predetermined rotational position difference limit value.

[0046] Both of the aforementioned limit values ​​have a value greater than zero.

[0047] Provided that the drive system is regulated to the aforementioned corresponding rotational speed (o_m) and at least one closing condition of the clutch is met, the following step can be carried out or permitted: closing the clutch (13,33) and establishing the drive connection between the drive system (16,36) and the vehicle wheel (W1, W2 / W3, W4).

[0048] Fig.Figure 5 shows a schematic representation of a switching clutch (13, 33). This clutch has a torque input (13a) and a torque output (13b) which are designed with corresponding clutch geometries, in particular in the form of a jaw clutch. At least one of these parts (torque input / torque output) can be moved between an active position, in which the switching clutch (13) is closed and the drive connection is established, and an inactive position, in which the switching clutch (13) is open and the drive connection is disconnected.

[0049] According to a preferred embodiment, the clutch (13, 33) can have a synchronizing device (13c) configured to supply drive energy via its own actuator in order to minimize or reduce to zero a residual speed difference and / or a residual rotational position difference between the torque input (13a) and the torque output (13b) of the clutch (13, 33). Such a synchronizing device (13c) can be configured in any way. For example, it can be formed by an actuator that applies a feed force in the closing direction, wherein a clutch geometry with a synchronizing element and / or with ramps is present at the torque input and / or the torque output.

[0050] The synchronization device (13c) can preferably be actuated as soon as the closing condition of the switching clutch (13, 33) is met. The residual speed difference and / or the residual rotational position difference can, in particular, be less than or equal to the speed difference limit value or the rotational position difference limit value mentioned above.

[0051] Alternatively or additionally, the shift clutch (13,33) may have a torque compensation device (13d) designed to temporarily absorb a torque momentarily transmitted between the torque input (13a) and the torque output (13b).

[0052] Such a torque compensation device (13d) can have any configuration. In particular, it can serve to facilitate the rapid opening of the switching clutch (13, 33). The (partial) absorption of the transmitted torque can be achieved, for example, by a device based on the effect of an eddy current brake or an induction brake. For this purpose, one or more electrically conductive bodies can be arranged on the circumference of a shaft section of the switching clutch (13, 33), to which an inductive braking device, for example, a coil, can be temporarily brought close. The magnitude and duration of the braking effect can be influenced on the one hand by the size or circumferential coverage of the braking body and on the other hand by the distance and the inductance characteristics.

[0053] The aforementioned device can be provided in one or two places, in particular once on the torque input side (13a) and once on the torque output side (13b), so that at any time the part of the clutch that delivers torque can be subjected to a short-term delay.

[0054] A preferred embodiment provides that the torque compensation device (13d) is activated when the trigger condition for a vehicle stability control intervention occurs. Furthermore, activation may require that the currently transmitted torque exceeds a torque limit value of the shift clutch (13, 33) and / or that an opening movement of the shift clutch (13, 33) is delayed by an impermissible amount.

[0055] Fig.Figure 1 illustrates a driving situation in which the motor vehicle (1) has a state at a time (t0) during an acceleration phase. In the example shown, the acceleration is positive, leading to an increase in speed (v). However, the example can be applied equally to a case with negative acceleration.

[0056] To the left and to the right side of the motor vehicle (1) are in Fig.Figure 1 shows a side view of each vehicle wheel (W1, W2, W3, W4) with the forces and moments currently acting on that wheel, and a Kahm circle (K) below each. The Kahm circle, with its circumference, indicates one limit for the maximum total force that can be transmitted in the static contact between the vehicle wheel (W1, W2, W3, W4) and the ground (U). Various factors can cause this total force to decrease, for example, a change in the ground properties and / or a reduction in the wheel's contact force.

[0057] In the example of Fig. 1. It is assumed that the road surface changes at the left front wheel (W1) of the vehicle (1), for example due to local wetness, slipperiness, or road contamination. As a result of the changed road surface, the limit for the maximum force (F_w) that can be transmitted at the wheel circumference decreases. In the example of Fig.1. It is assumed that the reduction in wheel grip is so great that the force (F_w) that can be transmitted to the ground becomes so small that the driving torque (M_d) transmitted to the vehicle wheel (W1) via the drivetrain (12) can no longer be fully supported. In other words, in the state (T0) according to Fig. 1 an impermissible slip or loss of traction between the vehicle wheel (W1) and the surface (U) is occurring or is imminent.

[0058] The consequence of this is shown in the diagram below. Fig. 1. that the circumferential speed (v_c) of the vehicle wheel (W1) deviates from the ground speed (v_e) of the vehicle wheel, i.e., from the speed of motion of the vehicle wheel (W1) over the ground (U), by an impermissible amount. In other words, in the example of Fig.1. The vehicle wheel (W1) rotates in the forward direction. As stated above, the example is equally applicable to the case where negative acceleration occurs. In such a case, the vehicle wheel (W1) would rotate too slowly relative to the ground speed (v_g) or even lock up (not shown).

[0059] Fig. 2 and Fig. 7A explain the phenomenon that can occur when or immediately afterwards an impermissible slip or loss of traction occurs, when the drive connection between the vehicle wheel (W1) and the drive unit (16) is closed.

[0060] According to Fig.7A The drive unit (16) delivers a torque that is transmitted via the drive train (12) to the vehicle wheel (W1) and initially further to the ground (U). The moment the wheel loses traction, the torque delivered by the drive unit (16) causes at least a significant increase in the rotational speed (o_w) of the vehicle wheel (W1). The circumferential speed (v_c) is calculated by multiplying the instantaneous rotational speed (o_w) of the vehicle wheel by the (dynamic) radius of the vehicle wheel (R_w).

[0061] As explained above, switching from torque output to torque absorption or generator operation is often not possible with the desired dynamics, especially in electric drive systems. Therefore, in the example of Fig.2. With the clutch (13) engaged, the wheel brake (B1) must be applied to limit the overspeed (v_o) of the vehicle wheel (W1) (i.e., the difference between the circumferential speed (v_c) of the vehicle wheel and the ground speed (v_g)). Even if the torque output of the drive (16) were reduced relatively quickly, a significant degree of overspeed (v_o) would remain. In systems known according to the prior art, wheel braking is automatically initiated upon detection of impermissible slip or loss of traction; that is, a control signal is issued to generate a braking force at the wheel brake (B1). However, a time window (Tb) elapses before the wheel brake (B1) can engage. This time window is necessary to build up the brake pressure at the wheel brake (B1), for example, to apply the brake shoes to a disc brake.

[0062] As soon as braking can be achieved, the rotational speed (o_w) of the vehicle wheel decreases, and consequently, so does the circumferential speed (v_c) of the vehicle wheel (W1). However, the degree of braking force depends considerably on situational conditions, such as brake temperature, dirt, wetness, wear, etc., meaning that the achievable braking force can only be specified imprecisely. As a result, overbraking usually occurs initially, meaning that the circumferential speed (v_c) of the vehicle wheel (W1) drops excessively and, in particular, falls below the ground speed (v_g), resulting in an overspeed (v_o) with the opposite sign. In known systems, the application of the wheel brake (B1) is then reduced or released, whereupon the vehicle wheel (W1) gradually accelerates again due to contact with the surface (U) until wheel traction is regained.

[0063] As explained above, during the aforementioned phenomenon, the control of the drive system (16) makes no positive contribution or even a parasitic one due to the insufficient dynamics. Furthermore, the wheel brake (B1) must also be activated in the example according to Fig. 2 when the shift clutch (13) is closed, not only provide the torque necessary to reduce the rotational speed of the vehicle wheel (o_w, v_c), but also support the torque required to decelerate the rotating masses of the drive train (12).

[0064] Fig. 3 and Fig. 7B explain an alternative development of the situation that can be achieved through the tax procedure according to the present disclosure.

[0065] In the example according to Fig. 3 will be at time t0 (comparison) Fig.1) that means, if the triggering condition is met, i.e., if an impermissible slip or loss of traction between the vehicle wheel (W1) and the surface (U) has occurred or is imminent, a signal to open the shift clutch (13) is sent.

[0066] This interrupts the drive connection between the drive unit (16) and the vehicle wheel (W1) within a time window (To) by interrupting the torque transmission between the torque input (13a) and the torque output (13b) of the shift clutch (13). As can be seen from Fig.As 7B results, in this example as well the force load on the wheel brake (B1) is minimized or mechanical braking can be completely avoided. This is because the rotating masses of the drivetrain located upstream of the shift clutch (13) no longer have a drive connection to the wheel brake (B1). They can be braked by a control of the drive system (16) as soon as its control allows the reduction of the torque output below the internal coefficient of friction of the drivetrain or – in the case of an electric drive system – the switch to generator operation, i.e., for example, with a time delay of approximately 500 ms to 1,000 ms after the activation condition is detected. In this process, the kinetic energy present in the mass rotation of the drivetrain can also be partially recuperated and fed back into an energy storage device (2) of the vehicle (1).

[0067] By opening the shift clutch (13), any torque initially transmitted by the drive (16) and / or from elasticities of the drivetrain (12) is no longer transmitted beyond the shift clutch (13, 33) towards the vehicle wheel (W1). Simultaneously, the moment of inertia (I) reduced at the vehicle wheel (W1) decreases as shown in the illustration. Fig. 4B has a significantly lower value. In other words, the vehicle wheel (W1) essentially assumes the state of a "free-rotating wheel".

[0068] Due to the residual forces (F_w) still transferable between the vehicle wheel (W1) and the ground (U), which are essentially frictional forces and thus counteract the excess speed (v_o), the circumferential speed (v_c) of the vehicle wheel (W1) approaches the ground speed (v_g) almost immediately. This approach does not require any additional force applied via the wheel brake (B1). Overbraking, as shown in the illustration of Fig. 2 can be advantageously avoided.

[0069] It follows that already at time t2 in the diagram below in Fig. 3. Wheel traction has been restored or wheel slip has decreased to the permissible level, and thus a stable driving condition has been achieved.

[0070] According to the above explanations regarding Fig. 4A and Fig.As can be seen in Figure 4B, positioning the shift clutch (13) in the drive train (12, 32) as close as possible upstream to the vehicle wheel (W1) and its associated wheel brake (B1) reduces the moment of inertia (I). On the other hand, the instantaneously transmitted torque at the shift clutch (13) can assume a greater value, according to the gear ratio in the drive train (12), the closer the shift clutch (13) is arranged to the vehicle wheel (W1) in the direction of power transmission. It has proven particularly advantageous to arrange a shift clutch (13) downstream of a transmission gearbox (15) and, more preferably, directly upstream or directly downstream of a wheel differential (14, 34).At this point, a suitable compromise is achieved between the achievable reduction of the moment of inertia and the effect that may need to be overcome to open the clutch, which can result from the instantaneously transmitted torque. Furthermore, it is possible at this point to open the drive connection to both vehicle wheels (W1, W2 / W3, W4) of the respective driving axle (11, 12) with only one clutch (13). Thus, the clutch (13) can be used jointly to improve driving stability in the event of a loss of traction at one vehicle wheel (W1) and / or the other vehicle wheel (W2).

[0071] A preferred embodiment provides that, before or simultaneously with the opening of the switching clutch (13, 33) as a result of the release condition, the instantaneous output torque of the drive (16, 36) is reduced, in particular by reducing the setpoint value for the output torque towards zero.

[0072] The aforementioned invention can be used on any type of vehicle with one or more drive systems. It is particularly suitable for use on motor vehicles with four-wheel drive and a secondary axle that is operated by an electric drive (16).

[0073] It is also particularly suitable for drive trains (12, 14) in which a transmission gearbox (15) with a fixed gear ratio is provided. It is also particularly suitable for drive trains (12, 32) that are designed as purely switchable drive trains without a torque-matching clutch.

[0074] According to a preferred embodiment, the motor vehicle (1) has at least two temporarily or permanently driven axles (11, 31) and two or more clutches (13, 33). These clutches can each be selectively opened and closed, with at least one clutch being assigned to each driven axle (11, 31) in order to interrupt the drive connection to the vehicle wheel(s) (W1, W2 / W3, W4) of that axle (11, 31).

[0075] It can be provided that several driven axles are present and that a switching clutch is provided for each axle or even each vehicle wheel, as described above. According to one embodiment of the invention, it can be provided that the opening of a switching clutch (13, 33) occurs only for the vehicle wheel or for the group of vehicle wheels on such a number of axles that at any given time at least one vehicle wheel, more preferably two vehicle wheels, or the vehicle wheels of at least one axle (11, 31) remain connected to a drive (16, 36). In this way, it can be ensured that at least a residual capability for the motor drive of the motor vehicle (1) is maintained at any given time, even if the release condition should occur simultaneously or overlapping in time for all driven vehicle wheels (W1, W2 / W3, W4).

[0076] Fig. 6A and Fig.6B explain a use case in which an unstable driving condition with loss of wheel traction occurs during cornering. This use case can occur, for example, if a driver - Excessive acceleration during a cornering maneuver with a constant steering angle, or - Increased steering angle while cornering at a constant speed, or - During cornering, a local reduction in wheel grip occurs due to external environmental influences, e.g., road surface contamination.

[0077] In the representation according to Fig. 6A a condition exists which is essentially the situation of the first vehicle wheel (W1) according to Fig. 1 corresponds. However, the vehicle wheel in the example of Fig.6A due to the steering angle, a wheel force (F_w) is transmitted, which includes a longitudinal force component extending in the circumferential direction of the vehicle wheel (W1), i.e. a longitudinal force (F_long), and essentially a lateral force component perpendicular to it, i.e. a transverse force or lateral guiding force (F_lat).

[0078] The representation according to Fig. 6B shows a state analogous to Fig. 3, i.e., with the clutch (13) open. Here too, due to the instantaneous overspeed (v_o) of the vehicle wheel (W1), a residual force exists which includes a longitudinal component (F_long) that counteracts the overspeed (v_o). In this driving condition as well, opening the clutch (13) promotes the fastest possible approximation of the circumferential speed (v_c) to the ground speed (v_b) in the longitudinal direction and thus a restoration of wheel traction and a stable driving condition.

[0079] It is evident that in the examples where a loss of wheel traction occurs or is imminent, a stable driving condition can be achieved within such a short time interval as a result of the opening of the shift clutch (13) that a supplementary intervention of the wheel brake (B1) is required to a far lesser extent or is no longer necessary at all. Thus, the control method according to the present disclosure can avoid the need for automatic individual wheel braking, which would otherwise be triggered by an ESP system, for example, or significantly reduce its extent or duration. If individual wheel braking is nevertheless required in a specific case, it is often already present at the beginning of the braking effect, i.e.,At the end of the time interval (Tb) a condition exists in which the affected vehicle wheel (W1) has regained a permissible slip or full wheel grip, so that the individual wheel braking can be carried out with a faster and better effect.

[0080] Simulations have shown that even in the WLTC test cycle, the use of the disclosed control method can achieve a 15% reduction in braking force application. This is expected to result in a reduction of brake dust emissions of at least 2% to 10%. Reference sign 1 Motor vehicle 4-wheel drive Motor vehicle, 4-wheel drive 2 Energy storage / traction battery / tank Energy storage / driving battery / tank 11 Driven axle / front axle (driven) Driven axle / Front axle (driven) 12 Drivetrain (front) Drive train (front) 13 front clutch Disconnect clutch (front) 13a Torque input Input / Torque input 13b Torque output Power take-off / torque output 13c Synchronization device Synchronization device 13d Torque compensation device Torque compensation device 14 Wheel differential (front) Wheel differential device (front) 15 Transmission gear (front) Transmission gearbox (front) 15a First gear stage First gear stage 15b Further gear stage Further gear stage 16 Drive system (front) Propulsion drive (front) 31 Front axle / rear axle (driven) Driven axle / Rear axle (driven) 32 Drivetrain (rear) Drive train (rear) 33 Rear clutch Disconnect clutch (rear) 34 Wheel differential (rear) Wheel differential device (rear) 35 Transmission gearbox (rear) Transmission gearbox (rear) 36 Drive system (front) Propulsion drive (rear) B1 Wheel brake (front left) / mechanical brake Wheel brake (front left) / mechanical brake B2 Wheel brake (front right) Wheel brake (front right) B3 Wheel brake (rear left) Wheel brake (rear left) B4 Wheel brake (rear right) Wheel brake (rear right) F_f Wheel force / transmitted force from vehicle wheel to ground Wheel force / force transmitted from wheel to underground F_lat Shear force Lateral force F_long longitudinal force Longitudinal force I Mass moment of inertia, reduced to vehicle wheel Mass of inertia, reduced to vehicle wheel K Kahm Circle Kahm's circle M_d Driving torque Driving torque o_m Rotational speed drive Rotational speed propulsion drive o_w Rotation speed of vehicle wheel Rotational speed vehicle wheel R_w (dynamic) radius of a vehicle wheel (dynamic) radius of vehicle wheel t Time time Tb Time window for brake pressure build-up Time window for brake pressure built-up To Time window for opening the clutch Time window for opening disconnector clutch Tr Time window after clutch release (for re-engaging) Time window after disconnectorclutch opening (for re-coupling) U Subsoil Underground v speed speed v_o Overspeed (difference between circumferential speed and ground speed) Over speed (difference of circumferential speed vs. ground speed) v_g Ground speed of vehicle wheel (speed of movement over ground) Ground speed vehicle wheel(movement speed over ground) v_c circumferential speed Circumferential speed W1 Vehicle wheel (front left) Vehicle wheel (front left) W2 Vehicle wheel (front right) Vehicle wheel (front right) W3 Vehicle wheel (rear left) Vehicle wheel (rear left) W4 Vehicle wheel (rear right) Vehicle wheel (rear right)

Claims

[1] Control method for a shift clutch (13, 33) arranged in a drive train (12, 32) of a motor vehicle (1) between a drive unit (16, 36) and a vehicle wheel (W1, W2 / W3, W4) driven by the drive unit to increase or restore the driving stability of the motor vehicle (1), wherein the motor vehicle (1) further comprises a mechanically retarding wheel brake (B1, B2 / B3, B4) for the individual vehicle wheel (W1, W2 / W3, W4), characterized by that the tax procedure includes the following steps: - Monitoring a trigger condition for a vehicle stability control intervention, wherein the trigger condition includes, ▪ that an impermissible slippage or loss of traction between the vehicle wheel (W1, W2 / W3, W4) and the surface (U) has occurred or is imminent; AND / OR ▪ that a target deceleration power at the vehicle wheel (W1, W2 / W3, W4) is greater than a maximum deceleration power that can be provided at present by the control of the drive system (16, 36); - Upon occurrence of the trigger condition: Opening of the switching clutch (13, 33), wherein the opening of the switching clutch (13, 33) takes place in a time window (To) that is shorter than a time window (Tb) that is necessary to build up the brake pressure required for a brake intervention at the wheel brake (B1, B2 / B3, B4). [2] Control method according to claim 1, further comprising the following steps, which are performed in particular in a time interval (Tr) directly after the clutch has been opened as a result of the release condition: - Monitoring the instantaneous rotational speed(s) (o_w) of the vehicle wheel(s) (W1, W2 / W3, W4) connected to a torque output (13b) of the open clutch (13, 33); - Controls of the drive system (16, 36) to a rotational speed (o_m) that corresponds to the instantaneous rotational speed (o_w) of the vehicle wheel or wheels (W1, W2 / W3, W4) such that a closing condition of the clutch (13, 33) is met, in particular that ▪ a difference between an instantaneous rotational speed of a torque input (13a) and the instantaneous rotational speed of the torque output (13b) of the shift clutch (13, 33) is less than or equal to a predetermined speed difference limit value, AND / OR that ▪ a difference between an instantaneous rotational position of a torque input (13a) and the instantaneous rotational position of the torque output (13b) of the shift clutch (13, 33) is less than or equal to a predetermined rotational position difference limit value. - Closing the shift clutch (13, 33) to restore the drive connection between the drive unit (16, 36) and the vehicle wheel (W1, W2 / W3, W4). [3] Control method according to one of the preceding claims, wherein the drive system (16, 36) is an electric drive system, in particular an electric drive system with singular assignment to the vehicle wheel (W1, W2 / W3, W4) or with singular assignment to only one driving axle (11 / 31) or only a part of the driveable driving axles (11, 31). [4] Control method according to one of the preceding claims, wherein the instantaneous output torque of the drive (16, 36) is reduced before or simultaneously with the opening of the switching clutch (13, 33) as a result of the release condition. [5] Control method according to one of the preceding claims, wherein the motor vehicle (1) has at least two temporarily or permanently driven axles (11, 31) and two or more clutches (13, 33) which are each selectively openable and closeable, wherein at least one clutch is assigned to each driven axle (11, 31) in order to interrupt the drive connection to the vehicle wheel(s) ((W1, W2 / W3, W4) of that axle (11, 31). [6] Control method according to one of the preceding claims, wherein the opening of a switching clutch (13, 33) is carried out only for the vehicle wheel(s) (W1, W2 / W3, W4) on such a number of driving axles (11, 31) that at any time at least the vehicle wheels (W1, W2 / W3, W4) of a driving axle (11, 31) remain connected to a driving drive (16, 36). [7] Control method according to one of the preceding claims, wherein the shift clutch (13, 33) in the drive train (12, 32) in the direction of power transmission from the drive unit (16, 36) to the vehicle wheel (W1, W2 / W3, W4) - downstream to a transmission gearbox (15, 35), in particular downstream to a wheel differential (14, 34); AND / OR - is located upstream of the wheel brake (B1, B2 / B3, B4). [8] Control method according to one of the preceding claims, wherein the switching clutch (13, 33) has a synchronizing device (13c) configured to supply drive energy via its own actuator in order to minimize a residual speed difference and / or a residual rotational position difference between the torque input (13a) and the torque output (13b) of the switching clutch (13, 33), and wherein in particular the synchronizing device (13c) is actuated as soon as the closing condition of the switching clutch (13, 33) is met. [9] Control method according to one of the preceding claims, wherein the switching clutch has a torque compensation device (13d) configured to temporarily absorb a torque momentarily transmitted between the torque input (13a) and the torque output (13b), wherein in particular the torque compensation device (13d) is activated when the release condition occurs and - the currently transmitted torque exceeds a torque limit value, OR - the opening movement of the shift clutch (13, 33) is delayed by an impermissible amount.

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