Control for an all-wheel drive clutch

Abstract

Method for controlling a clutch (AK) in the drive train between a primary axle (PA) and a secondary axle (SA) of a four-wheel drive, in which the drive torque of an engine (VKM) is distributed directly to the primary axle (PA) and via the adjustable clutch (AK) to the secondary axle (SA), taking into account the vehicle speed (v Fzg ) and the respective wheel speeds, the drive torque is distributed between the primary and secondary axles (PA, SA) by adjusting the torque that can be transmitted by the adjustable clutch (AK) as follows: - below a critical drive torque (M krit ) no torque is transferred to the wheels of the secondary axle (SA), - the critical drive torque (M krit ) is calculated from the dynamic tire radii (r PA , r SA ) of the wheels of the primary and secondary axles (PA, SA), where - as critical drive torque (M krit ) a drive torque is calculated, at which, for given dynamic tire radii (r PA , r SA ) of the wheels of the primary and secondary axles (PA, SA) and a determined tire stiffness value (k) which determines the speeds (n PA , n SA ) of the primary and secondary axes (PA, SA) are equal.

Description

[0001] The invention relates to a method for controlling a clutch in the four-wheel drive of a motor vehicle.

[0002] In vehicles driven by the front or rear axle, the front axle (average speed of the two front wheels) rotates slightly differently than the rear axle (average speed of the two rear wheels). The difference in speed is caused by different dynamic tire radii, manufacturing tolerances, varying tire wear, different axle loads, differences in tire pressure, mixed tire configurations, and wheel slip caused by the drive torque. When cornering, the difference in speed results from the different rolling distances traveled by the wheels.

[0003] In all-wheel drive vehicles - e.g. a vehicle with front-wheel drive and a rear-wheel drive that can be engaged via a clutch - a further difference in speed can occur due to a gear ratio difference in the axle drives.

[0004] When driving straight ahead at a constant speed, a speed difference of < 1% is typically observed. In this driving condition, all-wheel drive is not required, but which of the two axles is faster or slower depends primarily on the dynamic radii of the tires and is random. Therefore, with the all-wheel drive clutch engaged or the center differential locked, losses occur; the speed difference is reduced through tire slip.

[0005] If one considers the drive torque transmitted by the cardan shafts during acceleration, it is often evident that the torque of the front axle changes sign during the transition to constant speed, i.e., at the end of the acceleration process. The front axle transitions from traction to overrun. The front axle brakes, the rear axle now pushes a little more, and the drive system is under tension. This results in a reactive torque flow, which overall causes higher torques in the front and rear axles. In a transmission, the efficiency is usually greater in traction than in overrun, so the indicated losses are even greater.

[0006] DE 197 06 720 A1 describes a method for controlling a controllable clutch in the drivetrain between the front and rear axles of a four-wheel drive system. A wheel speed sensor is assigned to each wheel, and the signals from these sensors are evaluated. A theoretical clutch speed difference for wheel-slip-free driving is determined based on the driving speed, the curve radius, and the various wheel radii. A speed difference is determined at which wheel-slip-free cornering results, and the clutch is adjusted to this speed difference value using a generated control signal.

[0007] DE 36 26 025 A1 shows a similarly operating drive device for an all-wheel drive vehicle with a friction disc clutch for the variable transmission of drive torque to the front wheels.

[0008] In the four-wheel drive according to DE 37 21 626 C2, in order to improve braking performance, the clutch in the drive train between the front and rear axles is opened when a predetermined speed reduction gradient is exceeded.

[0009] DE 36 21 225 C1 describes an all-wheel drive system with permanent rear-wheel drive and a front-axle drive that can be engaged via an electrohydraulically controlled clutch. When a slip threshold is exceeded, the clutch is briefly opened, preventing tension in the drivetrain.

[0010] The control of a clutch in the drivetrain between the front and rear axles of a four-wheel drive vehicle according to DE 102 60 196 A1 ensures that the clutch is always subjected to torque, meaning it is never fully disengaged. This is intended to achieve a better transition when all-wheel drive is required.

[0011] In DE 690 25 487 T2, a correction factor is determined for the drive force distribution to determine the true speed based on the difference in speed and the measured speed.

[0012] US 5 752 211 A shows a control system for distributing drive power from an engine between the front and rear wheels of a vehicle. The controller is configured to reduce the drive power transmitted to the secondary drive wheels, which may be the front wheels, by modifying a drive power distribution control when the vehicle is in a predetermined steady-state speed condition in order to improve fuel economy during steady-state operation while maintaining a lively vehicle response to the driver's accelerator pedal input. Preferably, the controller determines an offset amount L having at least one cubic term proportional to the cube of a vehicle speed when the predetermined steady-state condition exists and increases the drive power to the secondary wheels as an excess of the wheel speed difference, the offset amount increasing from zero.

[0013] WO 2012 / 110659 A1 describes a control device configured to: control the auxiliary section to switch the drive system between the first and the second operating mode such that in the first mode the propeller shaft is disconnected from both the torque transmission path and the second group of one or more wheels, control the drive system to switch from the first operating mode to the second operating mode responsive to a value of at least one vehicle operating parameter, characterized in that the control device is configured to control the drive system not to switch back from the second operating mode to the first operating mode unless the at least one vehicle operating parameter does not have a value corresponding to a condition for the second operating mode to be assumed during a period of at least one disconnection delay period.

[0014] DE 601 28 763 T2 shows a front / rear wheel torque distribution control / regulation device of a four-wheel drive vehicle with an electronic control / regulation clutch for controlling / regulating a torque distribution to the front and rear wheels, in which either the front wheels or rear wheels are the main drive wheels and the others are the auxiliary drive wheels, wherein the front / rear wheel torque distribution control / regulation device comprises a driver-requested torque calculator for calculating a driver-requested torque that is transmitted from the main drive wheel to the auxiliary drive wheel via the electronic control / regulation clutch in response to a driver operation, a tire diameter difference measurement sensor for detecting a measurement of a diameter difference between a tire with a different diameter and a tire with a normal diameter,when tires with different diameters are mounted on at least one of the front or rear wheels; and a vehicle speed sensor for detecting a vehicle speed. The front / rear wheel torque distribution control device is such that it further comprises a starting-time limit speed determination unit for determining a starting-time limit speed according to the amount of diameter difference detected by the tire diameter difference sensor and determining a smaller starting-time limit torque when the amount of diameter difference detected by the tire diameter difference sensor is larger, and a starting-time torque distribution control unit that outputs the following instructions to the electronic control clutch until the vehicle speed becomes equal to the starting-time vehicle limit speed:If the vehicle is fitted with tyres of different diameters, the driver demand torque calculated by the driver demand torque calculator if the driver demand torque is less than or equal to the start-up time limit torque and the start-up time limit torque if the driver demand torque is greater than the driver demand torque.

[0015] The object of the present invention is to propose a method for controlling a clutch in the drive train in an improved embodiment compared to the known solutions.

[0016] This problem is solved by the features of method claim 1. Further developments arise from the subclaims.

[0017] The solution according to the invention thus consists in driving-situation-dependent control of the all-wheel-drive clutch to optimize driveline losses. The clutch to be used according to the invention is designed so that the lowest possible torque can be set, which is made possible, for example, by an over-ventilated position. The clutch thus makes it possible to approach a widely open position in which no or only a small amount of residual torque is transmitted between the front and rear axles. Such a low transmission torque can also be achieved, for example, by reducing the amount of lubricating and cooling oil in a clutch pack.

[0018] Depending on the driving situation, the optimal clutch position (in terms of loss) for the current driving condition is calculated and adjusted based on the wheel speeds. This avoids additional drivetrain losses caused by speed differences between the front and rear axles. The invention thus uses a calculation method for the current differences in tire radii between the front and rear axles, as well as a corresponding strategy for controlling the all-wheel drive clutch that distributes drive power to the front and rear wheels.

[0019] Based on wheel speed sensor signals, the difference in dynamic tire radii between the front and rear axles of the respective vehicle can be calculated based on the current tire size, load, and air pressure. Depending on the difference in dynamic tire radii between the front and rear axles, the state of the all-wheel drive clutch is adjusted according to the driving conditions to minimize driveline losses.

[0020] When driving at a constant speed in the lower speed range, it is advantageous according to the invention to completely open and over-ventilate the all-wheel drive clutch in order to avoid any additional drive train losses.

[0021] Above a certain speed (from a certain drive torque), the primary axle (e.g. the rear axle) reaches a slip that is so high that the front axle rotates slower than the rear axle. By setting a clutch torque on the all-wheel drive clutch, the front axle is no longer put into coasting mode, but the losses due to reactive torque can only be minimized. From this limiting speed, which depends on the torque to be transmitted, it no longer makes sense to fully disengage the all-wheel drive clutch. From this limiting speed, the all-wheel drive clutch is only operated with pilot control, which means that the front axle is always driven via the transfer case and the all-wheel drive clutch, and no longer exclusively via the road.

[0022] Furthermore, an embodiment of the invention is explained with reference to the drawings.

[0023] Fig.Figure 1 shows the basic components of an all-wheel-drive vehicle, consisting of an internal combustion engine (ICE), a downstream, automatically or manually shiftable transmission (SG), which drives the wheels of the rear axle (HA), in this case the primary axle (PA), via a cardan shaft (KHA). In the embodiment described here, the rear axle is to be understood as the main drive axle, but this is not intended to limit the general concept of the invention.

[0024] The output of the gearbox SG acts via a cardan shaft KHA on the rear wheels HA of a rear axle - the wheels of the primary axle. The output of the gearbox acts in parallel via an all-wheel drive clutch AK, which in turn transmits an adjustable drive torque via a cardan shaft KVA to the wheels of the front axle - in the example shown the secondary axle. When the all-wheel drive clutch AK is engaged, losses arise - e.g. due to differences in the radius of the wheels. The axles run at the same speed, i.e. there is a higher power loss compared to the power loss at a difference in speed corresponding to the tire radii. Due to the occurrence of tension in the drive train, increased torques in the axle drives and therefore increased losses.

[0025] Furthermore, the Fig.1 a controller S, which is in signal communication with the wheel speed sensors assigned to the wheels of the front and rear axles VA, HA, as indicated by the arrows. The controller S controls the clutch AK in such a way that a specific portion of the torque of the cardan shaft KHA (in the exemplary embodiment, the torque supplied to the primary axle) is diverted via the cardan shaft KVA to drive the front wheels (here, the secondary axle).

[0026] For a vehicle, the following variables and measurements are available or recorded: engine torque, wheel speeds (front and rear axle), steering angle, and the current overall gear ratio. The wheel speeds, in conjunction with the tire radii, also determine the vehicle speed.

[0027] From the wheel speeds, the ratio between primary and secondary axle tire radius r can be determined. PA / r SAcalculate, i.e., the ratio of the tire radii of the rear and front axles HA, VA. Furthermore, it is assumed that the slip in the relevant range is linearly (approximately or completely linearly) dependent on the torque.

[0028] To calculate the speed at which the wheel speeds (front axle VA, rear axle HA) are balanced, the following driving condition is considered: constant speed - all-wheel drive clutch open - drive only via the primary or main drive axle.

[0029] The following applies or results: Δn = n PA - n SA Difference between primary and secondary axle speed nPA=(vFzg / rPA)+(vSchlupf_PA / rPA)

[0030] This includes: v schlupf_PA = v Fzg . * M / k Slip speed at a given vehicle speed and axle torque M k is a linear assumed tire stiffness parameter Δn=vFzg. / rPA*(1+Mcrit / k)−vFzg. / rSA

[0031] If we set slip freedom Δn = 0 we get: rPA / rSA=(1+Mcrit / k)

[0032] From the detectable ratio between primary and secondary axle tire radius r PA / r SAthe critical drive torque and thus the vehicle speed can be determined at which Δn = 0. So: Mcrit=k*(rPA / rSA−1)

[0033] This calculation can be supplemented by taking into account a possible gear ratio difference between the axles: z Number of teeth iSA=zSA output / zSA input iPA=zPA output / zPA input Mkrit=k*((rPA*iSA) / (rSA*iPA)−1)

[0034] The Fig. Figure 2 shows a diagram of possible torque distributions between the primary axle PA and the secondary axle SA. The abscissa is divided into the torque of the axle drive – the primary and secondary axes. The ordinate represents the sum of the torques of the axle drives. The straight line M 50 / 50 corresponds to a 50 / 50 torque split, i.e. the all-wheel drive clutch AK or a corresponding differential (open) divides 50% of the drive power to the primary axle and 50% to the secondary axle.

[0035] The straight line M 100 / 0 corresponds to a torque distribution between the primary and secondary axles of 100% to 0%, meaning the entire drive power goes to the primary axle (here the rear axle), and it is a pure two-wheel drive. The BSA range corresponds to the reactive torque range of the secondary axle, and the BPA range corresponds to the reactive torque range of the primary axle. The torque distribution using the all-wheel drive clutch AK is shown in the diagram according to Fig. 2 within the area enclosed by the straight lines marked Primary and Secondary (dashed, dash-dotted).

[0036] It is at a drive torque less than M krit It is not possible to provide all-wheel drive torque without generating a reactive torque. Only with a drive torque greater than M krit it is possible to transfer drive torque to the road in a sensible way by dividing it between the primary and secondary axles.

[0037] As long as M krit(total torque) is not reached, the torque at the all-wheel-drive clutch AK is reduced as much as possible. Therefore, an all-wheel-drive system with an all-wheel-drive clutch AK is used, which allows the residual torque at the clutch to be reduced as much as possible, thus achieving the greatest possible torque separation.

[0038] If M krit is approximately reached (depending on tolerances to be taken into account, the point cannot be determined exactly), the reduction in the torque specification is removed and pure two-wheel drive is abandoned. Above the M krit At the corresponding limit speed, the drive torque is distributed between the primary and secondary axles according to the requirements of driving dynamics and traction.

[0039] The Fig. 3 shows possible torque setting ranges for a ratio of the dynamic tire radii (r PA * i SA ) / (r SA * i PA ) <1. If one sets i SA and iPA equal, then this is the state where the dynamic radius r SA the secondary axle SA (here the front axle) is greater than the dynamic radius r PA the primary axis PA. Due to the existing geometric conditions, the secondary axis SA rotates more slowly (since the radius r SA larger) than the primary axle PA. With the all-wheel drive coupling AK, it is therefore possible to drive even at speeds below the critical drive torque M krit to make a sensible torque distribution, i.e. to drive in real four-wheel operation. This condition is Fig. 3 is presented and also aimed for.

[0040] With M PA is the course of the moment distribution on the primary axis, with M SAthe torque distribution curve to the secondary axle is shown. In such a situation - there is a detectable tire radius difference - this can be seen even before the critical drive torque M is reached. krit or the corresponding speed, a torque distribution is carried out, ie it is not carried out up to M krit driven in strict or maximum two-wheel mode depending on the all-wheel drive clutch.

[0041] In the described case, even at moments below M krit A low clutch torque is always provided, meaning that a portion of the drive power can be distributed to the secondary axle. The all-wheel drive clutch is not completely disengaged. Furthermore, a small residual torque (distribution) is provided to compensate for the losses of the drivetrain elements to the secondary axle.

[0042] Compensation for the lost torques that arise in the secondary axle's angular drive is thus advantageously achieved via the direct path (transfer case, propshaft to the secondary axle) rather than via the primary axle (transfer case, propshaft to the primary axle, primary axle wheels, road, secondary axle wheels, secondary axle half shafts). The residual torque in the clutch should then correspond to the torque required to rotate the secondary drivetrain components. This generally results in efficiency advantages.

[0043] When applying the invention, it is therefore also advisable to ensure, when selecting or assembling wheels and tires, that the larger wheels are mounted on the secondary axle, in this case the front axle. This allows tires to be delivered to a vehicle assembly plant classified according to an actual size, and the classes with the larger rolling circumferences can be mounted on the secondary axle. List of reference symbols ICE engine, internal combustion engine SG transmission, manual or automatic transmission VA front axle SA secondary axle HA rear axle PA primary axis AK all-wheel drive clutch S Control KVA cardan shaft front axle KHA cardan shaft rear axle

Claims

[1] Method for controlling a clutch (AK) in the drive train between a primary axle (PA) and a secondary axle (SA) of a four-wheel drive, in which the drive torque of an engine (VKM) is distributed directly to the primary axle (PA) and via the adjustable clutch (AK) to the secondary axle (SA), taking into account the vehicle speed (v Fzg ) and the respective wheel speeds, the drive torque is distributed between the primary and secondary axles (PA, SA) by adjusting the torque that can be transmitted by the adjustable clutch (AK) as follows: - below a critical drive torque (M krit ) no torque is transferred to the wheels of the secondary axle (SA), - the critical drive torque (M krit ) is calculated from the dynamic tire radii (r PA , r SA ) of the wheels of the primary and secondary axles (PA, SA), where - as critical drive torque (M krit ) a drive torque is calculated, at which, for given dynamic tire radii (r PA , r SA ) of the wheels of the primary and secondary axles (PA, SA) and a determined tire stiffness value (k) which determines the speeds (n PA , n SA ) of the primary and secondary axes (PA, SA) are equal. [2] Method according to claim 1, wherein for determining the critical drive torque (M krit ) the wheel speed signals from the speed sensors assigned to the wheels of the primary and secondary axles (PA, SA) are evaluated. [3] Method according to claim 1, wherein in determining the critical drive torque (M krit ) the gear ratios (i PA , i SA ) between the primary and secondary axes (PA, SA) must be taken into account. [4] Method according to one of claims 1-3, wherein when the dynamic tire radii (r PA, r SA ) of the wheels of the primary and secondary axles (PA, SA) results in the condition that the radius (r SA ) of the wheels of the secondary axle (SA) is greater than the radius (r PA ) of the wheels of the primary axle (PA), a torque transfer to the secondary axle (SA) is ensured over the entire range of possible vehicle speed ( VFzg ) was carried out.

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