Electronically controlled limited slip differential

Abstract

The invention relates to an electronically controlled limited slip differential having: a torque receiving housing; a differential gear set having a first side gear and a second side gear for differentially distributing torque to the first side gear and the second side gear; a clutch interposed between the housing and the first side gear, the clutch configured to exert a frictional braking force on the first side gear against the housing to limit differential motion between the first side gear and the second side gear; an actuator configured to apply pressure to the clutch to control the differential motion; and an electronic control unit configured to calculate a required torque value from the input torque, compare the wheel load with a threshold value to determine a correction value, correct the required torque value by the correction value to obtain a required value, and control power supplied to the actuator in response to the required value.

Description

Technical Field

[0001] The disclosure herein relates to an electronically controlled limited slip differential, and more particularly to an electronically controlled limited slip differential capable of changing characteristics according to wheel loads. Background Art

[0002] Devices called limited-slip differentials (LSDs) are commonly used in vehicles to absorb differential motion between the left and right drive wheels and output torque to the other drive wheel even if one of the drive wheels loses traction. Differentials that use a physical mechanism to limit differential motion are sometimes called mechanical LSDs, while differentials that use any electronic device are sometimes called electronically controlled LSDs.

[0003] In an electronically controlled LSD, some additional parameters may be used to optimize the slip limiting force. For example, Japanese Patent Application Publication No. 2006-183784 discloses a technique for controlling the slip limiting force using a torque bias ratio and a lateral acceleration rate generated in a vehicle. Japanese Patent Application Publication No. 2003-329105 discloses a technique for precalculating the power (initial torque) to be applied. Summary of the invention

[0004] Any determined condition about the vehicle should be inferred at the time of actual use of any of the above-mentioned technologies. More specifically, while it is natural to reflect predetermined parameters such as the weight and wheelbase of the vehicle, it is also necessary to infer parameters that are variable in actual use, such as the average weight and number of passengers and the weight of luggage on the luggage compartment. If the inference is close to the actual state of the vehicle, the LSD can work in a preferred manner. However, the vehicle may carry heavier luggage than the inference, or going uphill and downhill may significantly change the weight distribution on the front and rear wheels. In some cases, such as when one vehicle is towing another vehicle, the traction may be heavy. Since the actual condition of the vehicle is different from the condition inferred at the time of design, adverse behavior may not be avoided.

[0005] The disclosure of this article relates to a device generated according to the above problems. According to one aspect, a differential for distributing an input torque to a pair of output shafts according to at least an input torque and a wheel load has: a housing that receives the input torque; a differential gear set that is accommodated in and supported by the housing, the differential gear set including a first side gear and a second side gear that are respectively connected to the output shaft, for differentially distributing the input torque to the first side gear and the second side gear; a clutch that is interposed between the housing and the first side gear and is configured to apply a friction braking force on the first side gear against the housing to limit differential movement between the first side gear and the second side gear; an actuator that is configured to apply pressure to the clutch to control differential movement; and an electronic control unit that is electrically connected to the actuator and includes a storage device that stores at least one threshold value, the electronic control unit being configured to calculate a required torque value according to the input torque, compare the wheel load with the threshold value to determine a correction value, correct the required torque value by the correction value to obtain the required value, and control the power supplied to the actuator in response to the required value. BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Figure 1 This is a block diagram of a vehicle including the differential according to the present embodiment.

[0007] Figure 2 is a block diagram of a vehicle having independent power sources for both the front and rear wheels.

[0008] Figure 3A is a schematic diagram showing the vehicle and wheel loads.

[0009] Figure 3B A schematic diagram of a vehicle and another vehicle drawn therefrom.

[0010] Figure 4 It is a cross-sectional front view of the differential.

[0011] Figure 5 is a diagram schematically showing calculations performed by the electronic control unit.

[0012] Figure 6 is a diagram schematically showing one example of modifying settings for calculation based on the load on the vehicle.

[0013] Figure 7 is a diagram schematically showing an example of multiplication by a coefficient based on the load on the vehicle.

[0014] Figure 8 is a diagram schematically illustrating another example of modifying settings for calculation based on the load on the vehicle.

[0015] Fig. 9Ais a graph schematically showing an example for selecting a correction value according to the load on the vehicle.

[0016] Fig. 9B is a graph schematically showing another example for selecting a correction value according to the load on the vehicle.

[0017] Fig.10 is a flow chart of the calculations and controls performed by the electronic control unit.

[0018] Fig.11 is a flow chart for improving NVH. DETAILED DESCRIPTION

[0019] Exemplary embodiments will be described hereinafter with reference to the accompanying drawings.

[0020] Reference Figure 3A , the engine or motor accounts for a large part of the vehicle's weight and in many cases may be mounted at the front of the vehicle 1. Therefore, its front wheel load Wf is generally somewhat greater than its rear wheel load (indicated by a solid arrow) Wr. In the prior art, the behavior of the differential is optimized under the premise of this weight balance. However, the vehicle 1 is not necessarily unladen, but may be loaded with heavy luggage on its back. Then, as its weight Wb increases, the rear wheel load Wr may become non-negligibly larger (indicated by a dotted arrow). This difference may be significant, especially in vehicle types such as convertible pickup trucks. Or, as Figure 3B In the example shown, the vehicle 1 may be towing another vehicle 9, such as a camper. In this case, the rear wheel load Wr may become larger. The disclosure below relates to an apparatus or system for achieving an optimal slip limiting force in any of a variety of vehicle conditions.

[0021] Reference Figure 1 A conventional vehicle 1 has a power source 3 consisting of an engine and / or a motor and a gear system including a transmission 5 for transmitting torque from the power source 3 to each axle, and also has a plurality of programmable electronic control units (ECUs) for electronically controlling these various parts.

[0022] Torque is extracted from the power source 3 through the transmission 5 and input to the differential 7, thereby distributing the torque to the two drive wheels. Figure 1 In the example shown, the power source 3 is installed at the front of the vehicle 1, and the rear wheels are driving wheels, but the power source 3 can of course be installed at the rear, and in either case, the front wheels can be driving wheels. In the case where the front wheels are used as driving wheels, it goes without saying that the front wheels have a differential 7'. This embodiment can also be applied to a four-wheel drive (or all-wheel drive) vehicle, in which both the front wheels and the rear wheels are used as driving wheels. Alternatively, in order to achieve Figure 2 The four-wheel drive (or all-wheel drive) vehicle shown not only for driving the front wheels but also for driving the rear wheels can be equipped with another set of power source 3', transmission 5 and differential 7', so that the front wheels and rear wheels can be driven independently. This embodiment can be applied to any of these aspects.

[0023] Each ECU has a storage device for storing commands and data and a microcontroller for reading commands from the storage device and executing them. Although the number of ECUs on an ordinary vehicle can reach dozens or more, Figure 1 and Figure 2 Only six of them are shown. These ECUs 11-17 are connected to each other via a bus 19, and exchange or share information with each other by, for example, communication using a so-called controlled area network (CAN).

[0024] Each ECU having information shared with other ECUs controls a corresponding part of the vehicle. ECU 11 can be connected to various acceleration sensors, for example, to read out the longitudinal acceleration, lateral acceleration and yaw acceleration acting on the vehicle. ECU 13 can read out and control the torque generated by the power source 3. ECU 15 can be connected to sensors for reading out load and angular velocity to read out the load and angular velocity on each axis. ECU 17 controls the differential 7, 7'. The readout information can be received via CAN communication through bus 19 and can be used for control by other ECUs, and one ECU can also send requests to other ECUs.

[0025] Reference Figure 4 , the applicable differential 7 is a generally available electronically controlled limited slip differential (LSD). Specifically, although the differential 7 is only an example, the differential 7 has a housing 21 rotatable about an axis X, a differential gear set 23 accommodated in the housing 21, a friction clutch 25 for limiting the differential movement of the differential gear set 23, and an actuator 27 for controllably applying pressure to the clutch 25. The ECU 17 is specifically used to control the actuator 27.

[0026] Although the following description generally relates to an example in which a helical gear differential gear set is applied to the differential 7, of course, a face gear type, a planetary gear type, or any other type may be used instead. Furthermore, although in this example, a multi-plate clutch is applied to a friction clutch, a cone clutch or any other type may be used instead. Furthermore, although the description relates to a gear cam actuator driven by an external motor, the motor may be built into the actuator to directly drive the rotating plate without an intermediate gear, or a hydraulic device may be used to generate thrust without using a cam mechanism, or any other type may be used instead.

[0027] Boss portions 21R and 21L protrude axially from both ends of the housing 21, and are supported by bearings such as ball bearings, respectively, so that the housing 21 can rotate around the axis X. A flange 21F radially extends from the outer periphery of the housing 21, and a ring gear, for example, is fixed to the flange 21F. The end of the transmission shaft is provided with a gear corresponding to the ring gear, and the two gears are meshed with each other, so that the housing 21 receives torque from the power source 3.

[0028] The differential gear set 23 has, for example, a pinion shaft 31 fixed to the housing 21, a pinion 33 rotatable about the shaft, and side gears 35R, 35L meshing with the pinion 33, respectively. For example, each side gear 35R, 35L has a spline on its inner circumference to connect with each output shaft. The output shafts are usually left and right shafts. As can be easily understood, the side gears 35R, 35L are allowed to rotate at a differential speed, so the differential gear set 23 distributes the torque received by the housing 21 to the side gears 35R, 35L and outputs it to the two axles, thereby allowing differential movement.

[0029] The clutch 25 is, for example, a multi-plate clutch interposed between the housing 21 and the right side gear 35R, which is provided with a set of inner plates and outer plates alternately stacked. The right side gear 35R is provided with lugs for engaging with the inner plates and the inner plates are provided with corresponding grooves, so as to mesh with each other. Similarly, the inner periphery of the housing 21 is provided with lugs and the outer plates mesh with the housing 21. When the actuator 27 applies pressure to the plate set, the clutch 25 applies a friction braking force to the right side gear 35R against the housing 21, thereby limiting the differential motion between the side gears 35R, 35L. Needless to say, instead of the right side gear 35R, the clutch 25 can apply a braking force on the left side gear 35L.

[0030] The actuator 27 is provided with, for example, a rotating plate 41, a supporting plate 43, and a cam mechanism 45 interposed therebetween to generate a thrust. The rotating plate 41 is rotatable about the axis X, for example, and is provided with gear teeth 41G around its outer periphery. Figure 4 , but the actuator 27 is also provided with a motor for driving itself, and when the shaft is engaged with the gear teeth 41G, the actuator 27 rotates the rotating plate 41. The support plate 43 is adjacent to the rotating plate 41 and has a structure for preventing rotation, which is engaged with the bearing to prevent rotation. The cam mechanism 45 is provided with, for example, cam slopes formed on the plate 41 or 43 or both, and cam balls rolling on these slopes. When the rotating plate 41 rotates relative to the support plate 43, the cam ball rolls and then goes up and down from the slope, so that the cam mechanism 45 converts the rotational force into thrust.

[0031] The actuator 27 is disposed so close to the clutch 25 as to apply the thrust to the clutch 25, and also close to the end wall of the housing 21. The actuator 27 may also be provided with a plunger 49 for outputting the thrust, and the plunger 49 passes through the end wall of the housing 21 to come into contact with the clutch 25. The actuator 27 thereby applies the thrust corresponding to the rotation angle given by the motor to the clutch 25.

[0032] Needless to say, the cam mechanism may not be formed by a cam ball, and any structure that converts a rotational force into a thrust is applicable. In addition, although the example shown in the figure relates to a structure in which the support plate 43 applies a thrust, the rotating plate 41 may also apply a thrust instead.

[0033] The actuator 27 configured in the manner described above generates thrust according to the rotation angle of the rotating plate 41, and the generated thrust acts on the clutch 25. Generally, as the rotation angle increases from the initial position, the thrust increases accordingly, so the torque ratio transmitted to the side gears 35R, 35L via the clutch 25 increases, while the transmission ratio via the gear set 23 decreases. On the contrary, if the rotation angle of the rotating plate 41 decreases, the transmission ratio via the clutch 25 decreases, while the transmission ratio via the gear set 23 increases.

[0034] The ECU 17 calculates the required torque value T using the data acquired via its communication and the data pre-stored in the storage device. req , and based on the required torque value, the actuator 27 is controlled to dynamically control the clutch torque.

[0035] Main reference Figure 5 , the torque T input to the differential 7 is notified to the ECU 17 via communication. diff The ECU 17 calculates the lock ratio f required to achieve the target TBR based on, for example, the differential torque ratio (or "torque bias ratio"; TBR). LSD , and for example by using the formula T diff ·f LSD To calculate the clutch torque T LSD The TBR may be pre-stored in the storage device of the ECU 17. Different TBRs depending on the forward and reverse directions may be used, and an example of such a set is, but is not limited to, forward TBR=2.7 and reverse TBR=2.3.

[0036] The ECU 17 can also calculate the clutch torque T by using another parameter LSD Correction is performed, and based on the corrected required torque value T reg To adjust the slip limiting force. For example, the required value can be corrected by considering the slip limiting force (initial torque) initially applied. Initial torque T iIt may be stored in advance in the storage device of the ECU 17, or may be stored according to the vehicle speed V v or any such parameters. Fig. 9A As shown by the broken line in v When the initial torque T is relatively low i can be relatively reduced to emphasize the realization of differential motion (or turning performance) between the left and right wheels. However, when the vehicle speed V v When the initial torque T is relatively high i It can be relatively increased to emphasize slip limitation. Therefore, the best slip limitation can be achieved according to the expected grip of the tire.

[0037] About the initial torque T i The calculation is used to convert the vehicle speed V v and initial torque T i The data table associated with each other is stored in advance in the storage device of the ECU 17, and the actual vehicle speed V can be read from the data table. v By comparison, the initial torque T i Alternatively, you can use any of the pre-supplied formulas to calculate. Figure 5 As shown in the example, the initial torque T i Add to clutch torque T LSD To the required torque value T req Make corrections, or use any larger value as the required torque value T req .

[0038] Refer again Figure 5 Similarly, the additional torque T can be considered according to the acceleration (deceleration) of the vehicle. Gx . T Gx It may also be stored in advance in the storage device of the ECU 17, or it may be stored according to the acceleration G x Needless to say, the calculation can be done by any data table or any formula.

[0039] In order to eliminate the required torque value T req In order to avoid excessive fluctuations or undulations in the required value, the corrected required value may be passed through an appropriate low pass filter (LPF). Instead of or in addition to this measure, any upper limit on the required value may be set so as not to apply an excessive input to the actuator 27. For example, the maximum value T may be preset. etm_Lim , and the calculated required torque value and maximum value T etm_Lim Any smaller value of is used as the required torque value T req .

[0040] The above calculation may not be performed by the ECU 17 but by any discrete calculation circuit because it can be formulated. Specifically, under the control of the ECU 17, any external calculation circuit may perform the above calculation.

[0041] Reference Figure 6 In this embodiment, the required torque value T is determined according to the load applied to the driving wheel. req For example, if the luggage box is loaded with heavy luggage, the required torque value T according to the empty state can be req Corrected to a larger value T req '. The correction may include comparing the load applied to the drive wheel with a pre-set appropriate threshold value (weight determination value) and making a determination based on the result to select and use T req or T req '. When the load on the driving wheel is greater than or equal to a certain value, the larger value T req ' can be used to increase the slip limiting force of the differential 7.

[0042] As a value to be compared with the weight determination value, as already described, an output value detected by a load sensor and acquired by the ECU 17 via the bus 19 is available. Alternatively, the load applied to the drive wheel may be estimated by calculation from other parameters and then used for comparison. For example, an output value of a stroke sensor or the like may be used for the calculation.

[0043] Required value T req ' can be calculated by using, for example, the value T req The calculation of is performed in a similar manner, but with the addition of a specific correction value. Alternatively, a set of data tables for the heavy-loaded state may be provided in advance separately from a set of data tables for the unloaded state, so that when the wheel load is less than the threshold value, the former may be used to calculate the value T req , and when the vehicle load is greater than the threshold, the latter can be used to calculate the value T req '.

[0044] Or, when it is greater than the threshold, the correction value T LSD 、T i and T Gx Any one of the following (such as T i ), can be multiplied by a coefficient not less than 1 to obtain the required value T req '. Of course, if Figure 7 As shown in the example, T req The total number can be multiplied by a coefficient not less than 1 to obtain the required value.

[0045] Alternatively, if Figure 8 As shown in the example in, the formula F for the no-load state can be provided in advance 1and the formula F for the overloaded state 2 ,Next, when the wheel load is less than the threshold, the former can be used to calculate the ,required value, and when the wheel load is greater than the threshold, the latter can be used to calculate the ,required value.

[0046] Alternatively, although Figures 6 to 8 The examples shown in involve selecting from two alternatives, but there may be three or more alternatives to choose from, and thus the threshold may not be limited to a single value, but may be two or more separate thresholds.

[0047] Main reference Fig. 9A According to the present embodiment, a control may be applied in which when the load on the drive wheel is relatively small, the required torque T is reduced. req (CV1), while when the load on the driving wheel is relatively large and thus the grip limit of the tire is high, increasing the required torque T req To enhance the sliding limiting force (CV2). Alternatively, Fig. 9B As shown in the example in , applicable is a control in which the required torque is not intentionally changed in a low speed region, while a large difference can be given in a high speed region.

[0048] The following reference Fig.10 The control steps performed by the ECU 17 are described. The ECU 17 first acquires or calculates parameters such as input torque and vehicle speed using CAN communication or the like (S1). Next, the ECU 17 selects any pre-stored data table or pre-provided compliance conditions to calculate the clutch torque (S2). If necessary, it calculates correction values ​​for initial torque, additional torque, etc. and these coefficients.

[0049] Next, it acquires or calculates the load (weight determination value) applied to the drive wheel (S3), and compares it with a threshold value provided in advance to determine a way to correct the clutch torque (any one of S5-1, S5-2, and S5-3). Based on the determined correction method, it corrects the required torque value to calculate the required value (S6). If necessary, the calculated value is subjected to filtering processing, etc. to obtain the final required value (S7).

[0050] Reference Fig.11, additional control can be performed. For example, when excessive differential movement occurs between the side gears 35R, 35L in the differential 7, or when the torque applied to the friction clutch 25 fluctuates excessively, noise, vibration, or harshness (commonly referred to as "NVH", named after the initials of the respective terms) often occurs. Therefore, any control for preventing the generation of NVH can be added. For example, any condition in which NVH may occur can be pre-set, and additional control can be performed to determine whether the condition is met (S8), maintaining the required value (S9-2) in the case where NVH will not occur (false), and reducing (S9-1) or increasing (S9-3) the required value in the case where NVH will occur (true).

[0051] According to the obtained required value, the ECU 17 applies electric power to the actuator to control the rotation angle of the rotating plate 41, thereby adjusting the slide restricting force. These steps are stably repeated, thereby dynamically adjusting the slide restricting force.

[0052] As can be understood from the above description, according to the present embodiment, the setting is selected according to the load applied to the drive wheel, so that the slip limiting force can be optimized regardless of the condition of the vehicle. For example, when the load is relatively small, it can achieve control that focuses mainly on the steering performance, and when the load is relatively large, it can also achieve control that focuses mainly on the traction. Regardless of the condition of the vehicle, the present embodiment embodies the optimal control.

[0053] While certain exemplary embodiments have been described above, modifications and variations of the embodiments will occur to those skilled in the art in light of the above teachings.

Claims

1. A differential for distributing an input torque to a pair of output shafts based at least on an input torque and a wheel load, the differential comprising: a housing receiving the input torque; a differential gear set accommodated in and supported by the housing, the differential gear set comprising a first side gear and a second side gear respectively connected to the output shaft for differentially distributing the input torque to the first side gear and the second side gear; a clutch interposed between the housing and the first side gear and configured to apply a friction braking force on the first side gear against the housing to limit differential movement between the first side gear and the second side gear; an actuator configured to apply pressure to the clutch to control the differential movement; as well as an electronic control unit electrically connected to the actuator and including a storage device storing at least one threshold value, the electronic control unit being configured to calculate a required torque value based on the input torque, compare the wheel load with the threshold value to determine a correction value, correct the required torque value by the correction value to obtain a required value, and control the power supplied to the actuator in response to the required value.

2. The differential according to claim 1, wherein: The storage device is configured to add the correction value to the required torque value to obtain the required value when the wheel load is greater than the threshold value.

3. The differential according to claim 1, wherein: The correction value includes a vehicle speed correction value, and the electronic control unit is configured to multiply the vehicle speed correction value by a coefficient not less than 1 to obtain the required value when the wheel load is greater than the threshold value.

4. The differential according to claim 3, wherein: The storage device stores a vehicle speed data table, and the electronic control unit is configured to compare the vehicle speed with the vehicle speed data table to obtain the vehicle speed correction value.

5. The differential according to claim 1, wherein: The storage device stores two or more vehicle speed data tables, and the electronic control unit is configured to select one of the vehicle speed data tables in response to the comparison of the wheel load with the threshold value, calculate a vehicle speed correction value based on the vehicle speed and the selected vehicle speed data table, and add the vehicle speed correction value to the required torque value to obtain the required value.

6. The differential according to any one of claims 2 to 5, wherein: The electronic control unit is connected to other electronic control units via a network, and is configured to calculate the vehicle speed, the input torque, and the wheel load based on data acquired from the network.

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