Traction control for an all wheel drive vehicle

Abstract

The invention provides traction control for an all-wheel drive vehicle. A method of controlling torque in an all-wheel drive vehicle limits the duration of traction loss. During a wheel slip event, the controller reduces the commanded torque from the driver demand level. The controller calculates a normalized wheel speed for each wheel based on the measured yaw rate, the measured wheel speed, and the calculated wheel diameter. The controller determines an end of the wheel slip event by comparing the normalized speeds of each wheel with each other.

Description

Technical Field

[0001] The present disclosure relates to vehicle control. More particularly, the present disclosure relates to controlling an all-wheel drive vehicle during periods of reduced or intermittent traction. Background Art

[0002] Like automobiles, ground vehicles are propelled by directing torque from a torque source (such as an electric drive unit or internal combustion engine and gear transmission) to the drive wheels. Intermediate components (such as differentials) can distribute power so that multiple wheels are driven. Alternatively, multiple torque sources can drive each axle or each wheel independently. When there is sufficient friction between the wheels and the ground, a large amount of propulsive force can be applied with a small degree of slip. However, when friction is limited, attempting to apply too much torque will result in the wheels accelerating without the vehicle accelerating. This wheel slip condition can result in excessive wheel speed, reduced vehicle controllability, and reduced propulsive force compared to what would be possible when the wheel speed matches the vehicle speed.

[0003] Many vehicles are equipped with traction control that limits the torque applied to the driven wheels when wheel slip is detected. Controlling the amount of wheel slip requires a way for the controller to calculate the wheel slip value. If the calculated wheel slip differs from the actual degree of wheel slip, the effectiveness of the traction control feature is reduced. In two-wheel drive vehicles, wheel slip can be calculated by measuring the vehicle speed using the non-driven wheels under the assumption that the non-driven wheels do not slip. Summary of the invention

[0004] In a first embodiment, a method controls torque in a vehicle. Torque is distributed from at least one torque source to all wheels of the vehicle. The commanded torque level of the at least one torque source is based on a driver demand torque. In response to detecting wheel slip, the commanded torque level is reduced and controlled based on the degree of wheel slip. A normalized speed of each wheel is calculated based on a measured rotational speed of each wheel. In response to a difference between a maximum normalized wheel speed and a minimum normalized wheel speed being less than a threshold, the commanded torque level is increased to the driver demand torque. The normalized speed of each wheel may be calculated by measuring a yaw rate and adding or subtracting a term proportional to the yaw rate to each measured wheel speed. The term may also be based on a lateral distance between the corresponding wheel and a point around which the yaw rate is measured. Calculating the normalized speed of each wheel may also include multiplying a factor based on a sensed diameter of the corresponding wheel. In response to detecting the absence of wheel slip, the sensed diameter may be updated based on the measured rotational speed. The controller may wait until the difference between the maximum normalized wheel speed and the minimum normalized wheel speed remains less than the threshold for a certain duration before increasing the commanded torque level to the driver demanded torque. The duration may be adjusted in response to vehicle settings, driving conditions and vehicle dynamics.

[0005] In a second embodiment, a method controls torque in a vehicle. Torque is distributed from at least one torque source, wherein a commanded torque level of the at least one torque source is based on a driver demanded torque. In response to detecting wheel slip, a vehicle speed estimate is updated, the commanded torque level is reduced, and a normalized speed of each wheel is calculated. The vehicle speed estimate is updated based on a signal from an accelerometer. The commanded torque level is reduced and controlled based on a difference between an average of measured wheel rotational speeds of each wheel and the vehicle speed estimate. The normalized speed is calculated based on the measured rotational speeds of the wheels. In response to a difference between a maximum normalized wheel speed and a minimum normalized wheel speed being less than a threshold, the commanded torque level is increased to the driver demanded torque. Calculating the normalized speed of each wheel includes measuring a yaw rate and adding or subtracting a term proportional to the yaw rate to each measured wheel speed. The term may also be based on a lateral distance between the corresponding wheel and a point around which the yaw rate is measured. Calculating the normalized speed of each wheel may also include multiplying by a factor based on a sensed diameter of the corresponding wheel. In response to detecting an absence of wheel slip, the sensed diameter may be updated based on the measured rotational speed.

[0006] A vehicle includes a torque source; four wheels; a transmission configured to distribute torque to the four wheels; and a processor. The processor is programmed to command the torque source to produce a certain torque level based on driver demand. In response to detecting wheel slip, the processor reduces the commanded torque level and controls the commanded torque level based on the degree of wheel slip. The processor calculates a normalized speed of each of the four wheels based on the measured rotational speed of each wheel. In response to the difference between the maximum normalized wheel speed and the minimum normalized wheel speed being less than a threshold, the processor increases the commanded torque level to the driver demanded torque. The processor can also be programmed to wait until the difference between the maximum normalized wheel speed and the minimum normalized wheel speed remains less than the threshold for a certain duration or distance, and then increase the commanded torque level to the driver demanded torque. The duration can be adjusted in response to vehicle settings, driving conditions, and vehicle dynamics. BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Figure 1A A block diagram of a first vehicle is shown;

[0008] Figure 1B showing a block diagram of a second vehicle;

[0009] Figure 2A , Figure 2B and Figure 2C is a flow chart of a method for controlling torque from a torque source based on driver demand when traction is good and maximizing traction when wheels are slipping.

[0010] Figure 3 is used to normalize the wheel speed FIG. 2A to FIG. 2C Flowchart of the subroutines of the method. DETAILED DESCRIPTION

[0011] Detailed embodiments of the present invention are disclosed herein; however, it should be understood that the disclosed embodiments are merely examples of the present invention that may be embodied in various forms and alternative forms. The drawings are not necessarily drawn to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, the specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching those skilled in the art to employ the present invention in various ways.

[0012] Figure 1AA first all-wheel drive (AWD) vehicle 10 is schematically shown. Torque for propelling the vehicle is generated by a torque source 12. The torque source may be, for example, an internal combustion engine or an electric motor. The torque source may also include a fixed or variable transmission, such as a transmission. The torque is transmitted to a center differential 14, which distributes the torque between the front wheels and the rear wheels. The power directed to the front wheels is further distributed between the left front wheel 18 and the right front wheel 20 by the front differential 16. The power directed to the rear wheels is further distributed between the left rear wheel 24 and the right rear wheel 26 by the rear differential 22. The differential distributes the torque between the two output shafts, but does not constrain the relative speed of the output shafts. The speed of the input shaft is proportional to the average speed of the output shaft. In addition to the torque distribution transmission, some differentials also include a torque multiplication transmission. Typically, the front wheels are steerable, while the rear wheels are not.

[0013] The controller 28 adjusts the torque output of the torque source 12 based on input from a number of sensors. These sensors include wheel speed sensors 30, 32, 34 and 36 associated with wheels 18, 20, 24 and 26, respectively. The wheel speed sensors measure the rotational speed of the axle to which the wheels are attached. The sensors also include an acceleration and yaw sensor 36. The sensors are configured to provide information from which the controller can determine the longitudinal acceleration rate (forward or rearward) and the yaw rate (turning). The sensors connected to the controller 28 may include Figure 1A Other sensors not shown above, such as steering angle sensor, pedal position sensor, etc.

[0014] Figure 1B A second all-wheel drive (AWD) vehicle 10' is schematically shown. In this vehicle, the front differential 16' and the rear differential 22' comprise torque sources. The two torque sources can be independently controlled based on signals from the controller 28'. In other aspects, Figure 1B Vehicles similar to Figure 1A of vehicles.

[0015] In all-wheel drive vehicles such as Figure 1A and Figure 1B In a vehicle with a wheel slip condition, there are no non-driven wheels that can provide a good estimate of the vehicle speed during a wheel slip condition. An alternative way to estimate the vehicle speed is to update the vehicle speed estimate at regular intervals based on the measured longitudinal acceleration. Unfortunately, any errors that may exist tend to accumulate during wheel slip events, so that the estimated vehicle speed tends to drift away from the actual vehicle speed.

[0016] During a wheel slip event, each torque is controlled based on the average wheel slip of the wheel corresponding to the torque source. (Some wheels may slip more than others.) The average wheel slip can be calculated by subtracting the vehicle speed converted to an equivalent rotational speed based on the wheel diameter from the average wheel speed. If the estimated vehicle speed is greater than the actual vehicle speed, the calculated wheel slip is less than the actual wheel slip. Conversely, if the estimated vehicle speed is less than the actual vehicle speed, the calculated wheel slip is greater than the actual wheel slip. In this condition, the controller can continue to limit torque based on the calculated wheel slip even when traction is restored on all wheels and the actual wheel slip is minimal.

[0017] FIG. 2A to FIG. 2C A flow chart of a traction control method performed by a controller is provided. The method is executed periodically, such as in response to an interrupt occurring every 100 ms. Some information, such as whether traction control is active and the estimated vehicle speed, is saved in memory between executions.

[0018] Figure 2A Operations that occur regardless of whether traction control is active or not are described. At 50, the controller reads the output of each of the wheel speed sensors. At 52, the controller calculates a normalized wheel speed for each wheel to adjust for factors that tend to cause one wheel to spin faster or slower, even under good traction. This calculation will be discussed in more detail below. At 54, the controller calculates the difference Δω between the fastest normalized wheel speed and the slowest normalized wheel speed. N At 56, the controller checks whether traction control is active and branches accordingly to Figure 2B Flowchart or Figure 2C Flowchart of the process.

[0019] Figure 2B describes the operation when traction control is not active. In other words, Figure 2B is performed when there is good traction between the wheels and the ground. The controller sets the torque of the torque source at 58 based on the driver demand. For example, the driver demand can be calculated based on the pedal position and the vehicle speed. At 60, the controller determines whether to enter traction control. This can be done in a variety of ways, including by comparing the wheel speed to the vehicle speed, and by comparing the normalized wheel speeds to each other, i.e., by using ΔωN. Therefore, a flag is set at 62 to indicate that the next time the method is executed, the traction control should be used. Figure 2C At 64, the Figure 2C The counter used in the program is initialized to zero.

[0020] Returning to step 60, the state of wheel slip can be used to inform other software components and use this fact to calculate the Figure 2C At 66, the controller notifies the vehicle speed ω veh During the non-slip condition, the vehicle speed ω veh The normalized wheel speeds may be substituted directly. The exact method of substitution may vary depending on vehicle dynamics, but may be a direct substitution, such as a median wheel speed, or a combination such as an average wheel speed. At 68, the controller informs the wheel normalization logic. Normalizing the wheel slip is necessary for the wheel comparison logic shown to function with high fidelity. If the raw wheel speeds were to be compared, it would be unclear whether the cause of the wheel speed difference was due to a slip event.

[0021] The wheel normalization logic referenced at 68 converts the raw wheel speeds to the values ​​of a reference (imaginary) tire located at the center of the vehicle. This is necessary because the diameters of vehicle tires will differ from one another. This can be caused by a variety of reasons, including manufacturing differences, tire wear, replacement, and intentional mismatches, such as in the case of spare tires. The logic works by determining the state when the raw tire speeds should match but do not. The state must be carefully selected to ensure that vehicle dynamics do not erroneously change the normalization process. If one wheel is always faster than the average wheel speed, the controller infers that the diameter of that wheel is smaller than the diameter of the other wheels. Conversely, if one wheel is always slower than the average, the controller infers that the diameter of that wheel is larger than the diameter of the other wheels. When normalizing the wheel speeds, a secondary consideration must be made. When the vehicle turns, the tire speeds naturally travel different distances and therefore rotate at different speeds. Again, this is not a result of slip and is expected. Therefore, vehicle geometry and dynamics are considered to appropriately adjust the raw wheel speeds during situations where the vehicle is not traveling in a straight line.

[0022] Figure 2C describes the operation when traction control is active. In other words, it is performed when at least one of the wheels is slipping relative to the ground. Figure 2C At 66 ′, the controller notifies the vehicle speed calculation logic to help accurately estimate the vehicle speed ω veh When it is assumed that the wheel is slipping, a different approach must be used than the corresponding step when the wheel is not slipping. At 66', an amount proportional to the sensed longitudinal acceleration is added to the previously calculated ω veh To calculate ω veh At 68, the average normalized wheel speed ω is calculated N aveNote that, depending on the architecture, this average may represent a vehicle average, or it may represent an axle average. At 70, the controller calculates the wheel slip ratio ω based on the calculated wheel slip ratio ω. N ave -ω veh To set the torque of the corresponding torque source. At 72, the difference Δω between the maximum normalized wheel speed and the minimum normalized wheel speed is calculated. N is compared with a second predetermined threshold. N If ω is less than the second threshold and remains less than the second threshold for a period of time, it indicates that traction may have been regained. ave More than ω veh When veh This may also be true due to possible accumulated errors in the calculation of . If ΔωN is less than the second threshold at 72, the counter is incremented at 74. Otherwise, the counter is decremented at 76. At 78, the counter is compared to a third predetermined threshold. If the counter exceeds the value, a flag is set at 80 to indicate that traction control is no longer active and torque should be increased to that demanded by the driver. Alternative embodiments may adjust Threshold 2 and / or Threshold 3 based on driving conditions such as driving mode, ambient temperature, pedal position, estimated friction coefficient, slip value, etc. Alternative embodiments may reset the counter to zero at 76. In another embodiment, the estimated vehicle speed may be used in conjunction with time, resulting in the exit criteria being based on the distance traveled.

[0023] Figure 3 More details are provided regarding the normalized wheel speeds calculated at 52. At 82, the yaw rate is measured by sensor 36. At 84, an adjustment is made to each measured wheel speed based on the yaw rate and the position of the wheel relative to the center of rotation. For example, if the vehicle is yawing to the left, one would expect the wheel on the right to rotate faster than the wheel on the left. To compensate for this, a certain amount is subtracted from the measured wheel speed of the right wheel and added to the wheel speed of the left wheel. At 86, an adjustment is applied to each wheel speed based on the calculated wheel diameter. If a wheel has a smaller diameter than the other wheels, one would expect it to rotate at a higher speed. To adjust for this, each wheel speed is multiplied by a factor that is proportional to the wheel diameter. The adjustments are not necessarily performed in the order shown. In some embodiments, other adjustments may be made. Some embodiments may omit Figure 3 The adjustments mentioned in .

[0024] Although exemplary embodiments are described above, these embodiments are not intended to describe all possible forms of the present invention. Rather, the words used in the specification are descriptive rather than restrictive words, and it should be understood that various changes may be made without departing from the spirit and scope of the present invention. In addition, the features of the various embodiments of implementation may be combined to form other embodiments of the present invention.

[0025] According to the present invention, a method for controlling torque in a vehicle includes: distributing torque from at least one torque source to all wheels of the vehicle according to a commanded torque based on a torque level required by a driver; reducing the commanded torque from the torque level required by the driver in response to detecting wheel slip; and after reducing the commanded torque and in response to a difference between a maximum normalized wheel speed and a minimum normalized wheel speed being less than a threshold, increasing the commanded torque to the torque level required by the driver.

[0026] In one aspect of the invention, the method includes controlling the commanded torque based on a degree of wheel slip after reducing the commanded torque and before increasing the commanded torque.

[0027] In one aspect of the invention, the normalized speed of each wheel is calculated by measuring the yaw rate and adding or subtracting a term proportional to the yaw rate from each measured wheel speed.

[0028] In an aspect of the invention, said term is further based on a lateral distance between the respective wheel and a point about which said yaw rate is measured.

[0029] In one aspect of the invention, calculating the normalized speed of each wheel further comprises multiplying by a factor based on the sensed diameter of the corresponding wheel.

[0030] In one aspect of the invention, the method comprises, in response to detecting an absence of wheel slip, updating the sensed diameter based on the measured rotational speed.

[0031] In one aspect of the invention, the method comprises waiting until a difference between the maximum normalized wheel speed and the minimum normalized wheel speed remains less than the threshold for a duration and then increasing the commanded torque level to the driver demanded torque.

[0032] In one aspect of the invention, the duration is adjusted in response to vehicle settings, driving conditions and vehicle dynamics.

[0033] According to the present invention, a method for controlling torque in a vehicle includes: allocating torque from at least one torque source according to a commanded torque based on a torque level required by a driver; in response to detecting wheel slip: updating a vehicle speed estimate based on a signal from an accelerometer; reducing the commanded torque from the torque level required by the driver based on a difference between an average value of measured wheel rotation speeds of each wheel and the vehicle speed estimate; and after reducing the commanded torque and in response to a difference between a maximum normalized wheel speed and a minimum normalized wheel speed being less than a threshold, increasing the commanded torque to the torque level required by the driver.

[0034] In one aspect of the invention, the method comprises calculating a normalized speed for each wheel by measuring the yaw rate and adding or subtracting a term proportional to the yaw rate from each measured wheel speed.

[0035] In an aspect of the invention, said term is further based on a lateral distance between the respective wheel and a point about which said yaw rate is measured.

[0036] In one aspect of the invention, calculating the normalized speed of each wheel further comprises multiplying by a factor based on the sensed diameter of the corresponding wheel.

[0037] In one aspect of the invention, the method comprises, in response to detecting an absence of wheel slip, updating the sensed diameter based on the measured rotational speed.

[0038] In one aspect of the invention, the method comprises waiting until the difference between the maximum normalized wheel speed and the minimum normalized wheel speed remains less than the threshold for a certain duration or distance and then increasing the commanded torque level to the driver demanded torque.

[0039] In one aspect of the invention, the duration is adjusted in response to vehicle settings, driving conditions and vehicle dynamics.

[0040] According to the present invention, a vehicle is provided, which has: a torque source; four wheels; a transmission device, which is configured to distribute torque to the four wheels; and a processor, which is programmed to: command the torque source to generate a certain torque based on a torque level required by a driver; in response to detecting wheel slip, reduce the commanded torque from the torque level required by the driver based on the degree of wheel slip; and after reducing the commanded torque and in response to the difference between the maximum normalized wheel speed and the minimum normalized wheel speed being less than a threshold, increase the commanded torque to the torque level required by the driver.

[0041] According to an embodiment, the processor is further programmed to wait until the difference between the maximum normalized wheel speed and the minimum normalized wheel speed remains less than the threshold for a certain duration or distance and then increase the commanded torque level to the driver demanded torque.

[0042] According to an embodiment, the duration is adjusted in response to vehicle settings, driving conditions and vehicle dynamics.

Claims

1. A method of controlling torque in a vehicle, the method comprising: distributing torque from at least one torque source to all wheels of the vehicle according to a commanded torque based on a driver demanded torque level; In response to detecting wheel slip, reducing the commanded torque from the driver demanded torque level; and After reducing the commanded torque and in response to a difference between a maximum normalized wheel speed and a minimum normalized wheel speed being less than a threshold, increasing the commanded torque to the driver demanded torque level.

2. The method according to claim 1, further comprising: After reducing the commanded torque and before increasing the commanded torque, the commanded torque is controlled based on the degree of wheel slip.

3. The method of claim 1 , calculating the normalized speed of each wheel by measuring the yaw rate and adding or subtracting a term proportional to the yaw rate to each measured wheel speed. 4 . The method of claim 3 , wherein the term is further based on a lateral distance between the respective wheel and a point about which the yaw rate is measured. 5 . The method of claim 3 , wherein calculating the normalized speed of each wheel further comprises multiplying by a factor based on a sensed diameter of the corresponding wheel.

6. The method according to claim 5, further comprising: In response to detecting an absence of wheel slip, the sensed diameter is updated based on the measured rotational speed. 7 . The method of claim 1 , further comprising waiting until a difference between the maximum normalized wheel speed and the minimum normalized wheel speed remains less than the threshold for a duration and then increasing the commanded torque level to the driver demanded torque.

8. The method of claim 7, wherein the duration is adjusted in response to vehicle settings, driving conditions, and vehicle dynamics.

9. A method of controlling torque in a vehicle, the method comprising: apportioning torque from at least one torque source according to a commanded torque based on a driver demanded torque level; In response to detecting wheel slip: updating a vehicle speed estimate based on a signal from an accelerometer; reducing the commanded torque from the driver demanded torque level based on a difference between an average of the measured wheel rotational speeds of each wheel and the vehicle speed estimate; and After reducing the commanded torque and in response to a difference between a maximum normalized wheel speed and a minimum normalized wheel speed being less than a threshold, increasing the commanded torque to the driver demanded torque level.

10. The method of claim 9 further comprising calculating a normalized speed for each wheel by measuring the yaw rate and adding or subtracting a term proportional to the yaw rate from each measured wheel speed.

11. The method according to claim 10, further comprising: In response to detecting an absence of wheel slip, updating the sensed diameter based on the measured rotational speed; and in Calculating the normalized speed for each wheel further includes multiplying by a factor based on the sensed diameter of the corresponding wheel.

12. The method of claim 9 further comprising waiting until a difference between the maximum normalized wheel speed and the minimum normalized wheel speed remains less than the threshold for a certain duration or distance before increasing the commanded torque level to the driver demanded torque.

13. The method of claim 12, wherein the duration is adjusted in response to vehicle settings, driving conditions, and vehicle dynamics.

14. A vehicle comprising: Torque source; Four wheels; a transmission configured to distribute torque to the four wheels; as well as A processor programmed to: commanding the torque source to produce a torque based on a driver demanded torque level; In response to detecting wheel slip, reducing the commanded torque from the driver demanded torque level based on a degree of wheel slip; and After reducing the commanded torque and in response to a difference between a maximum normalized wheel speed and a minimum normalized wheel speed being less than a threshold, increasing the commanded torque to the driver demanded torque level.

15. The vehicle of claim 14 wherein the processor is further programmed to wait until the difference between the maximum normalized wheel speed and the minimum normalized wheel speed remains less than the threshold for a certain duration or distance and then increase the commanded torque level to the driver demanded torque.

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