Patent Description:
<CIT> (<CIT>) describes a driving force controller for a four-wheel drive vehicle. The four-wheel drive vehicle includes a friction clutch provided in the middle of an engine drive-train to rear wheels. The driving force controller variably controls the driving force distribution ratio between front wheels and the rear wheels by controlling the clutch engagement force of the friction clutch. The driving force controller detects an acceleration slip of the wheels and executes traction control for reducing the driving force of drive wheels.

<CIT> (<CIT>) describes a method of calculating an estimated vehicle body speed. In a state where no excessive slip or spin is occurring in wheels, the estimated vehicle body speed is calculated from wheel speeds. On the other hand, when an excessive slip or spin is occurring in the wheels, the estimated vehicle body speed is calculated based on an output of a gravity accelerometer.

<CIT> (<CIT>) describes a vehicle body speed estimation apparatus for a vehicle. The vehicle includes a braking and driving force generator capable of generating a driving force and a braking force applied to all the wheels. The vehicle body speed estimation apparatus estimates a vehicle body speed from the wheel speeds of the wheels. The estimated vehicle body speed is used in traction control and anti-lock brake control. <CIT> discloses a transmission control system including a torque distribution control unit, a differential and clutch.

Further advantageous embodiments are set out in the dependent claims. Estimating the vehicle body speed of a four-wheel drive vehicle will be considered. When, for example, all the four wheels are in a slip, an estimated vehicle body speed that is estimated based on wheel speeds is higher than an actual vehicle body speed. In other words, an apparent increase of the estimated vehicle body speed occurs. Such a deviation of an estimated vehicle body speed from an actual vehicle body speed leads to a decrease in the accuracy of vehicle control or information processing using the estimated vehicle body speed.

The invention provides a vehicle body speed estimation method and a vehicle body speed estimation apparatus for ensuring the accuracy of an estimated vehicle body speed of a four-wheel drive vehicle.

A first aspect relates to a vehicle body speed estimation method applied to a four-wheel drive vehicle. The vehicle body speed estimation method includes: acquiring an estimated vehicle body speed based on wheel speeds or a longitudinal acceleration of the vehicle; determining whether an operation condition that includes at least a condition that the estimated vehicle body speed is higher than an operation determination speed is satisfied; performing torque limitation to reduce a torque of a portion of wheels of the vehicle when the operation condition is satisfied; determining whether a stop condition that includes a condition that the estimated vehicle body speed is lower than or equal to a stop determination speed or a condition that a duration of a state where a wheel acceleration of the portion of the wheels is higher than zero is longer than or equal to a set period of time; and stopping the torque limitation when the stop condition is satisfied.

A second aspect relates to a vehicle body speed estimation apparatus applied to a four-wheel drive vehicle. The vehicle body speed estimation apparatus includes one or plurality of processors. The one or plurality of processors is programmed to acquire an estimated vehicle body speed based on wheel speeds or a longitudinal acceleration of the vehicle; determine whether an operation condition that includes at least a condition that the estimated vehicle body speed is higher than an operation determination speed is satisfied; perform torque limitation to reduce a torque of a portion of wheels of the vehicle when the operation condition is satisfied; determine whether a stop condition that includes a condition that the estimated vehicle body speed is lower than or equal to a stop determination speed or a condition that a duration of a state where a wheel acceleration of the portion of the wheels is higher than zero is longer than or equal to a set period of time; and stop the torque limitation when the stop condition is satisfied.

In the first aspect, the torque limitation may reduce the torque of rear wheels without reducing the torque of front wheels.

In the first aspect, the operation determination speed may be calculated based on an integral of the longitudinal acceleration.

In the first aspect, the stop determination speed may be calculated based on an integral of the longitudinal acceleration.

In the first aspect, the stop determination speed may be the sum of a set value and a value calculated based on the integral of the longitudinal acceleration.

In the first aspect, the estimated vehicle body speed may be a first estimated vehicle body speed that is estimated based on the wheel speeds.

In the first aspect, the estimated vehicle body speed may be a second estimated vehicle body speed that is estimated based on an integral of the sum of the longitudinal acceleration and an offset.

In the first aspect, the vehicle body speed estimation method may further include acquiring a first estimated vehicle body speed based on the wheel speeds; calculating a second estimated vehicle body speed based on an integral of the sum of the longitudinal acceleration and an offset; determining whether a slip determination condition that includes at least a condition that the first estimated vehicle body speed is higher than the second estimated vehicle body speed is satisfied; setting the first estimated vehicle body speed for the estimated vehicle body speed when the slip determination condition is not satisfied; and setting the second estimated vehicle body speed for the estimated vehicle body speed when the slip determination condition is not satisfied.

In the first and second aspects, the vehicle body speed estimation method or the one or plurality of processors of the vehicle body speed estimation apparatus may further include determining whether a wheel slip is occurring, and the operation condition may include at least a condition that the wheel slip is occurring and a condition that the estimated vehicle body speed is higher than the operation determination speed.

In the first and second aspects, the operation condition may include a condition that the wheel slip occurs and a duration of a state where the estimated vehicle body speed is higher than the operation determination speed is longer than or equal to a set period of time.

In the first and second aspects, the determining whether the wheel slip is occurring may include: acquiring a first estimated vehicle body speed based on the wheel speeds; calculating a second estimated vehicle body speed based on an integral of the sum of the longitudinal acceleration and an offset; determining whether a slip determination condition that includes at least a condition that the first estimated vehicle body speed is higher than the second estimated vehicle body speed is satisfied; and determining that the wheel slip is occurring when the slip determination condition is satisfied.

According to the aspects of the invention, an estimated vehicle body speed is acquired based on wheel speeds or a longitudinal acceleration. When an operation condition that includes at least a condition that the estimated vehicle body speed is higher than an operation determination speed is satisfied, torque limitation to reduce a torque of a portion of wheels is performed. After that, when a stop condition is satisfied, the torque limitation stops. With such torque limitation, an apparent increase of the estimated vehicle body speed is eliminated, and the accuracy of the estimated vehicle body speed is recovered. In other words, the accuracy of the estimated vehicle body speed is ensured.

According to the aspects of the invention, there is a stop condition of torque limitation, so continuation of torque limitation more than necessary is suppressed. As a result, a stall of the vehicle is suppressed.

Embodiments of the invention will be described with reference to the accompanying drawings.

<FIG> is a conceptual view for illustrating the outline of a vehicle <NUM> and a vehicle body speed estimation apparatus <NUM> according to a first embodiment. The vehicle <NUM> includes a plurality of wheels <NUM>. The wheels <NUM> include front wheels 5F and rear wheels 5R. In the present embodiment, the vehicle <NUM> is configured to be able to run in four-wheel drive. In other words, the vehicle <NUM> is configured to be capable of driving the front wheels 5F and the rear wheels 5R. Typically, the vehicle <NUM> is capable of driving the front wheels 5F and the rear wheels 5R independently of each other.

The vehicle body speed estimation apparatus <NUM> estimates the vehicle body speed of the vehicle <NUM>. A vehicle body speed estimated is, hereinafter, referred to as "estimated vehicle body speed Ve". The estimated vehicle body speed Ve is estimated based on a vehicle status detected by sensors mounted on the vehicle <NUM>. Examples of the vehicle status of the vehicle <NUM> include wheel speeds and a longitudinal acceleration. Typically, the vehicle body speed estimation apparatus <NUM> is mounted on the vehicle <NUM> and acquires the estimated vehicle body speed Ve based on the vehicle status detected by the sensors. As another example, the vehicle body speed estimation apparatus <NUM> may be disposed in a distributed manner in the vehicle <NUM> and an external system.

The estimated vehicle body speed Ve can be used in various vehicle control and information processing. To accurately perform vehicle control or information processing, it is required to acquire a high-accuracy estimated vehicle body speed Ve.

For example, the estimated vehicle body speed Ve is used in traction control over the vehicle <NUM>. Traction control (hereinafter, which may be abbreviated as "TRC") is vehicle running control for reducing a spin of the wheels <NUM> (drive wheels) at the start or acceleration of the vehicle <NUM>. More specifically, a slip ratio of each wheel <NUM> is calculated based on the wheel speed of the wheel <NUM> and the estimated vehicle body speed Ve. When the slip ratio of any one of the wheels <NUM> exceeds a threshold, TRC is activated. The wheel <NUM> of which the slip ratio exceeds the threshold is, hereinafter, referred to as "TRC target wheel 5t". TRC reduces the torque of the TRC target wheel 5t such that the slip ratio of the TRC target wheel 5t decreases to a target slip ratio. An actual torque of the TRC target wheel 5t is able to be reduced by reducing the output of a drive device or applying a braking force. Thus, a spin of the TRC target wheel 5t is suppressed. In this way, the estimated vehicle body speed Ve is used to calculate the slip ratio of each wheel <NUM>. To highly accurately perform TRC, it is required to acquire a high-accuracy estimated vehicle body speed Ve.

In the first embodiment, the estimated vehicle body speed Ve estimated based on the wheel speeds of the wheels <NUM> will be considered. For the sake of convenience, the estimated vehicle body speed Ve that is estimated based on the wheel speeds is, hereinafter, referred to as "first estimated vehicle body speed Ve1". The first estimated vehicle body speed Ve1 based on the wheel speeds is known, and an estimation method therefor is not limited in the present embodiment. For example, the lowest one of the wheel speeds of the wheels <NUM> is acquired as the first estimated vehicle body speed Ve1. As another example, the second lowest one of the wheel speeds of the wheels <NUM> may be acquired as the first estimated vehicle body speed Ve1.

In a situation in which no wheel slip is occurring, the accuracy of the first estimated vehicle body speed Ve1 is high. However, when, for example, all the wheels <NUM> are in a slip, the first estimated vehicle body speed Ve1 that is estimated based on the wheel speeds is higher than an actual vehicle body speed Vt. In other words, an apparent increase of the first estimated vehicle body speed Ve1 occurs. Such a deviation of the first estimated vehicle body speed Ve1 from the actual vehicle body speed Vt undesirably leads to a decrease in the accuracy of vehicle control or information processing using the first estimated vehicle body speed Ve1.

The vehicle body speed estimation apparatus <NUM> according to the present embodiment has a function of reducing a deviation of the first estimated vehicle body speed Ve1 from the actual vehicle body speed Vt. More specifically, the vehicle body speed estimation apparatus <NUM> determines whether an apparent increase of the first estimated vehicle body speed Ve1 is occurring. When the vehicle body speed estimation apparatus <NUM> determines that an apparent increase of the first estimated vehicle body speed Ve1 is occurring, the vehicle body speed estimation apparatus <NUM> performs torque limitation to reduce the torque of a portion of the wheels <NUM>. As a result of performing torque limitation, the wheel speed of a portion of the wheels <NUM> decreases, and the apparent increase of the first estimated vehicle body speed Ve1 is eliminated.

<FIG> is a conceptual view for illustrating the details of torque limitation according to the first embodiment.

Initially, the vehicle body speed estimation apparatus <NUM> determines whether the operation condition of torque limitation is satisfied. The operation condition of torque limitation is that there is an apparent increase of the first estimated vehicle body speed Ve1. To detect such an apparent increase of the first estimated vehicle body speed Ve1, an operation determination speed Va is used.

The operation determination speed Va is set so as to be higher than the first estimated vehicle body speed Ve1 in a state where no wheel slip is occurring and lower than the first estimated vehicle body speed Ve1 in a state where a wheel slip is occurring. At least the operation determination speed Va is not dependent on the wheel speeds. For example, the vehicle body speed estimation apparatus <NUM> calculates the operation determination speed Va based on an integral of the longitudinal acceleration of the vehicle <NUM>. The operation determination speed Va based on the integral of the longitudinal acceleration of the vehicle <NUM> is expressed by, for example, the following mathematical expression (<NUM>).

In the mathematical expression (<NUM>), Gx is a longitudinal acceleration (after filtering) detected by an acceleration sensor mounted on the vehicle <NUM>. α is a gain greater than one. The gain α is set in advance in consideration of an error of the acceleration sensor. The operation determination speed Va expressed by the mathematical expression (<NUM>) may be regarded as an estimated vehicle body speed that is estimated based on the integral of the longitudinal acceleration Gx.

As another example, the operation determination speed Va may be expressed by the following mathematical expression (<NUM>).

In the mathematical expression (<NUM>), β is a set value (positive value). For example, the set value β is a value corresponding to a wheel slip amount by which an adequate grip is generated in the wheels <NUM>. The operation determination speed Va expressed by the mathematical expression (<NUM>) may be regarded as the sum of "the estimated vehicle body speed that is estimated based on the integral of the longitudinal acceleration Gx" and "the set value β corresponding to a set slip amount".

The operation determination speed Va described above is lower, that is, closer to the actual vehicle body speed Vt, than the first estimated vehicle body speed Ve1 in a state where a wheel slip is occurring. The fact that the first estimated vehicle body speed Ve1 exceeds the operation determination speed Va means that an apparent increase of the first estimated vehicle body speed Ve1 is occurring.

From the above viewpoint, the operation condition of torque limitation is set so as to include at least a condition that the first estimated vehicle body speed Ve1 is higher than the operation determination speed Va. The operation condition of torque limitation may include a condition that the duration of a state where the first estimated vehicle body speed Ve1 is higher than or equal to the operation determination speed Va is longer than or equal to a set period of time.

When the operation condition is satisfied, the vehicle body speed estimation apparatus <NUM> performs torque limitation. In torque limitation, the vehicle body speed estimation apparatus <NUM> reduces the torque of a portion of the wheels <NUM> as compared to the torque before performing torque limitation. For example, the vehicle body speed estimation apparatus <NUM> performs torque limitation by multiplying the torque of a portion of the wheels <NUM> by a gain less than one. For example, the vehicle body speed estimation apparatus <NUM> reduces the torque of the rear wheels 5R without reducing the torque of the front wheels 5F. Reducing the torque of the rear wheels 5R not the front wheels 5F is desirable from the viewpoint of vehicle stability. As another example, the vehicle body speed estimation apparatus <NUM> may reduce the torque of the front wheels 5F without reducing the torque of the rear wheels 5R.

As a result of performing such torque limitation, the wheel speed of a portion of the wheels <NUM> decreases and a slip of the portion of the wheels <NUM> is reduced. As a result, the first estimated vehicle body speed Ve1 also decreases and approaches the actual vehicle body speed Vt.

However, unnecessary continuation of torque limitation affects acceleration of the vehicle <NUM> and leads to a stall of the vehicle <NUM> in some cases. Therefore, after the start of torque limitation, the vehicle body speed estimation apparatus <NUM> determines whether a stop condition of torque limitation is satisfied. The stop condition of torque limitation is that the apparent increase of the first estimated vehicle body speed Ve1 is eliminated.

For example, the stop condition of torque limitation includes a condition that the first estimated vehicle body speed Ve1 is lower than or equal to a stop determination speed Vb. The stop determination speed Vb is set based on a similar viewpoint to the operation determination speed Va. For example, the stop determination speed Vb is expressed by the following mathematical expression (<NUM>) or mathematical expression (<NUM>). <MAT> <MAT>.

The vehicle body speed estimation apparatus <NUM> calculates the stop determination speed Vb based on an integral of the longitudinal acceleration Gx of the vehicle <NUM>. The stop determination speed Vb expressed by the mathematical expression (<NUM>) may be regarded as an estimated vehicle body speed that is estimated based on the integral of the longitudinal acceleration Gx. The stop determination speed Vb expressed by the mathematical expression (<NUM>) may be regarded as the sum of "the estimated vehicle body speed that is estimated based on the integral of the longitudinal acceleration Gx" and "the set value β corresponding to a set slip amount".

The operation determination speed Va and the stop determination speed Vb may be the same or may be different. The gain α in the mathematical expression (<NUM>) or the mathematical expression (<NUM>) and the gain α in the mathematical expression (<NUM>) or the mathematical expression (<NUM>) may be the same or may be different. The set value β in the mathematical expression (<NUM>) and the set value β in the mathematical expression (<NUM>) may be the same or may be different. It is possible to adjust the operation determination speed Va and the stop determination speed Vb as needed by adjusting the gain α or the set value β. In any case, the operation determination speed Va and the stop determination speed Vb are lower, that is, closer to the actual vehicle body speed Vt, than the first estimated vehicle body speed Ve1 in a state where a wheel slip is occurring.

When the first estimated vehicle body speed Ve1 is lower than or equal to the stop determination speed Vb, that is, when the stop condition is satisfied, the vehicle body speed estimation apparatus <NUM> stops (cancels) torque limitation. For example, the vehicle body speed estimation apparatus <NUM> stops multiplying the torque of a portion of the wheels <NUM> subjected to torque limitation by the gain less than one. As a result, the first estimated vehicle body speed Ve1 returns to the stop determination speed Vb close to the actual vehicle body speed Vt. In other words, the accuracy of the first estimated vehicle body speed Ve1 improves (recovers). In other words, the accuracy of the first estimated vehicle body speed Ve1 is ensured. Torque limitation is not continued more than necessary, so a stall of the vehicle <NUM> is also prevented.

Particularly, the stop determination speed Vb expressed by the mathematical expression (<NUM>) includes the set value β corresponding to a wheel slip amount by which an adequate grip is generated in the wheels <NUM>. It is possible to prevent an excessive return of the first estimated vehicle body speed Ve1 by using the stop determination speed Vb expressed by the mathematical expression (<NUM>). In other words, it is possible to reliably prevent a stall of the vehicle <NUM> while appropriately recovering the accuracy of the first estimated vehicle body speed Ve1.

The stop determination speed Vb can be lower than the actual vehicle body speed Vt due to factors, such as an acceleration sensor error and a road grade change. In this case, there is a possibility that the stop condition that the first estimated vehicle body speed Ve1 is lower than or equal to the stop determination speed Vb is not satisfied and torque limitation is continued. Therefore, as a modification to the stop condition of torque limitation, a condition that the duration of a state where the wheel acceleration of a portion of the wheels <NUM>, to which torque limitation is applied, is higher than zero is longer than or equal to a set period of time may be added. The fact that the duration of the state where the wheel acceleration is higher than zero is longer than or equal to a set period of time means that a wheel slip has stopped.

In this way, the stop condition of torque limitation according to the present embodiment includes the condition that the first estimated vehicle body speed Ve1 is lower than or equal to the stop determination speed Vb or the condition that the duration of the state where the wheel acceleration of a portion of the wheels <NUM>, to which torque limitation is applied, is higher than zero is longer than or equal to a set period of time. Since such a stop condition is provided, unnecessary continuation of torque limitation is prevented. As a result, a stall of the vehicle <NUM> is prevented.

<FIG> is a timing chart showing an example of torque limitation. The abscissa axis represents time, and the ordinate axis represents various vehicle body speeds (first estimated vehicle body speed Ve1, operation determination speed Va, stop determination speed Vb, and actual vehicle body speed Vt). While the vehicle <NUM> is accelerating, a wheel slip occurs, and an apparent increase of the first estimated vehicle body speed Ve1 occurs. When the duration of a state where the first estimated vehicle body speed Ve1 is higher than the operation determination speed Va is longer than or equal to a set period of time, torque limitation is activated. As a result of torque limitation, the first estimated vehicle body speed Ve1 decreases. When the first estimated vehicle body speed Ve1 is lower than or equal to the stop determination speed Vb, torque limitation stops. Thus, the first estimated vehicle body speed Ve1 returns to a value close to the actual vehicle body speed Vt.

As described above, according to the present embodiment, the first estimated vehicle body speed Ve1 is acquired based on wheel speeds. When the operation condition that includes at least the condition that the first estimated vehicle body speed Ve1 is higher than the operation determination speed Va is satisfied, torque limitation to reduce the torque of a portion of the wheels <NUM> is performed. After that, when the stop condition is satisfied, the torque limitation stops. As a result of such torque limitation, an apparent increase of the first estimated vehicle body speed Ve1 is eliminated, and the accuracy of the first estimated vehicle body speed Ve1 improves (recovers). In other words, the accuracy of the first estimated vehicle body speed Ve1 is ensured.

According to the present embodiment, the stop condition of torque limitation is provided. Therefore, continuation of torque limitation more than necessary is prevented. As a result, a stall of the vehicle <NUM> is prevented. Particularly, it is possible to prevent an excessive return of the first estimated vehicle body speed Ve1 by using the stop determination speed Vb expressed by the mathematical expression (<NUM>). In other words, it is possible to reliably prevent a stall of the vehicle <NUM> while appropriately recovering the accuracy of the first estimated vehicle body speed Ve1.

A portion of the wheels <NUM>, to which torque limitation is applied, may be not the front wheels 5F and may be the rear wheels 5R. Reducing the torque of the rear wheels 5R not the front wheels 5F is desirable from the viewpoint of vehicle stability.

Since the accuracy of the first estimated vehicle body speed Ve1 is ensured, the accuracy of vehicle control or information processing using the first estimated vehicle body speed Ve1 is also ensured.

For example, the first estimated vehicle body speed Ve1 is used in TRC. Specifically, the slip ratio of each wheel <NUM> is calculated based on the wheel speed of the wheel <NUM> and the first estimated vehicle body speed Ve1. Since the accuracy of the first estimated vehicle body speed Ve1 is ensured, the accuracy of TRC is also ensured. This is preferable from the viewpoint of acceleration performance and vehicle stability.

<FIG> is a block diagram showing an example of the configuration of a vehicle control system <NUM> according to the first embodiment. The vehicle control system <NUM> controls the vehicle <NUM>. Typically, the vehicle control system <NUM> is mounted on the vehicle <NUM>. Alternatively, at least part of the vehicle control system <NUM> may be included in a remote system outside the vehicle <NUM>, and the vehicle <NUM> may be remotely controlled. In other words, the vehicle control system <NUM> may be disposed in a distributed manner in the vehicle <NUM> and the remote system.

The vehicle control system <NUM> includes a sensor group <NUM>, a traveling apparatus <NUM>, and a controller <NUM>.

The sensor group <NUM> is mounted on the vehicle <NUM> and detects the vehicle status of the vehicle <NUM>. The sensor group <NUM> includes a wheel speed sensor <NUM>, an acceleration sensor <NUM>, and other sensors. The wheel speed sensor <NUM> detects the wheel speed of each wheel <NUM>. The acceleration sensor <NUM> detects various accelerations including the longitudinal acceleration Gx of the vehicle <NUM>.

The traveling apparatus <NUM> is mounted on the vehicle <NUM>. The traveling apparatus <NUM> includes a drive device <NUM> and a braking device <NUM>. The drive device <NUM> drives the wheels <NUM>. The drive device <NUM> is capable of driving the front wheels 5F and the rear wheels 5R independently of each other. For example, the drive device <NUM> includes a front wheel motor that drives the front wheels 5F and a rear wheel motor that drives the rear wheels 5R. The drive device <NUM> may include in-wheel motors respectively provided in the wheels <NUM>. The braking device <NUM> applies a braking force to each of the wheels <NUM> independently of one another.

The controller <NUM> is a computer that controls the vehicle <NUM>. The controller <NUM> includes one or plurality of processors <NUM> (hereinafter, simply referred to as processor <NUM>) and one or plurality of storage devices <NUM> (hereinafter, simply referred to as storage device <NUM>). The processor <NUM> executes various processing. For example, the processor <NUM> includes a central processing unit (CPU). The storage device <NUM> stores various pieces of information. Examples of the storage device <NUM> include a volatile memory, a nonvolatile memory, a hard disk drive (HDD), and a solid state drive (SSD). The controller <NUM> may include one or plurality of electronic control units (ECUs). Part of the controller <NUM> may be an information processing apparatus outside the vehicle <NUM>. In this case, part of the controller <NUM> communicates with the vehicle <NUM> and remotely controls the vehicle <NUM>.

Vehicle status information <NUM> indicates a vehicle status detected by the sensor group <NUM>. Specifically, the vehicle status information <NUM> includes the wheel speeds of the wheels <NUM>, detected by the wheel speed sensor <NUM>, the longitudinal acceleration Gx detected by the acceleration sensor <NUM>, and the like. The controller <NUM> (processor <NUM>) acquires the vehicle status information <NUM> from the sensor group <NUM>. The vehicle status information <NUM> is stored in the storage device <NUM>.

A vehicle control program <NUM> is a computer program that is run by the processor <NUM>. The controller <NUM> (processor <NUM>) implements various processing by running the vehicle control program <NUM>. The vehicle control program <NUM> is stored in the storage device <NUM>. The vehicle control program <NUM> may be recorded in a computer-readable recording medium.

The controller <NUM> (processor <NUM>) executes torque control (driving force control) over the wheels <NUM>. More specifically, the controller <NUM> calculates a target torque of each wheel <NUM>. The controller <NUM> controls the operations of the drive device <NUM> and braking device <NUM> such that the target torque of each wheel <NUM> is obtained.

The controller <NUM> (processor <NUM>) performs traction control (TRC) as needed. More specifically, the controller <NUM> acquires an estimated vehicle body speed Ve based on the vehicle status information <NUM>. The controller <NUM> calculates the slip ratio of each wheel <NUM> based on the wheel speeds of the wheels <NUM> and the estimated vehicle body speed Ve. When the slip ratio of any one or some of the wheels <NUM> exceeds a threshold, the controller <NUM> activates TRC. The TRC target wheel 5t is the wheel <NUM> of which the slip ratio exceeds the threshold. For example, the controller <NUM> executes torque control such that the slip ratio of the TRC target wheel 5t decreases to a target slip ratio (for example, <NUM>% to <NUM>%).

The controller <NUM> also has a function of the vehicle body speed estimation apparatus <NUM> according to the present embodiment. The controller <NUM> acquires an estimated vehicle body speed Ve based on the vehicle status information <NUM>. In addition, the controller <NUM> performs the above-described torque limitation as needed. Hereinafter, an example of a process related to torque limitation, executed by the controller <NUM>, according to the present embodiment will be described.

<FIG> is a flowchart showing an example of a process related to torque limitation, executed by the controller <NUM>, according to the first embodiment. The process flow shown in <FIG> is repeatedly executed at every predetermined cycle.

In step S100, the controller <NUM> executes a vehicle body speed computing process to calculate various vehicle body speeds based on the vehicle status information <NUM>. Specifically, the controller <NUM> estimates a first estimated vehicle body speed Ve1 based on wheel speeds. The controller <NUM> calculates an operation determination speed Va and a stop determination speed Vb based on the integral of the longitudinal acceleration Gx (see the mathematical expressions (<NUM>) to (<NUM>)). The controller <NUM> calculates a second estimated vehicle body speed Ve2 expressed by the following mathematical expression (<NUM>). Mathematical Expression (<NUM>): Ve2 = (Gx × α + OFST) × (Actual period of time).

In the mathematical expression (<NUM>), Gx is a longitudinal acceleration (after filtering) detected by the acceleration sensor <NUM>. α is a gain greater than one (for example, α = <NUM>). The gain α is set in consideration of an error of the acceleration sensor <NUM>. The offset OFST is added such that the second estimated vehicle body speed Ve2 is not undervalued at the time when the vehicle <NUM> runs from a flat road to a downhill road. The second estimated vehicle body speed Ve2 expressed by the mathematical expression (<NUM>) may be regarded as an estimated vehicle body speed that is estimated based on the integral of the sum of the longitudinal acceleration Gx and the offset OFST.

In step S200, the controller <NUM> executes a slip determination process to determine whether a wheel slip is occurring. For example, the slip determination condition includes at least the condition that the first estimated vehicle body speed Ve1 based on wheel speeds is higher than the second estimated vehicle body speed Ve2 based on the longitudinal acceleration. The slip determination condition may include the condition that the duration of a state where the first estimated vehicle body speed Ve1 is higher than the second estimated vehicle body speed Ve2 is longer than or equal to a set period of time.

When the slip determination condition is not satisfied, the controller <NUM> determines that no wheel slip is occurring. On the other hand, when the slip determination condition is satisfied, the controller <NUM> determines that a wheel slip is occurring. The result of the slip determination process is used later.

In step S300, the controller <NUM> executes an operation condition determination process to determine whether the operation condition of torque limitation is satisfied. When the operation condition of torque limitation is satisfied (Yes in step S300), the process proceeds to step S400. On the other hand, when the operation condition of torque limitation is not satisfied (No in step S300), the process skips step S400 and proceeds to step S500.

The operation condition of torque limitation is defined based on the relationship between the estimated vehicle body speed Ve and the operation determination speed Va. Hereinafter, various examples of the operation condition of torque limitation will be described. In the first embodiment, the estimated vehicle body speed Ve is the first estimated vehicle body speed Ve1 that is estimated based on wheel speeds (Ve = Ve1).

<FIG> is a flowchart for illustrating a first example of the operation condition of torque limitation. The first example of the operation condition of torque limitation is a condition that a wheel slip is occurring and the estimated vehicle body speed Ve is higher than the operation determination speed Va. In step S310, the controller <NUM> determines whether a wheel slip is occurring and the estimated vehicle body speed Ve is higher than the operation determination speed Va. When the result of step S310 is negative (No in step S310), the operation condition is not satisfied (No in step S300). On the other hand, when the result of step S310 is affirmative (Yes in step S310), the operation condition is satisfied (Yes in step S300).

<FIG> is a flowchart for illustrating a second example of the operation condition of torque limitation. The second example of the operation condition of torque limitation is a condition that TRC is in operation, a wheel slip is occurring, and the estimated vehicle body speed Ve is higher than the operation determination speed Va. In step S320, the controller <NUM> determines whether TRC is in operation, a wheel slip is occurring, and the estimated vehicle body speed Ve is higher than the operation determination speed Va. When the result of step S320 is negative (No in step S320), the operation condition is not satisfied (No in step S300). On the other hand, when the result of step S320 is affirmative (Yes in step S320), the operation condition is satisfied (Yes in step S300).

<FIG> is a flowchart for illustrating a third example of the operation condition of torque limitation. The third example is a modification of the first example. The third example of the operation condition of torque limitation is a condition that a wheel slip is occurring and the duration of a state where the estimated vehicle body speed Ve is higher than the operation determination speed Va is longer than or equal to a set period of time. When the result of step S310 is affirmative (Yes in step S310), the controller <NUM> increments a timer (step S330). On the other hand, when the result of step S310 is negative (No in step S310), the controller <NUM> resets the timer (step S340). When the timer is less than a threshold (No in step S350), the operation condition is not satisfied (No step S300). On the other hand, when the timer is greater than or equal to the threshold (Yes in step S350), the operation condition is satisfied (Yes in step S300).

<FIG> is a flowchart for illustrating a fourth example of the operation condition of torque limitation. The fourth example is a modification of the second example. The fourth example of the operation condition of torque limitation is a condition that TRC is in operation, a wheel slip is occurring, and the duration of a state where the estimated vehicle body speed Ve is higher than the operation determination speed Va is longer than or equal to a set period of time. When the result of step S320 is affirmative (Yes in step S320), the controller <NUM> increments the timer (step S330). On the other hand, when the result of step S320 is negative (No in step S320), the controller <NUM> resets the timer (step S340). When the timer is less than a threshold (No in step S350), the operation condition is not satisfied (No step S300). On the other hand, when the timer is greater than or equal to the threshold (Yes in step S350), the operation condition is satisfied (Yes in step S300).

In step S400, the controller <NUM> performs torque limitation. Specifically, the controller <NUM> reduces the torque of a portion of the wheels <NUM> as compared to that before performing torque limitation. For example, the controller <NUM> reduces the torque of the rear wheels 5R without reducing the torque of the front wheels 5F. Reducing the torque of the rear wheels 5R not the front wheels 5F is desirable from the viewpoint of vehicle stability. As another example, the controller <NUM> may reduce the torque of the front wheels 5F without reducing the torque of the rear wheels 5R.

<FIG> is a conceptual view for illustrating examples of limit torques at the time when torque limitation is performed. A front torque Tf and a rear torque Tr are respectively a target torque of the front wheels 5F and a target torque of the rear wheels 5R when TRC is not in operation. The front torque Tf and the rear torque Tr are calculated from an accelerator depression amount or the like. A front TRC torque Tf_trc and a rear TRC torque Tr_trc are respectively a target torque of the front wheels <NUM> and a target torque of the rear wheels 5R when TRC is in operation. The front TRC torque Tf_trc and the rear TRC torque Tr_trc are required from TRC. The front TRC torque Tf_trc and the rear TRC torque Tr_trc are respectively less than the front torque Tf and the rear torque Tr.

Initially, the case where target wheels to which torque limitation is applied are the rear wheels 5R will be described. A limit rear torque TRr is a target torque of torque limitation for the rear wheels 5R. When TRC is not in operation (see <FIG> and <FIG>), the limit rear torque TRr is a product of the front torque Tf and a gain γ or a product of the rear torque Tr and the gain γ. Here, the gain γ is a constant greater than zero and less than one (<NUM> < γ < <NUM>). On the other hand, when TRC is in operation (see <FIG> and <FIG>), coordination is performed. For example, a smaller one of the product of the front TRC torque Tf_trc and the gain γ and the rear TRC torque Tr_trc is output. Alternatively, the limit rear torque TRr may be the product of the rear TRC torque Tr_trc and the gain γ.

Next, the case where target wheels to which torque limitation is applied are the front wheels 5F will be described. A limit front torque TRf is a target torque of torque limitation for the front wheels 5F. When TRC is not in operation (see <FIG> and <FIG>), the limit front torque TRf is a product of the rear torque Tr and the gain γ or a product of the front torque Tf and the gain γ. On the other hand, when TRC is in operation (see <FIG> and <FIG>), coordination is performed. For example, a smaller one of the product of the rear TRC torque Tr_trc and the gain γ and the front TRC torque Tf_trc is output. Alternatively, the limit front torque TRf may be the product of the front TRC torque Tf_trc and the gain γ.

In step S500, the controller <NUM> determines whether torque limitation is in operation. When torque limitation is in operation (Yes in step S500), the process proceeds to step S600. When torque limitation is not in operation (No in step S500), the process in this cycle ends.

In step S600, the controller <NUM> executes the stop condition determination process to determine whether the stop condition of torque limitation is satisfied. When the stop condition of torque limitation is satisfied (Yes in step S600), the process proceeds to step S700. On the other hand, when the stop condition of torque limitation is not satisfied (No in step S600), the process in this cycle ends.

<FIG> is a flowchart for illustrating an example of the stop condition of torque limitation. For example, the stop condition of torque limitation includes a condition that the estimated vehicle body speed Ve is lower than or equal to the stop determination speed Vb or a condition that the duration of a state where the wheel acceleration of a portion of the wheels <NUM>, to which torque limitation is applied, is higher than zero is longer than or equal to a set period of time (see Section <NUM>-<NUM>). In step S610, the controller <NUM> determines whether the stop condition is satisfied.

In step S700, the controller <NUM> stops torque limitation. Thus, the accuracy of the estimated vehicle body speed Ve is ensured.

<FIG> is a conceptual view for illustrating torque limitation according to a second embodiment. In the second embodiment, the second estimated vehicle body speed Ve2 expressed by the mathematical expression (<NUM>) is used as the estimated vehicle body speed Ve. The second estimated vehicle body speed Ve2 is an estimated vehicle body speed that is estimated based on the integral of the sum of the longitudinal acceleration Gx and the offset OFST. The offset OFST is added such that the second estimated vehicle body speed Ve2 is not undervalued at the time when the vehicle <NUM> runs from a flat road to a downhill road. Since the offset OFST is added, the second estimated vehicle body speed Ve2 also tends to be estimated to be higher. In other words, there is a possibility that an apparent increase also occurs in the second estimated vehicle body speed Ve2. Thus, a torque limitation process is also effective to the second estimated vehicle body speed Ve2.

The operation determination speed Va and the stop determination speed Vb are similar to those in the case of the first embodiment. The configuration of the vehicle control system <NUM> is also similar to that in the case of the first embodiment. The description overlapping that of the first embodiment is omitted as needed.

<FIG> is a flowchart showing a process related to torque limitation according to the second embodiment. Step S250 is added between step S200 and step S300 shown in <FIG>.

As described above, in step S200, the controller <NUM> determines whether a wheel slip is occurring. For example, the slip determination condition includes the condition that the first estimated vehicle body speed Ve1 is higher than the second estimated vehicle body speed Ve2. The slip determination condition may include the condition that the duration of a state where the first estimated vehicle body speed Ve1 is higher than the second estimated vehicle body speed Ve2 is longer than or equal to a set period of time.

In step S250, the controller <NUM> switches the estimated vehicle body speed Ve in accordance with the result of step S200. More specifically, when a wheel slip is not occurring (No in step S251), the controller <NUM> sets the first estimated vehicle body speed Ve1 for the estimated vehicle body speed Ve. On the other hand, when a wheel slip is occurring (Yes in step S251), the controller <NUM> sets the second estimated vehicle body speed Ve2 for the estimated vehicle body speed Ve.

When no wheel slip is occurring, the accuracy of the first estimated vehicle body speed Ve1 is high. However, when a wheel slip is occurring, the accuracy of the first estimated vehicle body speed Ve1 that depends on wheel speeds decreases, and the accuracy of the second estimated vehicle body speed Ve2 that does not depend on wheel speeds is higher than that of the first estimated vehicle body speed Ve1. Therefore, the accuracy of the estimated vehicle body speed Ve improves by switching the estimated vehicle body speed Ve from the first estimated vehicle body speed Ve1 to the second estimated vehicle body speed Ve2.

However, as described above, the accuracy of the second estimated vehicle body speed Ve2 is also not always high. Since the offset OFST is added, there is a possibility that an apparent increase also occurs in the second estimated vehicle body speed Ve2. Therefore, torque limitation is also applied to the second estimated vehicle body speed Ve2. As a result of torque limitation, when the wheel speed of a portion of the wheels <NUM> decreases, a wheel slip is eliminated. When the wheel slip is eliminated (No in step S251), the estimated vehicle body speed Ve switches from the second estimated vehicle body speed Ve2 to the first estimated vehicle body speed Ve1 (step S252). In other words, the estimated vehicle body speed Ve is reset to the high-accuracy first estimated vehicle body speed Ve1. Thus, the accuracy of the estimated vehicle body speed Ve is ensured.

<FIG> shows a modification of the second embodiment. In the modification, two-type second estimated vehicle body speeds Ve21, Ve22 are used. The second estimated vehicle body speeds Ve21, Ve22 are respectively expressed by the following mathematical expressions (<NUM>) and (<NUM>). <MAT> <MAT>.

The gain α2 in the mathematical expression (<NUM>) is less than the gain α1 in the mathematical expression (<NUM>). The offset OFST2 in the mathematical expression (<NUM>) is less than the offset OFST1 in the mathematical expression (<NUM>). In other words, the second estimated vehicle body speed Ve22 is further lower than the second estimated vehicle body speed Ve21.

In step S200, the second estimated vehicle body speed Ve21 is used. When the duration of a wheel slip is not longer than or equal to a set period of time (No in step S254), the controller <NUM> sets the second estimated vehicle body speed Ve21 for the estimated vehicle body speed Ve (step S255). On the other hand, when the duration of a wheel slip is longer than or equal to the set period of time (Yes in step S254), the controller <NUM> sets the second estimated vehicle body speed Ve22 for the estimated vehicle body speed Ve (step S256).

Claim 1:
A vehicle body speed estimation method applied to a four-wheel drive vehicle (<NUM>), the vehicle body speed estimation method comprising:
acquiring a first estimated vehicle body speed based on wheel speeds;
calculating a second estimated vehicle body speed based on an integral of the sum of a longitudinal acceleration of the vehicle (<NUM>) and an offset;
determining whether a slip determination condition that includes at least a condition that the first estimated vehicle body speed is higher than the second estimated vehicle body speed is satisfied;
setting the first estimated vehicle body speed for an estimated vehicle body speed when the slip determination condition is not satisfied;
setting the second estimated vehicle body speed for the estimated vehicle body speed when the slip determination condition is satisfied;
determining whether an operation condition that includes at least a condition that the estimated vehicle body speed is higher than an operation determination speed is satisfied;
performing torque limitation to reduce a torque of a portion of wheels (<NUM>) of the vehicle (<NUM>) when the operation condition is satisfied;
determining whether a stop condition that includes a condition that the estimated vehicle body speed is lower than or equal to a stop determination speed or a condition that a duration of a state where a wheel acceleration of the portion of the wheels (<NUM>) is higher than zero is longer than or equal to a set period of time is satisfied; and
stopping the torque limitation when the stop condition is satisfied.