Patent Description:
A method of controlling a differential lock is disclosed in <CIT>.

In at least one embodiment a method of controlling a differential lock is provided. The method includes actuating the differential lock to lock a differential assembly when wheel slip of a first wheel assembly is detected and a duration of the wheel slip exceeds a pre-activation buffer. The pre-activation buffer is based on acceleration of the first wheel assembly and vehicle speed.

Wheel slip of the first wheel assembly may be detected by comparing a rotational speed of the first wheel assembly to a rotational speed of a second wheel assembly. Wheel slip of the first wheel assembly may be present when the rotational speed of the first wheel assembly is at least twice the rotational speed of the second wheel assembly. The differential lock may not be actuated to lock the differential assembly when wheel slip is not detected.

The pre-activation buffer may be determined after wheel slip of the first wheel assembly is detected. The pre-activation buffer may delay actuation of the differential lock to lock the differential assembly. A delay provided by the pre-activation buffer may decrease as acceleration of the first wheel assembly increases and wheel speed increases.

The differential lock may not be actuated to lock the differential assembly when wheel speed of the first wheel assembly has been monitored for a first predetermined period of time and the duration of the wheel slip does not exceed the pre-activation buffer.

The method may include determining a differential lock engagement buffer when the duration of the wheel slip exceeds the pre-activation buffer. The differential lock engagement buffer may be based on vehicle speed, acceleration of the first wheel assembly, or both. The differential lock engagement buffer may delay actuation of the differential lock to lock the differential assembly. The delay provided by the differential lock engagement buffer may increase as vehicle speed increases, as acceleration of the first wheel assembly increases, or both.

Actuating the differential lock may include actuating the differential lock to lock the differential assembly when an amount of time that rotational speeds of the first wheel assembly and a second wheel assembly are in the differential lock engagement range exceeds the differential lock engagement buffer.

The differential lock is not actuated to lock the differential assembly when wheel speed of the first wheel assembly has been monitored for a second predetermined period of time and the amount of time that the rotational speed of the first wheel assembly and the second wheel assembly are in the differential lock engagement range does not exceed the differential lock engagement buffer.

The method may include disengaging the differential lock. The differential lock may be disengaged when the vehicle speed exceeds a predetermined speed for a first period of time. The differential lock may be disengaged when the vehicle speed is zero for a second period of time. The first period of time may differ from the second period of time. The first period of time may be less than the second period of time.

The method may include not disengaging the differential lock when the vehicle speed does not exceed a predetermined speed for the first period of time and the vehicle speed is zero for the second period of time.

It will also be understood that, although the terms first, second, etc. are, in some instances, used herein to describe various elements, these elements should not be limited by these terms. For example, a first element could be termed a second element, and similarly a second element could be termed a first element without departing from the scope of the various described embodiments. The first element and the second element are both elements, but they are not the same element.

The terminology used in the description of the various described embodiments is for the purpose of describing particular embodiments only and is not intended to be limiting. As used in the description of the various described embodiments and the appended claims, the singular forms "a" and "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will also be understood that the term "and/or" as used herein refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be further understood that the terms "includes," "including," "comprises," and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof.

Referring to <FIG>, an example of a vehicle <NUM> is shown. The vehicle <NUM> may be a motor vehicle like a truck, farm equipment, military transport or weaponry vehicle, or cargo loading equipment for land, air, or marine vessels. The vehicle <NUM> may include a trailer for transporting cargo in one or more embodiments.

The vehicle <NUM> includes one or more axle assemblies <NUM>, such as a front axle assembly and a rear axle assembly. The axle assemblies <NUM> are illustrated as drive axle assemblies. A drive axle assembly may be configured to provide torque to wheel assemblies <NUM> that may be rotatably supported on the axle assembly <NUM>. A wheel assembly <NUM> may include a tire <NUM> disposed on a wheel <NUM>. An axle assembly <NUM> may be driven using any suitable power source or torque source, such as an internal combustion engine, an electric motor, or combinations thereof.

The axle assembly <NUM> may have any suitable configuration. In the example shown, the axle assembly <NUM> is illustrated as including a housing assembly <NUM>, a differential assembly <NUM>, a pair or axle shafts <NUM>, and a drive pinion <NUM>.

The housing assembly <NUM> receives various components of the axle assembly <NUM>. In addition, the housing assembly <NUM> may facilitate mounting of the axle assembly <NUM> to the vehicle <NUM>.

The axle housing <NUM> may receive and support the axle shafts <NUM>. In at least one configuration, the axle housing <NUM> may include a center portion <NUM> and at least one arm portion <NUM>.

The center portion <NUM> may be disposed proximate the center of the axle housing <NUM>. The center portion <NUM> may define a cavity that may receive the differential assembly <NUM>.

One or more arm portions <NUM> may extend from the center portion <NUM>. For example, two arm portions <NUM> may extend in opposite directions from the center portion <NUM> and away from the differential assembly <NUM>. The arm portions <NUM> may each have a hollow configuration or tubular configuration that may extend around and may receive a corresponding axle shaft <NUM> and may help separate or isolate the axle shaft <NUM> from the surrounding environment. A wheel hub may be rotatably disposed on an arm portion <NUM> and operatively connected to an axle shaft <NUM>. A wheel assembly <NUM> is mountable to the wheel hub.

The differential carrier <NUM> may be mounted to the center portion <NUM> of the axle housing <NUM>. The differential assembly <NUM> may be rotatably supported on the differential carrier <NUM>.

The differential assembly <NUM> is disposed in the housing assembly <NUM>. For instance, the differential assembly <NUM> may be disposed in the center portion <NUM> of the axle housing <NUM>. The differential assembly <NUM> may transmit torque to the axle shafts <NUM> of the axle assembly <NUM> and permit the axle shafts <NUM> and wheel assemblies <NUM> to rotate at different velocities when the differential assembly is unlocked in a manner known by those skilled in the art. For example, the differential assembly <NUM> may have a ring gear <NUM> that may be fixedly mounted on a differential case <NUM>. The ring gear <NUM> and the differential case <NUM> may be rotatable about a differential axis <NUM>. The differential case <NUM> may receive differential gears that may be operatively connected to the axle shafts <NUM>.

The differential assembly <NUM> may include or be associated with a differential lock <NUM>. The differential lock <NUM> may have any suitable configuration, some examples of which are disclosed in <CIT> and <CIT>, which are incorporated by reference in their entirety herein. The differential lock <NUM> is configured to permit the axle shafts <NUM> to rotate at different speeds when unlocked and is configured to inhibit the axle shafts <NUM> of the axle assembly <NUM> from rotating at different speeds when locked. For instance, the differential lock <NUM> may couple an axle shaft <NUM> to the differential case <NUM> so that the axle shaft <NUM> does not rotate with respect to the differential case <NUM> when in an engaged or locked position and may not couple an axle shaft <NUM> to the differential case <NUM> so that the axle shaft <NUM> is permitted to rotate with respect to the differential case <NUM> when in a disengaged or unlocked position.

The differential lock <NUM> may include or be operatively connected to a differential lock actuator <NUM>. The differential lock actuator is configured to actuate the differential lock <NUM> between the engaged (locked) and disengaged (unlocked) positions. The axle shafts <NUM> of the axle assembly <NUM> may rotate together at a common speed or velocity when the differential lock <NUM> is in the engaged or locked position, which may help increase traction. The axle shafts <NUM> of the axle assembly <NUM> may be permitted to rotate at different speeds or different velocities when the differential lock <NUM> is in the disengaged or unlocked position. Unlocking of the differential assembly <NUM> may aid in negotiating a turn and may help reduce tire wear.

The axle shafts <NUM> are configured to transmit torque between the differential assembly <NUM> and a corresponding wheel hub. For example, two axle shafts <NUM> may be provided such that each axle shaft <NUM> extends through a different arm portion <NUM> of axle housing <NUM>. The axle shafts <NUM> may be rotatable about an axis, such as a wheel axis or the differential axis <NUM>.

The drive pinion <NUM> operatively connects the differential assembly <NUM> to the power source or torque source. The drive pinion <NUM> may be received in the housing assembly <NUM>. The drive pinion <NUM> may be rotatable about an axis, such as the axis <NUM>, and may have a gear portion that has teeth that meshes with teeth of the ring gear <NUM> of the differential assembly <NUM>. Torque that is provided by the power source or torque source to the drive pinion <NUM> may be transmitted to the ring gear <NUM> and thus to the differential assembly <NUM>. A transmission may operatively connect the drive pinion <NUM> to the power source or torque source in one or more configurations.

The control system <NUM> controls operation of the axle assembly <NUM>. For example, the control system <NUM> may include one or more microprocessor-based control modules or controllers <NUM> that may be electrically connected to or communicate with components of the vehicle <NUM> and/or the axle assembly <NUM>, such as the differential lock actuator <NUM>. Control system connections are represented by straight arrowed lines in <FIG>. In addition, the control system <NUM> may also process input signals or data from various input devices or sensors, such as one or more speed sensors <NUM>.

A speed sensor <NUM> provides a signal indicative of the rotational speed of a wheel assembly <NUM>. For instance, the speed sensor <NUM> may provide a signal indicative of the rotational speed or rotational velocity of a wheel assembly <NUM> and an associated axle shaft <NUM>. In at least one configuration, the speed sensor <NUM> may be mounted to the axle assembly <NUM> and may detect rotation of a wheel hub upon which the wheel assembly <NUM> is mounted and thus may be indicative of wheel speed. Alternatively or in addition, a speed sensor <NUM> may detect rotation of an axle shaft <NUM>.

A speed sensor <NUM> may be associated with each wheel assembly <NUM> or axle shaft <NUM> of the axle assembly <NUM>. For instance, a speed sensor <NUM> may be provided with an anti-lock brake system or traction control system. As such, the speed sensor <NUM> may detect wheel slip or unexpected rotation of a wheel assembly <NUM> in a manner known by those skilled in the art.

A signal from one or more speed sensors <NUM> may be indicative of vehicle speed or speed of the vehicle <NUM>. For instance, a signal from a speed sensor that is associated with a non-spinning wheel may be indicative of vehicle speed. It is contemplated that a speed sensor <NUM> may be associated with a wheel assembly <NUM> that does not receive propulsion torque (and thus may be less susceptible to slipping), and that the signal from this speed sensor <NUM> may be indicative of vehicle speed. In <FIG>, communication between the controller <NUM> and the speed sensors <NUM> is represented by connection nodes W1 through W4.

Referring to <FIG> and <FIG>, flowcharts of a method of controlling a differential lock are shown. As will be appreciated by one of ordinary skill in the art, the flowcharts may represent control logic which may be implemented or affected in hardware, software, or a combination of hardware and software. For example, the various functions may be affected by a programmed microprocessor. The control logic may be implemented using any of a number of known programming and processing techniques or strategies and is not limited to the order or sequence illustrated. For instance, interrupt or event-driven processing may be employed in real-time control applications rather than a purely sequential strategy as illustrated. Likewise, parallel processing, multitasking, or multi-threaded systems and methods may be used.

Control logic may be independent of the particular programming language, operating system, processor, or circuitry used to develop and/or implement the control logic illustrated. Likewise, depending upon the particular programming language and processing strategy, various functions may be performed in the sequence illustrated, at substantially the same time, or in a different sequence while accomplishing the method of control. The illustrated functions may be modified, or in some cases omitted, without departing from the scope of the present invention. Method steps may be executed by the control system <NUM> and may be implemented as a closed loop control system.

As used below, the term "if" is, optionally, construed to mean "when" or "upon" or "in response to determining" or "in response to detecting," depending on the context. Similarly, the phrase "if it is determined" or "if [a stated condition or event] is detected" is, optionally, construed to mean "upon determining" or "in response to determining" or "upon detecting [the stated condition or event]" or "in response to detecting [the stated condition or event]," depending on the context.

As an overview, a tire can slip or lose traction with respect to the road or support surface that it engages when propulsion torque is provided to an associated wheel assembly. Such a loss of traction may result in unintended spinning of the wheel assembly, which may be referred to as wheel slip. Propulsion torque may be biased toward the slipping wheel when the differential lock is unlocked, which may allow the slipping wheel to continue to spin. Thus, when wheel slip is detected, the differential lock may be actuated to lock the differential assembly so that both axle shafts and their associated wheel assemblies are rotatable together to help reduce spinning and to help gain traction. However, wheel slip alone may not warrant actuating the differential lock the differential assembly. For instance, traction may be regained and wheel slip may stop before the differential assembly can be locked. As another example, data indicative of wheel slip may be inaccurate at startup or when there is limited data available to evaluate whether wheel slip is present. In addition, the differential assembly may not be able to be locked when sufficient synchronization is not attained for a sufficient period of time to engage the differential lock. The method below addresses such issues.

<FIG> is described below starting with the differential lock <NUM> in the disengaged or unlocked position. In addition, the method is described with respect to controlling the differential lock <NUM> of one axle assembly; however, it is to be understood that the method may be applied to multiple axle assemblies such that each differential lock can be independently controlled.

At block <NUM>, the method monitors and compares the wheel speeds of the axle assembly <NUM>. The wheel speeds may be monitored and compared using the speed sensors <NUM>. For example, the signal from a first speed sensor <NUM>, such as that associated with connection node W1, may be indicative of the rotational speed of a first wheel assembly <NUM>, while a signal from a second speed sensor <NUM>, such as that associated with connection node W2, may be indicative of the rotational speed of a second wheel assembly <NUM>. It is expected that both wheel assemblies will rotate at similar but not necessarily identical speeds in the absence of wheel slip.

At block <NUM>, the method determines whether wheel slip is occurring or is detected. Wheel slip may be determined by comparing the rotational speed of two wheel assemblies <NUM>, such as the rotational speed of a first wheel assembly <NUM> to the rotational speed of a second wheel assembly <NUM>. The first wheel assembly <NUM> and the second wheel assembly <NUM> may be mounted to the same axle assembly <NUM>. Wheel slip may be occurring when the rotational speed of the first wheel assembly <NUM> is not sufficiently close to the rotational speed of the second wheel assembly <NUM>. As a nonlimiting example, wheel slip may be present when the rotational speed of one wheel assembly <NUM> is at least twice as fast as another wheel assembly <NUM>. It is also contemplated that wheel slip may also be assessed using a speed sensor <NUM> of another axle assembly <NUM> or a speed sensor that is associated with a wheel assembly <NUM> that is not operatively connected to the differential assembly <NUM>. For instance, it is expected that the rotational speed of all wheel assemblies <NUM> that are in contact with the road or support surface may rotate at approximately the same rotational speeds when wheel slip is not present. Thus expanding on the previous example, wheel slip may be present when the rotational speed of one wheel assembly <NUM> is at least twice as fast as the rotational speed of another wheel assembly <NUM> that is part of another axle assembly, at least twice as fast as an average wheel speed of multiple wheel assemblies <NUM>, or the like. If wheel slip is not detected, then the method may continue at block <NUM>. If wheel slip is detected, then the method may continue at block <NUM>.

At block <NUM>, the differential lock <NUM> may not be actuated to lock the differential assembly <NUM> and thus may be kept in the disengaged or unlocked position. The method iteration may end at block <NUM>.

At block <NUM>, the method determines a pre-activation buffer (PAB). The pre-activation buffer may be variable value that may be based on the vehicle speed and wheel acceleration (i.e., acceleration of the wheel assembly that is slipping, which is the rate of change of the rotational speed provided by an associated speed sensor <NUM>). The pre-activation buffer may be calculated or may be determined using a lookup table and may be bounded by maximum and minimum values. As a nonlimiting example, the minimum buffer may be about <NUM> seconds and the maximum buffer may be about <NUM> seconds.

The pre-activation buffer is used to delay engagement of the differential lock <NUM> or actuation of the differential lock from the unlocked position to the locked position to lock the differential assembly <NUM>. The pre-activation buffer may help ensure that wheel slip is actually occurring. For instance, a false wheel slip condition may be detected when the vehicle is traveling on a paved surface and one wheel encounters an obstacle, such as a curb. As another example, a false wheel slip condition may be detected during low speed vehicle maneuvers, which may be due to misleading signals from a speed sensor <NUM>. Thus, it may be desirable to delay engagement of the differential lock at low vehicle speeds to obtain more confidence that wheel slip is actually occurring. As another example, it may be easier or more likely to regain traction at lower wheel accelerations as compared to higher wheel accelerations or at lower vehicle speeds as compared to higher vehicle speeds. Conversely, it may be harder less likely to regain traction at higher wheel accelerations and/or higher vehicle speeds. Thus, the delay provided by pre-activation buffer may be higher at low vehicle speeds and low wheel accelerations, lower at high vehicle speeds and higher wheel accelerations, and may be associated with intermediate values at low vehicle speeds and high wheel accelerations as well as at high vehicle speeds and low wheel accelerations. As such, the delay provided by the pre-activation buffer may decrease as acceleration and wheel speed of the slipping wheel assembly increases. The method may continue at block <NUM>.

At block <NUM>, the method determines whether the wheel slip duration is greater than the pre-activation buffer (PAB). The wheel slip duration, which may also be referred to as the duration of the wheel slip, may be indicative of the amount of time or length of time that wheel slip has been occurring or detected. If the wheel slip duration does not exceed or is not greater than the pre-activation buffer, then the method may continue at block <NUM>. If the wheel slip duration exceeds or is greater than the pre-activation buffer, then the method may continue at block <NUM>.

At block <NUM>, the method determines whether the wheel speed has been monitored for a first predetermined period of time. This comparison may help better identify whether wheel slip is recurring. For instance, to arrive at block <NUM>, a short duration wheel slip has been detected having a duration that is less than or equal to the pre-activation buffer. Further monitoring may help better interpret the terrain upon which the vehicle <NUM> is traveling. For instance, short duration wheel slips or smaller number of wheel slips may be more likely on a paved road as compared to muddy terrain. On muddy terrain a short duration wheel slip may occur and may be followed by another wheel slip having a longer duration. Accordingly, this step allows the method to check for longer duration wheel slips (that exceed the pre-activation buffer) for a limited amount of time (i.e. for the first predetermined period of time). The first predetermined period of time may be a constant that may be based on vehicle development testing. As a nonlimiting example, the first predetermined period of time may be a value greater than <NUM> seconds, such as <NUM> seconds. If the wheel speed has not been monitored for the first predetermined period of time, then the method may return to block <NUM>. If the wheel speed has been monitored for at least the first predetermined period of time, then the method may continue at block <NUM> and the differential lock <NUM> may remain disengaged.

At block <NUM>, the method determines a differential lock engagement buffer (DLEB). The differential lock engagement buffer may be used to delay actuation of the differential lock <NUM> to lock the differential assembly <NUM> to help improve the likelihood that the differential lock <NUM> can be successfully actuated or engaged to lock the differential assembly <NUM>. The differential lock engagement buffer may be based on vehicle speed and wheel acceleration (i.e., acceleration of the wheel assembly that is slipping); however, the differential lock engagement buffer may be more heavily weighted based on vehicle speed. For instance, the differential lock engagement buffer may be greater (longer) at higher vehicle speeds as compared to lower vehicle speeds. Thus, the delay associated with differential lock engagement buffer may be higher at high vehicle speeds and high wheel accelerations, lower at low vehicle speeds and low wheel accelerations, and may be associated with intermediate values at low vehicle speeds and high wheel accelerations as well as at high vehicle speeds and low wheel accelerations. As such, the delay provided by the differential lock engagement buffer may increase as acceleration and wheel speed of the slipping wheel assembly increases. The differential lock engagement buffer may be calculated or may be determined using a lookup table. The method may continue at block <NUM>.

At block <NUM>, the method compares an amount of time associated with a differential lock engagement range to the differential lock actuator buffer. This step evaluates whether the differential lock <NUM> can be successfully actuated to lock the differential assembly <NUM> after accommodating the delay associated with the differential lock engagement buffer.

For instance, this step may help determine whether the rotational speed of the differential lock is sufficiently close to the rotational speed of another component, such as the differential case or the other axle shaft <NUM> of the axle assembly <NUM>, remain close enough for a long enough period of time to permit engagement of the differential lock <NUM>. As an example, the differential lock <NUM> may be configured as a clutch collar that may rotate with a first axle shaft <NUM>. The differential lock <NUM> may have teeth that are configured to mesh with corresponding teeth on the differential case <NUM>. The teeth of the differential lock <NUM> may mesh with teeth of the differential case <NUM> when the rotational speeds of the differential lock <NUM> and the differential case <NUM> are sufficiently close to permit the differential lock <NUM> to move axially such that the teeth of the differential lock <NUM> can be inserted into corresponding gaps between the teeth on the differential case <NUM> and into meshing engagement.

The differential lock engagement range may be based on the rotational speed of each wheel assembly <NUM> or each axle shaft <NUM> of the axle assembly <NUM>. For instance, the differential lock <NUM> may be engageable when the rotational speed of the first wheel assembly <NUM> and the first axle shaft <NUM> is sufficiently close to the rotational speed of the second wheel assembly <NUM> and the second axle shaft <NUM>, such as when the rotational speeds are the same or within ±<NUM>% of each other (e.g., rotational speeds within ±<NUM>% of each other may result in the differential case <NUM> and differential lock <NUM> rotating at speed in which shifting of the differential lock <NUM> can be successful). The amount of time that the rotational speeds are sufficiently close may be measured by continuing to compare the rotational speeds of the wheel assemblies <NUM>. If the duration or length of the differential lock engagement range exceeds the differential lock actuator buffer or there is sufficient synchronization to actuate the differential lock <NUM> to lock the differential assembly <NUM> when the delay associated with differential lock actuator buffer has elapsed, then the method may continue at block <NUM>. Otherwise, then the method may continue at block <NUM>.

At block <NUM>, the method may determine whether the wheel speeds have been monitored for more than a second predetermined period of time. This block may be similar to block <NUM>. This block may help reduce the likelihood that the differential lock <NUM> is actuated during a transitioning stage in which traction shifts from one wheel assembly <NUM> of the axle assembly <NUM> to the other. The second predetermined period of time associated with block <NUM> may be the same as the first predetermined period of time associated with block <NUM> or may differ from the first predetermined period of time associated with block <NUM>. The second predetermined period of time may be a constant that may be based on vehicle development testing. As a nonlimiting example, the second predetermined period of time may be a value greater than <NUM> seconds, such as <NUM> seconds. If the wheel speeds have not been monitored for the second predetermined period of time, then the method may return to block <NUM>. If the wheel speeds have been monitored for at least the second predetermined period of time, then the method may continue at block <NUM> and the differential lock <NUM> may remain disengaged.

At block <NUM>, the differential lock <NUM> is engaged. The differential lock may be engaged by operating the differential lock actuator <NUM> to actuate the differential lock <NUM> from the disengaged or unlocked position to the engaged or locked position to lock the differential assembly <NUM>. It is contemplated that the differential lock <NUM> may be actuated in anticipation that sufficient synchronization will be present once the differential lock <NUM> reaches the differential assembly <NUM>. The method may continue at <FIG> as represented by flowchart connector symbol B.

Referring to <FIG>, method steps associated with potential disengagement of the differential lock <NUM> are shown. These steps may be associated with vehicle speed and at least one period of time.

At block <NUM>, the method may determine whether the vehicle speed exceeds a predetermined speed for at least a first period of time. The predetermined speed may be a constant or variable value. As a nonlimiting example, the predetermined speed may be a constant such as <NUM>/h. The first period of time may also be a constant or variable value. For instance, the first period of time may be a constant such as <NUM> seconds. Accordingly, the step may check to see whether the vehicle speed is exceeded over a first period of time. When the vehicle speed exceeds the predetermined speed for the first period of time, this is indicative that wheel slip is no longer occurring, or the wheel slip that is occurring is not significantly affecting the vehicle speed, or the wheel slip that is occurring is at least not causing the vehicle speed to drop below the predetermined speed. If the vehicle speed exceeds the predetermined speed for at least the first period of time, then the method may continue at block <NUM>. If the vehicle speed does not exceed the predetermined speed for at least the first period of time, then the method may continue at block <NUM>.

At block <NUM>, the differential lock may be disengaged or unlocked. The differential lock may be disengaged by operating the differential lock actuator <NUM> to actuate the differential lock <NUM> from the engaged or locked position to the disengaged or unlocked position.

At block <NUM>, the method may determine whether the vehicle is stationary for at least a second period of time. The vehicle may be stationary when the vehicle is not moving, such as when the vehicle speed is <NUM>/h. The second period of time may be a constant or variable value and may be the same as the first period of time or may differ from the first period of time. For instance, the second period of time may be greater than the first period of time. As a nonlimiting example, the second period of time may be greater than <NUM> seconds, such as <NUM> seconds. The step may check to see whether the vehicle has been parked or has remained stationary such as at a stoplight for a sufficiently long period of time. If the vehicle is stationary for at least the second period of time, then the method may continue at block <NUM> and the differential lock may be disengaged or unlocked. If the vehicle is not stationary for at least the second period of time, then the method may continue at block <NUM>.

At block <NUM>, the differential lock <NUM> may be kept in the engaged or locked position.

The system and method described above may allow a differential lock to be automatically controlled rather than manually controlled by the vehicle operator. This may avoid attempts to engage the differential lock when differential locking is not needed or when the differential lock cannot be engaged due to a lack of synchronization, thereby helping improve vehicle drivability and help increase the life of the differential lock in the differential lock actuator. In addition, automatic control may help avoid or eliminate situations in which a vehicle operator forgets to unlock the differential assembly, thereby avoiding unnecessary tire wear and helping increase the life of components of the axle assembly, such as the axle shafts. The system and method may use the same speed sensors as a vehicle antilock brake system, which may allow existing sensors to be used to help reduce cost and complexity.

Claim 1:
A method of controlling a differential lock (<NUM>), the method comprising:
actuating the differential lock (<NUM>) to lock a differential assembly (<NUM>) when wheel slip of a first wheel assembly (<NUM>) is detected and a duration of the wheel slip exceeds a pre-activation buffer,
characterized in that the pre-activation buffer is based on acceleration of the first wheel assembly (<NUM>) and vehicle speed.