Vehicle driveline torque managing process

A vehicle driveline torque managing process provides a motor driven actuator that manages clamping forces exerted on a clutch pack and regulates a coupling of power between a driveshaft and one of the axles of a vehicle, based on clutch positions. The process derives coupling torque values by stepping a motor from a maximum clutch separation position through a series of clutch positions while recording a motor driven commanded torque value at each step, up to a maximum clutch compression position having a maximum commanded motor torque value. Then, each motor driven commanded torque value is converted into respective clutch force values as a function of clutch position, thereby relating each clutch torque to a clutch position based on conversion tables stored in a controller that are associated with physical factors that influence clutch torque for each of the respective clutch actuator positions.

FIELD OF THE INVENTION

The present invention relates to a driveline control actuator managing torque conduction through a vehicle drive axle assembly. More particularly, the present invention relates to a driveline control actuator managing a clutch clamping force exerted on a vehicle clutch pack for coupling power between a driveshaft and one of the rear axles in a vehicle.

BACKGROUND OF THE INVENTION

It is known that hydraulic actuators are employed in various applications utilizing a source of a pressurized hydraulic fluid, for example, as in friction clutch assemblies and hydraulic brake systems. Also, it is common to create the hydraulic pressure to drive the hydraulic actuators with an electric motor, while utilizing a motor driven screw shaft, a non-rotatable nut threaded to the screw shaft, and a hydraulic piston slidably mounted within a hydraulic cylinder and fixed to the nut.

When the electric motor is actuated, the rotary motion of the screw shaft is transmitted to the nut that linearly travels along the screw shaft. Since the piston is fixed to the nut, it also moves along the screw shaft, thus generating a desired hydraulic pressure which translates to torque from the hydraulic piston applying a clamping force on a friction clutch, as taught in U.S. Pat. No. 8,118,571 to Krisher, which is incorporated herein by referenced.

Specifically, Krisher teaches that the torque capacity of a clutch pack is proportional to the hydraulic fluid pressure applied to a second piston, along the screw shaft. A small amount of torque generated by the electric motor can, therefore, result in a significant amount of force on the second piston. Torque amplification is realized by the lead of the motor drive screw, while force amplification is realized by the ratio of a surface area of the piston head of the first piston to a surface area of the second piston.

Although conventional hydraulic actuators have in the past adequately controlled the transfer of torque from a driveshaft to a vehicle rear wheel, it would be advantageous to have actuators that possess faster response times to control the transfer of torque to a vehicle rear wheel, while providing large travel distances to minimize disengage drag torque. Also, it would be a benefit to detect the position of initial clutch engagement and to compensate for mechanical wear due to friction, malfunction, and/or mis-assembly within a differential and its associated parts.

SUMMARY OF THE INVENTION

A driveline control actuator managing torque conduction through a vehicle drive axle assembly disconnect unit embedded controller (DUEC), which is comprised of an electronic circuit board with electronic components, manages torque conduction through a vehicle drive axle assembly, for example, a differential drive axle assembly. The DUEC controls an attached electric motor driven actuator that manages clamping forces exerted on a clutch pack. The clutch pack regulates a coupling of power between a driveshaft and one of the rear axles of a vehicle, based on clutch position. As a result, the coupling control provides a means to realize vehicle stability and mobility functions.

In a first embodiment, at a time of initialization the DUEC derives coupling torque values (aka coupling torque limits) (CTL), which are torque values from various clutch position measurements and motor torque estimations that serve as indices in a position/force table. To derive the CTL, a drive motor is driven from a full open maximum clutch separation zero reference position, where an estimated motor command torque is from zero through a series of “n” step positions, which would be measured at the motor or elsewhere in a clutch engagement mechanism, and where the motor command torque increases from zero torque to a maximum clutch compression position having a maximum command torque ((0, 1×Tmax/n, (2×Tmax/n), . . . (n×Tmax/n)).

Clutch torque transmission capacity is related to clutch compression, and clutch compression is related to motor torque at each motor position. Consequently, these 0 to “n” motor command torques are converted into 0 to “n” clutch forces, so the indices in the position/force table relate motor position to clutch force, and clutch force can then be used to determine clutch torque capacity.

Then, throughout normal operation, each measured clutch position serves as an index into the position/force table (created upon initialization) for looking-up a corresponding force. In other words, a look-up table is developed, which results in model parameters that are stored within the controller. This look-up table, which is developed in the referencing sequence, would contain motor position versus motor torque data (as shown inFIG. 6). Also, this look-up table would be recalibrated at intervals and used as an input to the controller to allow appropriate transformation from command motor torque to motor position across a range of environmental and machine conditions, throughout machine life given changing characteristics due to wear. Hence, the look-up table provides the position/force relationship to accurately select the present force.

Each present force is then converted into an individual CTL using a coefficient that accounts for robust assembly variations, along with a coefficient (kT) derived from clutch temperature that accounts for clutch variations associated with specific temperatures. Other factors that influence clutch variations could also be used to create additional coefficients for use in this manner. From these present force selections, improvements in accuracy over a clutch operating temperature range are then also realized.

In another embodiment, at the time of initialization, the DUEC derives coupling torque limits (CTL) by commanding a motor to drive the clutch pack to a fully open position, wherein the electric motor driven actuator has a mechanical stop that stops the motor in the maximum clutch separation position (see, for example, stop 102 in FIG. 1A of U.S. Pat. No. 8,490,769 to Pritchard, which is incorporated by reference herein). Once against the mechanical stop, a clutch position is set to 0. Next, the motor is caused to drive the clutch pack toward a fully compressed state where an algorithm based on feedback control (see the block diagram control system illustrated inFIG. 5, possibly a proportional-integral-derivative controller (PID control)) is utilized to drive the motor toward being fully compressed through a series of “k” incrementing clutch compressing positions (for example, k=32). The “Calculate Required Force” may use the equation Clamping force=CTL/(Friction Coefficient×Effective Radius×Number of Friction Surfaces) divided into steps toward the CTL of a fully compressed state.

During each of these “k” clutch positions, the DUEC waits for the motor to stop moving and then another clutch position is recorded along with a commanded motor torque. From these steps is developed a second embodiment table having 32 positions associated with each torque step, wherein it is then known what the clutch position is for each motor torque level of the step sequence.

Further, the amount of torque that can be transmitted through a clutch is Clamping Force×Clutch Friction Coefficient×Effective Radius×Number of Friction Surfaces, wherein the maximum amount of torque that can be transmitted through the clutch is the CTL value, and the clutch friction coefficient is affected by clutch temperature and other influences.

Then, throughout normal operation, each measured clutch position provides a CTL to the controller, which is calculated from the clamping force from the equation Clamping Force=CTL/(Friction Coefficient×Effective Radius×Number of Friction Surfaces). The friction coefficient comes from a separate table, wherein a read clutch temperature is used to look up the friction coefficient from this separate table. Other influences can be included in a similar manner.

In a third embodiment, at a time of initialization, the DUEC derives coupling torque limits (CTL) by commanding a motor to drive the clutch pack to a fully open clutch position, wherein the electric motor driven actuator has a mechanical stop, which stops the motor in the maximum clutch separation position. Once against the mechanical stop, a clutch position is set to zero. Next, the motor is set to drive the clutch pack toward a fully compressed state, where two levels of motor torque are applied. First, a low level (e.g., at approximately a ten percent) of motor torque is applied until the motor stalls, due to a counter force by way of clutch compression. This is followed by a high level (e.g., at approximately a ninety percent) of motor torque that is applied until the motor again comes to a stall due to the higher counter force of the clutch pack at higher compression, wherein at both of these stopped (stalled) points the motor torque and its clutch position are recorded.

Hence, at a given motor torque, the electric motor driven actuator will travel until it reaches equilibrium with resisting forces, at which point the motor torque is related to a clutch position. There, the low and high level of motor torque are where clamp force changes from essentially zero to a value where it begins to increase, and the clutch position where the clamp force is far up the stiff engagement line—as shown inFIG. 6, wherein the stalled position must occur at a static motor torque value.

A first variation to the third embodiment is to drive the clutch at a constant motor torque and monitor speed, wherein more power is required after initial clutch contact and speed would decrease. Analysis of the speed (i.e., position data) response would then be utilized to establish the model parameters.

A second variation to the third embodiment is to drive the clutch while utilizing the feedback control algorithm to a constant motor speed and then monitor motor torque (or current) required. Analysis of the current data would then be used to establish the model parameters.

Hence, two line segments are established from the two points, wherein a low-force line segment defines a relationship of position to motor torque at a condition of compliance when the clutch pack is beginning to be compressed and a high-force line segment defines a relationship of clutch position to motor torque at a condition of compliance when the clutch pack is compressed.

Consequently, the motor torque values at these line segments are converted into corresponding clutch compressing forces, wherein these conversions from motor torques to compressing forces can be done by an equation. Therefore, these lines segments describe the motor position as a function of force applied to the clutch and also a point in force where the position versus motor torque relationship changes from the low force line to the high force line, which concludes the initial calibration phase.

As a result, the amount of torque that can be transmitted through a clutch is Clamping Force×Clutch Friction Coefficient×Effective Radius×Number of Friction Surfaces, which is taken to be the maximum amount of torque that can be transmitted through the clutch and designated clutch Coupling Torque Limit (CTL).

As in the other embodiments, the clutch friction coefficient is affected by clutch temperature. Under normal conditions, the vehicle control provides a CTL to the DUEC, wherein the clamping force is calculated to be: Clamping Force=CTL/(Friction Coefficient×Effective Radius×Number of Friction Surfaces), where the friction coefficient comes from a table, wherein the clutch temperature is used to look up the friction coefficient from this table. Hence, with the required clamping force in hand, a desired position is needed to be achieved and retrieved from the line equations and the intersect point that were derived during initial calibration. From this information, the motor is commanded to go to that position.

Further objects and advantages of the present invention will be apparent from the following description and appended claims, reference being made to the accompanying drawings forming a part of a specification, wherein like reference characters designate corresponding parts of several views.

DESCRIPTION OF THE INVENTION

FIGS. 1-7illustrate a vehicle disconnect unit (DU)10, which may be a rear disconnect unit, that comprises a drive axle assembly16having a differential therein (not shown) and a driveshaft24with axles26,28. The DU10further comprises a motor driven actuator20that comprises a disconnect unit embedded controller (DUEC)12having an electronic circuit board14with electronic components14aand software.

The drive axle assembly16comprises a motor18, a clutch pack22, a ball ramp30having a plurality of balls38, gears32, and an encoder34, wherein the motor driven actuator20manages clamping forces exerted on the clutch pack22. The encoder34provides a motor rotational angle position of the motor magnets relative to the motor stator windings for managing the voltage applied to the motor phases, where the motor18is brushless.

Consequently, a logical clutch position indicator is formed from the encoder34in conjunction with linear positions determined by the DUEC12, which are provided by integrating the motor rotational angle, where linear position is proportional to the motor rotational angle. The motor driven actuator20manages clamping forces exerted on the clutch pack22, thereby regulating a coupling of power between the driveshaft24and one of the axles26,28, which may be on the rear of a vehicle (not shown but common in the art), based on position which is derived from a coupling torque values/limit CTL by stepping the motor18from a full open maximum clutch separation position through a series of clutch positions and recording a motor driven command torque value at each step to a maximum clutch compression position having a maximum command torque value, recording a drive motor command torque values (e.g., estimations) at each clutch position, and converting each drive motor driven command torque value into a respective clutch force value as a function of clutch position. Thereby, each clutch torque is related to a clutch position based on conversion tables stored in the controller DUEC12. These clutch positions, in turn, are associated with physical factors that influence clutch torque for each of the respective clutch actuator positions. These clutch positions may be based on clutch geometry for each of the respective actuator positions.

Specifically,FIG. 2illustrates an assembled differential drive axle assembly16with the electric motor18, the driveshaft24and the rear axles26,28of the vehicle.FIG. 3depicts a cross sectional view within the differential drive axle assembly16with the motor18, clutch pack22, ball ramp30, and gears32.FIGS. 4 and 7are further cross sectional views within the differential drive axle assembly16showing one of a plurality of balls38that functions in a known manner within the ball ramp30and a stop40.

Overall, the DU10regulates a coupling of power between the driveshaft24and one of the rear axles26,28at a time, where the coupling of power is based on clutch position P. In contrast, conventional actuators couple power based on torque TM, which is the case in the Krisher patent described above. Hence, the control strategy of Krisher simply controls electric motor torque TMto affect the clutch coupling torque limit. On the other hand, the instant invention has the advantage of controlling to a clutch position, which permits faster control response by performing an initialization process that is disclosed below.

The DUEC12is electrically powered by vehicle power VDC and exchanges data with vehicle devices (not shown but common in the art) over, as an example, a CAN bus that conforms to the ISO11898-2 specification. Between the clutch pack22and the motor18are the ball ramp30and the gears32, which mechanically cooperate with one another in the coupling of power between the driveshaft24and one of the rear axles26,28.

The DUEC12is capable of sensing motor current, supply voltage applied to the controller12and other physical factors; like motor position, motor temperature, clutch fluid temperature, an actuator control enable state, and command messages from the CAN bus, which can be stored in corresponding tables in the controller12. The DUEC12is also capable of operating in either of (2) control modes, Position or Coupling Torque. Control set-points are provided in received CAN message data.

Functionally, at a time of initialization, the DUEC12derives the coupling torque values taken from various clutch position measurements and motor torque estimations (see Table 1), that serve as indices in the position/force Table 2. To derive the clutch torque values CTL, the drive motor18is driven from a full open maximum clutch separation “zero reference position” P0, where a command torque value goes from zero through a series of “n” step positions, where a motor command torque is increasing ((0, 1×Tmax/n, (2×Tmax/n), . . . , Tmax), to a maximum clutch compression position having a maximum command torque (n×Tmax/n). In other words, clutch torque capacities are mapped to a series of positions (P0, P1, P2, . . . , Pn).

Then, throughout normal operation, each measured clutch position serves as an index into the position/force Table 2 (discussed above as being created during initialization) to look-up a present force. Subsequently, Table 2 is utilized to obtain motor torque TMassociated with clutch force FCthat is used to capture a potentially non-linear relationship between motor torque TMand clutch force FC. Hence, Table 2 provides the position/force relationship for accurately selecting an instant force.

Each present force is then converted into an individual CTL using a constant coefficient that accounts for individual clutch assembly characteristics, along with a coefficient derived from clutch temperature that accounts for clutch variations associated, for example, with specific temperatures. From these present force selections, improvements in accuracy over a clutch operating temperature range are then also realized.

It is noted that Tables 1 and 2 contain no less than 2×32 elements each and duration of a conversion algorithm does not exceed 1.0 second.

During normal operation, CTL varies as a function of vehicle dynamic control demands. The force required to achieve the given CTL is:
F=CTL/(Friction Coefficient×Effective Radius×Number of Friction Surfaces)

Once the force F is calculated, it is used as an index into the look-up table that was created during initialization to look up the clutch position P to where the motor18is required to drive. It is understood that a clutch friction coefficient CFC varies over temperature and other physical factors and that it is tracked during operation and stored in tables in the controller12.

Hence, the embedded controller DUEC12better regulates a coupling of power between a driveshaft and one of the axles of a vehicle. The control strategy of the known art, however, which is currently utilized in the industry, simply controls electric motor torque to affect the clutch coupling torque limit. Another of the advantages of the present invention is that it regulates the coupling of power according to clutch position, which permits faster control response time of the DU10. By monitoring the operating clutch positions and temperature of the DU10, the present invention can detect mechanical wear, malfunction, and/or mis-assembly within a differential and its associated parts.

Consequently, an actuator position determines the CTL, where the clutch position is controlled using the feedback control algorithm ofFIG. 5. Gain scheduling, as a function of clutch position, is employed in order to optimize response characteristics. The overall CTL control strategy is constructed as shown in the block diagram ofFIG. 5. Also, a force F may be converted into CTL by using the following equation CTL=kCkTF, where kCis a clutch assembly characteristics coefficient and kTis a clutch temperature coefficient. Required clutch position P uses Tables 1 and 2.

FIG. 6illustrates a graph of motor torque TMversus clutch pack position in accordance with a third embodiment of the invention. At a time of initialization, the DUEC derives coupling torque values (CTL) by commanding the motor18to drive the clutch pack22to a fully open clutch position, wherein the electric motor driven actuator20has the mechanical stop40(as seen inFIGS. 4 and 7), which stops the motor18in the maximum clutch separation position M. Once against the mechanical stop40, a clutch position is set to zero. Next, the motor18is set to drive the clutch pack22toward a fully compressed position C, where two levels of motor torque TMare applied. First, a low level motor torque is applied until the motor18stalls, due to a counter force by way of clutch pack compression. This is followed by a high level motor torque that is applied until the motor18again comes to a stall due to the higher counter force of the clutch pack22at higher compression, wherein at both of these stopped (stalled) points the motor torque and its clutch position are recorded.

Hence, the two line segments are established from the two points 0-1, 1-2 inFIG. 6. Therein, a low-force line segment defines a relationship of clutch position P along the vertical axis to motor torque TMalong the horizontal axis at a condition of compliance. When the clutch pack22begins to be compressed, a high-force line segment proceeds to define a relationship of clutch position to torque. This results in a condition of stiffer compliance, when the clutch pack22is compressed.

Consequently, the motor torque values at these line segments are converted into corresponding clutch compressing forces, wherein these conversions from motor torques to compressing forces can be done by an equation associated with the low-force line segment and the high-force line segment. Therefore, both of these line segments describe the motor position as a function of force applied to the clutch and also a point in force where the clutch position versus torque relationship changes from the low force line to the high force line, which concludes the initial calibration phase.

As a result, the amount of torque that can be transmitted through a clutch is Friction Coefficient×Effective Radius×Number of Friction Surfaces, which is taken to be the maximum amount of torque that can be transmitted through the clutch pack22and designated clutch Coupling Torque Limit (CTL).

As in the other embodiments, the clutch friction coefficient is affected by clutch temperature. Under normal conditions, the vehicle control provides a CTL to the DUEC, wherein the clamping force is calculated to be: Clamping force=CTL/(Friction Coefficient×Effective Radius×Number of Friction Surfaces), where the friction coefficient comes from a table, wherein the clutch temperature is used to look up the friction coefficient from this table. Hence, with the required clamping force in hand, a desired clutch position is needed to be achieved and gotten from the line equations (i.e., ofFIG. 6) and the intersect point1that were derived during initial calibration. From this information, the motor is commanded to go to that clutch position.

An added benefit of the above-described process, as a function of clutch position, is that at the low level motor torque TMpoint, the present invention allows for rapid movement through the first clutch position range, which has a low level clutch torque, since motor torque TMis not significant. Subsequently, more careful movement can be utilized in the higher motor torque TMclutch position range, where accurate control of clutch position provides quick responsiveness, instead of waiting for small differences in control forces to equalize out. For example, a rapid regulating of the coupling of power between a driveshaft and an axle occurs through the first ten percent of range clutch positions for deriving the coupling torque values therefrom.

There are other sources of variation from clutch force to clutch torque, including amount of oil in a friction material—which changes with compression time, relative slip speed across the friction surfaces, and condition of the friction material surface. Such variability can be compensated by stored look-up tables or by observations during operation or initialization.

In accordance with the provisions of the patent statutes, the principles and modes of operation of this invention have been described and illustrated in its preferred embodiments. However, it must be understood that the invention may be practiced otherwise than specifically explained and illustrated without departing from its spirit or scope.