Steering system with magnetic torque overlay lash compensation

Technical solutions for compensating for lash in a steering system are described. An example method includes determining a rack pressure value based on a driver torque value and a differential pressure across a rack of the steering system. The method also includes determining a compensation friction value based on a position of a handwheel of the steering system and a speed of a vehicle equipped with the steering system. The method also includes computing a pressure value based on the rack pressure value and the compensation friction value. The method also includes generating a torque command using the pressure value, the torque command being added to the driver assist torque for the steering system.

BACKGROUND

The present application relates to vehicle steering systems, and particularly for eliminating a degradation of steering-feel in case of a lash being present between a steering wheel and road wheels of a vehicle.

Typically, a steering system of a vehicle, such as a hydraulic steering system, uses a power steering pump to provide pressurized hydraulic fluid to either a recirculating ball style steering gear or a rack and pinion style steering gear. The steering system provides an assistive torque (or assist) to a driver during driving. A level of assist provided is determined by an amount of torque applied by the driver to a steering valve integrated into the steering gear. A resulting movement of a steering linkage in the vehicle results in angulation of the steerable wheels of the vehicle, thus steering the vehicle. The steerable wheels may be front wheels and/or rear wheels. It is desirable in a steering system to have a lash free connection between the steering wheel and the road wheels, especially in the center position of the steering wheel.

SUMMARY

According to one or more embodiments, a method for compensating for lash in a steering system includes determining a rack pressure value based on a driver torque value and a differential pressure across a rack of the steering system. The method also includes determining a compensation friction value based on a position of a handwheel of the steering system and a speed of a vehicle equipped with the steering system. The method also includes computing a pressure value based on the rack pressure value and the compensation friction value. The method also includes generating a torque command using the pressure value, the torque command being added to the driver assist torque for the steering system.

According to one or more embodiments, a steering system includes a control module that determines a rack pressure value based on a driver torque value and a differential pressure across a rack of a steering gear. The control module also determines a compensation friction value based on a position of a handwheel of the steering system and a vehicle speed. The control module also computes a pressure value based on the rack pressure value and the compensation friction value. Further, the control module generates a torque command using the pressure value, the torque command being added to a driver assist torque for the steering system.

According to one or more embodiments, a power steering system is configured to determine a rack pressure value based on a driver torque value and a differential pressure across a rack of a steering gear of the power steering system. The power steering system also determines a center friction value based on a usage parameter of the power steering system. The power steering system also determines a compensation friction value based on a position of a handwheel of the steering system, a vehicle speed, and the center friction value. The power steering system also computes a pressure value based on the rack pressure value and the compensation friction value. The power steering system further generates a torque command using the pressure value to compensate for a lash in the steering gear, the torque command being added to a driver assist torque for the power steering system.

DETAILED DESCRIPTION

As used herein the terms module and sub-module refer to one or more processing circuits such as an application specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that executes one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality. As can be appreciated, the sub-modules described below can be combined and/or further partitioned.

Referring now to the Figures, where the invention will be described with reference to specific embodiments, without limiting same,FIG. 1is an exemplary embodiment of a steering system10, such as a hydraulic steering system, in a vehicle. In various embodiments, the steering system12includes a handwheel14coupled to a steering shaft16. In one or more examples, a steering assist unit18couples to the steering shaft16of the steering system12and to tie rods20,22of the vehicle10. The steering assist unit18includes, for example, an integral steering gear, torsion-bar type of hydraulic assist system that furnishes power to reduce turning effort at the handwheel14. For example, during operation, as the handwheel14is turned by a vehicle operator, the steering assist unit18provides assistance to move the tie rods20,22which in turn moves steering knuckles24,26, respectively, coupled to roadway wheels28,30, respectively of the vehicle10.

In one or more examples, the vehicle10further includes various sensors31,32,33that detect and measure observable conditions of the steering system12and/or of the vehicle10. The sensors31,32,33generate sensor signals based on the observable conditions. For example, the sensor31is a position sensor that senses the angle of rotation of the handwheel14by the operator of the vehicle10. The position sensor generates a handwheel angle signal based thereon. The sensor32is a differential pressure sensor that senses assist provided by the steering assist unit. The sensor32generates a differential pressure signal based thereon. In yet another example, the sensor33is a vehicle speed sensor that senses a rotational speed of the wheel30. The sensor33generates a vehicle speed signal based thereon. A control module40controls the operation of the steering system12based on one or more of the sensor signals and further based on the steering control systems and methods of the present disclosure.

Referring now toFIG. 2, a cross section of a recirculating ball steering gear200is illustrated. The gear200has an input shaft210that is operably connected to the steering wheel14of the vehicle. When a torque is applied to the input shaft210, a valve assembly215is actuated to provide assist pressure in either cavity A220, or cavity B225, depending on a direction of the torque, to assist in steering the vehicle. A worm230, which is attached to the bottom of the valve assembly215, provides a thrust force on a rack235. When the valve assembly215is rotated in the steering gear200, the rack235translates in a rack bore along the axis of the worm230. The rack235has teeth which mate with teeth on a pitman shaft240. When the rack235moves axially in the rack bore, the pitman shaft240rotates along its axis. A pitman arm, which is attached to the bottom of the pitman shaft240, connects the pitman shaft240to a linkage in the vehicle. For example, the linkage may include the tie rods20,22, the steering knuckles24,26, and/or other components that facilitate transferring the force from the steering wheel14to the roadway wheels28,30. Accordingly, when the pitman arm rotates, the pitman arm swings through an arc, and this, in turn, moves the steering linkage in the vehicle, resulting in angulation of the wheels28,30in order to steer the vehicle.

The technical solutions described herein facilitate the steering system10to have a lash free connection between the steering wheel14and the road wheels28,30. A lash-free connection facilitates the steering system10to improve a steering feel for the driver, especially in the on-center position of the steering wheel14. Typically, for providing a lash free connection, the teeth on the pitman shaft240are manufactured with a taper. An adjuster mechanism (not shown) positions the pitman shaft240along its axis in order to remove any clearance between the pitman shaft teeth and the rack teeth. Further, to account for normal wear in the gear assembly, it is common to make the center tooth thicker than the outer teeth. When initially adjusted, with no hydraulics applied, the torque required to rotate such a gear assembly100is higher on-center than in the corners. Even with the initial preload on-center, normal wear in the steering gear100leads to lash conditions in the gear100that deteriorate the steering feel and functionality of the steering system10. Accordingly, it may be necessary to adjust the gear100in service, in order to remove or reduce the lash that has developed. The technical solutions described herein address the above described technical solution to provide a lash free connection, which is free of the loading conditions that lead to the lash-conditions described above.

Further, a steering system with magnetic torque overlay incorporates a magnetic actuator into the valve assembly215to enable controlling operations on a hydraulic system of the steering system. The operations may include variable effort, leads and pulls correction, active damping, active return, and the like or a combination thereof. The steering system equipped with the magnetic torque may be referred to as a Magnasteer with Torque Overlay (MTO) system.

FIG. 3shows a cross section of an MTO recirculating ball steering gear300. As can be seen, the assist and the connection from the steering wheel and the road wheels are functionally the same as a base gear (shown inFIG. 2). The MTO gear300has a magnetic actuator310incorporated into the valve assembly215, to provide additional effort to aid, or to resist the driver during certain steering conditions. In these conditions, a controller320provides current to a coil330, which determines the amount of torque provided by the magnetic actuator310. In one or more embodiments, the controller320may be part of the control module40(FIG. 1).

FIG. 4shows a high level block diagram of controls of the MTO equipped steering system10. The MTO steering system10includes an MTO Torque command module410that receives multiple input signals. For example, the input signals include a measurement of a differential pressure across the piston in the steering system10. The input signals may further include vehicle signals such as vehicle speed, hand wheel speed, and hand wheel angle, and an estimation of driver torque. The MTO Torque Command Module410uses the input signals to generate a desired MTO torque command.

The torque command is provided to a current command module420, which converts the torque command into a current command. The current command is applied to the magnetic actuator coil330in the MTO steering gear300. A driver torque estimation module430determines, or computes, a driver torque estimate based on the current command along with the measured differential pressure. The determined driver torque estimate is used in a next iteration, such as after a unit delay. In one or more embodiments, a delay module440facilitates delaying the driver torque estimate for use in the next iteration.

However in the MTO steering system10such as the above, the lash in the gear300is still controlled by the same mechanical features, is subject to the same normal wear as described herein, and thus fails to address the corresponding technical problems. As a result, the development of lash has the same undesirable effects on the steering feel, requiring to be adjusted in service.

Accordingly, technical solutions described herein provide a lash compensation for the MTO steering system, thus facilitate improvements to a steering feel in the case where lash has developed in steering gear of the steering system. Thus, the technical solutions address the technical problems of lash conditions associated with steering gear of a steering system.

Referring back toFIG. 3that shows a cross section of an MTO recirculating ball steering gear300, when a torque is applied to the input shaft210, motion of the rack235is resisted by friction of the rack235in the rack bore, and the friction and steering loads that rotate the pitman shaft240. As a result, the valve assembly215is actuated, generating assist pressure in either cavity A220or cavity B225, depending on the direction that the torque is applied. When the torque and pressure get high enough to overcome the resisting forces on the rack235, the rack235moves and rotates the pitman shaft240, and ultimately steers the vehicle. In this scenario, the driver feels the torque buildup, which provides a feeling for where the center position is in the vehicle.

When lash develops between the rack teeth and the pitman shaft teeth, the friction of the rack235in the rack bore is reduced. The rack235has to move axially in the rack bore until the clearance is removed between the rack teeth and the pitman shaft teeth before the steering loads begin to resist motion of the rack235. This creates a window of angular motion of the steering wheel14in which at least a portion of the torque is followed by a sharp increase in torque once the clearance in the teeth is removed. This creates an undesirable steering feel, such as a jerky, or disconnected steering feel. The condition described is typically referred to as a lash condition.

To improve such a condition, the technical solutions described herein utilize the MTO actuator310to apply pressure that opposes motion of the rack235when lash is present. When torque is applied in a direction that would pressurize the cavity A220, the valve assembly215is actuated to provide pressure in the Cavity B225to resist motion of the rack235. The driver thus has to increase the torque applied in order to move the rack235, minimizing the torque buildup change between the on center lash condition, and the condition when the rack teeth and the pitman shaft teeth are engaged. When torque is applied in a direction that would pressurize the cavity B225, the valve assembly215is actuated to provide pressure in the cavity A220to resist motion of the rack235, providing the same effect in the other direction.

FIG. 5shows a block diagram of example modules for providing an MTO lash compensation.FIG. 6illustrates a flowchart of an example method for providing the MTO lash compensation. It is understood that in one or more embodiments the modules used to implement the method for MTO lash compensation may be different from those illustrated inFIG. 5. Further, it is understood that in one or more embodiments, the operations illustrated inFIG. 6may be performed in a different order.

Referring toFIG. 6, the method for providing the MTO lash compensation includes receiving a driver torque signal, as shown at610. In one or more embodiments, the driver torque signal provides a measured value of the driver torque, such as measured using a torque measurement sensor. Alternatively, the driver torque signal provides the estimated driver torque value.

The method further includes computing a rack pressure signal that is equivalent to a rack force corresponding to the driver torque, as shown at620. In one or more embodiments, the driver torque signal is multiplied by a configurable value using a multiplier module510, as shown at622. The multiplier module510, using the configurable value, generates a pressure signal that is input to an adder module520. The adder module520adds the computed pressure signal, corresponding to the driver torque signal, and a measured differential pressure across the rack235to generate the equivalent rack pressure signal, as shown at624. In one or more embodiments, a low pass filter525is applied to the sum, providing the equivalent rack pressure signal, as shown at626.

The method further includes determining, using a friction module530, a compensation friction value, as shown at630. The friction module530receives input signals to generate the compensation friction, the input signals including a vehicle speed signal, a handwheel angle signal, and a center friction signal.

In one or more embodiments, determining the compensation friction value includes determining the center friction value, as shown at632. The center friction value represents a magnitude that compensates an occurrence of lashing conditions in the gear300due to friction. In one or more embodiments, the center friction value is a predetermined value, which is configurable prior to initiation of the method. Alternatively, the center friction value is a variable signal that is dynamically configured. For example, the center friction value represents an increased usage. Alternatively or in addition, a learning algorithm determines the friction of the steering system, and in turn, the center friction value is determined based on the result of the learning algorithm. An example of variable signal could be a signal that increments with each ignition cycle, or one that increases with vehicle mileage. For example, in response to each ignition cycle of the vehicle that is equipped with the steering system, the center friction value is incremented by a predetermined value. Alternatively, the center friction value is incremented by the predetermined value after a predetermined number of ignition cycles. Alternatively or in addition, upon ignition of the vehicle, the control module40determines a number of miles travelled (mileage) by the vehicle and configures the center friction value accordingly. In one or more embodiments, the control module40may store a look up table or any other data structure or algorithm that determines the center friction value corresponding to the mileage.

Alternatively, or in addition, the center friction value is determined based on one or more sensor measurements. For example, the steering system12may be configured with a predetermined target value for the center friction value, which may be specific for the vehicle10, and/or the steering system12. The control module40may accordingly adjust the center friction value to maintain the center friction value as per the target value. For example, as the lash condition develops the center friction value of the steering system12may reduce. The control module40monitors the center friction value and compares the measured value with the target value. The control module40, based on the difference, adjusts the center friction value to meet the target value. For example, the control module40receives pressure measurements, while steering, for the two cavities A and B (220and225). As the lash condition develops, the center friction value reduces, and the pressure difference betweenthe two cavities reduces in magnitude while steering. In one or more examples, the control module uses the difference as the center friction value, and/or to increment the center friction value. Thus, as the lash condition develops, the control module40applies different center friction values based on the pressure measurements in the cavities A and B (220and225).

FIG. 7illustrates an example block diagram and dataflow within the friction module530. The friction module530includes, among other components, a center-speed multiplier module705, an absolute value module710, a center shape module715, one or more limiting modules720and725, and a compensation friction multiplier module730. In one or more embodiments, the vehicle speed signal is used as an input to the center speed multiplier module705. The center speed multiplier module705may include a lookup table that outputs a scale factor based on the input vehicle speed signal. The limiting module720limits the scale factor within a predetermined range, such as 0 to 1.

In one or more embodiments, the handwheel angle is used as an input to the center shape module715. In one or more examples, the modulo module710determines and outputs an absolute value of the handwheel angle signal prior to inputting the handwheel angle signal to the center shape module715. In one or more embodiments, the center shape module uses a lookup table to output a scale factor based on the input handwheel angle signal. The limiting module725limits the scale factors within a predetermined range, such as 0 to 1.

The two scale factors, one from the center speed multiplier module705, and one from the center shape module715, are multiplied by each other along with the center friction value by the compensation friction multiplier module730. The resulting product provides the compensation friction value for the operating condition represented by the vehicle speed and handwheel angle.

Referring back to the flowchart inFIG. 6, the method further includes determining, using a pressure module540, a pressure value to compensate for the lashing condition, as shown at640. The pressure value is determined based on a handwheel speed signal, the equivalent rack pressure value, and the compensation friction value from the friction module530.

FIG. 8illustrates an example block diagram of the pressure module540. In one or more embodiments, the pressure module540includes, among other components, a static component810and a dynamic component820. The static component810determines a static-pressure value, as shown at642(FIG. 6). The static component810computes the static-pressure value by multiplying a predetermined value, such as −1, with the product of a predetermined static rate calibration value, and the equivalent rack pressure signal. In one or more examples, the predetermined static rate calibration value ranges between 0 and 1. The static component810includes a multiplier812that performs the above multiplication. The static component810may further include a limiting module814that limits the static-pressure value to +/− the compensation friction signal value.

The dynamic component820determines a dynamic-pressure value, as shown at644(FIG. 6). The dynamic component820computes the dynamic-pressure value by multiplying by a predetermined value, such as −1, the product of a predetermined dynamic rate calibration value, and the handwheel speed signal. The predetermined dynamic rate calibration value is a number greater than 0, such as 1, 5, 10, 0.5, 1.6, or any other positive value. The dynamic component includes a multiplier module822that performs the above multiplication.

The pressure module540further includes an adder830that computes a sum of the static-pressure value and the dynamic-pressure value, as shown at646(FIG. 6). In one or more embodiments, a limiting module840limits the resulting sum to +/− the compensation friction signal value to determine the pressure value signal.

Referring back toFIG. 6, the pressure value is supplied to a pressure to torque conversion module550that generates a torque command signal. The torque command is added to the MTO torque command inFIG. 4. In one or more embodiments, the pressure to torque conversion module550uses a look up table or any other conversion algorithm to determine the amount of torque that produces the pressure represented by the pressure value.

Accordingly, the torque command that is added to the MTO torque command compensates for the lashing condition that may occur in the gear300. Further, by generating the torque command using the one or more examples described herein, adjusting the gear periodically for compensating the lashing is reduced in frequency, if not entirely eliminated. Thus, the technical solutions described herein improves the operation of the steering system.

Aspects of the present technical solutions are described herein with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the technical solutions. It will be understood that each block of the flowchart illustrations and/or block diagrams, and combinations of blocks in the flowchart illustrations and/or block diagrams, can be implemented by computer readable program instructions.

It will also be appreciated that any module, unit, component, server, computer, terminal or device exemplified herein that executes instructions may include or otherwise have access to computer readable media such as storage media, computer storage media, or data storage devices (removable and/or non-removable) such as, for example, magnetic disks, optical disks, or tape. Computer storage media may include volatile and non-volatile, removable and non-removable media implemented in any method or technology for storage of information, such as computer readable instructions, data structures, program modules, or other data. Such computer storage media may be part of the device or accessible or connectable thereto. Any application or module herein described may be implemented using computer readable/executable instructions that may be stored or otherwise held by such computer readable media.

While the technical solutions are described in detail in connection with only a limited number of embodiments, it should be readily understood that the technical solutions are not limited to such disclosed embodiments. Rather, the technical solutions can be modified to incorporate any number of variations, alterations, substitutions, or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the technical solutions. Additionally, while various embodiments of the technical solutions have been described, it is to be understood that aspects of the technical solutions may include only some of the described embodiments. Accordingly, the technical solutions are not to be seen as limited by the foregoing description.