Methods, systems, and computer program products for steering travel limit determination for electric power steering

Methods, systems, and computer program products for steering travel limit determination for electric power steering. Exemplary embodiments include a method, computer program product and system incorporating the method, for implementing steering travel limit determination for electric power steering, including measuring end of travel positions of a handwheel of a vehicle, recording the end of travel positions, comparing the end of travel positions with current travel limit values and recording new travel limits with respect to the end of travel positions.

FIELD OF THE INVENTION

This invention relates to automobile steering systems, and more particularly, to methods, systems, and computer program products for steering travel limit determination for electric power steering.

BACKGROUND

Steering equipment for assisting a driver to steer an automobile is well known in the art. In conventional steering assemblies, the operator controls the direction of the vehicle with the aid of a steering wheel. This wheel is mechanically connected, usually through a gear assembly to the road wheels. To aid the operator, many systems utilize an auxiliary system to generate a force that is transmitted to a steering gear assembly. The additional force reduces the effort required by the operator in changing the direction of the vehicle. Typically, this auxiliary force is generated by either a hydraulic drive or an electric motor.

Furthermore, electric power steering systems can utilize knowledge of travel limits of the steering system in order to reduce thermal loads and mechanical loads, as well as to determine a center position of the steering system. However, the limit values are typically unique to each vehicle and can be asymmetrical, thereby making it difficult to use apriori calibrations to set the limits.

Therefore, is it considered advantageous to provide a steering control system that provides steering travel limit determination.

SUMMARY

Disclosed herein is an exemplary method in a vehicle having a handwheel, the method including measuring end of travel positions of the handwheel, recording the end of travel positions, comparing the end of travel positions with current travel limit values and recording new travel limits with respect to the end of travel positions.

Further disclosed herein is a computer program product for implementing steering travel limit determination for electric power steering, the computer program product including instructions for causing a computer to implement a method, the method including measuring end of travel positions of a handwheel of a vehicle, recording the end of travel positions, comparing the end of travel positions with current travel limit values and recording new travel limits with respect to the end of travel positions.

Further disclosed herein is a system, including a controller having a process with instructions to measure end of travel positions of a handwheel of a vehicle, record the end of travel positions, compare the end of travel positions with current travel limit values and record new travel limits with respect to the end of travel positions.

DETAILED DESCRIPTION

In exemplary embodiments, a steering travel limit determination method for electric power steering monitors the range of steering system travel during operation of the electric power steering system. The method further records unique values for the range of the steering system travel in non-volatile memory. Limits and checks are implemented to ensure that values selected for travel limits are reasonable with respect to operation of the steering system.

Turning now to the drawings in greater detail,FIG. 1illustrates a diagram of a vehicle100upon which steering travel limit determination may be implemented in exemplary embodiments. The vehicle100ofFIG. 1includes a handwheel102for controlling the angle of a left front road wheel104and a right front road wheel106in an exemplary front wheel steering configuration. The left and right front road wheels104and106are coupled through a rack and pinion steering mechanism108. The vehicle100also includes a left rear road wheel110and a right rear road wheel112coupled through a rear axle114. In exemplary embodiments, when a driver of the vehicle100rotates the handwheel102, the angular position of the handwheel102may be determined through a handwheel angle sensor (column position sensor)116. The handwheel102and the rack and pinion steering mechanism108may be coupled through a steering shaft118and a steering actuator motor and gearing120(steering actuator). The steering actuator120may include an electric motor in a steering actuation system, such as AFS, EPS, SBW, ARS, or AWS. In exemplary embodiments, the steering actuator120is part of an EPS system. While the steering actuator120is depicted inFIG. 1at the coupling junction between the steering shaft118and the rack and pinion steering mechanism108, in exemplary embodiments the steering actuator120may be located at or distributed between the steering shaft118and the rack and pinion steering mechanism108. Positional determinations of the steering actuator120may be determined through a motor position sensor121which may also be used to calculate the angular position of the handwheel (θHW). In exemplary embodiments, the vehicle100also includes rack position sensors109,111to determine the position of the rack and pinion steering mechanism108(e.g., the linear position of the rack).

In exemplary embodiments, the vehicle100also includes a controller122, which receives input signals from the handwheel angle sensor116, the motor position sensor121, the rack position sensors109,111, and other sensors, including but not limited to, a vehicle speed sensor124, a yaw rate sensor126, a lateral acceleration sensor128, etc. The controller122may generate output signals to the steering actuator120. Although only a single controller122is depicted, it is understood by those skilled in the art that the controller122may be distributed through multiple control units within the vehicle100. For example, there may be a local controller at the steering actuator120that receives vehicle information over a vehicle network from various smart sensors or a centralized control unit.

The vehicle sensors109,111,116,121,124,126,128depicted inFIG. 1may include various types of technologies to produce input signals with or without additional signal conditioning and processing by the controller122. The handwheel angle sensor116may be any type of sensor capable of generating an angular handwheel position (θHW). For example, the handwheel angle sensor116may be a resolver, a rotational variable differential transformer (RVDT), an encoder, or a potentiometer. The motor position sensor121may similarly be a resolver, a RVDT, an encoder, or a potentiometer, and a combination of other sensor types. The position sensors109,111may be a single or composite of multiple sensors on the rack and pinion steering mechanism108to determine the travel limits of the overall steering system. The position sensors109,111, the handwheel sensor116and the motor position sensor121may further include position switches as discussed further below.

The vehicle speed sensor124may be a single sensor or a composite of multiple sensors, which can be on the drive shaft, transmission and or each road wheel104,106,110,112. at each road wheel104,106,110, and112. In exemplary embodiments, the vehicle speed sensor124may utilize a magnetic pick-up to determine the speed (V) of the vehicle100. The yaw rate sensor126may determine the rate of vehicle100yaw (ω) with a gyroscope. The lateral acceleration sensor128may be an accelerometer. While vehicle sensors109,111,116,121,124,126,128are depicted as separate sensors, they may be merged into any combination of modules. Furthermore, the vehicle sensors109,111,116,121,124,126,128may be integrated in any combination with the steering actuator120or the controller122. Multiple sensors may be provided for redundancy or improved accuracy. In exemplary embodiments, equivalent vehicle information provided by the vehicle sensors109,111,116,121,124,126,128may be extracted or synthesized from a combination of vehicle sensors, including other vehicle sensors not depicted inFIG. 1. While sensors109,111,116,121,124,126,128are depicted inFIG. 1, not all sensors are needed for all embodiments. For example, in some embodiments only a handwheel angle individually, a rack and pinion position measurement individually, or a motor position measurement individually may be used to perform travel limit determination.

Turning now toFIG. 2, a block diagram200of the controller122is depicted in accordance with exemplary embodiments. The controller122may collect various vehicle signals to control the steering actuator120. The controller122may execute computer readable instructions for functions such as EPS control logic, vehicle stability logic, and travel limit determination logic, which may be embodied as computer program products. In exemplary embodiments, the controller122includes a central processing unit (CPU)202, a read-only memory (ROM)204(e.g., non-volatile EEPROM), a volatile memory such as a random access memory (RAM)206, and an I/O interface208. The CPU202operably communicates with the ROM204, the RAM206, and the I/O interface208. Computer readable media including the ROM204and the RAM206may be implemented using any of a number of known memory devices such as PROMs, EPROMs, EEPROMS, flash memory or any other electric, magnetic, optical or combination memory device capable of storing data, some of which represent executable instructions used by the CPU202. In exemplary embodiments, the CPU202communicates via the I/O interface208with the rack position sensors109,111, which can include sensing the state of position switches disposed in the position sensors109,111. The CPU can further communicate via the I/O interface208with the handwheel angle sensor116(and a state of the position switches), the motor position sensor121(and a state of the position switches), the vehicle speed sensor124, the yaw rate sensor126, the lateral acceleration sensor128, and the steering actuator120. While vehicle sensors109,111,116,121,124,126,128are depicted as separate inputs to the I/O interface208, the signals may be otherwise coupled, packetized, or encoded.

Exemplary embodiments are now described in reference to the handwheel102of the vehicle100ofFIG. 1. As a driver of the vehicle100rotates the handwheel102, creating a handwheel angle (θHW) relative to the vehicle, a tire steer angle (δf) is imparted upon the front wheels104and106. The controller122may be used in conjunction with the steering actuator120to control the tire steer angle (δf), keeping the angle within a set of upper and lower limits. In exemplary embodiments, the steering actuator120is an EPS motor used for driver assist and a travel limit determination function. In addition, the controller122can measure and record positional changes within the steering actuator120. Furthermore, as the driver of the vehicle100rotates the handwheel102a resultant relative displacement is imparted on the rack and pinion steering mechanism108. This displacement can be measured by the position sensors109,111and recorded to the controller122.

In exemplary embodiments, the vehicle sensors109,111,116,121,124,126,128can be implemented individually or in combination in any vehicle system in which knowledge of specific steering travel limits, which typically vary from vehicle to vehicle, enhance performance. For example, a determination of a true zero position of the handwheel102can be determined when a relative position sensor is used in conjunction with the handwheel angle sensor116. Furthermore, in another example, “soft” travel limit stops (rack stops on the rack and pinion steering mechanism108) can be implemented electronically via any of the sensors described herein. The controller122can measure travel limit stops via the position sensors and, in one implementation, signal the steering actuator120of the new soft travel limits. In still other examples, the vehicle sensors109,111,116,121,124,126,128can be implemented for parking and other autonomous steering methods.

In exemplary embodiments, the vehicle sensors109,111,116,121,124,126,128can be, implemented for overall vehicle100stabilization methods. For example, as a driver of the vehicle100rotates the handwheel102, creating the handwheel angle (θHW) relative to the vehicle100, the tire steer angle (δf) is imparted upon the front wheels104and106, as discussed above. The lateral rotation of the front wheels104and106applies a lateral tire force on each wheel. As the vehicle yaws in response to rotating the front wheels104and106, the portion of the road wheels104and106in contact with a travel surface (e.g., a road) may not align with the direction of travel of the vehicle. The actual change in lateral vehicle travel direction at the front of the vehicle100is referred to as a vehicle sideslip angle (βf) or a vehicle slip angle. The difference between the vehicle sideslip angle (βf) and the tire steer angle (δf) is the tire slip angle (αf), which is expressed by the equation: αf=δf−βf. A rapid maneuver can create a larger tire slip angle (αf) in a positive or negative direction, resulting in a larger positive or negative tire lateral force.

To provide overall vehicle stability, travel limit determinations can be implemented with one or more of the vehicle sensors, such as position sensors109,111as described above, the handwheel sensor116and the motor position sensors121. For example, in an AFS system, the tire steer angle (δf) can be the mechanical combination of two inputs, the handwheel angle (θHW) and the AFS motor angle. In exemplary embodiments, the steering actuator120is an AFS motor used for VR control and a travel limit determination function. Therefore, for example, an AFS motor angle command can be bounded by measured travel limit quantities from the handwheel sensor116and the motor position sensor121to prevent commands beyond the mechanical capability of the system. In further exemplary embodiments, position data can be measured from the position sensors109,111to provide further travel limit determinations to the controller122. In exemplary embodiments, travel limit learning can be used to bound the commands in an AFS system to prevent running into the travel stops.

In exemplary embodiments, systems and methods herein can determine unique travel limits for each direction (i.e., clockwise (CW) and counter-clockwise (CCW)) of a steering system via position monitoring via the vehicle position sensors109,111,116,121. As such, a system wherein a position sensor is attached to the steering system via the handwheel102, the steering actuator120, and the rack and pinion steering mechanism108, for example, via the vehicle position sensors109,111,116,121, the controller122can execute one or more algorithms to perform functions including, but not limited to: sensing or calculating angle of the handwheel102; determining a zero reference point with respect to the handwheel102and determining a total possible travel CW and CCW from the reference point. In exemplary embodiments, determining the zero reference point can be determined by a relative sensor reference point when only the change in position is known, and by an absolute reference point when the position with respect to a zero reference is known explicitly and read from one of the vehicle sensors (e.g., handwheel sensor116). In exemplary embodiments, determining the total possible travel CW and CCW from the reference point can be determined from actual angles measured, for example maximum (CW) measurements and minimum (CCW) measurements during usage of the steering system. In exemplary embodiments, determining the total possible travel CW and CCW from the reference point can be determined further if a particular measurement is less than the minimum or greater than the maximum. In such circumstances, the new measured values can become the new minimum and maximum, respectively.

In exemplary embodiments, the systems and methods described herein can further save data that has been measured by the vehicle sensors109,111,116,121,124,126,128across ignition cycles. As such, the ROM204(as non-volatile memory) and the controller122can be implemented to retain the measurement data. Furthermore, the controller122can include “keep-alive” power to maintain memory. In accordance with further exemplary embodiments, the controller122can include algorithms to implement error checking. For example, exemplary algorithms can determine minimum and maximum possible values for end of travel (EOT) limits (e.g., of the rack and pinion steering mechanism108) from mechanical constraints of the steering geometry. Exemplary algorithms can further check the calculated limits against the EOT limit values. Furthermore, exemplary algorithms can provide methods for resetting the limits when the geometry changes (e.g. component replacement, steering alignment, etc.)

As discussed above, the position sensors109,111, the handwheel sensor116and the motor position sensor121can further include position switches. As such, in exemplary embodiments, the systems and methods described herein can determine unique travel limits for each direction (CW and CCW) of the steering system via limit switches. For example, a switch can be placed at each travel limit. For example, when the switch is closed on the handwheel sensor116, the current handwheel angle θHWis determined to be the corresponding travel limit. In exemplary embodiments, the switches may be wired in parallel and the corresponding travel limit determined by the sign of the θHW. A unique switch circuit may be used for each travel stop and the limit determined explicitly.

In exemplary embodiments, the systems and methods described herein can further determine unique travel limits for each direction (CW and CCW) of a steering system via monitoring a handwheel102torque-angle gradient. In an exemplary embodiment, the slope of the handwheel102torque (y axis) versus the handwheel angle, θHW, (x axis) can be steeper when the steering system encounters a travel stop as compared to the slope when the steering system is constrained by an obstacle such as a curb due to the tire compliance. As such, by detecting the higher slope of the travel stop, the current handwheel102angle θHWis determined to be the corresponding travel limit.

It is appreciated that in accordance with exemplary embodiments, calibrations can be provided for the vehicle100. For example, an EOT maximum, that is, a maximum allowable absolute value of the EOT travel limit, and an EOT minimum, that is, the minimum allowable absolute value of the EOT travel limit, can be defined. In addition, the controller122, and alternatively the ROM204(e.g., EEPROM, etc.) can be programmed with variables such as Lcw, an EOT CW travel limit, and Lccw, an EOT CCW travel limit.FIG. 3illustrates a flow chart of a method300for steering travel limit determination for electric power steering in accordance with exemplary embodiments.

At step305, the steering system is powered on, which can correspond with an ignition cycle of the vehicle100. In addition, at step305, the controller122is initiated, which can include updating or erasing an EEPROM. At step306, the previously stored limits are checked for validity. If they are not valid, the stored values are erased at step308and, At step310, the variables, Lcw and Lccw are set to an EOT minimum calibration. If the stored values are determined to be valid at Step306, the values are read at Step307and used in further calculations. At step315, the method300determines whether or not an autocenter initialization has occurred, which determines an initial zero position of the handwheel102. In an exemplary embodiment, the method300loops at step315until the autocenter initialization has occurred. Once autocenter initialization has occurred at step315, at step320the current position of the handwheel102is measured. At step330, the method300determines whether or not the current position exceeds any of the current travel limits, which is provided by the controller122at step325(e.g., current travel limits stored in an EEPROM, etc.).

If at step330, travel limits have been exceeded, then at step335, if the current position is greater than the value Lcw, then Lcw is set to be the minimum of the current position and the EOT maximum. At step340, if the current position is less than the value Lccw, then Lccw is set to be the maximum of the current position and the negative of EOT maximum. At step345, the controller122can enter a service state in which the controller clears the learned limits and sets a new EOT maximum.

It is appreciated that the controller122can be initialized with travel limit values at the end of line (EOL) of the manufacturing line. In one exemplary embodiment, the autocenter initialization algorithm can obtain an accurate initial center value if the controller122is programmed during a rolling alignment, in which the vehicle is moving above a preset velocity threshold. In further exemplary embodiments, if a rolling alignment is not implemented, that is, a static alignment is implemented, then other exemplary algorithms can be implemented to learn current positions upon execution. For example, the controller122can be calibrated by performing a full lock-to-lock steer after alignment without cycling ignition to initially place a set of travel limits into the vehicle (e.g., non-volatile ROM204.)

The disclosed systems and methods can be embodied in the form of computer or controller implemented processes and apparatuses for practicing those processes. It can also be embodied in the form of computer program code containing instructions embodied in tangible media, such as floppy diskettes, CD-ROMs, hard drives, or any other computer-readable storage medium, wherein, when the computer program code is loaded into and executed by a computer or controller, the computer becomes an apparatus for practicing the method. The method may also be embodied in the form of computer program code or signal, for example, whether stored in a storage medium, loaded into and/or executed by a computer or controller, or transmitted over some transmission medium, such as over electrical wiring or cabling, through fiber optics, or via electromagnetic radiation, wherein, when the computer program code is loaded into and executed by a computer, the computer becomes an apparatus for practicing the method. When implemented on a general-purpose microprocessor, the computer program code segments configure the microprocessor to create specific logic circuits.

It will be appreciated that the use of first and second or other similar nomenclature for denoting similar items is not intended to specify or imply any particular order unless otherwise stated. It is further appreciated that references to left and right as well as number used for logic can be interchanged and used otherwise in other implementations.