Patent Application
20090150018
The present invention relates to a steering system and more particularly to detecting and identifying high friction characteristics in a steering system.
High friction characteristics in a steering system are highly undesirable as they may adversely affect steering system performance. Large increases in friction may lead to degraded steering performance.
In an electric power steering system, there are mechanical and electrical components of hardware. In the event of a failure, it is preferable to have the electrical system fail, or shut-down, resulting in a loss of electric power assist before failure of the mechanical system. This at least maintains the physical integrity of the system, allowing an operator to safely steer a vehicle, even though it may be manual, i.e., without the power assist.
Under the presence of a corrosive liquid, the mechanical portions of a steering system may corrode quickly and lead to a large increase in steering friction. Due to high output torque assist capacity of a steering system, this increase in friction may go unnoticed by a normal driver due to the system powering through the increase in friction. In the event that the torque assist is lost, the vehicle will become difficult to steer, due to the combined effect of loss assist and high friction.
In an electric power system, there is no guarantee that once the mechanical system has corroded the electrical system will not terminate, quite possibly unexpectedly, at some point during a vehicle's journey. There is a need to identify high friction characteristics and alert a vehicle operator in an appropriately safe manner to have the steering system serviced to correct the high friction condition.
The inventive subject matter is a method for detecting and identifying a high friction characteristic in a steering system according to the independent claims with variations as described in the dependent claims.
Elements and steps in the figures are illustrated for simplicity and clarity and have not necessarily been rendered according to any particular sequence. For example, steps that may be performed concurrently or in different order are illustrated in the figures to help to improve understanding of embodiments of the present invention.
While various aspects of the present invention are described with reference to a particular illustrative embodiment, the invention is not limited to such embodiments, and additional modifications, applications, and embodiments may be implemented without departing from the present invention. In the figures, like reference numbers will be used to illustrate the same components. Those skilled in the art will recognize that the various components set forth herein may be altered without varying from the scope of the inventive subject matter.
The measured vehicle states 104 are determined from direct measurement of signals 102 from the steering system. When used in calculations, the measured signals produce a measured vehicle state. The measured vehicle states 104 are considered an actual state of the steering system as the states are directly measuring the output of the steering system, regardless of the friction that may be within the steering system. Any signals available to the EPS may be used to measure the vehicle state. For example, a combination of any of the following signals may be readily available internally from the steering system: Input Torque, Assist Torque, Pinion Angle, Rack Travel, Steering System Gear Ratios, System Temperature, System Performance, and algorithms that run within the steering system.
Specifically, an example of a measured state for an EPS system that may be used in the inventive subject matter is described hereinafter. A rack load signal, Rload, representative of a total steering force, may be developed using known rack parameters according to the equation:
R
load=(AssistTq+InputTq)·(1/PinionRatio) (1)
Where, AssistTq is an assist torque output of the steering system in Nm, InputTq is an input torque from a vehicle operator in Nm, and PinionRatio, is the rack and pinion ratio in meters (m). The result is a measured rack load, or total steering force, that the system is producing at a current vehicle state for the EPS system
The idealized vehicle state 106 is determined from external signals 103 from the vehicle CAN and predetermined signals 102 from the steering system to predict a given idealized vehicle state, or a value for what the vehicle state should be assuming the presence of a nominal level of friction in the system. The predetermined signals from the steering system may be the same as those described with the measured vehicle state. However, the idealized vehicle state 106 also uses external signals 103, in addition to the measured vehicle state, to determine an idealized state value. The external signals 103 may be received from the vehicle CAN and may include, but are not limited to: Brake Control Module System (lateral acceleration, yaw rate, longitudinal acceleration, etc.), Powertrain Information (engine speed, engine torque, vehicle speed, etc.), Wheel Speeds, ABS and other safety systems, Vehicle temperatures, and System temperatures.
Using the signals and an appropriate governing engineering equation in conjunction with predetermined tunable parameters, a desired vehicle state can be calculated. In order to assure that the idealized value is accurate for the vehicle state, a confidence factor for the idealized state is created and applied as part of the idealized vehicle state calculation 108. The confidence factor is developed from the outside signals from the CAN and the steering system in order to “verify” (provide more or less confidence to the vehicle state) the idealized vehicle state value for predetermined vehicle conditions. The confidence factor will be described later herein.
An example of the prediction of the vehicle state Rack Load is provided by the Equation:
R
Load=LoadGain·ay (2)
Where LoadGain is an experimentally determined coefficient to convert lateral acceleration to rack load, ay is the vehicle's lateral acceleration in m/s2 as determined by the Equation:
ay=(u2/(Ku2+L))·δf (3)
Where u is vehicle velocity in m/s, K is an understeer coefficient in 1/(m/s2), L is wheelbase in m, and δf is front road wheel angle in radians given by:
δf=SWA·G·(π/180°) (4)
Where SWA is a steering wheel angle in degrees and G is an overall steering ratio.
The vehicle state comparison is made mathematically and the result is a difference between the measured and idealized vehicle states.
The confidence factor 420 is calculated based on the threshold comparisons and will weigh on the significance of the idealized value as it is used in the system and method of the inventive subject matter. For example, a low confidence factor, i.e., a value much less than one, will result in an idealized value that is not afforded much weight in the determination of friction according to the inventive subject matter. On the other hand, a high confidence factor, i.e., a factor very close to one, will result in a valuable idealized value.
Referring again to
An example of such a mathematical comparison may include filtering the measured and idealized values to isolate the frequency content of interest. Typically, the low frequency content is of interest. Filtering is performed to allow consideration of overall level changes between the two signals, ignoring the high frequency changes that occur in the signals. After filtering, an absolute difference between the two signals may be taken.
The estimate of friction will then be subjected to friction boundaries 110 to determine how much friction may be present in the system. For example, if the estimate of friction is greater than a predetermined friction boundary, then high friction may be present in the system. The friction boundaries 110 may be calculated from vehicle state conditions 102, 103 and limits that are tunable. For example, a friction boundary may be calculated according to vehicle speed to allow for a lower friction boundary at low speeds and a higher friction boundary at high speeds. The boundaries are also vehicle dependent, time dependent, and may have a variety of factors taken into consideration in their values.
According to the inventive subject matter, the duration of the existence of the estimate of friction is determined so as to qualify 112 the friction prediction. A predetermined time limit for an estimate of friction that exceeds the friction boundary is used to compare the duration of the existence of the estimate of friction. In the event the estimate of friction exceeds the friction boundary for a time that exceeds the predetermined time limit, the qualification of the friction detection is verified. In such event, a friction-acknowledge bit may be set 114, which may result in a fault signal being initiated by the vehicle. One skilled in the art is capable of applying any one of several methods for using the friction-acknowledge bit to notify an operator and/or a vehicle system that high friction has been detected. The scope of which is dependent upon the type of failure that may occur on which the vehicle and the type of steering system on the vehicle all factor into the desired method of notification.
In the foregoing specification, the invention has been described with reference to specific exemplary embodiments. Various modifications and changes may be made, however, without departing from the scope of the present invention as set forth in the claims. The specification and figures are illustrative, rather than restrictive, and modifications are intended to be included within the scope of the present invention. Accordingly, the scope of the invention should be determined by the claims and their legal equivalents rather than by merely the examples described.
For example, the steps recited in any method or process claims may be executed in any order and are not limited to the specific order presented in the claims. The equations may be implemented with a filter to minimize effects of signal noises. Additionally, the components and/or elements recited in any apparatus claims may be assembled or otherwise operationally configured in a variety of permutations and are accordingly not limited to the specific configuration recited in the claims.
Benefits, other advantages and solutions to problems have been described above with regard to particular embodiments; however, any benefit, advantage, solution to problem or any element that may cause any particular benefit, advantage or solution to occur or to become more pronounced are not to be construed as critical, required or essential features or components of any or all the claims.
The terms “comprise”, “comprises”, “comprising”, “having”, “including”, “includes” or any variation thereof, are intended to reference a non-exclusive inclusion, such that a process, method, article, composition or apparatus that comprises a list of elements does not include only those elements recited, but may also include other elements not expressly listed or inherent to such process, method, article, composition or apparatus. Other combinations and/or modifications of the above-described structures, arrangements, applications, proportions, elements, materials or components used in the practice of the present invention, in addition to those not specifically recited, may be varied or otherwise particularly adapted to specific environments, manufacturing specifications, design parameters or other operating requirements without departing from the general principles of the same.
This application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 61/012,552, filed Dec. 10, 2007, entitled Friction Plausibility Detection Algorithm for a Steering System, the entire disclosure of which is hereby incorporated by reference into the present disclosure.
| Number | Date | Country | |
|---|---|---|---|
| 61012552 | Dec 2007 | US |
