Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
2001-01-18
2002-04-09
Chin, Gary (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C701S042000, C180S443000, C180S446000
Reexamination Certificate
active
06370459
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates generally to power steering systems for vehicles, and more particularly to electrically powered steering systems which include an electric drive motor for providing a powered assist to the steering gear of the host vehicle.
II. Description of the Prior Art
A variety of electrically powered steering systems (known commonly as electric power steering systems or “EPS systems”) have been proposed for providing a powered assist to the steering of a motor vehicle. Conventional rack-and-pinion steering systems include a primary pinion/rack gear mesh interface for coupling the steering wheel of the vehicle to the steering system. EPS systems include an electric drive motor having a rotating element which is additionally mechanically or hydraulically coupled to the rack of the steering gear. EPS systems are said to provide fuel efficiency enhancement amounting to between about 2½ and 5 percent; this enhancement is usually on the lower end of this range for relatively larger vehicles. EPS systems are also said to incorporate software which is easily programmable to provide selected steering characteristics for any particular vehicular application. However, despite overall industry developmental commitments to date on the order of a billion dollars (U.S.), no EPS system is currently offered for sale in a mass produced automobile in the United States.
There are a variety of reasons why EPS systems are not provided on automobiles in this country. One reason is that EPS systems are generally subject to an “auto-steer” problem, in which an unintended steering event is possible. Another reason is that EPS systems generally provide unsatisfactory tactile feedback (or “feel”) during use; colloquially, EPS systems simply “feel funny” in operation. The art generally does not satisfactorily indicate the source or sources of these tactile feedback problems.
It is believed herein that the auto-steer problem occurs because the drive motor of such systems is directly linked to the host vehicle's steering linkage and both the magnitude and the direction of steering boost are determined in an open-loop manner. Moreover, prior EPS systems appear to lack even minimal safety feedback information, such as full time independent verification that instant values of the actual assistive force have been properly generated. Errors in the proper generation of the assistive force are not sensed by the driver and remain uncorrected by the system. Other complaints about prior BPS systems include “motor cogging,” lack of return ability and poor steering response to small input signals.
Several methods are known for coupling the electric drive motor of an EPS system to the steering linkage of the host vehicle. Preferred is a method comprising a hydraulic transmission for connecting the electric drive motor to the vehicle's steering gear as disclosed in companion U.S. Provisional Patent Application Serial No. 60/090,311 entitled “Feedback and Servo Control for Electric Power Steering System with Hydraulic Transmission”. The whole of that provisional patent application is expressly incorporated by reference herein. Specifically, that provisional patent application discloses connecting the electric drive motor to the vehicle's steering gear via the motor driving a pump which, in turn, is fluidly coupled to a cylinder portion of the steering gear.
Of course, a variety of mechanical methods is known for coupling the electric drive motor of an EPS system to the steering linkage of the host vehicle. One method entails the use of a drive motor having a hollow rotor in which the rack shaft of a rack-and-pinion steering gear is concentrically disposed. The drive motor and rack shaft are connected by a ball screw and ball nut assembly, the ball screw being positioned on the rack in place of the conventional power cylinder, and the ball nut being engaged with the ball screw and supporting one end of the rotor. The other end of the rotor is supported by a thrust bearing. Other methods for coupling the drive motor to the steering linkage include coupling the drive motor to a second gear rack, via a gear train and a second pinion/rack gear mesh interface generally similar to the primary pinion/rack gear mesh interface utilized for coupling the steering wheel to the steering system, or coupling the drive motor directly to the steering shaft via a gear train. The ball screw/ball nut configuration is problematic because it couples the motor torque into the rack along with the desired axial thrust. This torque is quite sufficient to overcome the preload of the rack into the pinion so additional rotational constraint must be applied to the rack. This causes an over constraint in the gear mesh interface relationship between the pinion and the rack which results in stick-slip tactile characteristics felt at the steering wheel.
Similarly, adding a second pinion/rack gear mesh interface provides over constraint between either pinion/rack gear mesh with similar deleterious results. If the second pinion/rack gear mesh is loaded by a yoke mechanism there is additional Coulomb friction which adversely effects return ability. Coupling the drive motor directly to the steering shaft via a gear train is limited to vehicles with light steering loads because of wear limitations in the primary pinion/rack gear mesh. With either gear train, backlash becomes a tactile issue because it can be felt at the steering wheel. And if such a gear train were loaded sufficiently to eliminate the backlash, sufficient coulomb friction would be added as to adversely effect on-center feel.
Perhaps most significantly, previously known EPS systems have serious tactile problems at very low frequencies, in particular, on the general order of 1 Hz. This includes system resonance, also typical of many standard power steering systems, which is apparently ignored within the art but can readily be recognized by a driver sensing an “over-center” type of instability wherein the driver must tightly grip the steering wheel in order to maintain precise control of vehicle tracking. Further exacerbating the tactile feel issue in EPS systems is another low frequency resonance traceable to the moment of inertia of the system's electric motor. This resonance also occurs at a frequency in the general order of 1 Hz, but instead results in a “notch” in the tactile characteristics.
One attempt to address some of these problems is provided in U.S. Pat. No. 5,473,539 (Shimizu et al., Dec. 5, 1995). That patent discloses an electrically operated power steering apparatus in a motor vehicle having a steering system. The apparatus comprises a steering torque detector for detecting a manual steering torque applied to the steering system, an electric motor for generating an assistive torque to be transmitted as a steering assistive force to the steering system, and an actual assistive torque detector which detects an actual assistive steering torque which is actually transmitted from the electric motor to the steering system. The apparatus also comprises a controller which generates a target value for the assistive torque to be generated by the electric motor, and which generates a control signal based on the difference between the actual assistive steering torque detected by the actual assistive torque detector and the target value, the control signal then being used to energize the electric motor.
In a first embodiment, the patent discloses an actual assistive torque detector
22
coupling the nut
11
a
of a ball screw mechanism
11
to a rack shaft
7
of the steering system, the assistive torque of the electric motor
10
being applied to the rack shaft
7
through the ball screw mechanism
11
. The patent indicates that the actual assistive torque detector
22
may be a pressure sensor comprising a resistance wire strain gage. In a second embodiment, the detector
22
is replaced with an actual assistive torque estimator which estimates an actual assistive torque from the voltage acro
Chin Gary
Gifford, Krass, Groh Sprinkle, Anderson & Citkowski, P.C.
Techco Corporation
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