Data processing: vehicles – navigation – and relative location – Vehicle control – guidance – operation – or indication – Vehicle subsystem or accessory control
Reexamination Certificate
1999-03-30
2001-08-28
Cuchlinski, Jr., William A. (Department: 3661)
Data processing: vehicles, navigation, and relative location
Vehicle control, guidance, operation, or indication
Vehicle subsystem or accessory control
C701S042000, C180S443000
Reexamination Certificate
active
06282472
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to improvements in electrical power assisted steering systems of the kind in which an electric motor is adapted to provide an assistance torque in response to an output signal from a torque sensor indicative of the input torque applied to the steering system by the vehicle driver.
BACKGROUND OF THE INVENTION
In a steering system of this kind, where the assistance torque is generated by an electric motor, a ‘boost curve’ is needed. The boost curve defines the static relationship between the driver applied input torque and the assistance torque produced by the motor. In a simple system, where a linear relationship exists between input torque and output torque, the boost curve may take the form of a straight line.
In a real application, a simple ‘straight line’ boost curve does not produce satisfactory results. A practical boost curve has several features which provide for an improved driver feel. Firstly, it is desirable to provide for a low-gain region of the curve for small input torques on each side of zero input torque. In the low-gain region, assistance torque is small. This ensures that the steering system is not unnecessarily sensitive to very small adjustments in input torque when travelling in a straight line, for example when maintaining the position of the vehicle in a lane on a highway, but can provide a “progressive” feel when cornering at high vehicle speeds.
Another desirable feature of a practical boost curve is that the gradient of the curve should be high at high levels of input torque. Such high input torques usually occur during parking manoeuvres, and this feature ensures that movement of the steering wheel is eased during such manoeuvres by providing a ‘light’ steering feel. High input torque could also occur at high vehicle speed during an emergency evassive manoeuvre such as a lane change If the gradient of the curve is high, the curve can be said to have a high gain.
A third desirable feature is that there should be a smooth transition between the low-gain region (for low input torques) and the high gain region (for high input torques). Any sudden step changes in gradient of the curve would be apparent to the driver and produce an unpredictable steering feel.
A final feature of a practical boost curve is that there should be a limit on the level of assistance torque which can be applied by the motor. In many systems, this limit is imposed by limitations of the hardware (i.e. maximum torque of the motor). In other systems, the limit may need to be artificially introduced. Where such a limit exists, the boost curve will be flat (i.e. output torque constant) for input torques which would otherwise result in an assistance torque in excess of the limiting value.
Many of these features are well-known from hydraulic power assisted steering systems. Typically, in a hydraulic system the shape of the boost curve is determined by the profile of a rotary valve which permits varying flow rates of hydraulic fluid at different steering input torques. In an electrical power steering system, the boost curve is preferably provided electronically, for example by a software algorithm. This has the advantage that by providing a speed sensor to measure vehicle speed, it becomes possible to alter the size and shape of the boost curve for varying speeds. One such alteration might be an increase in the width of the low-gain region and reduction in the gain of the low gain region at higher speeds to provide an increase in the perceived stability of the vehicle.
In the past, the boost curve has been generated in the form of a piecewise linear approximation of a curve. Such a boost curve comprises a number of portions, each defined by a linear equation. By altering the parameters of the linear equation over each portion, a complex boost curve can be constructed. An example of one such curve is shown in FIG.
1
(
a
) of the drawings.
A disadvantage of the use of a boost curve in which each portion is defined by a linear equation is that it is not smooth as sudden step changes in gradient occur at the transition from one linear portion to another. This is clearly apparent when a plot is made of the derivative of the boost curve, which features many discontinuities as shown in FIG.
1
(
b
). An improvement in the smoothness of the curve can be obtained by increasing the number of linear equations used and thereby reducing the width of each portion, but this considerably increases the complexity of the curve. Furthermore, changes in the shape of the boost curve cannot readily be achieved due to the high number of variables required to generate the entire curve.
SUMMARY OF THE INVENTION
According to a first aspect, the invention provides an electrical power assisted steering system comprising an input member, an output member, an electric motor coupled to the output member and a torque sensor, the electric motor being adapted to apply an assistance torque to the output member in response to an output signal from the torque sensor indicative of the torque applied to the input member by a driver of the vehicle, characterised in that the relationship between the indicated input torque and the output torque applied by the motor is defined by a boost curve that has inverse symmetry about zero input torque in which the curve on each side of zero input torque comprises at least a first portion defined by a first polynomial equation corresponding to a range of input torque values between a zero value and a first value and a second portion defined by a second polynomial equation corresponding to a range of input torque values between a second value and a third value, in which the gradient of the first portion of the curve at the first value is substantially equal to the gradient of the second portion of the curve at the second value.
The input member and output member may comprise parts of a single unitary member, for example opposite ends of a steering column shaft.
Preferably, a third portion of the curve is defined between the first value and the second value. This portion may be defined by a linear equation. The gradient of the linear equation may be the same as the gradient of the first portion of curve at the first value (and hence by implication the same as the gradient of the second portion of the curve at the second value).
The first polynomial and second polynomials may be of a different form. Indeed, it is preferred that they are. Alternately, they may be the same. The polynomials may be quadratics.
The first and second values may be substantially the same. The zero value may correspond to zero input torque.
A fourth portion of the curve may be defined between the third value and a fourth value. This may be defined by a linear equation.
A fifth portion may be defined between the fourth value and a maximum input magnitude. This portion preferably has zero gradient.
One or more of the first, second, third and fourth values may be variable in use. They may vary with vehicle speed. A memory may be provided and the one or more values may be stored in the memory. They may be stored in an array in an electronic processing unit.
Additionally, the gradient of the boost curve at one or more of the first, second, third and fourth values may be stored in the memory. The gradient value(s) may be variable with vehicle speed. Also, the width of one or more of the portions of the boost curve may be stored in a memory.
The boost curve for the electrical power steering system of the present invention is advantageous over the prior art in that it does not have any sudden changes in gradient as occur when the portions are defined by linear piecewise equations. This ensures smooth steering feel over the whole range. The entire boost curve can also be defined by a minimum of variable or constant parameters.
Preferably, the boost curve has inverse symmetry about the zero input torque.
Preferably the second value of input torque is higher than the first value, the third value of input torque may be higher than the second value, and the fourth value may be higher
Jones Russell Wilson
Williams Andrew James Stephen
Cuchlinski Jr. William A.
Gibson Eric M.
Tarolli, Sundheim, Covel Tummino & Szabo L.L.P.
TRW LucasVarity Electric Steering Ltd.
LandOfFree
Electric power steering system with boost curve having... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Electric power steering system with boost curve having..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Electric power steering system with boost curve having... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2471943