Steering system for a motor vehicle

A steering system for a motor vehicle, comprising at least one steered axle, an actuator, an actuator motor and a steering mechanism. The actuator and the steering mechanism, respectively, each have a mechanical gear ratio. The steered axle, the actuator motor, the actuator and the steering mechanism are operatively connected with one another. Steering movements initiated by the driver and movements initiated by the actuator motor are superimposed by the actuator. The superimposed movement is passed to the steering gear, which relays this movement as steering angle to the wheels of the steered axle. The steering gear mechanical gear ratio is configured to be variable which enables selectively adjustable relationships between the steering wheel movements and the steering angles of the steered wheels and avoids the loss of good steering feel for the driver.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an automotive steering system. 
2. Related Art 
In conventional steering systems, the steering movements initiated by the 
driver via the steering wheel are transferred to the steered wheels by 
means of a steering gear, and as the case may be, aided by an assist 
force. Moreover, there are steering systems where the steering movements 
initiated by the driver are superimposed upon further steering movements 
initiated by an actuator. Such a system is known from DE-OS 40 31 316 
(corresponding to U.S. Pat. No. 5,205,371). In this system, an actuator is 
provided between the steering wheel actuated by the driver and the 
steering gear, with the aid of which actuator an actuator movement can be 
superimposed on the driver's steering movements. Such a system assures 
that the steering angle of the steered wheels of a vehicle is not at a 
fixed ratio to the turning angle of the steering wheel actuated by the 
driver. 
This makes possible, among others, a steering assist for the driver that is 
dependent on the steering angle and in addition, there are many options of 
variation given for having additional steering signals act upon the 
steering system. Envisaged with the additional steering signals, 
specifically, are increased road safety and/or travel comfort. 
A problem that may be involved with such power-assisted steering that 
employs superimposition of movement is that the selection of a constant 
gear ratio for the steering gear as established in its design and that the 
actuator fails to achieve acceptable steering wheel moments or a good 
steering feel for all states of travel. In particular, the steering wheel 
moment must at large steering angles not become excessive while the 
vehicle is at rest, whereas with the vehicle traveling at high speed the 
steering wheel moment must not become insufficient. 
An overlay steering mechanism of other design can be seen from DE-OS 42 43 
267. 
The objective underlying the present invention is to improve the driver's 
steering feel in case of a steering that employs superimposition of 
movement. 
SUMMARY OF THE INVENTION 
The present invention is based on a steering system, or steering device, 
for a motor vehicle with at least one steered axle, an actuator motor, an 
actuator and a steering gear. The overlay transmission and the steering 
gear each feature a mechanical gear ratio. The steered axle, the actuator, 
the actuator and the steering system are in working connection with one 
another. A superimposition of the steering movement initiated by the 
driver, upon the movements initiated by the actuator, takes place by means 
of the overlay transmission. The overall movement thus obtained is fed to 
the steering gear, which relays said overall movement as steering angle to 
the wheels of the steered axle. 
The core of the invention is that the mechanical gear ratio of the steering 
gear is designed variable. Hence, the invention consists of a combination 
of a feedback transmission with a steering gear, or with a steering 
mechanism, with variable gear ratio. The inventional combination, for one, 
leads favorably to selectively adjustable relations between the steering 
wheel movements and the steering angles of the steered wheels, without the 
necessity, for another, of losing good steering feel on the driver's part. 
Obtained as compared to an overlay transmission with a constant steering 
gear ratio, specifically, are more favorable steering wheel moments, or a 
better steering feel. 
The invention can be used, e.g., in steering systems such as cited in the 
initially mentioned DE-OS 40 31 316 or in DE-OS 42 43 267. Envisaged in 
DE-OS 40 31 316 is a variable gear ratio of the steering system 6 shown 
there in FIG. 1, while in DE-OS 42 43 267 a variable gear ratio is 
envisaged of the steering gear (L) illustrated there in FIG. 1. Steering 
gears with variable gear ratio are known from the prior art (refer to, 
e.g., "EVOLUTION in der Lenkungstechnologie" Evolution in Steering 
Technology!, Automobilindustrie April/May-91, pages 315 through 321). 
Named as an advantage of the invention, in particular, is that the 
corrective steering interventions, for instance for vehicle stabilization, 
are possible also, without restriction, with the gear ratio of the 
steering gear configured to be variable. 
A favorable embodiment of the invention provides for a design of the gear 
ratio of the steering gear such that the gear ratio is smaller with 
smaller steering angles than with steering angles of greater magnitude. 
This embodiment of the invention enables the aforementioned good steering 
feel of the driver. 
The steering movement initiated by the driver can be represented by the 
steering wheel angle of the steering wheel actuated by the driver. A 
turning angle initiated by the actuator motor is then superimposed on said 
steering wheel angle by means of the actuator. 
According to the above mentioned prior art, a turning angle of the actuator 
is superimposed on the steering wheel angle, the actuator being activated 
depending on the steering wheel angle and/or depending on vehicle 
movements. A favorable embodiment of the invention provides for the 
turning angle initiated by the actuator motor, furthermore, to be 
dependent on the variable-design mechanical gear ratio of the steering 
gear. 
Acting on the steered wheels, as known, is a reactive moment which via the 
steering gear and the actuator acts as a steering wheel moment on the 
steering wheel actuated by the driver. An inventional design option is 
configuring the mechanical gear ratio of the steering gear in such a way 
that said reactive moment, at smaller steering angles, is less heavily 
effective on the steering wheel than it is with larger steering angles. 
Furthermore, the invention relates to a steering device for a motor vehicle 
with a steering wheel that is actuated by the driver and is in working 
connection with a first input shaft of an actuator. The output of the 
actuator is in working connection with the input shaft of a steering gear, 
and the steering gear is via the steering linkage connected to the steered 
wheels. Moreover, the actuator features a second input shaft that is in 
working connection with the actuator motor. The core of the inventional 
steering connection is that the mechanical gear ratio of the steering gear 
is of variable design. 
Other advantages of the inventional steering device include a gear ratio of 
the steering gear designed such that the gear ratio in the case of smaller 
turns of the input shaft of the steering gear will be smaller than it is 
with larger turns of the input shaft of the steering gear. 
A further advantage of the invention is that straight-ahead travel of the 
vehicle requires only a low energy consumption.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The invention is illustrated hereafter in detail with the aid of an 
exemplary embodiment. 
Shown in FIGS. 1 and 2 is a combination of an actuator with a steering gear 
in a motor vehicle, such as is known from the aforementioned prior art. 
Referenced 11 is the steering wheel actuated by the driver. By actuation 
of the steering wheel, the driver applies a steering angle .delta..sub.L 
and a steering wheel moment M.sub.L which is passed to the actuator 12. 
The turning angle .delta..sub.M of the actuator motor 13 is superimposed 
on the movement .delta..sub.L initiated by the driver of the vehicle, by 
means of the actuator 12. The turning movement .delta..sub.L ' created by 
this superimposition is passed to the steering gear 14 by its input shaft. 
The steering gear 14 then relays this superimposed movement .delta..sub.L 
to the steered wheels 15a and 15b for adjustment of a steering angle 
.delta..sub.v. The mechanical gear ratio of the actuator 12 for 
.delta..sub.M =0 is referenced i.sub.u =.delta..sub.L '/.delta..sub.L, 
while the mechanical gear ratio of the steering gear 14 is referenced 
i.sub.L. The steered wheels 15a and 15b are acted upon by the reactive 
moment M.sub.v influenced by the road. Shown in FIG. 2, additionally, are 
sensors 26 sensing the vehicle 25 movements, such as yaw movements. The 
signals of sensors 26 are processed, by means of the control unit 27, to 
activation signals for the actuator motor 13. Furthermore, element 28 is a 
steering wheel angle sensor, whose signals are fed to the control unit 27. 
The known relations: 
EQU .delta..sub.v =.delta..sub.L /i.sub.u +.delta..sub.M !/i.sub.L (1) 
and 
EQU M.sub.L =M.sub.v /(i.sub.L *i.sub.u) (2) 
apply between the angles and torques illustrated in FIG. 1, or 2. 
A power-assisted steering function is obtained with the steering system 
shown in FIGS. 1 and 2 in that the steering wheel moment M.sub.L is 
reduced by a large overall gear ratio (i.sub.L *i.sub.u), that is, by a 
very indirect steering. Superimposed on the steering wheel angle 
.delta..sub.L is a motor angle .delta..sub.M according to the above 
equation (1), allowing adjustment of a desired front wheel steering angle 
.delta..sub.v with a not overly large steering wheel angle. Suitable 
activation of the actuator motor 13 enables adjustment of selective 
relations between the steering wheel angle and the steering angle, which 
relations may depend also on the state of travel, e.g., the road speed or 
the steering wheel angle. 
According to the above equation (2), the steering wheel moment M.sub.L 
depends only on the reactive moment M.sub.v on the steered wheels and, 
thus, cannot be influenced by motor intervention. Resulting thereof is the 
previously mentioned problem, that selecting a constant gear ratio of the 
steering gear cannot accomplish a steering wheel moment, or steering feel, 
that is acceptable for all states of travel. Especially, the steering 
wheel moment M.sub.L must not become too large with large steering angles 
with the vehicle at rest, whereas when traveling at high speed it must not 
become too small. 
Proposed for solving the problem, as can be seen from FIG. 3, is providing 
the steering gear 14 with a variable gear ratio. FIG. 3 shows the gear 
ratio i.sub.L of the steering gear 14 dependent on the turning input angle 
.delta..sub.L '. The gear ratio i.sub.L is, in the range of small turning 
input angles .delta..sub.L ', selected to be smaller than for larger 
turning input angles. The gear ratio of the steering gear 14 is thus 
stroke-dependent. For exemplary mechanical configuration of such steering 
gear that is geared depending on stroke, reference is made to the 
initially mentioned article "Evolution in the Steering Technology," 
Automobilindustrie April/May-91, pages 315 through 321. 
A stroke-dependent gear ratio i.sub.L of the steering gear as illustrated 
in FIG. 3 produces the patterns shown in FIGS. 4-6. 
FIG. 4a illustrates the steering angle .delta..sub.v as a function of the 
steering wheel angle .delta..sub.L. The assumption in the bottom curve of 
FIG. 4a is that the tuning movement .delta..sub.M initiated by the 
actuator motor 13 equals zero. The nonlinear relationship between the 
turning movements .delta..sub.L and .delta..sub.v illustrated in the 
bottom curve of FIG. 4a is thus arrived at. 
FIG. 4b depicts the turning movement .delta..sub.M, initiated by the 
actuator motor 13 dependent on the steering wheel angle .delta..sub.L, the 
assumption being that a linear relationship is to be established between 
.delta..sub.L and .delta..sub.v. According to the prior art (i.sub.L 
=constant), a proportional movement .delta..sub.M is superimposed to 
assist the steering wheel movement .delta..sub.L. This can be seen in the 
upper, rough pattern of FIG. 4b. When in accordance with the invention the 
gear ratio i.sub.L of the steering gear 14 is chosen to be variable (FIG. 
3), the actuator motor 13 is inventionally activated by the controller 27 
as a function of the steering wheel movement .delta..sub.L, such as 
illustrated in the bottom pattern of FIG. 4b. 
The activation of the actuator motor 13 shown in the bottom curve pattern 
of FIG. 4b produces the dependence between the steering angle 
.delta..sub.v and steering wheel angle .delta..sub.L as shown in the top 
curve pattern of FIG. 4a. This dependence is essentially linear, as 
desired. 
An appropriate activation of the actuator motor 13 (FIG. 4b, bottom 
pattern) engenders a quasi "compensation" of the variable-design gear 
ratio i.sub.L of the steering gear (FIG. 3) as regards the turning angle. 
That is, due to the variable gear ratio i.sub.L the driver senses, with 
respect to link between the steering wheel angle .delta..sub.L and the 
steering angle .delta..sub.v, no change as compared to the prior art. This 
is evident also from the aforementioned equation 1. 
The situation is different when viewing the respective moments. Here, FIG. 
5 illustrates the steering wheel moment M.sub.L as as a function of 
reactive moment M.sub.v that is active on the steered wheels. As is 
evident from equation 2, the linear relationship between the moments 
illustrated in FIG. 5 changes depending on the gear ratio i.sub.L of the 
steering gear and, thus, depending on the steering wheel angle 
.delta..sub.L. 
A different presentation of these circumstances derives from FIG. 6. Here, 
the steering wheel moment M.sub.L is shown as a function of steering wheel 
angle .delta..sub.L in the case when the reactive moment M.sub.v is 
assumed to be constant. FIG. 6 clearly evidences that the steering wheel 
moment M.sub.L felt by the driver is at constant reactive moment M.sub.v 
greater in the range of small steering wheel angles than it is in the 
range of large steering wheel angles. 
Achieved with the variable gear ratio of the steering gear 14 according to 
the invention, for one, and with the appropriate activation of the 
actuator motor 13 for another, is that large steering wheel moments with 
large steering angles are avoided with the vehicle at rest, but without 
losing, at high travel speeds and small steering wheel angles the driver's 
good steering feel (large steering wheel moments). 
As described in the aforementioned articles, a stroke-dependent gear ratio 
of the steering gear can be realized, e.g., with rack-and-pinion drives. 
Another option of making the gear ratio stroke-dependent consists in a 
suitable configuration of the gear mechanism between steering gear and 
wheels.