The present invention relates to a method of setting or establishing an assisting or auxiliary force in power-assisted steering as a function of a plurality of driving-condition parameters such as transverse acceleration and vehicle speed.
In a known assisting force method as shown in Japanese Preliminary Published Specification 51-149,632, the manual torque to be applied by the vehicle driver during a steering action is increased as transverse acceleration increases and as vehicle speed increases. The effect of the transverse acceleration and vehicle speed variable on the change in the manual torque to be applied is non-interacting. In other words, the extent of the increase in the manual torque to be applied as vehicle speed increases is independent of the magnitude of the transverse acceleration, and the extent of the increase in the manual torque to be applied as transverse acceleration increases is independent of the magnitude of the vehicle speed.
Such known method may be disadvantageous in that, from the increase in the manual torque to be applied as vehicle speed increases, no direct information is available to the vehicle driver as to whether the vehicle is in or close to a critical state. The same also applies to the increase in the manual torque to be applied as transverse acceleration increases.
In addition, another known method disclosed in DE-2,834,283 A1, varies the assisting force in the power-assisted steering as a function of the transverse acceleration in such a way that no assisting force appears in the power-assisted steering if the transverse acceleration exceeds a certain limit value. This assisting force is here cut off abruptly when the limit value is reached.
A disadvantageous effect in this method is that the vehicle driver can be forced into panic reactions by the sudden change in the force which can be felt at the steering wheel. In addition, as transverse acceleration increases, the force which can be felt at the steering wheel in a vehicle without power-assisted steering clearly drops when the transversely dynamic limit region is met. If, in this method, there is now a large assisting force in the power-assisted steering before the vehicle reaches the threshold value of the transverse acceleration, the effect just described for a vehicle without power-assisted steering is reversed. If the full force of the steering then reacts upon the steering wheel, since the assisting force has been removed, a greater force will be felt by the vehicle driver as a function of the size of the previously assisted force. For a moment, the vehicle driver will be surprised by the greater force, and the steering of the vehicle will be set back until the vehicle driver reacts in a compensating manner, i.e., the steering angle will be reduced and the vehicle will be unintentionally steered in a direction out of the bend. This encourages instability, which occurs anyway, of the driving behavior of a vehicle when the transversely dynamic limit region is reached, at which point the vehicle slides, via the wheels, out of the bend.
Furthermore, it is known from DE-3,424,826 A1 and De-3,504,561 A1 to deduce a general driving state with the use of suitable variables by taking an average. In particular, a distinction is to be made between driving in town, driving on the motorway, driving on an ordinary road and driving on a mountain road, and in particular driving on a winding road. The vehicle speed and the steering angle are used as characteristic variables. A load sensor is also used. The general driving state is determined from these variables by taking the average. For this driving state, a corresponding assisting force is set in the power-assisted steering.
In the method disclosed in the two last-mentioned publications, a disadvantage arises because adaption of the assisting force in the power-assisted steering to the existing instantaneous driving conditions might take place too late. In addition, the subdivision with regard to the driving conditions is only very rough, since the instantaneous driving situation is adapted to that driving condition to whose parameters it is nearest.
In addition, it is generally known in power-assisted steering systems to reduce the assisting force in a powerassisted steering as vehicle speed increases so that the vehicle driver is given considerable assistance (maneuvering) in the region of low vehicle speeds and has a more direct contact with the roadway as vehicle speed increases.
An object of the invention is, therefore, to vary an assisting force in the steering in such a way that the force to be applied by the vehicle driver during steering assumes such an amount that a critical driving state can be estimated, and a suitable assisting force in the steering is immediately made available to the vehicle driver under the instantaneous driving conditions.
This object has been achieved by providing a method of setting an assisting force in a power-assisted steering as a function of a plurality of instantaneous driving-condition parameters, such that at a transverse acceleration, a.sub.y2, a greater reduction in the power-assisted steering, as vehicle speed, v.sub.F, increases, is effected than at a transverse acceleration, a.sub.y1, where a.sub.y1 &lt;a.sub.y2.
A significant advantage of the present invention is that the vehicle driver obtains a better feel for the particular driving situation. In addition, a more accurate setting of the assisting force in the power-assisted steering as a function of the driving conditions is possible, since allocation to a few discrete driving conditions is not carried out, but a discretization is effected in the form of subdivisions with regard to at least one variable which characterizes the driving conditions and which, if need be, is supplemented by an interpolation between these discrete values.
The driving conditions are mainly characterized by two parameters, namely vehicle speed, v.sub.F, and transverse acceleration, a.sub.y. In the method according to the invention, the two parameters are taken into account simultaneously by the relationship between manual torque, T.sub.H, and steering-gear arm torque, T.sub.H, being plotted against these two parameters in a characteristic diagram. With this desired value, the assisting force is then set or established in the power-assisted steering system, which assisting force results in modulation of the manual torque, T.sub.H, as a function of the driving conditions. The relationship between manual torque, T.sub.H, and steering-gear arm torque, T.sub.L, is here selected in such a way that a manual torque, T.sub.H, which is convenient for the vehicle driver is set as a function of the driving conditions.
In the maneuvering region, a large assisting force in the power-assisted steering is to made available to the vehicle driver. The maneuvering region is recognized by the occurrence of both a low vehicle speed, v.sub.F, and a low transverse acceleration, a.sub.y. If the vehicle speed, v.sub.F, and the transverse acceleration, a.sub.y, are zero, a large assisting force in the power-assisted steering is made available to the vehicle driver, which assisting force is then reduced when the vehicle starts to move (vehicle speed, v.sub.F, not 0). The desired value for the manual torque, T.sub.H, then increases digressively with both the vehicle speed, v.sub.F, and the transverse acceleration, a.sub.y. The subdivision with regard to the vehicle speed, v.sub.F, when taking into account an interval of 0-v.sub.F MAX (maximum vehicle speed), takes place advantageously by a variable configuration of a step width, e.g. the step width being at 2 km/h in the region of low vehicle speeds, v.sub.F, and increasing to 20 km/h as vehicle speed, v.sub.F, increases. A convenient subdivision with regard to the transverse acceleration, a.sub.y, when taking into account an interval of 0-8 m/s.sup.2, is obtained by the step width being about 0.25 m/s.sup.2 in the region of low transverse acceleration, a.sub.y, and increasing to about 1-1.5 m/s.sup.2 as transverse acceleration, a.sub.y, increases. About 20 to 30 speed classes and about 10 transverse-acceleration classes are therefore obtained. It is in principle also conceivable to use other step widths or other classes for the vehicle speed, v.sub.F, and/or the transverse acceleration, a.sub.y.
Before the transversely dynamic limit region is reached, the desired value for the manual torque, T.sub.H, here has such an order of magnitude that the drop in the reactive force can clearly be recognized as an indication of the transversely dynamic limit region being reached.
In order to maintain the feel for the possible occurrence of roadway slipperiness in particular in the region of low vehicle speed, v.sub.F, and low transverse acceleration, a.sub.y, on account of the order of magnitude of the assisting force, the method of setting an assisting force in the power-assisted steering according to the present invention is advantageously performed in such a way that, when roadway slipperiness is recognized, the assisting force in the power-assisted steering system is reduced to an amount which corresponds to the amount at a higher vehicle speed, v.sub.F, or a higher transverse acceleration, a.sub.y. This reduction in the assisting force in power-assisted steering enables a vehicle driver to recognize the drop in the force when the transversely dynamic limit region is reached.
If roadway slipperiness increases, the transversely dynamic limit region is reached at decreasing transverse acceleration, a.sub.y. Thus, different roadway states can be taken into account when determining the assisting force in the power-assisted steering by the assisting force being reduced as a function of the difference in the transverse acceleration, a.sub.y. This reduced assisting force results, on one hand, from the transverse acceleration, a.sub.y, when the transversely dynamic limit region is reached under normal roadway conditions and, on the other hand, from the transverse acceleration, a.sub.y, when the transversely dynamic limit region is reached under the instantaneously determined roadway conditions.
Roadway slipperiness which occurs can also be recognized by a deviation of the force in the steering linkage when cornering from a reference value of the force in the steering linkage under optimum roadway conditions or from the appearance of signals, or their effects, from a drive-slip control (DSC) or from the appearance of signals, or their effects, from an antilock braking system (ABS). The effects of the signals associated with the ABS can be the brake pressures appearing at the individual wheels or the forces on the brake calipers or brake drums determined in a manner known per se. In addition, a possible effect of the signals from a DSC is a reduction in the engine torque, which reduction is preferably produced by the position of the throttle valve or of the control rod being manipulated. In principle, the extent of the requisite reduction in the assisting force in the power-assisted steering can be derived from any of these specified means of determining the roadway slipperiness.
Differences in the steering behavior of the vehicle which result from different loading states of the vehicle are compensated in an advantageous manner by suitably varying the assisting force in the power-assisted steering as a function of the loading state. In principle, it is possible to vary the characteristic diagram of the assisting force in the power-assisted steering by a discretization with regard to the loading state of the vehicle. However, an accumulation of a large quantity of data on account of the discretization with regard to the loading state can be avoided by two characteristic diagrams of the assisting force being included in the power-assisted steering, in accordance with the "empty" and "full" loading states. The two values of the assisting force in the powerassisted steering are then selected for the instantaneous driving conditions (vehicle speed, v.sub.F, and transverse acceleration, a.sub.y). If the actual loading state is known, by an interpolation between these two values of the assisting force in the power-assisted steering, an assisting force can be controlled in such a way that a steering behavior appears on the vehicle which is independent of the actual loading state. This interpolation between the two extreme loading states can be performed as a linear interpolation.
It is also possible, in preliminary tests, to experimentally determine in a more accurate manner the dependency of the steering behavior on the loading state, e.g. by the discretization described above, in order to reach an improved balance with regard to loading states by a complicated profile of the assisting force in the power-assisted steering being approximated by a suitable function. The parameters of this function are determined in a manner known per se, for example by the method of parameter identification. The actual loading state can here be determined by measuring the weight, F.sub.VA, on the steered axle, which weight, F.sub.VA, is determined by wire strain gauges or, in vehicles having air suspension, by measuring the pressure of the air spring. With this method, the loading, which has an influence on the steering behavior of the vehicle, is taken into account when the method according to the present invention is performed. For example, a change in the steering behavior when driving downhill (greater weight on the steered axle) and when driving uphill (less weight on the steered axle) is also taken into account here.
Furthermore, the height of the center of gravity can in principle be take into account when determining the requisite assisting force in the power-assisted steering. An increasing height of the center of gravity during cornering causes the weight to be displaced towards the outside of the bend. This necessitates a greater skew angle; in other words, the force reacting on the steering wheel assumes its maximum value at a low transverse acceleration, a.sub.y.
It is also possible to measure the height of the center of gravity during driving and to derive from this measured variable a change in the assisting force in the power-assisted steering in such a way that the drop in the force reacting on the steering wheel can be recognized by the vehicle driver when the transversely dynamic limit region is reached, i.e. that the assisting force in the power-assisted steering is reduced as the height of center gravity increases. The determination of the height of the center of gravity during driving can be dispensed with if the assisting force in the power-assisted steering definitely does not exceed such an amount that, under all relevant loading states, the drop in the reacting force on the steering wheel, when the transversely dynamic limit region is reached, can be recognized by the vehicle drive under all roadway states.
If the angle of the steering wheel is recorded in the vehicle, the angular velocity of the steering wheel can be obtained therefrom by differentiation. From a high angular velocity of the steering wheel, a situation can then be deduced which, for example, in a danger situation, requires an increasing assisting force in the power-assisted steering. This increase in the assisting force in the power-assisted steering can here increase linearly with the angular velocity of the steering wheel; in another embodiment, the increase in the steering force in the power-assisted steering can increase progressively with the angular velocity of the steering wheel.
A functional check on power-assisted steering can be carried out by measuring the manual torque, T.sub.H, to be applied to the steering wheel by the vehicle driver (for example by the torsional moment in the steering column tube) and comparing the manual torque, T.sub.H, with the desired value for the manual torque, T.sub.H, which desired value corresponds to the driving conditions. In the event of a defect in the power-assisted steering, a deviation occurs between the measured manual torque, T.sub.H, and the desired value.