WO2007031817A1

WO2007031817A1 - Method of controlling a steer by wire steering system

Info

Publication number: WO2007031817A1
Application number: PCT/IB2005/003561
Authority: WO
Inventors: François DECHAMP
Original assignee: Renault Trucks
Priority date: 2005-09-16
Filing date: 2005-09-16
Publication date: 2007-03-22

Abstract

The invention relates a method of determining a steering ratio in a steer-by-wire system for a vehicle comprising a control member (2) for controlling at least one steerable vehicle wheel (7). Said steering ratio is the ratio between an angle a of the control member and a steering angle θ of the steerable wheel. The method according to the invention comprises the step of detecting the speed of said vehicle; and the further step of determining a maximum allowable steering angle θ of the steerable wheel based on said detected speed and a predetermined maximum transverse acceleration criterion. The method defines for any vehicle speed a maximal angle for the steerable wheel. Therefore, the method of the invention makes it possible to ensure that when the steering control member is oriented at full travel the steerable wheel angle stays within an angular area defined as a function of a transverse acceleration area.

Description

METHOD OF CONTROLLING A STEER BY WIRE STEERING SYSTEM

Technical field The present invention relates to a method of controlling a steer-by-wire steering system.

Technical background

Conventionally in a land-based vehicle, the steering system, that is to say the combination of means allowing the course of the vehicle to be directed, is implemented by a mechanism that connects a control member, generally a steering wheel, to at least one steered axle generally comprising two steerable wheels, and which allows the orientation of the wheels relative to the vehicle to be modified. The mechanism may be assisted by electrical or hydraulic means in order to make it easier to transmit the rotation force from the control member to each wheel of the steering wheel axle.

There also exists another type of steering system in which the control member is mechanically decoupled from the steering wheel axle.

Schematically, a steer-by-wire steering system comprises a control member (steering wheel, lever, or joystick) on which a driver can act. By acting on the control member, the driver fixes a setpoint direction to be given to the vehicle. The movement that the driver applies to the control member is identified by a position sensor (an angle sensor in the case in which the control member is a steering wheel). The signal coming from the position sensor of the control member is suitably processed and then sent to one or more actuators that act on the orientation of the wheels relative to the vehicle, depending on the action that the driver has performed on the control member.

This type of steering has many advantages over conventional steering systems. In particular, this type of steering reduces the risk of injury by contact with the steering column in the event of an accident, and can be easily parameterized. It is also particularly advantageous in the case of industrial vehicles, of the truck type, which have considerable architectural constraints and generally have a cab decoupled from the chassis. A steer-by-wire steering system especially avoids having to use a conventional steering column that has to be rigidly fixed both to the cab and to the chassis.

It should be pointed out that a resistive torque is applied to the control member so that the driver has a sensation representative of the force exerted by the wheel on a running surface.

In a steer-by-wire steering system, it is possible to parameterize the steering in such a way that the steering angle of the steerable wheels is a function of the angle of the control member and of the speed of the vehicle.

A steering system parameterised in this manner may be called a variable steering ratio system. Taking a mechanical steering system as reference, for which the steering angle of the steerable wheels is a substantially linear function of the command angle applied to the control member, in contrast a steer-by-wire steering system allows the steering ratio to vary according to the vehicle speed and according to the command angle applied to vehicle control member.

A widely spread method to define the steering ratio in a steer-by-wire steering system consists of defining a steering ratio which increases with the speed of the vehicle.

This means, taking again a mechanical steering system as reference that, for a low speed, the driver has to deploy a slighter action on the control member, this being an important comfort factor, whereas at high speed, especially on a motorway, any inopportune action on the control member will be moderated as regards the steering angle of the steerable wheels, this being an important safety factor.

A possible drawback of this kind of method is that a speed variation

- acceleration or deceleration - while the vehicle is in a curve has an effect on the steering ratio; the consequence is that, an acceleration or a deceleration in a curve may cause, for a given orientation of the steering wheel, an alteration in the orientation of the steerable wheels. It turns out that, in a steer-by-wire steering system, the definition of the steering ratio which links an action applied onto the control member and the steer angle according to which a vehicle steerable wheel is oriented could be enhanced.

Summary of the invention

One object of the invention is to propose a method of defining a steering ratio in a steer-by-wire steering system that improves the vehicle stability.

The subject of the invention is therefore a method of determining a steering ratio in a steer-by-wire system for a vehicle comprising a control member for controlling at least one steerable vehicle wheel. Said steering ratio is the ratio between an angle α of the control member and a steering angle θ of the steerable wheel. The method according to the invention comprises the step of detecting the speed of said vehicle; and the further step of determining a maximum allowable steering angle θ of the steerable wheel based on said detected speed and a predetermined maximum transverse acceleration criterion.

The point of the invention is to define for any vehicle speed a maximal angle for the steerable wheel. The premise of the method according to the invention is that the steering control member has a movement of fixed amplitude; in most cases where a vehicle has a steering wheel as a steering control member, the steering wheel has a fixed angular travel. Therefore, the method of the invention makes it possible to ensure that when the steering control member is oriented at full travel the steerable wheel angle stays within an angular area defined as a function of a transverse acceleration area.

More precisely, the method can comprise the act of orienting a steering control member by an angle α; and converting said command angle α into a steering angle θ of at least one steerable wheel according to a steering ratio R equal to α/θ, wherein the minimal steering ratio R_min is equal to α_maχ/θ_maχ_, where the maximum angle of the steering wheel α_maχ is a vehicle construction value and the maximum steering angle θ_max is a function of a transverse acceleration criterion.

In one possible implementation of the method, the steering ratio Rmin for at least one steerable wheel can be equal to α_maχ/θ_max, where θ_max is the maximum steering angle, is approximately determined by the formula: eAy/V², where e is a geometrical constant associated with the vehicle and AY is a rollover transverse acceleration criterion. Thus, for a given speed the method of the invention defines a maximum steerable wheel angle θ_max; this, with the fact that the vehicle command member can be oriented within the limit of a maximal angle α_maχ makes it possible to define a minimal steering reduction.

According to one way of implementing the method according to the invention, the reduction ratio R_mj_n may be a function of a transverse acceleration criterion for vehicle low and intermediate speeds, whereas for vehicle very low speeds the steering ratio α/θ may be constant and equal to a vehicle geometrical reduction ratio Rn, and for vehicle high speeds the steering ratio α/θ may be constant and equal to a predetermined reduction ratio Rm.

These choices take into account the fact that, at very low speed, the transverse acceleration has little influence on the vehicle and the actual construction of the vehicle means that the steerable wheels present physical stops that set the maximum steering angle. However, at high speeds, for driving comfort, it may be desirable to set the reduction ratio at a constant value in order to avoid the reduction ratio increasing without limit as the speed increases.

In one possible way of implementing the method according to the invention, for determining θ_max the transverse acceleration criterion is less than the transverse rollover acceleration. This approach amounts to enhancing the operational safety of the vehicle in so far as the steering angle of the steerable wheels cannot reach a value that would make the vehicle reach its limiting transverse acceleration. In practice, this means that the driver is limited to being able to steer his vehicle within the range of safe movements of the vehicle. In another way of implementing the method according to the invention, for determining θ_max the transverse acceleration criterion is greater than the transverse rollover acceleration. This approach makes it possible to take into account a transverse acceleration that exceeds, very briefly, the tolerable transverse rollover acceleration threshold without thereby causing the vehicle to roll over.

It may also be considered to define the transverse acceleration criterion as a fraction of the transversal rollover acceleration (i.e. 80%, 90%, 100% or 120% of the rollover transverse acceleration), a rate of closure to the rollover transversal acceleration or a function of the rollover transversal acceleration or a combination of two or more of these data.

In an embodiment of the invention, the transverse acceleration criterion is periodically weighted by the vehicle operational parameters. Thus, the transverse acceleration criterion is dynamically adjusted to the vehicle operational vehicle at one time.

For example the vehicle operational parameters can be taken from the group of the weight of vehicle load and/or the quantity of petrol in the vehicle tank. The transverse acceleration criterion can be weighted by the loading of the vehicle. This allows any loading of a vehicle, which may quite appreciably modify the dynamics of the vehicle, to be taken into account. It should be noted that, on a vehicle of the industrial vehicle type, which generally employs air suspension, it is possible by measuring the deflection of the suspension to deduce the mass of an on-board load.

Brief description of the drawings

In order for the invention to be properly understood, it will be described with reference to the figures appended hereto, in which:

- Figure 1 is a block diagram of a steer-by-wire steering system; and

- Figure 2 is a diagram showing the steering ratio according to the invention as a function of the speed. Description of the invention

Figure 1 shows schematically a steering system of the steer-by-wire type. This steering system therefore includes a control member 2 which, in the example illustrated, is a steering wheel.

This control member 2 may be oriented at an angle α by the driver of the vehicle.

A position sensor 3, an angle counter in the example shown, records the angular displacement of the control member 2 and generates a signal that is sent to a CPU 4.

The CPU 4 can suitably include a microprocessor, a data memory such as a RAM memory and a program memory such as a ROM memory.

In addition to receiving a signal representative of the angle α of the control member 2, the computer 4 also receives a signal representative of the speed of the vehicle. This signal may come from a speed sensor 5.

From the angle α and the speed of the vehicle, the CPU 4 generates an output signal that is sent to an actuator 6 mechanically linked to the steerable wheels 7 of the vehicle.

Figure 1 shows only one steerable wheel, however, it is understood that a vehicle generally is equipped with two or more steerable wheels.

Not shown in the figure are the means for generating a resistive torque, which are generally present in a steer-by-wire steering system but which has nothing to do with the present invention.

As indicated, the steering angle θ depends on the angle α and on the speed of the vehicle. The ratio of the angle α to the angle θ therefore defines the reduction ratio by analogy with the reduction ratio of a mechanical steering system.

In the case of a steer-by-wire steering system, the reduction ratio may vary depending on the speed, something which, of course, cannot be obtained with a mechanical steering system in which the reduction ratio is set by gear ratios.

The benefit of a steering system with a variable reduction ratio is known, this making it possible to have very direct steering at low speed or at intermediate speed, and little direct steering at high speed. For the driver, this means that, at low speed or at intermediate speed, he or she can steer the steerable wheels by a relatively limited action on the steering control member. In contrast, at high speed the reduction ratio is such that an angle through which the steering control member is turned corresponds to a relatively small angle through which the wheels are turned.

Figure 2 illustrates the minimum reduction ratio as a function of the speed of the vehicle. The term "minimum reduction ratio" is understood to mean the ratio of the maximum angle α_max that the driver can turn the steering wheel to the maximum angle θ_maχ that the steerable wheels can adopt for a given speed.

The determination of the ratio R_min according to the criteria of the invention is more particularly of benefit in the speed range I, which may be termed as a low and intermediate speed range, with speeds possibly ranging from 15 km/h to 65 km/h. These speed values are given purely by way of indication and in no case must be considered as parameters of the invention.

In this speed range, the invention provides for the steering ratio Rmin to be set by a steering angle θ_maχ which is a function of a transverse acceleration criterion. Thus, the fact of determining the steering angle θ_max makes it possible to take into account the transverse rollover acceleration. According to one possibility, θm_ax is determined by the formula eA_y/V², in which: - e is a geometrical constant of the vehicle, which involves the wheelbase, that is to say the distance separating a steering front axle from a non-steered rear axle.; and

- Ay is a transverse acceleration criterion, this being a datum associated with the architecture of the vehicle (especially the mass, height and length of the vehicle). The transverse acceleration may possibly be weighted by a load of the vehicle. The fact that a vehicle may carry or not a load is a factor that influences the behaviour of a vehicle, more especially a vehicle with a high centre of gravity, such as a lorry or truck.

It can be pointed out that it is possible to set the transverse acceleration criterion to be less than the transverse rollover acceleration.

In such a situation, the steering angle of the steerable wheels cannot reach a value that would make the vehicle reach its limiting transverse acceleration. In practice, this amounts to limiting the driver to being able to steer his vehicle within the range of safe movements of the vehicle.

However, it is possible to choose to set the transverse acceleration criterion to be greater than the transversal rollover acceleration. In such a situation, the method may permit a steerable wheel angle that results in a vehicle transverse acceleration that exceeds the transverse rollover acceleration. It is then the responsibility of the driver to determine the maximum steering angle to be given to the vehicle.

For the very low speed range Il (for example below 15 km/h), it is possible to provide a constant reduction ratio. Firstly, the transverse acceleration has little influence at low speeds. In addition, the actual construction of the vehicle means that the steerable wheels present physical stops that set the maximum steering angle.

For the high speed range III (for example above 65 km/h), it is also possible to envisage providing a constant reduction ratio. This is because it may be desirable for high speeds to set the reduction ratio at a constant value, which may by way of indication be between 17 and 22, in order to prevent the reduction ratio from increasing without limit as the speed increases. This choice makes it possible to avoid, at high speeds, having an excessive reduction ratio, this being desirable for the average driver.

Thus, it may be seen that this limitation provides a method that allows a minimum reduction ratio to be set according to a transverse acceleration criterion, thereby making it possible in particular to make the behaviour of a vehicle safe by limiting the steering angle of the steerable wheels to a range within which the transverse acceleration of the vehicle remains less than its limiting rollover acceleration.

Of course, the invention is not restricted to the embodiment described above by way of non-limiting example, but on the contrary it encompasses all embodiments thereof.

Claims

1. A method of determining a steering ratio in a steer-by-wire system for a vehicle comprising a control member (2) for controlling at least one steerable vehicle wheel (7), said steering ratio being the ratio between an angle (α) of the control member (2) and a steering angle (θ) of the steerable wheel (7) characterized in that the method comprises the steps of:

- detecting the speed of said vehicle - determining a maximum allowable steering angle (θ) of the steerable wheel (7) based on said detected speed and a predetermined maximum transverse acceleration criterion.

2. The method according to claim 1 characterized in that the method comprises the act of:

- orienting a steering control member (2) by a command angle α;

- converting said command angle (α) into a steering angle (θ) of at least one steerable wheel according to a steering ratio R equal to α/θ, wherein the minimal steering ratio R_mj_n is equal to αmaχ/θ_max, where the maximum angle of the steering wheel α_maχ is a vehicle construction value and the maximum steering angle θ_maχ is a function of a transverse acceleration criterion.

3. The method according to claim 2, characterized in that the steering ratio R_mι_n for at least one steerable wheel is equal to α_maχ/θ_maχ, where θmax is substantially determined by the formula: eA_y/V², where e is a geometrical constant associated with the vehicle and A_y is a transverse acceleration criterion.

4. The method according to one of claims 2 to 3, characterized in that for vehicle low and intermediate speeds, the steering ratio is equal to α_maχ/θ_maχ, where the maximum steering angle θ_max is a function of a transverse acceleration criterion.

5. The method according to one of the claims 2 to 4, characterized in that, for vehicle very low speeds, the steering ratio α/θ is constant and equal to a geometrical reduction ratio R|.

6. The method according to one of the claims 2 to 5, characterized in that, for vehicle high speeds, the steering ratio α/θ is constant and equal to a predetermined reduction ratio Rm.

7. The method according to one of claims 1 to 6, characterized in that, the transverse acceleration criterion Ay has a preset value which is less than the vehicle rollover transverse acceleration.

8. The method according to one of claims 1 to 6, characterized in that, the transverse acceleration criterion Ay has a preset value which is greater than the vehicle rollover transverse acceleration.

9. The method according to one of the claim 1 to 6, characterized in that the transverse acceleration criterion Ay is a fraction of the rollover transverse acceleration, a rate of closure to the rollover transverse acceleration or a function of the rollover transverse acceleration or a combination of two or more of these data.

10. The method according to claim 9, characterized in that the transverse acceleration criterion A_y is periodically weighted by the vehicle operational parameters.

11. The method according to claim 9, characterized in that the vehicle operational parameters are taken from the group of:

- the weight of vehicle load,

- the quantity of petrol in the vehicle tank.