Abstract:
A method of steering a vehicle with a superimposed steering system, wherein a steering angle input by the driver and an additional angle (additional steering angle) is determined and wherein the additional steering angle can override the input steering angle according to further quantities, in particular diving-dynamics quantities, by means of an electric motor, is characterized in that the method includes a steering angle control with a subordinated current or torque control of the electric motor.

Description:
TECHNICAL FIELD 
   The invention relates to a method of steering a vehicle with a superimposed steering system, wherein a steering angle input by the driver and an additional angle (additional steering angle) is determined and wherein the additional steering angle can override the input steering angle according to further quantities, in particular diving-dynamics quantities, by means of an electric motor. 
   BACKGROUND OF THE INVENTION 
   Up-to-date motor vehicles, in particular passenger vehicles, are generally equipped with hydraulic or electrohydraulic servo steering systems, wherein a steering wheel is compulsively coupled mechanically with the steerable vehicle wheels. The servo assistance is devised such that actuators, e.g. hydraulic cylinders, are arranged in the mid-portion of the steering mechanism. A force generated by the actuators assists in the actuation of the steering mechanism in response to the turning of the steering wheel. This reduces the force the driver has to apply during the steering operation. 
   Superimposed steering systems are known in the art. They are characterized in that the steering angle input by the driver can be overridden in case of need by another steering angle (additional steering angle) by means of an actuator. Usually electric actuators are employed which act on an overriding drive and adjust the additional steering angle largely independently of the driver. 
   The additional steering angle is controlled by an electronic controller and is e.g. used to increase the stability and agility of the vehicle. According to a prior art control concept, as described in DE 197 51 125 A1, the steering components of the superimposed steering angle are produced irrespective of each other. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the invention is to provide a method of steering a vehicle with a superimposed steering that is safe and reliable in operation. 
   According to the invention, this object is achieved in that the method includes a steering angle control with a subordinated current or torque control of the electric motor. 
   To this end, a nominal current or a nominal motor torque is produced by means of which the electric motor introduces an additional steering angle into the steering system. Due to the angle superimposed on the steering actuation, the desired steering angle and, hence, also the additional steering angle is adjusted, which latter is additionally demanded by other vehicle control systems, as the case may be. 
   It is arranged for in the invention that an actual steering angle value and a nominal steering angle value is determined and, according to a comparison between the actual steering angle value and the nominal steering angle value, a nominal current or a nominal motor torque is produced by which the electric motor introduces the additional steering angle into the steering system. 
   A favorable embodiment of the method of the invention includes that a steering request of the driver δ DRV  is determined on the basis of a steering wheel angle δ H  adjusted by the driver, wherein the driver&#39;s steering request δ DRV  is composed of the adjusted steering wheel angle δ H  and an invariably or variably predeterminable gear ratio factor and the gear ratio factor is chosen corresponding to the current driving situation, in particular a detected longitudinal vehicle speed and/or a steering wheel turning angle, and that a nominal steering angle value δ nominal  is determined on the basis of the so calculated steering request of the driver and sent to the steering control. 
   According to another embodiment of the invention, the driver&#39;s steering angle δ H  is determined and, in connection with a gear ratio factor i L1  by which the driver&#39;s steering angle acts directly on the steering gear, an additional steering angle δ M  is additively superimposed thereon in connection with a second gear ratio i L2 , and a superimposed steering angle δ L  is determined and sent as an actual value δ L,actual  to the steering control, with said superimposed steering angle δ L  being determined according to the following formula:
 
δ L   =i   L1 *δ H   +i   L2 *δ M .
 
   The invention provides that a driving dynamics control (ESP system) cooperates with the steering control and that an additional steering angle Δδ responsive to driving dynamics is determined when the necessity of a stabilizing intervention is detected by driving dynamics control. 
   Preferably, the additional steering angle Δδ responsive to driving dynamics that is produced on the basis of a correcting intervention of a driving dynamics controller is additively superimposed on the driver&#39;s steering request δ DRV . 
   The control of the superimposed steering is improved by this embodiment of the method of the invention in particular in highly dynamic driving situations. The term ‘highly dynamic driving situation’ refers to all driving situations with a relatively quick change of the vehicle direction and/or the vehicle speed, which can cause instability of the vehicle or the desired vehicle movement. Driving situations in the frontier of driving dynamics, such as skidding maneuvers, demand too much from many drivers regarding a suitable steering performance. 
   It is arranged for by the invention that based on the series steering ratio i L,series  and due to a boosting factor K 1  responsive to a steering wheel angle and a boosting factor K 2  responsive to the vehicle speed, a resulting steering ratio I L,ESAS  which corresponds to the ratio between the steered wheels δ V  and the driver&#39;s steering angle δ H  is determined according to the following formula:
 
 i   L,ESAS =δ V /δ H   =i   L,series /( K 1* K 2)
 
   According to the invention, an anticipatory control of the nominal speed of the motor ω M,nominal  is executed, which is determined from a motor speed specification ω M,spec  and a motor speed preset value ω M,reg , and the motor speed preset value ω M,reg  is determined on the basis of a comparison between a nominal steering angle value δ L,nominal  and a determined actual steering angle value δ L,actual , and the motor speed specification ω M,spec  is determined from the time derivative of the nominal steering angle value δ L,nominal  and the driver&#39;s steering angle δ H  and a gear ratio factor i L2  by means of the following formula:
 
ω M,spec =({dot over (δ)} L,nominal   −i   L1 {dot over (δ)} H )/ i   L2 .
 
   According to the invention, the control of the motor of the superimposed steering is realized by a computer program which includes appropriate program steps for implementing the described method. 
   The above object is also achieved by a steering system for a vehicle, comprising a steering wheel arranged at a steering column, a steering gear, a steering angle sensor arranged at the steering column, an overriding motor that acts on the steering column by way of an overriding gear, an electric steering control element, a sensor for measuring the position of the steered wheels, and a steering control device, in which steering system the steering control device includes a means for implementing the method of the invention described hereinabove. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, 
       FIG. 1  is a block diagram of the basic structure of the method of the invention. 
       FIG. 2  is a block diagram of the structure of the method of the invention. 
       FIG. 3  is a block diagram for determining a nominal value and an actual value as input quantities of the steering angle control according to the invention. 
       FIG. 4  is a block diagram for determining a nominal value and an actual value as input quantities of the steering angle control according to the invention. 
       FIG. 5  is a block diagram for determining a motor torque specification for the electric motor for adjusting the overriding angle according to the invention. 
       FIG. 6  is a block diagram for determining a field weakening current and a nominal current for actuating the electric motor according to the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The basis structure of the method of the invention is represented in  FIG. 1 . 
   Based on the steering wheel angle δ H    50  adjusted by the driver, the driver&#39;s steering request δ DRV    52  is calculated in the basic steering function as a nominal steering angle value  53  δ L,nominal  (input quantity) for the steering control circuit  54  by way of a variably or invariably predeterminable gear ratio i L,ESAS    51 . In this arrangement, the basic steering function generally comprises the selection of a steering ratio i L,ESAS  corresponding to the current driving situation, e.g. the detected longitudinal vehicle speed. The actuator of the steering system is then driven corresponding to a steering angle δ L    55  (output quantity of the control circuit  54 ). 
   Driving stability and agility of the vehicle can be enhanced by means of adapting the position of the steered wheels, principally irrespective of the driver&#39;s request. To this end, an additional steering angle Δδ  56  responsive to driving dynamics is additively superimposed  58  on the driver&#39;s steering request δ DRV    52  on the basis of a correcting intervention of a driving dynamics controller  57 . The result is the nominal steering angle value δ L,nominal . 
     FIG. 2  shows the structure of the method. The driver&#39;s steering angle δ H  acts in the overriding gear as an input quantity  1  by way of a mechanical gear  2  with a gear ratio factor i L1  directly on the steering gear  3  (i L1 *δ H )  19 . The additional steering angle δ M    16  adjusted by a motor acts by way of a second gear  17  with a gear ratio factor i L2  and is additively superimposed on the geared steering angle of the driver:
 δ L   =i   L1 *δ H   +i   L2 *δ M . 
   The steering gear  3  generates as an output quantity a resulting steering angle δ V  that acts upon the vehicle. 
   The driving dynamics of the vehicle  5 , especially the yaw torque about the vertical axis of the vehicle  5 , and the transverse acceleration are determined. The driving-dynamics quantities  7  and the driver&#39;s steering angle δ H    8  are sent as input quantities to a driving dynamics controller  6 . Driving-dynamics-related steering interventions in the capacity of an additional steering angle Δδ  9  are sent as an input quantity to a steering controller  10  by means of the driving dynamics controller  6 . Likewise, the driver&#39;s steering angle δ H    11  and a value for the present vehicle speed  12 , in particular the vehicle reference speed from the driving dynamics controller  6  or an ABS controller, is sent as an input quantity to the steering controller  10 . Said steering controller  10  drives the actuator  14  of the overriding steering function  15 . 
   The actuator, in particular an electric motor  14 , produces an additional steering angle δ M , which acts by way of a gear  17  with a gear ratio factor i L2  on the steering gear  3  (i L2 *δ M )  18 . Gear  2  and gear  17  are illustrated herein as two individual ‘gears’ only for representation purposes. However, the two gear ratios of gears  2  and  17  are preferably realized by way of one single gear unit, in particular a planetary gear. 
   As can be taken from  FIG. 1  already, the additional steering angle Δδ which shall be considered as an external intervention of the driving dynamics controller  6  is additively superimposed at  58  on the nominal steering angle δ DRV  of the basic steering function. The nominal steering angle value δ L,nominal  resulting from this addition is sent to the control of the superimposed steering. 
   A sum steering angle δ L    21  is the result of the additive superposition of driver&#39;s steering angle and superimposed steering angle generated by the actuator, from which sum steering angle a resulting steering angle δ V  is produced by the steering gear  3  as a resulting output quantity and acts on the vehicle corresponding to the desired learning function. 
   The sum steering angle δ L    21  is furnished to the steering controller  10  as an input quantity at  22 , just as the additional steering angle δ M    23 . The sum steering angle δ L    21  is also sent  26  as an input quantity to the driving dynamics controller  6 . Signals or measured quantities of the actuator means, the electric motor  14 , are also sent to the steering controller  10  at  24 . 
     FIG. 3  shows the determination of the nominal steering angle value δ L,nominal  and, if needed, a motor speed specification ω M,spec    44  in a nominal value producing means  30  and the determination of the actual value δ L,actual  in an actual value producing means  31 , said values being used as input quantities  32 ,  33  of the steering controller  34  under consideration. A motor torque M mot,nominal    35  to be adjusted or a torque-producing motor current l q,nominal  is produced from output quantities. These quantities are associated with the electric motor, exactly as a commutation of the motor (in the case of an electronic commutation). 
   In this arrangement, the control quantity of the steering controller  34  is the steering angle δ L , which is either directly measured and sent  36  to the actual value producing means  31 , or which can be calculated in the actual value producing means  31  by means of the motor angle δ M    37  and the driver&#39;s steering angle δ H    38  in consideration of the gear ratio of the overriding gear. The motor speed ω M,actual    40  which can be calculated from the measured motor angle by differentiation is used as internal control quantity. 
   The driver&#39;s steering angle δ H    41  and the additional steering angle Δδ  42  and the vehicle speed V VEH    43  are also sent to the nominal value producing means. 
     FIG. 4  shows the determination of the nominal steering angle δ L,nominal    32  in greater detail. 
   The resulting steering ratio i L,ESAS    60  corresponds to the ratio between the angle of the steered wheels (wheel turning angle) δ V  and the driver&#39;s steering angle δ H . It results from two boosting factors K 1   61  and K 2   62  which are multiplicatively combined with the series steering gear ratio i L,series  by the following formula:
 
 i   L,ESAS =δ V /δ H   =i   L,series /( K 1* K 2)
 
   The boosting factors represent a component K 1  responsive to the steering wheel angle  63  and a component K 2  responsive to the vehicle speed  64 . They can be chosen freely according to aspects related to driving dynamics or specifications by the driver. To calculate the nominal steering angle value δ L,nominal  and the motor speed specification ω M,spec    66 , the additional steering angle Δδ  67  is also taken into consideration, and a corrected additional steering angle Δδ IPO    69  is superimposed at  71  on the driver&#39;s request δ nominal,DRV    70  after an interpolation and limitation of rise  68 . 
   The motor speed specification ω M,spec    66  is calculated from the time derivative of the nominal steering angle value δ L,nominal  and the steering angle of the driver δ H  by the following formula  72 :
 
ω M,spec =({dot over (δ)} L,nominal   −i   L1 {dot over (δ)} H )/ i   L2 .
 
     FIG. 5  shows the steering angle control in greater detail. Said control is a cascade control in its basic structure. An anticipatory control of the nominal speed of the motor is executed to enhance the dynamics of the control circuit. The nominal speed ω M,nominal  is produced  83  from the motor speed specification ω M,spec    81  and the motor speed preset value ω M,reg    93  being determined as an output quantity of the angle controller based on the comparison between the nominal steering angle value δ L,nominal  and the actual steering angle value δ L,actual  determined. To prevent impairment of the steering comfort by the anticipatory control especially during slow steering movements, the anticipatory control value is weighted depending on the desired motor speed at  83 ,  84 . 
   The nominal motor torque M mot,nominal    86  or a torque-producing nominal motor current I q,nominal    87  by which the motor shall be driven, is produced from the nominal speed ω M,nominal    80  and the comparison with the actual motor speed ω M,actual    88  determined by way of a motor speed controller  85 . 
   A higher motor speed than available may be required in certain cases of operation. In this case, a demand-responsive brief increase of the motor speed without reduction of the available motor torque can be reached by using a field weakening. A brief increase of the current consumption is related thereto. In particular the existence of a very direct steering ratio and a high nominal speed on the part of the driver or the driving dynamics control system is considered as a case of need. The resulting controller structure represents an extension of the structure shown in  FIG. 5  and is illustrated in  FIG. 6 . Therefore, all steps and elements corresponding to  FIG. 5  have been assigned equal reference numerals in  FIG. 6  and will not be explained in detail in the following. 
   A decision about the use of the field weakening and the magnitude of the field weakening current is taken  104  based on the present actual condition of the steering system, that means the prevailing actual motor speed ω M,actual    100  and the prevailing steering angle value δ L,actual    101  as well as the desired nominal condition, i.e. the motor speed specification ω M,spec    102  and the nominal steering angle value δ L,nominal    103  and the boosting factors of the steering ratio  106 . In case field weakening of the motor is not necessary, the resulting field weakening current I d,nominal    105  is zero, i.e. 0 A. The torque control of the electronically commutated motor is then required to control the field-weakening current value Id in addition to the torque-producing current Iq  87 .