Abstract:
A steering system for a motor vehicle with at least one steerable wheel, an actuator, and an auxiliary drive. The steering motion initiated by the driver of the vehicle and the motion initiated by the actuator are superimposed by the auxiliary drive to generate the steering motion of the steerable wheel. At least two steering components are computed and a control signal is generated to control the actuator by superimposing the computed steering components. The steering components are computed in parallel and independently of one another so that several functions are beneficially combined to control running behavior. This provides an improvement of running dynamics by means of steering interventions.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a method and an apparatus for operating a steering system for a motor vehicle. 
     A prior art steering system is disclosed by DE-OS 40 31 316 (corresponding to U.S. Pat. No. 5,205,371) and to the extent it is relevant to understanding this invention, will be described with reference to FIGS. 1 and 2. In such a steering system the motions of the actuator  13  or  23 , the motor angle δ M , are superimposed in the auxiliary drive  12  or  22  on the steering motions imparted by the driver through the steering wheel  11  or  21 , the steering wheel angle δ L  detected by the sensor  28 . The total motion δ L ′ thus obtained is transmitted through the steering mechanism  14  and the steering linkage  16  to the steerable wheels  15   a  and  15   b  to set the steering angle δ V . The actuator  13  or  23  can be an electric motor. The operating principle of such a steering system consists of determining the motor angle δ M  dependent on the steering wheel angle δ L  and dependent on signals Sm to control the dynamic behavior of the vehicle, with the steering wheel angle δ L  being determined through the sensor  28  and with the signals Sm representing vehicle motions detected by the sensors  26 . The total steering angle is obtained according to the equation 
     
       
         δ L ′=δ L   /i   ü +δ M ,  
       
     
     in which the transmission ratio can be i ü =1 or i ü ≈1. 
     DE-OS 40 38 079 (corresponding to U.S. Pat. No 5,316,379) shows the superimposition of a steering component (compensating steering angle) on the front wheel and/or the rear wheel. The compensating steering angle, which depends on the differences between brake pressures, compensates for the yawing motion in the case of so-called ì-split braking, i.e. braking in which the coefficients of roadway friction are significantly different on the right and left sides of the vehicle. 
     DE 196 01 825.0, not a prior art publication, shows a steering system that is similar to DE-OS 40 31 316 described above (corresponding to U.S. Pat. No. 5,205,371). In this case two steering components are superimposed to obtain a desired output rotational motion for the servomotor, with a first steering component serving for steering assistance, while a second steering component depends on the angular yaw rate, transverse acceleration, and/or the vehicle&#39;s longitudinal speed. 
     DE A1 36 25 392 shows the generation of a correction signal to a servomotor that controls the front wheel steering angle. The correction signal in this case depends on a desired-actual yaw rate difference. 
     GB-PS 1,414,206 shows compensation of crosswind effects by a superimposed steering angle intervention. 
     The purpose of this invention is to combine various control and regulation functions relating to a steering system in a suitable fashion. 
     SUMMARY OF THE INVENTION 
     As already mentioned hereinabove, this invention relates to a steering system for a motor vehicle with at least one steerable wheel, an actuator, and an auxiliary drive. The steering motion initiated by the driver of the vehicle and the motion initiated by the actuator are superimposed by the auxiliary drive to generate the steering motion of the steerable wheel. At least two steering components are computed and a control signal is generated by superimposing the computed steering components to control the actuator. The core of the invention consists of computing the steering components in parallel and independently of one another. 
     The invention makes it possible to combine several functions beneficially to control road behavior. This provides an improvement of running dynamics by means of steering interventions. 
     The individual steering components are advantageously computed by providing that each of the steering components is computed to achieve a predetermined control or regulation objective. It is provided in particular that the control or regulation objectives for the individual steering components are stated differently. The steering components in this case can be computed at the same time, or in each sensing step when accomplished in a digital processor. 
     Thus, different individual functions of steering intervention are combined with one another. 
     The combination of individual steering components pursuant to the invention produces all of the advantages that the steering system permits, and thus exhausts the entire possible potential of steering interventions. The different individual functions supplement and assist one another mutually. Furthermore, the invention guarantees ease of application since the individual functions affect only some running dynamics characteristics of the vehicle and can be individually fine-tuned. For this reason there are few objective conflicts in application since the different functions provide adequate degrees of freedom for the selective control of individual characteristics and to produce definite benefits. The individual functions can be combined freely as needed or desired. 
     An especially beneficial refinement of the invention provides that the steering components are computed by 
     regulating the yawing motion of the motor vehicle, particularly the angular yaw rate, to a definite desired value (yaw rate control), and/or 
     counteracting a yawing motion caused by a motor vehicle braking process (yaw moment compensation), and/or 
     counteracting crosswind effects (crosswind compensation), and/or 
     a steering component dependent on the speed of rotation of the steering wheel) (corrective steering). 
     It can be provided in particular that 
     in the case of yaw rate control, the instantaneous angular yaw rate is detected, the desired value is determined at least dependent on the detected rotation of the steering wheel and the detected vehicle speed, and a steering component is computed dependent on the difference between the instantaneous angular yaw rate and the desired value, and/or 
     in the case of yaw moment compensation, a steering component is computed that is dependent on the detected brake pressures applied to the vehicle&#39;s brakes on different sides of the vehicle, and/or 
     in the case of crosswind compensation, a steering component is computed dependent on air pressures detected at various positions on the vehicle body. 
     It can also be provided pursuant to the invention that the computed steering components be coordinated differently, in particular weighted, dependent on the instantaneous running conditions. The underlying idea for this refinement is to control some or all of the prevailing individual steering functions through a priority coordinator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 show schematically the steering system pursuant to the prior art, 
     FIG. 3 represents the control and regulation strategy of such a steering system, 
     FIG. 4 shows a yaw rate controller, 
     FIG. 5 shows a yaw moment compensator, while 
     FIG. 6 depicts the entire vehicle control with reference to a block diagram. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The invention will be discussed below with reference to an example of a preferred embodiment. The starting point by way of example is an auxiliary steering system as mentioned hereinabove. 
     FIGS. 1 and 2 with the reference symbols  11  and  21 , respectively, show a steering wheel operable by the driver of the vehicle. By operating the steering wheel  11  or  21 , the steering wheel angle δ L  is transmitted to the auxiliary drive  12  or  22  through the connection  101 . At the same time, the motor angle δ M  of the actuator  13  or  23  is transmitted to the auxiliary drive  12  or  22  through the connection  104 ; the actuator can be an electric motor. At the output of the auxiliary drive  12  or  22 , the superimposed motion δ L ′ is transmitted through the connection  103  to the steering mechanism  14  or  24 , which in turn impacts the steerable wheels  15   a  and  15   b  with the steering angle δ V  corresponding to the total angle δ L ′ through the steering linkage  16 . Sensors  28  and  26  are also shown in FIG. 2, with the sensor  28  detecting the steering wheel angle δ L  and transmitting it to the control instrument  27 , while sensors labeled  26  sense the motions of the vehicle  26  (e.g. yawing motions, transverse acceleration, vehicle speed, etc.) and transmit corresponding signals Sm to the control instrument  27 . The control instrument  27 , dependent on the detected steering wheel angle δ L  and, as the case may be, dependent on the vehicle motions, determines a correcting variable u to control the actuator  13  or  23 . 
     FIG. 3, with reference to a block diagram, shows how the steering system functions when the vehicle is running. The steering wheel angle δ L  detected by the sensor  28  is transmitted to the servo steering control  41  for this purpose. The servo steering control  41 , dependent on the steering wheel angle δ L  and, as the case may be, dependent on the vehicle&#39;s longitudinal speed V x , determines a steering component δ Komf . The vehicle regulator  34 , dependent on the vehicle motions detected by the sensors  26  (the vehicle&#39;s longitudinal speed V x , yaw rate ù, transverse acceleration, etc.), also determines another, dynamic steering component δ dyn . The steering component δ Komf  that takes into consideration the servo steering function, and the steering component δ dyn , by which the running dynamics of the vehicle are optimized, are superimposed at Point  35  to give a desired motor angle δ M,soll  that is transmitted to the input side of the positioning control  32 . The instantaneous motor angle δ M,ist  of actuator  13  or  23  (e.g., an electric motor is also transmitted to the positioning control  32 . Based on a desired-actual comparison, the underlying positioning control  32  determines the desired motor current I soll  in a known way. The current regulator  33 , based on the difference between the desired motor current I soll  and the actual current I ist  detected on the vehicle actuator  13  or  23  (subtraction  36 ), determines a corresponding control signal u for the actuator  13  or  23  to bring about the desired motor angle δ M,soll . 
     Before the superimposition of the individual steering components δ (i)   M,soll  is described with reference to FIG. 6, some individual running dynamics functions will first be discussed below in connection with steering interventions. 
     Corrective Steering 
     The steering intervention known as corrective steering is a control in which the added angle δ (1)   M,soll  is proportional to the speed of rotation of the steering wheel d(δ L )/dt, i.e. the differential of the steering wheel angle δ L . Corrective steering improves the response behavior of the vehicle to the driver&#39;s operation of the steering wheel. 
     Yaw Rate Control 
     FIG. 4 shows a yaw rate controller. Yaw rate means the rotational motion of a vehicle around a vertical axis. A desired yaw rate ù soll  is calculated here from the vehicle speed V x , the steering wheel angle δ L , and any other variables in Block  41 . If the vehicle&#39;s yaw rate ù measured on the vehicle  45  differs from this, the yaw rate controller  42  determines a desired added angle δ (2)   M,soll  so that the yaw rate deviation is reduced. By comparison with the instantaneous motor angle δ M  that is set, the motor  44  is impacted through the underlying positioning controller  43  so that the appropriate added angle δ M  is set and is superimposed on the steering motion δ L  of the driver. The yaw rate controller increases the damping of the yawing motion, improves running stability, and reduces the influence of vehicle parameters. 
     Yaw Moment Compensation 
     When a severe braking process, particularly antilock braking (ABS braking), takes place on a roadway that shows very different coefficients of friction on the right and left sides of the vehicle, a yaw moment arises in a known way. In the case of the yaw moment compensator shown in FIG. 5, automatic countersteering occurs during ABS braking on such an asymmetric roadway, by which undesirable yawing (rotation of the vehicle around its vertical axis) is prevented. 
     For yaw moment compensation, the brake pressures p vl  and p vr  on the left and right front wheels are first either sensed directly or calculated from available measured data (e.g., valve opening times, bias pressure). These wheel brake pressures are then filtered in the filters  51  and  52  to cancel out interference effects. The difference between the brake pressures filtered in this way is then processed in Block  53  (proportional amplifier with dead band). The magnitude of the steering intervention δ (3)   M,soll  is then determined from the difference thus processed by means of a constant amplification factor and a time-variable amplification factor (Blocks  54  and  55 ). In case of ABS individual control at the rear axle, the brake pressures at the rear wheels can also be taken into consideration. 
     Compensation for Crosswind Effects 
     A compensator for crosswind effects is known, for example according to Tran, V. T.: Crosswind Feedforward Control—A Measure to Improve Vehicle Crosswind Behavior, Vehicle Systems Dynamics 23 (1993), pp. 165-205. The added angle δ (4)   M,soll  in this case is calculated from air pressures p i , i=1, . . . , n, at various points on the body. This function automatically countersteers in case of crosswind and thus prevents unwanted yawing of the vehicle. 
     Each of the individual functions described above produces only some of the desired benefits. To obtain all of the benefits, pursuant to the invention the functions are combined as shown in FIG.  6 . 
     The individual functions  3401  to  3404  are executed in parallel at the same time. When implemented in a digital processor, the algorithns of the individual functions  3401  to  3404  are processed in each sensing step. Each individual function  3401  to  3404  provides a contribution δ (i)   M,soll  to the desired value δ M,soll  of the added steering angle. These contributions are superimposed at Point  3405 . 
     FIG. 6 shows an example of a block diagram of a vehicle regulator  34  that consists of a combination of the individual functions  3401  to  3404  described above. The contributions of the individual functions to the added angle are labeled δ (i)   M,soll , i=1, . . . , m. Which individual functions the vehicle regulator  34  includes can be established according to the particular requirements or desires. In this sense the vehicle regulator does not have to include all of the individual functions mentioned, but others can also be added. 
     It is also possible for some or all of the individual functions that are present to be controlled by a priority coordinator. In this case, it can be provided that it is not a simple superimposition of the individual steering components δ (i)   M,soll  that takes place at Point  3405 , but that the steering components δ (i)   M,soll  are coordinated, or especially weighted, individually depending on the instantaneous running state. 
     In summary, the following benefits of the invention can be pointed out: 
     Combination of several functions to control vehicle behavior and improve running dynamics by steering intervention at the front axle. 
     Simple superimposition of the steering interventions of the individual functions, i.e. addition of the corresponding desired values of the added steering angles. 
     Provision of all of the benefits that the steering system permits; full utilization of potential. 
     Individual functions supplement and assist one another mutually. 
     Simple applicability, since the individual functions control only some running dynamics characteristics of the vehicle and can be fine-tuned individually. 
     Few objective conflicts in application, since the different functions offer adequate degrees of freedom for selective control of individual characteristics and for provision of definite benefits. 
     Individual functions can be combined freely as needed or desired. 
     Additional coordination of some or all of the individual functions is possible.