Abstract:
In an automatic power steering system incorporated with a power steering system and an automatic steering system, a continuous switching unit carries out a switch over between a power steering mode and an automatic steering mode in a continuous manner depending on the magnitude of the manual steering torque applied to the steering wheel. Thus, a smooth transition from one of the steering modes to the other can be accomplished without involving any absence of control.

Description:
TECHNICAL FIELD 
     The present invention relates to a vehicle steering system which combines a power steering system which assists the manual effort required by the vehicle operator to steer the vehicle and an automatic steering system which automatically steers the vehicle according to given road information. 
     BACKGROUND OF THE INVENTION 
     Various forms of automatic steering systems have recently been proposed which automatically steer the vehicle according to the configuration of the road. The road information can be derived from a GPS system incorporated with a map of the area, and/or from a television camera which may detect the position of a white line marked on the road or the shape of the road. It is also possible to use other guidance systems for guiding the vehicle along the road. According to such road information, a target value of a dynamic variable such as yaw rate, lateral acceleration, and steering angle is computed, and the vehicle steering system is controlled in such a manner that the deviation between the target value of the dynamic variable and the actual value of the dynamic variable is minimized. 
     It is now customary for a motor vehicle to be equipped with a power steering system which assists the manual steering input applied to a steering wheel by using a hydraulic cylinder or an electric motor. FIG. 3 shows a typical electric power steering system. A steering wheel  21  is attached to an upper end of a steering shaft  22 , and a lower end of the steering shaft  22  is connected to a pinion  24  via a connecting shaft  23  which is provided with universal joints  23   a  and  23   b  at two ends thereof. The pinion  24  meshes with a rack  27  which extends laterally of the vehicle body and is guided to move along a longitudinal axial line thereof. The two ends of a rack shaft  28  carrying the rack  27  are connected to knuckle arms  26  of right and left front wheels W via tie rods  25 . To provide a power assist to this steering system, an electric motor  29  is coaxially combined with the rack  27  for axially actuating the rack  27  via a ball and nut mechanism  30 . 
     A steering torque sensor  31  is provided in an appropriate part of the steering system to detect the magnitude of the steering effort applied to the steering wheel  21  by the vehicle operator. The electric motor  29  is controlled by a controller  32  which receives an output signal from the steering torque sensor  31  so that a desired target steering torque may be produced. 
     The automatic steering system and the power steering system described above have various common components, and a significant reduction in size, cost, and complexity of the overall system can be achieved by sharing these common components by the two systems. The simplification of the overall system also contributes to an improvement in the reliability of the operation of these systems. 
     However, when these two systems are simply combined so as to share various component parts, certain inconveniences may arise. For instance, when both the automatic steering system and the power steering system are in operation, a manual effort to change the driving lane of the vehicle gives rise to an actual dynamic variable such as yaw rate, lateral acceleration, and steering angle which deviates from the target value of the dynamic variable, and the system tends to resist the manual effort. 
     To resolve such an inconvenience, it has been previously proposed in Japanese patent laid open publication No. 7-205831 to allow an automatic steering mode and a power steering mode to be selected in a mutually exclusive manner, and to provide a certain time lag in a switch-over between the two modes. However, according to this previous proposal, the time lag produces a blank period during which there is no steering control so that the transition from one mode to the other is not very smooth, and the vehicle operator may experience an undesirable sudden change in the handling of the vehicle. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of such problems of the prior art, a primary object of the present invention is to provide an automatic power steering system which allows an automatic steering system and a power steering system to share various components so as to simplify the structure. 
     A second object of the present inventions to provide an automatic power steering system which allows a smooth transition from one of the operation modes to the other without involving any absence of control. 
     According to the present invention, these and other objects can be accomplished by providing a steering control system for a vehicle, comprising: a steering mechanism; power steering means for applying an assist steering torque to the steering mechanism according a magnitude of manual steering effort; automatic steering means for applying an automatic steering torque to the steering mechanism according to road information; continuous switching means for continuously changing weights of the assist steering torque and the automatic steering torque in an overall steering torque according to the magnitude of the manual steering effort. Typically, the manual steering effort is given by an output of a steering torque sensor for measuring a steering torque applied to a steering wheel. 
     Thus, it is possible to avoid any interference between a power steering mode and an automatic steering mode, and to switch over between them without any discontinuity. 
     According to a preferred embodiment of the present invention, the power steering means comprises steering torque detecting means for detecting a manual steering torque applied to a steering wheel, an actuator for producing the assist steering torque, and first target drive torque determining means for determining the assist steering torque according to a magnitude of the manual steering torque. Additionally, the automatic steering means may comprise road configuration detecting means for detecting a configuration of a road on which the vehicle is traveling, target dynamic variable determining means for determining a target dynamic variable for the vehicle according to an output from the road configuration detecting means, and second target drive torque determining means for determining the automatic steering torque which is to be produced by the actuator according an output from the target dynamic variable determining means. Also, the continuous switching means may comprise actual dynamic variable detecting means for measuring an actual value of the dynamic variable, estimated dynamic variable computing means for computing a hypothetical dynamic variable which would be produced when the output of the second target drive torque determining means is applied to the steering mechanism, and dynamic variable weighting means for weighting an output of the estimated dynamic variable computing means and an output of the actual dynamic variable detecting means according to an output of the steering torque detecting means, an output of the dynamic variable weighting means being deducted from the output of the target dynamic variable determining means which is applied to the second target drive torque determining means. 
     Thus, the continuous switching means can identify the point of switching of switching from one steering mode to the other in an unambiguous manner. Also, a significant part of sensors and actuators can be shared by the automatic steering system and the power steering system so that a significant reduction in cost and size can be achieved with the added advantage of an improvement in reliability. Also, the vehicle operator can regain the manual override of the steering system whenever necessary. Therefore, the vehicle operator can take sudden and extreme steering actions when such a need arises. Conversely, when the vehicle operator wishes to return to the automatic steering mode, all he is required to do is to let go of the steering wheel, and the steering system takes over the control in a highly smooth manner. 
     The dynamic variable which is given as a variable for representing the navigational movement of the vehicle may consist of a yaw rate, a lateral acceleration, a steering angle of a steerable wheel of the vehicle, or any combination of these variables. The road information can be obtained in any of a number of possible ways. For instance, the vehicle may be equipped with a GPS system incorporated with a road map. The road information required for the operation of the present invention may be produced from any guidance system which is capable of identifying the position of the vehicle with respect to the road on which the vehicle is traveling. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Now the present invention is described in the following with reference to the appended drawings, in which: 
     FIG. 1 is a block diagram of the control system for the automatic power steering system according to the present invention; 
     FIG. 2 is a weighting function for determining the relative weights of the power steering mode and the automatic steering mode; and 
     FIG. 3 is a steering system to which the present invention may be applied. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 generally illustrates the overall arrangement of the control system of an embodiment of the present invention. The mechanical arrangement of the steering system is not different from the conventional arrangement illustrated in FIG. 3, and reference should be made to FIG. 3 wherever appropriate. 
     An electric power steering control unit  1  includes a first target drive torque determining unit  3  which produces an appropriate assist steering torque target value for reducing the torque required to manually steer the vehicle according to an output from the steering torque sensor  31 . 
     An automatic steering control unit  4  includes a road configuration detecting unit  5  for detecting the direction and the curvature of the road, on which the vehicle is traveling, according to map information stored in a GPS car navigation system, a target dynamic variable determining unit  6  for determining a target value of a dynamic variable such as yaw rate, lateral acceleration, or steering angle of the steerable wheels which is required for traveling the road, an actual dynamic variable detecting unit  7  for detecting the actual value of the dynamic variable of the vehicle, and a second target drive torque determining unit  8  for producing a target value of the steering torque which is required to follow the configuration of the road detected by the road configuration detecting unit  5 . 
     The electric motor  9  is controlled by electric motor drive control unit  10  according to the sum of the output from the first drive torque determining unit  3  of the power steering control unit  1  and the second target drive torque determining unit  8  of the automatic steering control unit  4 . The output torque of the electric motor  9  is converted into a thrust of the rack shaft  28  via a torque/thrust converting mechanism such as the ball and nut mechanism  11 . 
     The electric power steering control unit  1  and the automatic steering control unit  4  are adapted to take over each other in a smooth manner by continuous switching unit  14  comprising an estimated dynamic variable computing unit  12  for estimating the dynamic variable of the vehicle motion according to an output a of the second target drive torque determining unit  8 , and a dynamic variable weighting unit  13  for weighting an output e of the estimated dynamic variable computing unit  12  and an output c of the actual dynamic variable detecting unit  7  according to an output b of the steering torque sensor  31 . 
     Now the operation of this system is described in the following. 
     The estimated dynamic variable computing unit  12  computes a value e of the estimated dynamic variable from the output a of the second target drive torque determining unit  8 , and the estimated dynamic variable value e is given by a transfer function of a vehicle model in the following form. 
     
       
           e=[ (β n   ·S   n +β n−1   ·S   n−1 +. . . +β 2   ·S   2 +β 1   ·S+β   0 ) /(α n   ·S   n +α n−1   ·S   n−1 +. . . +α 2    ·S   2 +α 1   ·S+α   0 )]· a   (1) 
       
     
     For practical purpose, this transfer function may be approximated by a second order or a third order equation. 
     The output d of the state variable weighting unit  13  for weighting the output e of the estimated dynamic state variable computing unit  12  and the output c of the actual dynamic variable detecting unit  7  according to the output b of the steering torque sensor  31  can be given by the following equation. 
     
       
           d=c· (1− k ( b ))+ k ( b )· e   (2) 
       
     
     where 0≦k(b)≦1 
     Here, k(b) is a function of the steering torque input which is given by the output b of the steering torque sensor  31 . When the output b is small, or in the automatic steering mode where the vehicle operator is not applying any torque to the steering wheel  21 , the weight of the output c of the actual dynamic variable detecting unit  7  is relatively large in the output d of the state variable weighting unit  13 . Conversely, when the output b is large, or in the power steering mode where the vehicle operator is actively applying a manual torque to the steering wheel  21 , the weight of the output e of the estimated state variable computing unit  12  is relatively large in the output d of the dynamic variable weighting unit  13 . In this embodiment, the estimated dynamic variable computing unit  12  is represented by a transfer function of a vehicle model, but may also be represented by a state equation. Also, the dynamic variable weighting unit  13  may be represented by a membership function based on the fuzzy control theory. 
     Thus, in the automatic steering mode, because the vehicle operator does not apply any significant steering torque to the steering wheel  21 , the output b of the steering torque sensor  31  is substantially zero, and so is the output g of the first target drive torque generating unit  3 . Therefore, in this case, the electric motor control unit  10  receives the output a of the second target drive torque determining unit  8  which depends on the deviation between the output d of the dynamic variable weighting unit  13 , in which the output c of the actual dynamic state variable detecting unit  7  is dominant and the output f of the target dynamic variable determining unit  6  which depends on the output of the road configuration detecting unit  5 . As a result, the corresponding output of the electric motor  9  is applied to the manual steering unit via the torque/thrust converting mechanism  11  and the steering angle of the steerable wheels is controlled in such manner that the actual dynamic variable c and the target dynamic variable f are made to coincide with each other. 
     In the automatic steering mode, the output e of the estimated dynamic variable computing unit  12  substantially coincides with the output c of the actual dynamic variable detecting unit  7 . 
     When the steering wheel  21  is turned in this state, the output b of the steering torque sensor  31  increases, and the value of k(b) in FIG. 2 approaches  1  with the result that the weight of the output e of the estimated dynamic variable computing unit  12  in the output d of the dynamic variable weighting unit  13  becomes predominant. It means that a change in the output c of the actual dynamic variable detecting unit  7  would not cause any substantial increase in the deviation between the output d of the dynamic variable weighting unit  13  and the output f of the target dynamic variable determining unit  6 . Thus, the output a of the second target drive torque determining unit  8  is reduced in magnitude, and the electric motor  9  is mostly actuated according to the output g of the first target drive torque determining unit  3 . Any reduction in the manual steering torque applied to the steering wheel  21  is made up for by the increase in the weight of the output c of the actual dynamic variable detecting unit  7  in the output d of the dynamic variable weighting unit  13  so that a smooth transition to and from the automatic steering mode can be achieved. 
     The dynamic variable may consist of lateral acceleration, yaw rate, steering angle of the steerable wheels, or any combination of these variables. Obviously, the actual value, target value and estimated value are required to be similarly associated with each other without regard to any particular selection of the dynamic state variable. 
     The switching between the two steering modes was carried out according to the magnitude of the manual steering input to the steering wheel in the above described embodiment, but it is also possible to use other measures of steering effort such as steering speed, and steering acceleration in addition to or instead of the steering torque. 
     Although the present invention has been described in terms of a specific embodiment thereof, it is possible to modify and alter details thereof without departing from the spirit of the present invention.