Abstract:
Provided is a reaction force control device for reducing discomfort experienced by the driver operating the accelerator pedal when continuous curves are being traveled, and setting the characteristics of the reaction force on the accelerator pedal in accordance with the curves. In the case that the target reaction force, which has been set prior to entering a subsequently traveled second curve, is greater than the current reaction force acting on the acceleration pedal and being generated during the turn through the first curve, a reaction force controller controls the operation so as to reduce the target reaction force imparted on the second curve until the vehicle leaves the first curve.

Description:
TECHNICAL FIELD 
     The present invention relates to a reaction force control apparatus (device) for applying a reaction force to an accelerator pedal of a vehicle to prompt the driver of the vehicle to decelerate the vehicle when there is a curve ahead along the road on which the vehicle is driving. 
     BACKGROUND ART 
     Japanese Laid-Open Patent Publication No. 1999-229917 (hereinafter referred to as “JP1999-229917A”) discloses a vehicle control apparatus for controlling the drive power of a vehicle depending on a curve that is ahead on the road. When a navigation device on the vehicle detects a curve, the vehicle control apparatus sets a deceleration control permission range based on the detected curve. When the vehicle enters the deceleration control permission range, the vehicle control apparatus incrementally corrects an amount of engine braking in order to perform a deceleration control process (see paragraphs [0002] and [0003] of JP1999-229917A). 
     According to JP1999-229917A, in order to prevent the deceleration control process from being switched on and off frequently in a situation where there is a succession of curves along a path such as a mountain path, for example, if the distance between adjacent deceleration control permission ranges is small, then the vehicle control apparatus joins the adjacent deceleration control permission ranges into a single deceleration control permission range (see paragraph [0007] of JP1999-229917A). 
     SUMMARY OF INVENTION 
     According to JP1999-229917A, however, as can be understood from the fact that the deceleration control process is performed even if the accelerator opening is zero (see paragraphs [0017], [0035], and [0040] of JP1999-229917A), no consideration is given to a so-called accelerator pedal reaction force control process, which generates a reaction force on the accelerator pedal while the driver presses the accelerator pedal, to thereby allow the driver to sense the reaction force and prompt the driver to return the accelerator pedal. 
     When an accelerator pedal reaction force is generated in order to notify the driver concerning a succession of curves up ahead, or to make the driver reduce the accelerator opening prior to the vehicle entering each of the curves, it may be advisable to perform a control process for generating a reaction force, which is proportional to the curvature of the curve. 
     However, when such a control process is performed to generate a reaction force that is proportional to the curvature of each of the curves prior to the vehicle entering the curves, the driver may feel strange and uncomfortable concerning operation of the accelerator pedal while the vehicle is making a turn along a curve, since a reaction force proportional to the curvature of a subsequent curve is generated while driving within the present curve. 
     The present invention has been made in view of the above problems. It is an object of the present invention to provide a reaction force control apparatus, which minimizes the tendency to make the driver of a vehicle feel strange and uncomfortable concerning operation of the accelerator pedal while the vehicle is traveling along a curve, especially a succession of curves. 
     According to the present invention, there is provided a reaction force control apparatus comprising an accelerator pedal, a reaction force controller for controlling a reaction force generated by an actuator, the reaction force being applied to the accelerator pedal, and a curve detector for detecting curves along a path traveled by a host vehicle, wherein when the host vehicle travels along at least two curves including a first curve and a second curve subsequent to the first curve, the reaction force controller sets respective target reaction forces for the first curve and the second curve, and the reaction force controller corrects the target reaction force for one of the first curve and the second curve based on the target reaction force for another of the first curve and the second curve. 
     According to the present invention, when the host vehicle travels along a succession of curves, the driver feels less strange and uncomfortable concerning operation of the accelerator pedal, and reaction force characteristics for the accelerator pedal are established depending on the nature of the curves. 
     The reaction force controller may set a target reaction force for the accelerator pedal depending on a curvature of each of the curves before the host vehicle enters the curves, and when the reaction force controller generates and applies the set target reaction force to the accelerator pedal before the host vehicle enters the curves, if a target reaction force, which is set before the host vehicle enters the second curve, is greater than a present reaction force generated and applied to the accelerator pedal while the host vehicle is turning along the first curve, the reaction force controller may reduce the target reaction force for the second curve and apply a reduced reaction force to the accelerator pedal until the host vehicle has finished traveling through the first curve. 
     According to the present invention, when the host vehicle travels along a succession of curves, the driver feels less strange and uncomfortable concerning operation of the accelerator pedal, and reaction force characteristics for the accelerator pedal are established depending on the nature of the curves. 
     The reaction force controller may make an amount of a reduction in the target reaction force for the second curve smaller as the distance between an exit of the first curve and an entrance of the second curve, which are detected by the curve detector, becomes shorter. 
     If the distance between the first curve and the second curve is shorter, the driver is required to decelerate the host vehicle more than if the distance between the first curve and the second curve were greater. Since the reduction in the target reaction force for the second curve becomes smaller as the distance is shorter, the driver is prompted to decelerate the vehicle in preparation for the second curve, or the driver is made aware of the presence of the second curve, while feeling less strange and uncomfortable while the vehicle travels along the first curve. Consequently, the driver is able to maneuver the vehicle more easily while the host vehicle travels along a succession of curves that are spaced by short distances. 
     The reaction force controller may reduce the amount by which the target reaction force is reduced for the second curve if the curvature of the second curve is detected by the curve detector as being smaller than the curvature of the first curve, or if the radius of curvature of the second curve is detected by the curve detector as being greater than the radius of curvature of the first curve. 
     More specifically, if the curvature of the second curve is smaller, i.e., if the radius of curvature of the second curve is greater, then since the driver feels less strange and uncomfortable concerning operation of the accelerator pedal due to the generated reaction force, the driver can prepare for turning along the second curve, which has a smaller curvature, i.e., a greater radius of curvature, even if a reduction in the target reaction force for the second curve is reduced. 
     The reaction force controller may not reduce the target reaction force for the second curve if the curvature of the second curve is detected by the curve detector as being smaller than the curvature of the first curve, or if the radius of curvature of the second curve is detected by the curve detector as being greater than the radius of curvature of the first curve. 
     More specifically, if the curvature of the second curve is smaller, i.e., if the radius of curvature of the second curve is greater, then since the driver feels less strange and uncomfortable concerning operation of the accelerator pedal due to the generated reaction force, the driver can prepare for turning along the second curve, which has a smaller curvature, i.e., a greater radius of curvature, even if the target reaction force for the second curve is not reduced. 
     The reaction force controller may reduce an amount by which the target reaction force for the second curve is reduced if the curvature of the second curve is detected by the curve detector as being smaller than a prescribed curvature, i.e., if the radius of curvature of the second curve is detected by the curve detector as being greater than a prescribed radius of curvature. 
     More specifically, if the curvature of the second curve is smaller than a prescribed curvature, i.e., if the radius of curvature of the second curve is greater than a prescribed radius of curvature, then since the driver feels less strange and uncomfortable concerning operation of the accelerator pedal due to the generated reaction force, the driver can prepare for turning along the second curve, which has a smaller curvature, i.e., a greater radius of curvature, even if a reduction in the target reaction force for the second curve is reduced. 
     The reaction force controller may not reduce the target reaction force for the second curve if the curvature of the second curve is detected by the curve detector as being smaller than a prescribed curvature, i.e., if the radius of curvature of the second curve is detected by the curve detector as being greater than a prescribed radius of curvature. 
     More specifically, if the curvature of the second curve is smaller than a prescribed curvature, i.e., if the radius of curvature of the second curve is greater than a prescribed radius of curvature, then since the driver feels less strange and uncomfortable concerning operation of the accelerator pedal due to the generated reaction force, the driver can prepare for turning along the second curve, which has a smaller curvature, i.e., a greater radius of curvature, even if the target reaction force for the second curve is not reduced. 
     The reaction force control apparatus may further comprise a vehicle speed sensor for measuring a vehicle speed of the host vehicle, and a target speed calculator for calculating a speed at which the host vehicle travels or turns along each of the curves, as a turning target speed depending on a curvature of each of the curves that are detected, wherein the target speed calculator calculates a reduction characteristic for reducing the target speed from the present vehicle speed measured by the vehicle speed sensor to the turning target speed, and the reaction force controller sets the target reaction force based on a present vehicle speed measured by the vehicle speed sensor and the turning target speed, and the reduction characteristic for reducing the target speed, which are calculated by the target speed calculator. 
     According to the present invention, there is provided a reaction force control apparatus comprising an accelerator pedal, a reaction force controller for controlling a reaction force generated by an actuator, the reaction force being applied to the accelerator pedal, and a curve detector for detecting curves along a path traveled by a host vehicle, wherein if the curve detector detects only one curve, the reaction force controller sets a target reaction force depending on the detected one curve, and gradually reduces the reaction force when the host vehicle reaches an end point of the detected one curve. 
     According to the present invention, the driver is less likely to feel strange and uncomfortable concerning operation of the accelerator pedal when the host vehicle travels along the curve, as well as immediately after the host vehicle has traveled along the curve. 
     If the curve detector detects the second curve while the host vehicle is traveling or turning along the first curve, the reaction force controller may calculate a second curve target reaction force based on the second curve, and if the calculated second curve target reaction force is greater than a reaction force presently applied to the accelerator pedal while the host vehicle is traveling along the first curve, the reaction force controller may perform a rate limiting process on a reaction force characteristic, depending on a difference between a present vehicle speed measured by the vehicle speed sensor and a target speed set for the second curve. With such an arrangement, the driver feels less strange and uncomfortable concerning operation of the accelerator pedal at the time that the host vehicle travels along a succession of curves. 
     According to the present invention, when the host vehicle travels along a succession of curves, the driver feels less strange and uncomfortable concerning operation of the accelerator pedal, and depending on the nature of the curves, reaction force characteristics for the accelerator pedal can be established. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram of a reaction force control apparatus according to an embodiment of the present invention; 
         FIG. 2  is a flowchart (1 of 2) of an operation sequence of the reaction force control apparatus shown in  FIG. 1 ; 
         FIG. 3  is a flowchart (2 of 2) of an operation sequence of the reaction force control apparatus shown in  FIG. 1 ; 
         FIG. 4A  is a diagram of a path that illustrates a reaction force control process, which is performed when a first curve is not followed by a second curve within a prescribed distance after exiting from the first curve; 
         FIG. 4B  is a diagram that illustrates setting of a target speed with respect to distances in the absence of the second curve; 
         FIG. 4C  is a diagram that illustrates setting of reaction forces with respect to distances in the absence of the second curve; 
         FIG. 5  is a diagram that shows the manner in which a reaction force is applied depending on a speed difference in the presence of a single curve; 
         FIG. 6A  is a diagram of a path, which illustrates a reaction force control process that is performed when a first curve is followed by a second curve within a prescribed distance after exiting from the first curve; 
         FIG. 6B  is a diagram that illustrates setting of a target speed with respect to distances in the presence of the second curve, which follows within the prescribed distance; 
         FIG. 6C  is a diagram that illustrates setting of reaction forces with respect to distances in the presence of the second curve, which follows within the prescribed distance; 
         FIG. 7  is a diagram showing the manner in which reaction forces are applied depending on a speed difference in the presence of a succession of curves; 
         FIG. 8A  is a diagram of a path that illustrates a reaction force control process, which is performed when a first curve is followed by a second curve immediately after exiting from the first curve; 
         FIG. 8B  is a diagram that illustrates setting of a target speed with respect to distances in the presence of the second curve, which follows immediately after exiting from the first curve; and 
         FIG. 8C  is a diagram that illustrates setting of reaction forces with respect to distances in the presence of the second curve, which follows immediately after exiting from the first curve. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present invention will be described below with reference to the drawings. 
       FIG. 1  shows in block form a reaction force control apparatus  10  according to an embodiment of the present invention. The reaction force control apparatus  10  is incorporated in a vehicle (hereinafter referred to as a “host vehicle”)  11  such as a four-wheeled passenger car or the like. Basically, the reaction force control apparatus  10  includes an accelerator pedal  12  for adjusting an opening of a throttle valve, not shown, an operation amount sensor  14  (accelerator pedal operation amount sensor), a vehicle speed sensor  16  (vehicle speedometer), a navigation device  20 , an ECU (Electronic Control Unit)  22 , and a reaction force applying mechanism  24 . 
     The accelerator pedal operation amount sensor  14  detects an operation amount (accelerator pedal operation amount θ [°]) from an original position (θ=0 [°]) of the accelerator pedal  12  with a potentiometer or the like, and outputs the detected accelerator pedal operation amount θ to the ECU  22 . 
     The vehicle speed sensor  16  measures a vehicle speed (present speed) V [km/h] of the host vehicle  11  and outputs the measured vehicle speed V to the ECU  22 . 
     The navigation device  20  detects the position of the host vehicle  11  using GPS (Global Positioning System), performs a map matching process for comparing the detected position with a present position contained in map data stored in a memory  30 , and displays the present position of the host vehicle  11  on a map as a result of the map matching process, within a given range that is displayed on a display unit  20 A. 
     The navigation device  20  also functions as a curve detecting device for detecting a curve or a plurality of curves on the road traveled by the host vehicle  11 , i.e., a path along which the host vehicle  11  is guided to a preset destination. In the present embodiment, the navigation device  20 , which functions as the curve detecting device, sends information to a curve detector  22 A of the ECU  22  in relation to (the position of) the entrance of a curve, the distance along the curve (zone), the curvature (the reciprocal of the radius of curvature) of the curve, (the position of) the exit of the curve, (the position of) the entrance of a next curve subsequent to the aforementioned curve, the distance along the next curve (zone) and the curvature thereof, and (the position of) the exit of the next curve. 
     According to the present embodiment, the curve detector  22 A detects (information concerning) the curves from the navigation device  20 . However, the curve detector  22 A may detect (information concerning) a curve, e.g., the entrance and exit of the curve, and the curve (curved section), from a handle angle (steering angle) of a handle (steering wheel), not shown, a maintained steering angle, a time during which the steering wheel is turned, a time-dependent change in a lateral G-force that is applied to the host vehicle  11 , which is detected by a non-illustrated lateral G-force sensor, and a time-dependent change in a yaw rate of the host vehicle  11 , which is detected by a non-illustrated yaw rate sensor. 
     The ECU  22  includes a target speed calculator  22 B, which calculates a speed at which the host vehicle  11  travels (turns) along a detected curve depending on the curvature (the reciprocal of the radius of curvature) of the detected curve, as a turning target speed Vtc. The target speed calculator  22 B also calculates a reduction characteristic for reducing a target speed Vt, in order to change the present speed (present speed or actual speed) V to the turning target speed Vtc, until the host vehicle  11  reaches the entrance of the detected curve. 
     The ECU  22  includes a reaction force controller  22 C, which compares the present speed V and the reduction characteristic for the target speed Vt and the turning target speed Vtc with each other, and calculates a reaction force Fr to be applied to the accelerator pedal  12  based on the compared result. The reaction force controller  22 C also generates a control signal representing the calculated reaction force Fr, and supplies the control signal to the reaction force applying mechanism  24 , thereby instructing the reaction force applying mechanism  24  to apply the reaction force Fr. 
     The reaction force applying mechanism  24 , which comprises a non-illustrated motor or the like (not shown) that is connected to the accelerator pedal  12 , applies the reaction force Fr to the accelerator pedal  12  depending on the control signal received from the ECU  22 . 
     The accelerator pedal  12  receives the reaction force Fr from the reaction force applying mechanism  24 , in addition to an original position restoring force Forg, which is applied to the accelerator pedal  12  by a spring or the like (i.e., a force tending to restore the accelerator pedal  12  to the original position, by which the accelerator pedal  12  returns to an original position at which the accelerator pedal operation amount θ=0 [°] when the driver releases his or her foot from the accelerator pedal  12 ). 
     The ECU  22  operates as a function realizing component (function realizing means), which realizes various functions when a CPU executes programs stored in a memory (storage unit)  23  such as a ROM or the like, based on various input signals. According to the present embodiment, in addition to functioning as the curve detector  22 A, the target speed calculator  22 B, and the reaction force controller  22 C, the ECU  22  also functions as a timer  22 D for measuring time intervals. Rather than a ROM, the memory  23  may be a flash memory, an EEPROM, a RAM, a hard disk, or the like. 
     The reaction force control apparatus  10  according to the present embodiment basically is constructed as described above. A process of setting and controlling a reaction force that is applied to the accelerator pedal  12 , which is carried out by the ECU  22 , will be described in detail below with reference to the flowchart shown in  FIG. 2  and the distance charts shown in  FIGS. 4A ,  4 B, and  4 C. 
     [First Process: Reaction Force Control Process in the Absence of a Second Curve within a Prescribed Distance after Exiting From a First Curve] 
     As shown in  FIG. 4A , the host vehicle  11  travels along a path  50  up to a destination that is set by the navigation device  20 . In  FIG. 4A , the present position of the host vehicle  11  is indicated by a solid triangular mark. In step S 1 , while the host vehicle  11  travels along the path  50 , the navigation device  20 , which functions as a curve detecting device, detects whether or not there is a first curve  51  within a prescribed distance on the path  50  ahead of the host vehicle  11 . If the navigation device  20  detects a first curve  51 , then the navigation device  20  sends information to the curve detector  22 A concerning the curve, including the distance from the host vehicle  11  to a first curve entrance point P 2 , the position of the first curve entrance point P 2 , the position of a first curve exit point P 3 , and a curvature Cu 1  of the first curve  51  (first curve curvature). 
     The curve detector  22 A transfers the received information concerning the curve to the target speed calculator  22 B, which functions as a first curve target speed calculator. 
     In step S 2 , the target speed calculator  22 B calculates, at a point P 0  where the target speed calculator  22 B received the information concerning the curve, a turning target speed Vtc 1  for the first curve  51  (also referred to as a “first curve turning target speed”), and also calculates a before-entering-curve target speed characteristic (also referred to as a “before-entering-curve target speed”) Vtf 1  from the calculated turning target speed Vtc 1  and the actual speed V of the host vehicle  11 . 
     The turning target speed Vtc 1  and the before-entering-curve target speed characteristic Vtf 1  make up a first curve target speed Vt 1 . The turning target speed Vtc 1  is set at a constant speed that enables the host vehicle  11  to turn safely along the first curve  51  having the first curvature Cu 1 , depending on the first curvature Cu 1 . The before-entering-curve target speed Vtf 1  is set as a speed for calculating a reaction force in a curve entering zone, i.e., a zone from a first curve deceleration start point P 1  to the first curve entrance point P 2 , for the host vehicle  11 , so that the host vehicle  11  will travel at the turning target speed Vtc 1  at the first curve entrance point P 2 . 
     More specifically, the before-entering-curve target speed Vtf 1  is set to a speed gradient (linear or curved) in order to gradually reduce the present speed V to the turning target speed Vtc 1  over a distance between the first curve entrance point P 2  and the first curve deceleration start point P 1 , which is a prescribed distance prior to the first curve entrance point P 2 , i.e., a distance from the point P 1  to the point P 2  shown in  FIG. 4B . 
     According to the present embodiment, the turning target speed Vtc 1  is set to a smaller (lower) value in proportion to the curvature (the reciprocal of the radius of curvature). In other words, as the curvature is greater, the turning target speed Vtc 1  becomes smaller. 
     In step S 3 , the reaction force controller  22 C determines a reaction force (also referred to as a “first curve target reaction force”) Fr to be applied to the accelerator pedal  12  according to a reaction force characteristic  61  (see  FIG. 5 ), which is used to determine a reaction force Fr based on a speed difference ΔV. As shown in  FIG. 4B , the speed difference ΔV represents a difference between the actual speed (present speed) V and the before-entering-curve target speed characteristic Vtf 1  (ΔV=V−Vtf 1 ). 
     In step S 4 , the reaction force Fr, which is calculated in the foregoing manner, is applied to the accelerator pedal  12  by the reaction force applying mechanism  24 . As indicated by the reaction force characteristic  61  shown in  FIG. 5 , if the speed difference ΔV is smaller than a threshold value ΔVth, it is determined that the reaction force Fr does not need to be applied, and hence, the reaction force Fr is not applied to the accelerator pedal  12 . 
     In step S 5 , it is judged whether or not the host vehicle  11  has reached the first curve entrance point P 2 . Until the host vehicle  11  reaches the first curve entrance point P 2 , a reaction force Fr, which was determined in step S 3  and output to the accelerator pedal  12  in step S 4 , and has the characteristic  102  shown in  FIG. 4C , is calculated based on the speed difference ΔV between the actual speed V and the before-entering-curve target speed characteristic Vtf 1  shown in  FIG. 4B . 
     If the answer to step S 5  is affirmative, i.e., if the host vehicle  11  has reached the first curve entrance point P 2 , then in step S 6 , the reaction force Fr applied to the accelerator pedal  12  upon arrival at the first curve entrance point P 2  is maintained, i.e., the reaction force Fr continues to be applied to the accelerator pedal  12 . 
     The host vehicle  11 , in which the reaction force Fr applied to the accelerator pedal  12  upon arrival at the first curve entrance point P 2  is maintained at a constant level, as indicated by the characteristic  104 , starts to turn along a first curve turning zone Xc 1 . In step S 7 , it is monitored whether or not the host vehicle  11  has finished turning along the first curve turning zone Xc 1 . In step S 8 , while the host vehicle  11  turns along the first curve turning zone Xc 1 , the navigation device  20  detects whether or not there is a second curve  52  within a prescribed distance on the path  50  ahead of the host vehicle  11 . 
     While the host vehicle  11  turns along the first curve  51 , since the reaction force Fr applied to the accelerator pedal  12  does not change, the driver of the host vehicle  11  can operate the accelerator pedal  12  without feeling strange and uncomfortable. 
     According to the distance charts shown in  FIGS. 4A ,  4 B, and  4 C, since a second curve  52  does not exist up to a point P 5  at the end of the prescribed distance, the answer to step S 7  eventually becomes affirmative, i.e., the host vehicle  11  reaches the first curve exit point P 3  at the end of the first curve turning zone Xc 1 . Thereafter, a reaction force fading process is carried out. In the reaction force fading process, as indicated by a reaction force characteristic  106 , the reaction force Fr is gradually reduced to zero over a prescribed time or distance from the first curve exit point P 3  shown in  FIG. 4C . 
     [Second Process: Reaction Force Control Process in the Presence of a Second Curve within a Prescribed Distance after Exiting from a First Curve] 
     A process of setting a reaction force applied to the accelerator pedal  12 , which is carried out by the ECU  22 , will be described in detail below with reference to the flowchart shown in  FIG. 3  and the distance charts shown in  FIGS. 6A ,  6 B, and  6 C. 
     The process includes steps S 1  through S 8 , which are identical to those of the first process shown in  FIG. 2 . Steps S 1  through S 8  will be described briefly below. 
     As shown in  FIG. 6A , the host vehicle  11  travels along a path  50 A. While the host vehicle  11  travels along the path  50 A, in step S 1 , the navigation device  20  detects whether or not there is a first curve  51  within a prescribed distance on the path  50 A ahead of the host vehicle  11 . If the navigation device  20  detects the first curve  51 , the navigation device  20  sends information concerning the curve, including the distance from the host vehicle  11  to a first curve entrance point P 2 , the position of the first curve entrance point P 2 , the position of a first curve exit point P 3 , and the curvature Cu 1  of the first curve  51  (first curve curvature), to the curve detector  22 A. 
     The curve detector  22 A then transfers the received information concerning the curve to the target speed calculator  22 B, which functions as a first curve target speed calculator. 
     In step S 2 , the target speed calculator  22 B calculates, at a point P 0  where the target speed calculator  22 B received the information concerning the curve, a turning target speed Vtc 1  for the first curve  51 . The target speed calculator  22 B also calculates a before-entering-curve target speed characteristic (also referred to as a “before-entering-curve target speed”) Vtf 1  from the calculated turning target speed Vtc 1  and the actual speed V. The turning target speed Vtc 1  and the before-entering-curve target speed characteristic Vtf 1  make up a first curve target speed Vt 1 . The turning target speed Vtc 1  is set to a constant speed at which the host vehicle  11  is capable of turning safely along the first curve  51  having the first curvature Cu 1 , depending on the first curvature Cu 1 . The before-entering-curve target speed Vtf 1  is set as a speed for calculating a reaction force in a curve entering zone for the host vehicle  11 , i.e., a zone from a first curve deceleration start point P 1  to the first curve entrance point P 2 , so that the host vehicle  11  will travel at the turning target speed Vtc 1  at the first curve entrance point P 2 . 
     In step S 3 , the reaction force controller  22 C determines a reaction force Fr to be applied to the accelerator pedal  12  according to a reaction force characteristic  61  (see  FIG. 7 , which is the same as the reaction force characteristic  61  shown in  FIG. 5 ), which is used to determine a reaction force Fr based on a speed difference ΔV. As shown in  FIG. 6B , the speed difference ΔV represents the difference between the actual speed (present speed) V and the before-entering-curve target speed characteristic Vtf 1  (ΔV=V−Vtf 1 ). 
     In step S 4 , the reaction force Fr, which was calculated in the foregoing manner, is applied to the accelerator pedal  12  by the reaction force applying mechanism  24 . 
     In step S 5 , it is judged whether or not the host vehicle  11  has reached the first curve entrance point P 2 . Until the host vehicle  11  reaches the first curve entrance point P 2 , a reaction force Fr, which was determined in step S 3  and output to the accelerator pedal  12  in step S 4 , and has the characteristic  102  shown in  FIG. 6C , is calculated based on the speed difference ΔV between the actual speed V and the before-entering-curve target speed characteristic Vtf 1  shown in  FIG. 6B . 
     If the answer to step S 5  is affirmative, i.e., if the host vehicle  11  has reached the first curve entrance point P 2 , then in step S 6 , the reaction force Fr according to the characteristic  104  upon arrival at the first curve entrance point P 2  is maintained, i.e., the reaction force Fr continues to be applied to the accelerator pedal  12 . 
     The host vehicle  11 , in which the reaction force Fr continues to be applied to the accelerator pedal  12 , starts to turn along a first curve turning zone Xc 1 . In step S 7 , it is monitored whether or not the host vehicle  11  has finished turning along the first curve turning zone Xc 1 . In step S 8 , while the host vehicle  11  turns along the first curve turning zone Xc 1 , the navigation device  20  detects whether or not there is a second curve  52  within a prescribed distance on the path  50  ahead of the host vehicle  11 . 
     According to the distance charts shown in  FIGS. 6A ,  6 B, and  6 C, since a second curve  52  exists, the answer to step S 8  is affirmative, and control proceeds to step S 9  in  FIG. 3 . 
     In step S 9 , similar to step S 2 , the target speed calculator  22 B calculates, at a point where the target speed calculator  22 B has received information concerning the curve, i.e., at a point between the point P 2  and the point P 3 , a turning target speed Vtc 2  for the second curve  52 . The target speed calculator  22 B also calculates a before-entering-curve target speed characteristic (also referred to as a “before-entering-curve target speed”) Vtf 2  from the calculated turning target speed Vtc 2  and the actual speed V of the host vehicle  11 . 
     The turning target speed Vtc 2  is set to a constant speed at which the host vehicle  11  is capable of turning safely along the second curve  52  having a second curvature Cu 2 , depending on the second curvature Cu 2 . In addition, the before-entering-curve target speed Vtf 2  is set as a speed for calculating a reaction force for the host vehicle  11  in a curve entering zone, i.e., a zone from a point Pa, which is a prescribed distance ahead of the entrance to the second curve  52  and at which the navigation device  20  has detected the second curve  52 , to a second curve entrance point P 5 , so that the host vehicle  11  will travel at the turning target speed Vtc 2  at the second curve entrance point P 5 . 
     More specifically, the before-entering-curve target speed Vtf 2  is set to a speed gradient (linear or curved) in order to gradually reduce the present speed V to the turning target speed Vtc 2  over a distance between the point Pa, which is a prescribed distance ahead of the entrance of the second curve  52  and at which the navigation device  20  has detected the second curve  52  while the host vehicle  11  is traveling along the first curve  51 , and the second curve deceleration start point P 2 , i.e., the distance from the point Pa to the point P 5  shown in  FIG. 6B . 
     In step S 10 , the reaction force controller  22 C determines a reaction force (also referred to as a “second curve target reaction force”) Fr to be applied to the accelerator pedal  12 , which has a reaction force characteristic between a reaction force characteristic  62   a  and a reaction force characteristic  62   b , and which is smaller than the reaction force Fr according to the reaction force characteristic  61 . The reaction force Fr is determined using the reaction force characteristic  62   a  (in which the distance between the first and second curves is small) and the reaction force characteristic  62   b  (in which the distance between the first and second curves is large) shown in  FIG. 7 , which are used to determine the reaction force Fr based on a speed difference ΔV. 
     More specifically, the reaction force characteristic  62   a  is applied in the case that the distance between the first curve  51  and the second curve  52  (inter-curve distance) is small, whereas the reaction force characteristic  62   b  is applied in the case that the distance between the first curve  51  and the second curve  52  is large. Depending on the distance between the first curve  51  and the second curve  52 , a characteristic having a gradient, which lies between the gradients of the reaction force characteristics  62   a  and  62   b , is interpolated. 
     In step S 11 , it is judged whether or not the second curve target reaction force Fr is greater than the presently applied reaction force Fr. If the second curve target reaction force Fr is smaller than the presently applied reaction force Fr, the second curve target reaction force Fr, i.e., the reaction force having the characteristic  104 , is output in step S 15 . Then it is judged whether or not the host vehicle  11  has reached the second curve entrance point. 
     In contrast thereto, in step S 11 , if the second curve target reaction force Fr, i.e., the reaction forced Fr having a characteristic  108 , is greater than the presently applied reaction force Fr, i.e., the reaction force having the characteristic  104  (see point Pa in  FIG. 6C ), then since the host vehicle  11  is traveling along the first curve  51 , the reaction forced Fr having the characteristic  108 , which corresponds to the second curve target reaction force Fr, is not simply applied as is, but rather, the second curve target reaction force Fr is reduced depending on the speed difference ΔV by a rate limiting process by referring to a reaction force characteristic between the reaction force characteristics  62   a  and  62   b  shown in  FIG. 7 , as indicated by a reaction force Rr having a characteristic  110 . In addition, the reduced reaction force is applied to the accelerator pedal  12 , so that the driver will not feel strange and uncomfortable on account of the applied reaction force Fr. 
     While the rate limiting process (between points Pa and P 3 ) is performed, in step S 13 , it is judged whether or not the host vehicle  11  has finished turning along the first curve turning zone Xc 1 . In step S 14 , at point P 3 , after the host vehicle  11  has finished turning along the first curve turning zone Xc 1 , the rate limiting process is canceled. In step S 15 , a second curve target reaction force Fr is output having a characteristic  112  ( FIG. 6C ) calculated depending on the speed difference ΔV by referring to the reaction force characteristic  61  ( FIG. 7 ). Then, in step S 16 , it is judged whether or not the host vehicle  11  has reached the second curve entrance point P 5 . If the host vehicle  11  has reached the second curve entrance point P 5 , then control proceeds to step S 17  in which “first” is replaced with “second”. Thereafter, control returns to step S 6  and the reaction force is maintained (point P 5 ). 
     According to the distance charts shown in  FIGS. 6A ,  6 B, and  6 C, since the speed difference ΔV=V−Vtf 2  is zero at a point Pb prior to the second curve entrance point P 5  and the speed difference ΔV is negative (ΔV&lt;0) at the second curve entrance point P 5 , the reaction force Fr is zero (Fr=0) and the reaction force Fr=0 is maintained at the second curve entrance point P 5 . 
     Therefore, the reaction force Fr is maintained at zero within the second curve turning zone Xc 2  from the second curve entrance point P 5  to the second curve exit point P 6 . 
     While the host vehicle  11  is in the second curve turning zone Xc 2  from the second curve entrance point P 5  to the second curve exit point P 6 , since the actual speed V is smaller than the turning target speed Vtc 2  for the second curve  52 , as shown in  FIG. 6B , the host vehicle  11  can turn safely along the second curve turning zone Xc 2 . 
     If the distance between the exit point P 3  of the first curve  51  and the entrance point P 5  of the second curve  52  is short, as shown in  FIGS. 8A ,  8 B, and  8 C, a reaction force Fr having a characteristic  118  is not as small as the reaction force Fr having the characteristic  110  ( FIG. 6C ). In other words, as the distance between the first curve  51  and the second curve  52  subsequent to the first curve  51  becomes shorter, the degree to which the reaction force Fr is reduced by the rate limiting process is not decreased significantly. In  FIGS. 8A ,  8 B, and  8 C, from the exit point P 3  of the first curve  51  (=the entrance point P 5  of the second curve  52 ), a reaction force Fr is calculated by referring to the reaction force characteristic  61  shown in  FIG. 7 , depending on the difference ΔV between the turning target speed Vtc 2  of the second curve  52  and the actual speed V. The calculated reaction force Fr is applied to the accelerator pedal  12  by the reaction force applying mechanism  24 . 
     Furthermore, as shown in  FIGS. 8A ,  8 B, and  8 C, since no curve is detected along a path  50 B from the point P 6 , through the point P 8 , to the point P 9 , the reaction force Fr remains zero, as indicated by a characteristic  124 . The reaction force Fr also remains zero from a point Pc at which the actual speed V is lower than the turning target speed Vtc 2  of the second curve  52 , i.e., a point where the characteristic  122  changes to the characteristic  124 , to the point P 6 . 
     [Review of the Present Embodiment] 
     As described above, the reaction force control apparatus  10  according to the present embodiment includes the accelerator pedal  12  for adjusting the opening of the throttle valve, not shown, (or for adjusting an amount of current for energizing a motor if the vehicle is an electric vehicle driven by the motor), the reaction force controller  22 C for controlling a reaction force Fr applied to the accelerator pedal  12 , which is generated by the reaction force applying mechanism  24  that serves as an actuator, and the curve detector  22 A for detecting the curves  51  and  52  along the paths  50 ,  50 A, and  50 B on which the host vehicle  11  travels. 
     Before the host vehicle  11  enters the curves  51  and  52 , the reaction force controller  22 C sets a target reaction force Fr to be applied to the accelerator pedal  12  depending on the curvatures Cu 1 , Cu 2  of the curves  51  and  52 . For generating the set target reaction force Fr before the host vehicle  11  enters the curves  51  and  52 , and by applying the generated target reaction force Fr, when the host vehicle  11  is traveling and is about to travel along at least the two curves  51  and  52 , i.e., the first curve  51  and the second curve  52  subsequent to the first curve  51 , if the target reaction force Fr (the reaction force according to the characteristic  108  shown in  FIG. 6C ), which is set before the host vehicle  11  enters the second curve  52 , is greater than the present reaction force Fr (the reaction force according to the characteristic  104  shown in  FIG. 6C ) generated and applied to the accelerator pedal  12  while the host vehicle  11  travels and turns along the first curve  51 , the target reaction force Fr for the second curve  52  is reduced, and the reaction force according to the characteristic  110  shown in  FIG. 6C  is applied to the accelerator pedal  12  until the host vehicle  11  has finished traveling through the first curve  51 . The reaction force Fr also remains zero from a point Pb at which the actual speed V is lower than the turning target speed Vtc 2  of the second curve  52 , i.e., a point where the characteristic  114  changes to the characteristic  116 . 
     According to the present embodiment, when the host vehicle  11  travels along successive curves  51  and  52 , the driver feels less strange and uncomfortable concerning operation of the accelerator pedal  12 , and reaction force characteristics for the accelerator pedal  12  are established depending on the nature of the curves  51  and  52 . 
     When the host vehicle  11  travels along successive curves  51  and  52 , the target reaction force Fr for the second curve  52  may be corrected (calculated) based on the target reaction force Fr for the first curve  51 . Conversely, the target reaction force Fr for the first curve  51  may be corrected (calculated) based on the target reaction force Fr for the second curve  52 . In other words, the target reaction force Fr for one of the curves may be corrected based on the target reaction force Fr for the other curve. 
     In such a case, as shown in  FIGS. 8A ,  8 B, and  8 C, the reaction force controller  22 C may lessen the amount by which the target reaction force Fr for the second curve  52 , i.e., the difference between the reaction forces according to characteristics  108  and  118 , is reduced, as the distance between the exit point P 3  of the first curve  51  and the entrance point P 5  of the second curve  52 , which are detected by the curve detector  22 A, becomes shorter. 
     If the distance between the first curve  51  and the second curve  52  is shorter, the driver is required to decelerate the host vehicle  11  more quickly than if the distance between the first curve  51  and the second curve  52  were greater. Since the reduction in the target reaction force Fr for the second curve  52  becomes smaller as the distance is shorter, the driver is prompted to decelerate the host vehicle  11  in preparation for the second curve  52 , or the driver is made aware of the presence of the second curve  52 , without being made to feel strange and uncomfortable while traveling along the first curve  51 . Consequently, the driver is able to maneuver the host vehicle  11  more easily while the host vehicle  11  travels along a succession of curves, which are separated by short distances therebetween. 
     If the curve detector  22 A detects that the curvature Cu 2  of the second curve  52  subsequent to the first curve  51  is smaller than the curvature Cu 1  of the first curve  51  (Cu 2 &lt;Cu 1 ), i.e., the radius of curvature of the second curve  52  is greater than the radius of curvature of the first curve  51 , or that the curvature Cu 2  of the second curve  52  is smaller than a prescribed curvature (curvature threshold value) Cuth, i.e., the radius of curvature of the second curve  52  is greater than a prescribed radius of curvature, (Cu 2 &lt;Cuth), then the reaction force controller  22 C may reduce the amount by which the target reaction force Fr is reduced for the second curve  52 , or may not reduce the target reaction force Fr for the second curve  52 . 
     More specifically, if the curvature Cu 2  of the second curve  52  is small, i.e., if the radius of curvature of the second curve  52  is large, then since the driver feels less strange and uncomfortable concerning operation of the accelerator pedal  12  due to the generated reaction force, the driver can prepare for traveling along the second curve  52 , which has a small curvature Cu 2  or a large radius of curvature, even if the amount by which the target reaction force Fr for the second curve  52  is reduced is smaller, or even if the target reaction force Fr for the second curve  52  is not reduced. 
     The above embodiment may include an arrangement, which includes a reaction force control method (reaction force control apparatus) as described below. 
     A reaction force control method (reaction force control apparatus) sets turning target speeds Vtc 1 , Vtc 2  for the curves  51  and  52  depending on respective curvatures Cu 1 , Cu 2  of the curves  51  and  52  before the host vehicle  11  enters the curves  51  and  52 . The reaction force control method (reaction force control apparatus) applies a reaction force Fr, which depends on a speed difference (error) ΔV between the set turning target speed Vtc 1  and an actual speed (present speed) V, so as to prompt the driver of the host vehicle  11  to decelerate the host vehicle  11  to the turning target speeds Vtc 1 , Vtc 2  until the host vehicle  11  arrives at the entrance points P 2 , P 5  of the curves  51  and  52 . The reaction force control method (reaction force control apparatus) comprises a step (step S 11 , comparing means, comparing section) of comparing the turning target speed Vtc 2  for the second curve  52  and the turning target speed Vtc 1  for the first curve  51  with each other when the turning target speed Vtc 2  is set for the second curve  52 , which is subsequent to the first curve  51  within a prescribed distance. In addition, if the turning target speed Vtc 2  for the second curve  52  is smaller than the turning target speed Vtc 1  for the first curve  51 , the reaction force control method (reaction force control apparatus) also comprises a step (step S 19 , reduced reaction force applying means, reduced reaction force applying section) of applying a reaction force Fr (characteristic  118 ) representing a reduced value of the reaction force Fr (characteristic  108 ) depending on the speed difference ΔV between the actual speed V and the turning target speed Vtc 2  for the second curve  52 . 
     With the above arrangement, when the host vehicle  11  travels along the successive curves  51  and  52 , the driver feels less strange and uncomfortable concerning operation of the accelerator pedal  12 , and reaction force characteristics for the accelerator pedal  12  are established depending on the nature of the curves  51  and  52 . 
     The present invention is not limited to the above embodiment, and various arrangements may be adopted based on the disclosure of the present description. 
     For example, the present invention may be utilized as a reaction force control apparatus  10  having a reaction force controller  22 C for controlling a reaction force Fr, the reaction force Fr being applied to the accelerator pedal  12 . The reaction force Fr is generated by the reaction force applying mechanism  24 , which serves as an actuator, and a curve detector  22 A for detecting a curve on a path  50  traveled by a host vehicle  11 . In this case, as shown in  FIG. 5A , if the curve detector  22 A detects only one curve  51 , the reaction force controller  22 C sets a target reaction force Fr depending on the detected one curve  51 . Then, when the host vehicle  11  reaches a first curve exit point (end point) P 3  of the one curve  51 , the reaction force controller  22 C gradually reduces the reaction force Fr to zero according to a reaction force characteristic  106 . With such a reaction force control apparatus  10 , the driver is less likely to feel strange and uncomfortable concerning operation of the accelerator pedal  12  when the host vehicle  11  is traveling along the curve  51 , and also immediately after the host vehicle  11  has traveled along the curve  51 .