Patent Publication Number: US-7711465-B2

Title: Vehicular steering control apparatus and control method thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This is a Division of application Ser. No. 11/581,460 filed Oct. 17, 2006, which claims the benefit of Japanese Patent Application No. 2005-319512 filed on Nov. 2, 2005. The entire disclosure of the prior applications is hereby incorporated by reference herein in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to a steering control apparatus of a vehicle that includes variable steering ratio controlling means for changing a relationship between the steering angle of steered wheels and the steering angle of the steering by applying a driving force to the steering input. 
   2. Description of the Related Art 
   A steering control apparatus (VGRS: Variable Gear Ratio Steering) that includes variable steering ratio controlling means that can change the ratio between the steering angle of a steering wheel and the steering angle of steered wheels is known (for example, refer to Japanese Patent Application Publication No. JP-A-H11-321684). This technology aims at realizing desired steering response under various operational conditions of the vehicle by, for example, changing the steering ratio according to the vehicle speed. To realize such variable steering ratio controlling means, for example, a transmission portion such as a gear is provided at a connection point between the input shaft in the steering wheel side and the output shaft in the tie rods side, and the relative rotation angles of the input shaft and the output shaft are changed by applying driving force to the transmission portion by an actuator. It should be noted that the actuator that applies the driving force to the transmission portion may constitute the transmission portion. Note that for such steering ratio control there is a method in which feedback control is performed so as to achieve a target steering angle. 
   In feedback control for variable steering ratio control as described above, the accuracy of position control is improved by setting the control gain high. However, when the driver is not holding the steering wheel (will be referred to as a “non-holding state” where necessary), the reaction force from the actuator is not received by the steering wheel side, and therefore the control system becomes unstable. As a result, the steering wheel may vibrate. Such a non-holding state may occur while the steering wheel is being returned to the neutral position. 
   In the technology disclosed in Japanese Patent Publication Application No. JP-A-H11-321684, for the purpose of reducing vibration when the driver is not holding the steering wheel, such a non-holding state is detected based on the torque that the driver applies to the steering wheel, and when it is detected, the control gain is reduced. 
   However, when the control gain is reduced in this manner, the accuracy of position control and the steering response are deteriorated, and thus there is the problem that the positional deviation due to external disturbances becomes large. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide a vehicular steering control apparatus and a control method thereof that can suppress vibration which occurs when the driver is not holding the steering wheel, while reducing the positional deviation due to external disturbances. 
   A first aspect of the invention relates to a vehicular steering control apparatus including: (1) a variable steering ratio controlling device that controls a steering angle of steered wheels by applying a driving force to a steering input; (2) an input torque obtaining unit that obtains an input torque that is applied to a steering train of a steering system by external forces; (3) an operation torque calculating unit that calculates an operation torque that a driver applies to a steering; and (4) a target steering angle adjusting unit that sets a target steering angle of the variable steering ratio controlling device smaller when a first condition which is satisfied if the operation torque is smaller than a first predetermined value and a second condition which is satisfied if the input torque is smaller than a second predetermined value are both satisfied, than when at least one of the first and the second conditions is not satisfied. 
   A second aspect of the invention relates to a vehicular steering control apparatus that includes a phase adjusting unit that, when a first condition which is satisfied if the operation torque is smaller than a first predetermined value and a second condition which is satisfied if the input torque is smaller than a second predetermined value are both satisfied, sets a phase of a target steering angle of the variable steering ratio controlling device to a phase that is different from a phase set when at least one of the first and the second conditions is not satisfied. In this apparatus, the phase adjusting unit is provided instead of the target steering angle adjusting unit of the apparatus according to the first aspect of the invention, which sets a target steering angle of the variable steering ratio controlling device smaller when a first condition which is satisfied if the operation torque is smaller than a first predetermined value and a second condition which is satisfied if the input torque is smaller than a second predetermined value are both satisfied, than when at least one of the first and the second conditions is not satisfied. 
   A third aspect of the invention relates to a steering control method for a vehicle including a variable steering ratio controlling device that controls a steering angle of steered wheels by applying a driving force to a steering input. In the steering control method of the vehicle, an input torque that is applied to a steering train of a steering system by external forces and an operation torque that the driver applies to a steering are obtained, and a target steering angle of the variable steering ratio controlling device is set smaller when a first condition which is satisfied if the operation torque is smaller than a first predetermined value and a second condition which is satisfied if the input torque is smaller than a second predetermined value are both satisfied, than when at least one of the first and the second conditions is not satisfied. 
   A fourth aspect of the invention relates to a steering control method for a vehicle including a variable steering ratio controlling device that controls a steering angle of steered wheels by applying a driving force to a steering input. In the steering control method of the vehicle, an input torque that is applied to a steering train of a steering system by external forces and an operation torque that a driver applies to a steering are obtained, and when a first condition which is satisfied if the operation torque is smaller than a first predetermined value and a second condition which is satisfied the input torque is smaller than a second predetermined value are both satisfied, a phase of a target steering angle of the variable steering ratio controlling device is set to a phase that is different from a phase set when at least one of the first and the second conditions is not satisfied. 
   The aforementioned vibration that occurs in the steering system including the variable steering ratio controlling device when the driver is not holding the steering wheel tends to intermittently occur near the equilibrium point after the steering wheel returns to around the neutral position. According to the invention, the vibration reduction control is performed when the driver is not holding the steering wheel (i.e., the operation torque is smaller than the first predetermined value) while the equilibrium point is reached (i.e., the input torque is smaller than the second predetermined value). It should be noted that the term “equilibrium point” herein indicates the point where the torque input to the steering side and the reaction force from the steered wheels are in equilibrium. 
   The vibration reduction control according to the invention varies the response of the target steering angle, and in the first method, vibration is forcibly attenuated by setting the target steering angle smaller. In the second method, attenuation of vibration is promoted by changing the phase of the target steering angle from one which causes vibration. As the method to change the phase, for example, the filtering frequency for calculating the target steering angle may be changed. 
   According to the invention, when the driver is not holding the steering wheel around the equilibrium point, attenuation of vibration is promoted and occurrence of vibration is prevented by changing the response of the target steering angle. As a result, it is possible to effectively prevent or reduce vibration around the equilibrium point. In addition, the vibration reduction control is performed only when the driver is not holding the steering wheel while the equilibrium point is reached, and therefore the accuracy of position control and the steering response are not deteriorated. When there are some external disturbances, the vibration reduction control is not performed, and thus the positional deviation due to the external disturbances can also be prevented. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and further objects, features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
       FIG. 1  is a view schematically showing the steering system that includes a vehicular steering control apparatus according to the first embodiment of the invention. 
       FIG. 2  is a control block diagram showing the configuration of the first embodiment. 
       FIG. 3  is a graph showing a comparison between the result of the steering control of the first embodiment and the result of steering control in which no vibration reduction control is performed. 
       FIG. 4  is a view schematically showing a steering system as modified from the first embodiment. 
       FIG. 5  is a control block diagram showing the configuration of the second embodiment. 
       FIG. 6  is a graph showing a comparison between the result of the steering control of the second embodiment and the result of steering control in which no vibration reduction control is performed. 
       FIG. 7  is a view schematically showing the steering system that includes a vehicular steering control apparatus according to the third embodiment of the invention. 
       FIG. 8  is a control block diagram showing the configuration of the third embodiment. 
       FIG. 9  is a view schematically showing a steering system including variable steering ratio controlling device as modified from the first embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Preferred embodiments of the invention will be hereinafter described in detail with reference to the attached drawings. In order to facilitate understanding of the description, the same reference numerals are used for the same elements in the drawings as much as possible, and same descriptions of the elements are omitted. 
   As shown in  FIGS. 1 and 2 , a steering system functions to turn a right front wheel FR and a left front wheel FL, which are steered wheels, as a steering wheel  10  is turned. Rotation of a steering shaft that is connected to the steering wheel  10  is converted to a linear motion in the horizontal direction at a steering gear box  13 , and is then transmitted to the front wheels FR, FL through a tie rod  14 . 
   The steering shaft includes an input shaft  11  connected to the steering wheel  10  and an output shaft  12  connected to the steering gear box  13 . A gear mechanism  20  is provided between the input shaft  11  and the output shaft  12 . An electric motor  21  is connected to the gear mechanism  20 . They constitute variable steering ratio controlling device. 
   An input angle sensor  30  and a torque sensor  31  are provided on the input shaft  11 . The input angle sensor  30  detects an operation angle θh of the steering wheel  10 , and the torque sensor  31  detects an input torque T to the steering train of the steering system. The input torque T is resultant force of external forces which are applying to the steering train of the steering system other than an operation torque Th (as will hereinafter be described in detail). A motor output angle sensor  32  that detects a motor output shaft turning angle θp is provided on the motor output shaft of the motor  21 . The turning angle of the output shaft  12  is the sum of θh and θp (θh+θp), and this value has a predetermined relationship with the steering angle of the front wheels FR, FL. 
   A steering control unit  4  includes a CPU, a ROM, a RAM, an electric circuit etc., and the output signals from the input angle sensor  30 , the torque sensor  31 , the motor output angle sensor  32 , and a vehicle speed sensor  50  that detects a vehicle speed V of the vehicle are input to the steering control unit  4 . The steering control unit  4  controls the driving of the motor  21 . 
   As shown in  FIG. 2 , the steering control unit  4  further includes a target steering angle setting portion  41 , an operation torque estimating portion  42 , a non-holding state detecting portion  43 , an equilibrium detecting portion  44 , a target steering angle adjusting portion  45 , a switching portion  46 , a compensator  23 , a motor drive circuit  22 , multipliers  47 ,  48 , and an adder  49 . 
   The target steering angle setting portion  41  sets a target output angle θpm based on the vehicle speed V detected by the vehicle speed sensor  50  and the operation angle θh detected by the input angle sensor  30 . The turning angle θh+θp of the output shaft  12  and the steering angle δ of the steered wheels have a predetermined relationship defined by the characteristics of the steering gear box  13 . Therefore, setting the target output angle θpm in accordance with the operation angle θh is synonymous with setting the target steering angle δm. 
   The operation torque estimating portion  42  estimates an operation torque Th applied by a driver using an equation (1) that factors in the input torque T at the steering train of the steering system which is detected by the torque sensor  31 , and the operation angle θh which is detected by the input angle sensor  30 . 
   
     
       
         
           
             
               
                 Th 
                 = 
                 
                   
                     Ih 
                     ⁢ 
                     
                       
                         
                           
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                             2 
                           
                           ⁢ 
                           θ 
                         
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                         ⁢ 
                         h 
                       
                       
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                     Ch 
                     ⁢ 
                     
                       
                         
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                         h 
                       
                       
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                 ( 
                 1 
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   The reference symbol Ih denotes a moment of inertia at the steering wheel  10 , and Ch denotes a coefficient of viscous friction against the rotation of the input shaft  11 . The inertia moment Ih and the viscous friction coefficient Ch have values specific to the type or structure of each steering system. Therefore, by obtaining the inertia moment Ih and the viscous friction coefficient Ch in advance and storing them in the operation torque estimating portion  42 , it is possible to estimate the operation torque Th from the input torque T, an angular velocity and an angular acceleration of the operation angle θh. 
   Then, based on the input torque T and the operation torque Th, the non-holding state detecting portion  43  detects a non-holding state which is the state where the driver is not holding the steering  10  and the equilibrium detecting portion  44  determines whether an equilibrium point, to be described later, is reached. Specifically, the non-holding state detecting portion  43  detects the non-holding state by comparing the absolute value of the operation torque Th and a threshold value ε 1 . The threshold value ε 1  is set to a value at and above which it is considered that the driver is holding the steering wheel  10  controllably. That is, if the absolute value |Th| is equal to or larger than the threshold value ε 1 , the non-holding state detecting portion  43  determines that the non-holding state is not present and therefore outputs “0” to the multiplier  47 . When the absolute value |Th| is smaller than the threshold value ε 1 , conversely, the non-holding state detecting portion  43  determines that the non-holding state is present and therefore outputs “1” to the multiplier  47 . Meanwhile, the equilibrium detecting portion  44  determines whether an equilibrium point is reached by comparing the absolute value of the input torque T and a threshold value ε 2 . The threshold value ε 2  is set to a value corresponding to a state where the respective inputs to the steering train of the steering system, i.e., the input from the driver to the steering wheel  10  is, equilibrates to the assist torque from the motor  21  and the reaction force from the steered wheels FL, FR side. That is, if the absolute value |T| is equal to or larger than the threshold value ε 2 , the equilibrium detecting portion  44  determines that the equilibrium point is not reached, and outputs “0” to the multiplier  47 . If the absolute value |T| is smaller than the threshold value ε 2 , the equilibrium detecting portion  44  determines that the equilibrium point is reached and outputs “1” to the multiplier  47 . 
   The output values (0 or 1) from the non-holding state detecting portion  43  and the equilibrium detecting portion  44  are multiplied by the multiplier  47 . In this way, “1” is output to the switching portion  46  only when the non-holding state is present and the equilibrium point is reached. When either or both of the these conditions is not satisfied, such as when the driver is holding the steering wheel  10 , or when the driver is not holding the steering wheel  10  but the equilibrium point is not reached, “0” is output to the switching portion  46 . 
   The switching portion  46  is connected to the output side of the target steering angle setting portion  41 , and the operation of the switching portion  46  is controlled by the output from the multiplier  47 . More specifically, when the output value of the multiplier  47  is 0, the switch is set to the upper position in the switching portion  46 , as seen in the drawing, and θpm output from the target steering angle setting portion  41  is directly input to a positive input terminal of the adder  49 . When the output value of the multiplier  47  is 1, the switch is set to the lower position in the switching portion  46 , as seen in the drawing. A multiplier  48  is provided between the positive input terminal of the adder  49  and the switching portion  46 , and θpm′ calculated by the multiplier  48  is input to the positive input terminal of the adder  49 . The output value of a target steering angle adjusting portion  45  is input to the multiplier  48 . The target steering angle adjusting portion  45  sets a gain G in accordance with the input torque T detected by the torque sensor  31 . The gain G is 1 when the absolute value of the input torque T is equal to the threshold value ε 2 , and approaches 0 as the input torque T decreases. That is, θpm′, which is the output value of the multiplier  48 , is obtained by multiplying G with θpm (G×θpm), and therefore it approaches 0 as the input torque T decreases. 
   θp output from the motor output angle sensor  32  is input to a negative input terminal of the adder  49 . Therefore, when it is determined that the driver is not holding the steering wheel  10  and the equilibrium point is reached, the output from the adder  49  is θpm′-θp. In other cases, the output value of the adder  49  is θpm-θp. That is, the difference between the adjusted target output angle and the actual output angle is output to the compensator  23 . 
   The compensator  23  calculates the driving amount of the motor  21  in accordance with the difference between the target output angle and the actual output angle, and transmits the calculated driving amount to the motor drive circuit  22 . The motor drive circuit  22  then drives the motor  21  according to the driving amount. In this way, by bringing the actual output angle closer to the target output angle, the steering angle is controlled to be come a desired angle. 
     FIG. 3  is a graph showing a comparison between the result of the steering control of the first embodiment and the result of steering control in which no vibration reduction control is performed, when the driver takes his or her hands off the steering wheel  10  so as to allow the steering wheel  10  to turn back to the neutral position after turning the steering wheel  10  to the maximum steering angle. In the graph, θpm 1  shows changes with time in the target output angle and θp 1  shows changes with time in the actual output angle during the steering control with no vibration reduction control. On the other hand, θpm 2  shows changes with time in the target output angle and θp 2  shows changes with time in the actual output angle during the steering control of the embodiment. 
   Unavoidably, the actual output angle changes with delay in response to a change in the target output angle due to, for example, deformation and friction in the steering train of the steering system. Because of such delay, vibration occurs around the neutral position during the steering control with no vibration reduction control. To counter this, in the steering control of the embodiment, when the steering reaches around the neutral position, which corresponds to the equilibrium point, the target output angle is reduced to reduce the target steering angle (see θpm 2 ). As a result, the operation amount of the motor  21  is reduced, so that vibration is effectively suppressed (see θp 2 ). In this case, the control gain is substantially the same as used in a regular control, and therefore good steering response is maintained. Meanwhile, in the case where an external force is acting on the steering train of the steering system and thus the equilibrium point is not reached, the regular control is performed, and therefore the accuracy of position control and the response can be maintained. Accordingly, the steering control apparatus of the embodiment improves the overall steering feeling. 
   In the first embodiment, the torque sensor  31  is provided on the input shaft  11 , but it may alternatively be provided on the output shaft  12  as shown in  FIG. 4 . In this case, too, it is possible to estimate the operation torque Th based on the equation (1). 
     FIG. 5  is a block diagram showing the configuration of a vehicular steering control apparatus according to the second embodiment of the invention. This apparatus is used as a steering control apparatus for the steering system shown in  FIG. 1  or  FIG. 4 . In the steering control unit  4  of the first embodiment shown in  FIG. 2 , as mentioned above, the switching portion  46  is provided between the target steering angle setting portion  41  and the adder  49 , and it switches, in accordance with the output value of the multiplier  47 , between the mode where the output of the target steering angle setting portion  41  is directly input to the adder  49  and the mode where the output of the target steering angle setting portion  41  is adjusted using the output value of the target steering angle adjusting portion  45 . Meanwhile, in a steering control unit  4   a  of the second embodiment, the switching portion  46  is provided between the input angle sensor  30  and the target steering angle setting portion  41 , and a filter for filtering the input angle signals input to the target steering angle setting portion  41  is switched between a first filter  51  and a second filter  52 . 
   The first filter  51  and the second filter  52  have different filter frequencies, therefore the phase characteristic of the target output angle set by the target steering angle setting portion  41  is different depending on which of the first filter  51  or the second filter  52  is used. The use of such different phase characteristics, in particular, makes it possible to suppress resonance caused by a phase difference between the target output angle input to the adder  49  and the measured output angle when vibration occurs. The phase characteristic of the target output angle obtained with the second filter  52  can be made different from that obtained with the first filter  51  by setting the filter frequency of the second filter  52  higher or lower than the filter frequency of the first filter  51 . 
     FIG. 6  is a graph showing a comparison between the result of the steering control of the embodiment and the result of steering control in which no vibration reduction control is performed, when the driver takes his or her hands off the steering wheel  10  so as to allow the steering wheel  10  to turn back to the neutral position after turning the steering wheel  10  to the maximum steering angle. In the graph, θpm 3  shows changes with time in the target output angle and θp 3  shows changes with time in the actual output angle during the steering control with no vibration reduction control. On the other hand, θpm 4  shows changes with time in the target output angle and θp 4  shows changes with time in the actual output angle during the steering control of the embodiment. 
   In the steering control with no vibration reduction control, when vibration occurs, the vibration cannot be reduced because the phase relationship between the target output angle and the actual output angle promotes resonance, and as a result, the vibration continues for a long time. In order to prevent such resonance, during the steering control of the embodiment, the phase characteristic of the target output angle is selectivelyset. Therefore, when vibration is occurring, the vibration is effectively and immediately attenuated. Similarly to the first embodiment, in the case where an external force is acting on the steering train of the steering system and thus the equilibrium point is not reached, i.e., one of the conditions for performing the vibration reduction control is not satisfied, the regular control is performed, and therefore the accuracy of position control and the response are not deteriorated. Accordingly, the steering control apparatus of the embodiment improves the overall steering feeling. 
     FIG. 7  is a view schematically showing the configuration of a steering system incorporating a steering control apparatus according to the third embodiment of the invention, and  FIG. 8  is a block diagram showing the configuration of the steering control apparatus. The steering system in the third embodiment, as shown in  FIG. 7 , differs from those shown in  FIGS. 1 and 4  in that the torque sensor  31  is removed and a motor current sensor  24  that detects current supplied to the motor  21  is provided. 
   As shown in  FIG. 8 , a steering control unit  4   b  has basically the same configuration as the steering control unit  4  in the first embodiment. In the steering system shown in  FIG. 2 , as mentioned above, the torque sensor  31  serves as an input torque obtaining portion that detects the input torque T that is applied to the steering train of the steering system by external forces, and the output from the torque sensor  31  is directly input to the operation torque estimating portion  42  and the equilibrium detecting portion  44 . In contrast, in the steering system of the third embodiment shown in  FIG. 8 , a torsional force estimating portion  25  serves as the input torque obtaining portion that receives a current value Ir output from the motor current sensor  24  and estimates the input torque T based on it is provided. 
   In the steering system in  FIG. 7 , the steering wheel  10  does not receive the torque generated by the motor  21 . Therefore, the torsional force acting on the input shaft  11  is estimated using an equation (2) which factors in the inertia moment Im of the motor  21  and the gear mechanism  20  and the viscous friction coefficient Cm. 
   
     
       
         
           
             
               
                 T 
                 = 
                 
                   KmGmIr 
                   - 
                   
                     Im 
                     ⁢ 
                     
                       
                         
                           
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                             2 
                           
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                         p 
                       
                       
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                     Cm 
                     ⁢ 
                     
                       
                         
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                         p 
                       
                       
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                 ( 
                 2 
                 ) 
               
             
           
         
       
     
   
   Km denotes the torque constant of the motor  21 , and Gm denotes the transmission ratio of the variable steering ratio control mechanism. The operation torque applied by the driver can be estimated by assigning the result of the equation (2) into the equation (1). 
   In the third embodiment, similar effects and advantages to those in the first embodiment can be obtained. The steering control apparatus according to the third embodiment, for example, may be advantageously employed in a hydraulic power steering apparatus. On the other hand, the steering control apparatus according to the first embodiment may be advantageously employed in an electric power steering apparatus. 
   The variable steering ratio control mechanism is not limited to those described above, but may be constructed such that, for example, the input shaft directly receives torque from the electric motor as shown in  FIG. 9 . In this case, since the equation of motion for the steering system is different from those in the foregoing embodiments, different calculation methods are used for estimating the operation torque of the driver and the input torque. However, the basic concept is the same. 
   While the invention has been described with reference to what are considered to be preferred embodiments thereof, it is to be understood that the invention is not limited to the disclosed embodiments or constructions. On the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the disclosed invention are shown in various combinations and configurations, which are exemplary, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.