Abstract:
An electric power steering device includes a steering link that includes a plurality of connected steering shafts that connect a steering wheel to a steered wheel, a motor that provides a steering torque to assist the steering link, a torque sensor which is mounted to one of the steering shafts and which detects the steering torque of that steering shaft, a steering angle sensor which is mounted to one of the steering shafts and which detects a steering angle of that steering shaft, a calculating unit that calculates a steering assist amount based on the steering torque detected by the torque sensor, the steering angle detected by the steering angle sensor, and predetermined crossed axes angles between the connected steering shafts, and a motor controller that drives the motor so as to apply a steering assist force to the steering link based on the steering assist amount. As a result, it is possible to correct the torque by a simple method.

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2001-263429 filed on Aug. 31, 2001, including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to an electric power steering device that corrects, via a motor, a torque variation in a steering link in an automobile, as well as to a control method thereof. 
     2. Description of Related Art 
     An electric power steering device that generates a steering assist force using a motor, such as that disclosed in Japanese Patent Application Laid-Open Publication No. 9-254804, has conventionally been known. In this electric power steering device, a first shaft, which is connected to a steering wheel, is connected to a second shaft by a first universal joint, and the second shaft is connected to a third shaft by a second universal joint, such that a steering link having a plurality of steering shafts is constructed. A torque sensor and a steering angle sensor are then provided on the first shaft and a motor that applies a steering assist force to a link that transmits rotational torque from the steering link to the wheels is provided on that link. 
     This electric power steering device is also provided with a control unit that includes a control circuit, memory, and a drive circuit. In the memory is stored mapped torque correction data in order to obtain a rotational torque variation of the third shaft with respect to the steering force input to the first shaft. The control circuit determines an assist torque value based on a torque detection signal from the torque sensor, an angle detection signal from the steering angle sensor, and the torque correction data from the memory. The drive circuit then drives the motor based on that assist torque value. It is in this way that the steering assist force is obtained. 
     With the foregoing electric power steering device, however, because the torque is corrected using mapped torque correction data, new torque correction data must be created every time the configuration is changed, e.g., every time the length of the steering shafts or the angle between them and the like are changed, that corresponds to that change, which is troublesome. Moreover, creating that torque correction data based on experimental values increases the number of man-hours increases even more. 
     Still further, with an electric power steering device that is provided with a variable-ratio steering mechanism that changes the phase between the steering wheel and the end of the steering link opposite the steering wheel, it is extremely difficult to create appropriate correction data, and what is more, the mounting positions of the torque sensor and the steering angle sensor are limited. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing problems, it is an object of this invention to provide an electric power steering device i) in which torque correction is possible by a simple method, ii) in which a torque sensor, a steering sensor, and a motor, and the like can be mounted to any portion of one of the steering shafts, and iii) which can also be provided with a variable-ratio steering mechanism. 
     In order to achieve the foregoing objects, an electric power steering device is provided that includes i) a steering link which includes a plurality of connected steering shafts that connect the steering wheel with the steered wheels, ii) a motor that provides a steering torque to assist the steering link, iii) a torque sensor which is provided on one of the steering shafts and which detects a steering torque of the steering shaft, iv) a steering angle sensor which is provided on one of the steering shafts and which detects a steering angle of the steering shaft, v) a calculating unit that calculates a steering assist amount based on the steering torque detected by the torque sensor, the steering angle detected by the steering angle sensor, and predetermined crossed axes angles between the connected steering shafts, and vi) a motor controller that drives the motor so as to apply a steering assist force to the steering link. 
     In addition, a control method is provided for an electric power steering device including a steering link that includes a plurality of connected steering shafts that connect the steering wheel with the steered wheels, a motor that provides a steering torque to assist the steering link, a torque sensor which is provided on one of the steering shafts and which detects a steering torque of that steering shaft, and a steering angle sensor which is provided on one of the steering shafts and which detects a steering angle of that steering shaft, includes the steps of calculating a steering assist amount based on the steering torque detected by the torque sensor, the steering angle detected by the steering angle sensor, and predetermined crossed axes angles between the connected steering shafts, and driving the motor so as to apply a steering assist force to the steering link based on that steering assist amount. 
     According to an electric power steering device having the above described configuration and the above-described control method thereof, because the steering assist amount is obtained by calculation from the steering torque, the steering angle, and the crossed axes angle between each of the steering shafts, an appropriate assist force can always be applied to the steering link irrespective of the positional relationship, e.g., the crossed axes angle, between each of the steering shafts, thus giving the driver a uniformly stable steering feel. In addition, because the steering assist amount is calculated based on the steering torque that is detected by the torque sensor, the steering angle that is detected by the steering angle sensor, and the crossed axes angle between the steering shafts, the mounting locations of the torque sensor and the steering angle sensor are able be anywhere on the steering shafts, which improves the degree of freedom in the design of the construction. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-mentioned embodiment and other embodiments, objects, features, advantages, technical and industrial significance of this invention will be better understood by reading the following detailed description of the preferred embodiments of the invention, when considered in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic view of an electric power steering device according to a first embodiment of the invention; 
     FIG. 2 is a block diagram of the functions of the electric power steering device according to the first embodiment of the invention; 
     FIG. 3 is an explanatory view showing a state in which torque is being transferred between the steering shafts according to the first embodiment of the invention; 
     FIG. 4 is a line graph of the characteristics of the torque ratio with respect to the shaft rotation according to the first embodiment of the invention; 
     FIG. 5 is a schematic view of an electric power steering device according to a second embodiment of the invention; and 
     FIG. 6 is a block diagram of the functions of the electric power steering device according to the second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description and the accompanying drawings, the present invention will be described in more detail in terms of exemplary embodiments. 
     Hereinafter, a first exemplary embodiment of the invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic view of an electric power steering device  10  according to a first embodiment of the invention, and FIG. 2 is a block diagram of the functions of the electric power steering device  10  shown in FIG.  1 . 
     In the electric power steering device  10 , a lower end portion of a main shaft  12 , which is connected to a steering wheel  11 , is connected to an upper end portion of a mid shaft  14  via a first universal joint  13 . A lower end portion of the mid shaft  14  is connected to an upper end portion of an extension shaft  16  via a second universal joint  15 . A lower end portion of the extension shaft  16  is connected to a pinion gear which is housed within a gear box  17  and which engages with a rack bar  18 . 
     Therefore, when the steering wheel  11  is turned, the rotational force is transmitted to the extension shaft  16  via the main shaft  12 , first universal joint  13 , mid shaft  14 , and second universal joint  15  so as to turn the pinion gear in the gear box  17 . The turning of the pinion gear in the gear box  17  selectively moves the rack bar  18  in the directions of arrows a and b, which in turn changes the direction of the steered wheels, not shown. The steering link includes the three steering shafts, which are the main shaft  12 , the mid shaft  14 , and the extension shaft  16 . Also in the figure, the crossed axes angle of the main shaft  12  and the mid shaft  14  is denoted as α, and the crossed axes angle of the mid shaft  14  and the extension shaft  16  is denoted as β. 
     The first universal joint  13  is constructed such that a yoke joint  12   a , which is connected to the lower end of the main shaft  12  and which rotates integrally therewith, is connected via a cross-pin  13   a  to a yoke joint  14   a , which is connected to the upper end of the mid shaft  14  and which rotates integrally therewith. Also, the second universal joint  15  is constructed such that a yoke joint  14   b , which is connected to the lower end of the mid shaft  14  and which rotates integrally therewith, is connected via a cross-pin  15   a  to a yoke joint  16   a , which is connected to the upper end of the extension shaft  16  and which rotates integrally therewith. 
     Mounted to the main shaft  12  are a steering angle sensor  19  that detects a rotation angle of the main shaft  12  when the steering wheel  11  is turned, a torque sensor  20  that detects a steering torque of the main shaft  12 , and an assist motor  21  which is a motor that applies a steering assist force to the main shaft  12 . The steering angle sensor  19 , torque sensor  20 , and assist motor  21  are each connected to a power steering ECU (electronic control unit)  22 . 
     The power steering ECU  22  utilizes a computer, which is run according to a program routine, as its main component. When shown in a function block diagram such as that in FIG. 2, the power steering ECU  22  includes a torque variation correction calculating portion  22   a , an assist amount calculating portion  22   b , and a motor driver  22   c  that functions as motor driving means. The torque variation correction calculating portion  22   a  receives as a steering angle signal a rotation angle θ of the main shaft  12  that is detected by the steering angle sensor  19  and calculates a correction value using the prerecorded crossed axes angles α and β and a (initial) phase of the upper and lower yokes. The torque variation correction calculating portion  22   a  then sends this correction value to the assist amount calculating portion  22   b.    
     The assist amount calculating portion  22   b  receives this correction value sent from the torque variation correction calculating portion  22   a  as well as a signal indicative of a torque value T detected by the torque sensor  20 . The assist amount calculating portion  22   b  then calculates an assist amount based on these signals and sends a signal indicative of that assist amount to the motor driver  22   c . The motor driver  22   c  then sends the signal received from the assist amount calculating portion  22   b  to the assist motor  21  as a signal indicative of a drive current so as to drive the assist motor  21 . 
     In the above configuration, when the crossed axes angle of the main shaft  12  and the mid shaft  14 , or the crossed axes angle of the mid shaft  14  and the extension shaft  16  is denoted as α, the input/output relationship of the torque with the first universal joint  13  or the second universal joint  15  can be expressed with Expression 1. 
     
       
           f (α, θin)= T out/ T in=cos α/(1−sin 2  θin×sin 2  α)  [Expression 1] 
       
     
     In both FIG.  3  and Expression 1, Tin denotes the torque of the shaft on the input side (the main shaft  12  or the mid shaft  14 ) and Tout denotes the torque of the shaft on the output side (the mid shaft  14  or the extension shaft  16 ). Also, θin denotes the rotation angle of the shaft on the input side. In this case, the torque variation Tout/Tin with respect to the input angle θin shown in Expression 1 changes in 180 degree cycles, as shown in FIG.  4 . 
     Here, the input/output ratio of torque with the first universal joint  13  shown in FIG. 1 with respect to the angle of the yoke joint  12   a  referenced when the yoke joint  12   a  is orthogonal to a plane A that includes the main shaft  12  and the mid shaft  14  is expressed with curve a in FIG.  4 . Also, the input/output ratio of torque with the second universal joint  15  with respect to the angle of the yoke joint  14   b  referenced when the yoke joint  14   b  is orthogonal to a plane B that includes the mid shaft  14  and the extension shaft  16  is expressed with curve b in FIG.  4 . The phase difference between curve a and curve b is denoted as γ. The rotation angle of the input/output shaft at each yoke is expressed with Expression 2 below. 
     
       
           g (α, θin)=θout=tan −1 {tan θin×cos α}  [Expression 2] 
       
     
     θout denotes the rotation angle of the shaft on the output side, which in this case, is the rotation angle of the mid shaft  14  or the extension shaft  16 . In this way, because the torque ratio and the rotation angle can be expressed with Expression 1 and Expression 2, Expression 1 and Expression 2 can be used to obtain the torque variation of the main shaft  12  and the extension shaft  16 . That is, by denoting the torque of the main shaft  12  as a column shaft torque T 1  and its rotation angle as θ1, denoting the torque of the mid shaft  14  as T 2  and its rotation angle as θ2, and denoting the torque of the extension shaft  16  as T 3  and its rotation angle as θ3, the mutual relationships between the torque T 1 , the torque T 2 , and the torque T 3  can be expressed with the following expressions. First, the torque variation T 2 /T 1  of the main shaft  12  and the mid shaft  14  can be expressed by the function in Expression 3 below. 
     
       
           T   2 / T   1 = f (α, θ1)  [Expression 3] 
       
     
     Also, a rotation angle variation expression g is expressed by θ2=g(α, θ1). Next, the torque variation T 3 /T 2  of the mid shaft  14  and the extension shaft  16  can be expressed by the function in Expression 4 below. 
     
       
           T   3 / T   2 = f (β, γ+θ2)  [Expression 4] 
       
     
     Therefore, the torque variation T 3 /T 1  of the main shaft  12  and the extension shaft  16  can be obtained by multiplying Expression 3 by Expression 4, which results in Expression 5 below. 
     
       
           T   3 / T   1 = f (α, θ1)× f (β, γ+θ2)× F (θ1)  [Expression 5] 
       
     
     In the foregoing expressions, f denotes the torque ratio expression and g denotes the rotation angle variation expression. Therefore, according to Expression 5 it is possible to obtain the torque variation of the main shaft  12  and the extension shaft  16 . The target assist amount when there is no torque variation in the steering link will be referred to as “basic assist amount.” The correction value that corrects the assist motor  21  is obtained from the obtained torque variation. The motor driving current corresponding to the basic assist amount and the correction value flows to the assist motor  21  so as to drive the assist motor  21 . As a result, the target assist is able to be implemented without generating torque variation in the main shaft  12 . 
     The assist amount reduces the steering force required by a driver to turn the steering wheel such that the sum of this assist amount and manual torque provided by the driver is the output torque applied to the main shaft  12 . Therefore, the relationship between this manual torque M, the motor assist torque PS (M, θ1) and column shaft torque T 1  (θ1) is expressed with the following expressions. 
     
       
           M+PS ( M, θ 1)= T   1 (θ1)  [Expression 6] 
       
     
     
       
           T   3 = F (θ1)= F (θ1)×( M×PS ( M, θ 1))  [Expression 7] 
       
     
     
       
           PS ( M, θ 1)= T   3 / F (θ1)− M   [Expression 8] 
       
     
     By making the target value of T 3  with respect to M an assist ratio As(M) function, Expression 9 below can be obtained. Here, the assist ratio As(M) function is usually determined by a preset expression or map. 
     
       
           T   3 = As ( M )  [Expression 9] 
       
     
     
       
         ⊂ PS ( M, θ 1)= As ( M )/ F (θ1)− M   
       
     
     The manual torque M is a torque value detected by the torque sensor  20 . Therefore, by controlling the assist amount output to the assist motor  21  based on this theoretical expression so as to become PS(M, θ1), the driver is able to steer smoothly without an unpleasant sensation from torque variation. In this case, the calculation process for the torque variation T 3 /T 1 =F(θ1) is performed by the torque variation correction calculating portion  22   a  and the calculation process for the motor assist torque PS(M, θ1) is performed by the assist amount calculating portion  22   b.    
     Accordingly, with the electric power steering device  10  according to this exemplary embodiment, the assist amount is obtained by a calculation based on the crossed axes angle α of the main shaft  12  and the mid shaft  14 , the crossed axes angle β of the mid shaft  14  and the extension shaft  16 , the rotation angle of the main shaft  12  that is detected by the steering angle sensor  19 , and the torque of the main shaft  12  that is detected by the torque sensor  20 . Then, assist force is output from the assist motor  21  in accordance with this assist amount. Obtaining the assist amount based on the theoretical expression in this way enables the torque correction to be obtained simply without the need to create large amounts of data and perform complicated control, as is the case with the related art. Also, because the calculation process can be performed in accordance with the configuration of the steering link, the torque is able to be corrected irrespective of the configuration of the steering link. 
     The control described above is with a column-type electric power steering device in which the steering angle sensor  19 , the torque sensor  20 , and the assist motor  21  are mounted to the main shaft  12 . According to another exemplary embodiment, however, the steering angle sensor  19 , the torque sensor  20 , and the assist motor  21  can also be mounted to another portion other than the main shaft  12 . For example, with a pinion-type electric power steering device in which only the steering angle sensor  19  is mounted to the main shaft  12  and the torque sensor  20  and the assist motor  21  are provided on a portion where the pinion gear in the gear box  17  and the rack bar  18  are connected, the assist amount can be obtained according to the following Expression 10. 
     
       
           T   3 + Ps (θ1 =Tp, Tp=As ( M )  [Expression 10] 
       
     
     Tp in Expression 10 denotes a pinion engaging torque. From Expression 5 is obtained the expression T 3 =F(θ1)×T 1 , and from this expression and Expression 10 is obtained the expression Tp=F(θ1)×T 1 +Ps(θ1), such that Expression 11 below is obtained. 
     
       
           PS ( M, θ 1)= Tp−F (θ1)× T   1 = Tp−F (θ1)× M   [Expression 11] 
       
     
     
       
         ⊂ PS ( M, θ 1)= As ( M )− F (θ1)× M   
       
     
     By obtaining F(θ1) using Expressions 1 and 5, the motor assist torque PS(M, θ1) can be obtained. 
     In this way, with the electric power steering device  10  according to the first exemplary embodiment, the torque can be corrected appropriately even when the torque sensor  20  and the assist motor  21  are mounted to a portion other than the main shaft  12 . Further, the torque sensor  20  and the assist motor  21  can be mounted to the mid shaft  14  and the steering angle sensor  19  can also be mounted to a portion other than the main shaft  12 . In this case, a suitable assist amount can be calculated by simply modifying the foregoing expression such that the mounting positions of the steering angle sensor  19 , the torque sensor  20 , and the assist motor  21  no longer become restricted. 
     Further, FIG.  5  and FIG. 6 show an electric power steering device  30  according to a second exemplary embodiment of the invention. With this electric power steering device  30 , a steering angle control actuator  32  is mounted to a mid shaft  31  so as to enable a phase of an upper side portion  31   a  and a lower side portion  31   b  of the mid shaft  31  to change. This mid shaft  31  and steering angle control actuator  32  together serve as a variable-ratio steering mechanism which enables control such that when the steering wheel  11  is turned one complete revolution the extension shaft  16  will rotate two complete revolutions, for example. 
     The steering angle control actuator  32  is then connected to a steering angle control ECU  33  that controls the steering angle control actuator  32 . The steering angle control ECU  33  is also connected to a vehicle speed sensor  34 , the steering angle sensor  19 , and the power steering ECU  22 . The configuration of other parts in this electric power steering device  30  is the same as with the aforementioned electric power steering device  10  and like parts will be denoted with like reference numerals. 
     The steering angle control ECU  33  receives a steering angle signal indicative of the rotation angle of the main shaft  12  that is detected by the steering angle sensor  19 . The steering angle control ECU  33  also receives a vehicle speed signal indicative of the vehicle speed that is detected by the vehicle speed sensor  34 . The steering angle control ECU  33  then calculates a current value based on the steering angle signal and the vehicle speed signal to actuate the steering angle control actuator  32 , and then sends a signal indicative of the calculated current value to the steering angle control actuator  32  so as to actuate it. The steering angle control ECU  33  also receives a signal indicative of the operation angle of the mid shaft  31  that rotates with the actuation of the steering angle control actuator  32 , and then outputs it to the power steering ECU  22 . 
     Further, the steering angle control ECU  33  controls the actuation angle of the steering angle control actuator  32  so that it, for example, rotates a large amount in the direction of the main shaft rotation with respect to the main shaft rotation angle when the vehicle is running at slow speeds, and only slightly, or in the reverse direction, when the vehicle is running at high speeds. 
     Also, when the steering angle control actuator  32  is actuated and the phase of the upper side portion  31   a  and the lower side portion  31   b  of the mid shaft  31  have changed, the steering angle control ECU  33  takes the actuation angle of the mid shaft  31  as a relative angle by a calculation process and sends a signal indicative of that actuation angle to the torque variation correction calculating portion  22   a . This actuation angle is an actuation angle sensor value within the steering angle control actuator  32  that is generated as a result of the steering angle control ECU  33  driving the steering angle control actuator  32 , or an actuation command value of the steering angle control ECU  33  (or a drive control value). 
     The torque variation correction calculating portion  22   a  then obtains the torque variation by a calculation process using the steering angle signal from the steering angle sensor  19  and the actuation angle signal from the steering angle control ECU  33 , and sends a signal indicative of the obtained data to the assist amount calculating portion  22   b . This is done by modifying Expression 4 to T 3 /T 2 =f(β,γ+θ2+ε). ε denotes the actuation angle of the steering angle control actuator  32 . Thereafter, just as with the control in electric power steering device  10 , the assist amount calculating portion  22   b  receives a signal sent from the torque variation correction calculating portion  22   a  and a torque signal indicative of the torque that is detected by the torque sensor  20 , and calculates the assist amount from these signals. The assist amount calculating portion  22   b  then outputs a signal indicative of that assist amount to the motor driver  22   c , which in turn sends the received signal as a drive current signal to the assist motor  21  to drive the assist motor  21 . 
     The torque variation T 3 /T 1  and the motor assist torque PS(M, θ1) in this case are able to be obtained using Expressions 1 through 11. In this way, with the electric power steering device  30  according to this exemplary embodiment, the torque is able to be corrected appropriately even when a variable-ratio steering mechanism is provided, so that the driver is always able to have a steering sensation with no unpleasantness. Further, this can also be applied in the same way and with the same advantages when there are three or more joints. 
     While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the preferred embodiments are shown in various combinations and configuration, 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.