Abstract:
A steering system includes a steering wheel, a steering angle sensor detecting a steering angle, a steering motor steering road-wheels, a reaction motor imparting a reaction force to the steering wheel, an ECU driving the steering motor with respect to a detected steering angle and a temperature detecting unit detecting temperatures of the steering motor, the reaction motor or a constituent member involved in the temperatures of the motors. When the temperature detected is larger than a predetermined value, the ECU performs such that a ratio of the rotating angle to the steering angle is made smaller than one that is to result when the temperature is equal to or smaller than the predetermined value, or a reaction force that is to be imparted to the steering wheel is made larger.

Description:
The present invention claims foreign priority under 35 USC 119 based on Japanese patent application No. P.2004-319289, filed on Nov. 2, 2004, the content of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a steering system, in which an operating element that is to be operated by a driver, is not mechanically connected to steered road-wheels to be steered or rotated to steer a vehicle. 
   2. Description of the Background Art 
   In a so-called SBW (steer-by-wire) type steering system in which a steering wheel (an operating element) that is to be operated by the driver, and steered road-wheels are not mechanically connected to each other, the steered road-wheels are turned by a steering motor and a reaction force is imparted to the operating element by a reaction motor. These motors are controlled independently, whereby a ratio of a rotating angle of the steered road-wheels to a steering input to the operating element is varied depending on vehicle speeds to thereby largely contribute to the stabilization of the behaviors of a vehicle. 
   In this SBW type steering system, since it is only the drive force of the steering motor that turns the steered road-wheels with no steering effort that is inputted to the operating element by the driver imparted to the steered road-wheels, the steering motor of the SBW type steering system needs a larger output than that of a steering motor for a steering system in which the operating element and the steered road-wheels are mechanically connected (hereinafter, referred to as an electric power steering). 
   Consequently, even compared in the same driving conditions, more heat is generated in the steering motor and a power drive unit which controls the drive of the steering motor in the SBW type steering system than in the electric power steering. Due to this, in such a situation that a large load is applied to a rotating system as when the steered wheels are rotated about their swivel pins at extremely low speeds or during stationary steering, the heat generation in a steering system needs to be suppressed. 
   Japanese Patent Unexamined Publication No. JP-A-2004-194385 discloses a technique to reduce power supplied to the reaction motor when the temperature of the reaction motor reaches or exceeds a predetermined temperature. 
   However, when the supply of power to the reaction motor is reduced when the temperature of the reaction motor reaches or exceeds the predetermined temperature as described the JP-A-2004-194385, while the increase in the temperature of the reaction motor can be suppressed, since the reaction force is reduced, the driver is then allowed to increase the steering input to the operating element with ease, and as a result, the load to the steering motor is increased, causing a problem that the generation of heat in the steering motor is promoted. Note that in the SBW type steering system, the steering motor is generally required to have a larger output than the reaction motor. 
   SUMMARY OF THE INVENTION 
   The present invention was made to provide a steering system which can suppress the increase in the temperature of a steering system in an ensured fashion. 
   With a view to solving the problem, according to a first aspect of the invention, there is provided a steering system (for example, a steering system  1  in an embodiment which will be described later on) comprising: 
   an operating element (for example, a steering wheel  11  in the embodiment which will be described later on) operated by a driver; 
   a steering input detecting unit (for example, a steering angle sensor  13  in the embodiment which will be described later on) detecting a steering input that is inputted to the operating element; 
   a steering actuator (for example, a steering motor  25  in the embodiment which will be described later on) steering steered road-wheels (for example, road-wheels  21  in the embodiment which will be described later on) which are mechanically disconnected from the operating element; 
   a reaction actuator (for example, a reaction motor  12  in the embodiment which will be described later on) which imparts a reaction force to the operating element; 
   a control unit (for example, an ECU  40  in the embodiment which will be described later on) which drives the steering actuator in accordance with the steering input detected by the steering input detecting unit; and 
   a temperature detecting unit (for example, the ECU  40  in the embodiment which will be described later on) for detecting temperature of the steering actuator, temperature of the reaction force actuator or temperature of a constituent member (for example, a reaction motor power drive unit  16 , a steering motor power drive unit  28  in the embodiment which will be described later on) that is involved in the temperatures of the steering and reaction force actuators, 
   wherein the control unit determines a ratio of the rotating angle to the steering input or a reaction force to be imparted to the operating element such that: 
   in the event that a temperature detected by the temperature detecting unit is equal to or smaller than a primary predetermined temperature, the ratio of the rotating angle or the reaction force is made a first value; and 
   in the event that the temperature detected by the temperature detecting unit is larger than the primary predetermined temperature, the ratio of the rotating angle is made smaller than the first value, or the reaction force is made larger than the first value. 
   By adopting the above configuration, the load applied to the steering actuator can be reduced by reducing a ratio of the rotating angle to a steering input. In addition, excessively quick steering by the driver can be suppressed by increasing the reaction force imparted to the operating element, and as a result, the increase in the load applied to the steering actuator can be suppressed. 
   According to a second aspect of the present invention in addition to the features set forth in the first aspect of the present invention, it is preferable that the steering system further comprises 
   a connecting unit (for example, a clutch  30  in the embodiment which will be described later on) which is capable of establishing mechanical connection and disconnection between the operating element and the steered road-wheels, 
   wherein in the event that the temperature detected by the temperature detecting unit is equal to or larger than a secondary predetermined temperature, which is larger than the primary predetermined temperature, the connecting unit establishes the mechanical connection between the operating element and the steered road-wheels. 
   By adopting the above configuration, the steering effort inputted to the operating element by the driver can be utilized as a drive force which steers the drive road-wheels about their swivel pins by bringing the connecting unit into engagement, and as a result, the load applied to the steering actuator can be reduced. 
   According to a third aspect of the present invention, in addition to the features set forth in the second aspect of the present invention, it is preferable that the control unit controls a second value of the ratio of the rotating angle to the steering input or the reaction force in such a manner that: 
   as the temperature increases to the secondary predetermined temperature, 
   the second value of the ratio of the rotating angle to the steering input or the reaction force is made substantially equal to a value of the ratio of the rotating angle to the steering input or the reaction force which is obtained when the mechanical connection between the operating element and the steered road-wheels is established. 
   According to a fourth aspect of the present invention, in addition to the features set forth in the first aspect of the present invention, it is preferable that the control unit controls both the ratio of the rotating angle to the steering input and the reaction force. 
   According to the first aspect of the invention, since the load applied to the steering actuator can be reduced by reducing the ratio of the rotating angle to the steering input, and additionally, excessively quick steering by the driver can be suppressed by increasing the reaction force imparted to the operating element, the increase in the load applied to the steering actuator can be suppressed, and the increase in the temperatures of the steering actuator and the constituent member involved therein can be prevented, thereby making it possible to eliminate the generation of a drawback attributed to heat that would otherwise be generated. 
   According to the second aspect of the invention, since the load applied to the steering actuator can be reduced by bringing the connection unit into engagement, the increase in the temperatures of the steering actuator and the constituent member involved therein can be prevented, thereby making it possible to eliminate the generation of a drawback attributed to heat that would otherwise be generated. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram which shows the configuration of an SBW type steering system according to the invention; 
       FIG. 2  is a flowchart (Part  1 ) which illustrates an embodiment of a temperature suppressing control; 
       FIG. 3  is a flowchart (Part  2 ) which illustrates the embodiment of the temperature suppressing control; 
       FIG. 4  is a chart which shows an example of a rotating angle compensation factor map used in the temperature increase suppressing process; and 
       FIG. 5  is a chart which shows an example of a reaction force compensation factor map used in the temperature increase suppressing process. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An embodiment of a steering system according to the invention will be described below by reference to  FIGS. 1 to 5 . 
   As shown in  FIG. 1 , an SBW type steering system  1  includes a steering input unit (a steering unit)  10  and a steering output unit (a steering unit)  20  which steers steered road-wheels, and the steering input unit  10  and the steering output unit  20  are made to be mechanically connected to and disconnected from each other via a clutch unit  30 . In addition, the steering output unit  20  is electrically controlled by a steering electronic control unit (ECU)  40  based on a steering input to the steering input unit  10 . 
   The steering input unit  10  is made up of a steering wheel  11  that is operated by a driver, a reaction motor  12  which imparts a reaction torque to the steering wheel  11 , a steering angle sensor  13  which detects a steering angle of the steering wheel  11  and outputs an electric signal corresponding to the steering angle so detected to the ECU  40 , a steering torque sensor  14  which detects a steering torque applied to the steering wheel  11  and outputs an electric signal corresponding to the steering torque so detected to the ECU  40  and a steering shaft  15  which is directly connected to the steering wheel  11 . Power is supplied to the reaction motor  12  via a reaction motor power drive unit  16  which is controlled by the ECU  40 . The reaction motor power drive unit  16  includes a current sensor  17  which detects a current supplied to the reaction motor  12  and outputs an electric signal corresponding to the current so detected to the ECU  40 . 
   Note that the steering wheel  11  is spring biased towards a neutral position at all times by an appropriate spring mechanism or the like (not shown). 
   The steering output unit  20  is made up of a steering rod  24  connected to left and right road-wheels (steered road-wheels)  21  via knuckle arms  22  and tie-rods  23 , a steering motor (a steering actuator)  25  which drives the steering rod  24  in an axial direction via a gear mechanism (not shown) so as to steer the road-wheels  21 , a sub-steering shaft  26  which can drive the steering rod  24  in the axial direction via a rack-and-pinion mechanism  26   a , and a rotating angle sensor  27  which detects a rotating angle of the road-wheels  21  and outputs an electric signal corresponding to the rotating angle so detected to the ECU  40 . The steering rod  24  is moved to the left and right in the axial direction by rotating the steering motor  25  clockwise and counterclockwise, so that the road-wheels  21  can be steered angularly to the left and right. Power is supplied to the steering motor  25  via a steering motor power drive unit  28  which is controlled by the ECU  40 . The steering motor power drive unit  28  detects a current supplied to the steering motor  25  and outputs an electric signal corresponding to the current so detected to a current sensor  29 . 
   The clutch unit  30  is made to mechanically connect and disconnect the steering shaft  15  of the steering input unit  10  and the sub-steering shaft  26  of the steering output unit  20  to and from each other, and the engagement/disengagement or release of the clutch unit  30  is controlled by the ECU  40 . The ECU  40  releases the clutch unit  30  from an engaged state when the steering system  1  is made to function as the originally intended SBW type steering system, and brings the clutch unit  30  into engagement when the road-wheels  21  are made to be steered by a steering effort inputted to the steering wheel  11 . 
   A vehicle speed sensor  31  which detects a speed of the vehicle is mounted at an appropriate position on a body of the vehicle and outputs an electric signal corresponding to the speed of the vehicle detected by the sensor to the ECU  40 . 
   The ECU  40  calculates a target rotating angle based on signals from the steering angle sensor  13  and the vehicle speed sensor  31 , calculates a target current of power that is caused to flow to the steering motor  25  such that an output value (namely, an actual rotating angle of the road-wheels  21 ) of the rotating angle sensor  27  coincides with the target rotating angle so calculated and outputs the target current so calculated to the steering motor power drive unit  28 . The steering motor power drive unit  28  then receives the target current to thereby supply power to the steering motor  25  at the target current, whereby a rotating angle ratio is set in accordance with the steering angle inputted to the steering wheel  11  and the vehicle speed in the SBW type steering system  1 . Note that in this application, the ECU  40  functions as a rotating angle ratio determining unit, and sets a ratio of rotating angle relative to steering angle (rotating angle/steering angle). 
   In addition, the ECU  40  calculates a target steering reaction force based on signals from the vehicle speed sensor  31  and the rotating angle sensor  27 , as well as a deviation signal between the target rotating angle and an actual rotating angle (hereinafter, referred to a rotating angle deviation signal), calculates a target current of power that is caused to flow to the reaction motor  12  such that an output value (namely, an actual steering torque) of the steering torque sensor  14  coincides with the target steering reaction force so calculated and outputs the target current so calculated to the reaction motor power drive unit  16 . Then, the reaction motor power drive unit  16  receives the target current to thereby supply power to the reaction motor  12  at the target current, whereby a steering reaction force according to the vehicle speed, rotating angle and rotating angle deviation signal is applied to the steering wheel  11  in the SBW type steering system  1 . 
   Furthermore, in the SBW type steering system  1 , in order to prevent overheating of the steering motor  25 , the steering motor power drive unit  28 , the reaction motor  12  and the reaction motor power drive unit  16 , a temperature increase suppressing process is implemented in steps according to temperatures. 
   Namely, when the temperature is higher than the predetermined temperature, in a first-step temperature increase suppressing process, the steering reaction force applied to the steering wheel  11  is made larger than one resulting when the temperature is normal, whereby excessively quick steering by the driver is suppressed, and an increase in load to the steering output unit  20  is suppressed, whereby the increase in the temperatures of the steering motor  25  and the steering motor power drive unit  28  is suppressed. 
   In the event that the temperatures of the steering motor  25  and the steering motor power drive unit  28  still continue to rise even after the first-step temperature increase suppressing process has been implemented, or in the event that the temperature of the reaction motor  12  or the reaction motor power drive unit  16  has increased to a predetermined temperature as a result of the implementation of the first-step temperature increase suppressing process, the rotating angle ratio is reduced gradually so as to become smaller than when the temperature is normal as a second-step temperature increase suppressing process while the first-step temperature increase suppressing process continues to be implemented, whereby the load on the steering output unit  20  is made to continue to be reduced, so that the increase in the temperatures of the steering motor  25  and the steering motor power drive unit  28  is suppressed. 
   In the event that the temperatures still continue to rise to thereby reach a predetermined threshold temperature even after the second-step temperature increase suppressing process has been implemented, the steering wheel  11  and the steering rod  24  are connected to each other via the rack-and-pinion mechanism  26   a  by bringing the clutch unit  30  into engagement as a third-step temperature increase suppressing process, whereby a steering effort inputted to the steering wheel  11  by the driver comes to be inputted to the steering output unit  20  as a drive force which steers the road-wheels  21 , and as a result, the output of the steering motor  25  is reduced so as to suppress the increase in the temperatures of the steering motor  25  and the steering motor power drive unit  28 . In addition to this, the output of the steering motor  25  is aggressively restricted (stopped or reduced) so as to suppress the increase in the temperatures of the steering motor  25  and the steering motor power drive unit  28 . 
   Note that when the clutch  30  is brought into engagement in the third-step temperature increase suppressing process, the rotating angle ratio and the steering reaction force are made in advance to coincide with or approximate a rotating angle ratio and a steering reaction force (hereinafter, referred to as a rotating angle ratio in connection and a steering reaction force in connection, respectively) which result when the steering shaft  15  and the sub-steering shaft  26  are connected to each other via the rack-and-pinion mechanism  26   a  before the clutch is engaged in order to prevent the driver from perceiving a sensation of physical disorder. 
   In addition, in the event that the temperatures of the steering motor  25  and the steering motor power drive unit  28  are lowered as a result of the implementation of the third-step temperature increase suppressing process, the clutch  30  is released or disengaged so as to mechanically disconnect the steering input unit  10  from the steering output unit  20  to thereby restore the originally intended SBW type steering system. Also, as this occurs, in order to prevent the driver from perceiving a sensation of physical disorder, the rotating angle ratio and the steering reaction force are set in advance to optimal values before the clutch  30  is released. 
   Next, referring to flowcharts in  FIGS. 2 and 3 , the temperature increase suppressing process of the embodiment will be described. 
   A temperature increase suppressing process routine shown in flowcharts in  FIGS. 2 and 3  is executed constantly by the ECU  40  every certain length of time. 
   Firstly, in step S 101 , the current that has flowed to the steering motor  25  is calculated based on a current value detected by the current sensor  29  of the steering motor power drive unit  28 , and the current that has flowed to the reaction motor  12  is calculated based on a current value detected by the current sensor  17  of the reaction motor power drive unit  16 . 
   Next, proceeding to step S 102 , a temperature T 1  of the steering motor  25  is estimated based on a current value calculated as having flowed to the steering motor  25  in step S 101 , and a temperature T 2  of the reaction motor  12  is estimated based on a current value calculated as having flowed to the reaction motor  12  in step S 101 . 
   Next, proceeding to step S 103 , a temperature T 3  of the steering motor power drive unit  28  is estimated based on the current value calculated as having flowed to the steering motor  25  in step S 101 , and a temperature T 4  of the reaction motor power drive unit  16  is estimated based on the current value calculated as having flowed to the reaction motor  12  in step S 101 . 
   Note that in this embodiment, a temperature detecting unit is realized when the ECU  40  executes the processes in steps S 101  to S 103 . 
   Next, proceeding to step S 104 , a highest temperature among the temperature T 1  of the steering motor  25  and the temperature T 2  of the reaction motor  12 , which are estimated in step S 102 , and the temperature T 3  of the steering motor power drive unit  28  and the temperature T 4  of the reaction motor power drive unit  16 , which are estimated in step S 103  is made to be a representative temperature T. 
   Next, proceeding to step S 105 , whether or not a temperature determination flag F is “0” is determined. As will be described later on, this temperature determination flag F is set to “0” in step S 113  and is then set to “1” in step S 117 . 
   If the result of the determination in step S 105  is “YES”, then proceeding to step S 106 , a rotating angle compensation factor corresponding to the representative temperature T determined in step S 104  is calculated by reference to a rotating angle compensation factor map shown in  FIG. 4 . In the rotating angle compensation factor map in this embodiment, a rotating angle compensation factor R is 1.0 constantly when the representative temperature T is equal to or lower than a temperature t 2 , and when the representative temperature T exceeds the temperature t 2 , the rotating angle compensation factor R reduces gradually continuously as the temperature increases. A rotating angle compensation factor r 1  corresponding to a temperature t 3  is set such that the rotating angle ratio substantially equals the steering reaction force in connection. Note that the temperature t 3  is higher than the temperature t 2  (t 3 &gt;t 2 ) and the rotating angle compensation factor r 1  is smaller than 1 (r 1 &lt;1). 
   Next, proceeding to step S 107 , a reaction force compensation factor G which corresponds to the representative temperature T determined in step S 104  is calculated by reference to a reaction force compensation factor map shown in  FIG. 5 . In the reaction force compensation factor map in this embodiment, the reaction force compensation factor G is 1.0 constantly when the representative temperature T is equal to or lower than a temperature t 1 , and when the representative temperature T exceeds the temperature t 1 , the reaction force compensation factor G increases gradually continuously as the temperature increases. A reaction force compensation factor g 1  at a temperature t 3  is set such that the steering reaction force substantially equals the steering reaction force in connection. Note that the temperature t 1  is lower than a temperature t 2  (t 1 &lt;t 2 ) and the reaction force compensation factor g 1  is larger than 1 (g 1 &gt;1). 
   Next, proceeding to step S 108 , a compensated target rotating angle is calculated by multiplying the target rotating angle calculated based on the signals from the steering angle sensor  13  and the vehicle speed sensor  31  by the rotating angle compensation factor R calculated in step S 106 . In addition, a current control is executed on the steering motor  25  such that an output value (namely, an actual rotating angle of the road-wheels  21 ) of the rotating angle sensor  27  coincides with the compensated target rotating angle so calculated. In further addition, a compensated target steering reaction force is calculated by multiplying the target steering reaction force calculated based on the signals from the vehicle speed sensor  31  and the rotating angle sensor  27  and the rotating angle deviation signal by the reaction force compensation factor G calculated in step S 107 , and a current control is executed on the reaction motor  12  such that an output value (namely, an actual steering torque) of the steering torque sensor  14  coincides with the compensated target steering reaction force so calculated. 
   Note that for the sake of a better development of the following discussion, the target rotating angle that is calculated based on signals from the steering angle sensor  13  and the vehicle speed sensor  31  and which has not yet been compensated is referred to as a “basic target rotating angle” and the target steering reaction force that is calculated based on signals from the vehicle sensor  31  and the rotating angle sensor  27  and which has not yet been compensated is referred to as a “basic target steering reaction force.” 
   Next, proceeding to step S 109 , whether or not the representative temperature T is equal to or higher than the temperature t 3  (T·t 3 ) is determined. 
   If the result of the determination in step S 109  is “NO” (T&lt;t 3 ), the implementation of the routine is tentatively stopped there. Namely, the processes in steps S 101  to S 108  are implemented repeatedly while the representative temperature T remains lower than the temperature t 3 . 
   Then, when the representative temperature T is equal to or lower than the temperature t 1  during the implementation of the series of processes in steps S 101  to S 108 , since the rotating angle compensation factor R and the reaction force compensation factor G are both “1”, the compensated target rotating angle coincides with the basic target rotating angle, and the compensated target steering reaction force coincides with the basic target steering reaction force. Namely, in a temperature area where the representative temperature T is equal to or lower than the temperature t 1 , both the rotating angle ratio and the steering reaction force become the rotating angle ratio and the steering reaction force which result when the temperature is normal. 
   In addition, in a temperature area where the representative temperature T is higher than the temperature t 1  but is equal to or lower than the temperature t 2 , while the rotating angle compensation factor R remains at “1”, the reaction force compensation factor G gradually becomes larger than 1. Namely, in the temperature area where the representative temperature T is higher than the temperature t 1  but is equal to or lower than the temperature t 2 , while the rotating angle ratio is set to one, which results when the temperature is normal, the steering reaction force is set to a value which is larger than the steering reaction force which results when the temperature is normal. Namely, the first-step temperature increase suppressing process is implemented. 
   In addition, in a temperature area where the representative temperature T is higher than the temperature t 2  but is equal to or lower than the temperature t 3 , the reaction compensation factor G continues to gradually increase, and in addition to this, the rotating angle compensation factor R gradually becomes smaller than 1. Namely, in the temperature area where the representative temperature T is higher than the temperature t 2  but is equal to or lower than the temperature t 3 , the steering reaction force is set to a value which is larger than the steering reaction force which results when the temperature is normal, and the rotating angle ratio is set to a value which is smaller than the rotating angle ratio which results when the temperature is normal. Namely, the second-step temperature increase suppressing process is implemented. 
   Then, if the result of the determination in step S 109  is “YES” (T·t 3 ), proceeding to step S 110 , the clutch  30  is brought into engagement, so that the steering shaft  15  and the sub-steering shaft  26  are connected to each other via the rack-and-pinion mechanism  26   a , whereby the steering effort inputted to the steering wheel  11  by the driver comes to be inputted to the steering output unit  20  as a drive force which rotates the road-wheels  21  about their swivel pins. As a result, the output of the steering motor  25  can be reduced, thereby making it possible to suppress the increase in the temperatures of the steering motor  25  and the steering motor power drive unit  28 . Namely, the third-step temperature increase suppressing process is implemented. 
   Note that since the rotating angle compensation factor R is set to a value which allows the rotating angle ratio to substantially equal the rotating angle ratio in connection and the reaction force compensation factor G is set to a value which allows the steering reaction force to substantially equal the steering reaction force in connection just before the representative temperature T becomes the temperature t 3  (namely, just before the clutch  30  is brought into engagement), there is no case where the driver feels a sensation of physical disorder when the clutch  30  is engaged. 
   Next, proceeding to step S 111 , the steering motor  25  is stopped or the output thereof is reduced. Namely, the output of the steering motor  25  is limited, whereby the increase in the temperatures of the steering motor  25  and the steering motor power drive unit  28  can be suppressed. 
   Next, proceeding to step S 112 , whether the representative temperature T is equal to or higher than the temperature t 3  (T·t 3 ) is determined. If the result of the determination in step S 112  is “YES” (T·t 3 ), then proceed to step S 117 , where the temperature determination flag F is set to “1” and the execution of the routine is tentatively stopped there. Namely, in this case, the series of processes in steps S 101  to S 105  (the third-step temperature increase suppressing process) continues to be implemented in such a state that the clutch  30  continues to be in engagement. 
   On the other hand, if the result of the determination in step S 112  is “NO” (T&lt;t 3 ), since this means that the temperatures of the steering motor  25  and the steering motor power drive unit  28  are reduced, then proceed to step S 113 , where the temperature determination flag F is set to “0”, and furthermore, proceed to steps S 114  and S 115  for preparation for disengagement of the clutch  30 . Namely, in order for the driver not to have to feel a sensation of physical disorder when the clutch is released or disengaged, in steps S 114  and S 115 , a target rotating angle and a target steering reaction force are respectively set which are optimum to the restoration of the originally intended SBW type steering system in which the clutch is in the released or disengaged state. 
   Next, proceeding to step S 116 , the clutch  30  is released, so that the originally intended SBW type steering system is restored, and the execution of the routine is tentatively stopped there. 
   According to the SBW type steering system  1  that is configured as has been described heretofore, since the rotating angle ratio and the steering reaction force are set such that the loads on the steering input unit  10  and the steering output unit  20  are reduced in accordance with the temperature conditions thereof, heat generated in the steering motor  25 , the steering motor power drive unit  28 , the reaction motor  12  and the reaction motor power drive unit  16  can be reduced without causing the driver to feel a sensation of physical disorder abruptly even when the steered road-wheels  21  are angularly rotated at extremely low speeds as when the vehicle is parked in the garage or during stationary steering in which heat is easily generated. 
   In addition, in the event that the temperatures still continue to rise even in that situation, the steering input unit  10  and the steering output unit  20  are brought into mechanical connection with each other without causing the driver to feel a sensation of physical disorder so as to reduce the load on the steering motor  25  or the like, thereby making it possible to reduce heat generated. Then, when the mechanical connection between the steering input unit  10  and the steering output unit  20  is no longer required due to the heat generated being so reduced, the steering input unit  10  is mechanically disengaged from the steering output unit  20  so that the originally intended SBW type steering system can be restored. 
   OTHER EMBODIMENTS 
   Note that the invention is not limited to the embodiment that has been described heretofore. 
   For example, while in the aforesaid embodiment, the temperatures of the reaction motor  12 , the reaction motor power drive unit  16 , the steering motor  25  and the steering motor power drive unit  28  are estimated through the calculation of current, these temperatures may be detected direct by temperature sensors (temperature detecting unit). 
   While the present invention has been described in connection with the preferred embodiments thereof, it will be obvious to those skilled in the art that various changes and modification may be made therein without departing from the present invention, and it is aimed, therefore, to cover in the appended claim all such changes and modifications as fall within the true spirit and scope of the present invention.