Patent Publication Number: US-2020283060-A1

Title: Auxiliary electric source device and steering device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2019-042393 filed on Mar. 8, 2019, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The disclosure relates to an auxiliary electric source device and a steering device. 
     2. Description of Related Art 
     As a device that causes an electric feed object to act with electric feed from a main electric source and thereby fulfill a desired function, for example, there is a device that causes a motor to act with the electric feed from the main electric source and thereby fulfill a function to give rotation power of the motor to a steering mechanism of a vehicle as steering assist power in order to assist a steering operation by a driver (Japanese Patent Application Publication No. 2018-196203 (JP 2018-196203 A)). 
     In JP 2018-196203 A, there is provided an auxiliary electric source device including an auxiliary electric source that backs up the electric feed from the main electric source in order to maintain the function to give the steering assist power in the case where the main electric source breaks down and the electric feed from the main electric source to the motor is impossible. 
     SUMMARY 
     In JP 2018-196203 A, in order to fulfill the function to give the steering assist power in the case of the backup of the electric feed from the main electric source, it is necessary to prepare an auxiliary electric source having an electric source performance equivalent to the electric source performance of the main electric source, for maintaining a function equivalent to the function to give the steering assist power with the electric feed from the main electric source. However, for the auxiliary electric source, it is also demanded to restrain increase in electric source performance. 
     The disclosure provides an auxiliary electric source device and a steering device that make it possible to restrain the increase in the electric source performance of the auxiliary electric source. 
     An auxiliary electric source device according to a first aspect of the disclosure is provided on an electric feed path between a main electric source and an electric feed object of the main electric source. The auxiliary electric source device includes an auxiliary electric source. The electric feed object performs an action of fulfilling a desired function with electric feed from the main electric source, and performs a minimal action of fulfilling a desired minimal function with electric feed from the auxiliary electric source when the main electric source has broken down. An electric source performance of the auxiliary electric source is set based on an electric power necessary for the minimal action. 
     With the above configuration, when the auxiliary electric source backs up the electric feed from the main electric source in the case where the main electric source has broken down, it is possible to secure the minimal action of the electric feed object with the electric feed from the auxiliary electric source. That is, when the auxiliary electric source backs up the electric feed from the main electric source in the case where the main electric source has broken down, it is possible to fulfill at least the desired minimal function with the electric feed from the auxiliary electric source, although it is not possible to maximally fulfill the desired function. Thereby, when the auxiliary electric source backs up the electric feed from the main electric source in the case where the main electric source has broken down, the electric feed object can fulfill the desired function, even if the electric source performance of the auxiliary electric source is not equivalent to the electric source performance of the main electric source. Accordingly, it is not necessary to prepare an auxiliary electric source having an electric source performance equivalent to the electric source performance of the main electric source, and it is possible to restrain the increase in the electric source performance of the auxiliary electric source. 
     In the above aspect, the electric source performance of the auxiliary electric source may be set so as to be above a maximal supply voltage of the auxiliary electric source for which noise to be generated at a time of the electric feed from the auxiliary electric source is considered with a supply voltage of the auxiliary electric source necessary for the minimal action when the electric source performance of the auxiliary electric source is below a maximal supply voltage of the main electric source to the electric feed object. 
     With the above configuration, in the case where the auxiliary electric source performs the electric feed to the electric feed object at the maximal supply voltage, it is possible to secure the minimal action of the electric feed object, even if noise is generated at the time of the electric feed. That is, when the auxiliary electric source backs up the electric feed from the main electric source in the case where the main electric source has broken down, it is possible to fulfill at least the desired minimal function. 
     In the above configuration, the auxiliary electric source may be configured to change a supply voltage to the electric feed object based on a remaining electric power that is able to be fed to the electric feed object when the electric source performance of the auxiliary electric source is above the supply voltage necessary for the minimal action. 
     With the above configuration, for example, in the case where the remaining electric power is low, the supply voltage to the electric feed object is decreased. Thereby, in the case where the main electric source has broken down, it is possible to adjust the manner of the backup, for example, by increasing a period for which the electric feed object can fulfill the desired function with the electric feed from the auxiliary electric source in the case where the main electric source has broken down. 
     A steering device according to a second aspect of the disclosure includes: the auxiliary electric source device according to the above first aspect; and a motor that performs an action of giving a dynamic power to a steering mechanism of a vehicle, as the desired function. An electric source object of the auxiliary electric source device is the motor. 
     With the above configuration, the electric source performance of the auxiliary electric source is set such that the minimal function that needs to be fulfilled by the steering device can be secured, and therefore it is possible to realize a steering device in which the increase in the electric source performance of the auxiliary electric source is restrained. 
     With the auxiliary electric source device and the steering device in the above aspects, it is possible to restrain the increase in the electric source performance of the auxiliary electric source. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG. 1  is a diagram showing a schematic configuration of a steering device equipped with an auxiliary electric source device; 
         FIG. 2  is a diagram showing an electric configuration of the auxiliary electric source device; 
         FIG. 3  is a flowchart showing a processing procedure in an electronic control unit; and 
         FIG. 4  is a graph showing a relation between a remaining electric power of an auxiliary electric source and a discharge voltage of the auxiliary electric source. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment in which an auxiliary electric source device is applied to an electric power steering device (referred to as an “EPS” hereinafter) will be described. As shown in  FIG. 1 , an EPS  1  in the embodiment includes a steering mechanism  2  that steers steered wheels  16  based on an operation of a steering wheel  10  by a driver, and an assist mechanism  3  that includes a motor  20  for assisting the steering operation by the driver. The EPS  1  has a function to assist the steering operation by the driver, by giving the motor torque of the motor  20  to the steering mechanism  2  as steering assist power. 
     The steering mechanism  2  includes a steering shaft  12  having one end at which the steering wheel  10  is fixed and the other end at which a pinion gear  11  is formed, and a rack shaft  14  on which a rack gear  13  that engages with the pinion gear  11  is formed. The pinion gear  11  and the rack gear  13  constitute a rack-and-pinion mechanism. A rotation motion of the steering shaft  12  is transformed into a reciprocal linear motion of the rack shaft  14  in an axial direction of the rack shaft  14 , by the rack-and-pinion mechanism. The EPS  1  is incorporated in a vehicle, such that the axial direction of the rack shaft  14  is a vehicle width direction. The reciprocal linear motion of the rack shaft  14  is transmitted to the right and left steered wheels  16  through tie-rods  15  coupled with both ends of the rack shaft  14  respectively. Thereby, steer angles of the steered wheels  16  are changed, and the movement direction of the vehicle is altered. 
     To the steering shaft  12 , a torque sensor  17  is attached. The torque sensor  17  measures a steering torque TR that is applied to the steering shaft  12  by the operation of the steering wheel  10 . The torque sensor  17  in the embodiment detects the torsion amount of a torsion bar that constitutes a part of the steering shaft  12 , and measures the steering torque TR based on the detected torsion amount. 
     The assist mechanism  3  includes the motor  20  for steering assist and a reducer  21 . The motor  20  is coupled with the steering shaft  12  through the reducer  21 . The reducer  21  reduces the rotation of the motor  20 , and transmits the reduced rotation power to the steering shaft  12 . As the motor  20  in the embodiment, a three-phase brushless motor is employed. As the reducer  21  in the embodiment, a worm gear mechanism is employed. 
     The EPS  1  includes a drive circuit  30 , an auxiliary electric source device  40  and an electronic control unit  50 . As the drive circuit  30 , a known circuit including two switching elements for each of the phases (U-phase, V-phase and W-phase) of the motor  20  is employed. Each of the auxiliary electric source device  40  and the electronic control unit  50  is connected to a main electric source  60  equipped in the vehicle, at the time when the EPS  1  is incorporated in the vehicle. 
     The electronic control unit  50  causes the EPS  1  to fulfill a desired function to assist the steering operation by the driver, by controlling the action of the motor  20  that is an electric feed object with the electric feed from the main electric source  60 . The electronic control unit  50  includes an arithmetic processing circuit  51  that executes arithmetic processing, and a memory  52  in which programs and data for control are stored. To the electronic control unit  50 , the above-described torque sensor  17  and a speed sensor  18  are connected. The speed sensor  18  detects a traveling speed VS of the vehicle. In the control of the steering assist power, the electronic control unit  50  decides a target steering assist power that is a target value of the steering assist power, based on the steering torque TR and the traveling speed VS. The electronic control unit  50  controls actions of the drive circuit  30  and the auxiliary electric source device  40 , in order to generate a steering assist power equivalent to the target steering assist power. 
     As shown in  FIG. 2 , the auxiliary electric source device  40  includes an input port  41  and an output port  42 . The input port  41  is connected to the main electric source  60 , at the time when the auxiliary electric source device  40  is incorporated in the EPS  1 . The output port  42  is connected to the motor  20  that is the electric feed object, through the drive circuit  30 , at the time when the auxiliary electric source device  40  is incorporated in the EPS  1 . 
     The auxiliary electric source device  40  includes a first line  43  that is an electric feed path at the time of a later-described keeping state or charging state. The first line  43  is connected to the input port  41  and the output port  42 . The auxiliary electric source device  40  includes an auxiliary electric source  100  that can be charged and discharged with electric power. The auxiliary electric source  100  is connected between the main electric source  60  and the motor  20  that is the electric feed object. The auxiliary electric source  100  is constituted by three capacitors  101 ,  102 ,  103 . The three capacitors  101 ,  102 ,  103  are connected to each other in series. A discharge voltage that is the electric source performance of the auxiliary electric source  100  is the sum of discharge voltages of the three capacitors  101 ,  102 ,  103 . Maximal discharge voltages that are electric source performances of the three capacitors  101 ,  102 ,  103  are equal to each other. Each of the capacitors  101 ,  102 ,  103  includes an electrode plate having a positive polarity and an electrode plate having a negative polarity. A first terminal T 1  of an electrode plate  101   a  on the negative side of the capacitor  101  is connected to the first line  43 , and a second terminal T 2  of an electrode plate  103   a  on the positive side of the capacitor  103  is connected to a second line  44  that is an electric feed path at the time of a later-described backup or boost. The first terminal T 1  on the electrode plate  101   a  on the negative side of the capacitor  101  is connected to the input port  41  through a fifth switching element (referred to as a “FET 5 ” hereinafter), on the first line  43 . When the auxiliary electric source  100  is viewed as a whole, the charge of the auxiliary electric source  100  with electric power is performed between the electrode plate  101   a  on the negative side of the capacitor  101  and the electrode plate  103   a  on the positive side of the capacitor  103 . In the embodiment, as each of the capacitors  101 ,  102 ,  103 , a lithium-ion capacitor is employed. The lithium-ion capacitor has advantages of high heat resistance, long life, high charge-discharge performance, high energy density and high safety. 
     The auxiliary electric source device  40  includes a first switching element (referred to as a “FET 1 ” hereinafter) and a second switching element (referred to as a “FET 2 ” hereinafter). One end of the FET 1  is connected to the ground, and the other end of the FET 1  is connected to the second line  44  through the FET 2 . 
     The auxiliary electric source device  40  includes a third switching element (referred to as a “FET 3 ” hereinafter) for switching connection between the second line  44  and the motor  20  and a fourth switching element (referred to as a “FET 4 ” hereinafter) for switching connection between the first line  43  and the motor  20 . The FET 3  is provided between the FET 2  and the output port  42  on the second line  44 . The FET 4  is provided between the FET 5  and the output port  42  on the first line  43 . A connection point P 2  between the FET 5  and the FET 4  on the first line  43  is connected to a connection point P 1  between the FET 1  and the FET 2 . A connection point P 3  between the connection point P 2  and the FET 4  on the first line  43  is connected to the first terminal T 1  of the electrode plate  101   a  on the negative side of the capacitor  101 . A connection point P 4  between the FET 2  and the FET 3  on the second line  44  is connected to the second terminal T 2  of the electrode plate  103   a  on the positive side of the capacitor  103 . 
     As each of the FET 1  to FET 5 , a metal-oxide-semiconductor field-effect transistor (MOS-FET) is employed. Gates of the FET 1  to FET 5  are connected to the electronic control unit  50 . The electronic control unit  50  outputs drive signals to the gates of the FET 1  to FET 5 , and thereby switches the FET 1  to FET 5  between an on-state and an off-state depending on the drive signals. By switching the FET 1  to FET 5  between the on-state and the off-state, the electronic control unit  50  switches the action state of the auxiliary electric source  100  among the keeping state, the charging state and a discharging state. 
     The keeping state is an action state for keeping the electric power of the auxiliary electric source  100 . When the electronic control unit  50  switches the action state of the auxiliary electric source  100  to the keeping state, the electronic control unit  50  puts the FET 1  to FET 3  into the off-state, and puts the FET 4  and FET 5  into the on-state. In this case, the main electric source  60  is connected to the motor  20  through the first line  43 . Thereby, electric power is fed to the motor  20 , based on a battery voltage Vb that is an electric feed voltage from the main electric source  60 . The electric power of the auxiliary electric source  100  is kept. 
     The charging state is an action state for charging the auxiliary electric source  100  with electric power. When the electronic control unit  50  switches the action state of the auxiliary electric source  100  to the charging state, the electronic control unit  50  puts the FET 3  into the off-state, puts the FET 4  and FET 5  into the on-state, and performs PWM drive of the FET 1  and FET 2 , such that the FET 1  and the FET 2  alternately become the off-state. Thereby, electric charges are stored in the capacitors  101 ,  102 ,  103 , so that the auxiliary electric source  100  is charged with electric power. 
     The discharging state is an action state for discharging the auxiliary electric source  100  with electric power. When the electronic control unit  50  switches the action state of the auxiliary electric source  100  to the discharging state, the electronic control unit  50  puts the FET 1 , FET 2  and FET 4  into the off-state and puts the FET 3  and FET 5  into the on-state, or the electronic control unit  50  puts the FET 2 , FET 4  and FET 5  into the off-state and puts the FET 1  and FET 3  into the on-state. 
     Specifically, in the case where the electronic control unit  50  puts the FET 1 , FET 2  and FET 4  into the off-state and puts the FET 3  and FET 5  into the on-state, the first terminal T 1  of the auxiliary electric source  100  is connected to the connection point P 3  of the first line  43 , and is connected to the main electric source  60  through the first line  43  and the FET 5 . The second terminal T 2  of the auxiliary electric source  100  is connected to the drive circuit  30  and the motor  20  through the second line  44  and the FET 3 . That is, the auxiliary electric source  100  is connected to the main electric source  60  in series between the main electric source  60  and the motor  20 , and the battery voltage Vb of the main electric source  60  is boosted. Thereby, electric power is fed to the motor  20 , based on the boosted battery voltage Vb of the main electric source  60 . That is, the discharging state in the case where the FET 1 , FET 2  and FET 4  are put into the off-state is a discharging state at the time of a boost in which the battery voltage Vb of the main electric source  60  is boosted. 
     In the case where the electronic control unit  50  puts the FET 2 , FET 4  and FET 5  into the off-state and puts the FET 1  and FET 3  into the on-state, the first terminal T 1  of the auxiliary electric source  100  is connected to the connection points P 1  to P 3 , and is connected to the ground through the FET 1 . The second terminal T 2  of the auxiliary electric source  100  is connected to the output port  42  through the FET 3 . In this case, the main electric source  60  is isolated from the motor  20 , and the auxiliary electric source  100  is connected to the motor  20 . Thereby, the electric power of the auxiliary electric source  100  is fed to the motor  20 . That is, the discharging state in the case where the FET 2 , FET 4  and FET 5  are put into the off-state and the FET 1  and FET 3  are put into the on-state is a discharging state at the time of a backup in which the auxiliary electric source  100  backs up the electric feed from the main electric source  60 . The action state is switched to the discharging state at the time of the backup, when the main electric source  60  has broken down, for example, due to disconnection between the main electric source  60  and the input port  41  and decrease in the battery voltage Vb caused by degradation of the main electric source  60 . 
     Next, the electric source performance of the auxiliary electric source  100  will be described. The maximal discharge voltage that is the electric source performance of the auxiliary electric source  100  is set based on an electric power that is necessary for the motor  20  to perform a minimal action in order for the EPS  1  to fulfill a minimal function. 
     In the embodiment, the minimal function to be fulfilled by the EPS  1  is a function to assist the steering operation by the driver in the case where the vehicle performs an ordinary traveling at a speed in an ordinary speed region that is a certain level of speed. This is because the minimal function is important for assisting the steering operation by the driver from the standpoint of the securement of the safety of the driver. The case where the vehicle performs the ordinary traveling is a case where the vehicle performs a grip traveling. 
     It is desirable to assist the steering operation by the driver, even in the case where the vehicle performs a low-speed traveling at a relatively low speed, for example, in the case of the stop of the vehicle. However, from the standpoint of the securement of the safety of the driver, it is thought that the priority is low compared to the case where the vehicle performs the ordinary traveling. Therefore, in the embodiment, in the case of the breakdown of the main electric source  60 , for example, in the case of the discharging state at the time of the backup, the assist of the steering operation by the driver in the case where the vehicle performs the ordinary traveling as the minimal function that needs to be fulfilled by the EPS  1  is preferentially secured. 
     A so-called static steering operation, which is a steering operation in the case where the vehicle performs the low-speed traveling, requires a higher power for the steering operation because of a higher frictional force between the steered wheels  16  and a road surface, compared to the steering operation in the case where the vehicle performs the ordinary traveling. Therefore, in the case where the vehicle performs the low-speed traveling, the motor torque that needs to be given to the steering mechanism  2  as the steering assist power is higher than in the case where the vehicle performs the ordinary traveling. That is, in the case where the vehicle performs the low-speed traveling, the electric power that needs to be fed to the motor  20  for giving the steering assist power is higher. 
     Thereby, in the embodiment, the electric source performance of the auxiliary electric source  100 , which is an electric source to be used for minimally securing the function to assist the steering operation by the driver in the case where the vehicle performs the ordinary traveling, is set so as to be lower than the electric source performance of the main electric source  60 , which is an electric source to be used for securing also the function to assist the steering operation in the case where the vehicle performs the low-speed traveling. 
     Specifically, when a supply voltage Vmin is defined as a minimal supply voltage for feeding an electric power that is necessary for the motor  20  to perform the minimal action, the maximal supply voltage Vmax that is fed to the motor  20  by the auxiliary electric source  100  is set to a voltage resulting from adding a surplus voltage Vsu for which noise to be generated at the time of the electric feed from the auxiliary electric source  100  is considered, to the minimal supply voltage Vmin. Further, the maximal discharge voltage of the auxiliary electric source  100  is set so as to be above the maximal supply voltage Vmax. 
     For example, the maximal supply voltage Vmax is set to 90% of the maximal discharge voltage of the auxiliary electric source  100 . That is, when the minimal supply voltage Vmin is 9 V and the surplus voltage Vsu is 0.7 V, the maximal supply voltage Vmax is 9.7 V. In this case, the maximal discharge voltage of the auxiliary electric source  100  is 10.8 V (9.7 V/0.9). That is, as the capacitors, three cells in each of which the maximal discharge voltage is 3.6 V (10.8 V/3) are employed. 
       FIG. 3  shows a processing procedure in the electronic control unit  50 , mainly for the switching of the action state to the discharging state at the time of the backup. The process shown in  FIG. 3  is realized when the arithmetic processing circuit  51  repeatedly executes a program stored in the memory  52  of the electronic control unit  50  with a predetermined period. The electronic control unit  50  executes the following process after a control start when an ignition switch is turned on and before a control end when the ignition switch is turned off. Here, it is assumed that the main electric source  60  has not broken down at the time of the control start. 
     As shown in  FIG. 3 , the electronic control unit  50  determines whether the battery voltage Vb is lower than a threshold Vth (step S 1 ). The threshold Vth is set to a value in a voltage range that is decided based on a specification of the electric feed object. For example, when the operating voltage of the electronic control unit  50  is 9 to 11 V, the threshold Vth is set to 9 V. 
     In the case where the battery voltage Vb is equal to or higher than the threshold Vth (NO in step S 1 ), the electronic control unit  50  determines that the main electric source  60  has not broken down, and executes a main electric source setting process (step S 2 ). The main electric source setting process is a process for setting the main electric source  60  as the electric source that is used for the action of the motor  20 . In this case, the electronic control unit  50  switches the action state of the auxiliary electric source  100 , to the keeping state, the charging state or the discharging state at the time of the boost. After completion of the main electric source setting process, the electronic control unit  50  returns to the process of step S 1  again. 
     In the case where the battery voltage Vb is lower than the threshold Vth (YES in step S 1 ), the electronic control unit  50  determines that the main electric source  60  has broken, and determines whether the supply voltage Vc is lower than the minimal supply voltage Vmin in the auxiliary electric source  100  (step S 3 ). The supply voltage Vc is a supply voltage that is actually being supplied to the motor  20  by the auxiliary electric source  100 , and is the voltage value at the connection point P 4  on a higher potential side of the auxiliary electric source  100 . 
     In the case where the supply voltage Vc is lower than the minimal supply voltage Vmin in the auxiliary electric source  100  (YES in step S 3 ), the electronic control unit  50  determines that the auxiliary electric source  100  cannot feed the electric power that is necessary for the motor  20  to perform the minimal action, and ends the process shown in  FIG. 3 . In this case, the auxiliary electric source  100  cannot be switched to the action state at the time of the backup, and therefore a predetermined fail-safe process is executed. 
     In the case where the supply voltage Vc is equal to or higher than the minimal supply voltage Vmin in the auxiliary electric source  100  (NO in step S 3 ), the electronic control unit  50  determines that the auxiliary electric source  100  can feed the electric power that is necessary for the motor  20  to perform the minimal action, and executes an auxiliary electric source setting process (step S 4 ). The auxiliary electric source setting process is a process for setting the auxiliary electric source  100  as the electric source that is used for the action of the motor  20 . In this case, the electronic control unit  50  switches the action state of the auxiliary electric source  100  to the discharging state at the time of the backup. 
     After completion of the process of step S 4 , the electronic control unit  50  determines whether a remaining electric power Wr is lower than an electric power threshold Wth in the auxiliary electric source  100  (step S 5 ). In the embodiment, the voltage value at the connection point P 4  on the higher potential side of the auxiliary electric source  100  is used as the remaining electric power Wr. The electric power threshold Wth is set to the same value as the maximal supply voltage Vmax of the auxiliary electric source  100 . 
     In the case where the remaining electric power Wr is equal to or higher than the electric power threshold Wth in the auxiliary electric source  100  (NO in step S 5 ), the electronic control unit  50  determines that the remaining electric power Wr of the auxiliary electric source  100  is sufficient, and executes a first electric feed control (step S 6 ). As shown in  FIG. 4 , in the first electric feed control, the supply voltage Vc of the auxiliary electric source  100  is set as the maximal supply voltage Vmax. As shown in  FIG. 4 , the supply voltage Vc is lower as the remaining electric power Wr is higher. 
     In the case where the remaining electric power Wr is lower than the electric power threshold Wth in the auxiliary electric source  100  (YES in step S 5 ), the electronic control unit  50  determines that the remaining electric power Wr of the auxiliary electric source  100  is insufficient, and executes a second electric feed control (step S 7 ). As shown in  FIG. 4 , in the second electric feed control, the maximal supply voltage Vmax is adopted as the upper limit of the supply voltage Vc of the auxiliary electric source  100 , and the supply voltage Vc is set to a lower value as the remaining electric power Wr is lower. 
     Operations and effects of the embodiment will be described. For example, in the case where the battery voltage Vb of the main electric source  60  is 12 V, the auxiliary electric source  100  needs to be constituted by four capacitors in each of which the maximal discharge voltage is 3.6 V, for preparing the auxiliary electric source  100  having an electric source performance equivalent to the electric source performance of the main electric source  60 . 
     In response, in the embodiment, even in the case where the battery voltage Vb of the main electric source  60  is 12 V, it is only necessary to prepare the auxiliary electric source  100  having an electric source performance of 9 V as the minimal supply voltage Vmin. Therefore, the auxiliary electric source  100  only needs to be constituted by three capacitors in each of which the maximal discharge voltage is 3.6 V. 
     Therefore, in the embodiment, when the auxiliary electric source  100  backs up the electric feed from the main electric source  60  in the case where the main electric source  60  has broken down, it is possible to secure the minimal action of the motor  20  with the electric feed from the auxiliary electric source  100 . That is, when the auxiliary electric source  100  backs up the main electric source  60  in the case where the main electric source  60  has broken down, it is possible to fulfill at least the desired minimal function with the electric feed from the auxiliary electric source  100 , although it is not possible to maximally fulfill the desired function. Thereby, when the auxiliary electric source  100  backs up the electric feed from the main electric source  60  in the case where the main electric source  60  has broken down, it is possible to fulfill the desired function of the motor  20 , even if the discharge voltage that is the electric source performance of the auxiliary electric source  100  is not equivalent to the battery voltage Vb that is the electric source performance of the main electric source  60 . Accordingly, it is not necessary to prepare the auxiliary electric source  100  having an electric source performance equivalent to the electric source performance of the main electric source  60 , and it is possible to restrain the increase in the electric source performance of the auxiliary electric source  100 . 
     As described above, in the embodiment, it is possible to decrease the number of capacitors that constitute the auxiliary electric source  100 , and it is possible to decrease the number of parts of the auxiliary electric source device  40  and to reduce the cost, compared to the case of preparing the auxiliary electric source  100  having an electric source performance equivalent to the electric source performance of the main electric source  60 . In this case, in addition to the decrease in the number of capacitors that constitute the auxiliary electric source  100 , it is possible to decrease the number of balancing circuits that have a function to adjust the discharge voltages of the capacitors. 
     In the case where the auxiliary electric source  100  performs the electric feed to the motor  20  at the maximal supply voltage Vmax, it is possible to secure the minimal action of the motor  20 , even if the noise is generated at the time of the electric feed. That is, when the auxiliary electric source  100  backs up the electric feed from the main electric source  60  in the case where the main electric source  60  has broken down, it is possible to fulfill at least the desired minimal function. 
     In the case where the remaining electric power Wr is lower than the electric power threshold Wth and the remaining electric power Wr is relatively low in the auxiliary electric source  100 , the auxiliary electric source  100  can maintain the discharging state at the time of the backup, for a longer time, by setting the supply voltage Vc to a lower value as the remaining electric power Wr is lower. 
     The electric source performance of the auxiliary electric source  100  is set so as to allow the securement of the minimal function that needs to be fulfilled by the EPS  1  to assist the steering operation by the driver in the case where the vehicle performs the ordinary traveling, and therefore it is possible to realize the EPS  1  in which the increase in the electric source performance of the auxiliary electric source  100  is restrained. 
     In the embodiment, the discharge voltage of the auxiliary electric source  100  is set so as to be above the maximal supply voltage Vmax, and therefore the auxiliary electric source  100  can maintain the discharging state at the time of the backup, for a longer time, even if the maximal supply voltage Vmax is maintained. 
     The embodiment may be modified as follows. The following other embodiments can be combined with each other, in a range in which there is no technical inconsistency. In the processing procedure shown in  FIG. 3 , in steps S 5  to S 7 , the supply voltage Vc of the auxiliary electric source  100  can be changed based on the remaining electric power Wr. However, the supply voltage Vc may be constant regardless of the change in the remaining electric power Wr, or may be set to a lower value as the remaining electric power Wr is lower. In this case, step S 5  can be removed. 
     The discharge voltage of the auxiliary electric source  100  is set so as to be above the maximal supply voltage Vmax. However, the discharge voltage of the auxiliary electric source  100  may be set so as to be equal to the maximal supply voltage Vmax. The discharge voltage of the auxiliary electric source  100  is set to the voltage resulting from adding the surplus voltage Vsu for which the noise is considered to the minimal supply voltage Vmin. However, the discharge voltage of the auxiliary electric source  100  only needs to be set to a value equal to or higher than the minimal supply voltage Vmin. 
     The example in which the maximal supply voltage Vmax is 90% of the maximal discharge voltage of the auxiliary electric source  100  has been shown. However, the maximal supply voltage Vmax may be altered to a value lower than 90% of the maximal discharge voltage of the auxiliary electric source  100  or a value equal to or higher than 90% of the maximal discharge voltage of the auxiliary electric source  100 . For example, in the case where the maximal supply voltage Vmax is altered to a value lower than 90% of the maximal discharge voltage of the auxiliary electric source  100 , the auxiliary electric source  100  can maintain the discharging state at the time of the backup, for a longer time, than in the case where the maximal supply voltage Vmax is 90% of the maximal discharge voltage of the auxiliary electric source  100 . 
     In the case where the lowest operating voltage of the electronic control unit  50  is 9 V, various setting values are set so as to correspond to 9 V. However, in the case where the lowest operating voltage is 8 V, the various setting values may be set so as to correspond to 8 V. 
     The electronic control unit  50  switches the action state of the auxiliary electric source  100  to the discharging state at the time of the backup, when the main electric source  60  has broken down. However, even when the main electric source  60  has not broken down, the electronic control unit  50  may preliminarily switch the action state of the auxiliary electric source  100  to the discharging state at the time of the backup, before the main electric source  60  has broken down. 
     In the above embodiment, the number of capacitors that constitute the auxiliary electric source  100  can be altered to an appropriate number, for example, to two, four or more, or the like, depending on the electric source performance necessary for the securement of the minimal function that needs to be fulfilled by the EPS  1 . The number of capacitors that constitute the auxiliary electric source  100  is altered also depending on the maximal discharge voltage of a capacitor to be used. 
     The auxiliary electric source  100  may be constituted by a single capacitor. Also with the modification, it is possible to restrain the increase in the electric source performance of the auxiliary electric source  100 . In this case, it is possible to decrease the electric source performance of the capacitor that constitutes the auxiliary electric source  100 , and to reduce the cost of the auxiliary electric source device  40 , compared to the case of preparing the auxiliary electric source  100  having an electric source performance equivalent to the electric source performance of the main electric source  60 . Further, since the auxiliary electric source  100  is constituted by a single capacitor, it is unnecessary to provide the balancing circuit that has a function to adjust the discharge voltages of a plurality of capacitors. 
     In the embodiment, the electric source performance of the auxiliary electric source  100  to be set for securing the minimal function that needs to be fulfilled by the EPS  1  is the maximal discharge voltage. However, the electric source performance of the auxiliary electric source  100  may be the electric source capacity of the auxiliary electric source  100 , or may be the electric current value of the auxiliary electric source  100 . 
     In the embodiment, as each of the capacitors  101 ,  102 ,  103 , the lithium-ion capacitor is used. However, each of the capacitors  101 ,  102 ,  103  is not limited to the lithium-ion capacitor. That is, each of the capacitors  101 ,  102 ,  103  may be an electric double layer capacitor (EDLC), a lithium-ion battery or a lead storage battery. 
     Some or all of the switching elements (FET 1  to FET 5 ) may be constituted by switching elements other than the MOS-FET. Examples of the switching element other than the MOS-FET include an insulated gate bipolar transistor (IGBT). 
     The motor  20  is not limited to the three-phase brushless motor. For example, the motor  20  may be a brush motor. In the embodiment, the EPS  1  to which the auxiliary electric source device  40  is applied is configured as an EPS in which the motor  20  is coupled with the steering shaft  12  through the reducer  21 . However, the EPS  1  may be configured as an EPS in which the motor  20  is coupled with the rack shaft  14  through the reducer  21 . Further, without being limited to the EPS to which the auxiliary electric source device  40  is applied, the auxiliary electric source device  40  may be applied to a steer-by-wire steering device, for example. 
     The auxiliary electric source device  40  may be applied to an uninterruptible power supply, may be equipped in an unmanned transport vehicle, may be equipped in an overhead wire, or may be equipped in an industrial machine.