Patent Description:
There is known an electric power steering system (hereinafter referred to as "EPS") that assists a driver's steering operation by applying a rotational force of a motor as a steering assist force to a steering mechanism of a vehicle. A control device of the EPS controls driving of the motor by controlling power supply to the motor. The control device of the EPS is operated by power supplied from a battery.

With enhancing performance of the EPS, the power required to drive the motor is increasing. Thus, there is a demand for higher output of the power supplied to the motor. When the battery fails and the power supplied from the battery to the motor cannot be obtained, a steering operation assisting function cannot be continued. Reflecting these circumstances, in the related art disclosed in <CIT> (<CIT>), the EPS is provided with an auxiliary power source connected in series with the battery and an auxiliary power source connected in parallel to the battery. When high output is required, the auxiliary power source connected in series with the battery is used to achieve a function of boosting the power supplied from the battery to the motor. When the battery fails, the auxiliary power source connected in parallel to the battery is used to achieve a function of backing up the power supplied to the motor. <CIT> discloses an electric power steering system having the features of the preamble of claim <NUM>. Electric power steering systems are known from <CIT> and <CIT>.

A circuit is configured such that, for the boosting and the backup, when a voltage required for the backup is larger than that required for the boosting and when the voltages of the auxiliary power sources are the same, the number of auxiliary power sources used for backup is larger than the number of auxiliary power sources used for boosting. In order to make the circuit configuration compact, there is a case that requires shared auxiliary power sources that can be used for both boosting and backup. In this case, a configuration in which a part of the auxiliary power source used for backup is also used for boosting can be considered. In this configuration, for boosting, the auxiliary power source that is shared is in a discharging state, while the auxiliary power source that is not shared is maintained in a holding state. Therefore, for backup, the power supply to the motor is backed up using the auxiliary power sources with different charging states. In view of this, a voltage of the power supplied from the auxiliary power sources to the motor varies between cases where the power supply to the motor is backed up using the auxiliary power sources with different charging states, and where the power supply to the motor is backed up using the auxiliary power sources with the same charging states. Thus, it has been difficult to share the auxiliary power source between boosting and backup.

The invention allows the auxiliary power source to be shared between boosting and backup.

An auxiliary power supply device according to a first aspect which is not claimed and is a background aspect helpful for understanding the present invention includes an auxiliary power source and a booster circuit for boosting input voltage. The auxiliary power supply device is provided in a power supply path extending from a main power source to a power supply target, and configured to be switched among a charging state for charging the auxiliary power source, a holding state for holding a charge voltage of the auxiliary power source, and a discharging state for discharging power from the auxiliary power source. The auxiliary power source is configured to be switched between a serial connection mode and a parallel connection mode. The serial connection mode is a mode in which a first terminal is connected to the main power source and a second terminal is connected to the power supply target, and the auxiliary power source is connected in series between the main power source and the power supply target. The parallel connection mode is a mode in which the first terminal is connected to a part between the main power source and the power supply target, at which the main power source and the power supply target are connected in series. The auxiliary power source is configured to boost a voltage supplied from the main power source by a discharge voltage of the auxiliary power source to supply power to the power supply target when the auxiliary power source is switched to the serial connection mode. The auxiliary power source is configured such that the second terminal is connected to an input terminal of the booster circuit, the discharge voltage of the auxiliary power source, which is an input voltage of the booster circuit, is boosted, and backup to supply the boosted discharge voltage to the power supply target is performed when the auxiliary power source is switched to the parallel connection mode.

According to the above configuration, in order to make the circuit configuration of the auxiliary power supply device compact, the auxiliary power source is configured to be switched between the serial connection mode and the parallel connection mode. The serial connection mode is a mode in which the auxiliary power source is connected in series between the main power source and the power supply target. The parallel connection mode is a mode in which the first terminal is connected to a part between the main power source and the power supply target, at which the main power source and the power supply target are connected in series. The serial connection mode and the parallel connection mode share the same auxiliary power source. In other words, the shared auxiliary power source is used for both backup and boosting. Assuming that the auxiliary power source has a smaller power source capacitance than the main power source does, in the configuration above, the booster circuit boosts the discharge voltage from the auxiliary power source, which is the input voltage of the booster circuit, for backup. As a result, since all the auxiliary power sources are in the discharging state for boosting, for backup, the power supply to the power supply target is not backed up using an auxiliary power source with different charging states. Thus, since it is possible to suppress variation in the discharge voltage from the auxiliary power source, it is possible to stabilize the discharge voltage from the auxiliary power source for backup.

In the auxiliary power supply device according to the above aspect, the auxiliary power source may be configured to be switched to the parallel connection mode, when power is supplied from the main power source to the power supply target, such that the second terminal is connected to an output terminal of the booster circuit, the input terminal of the booster circuit is connected to the main power source, an output voltage of the main power source, which is the input voltage of the booster circuit, is boosted, and the auxiliary power source is charged by the boosted output voltage.

According to the above configuration, the shared booster circuit is used for both backup and charging the auxiliary power source. Thus, compared with the case where the booster circuit used for charging the auxiliary power source and the booster circuit used for backup are separately provided in the auxiliary power supply device, the circuit configuration of the auxiliary power supply device can be made further compact because the configuration of the booster circuit is shared.

An electric power steering system according to a second aspect which is according to the present invention includes a motor serving as a power supply target, a main power source, an auxiliary power supply device provided in a power supply path extending from the main power source to the motor and including an auxiliary power source and a booster circuit for boosting an input voltage, and an electronic control unit. The electronic control unit is configured to i) switch the auxiliary power source between a serial connection mode and a parallel connection mode, the serial connection mode being a mode in which a first terminal is connected to the main power source and a second terminal is connected to the motor, and the auxiliary power source is connected in series between the motor and the main power source, the parallel connection mode being a mode in which the first terminal is connected to a part between the main power source and the power supply target, at which the main power source and the power supply target are connected in series, ii) switch among a charging state for charging the auxiliary power source, a holding state for holding a charge voltage of the auxiliary power source, and a discharging state for discharging power from the auxiliary power source, and iii) switch the auxiliary power source between the serial connection mode and the parallel connection mode. The auxiliary power source is configured to boost a voltage supplied from the main power source by a discharge voltage of the auxiliary power source to supply power to the motor when the electronic control unit switches the auxiliary power source to the serial connection mode. The auxiliary power source is configured such that the second terminal of the auxiliary power source is connected to an input terminal of the booster circuit, the discharge voltage of the auxiliary power source, which is an input voltage of the booster circuit, is boosted, and backup to supply the boosted discharge voltage to the motor is performed when the electronic control unit switches the auxiliary power source to the parallel connection mode.

According to the above configuration, in order to make the circuit configuration of the auxiliary power supply device compact, the electronic control unit is configured to switch the auxiliary power source between the serial connection mode and the parallel connection mode. The serial connection mode is a mode in which the auxiliary power source is connected in series between the motor and the main power source. The parallel connection mode is a mode in which the first terminal of the auxiliary power source is connected to a part between the main power source and the motor, at which the main power source and the motor are connected in series. The serial connection mode and the parallel connection mode share the same auxiliary power source. In other words, the shared auxiliary power source is used for both backup and boosting. Assuming that the auxiliary power source has a smaller power source capacitance than the main power source does, in the configuration above, the booster circuit boosts the discharge voltage from the auxiliary power source, which is the input voltage of the booster circuit, for backup. As a result, since all the auxiliary power sources are in the discharging state for boosting, for backup, the power supply to the power supply target is not backed up using an auxiliary power source with different charging states. Thus, since it is possible to suppress variations in the discharge voltage from the auxiliary power source, it is possible to stabilize the discharge voltage from the auxiliary power source for backup. Thus, the electric power steering system capable of stabilizing the discharge voltage from the auxiliary power source can be achieved.

In the electric power steering system according to the above aspect, the electronic control unit may be configured to switch the auxiliary power source to the parallel connection mode, when the auxiliary power source supplies power from the main power source to the power supply target, such that the second terminal is connected to an output terminal of the booster circuit, the input terminal of the booster circuit is connected to the main power source, an output voltage of the main power source, which is the input voltage of the booster circuit, is boosted, and the auxiliary power source is charged by the boosted output voltage.

According to the first and second aspects described above, it is possible to share the auxiliary power source between boosting and backup.

A first embodiment in which an auxiliary power supply device is applied to an electric power steering system (hereinafter referred to as "EPS") will be described. As shown in <FIG>, an EPS <NUM> of the embodiment includes a steering shaft <NUM>. A steering wheel <NUM> is fixed to one end of the steering shaft <NUM>, and a pinion gear <NUM> is provided to the other end thereof. The pinion gear <NUM> meshes with a rack gear <NUM> provided on a rack shaft <NUM>. The pinion gear <NUM> and the rack gear <NUM> constitute a rack and pinion mechanism. A rotational motion of the steering shaft <NUM> is converted into a reciprocating linear motion of the rack shaft <NUM> in a longitudinal direction of the rack shaft <NUM> (in a right and left direction in <FIG>) via the rack and pinion mechanism. The EPS <NUM> is assembled to a vehicle so that the longitudinal direction of the rack shaft <NUM> matches a vehicle width direction. The reciprocating linear motion of the rack shaft <NUM> is transmitted to the right and left steered wheels <NUM> via tie rods <NUM> connected to both ends of the rack shaft <NUM>. Thereby, a steered angle of the steered wheels <NUM> is changed, and a traveling direction of the vehicle is changed.

The steering shaft <NUM> is equipped with a torque sensor <NUM> for measuring a steering torque TR applied to the steering shaft <NUM> by an operation of the steering wheel <NUM>. The torque sensor <NUM> of the embodiment detects a torsion amount of a torsion bar that constitutes a part of the steering shaft <NUM> and measures the steering torque TR based on the torsion amount.

A steering assist motor <NUM> is connected to the steering shaft <NUM> via a speed reducer <NUM>. The speed reducer <NUM> reduces speed of rotation output from the motor <NUM> and transmits a rotational force with reduced speed to the steering shaft <NUM>. A three-phase brushless motor is employed as the motor <NUM> in the embodiment. A worm gear mechanism is employed as the speed reducer <NUM> in the embodiment.

The EPS <NUM> includes a drive circuit <NUM>, an auxiliary power supply device <NUM>, and an electronic control unit <NUM>. A known circuit including two switching elements for each phase (U phase, V phase, and W phase) of the motor <NUM> is employed as the drive circuit <NUM>. When the EPS <NUM> is assembled to the vehicle, the auxiliary power supply device <NUM> and the electronic control unit <NUM> are connected to an in-vehicle battery <NUM> serving as a main power source.

The electronic control unit <NUM> includes an arithmetic processing circuit <NUM> that executes arithmetic processing related to the control of the EPS <NUM>, and a memory <NUM> in which a program and data for the control are stored. The torque sensor <NUM> described above is connected to the electronic control unit <NUM>. When the EPS <NUM> is assembled to the vehicle, a vehicle speed sensor <NUM> is connected to the electronic control unit <NUM>. The vehicle speed sensor <NUM> is installed in the vehicle and detects a traveling speed VS of the vehicle. Further, a current sensor <NUM> is connected to the electronic control unit <NUM>. The current sensor <NUM> detects an actual current value I of a power supply from the auxiliary power supply device <NUM> to the drive circuit <NUM>. Specifically, the current sensor <NUM> detects the actual current value I of an output port <NUM> (see <FIG>) of the auxiliary power supply device <NUM>.

With the EPS <NUM> assembled to the vehicle, the electronic control unit <NUM> controls a steering assist force applied by the motor <NUM>. When controlling the steering assist force, the electronic control unit <NUM> determines a target steering assist force that is a target value of the steering assist force based on the steering torque TR and the traveling speed VS. The electronic control unit <NUM> controls the operation of the drive circuit <NUM> and the auxiliary power supply device <NUM> so as to generate a steering assist force corresponding to the target steering assist force.

As shown in <FIG>, the auxiliary power supply device <NUM> includes an input port <NUM> and the output port <NUM>. The input port <NUM> is connected to the in-vehicle battery <NUM> when the EPS <NUM> is assembled to the vehicle. The output port <NUM> is connected, via the drive circuit <NUM>, to the motor <NUM> that is a power supply target when the auxiliary power supply device <NUM> is assembled to the EPS <NUM>.

The auxiliary power supply device <NUM> includes a first line <NUM>, which is a line serving as a power supply path in a holding state and a charging state described later. The first line <NUM> is a wiring connected to the input port <NUM> and the output port <NUM>.

The auxiliary power supply device <NUM> includes an auxiliary power source <NUM> capable of charging and discharging electric charges. The auxiliary power source <NUM> is connected between the in-vehicle battery <NUM> and the motor <NUM> so as to be switched between a serial connection mode and a parallel connection mode. In the serial connection mode, the in-vehicle battery <NUM> and the motor <NUM> are connected in series. In the parallel connection mode, a first terminal T1 is connected to a connection point P2 on the first line <NUM>. The connection point P2 is a part between the in-vehicle battery <NUM> and the motor <NUM>, at which the in-vehicle battery <NUM> and the motor <NUM> are connected in series. The auxiliary power source <NUM> includes two capacitors 24a and 24b. The number of capacitors 24a and 24b provided in the auxiliary power source <NUM> is determined in consideration of a capacitance of the capacitors 24a and 24b with respect to a voltage required by the EPS <NUM>. The capacitors 24a and 24b each include an electrode plate having a positive polarity and an electrode plate having a negative polarity. The first terminal T1 of a negative electrode plate 24c of the capacitor 24a is connected to the first line <NUM>, and a second terminal T2 of a positive electrode plate 24d of the capacitor 24b is connected to a second line <NUM>, which is a line serving as a power supply path used for boosting and backup described later. When the auxiliary power source <NUM> is considered as a whole, the auxiliary power source <NUM> is charged between the negative electrode plate 24c of the capacitor 24a and the positive electrode plate 24d of the capacitor 24b. When the auxiliary power source <NUM> is switched to the serial connection mode, the first terminal T1 is connected to the in-vehicle battery <NUM> via the first line <NUM>, and the second terminal T2 is connected to the motor <NUM> via the second line <NUM>. When the auxiliary power source <NUM> is switched to the parallel connection mode, the first terminal T1 is connected to the ground via the first line <NUM>, and the second terminal T2 is connected to the motor <NUM> via the second line <NUM>. In the embodiment, lithium ion capacitors are employed as the capacitors 24a and 24b.

The lithium ion capacitor has advantages of good heat resistance, long life, good charge/discharge performance, high energy density, and high safety. Meanwhile, an electric double layer capacitor has advantages of good heat resistance, long life, good charge/discharge performance, and high safety, but has disadvantages of low energy density and a likeliness to increase in size when heat resistance is increased. In the embodiment, since the lithium ion capacitors are employed as the capacitors 24a and 24b, the capacitors 24a and 24b have the above advantages.

The auxiliary power supply device <NUM> includes a chopper coil <NUM> that is a boosting coil. One end of the chopper coil <NUM> is connected to the ground via a first switching element (hereinafter referred to as "FET <NUM>") and to the second line <NUM> via a second switching element (hereinafter referred to as "FET <NUM>"). The chopper coil <NUM>, the FET <NUM>, and the FET <NUM> constitute a booster circuit <NUM> that boosts an input voltage. The booster circuit <NUM> is connected so as to be switched to the in-vehicle battery <NUM> or the second terminal T2 of the auxiliary power source <NUM>. The other end of the chopper coil <NUM> is connected to the first line <NUM> via a fifth switching element (hereinafter referred to as "FET <NUM>"). The second line <NUM> is a wiring that connects the FET <NUM> and the electrode plate 24d of the capacitor 24b via a sixth switching element (hereinafter referred to as "FET <NUM>").

The auxiliary power supply device <NUM> includes a third switching element (hereinafter referred to as "FET <NUM>") and a fourth switching element (hereinafter referred to as "FET <NUM>"). The FET <NUM> is provided between the second line <NUM> and the output port <NUM> and switches the connection between the second line <NUM> and the motor <NUM>. The FET <NUM> is provided between the first line <NUM> and the output port <NUM> and switches the connection between the first line <NUM> and the motor <NUM>. The FET <NUM> is connected to a connection point P1 provided between the FET <NUM> and the FET <NUM> on the second line <NUM> so as to switch the connection between the second terminal T2 and the second line <NUM>. The connection point P2 provided on the first line <NUM> and between the FET <NUM> and the electrode plate 24c of the capacitor 24a is connected to a seventh switching element (hereinafter referred to as "FET <NUM>"). The FET <NUM> switches the connection between the first terminal T1 and the ground. A connection point P3 provided on the first line <NUM> and between the chopper coil <NUM> and the FET <NUM> is connected to an eighth switching element (hereinafter referred to as "FET <NUM>"). The FET <NUM> switches the connection between the second terminal T2 and the chopper coil <NUM>.

Metal-oxide-semiconductor field-effect transistors (MOS-FETs) are employed as the FET <NUM> to the FET <NUM>. The electronic control unit <NUM> is connected to gates of the FET <NUM> to the FET <NUM>. The electronic control unit <NUM> outputs drive signals to the gates of the FET <NUM> to the FET <NUM>, thereby switching the FET <NUM> to the FET <NUM> between an on state and an off state according to the drive signals. The electronic control unit <NUM> switches the auxiliary power source <NUM> to the serial connection mode or the parallel connection mode by switching the FET <NUM> to FET <NUM> to the on state or the off state.

A power supply control of the auxiliary power supply device <NUM> executed by the electronic control unit <NUM> will be described. <FIG> shows operation states of the FET <NUM> to the FET <NUM> of the auxiliary power supply device <NUM> in the charging state, the holding state, and a discharging state. The charging state is a state in which the power supply is controlled so that the auxiliary power source <NUM> is charged. The holding state is a state in which the power supply is controlled so that a charging voltage of the auxiliary power source <NUM> is held. The discharging state is a state in which the power supply is controlled so that power is discharged from the auxiliary power source <NUM>. In the embodiment, there are two discharging states: a discharging state for boosting and a discharging state for backup. For boosting, a battery voltage Vb supplied from the in-vehicle battery <NUM> is boosted by a capacitor voltage Vc that is a discharge voltage of the auxiliary power source <NUM>. For backup, the discharge voltage of the auxiliary power source <NUM> is boosted as the input voltage of the booster circuit <NUM>, and the boosted discharge voltage is supplied to the motor <NUM>. Thereby, even when the power supply from the in-vehicle battery <NUM> is interrupted, the power supply to the motor <NUM> can be continued.

The electronic control unit <NUM> determines the target steering assist force, which is the target value of the steering assist force, based on the steering torque TR and the traveling speed VS at the time of the power supply control. The electronic control unit <NUM> calculates a required voltage Vr that is a voltage necessary for the motor <NUM> to generate a steering assist force corresponding to the target steering assist force. The electronic control unit <NUM> switches a state of the power supply control among the charging state, the holding state, and the discharging state in accordance with the battery voltage Vb, the required voltage Vr, and a first voltage V1. The battery voltage Vb is a voltage of the power supplied from the in-vehicle battery <NUM> to the auxiliary power supply device <NUM>. The first voltage V1 is a voltage at the connection point P1.

As shown in <FIG> and <FIG>, the electronic control unit <NUM> switches the state of the power supply control to the holding state when the required voltage Vr is equal to or smaller than the battery voltage Vb and the first voltage V1 is equal to or larger than a specified maximum charging voltage. The maximum charging voltage is a voltage when the auxiliary power source <NUM> is fully charged. When switching the state of the power supply control to the holding state, the electronic control unit <NUM> turns off the FET <NUM> to the FET <NUM>, the FET <NUM>, the FET <NUM>, and the FET <NUM>, and turns on the FET <NUM> and the FET <NUM>. In this case, since the FET <NUM> is turned on, the in-vehicle battery <NUM> is connected to the motor <NUM> via the first line <NUM>. A second voltage V2 that is a voltage of the output port <NUM> of the auxiliary power supply device <NUM> is almost equal to the battery voltage Vb supplied from the in-vehicle battery <NUM>. The battery voltage Vb of the in-vehicle battery <NUM> is supplied to the motor <NUM> via the first line <NUM>. Meanwhile, since the FET <NUM>, the FET <NUM>, and the FET <NUM> are turned off, the electric charge accumulated in the auxiliary power source <NUM> is not discharged, and the charge voltage of the auxiliary power source <NUM> charged until then is held. Further, the booster circuit <NUM> is disconnected from the in-vehicle battery <NUM> because the FET <NUM> is turned off, and is disconnected from the auxiliary power source <NUM> because the FET <NUM> is turned off.

As shown in <FIG> and <FIG>, the electronic control unit <NUM> switches the state of the power supply control to the charging state when the required voltage Vr is equal to or smaller than the battery voltage Vb and the first voltage V1 is smaller than the maximum charging voltage. When switching the state of the power supply control to the charging state, the electronic control unit <NUM> turns off the FET <NUM>, the FET <NUM>, and the FET <NUM> and turns on the FET <NUM> to the FET <NUM>. Meanwhile, in the charging state, the electronic control unit <NUM> performs Pulse Width Modulation (PWM) driving so that the FET <NUM> and the FET <NUM> are alternately turned off. In this case, since the FET <NUM> is turned on, the in-vehicle battery <NUM> is connected to the motor <NUM> via the first line <NUM>. The second voltage V2 that is the voltage of the output port <NUM> of the auxiliary power supply device <NUM> is almost equal to the battery voltage Vb supplied from the in-vehicle battery <NUM>. The battery voltage Vb of the in-vehicle battery <NUM> is supplied to the motor <NUM> via the first line <NUM>. Meanwhile, since the FET <NUM> and the FET <NUM> are turned on and the PWM driving is performed for the FET <NUM> and the FET <NUM>, the auxiliary power source <NUM> is connected to the in-vehicle battery <NUM>. Since the FET <NUM> is turned on, an input terminal T3 of the booster circuit <NUM> is connected to the in-vehicle battery <NUM>, and since the FET <NUM> is turned off, the input terminal T3 of the booster circuit <NUM> is not connected to the second terminal T2 of the auxiliary power source <NUM>. Meanwhile, since the FET <NUM> is turned on, an output terminal T4 of the booster circuit <NUM> is connected to the second terminal T2 of the auxiliary power source <NUM>, and since the FET <NUM> is turned off, the output terminal T4 of the booster circuit <NUM> is not connected to the motor <NUM>. Since the FET <NUM> to the FET <NUM> are turned on and the PWM driving is performed for the FET <NUM> and the FET <NUM>, the battery voltage Vb that is the output voltage of the in-vehicle battery <NUM> is supplied to the auxiliary power source <NUM> via the chopper coil <NUM>. The battery voltage Vb is input to the booster circuit <NUM> as the input voltage. The booster circuit <NUM> boosts the input battery voltage Vb to the first voltage V1, and outputs the boosted first voltage V1 as the output voltage. The auxiliary power source <NUM> is charged by the first voltage V1 boosted by the booster circuit <NUM>.

As shown in <FIG> and <FIG>, when the traveling speed VS is equal to or smaller than a traveling speed threshold VS0 and the actual current value I of the output port <NUM> is equal to or larger than a current threshold <NUM>, the electronic control unit <NUM> switches the state of the power supply control to the discharging state for boosting. The traveling speed threshold VS0 is a threshold determined in view of determining whether it is not preferable to inevitably stop assisting the steering operation. A case where the traveling speed VS is equal to or smaller than the traveling speed threshold VS0 is a case where the vehicle is stopped or is traveling at a low speed. When the vehicle is stopped or is traveling at a low speed, as compared with the case where the vehicle is traveling at a high speed, an influence on a driver is small even when the assist of the steering operation is inevitably stopped. However, the required voltage Vr required by the EPS <NUM> for the motor <NUM> to generate the steering assist force corresponding to the target steering assist force is likely to increase. The current threshold I0 is a threshold determined in view of determining whether the actual current value I that actually flows through the output port <NUM> with the assist of the steering operation indicates a rapid steering operation. The actual current value I that is equal to or larger than the current threshold I0 indicates the rapid steering operation, and it can be considered that the required voltage Vr is exceeding the battery voltage Vb. When switching the state of the power supply control to the discharging state for boosting, the electronic control unit <NUM> turns off the FET <NUM>, the FET <NUM>, the FET <NUM>, the FET <NUM>, the FET <NUM>, and the FET <NUM> and turns on the FET <NUM> and the FET <NUM>. Thereby, the electronic control unit <NUM> switches the connection mode of the auxiliary power source <NUM> to the serial connection mode. When the auxiliary power source <NUM> is switched to the serial connection mode, the first terminal T1 of the auxiliary power source <NUM> is connected to the connection point P2 on the first line <NUM> and is connected to the in-vehicle battery <NUM> via the first line <NUM>. Since the FET <NUM> and the FET <NUM> are turned on, the second terminal T2 of the auxiliary power source <NUM> is connected to the motor <NUM> via the second line <NUM>. As a result, the battery voltage Vb of the in-vehicle battery <NUM> is supplied to the motor <NUM> via the first line <NUM>, the auxiliary power source <NUM>, and the second line <NUM>. As described above, the in-vehicle battery <NUM> and the auxiliary power source <NUM> are connected in series, and the battery voltage Vb of the in-vehicle battery <NUM> is boosted by the capacitor voltage Vc that is a discharge voltage of the auxiliary power source <NUM> so as to be supplied to the motor <NUM>. Since the in-vehicle battery <NUM> and the auxiliary power source <NUM> are connected in series, the total voltage of the battery voltage Vb and the capacitor voltage Vc is output from the second terminal T2 of the auxiliary power source <NUM>. Thereby, the power supply to the motor <NUM> that is the power supply target is boosted.

As shown in <FIG> and <FIG>, when the traveling speed VS exceeds the traveling speed threshold VS0 and the required voltage Vr exceeds the battery voltage Vb, the electronic control unit <NUM> switches the state of the power supply control to the discharging state for backup. A case where the traveling speed VS exceeds the traveling speed threshold VS0 is a case where the vehicle is traveling at a high speed, and the required voltage Vr required by the EPS <NUM> for the motor <NUM> to generate the steering assist force corresponding to the target steering assist force is likely to decrease, but it is not preferable to stop assisting the steering operation. A case where the required voltage Vr exceeds the battery voltage Vb occurs when the in-vehicle battery <NUM> fails, such as when the in-vehicle battery <NUM> is disconnected and damaged or when the battery voltage Vb of the in-vehicle battery <NUM> is lower than a specified value. When switching the state of the power supply control to the discharging state for backup, the electronic control unit <NUM> turns off the FET <NUM> to the FET <NUM> and turns on the FET <NUM>, the FET <NUM>, and the FET <NUM>. In the discharging state for backup, the electronic control unit <NUM> performs the PWM driving so that the FET <NUM> and the FET <NUM> are alternately turned off. Thereby, the electronic control unit <NUM> switches the connection mode of the auxiliary power source <NUM> to the parallel connection mode. When the auxiliary power source <NUM> is switched to the parallel connection mode, the FET <NUM> is turned on. Thus, when the auxiliary power source <NUM> is switched to the parallel connection mode, the first terminal T1 of the auxiliary power source <NUM> is connected to the connection point P2 and to the ground via the FET <NUM>. Since the FET <NUM> is turned on, the second terminal T2 of the auxiliary power source <NUM> is connected to the input terminal T3 of the booster circuit <NUM>. Since the FET <NUM> is turned off, the second terminal T2 of the auxiliary power source <NUM> is not connected to the connection point P1 on the second line <NUM>, and thus is not connected to the output terminal T4 of the booster circuit <NUM>. Since the FET <NUM> and the FET <NUM> are turned off, the battery voltage Vb of the in-vehicle battery <NUM> is not supplied to the motor <NUM>. Since the FET <NUM> is turned on, the output terminal T4 of the booster circuit <NUM> is connected to the motor <NUM>. Since the FET <NUM>, the FET <NUM>, and the FET <NUM> are turned on and the PWM driving is performed for the FET <NUM> and the FET <NUM>, the capacitor voltage Vc that is the discharge voltage from the auxiliary power source <NUM> is supplied to the motor <NUM> via the chopper coil <NUM>. The capacitor voltage Vc from the auxiliary power source <NUM> is input to the booster circuit <NUM> as the input voltage. The booster circuit <NUM> boosts the input capacitor voltage Vc to the first voltage V1, and outputs the boosted first voltage V1 as the output voltage. Thus, the motor <NUM> is supplied with the first voltage V1 boosted by the booster circuit <NUM>.

A procedure for determining the state of the power supply control executed by the electronic control unit <NUM> will be described with reference to a flowchart. A determination process of the state of the power supply control is repeatedly executed at predetermined cycles. As shown in <FIG>, the electronic control unit <NUM> determines whether the traveling speed VS is equal to or smaller than the traveling speed threshold VS0 (step S <NUM>).

When the traveling speed VS is equal to or smaller than the traveling speed threshold VS0 (YES in step S1), the electronic control unit <NUM> determines whether the actual current value I is equal to or larger than the current threshold I0 (step S2).

If the actual current value I is equal to or larger than the current threshold I0 (YES in step S2), the electronic control unit <NUM> switches the state of the power supply control to the discharging state for boosting (step S3). When the actual current value I is smaller than the current threshold I0 (NO in step S2), the electronic control unit <NUM> ends the process. In this case, the electronic control unit <NUM> switches the state of the power supply control to the holding state or the charging state.

When the traveling speed VS exceeds the traveling speed threshold VS0 (NO in step S <NUM>), the electronic control unit <NUM> determines whether the battery voltage Vb is equal to or smaller than the required voltage Vr (step S4).

When the battery voltage Vb is equal to or smaller than the required voltage Vr (YES in step S4), the electronic control unit <NUM> switches the state of the power supply control to the discharging state for backup (step S5).

When the battery voltage Vb exceeds the required voltage Vr (NO in step S4), the electronic control unit <NUM> ends the process. In this case, the electronic control unit <NUM> switches the state of the power supply control to the holding state or the charging state.

The determination process of the state of the power supply control executed by the electronic control unit <NUM> is thus completed. The operation and effects of the embodiment will be described.

In order to make the circuit configuration of the auxiliary power supply device <NUM> compact, the auxiliary power source <NUM> can be switched between the serial connection mode in which the motor <NUM> that is the power supply target and the in-vehicle battery <NUM> are connected in series, and the parallel connection mode in which the first terminal T1 of the auxiliary power source <NUM> is connected to the connection point P2. In addition, the serial connection mode and the parallel connection mode share the same auxiliary power source <NUM>. In other words, the shared auxiliary power source <NUM> is used for both backup and boosting. Assuming that the auxiliary power source <NUM> has a smaller power supply capacity than the in-vehicle battery <NUM> does, in the embodiment, the booster circuit <NUM> is used to boost the capacitor voltage Vc from the auxiliary power source <NUM> for backup. As a result, since all the auxiliary power sources <NUM> are in the discharging state for boosting, the power supply to the power supply target for backup is not backed up using an auxiliary power source with different charging states. For this reason, since it is possible to suppress variation in the capacitor voltage Vc, which is a discharge voltage from the auxiliary power source <NUM>, it is possible to stabilize the discharge voltage from the auxiliary power source <NUM> for backup.

The shared booster circuit <NUM> is used for both backup and for charging the auxiliary power source <NUM>. Thus, compared with the case where the booster circuit <NUM> used for charging the auxiliary power source <NUM> and the booster circuit <NUM> used for backup are separately provided in the auxiliary power supply device <NUM>, the circuit configuration of the auxiliary power supply device <NUM> can be made further compact because the configuration of the booster circuit <NUM> is shared.

Thus, the EPS <NUM> capable of stabilizing the capacitor voltage Vc from the auxiliary power source <NUM> can be achieved. In addition, the embodiment may be modified as follows. The following other embodiments can be combined with each other within a technically consistent range.

The electronic control unit <NUM> determines whether the actual current value I is equal to or larger than the current threshold I0 in step S2 of <FIG>. However, the invention is not limited thereto. For example, the electronic control unit <NUM> may make a determination based on whether the steering speed of the steering wheel <NUM> is equal to or larger than a predetermined threshold in step S2 of <FIG>.

In <FIG>, the electronic control unit <NUM> determines whether the traveling speed VS is equal to or smaller than the traveling speed threshold VS0 in step S1. However, this determination need not be made. In this case, for example, the electronic control unit <NUM> determines whether the battery voltage Vb is equal to or smaller than the required voltage Vr. When the battery voltage Vb is equal to or smaller than the required voltage Vr, the electronic control unit <NUM> switches the state of the power supply control to the discharging state for backup. In contrast, when the battery voltage Vb exceeds the required voltage Vr, the electronic control unit <NUM> determines whether the actual current value I is equal to or larger than the current threshold I0. When the actual current value I is equal to or larger than the current threshold I0, the electronic control unit <NUM> switches the state of the power supply control to the discharging state for boosting. When the battery voltage Vb exceeds the required voltage Vr and the actual current value I is smaller than the current threshold I0, the electronic control unit <NUM> ends the process. In this case, the electronic control unit <NUM> switches the state of the power supply control to the holding state or the charging state.

The current threshold I0 used in the determination process in step S2 of <FIG> may be determined in view of determining whether a load of the steering operation is large, including stationary steering. The electronic control unit <NUM> switches the state of the power supply control to the discharging state for backup when the in-vehicle battery <NUM> has failed. However, the electronic control unit <NUM> can switch the state of the power supply control to the discharging state for backup regardless of whether the in-vehicle battery <NUM> has failed.

At least one of or all of the switching elements (the FET <NUM> to the FET <NUM>) may be constituted by switching elements other than the MOS-FETs. Examples of the switching elements other than the MOS-FETs include insulated-gate bipolar transistors (IGBTs).

The motor <NUM> is not limited to a three-phase brushless motor. For example, the motor <NUM> may be a brushed motor. In the embodiment, the booster circuit <NUM> used for charging the auxiliary power source <NUM> and the booster circuit <NUM> used for backup are shared, but the invention is not limited to this. That is, the booster circuit <NUM> used for backup may be provided separately from the booster circuit <NUM> used for charging the auxiliary power source <NUM>.

In the embodiment, the current sensor <NUM> detects the actual current value I of the output port <NUM> of the auxiliary power supply device <NUM>. However, the invention is not limited to this. For example, the current sensor <NUM> may detect the actual current value I of the input port <NUM>.

In the embodiment, the auxiliary power source <NUM> of the auxiliary power supply device <NUM> includes two capacitors 24a, 24b, due to the relationship between the voltage required by the EPS <NUM> and the capacitances of the capacitors 24a and 24b. However, the number of the capacitors may be changed as appropriate. For example, the auxiliary power source <NUM> may be composed of one capacitor or three or more capacitors.

In the embodiment, the lithium ion capacitor is used as the auxiliary power source <NUM>. However, the invention is not limited to this. That is, the auxiliary power source <NUM> may be an electric double layer capacitor (EDLC), a lithium ion battery, or a lead storage battery.

In the embodiment, the EPS <NUM> is configured as an EPS in which the motor <NUM> is connected to the steering shaft <NUM> via the speed reducer <NUM>. Alternatively, the EPS <NUM> may be configured as an EPS in which the motor <NUM> is connected to the rack shaft <NUM> via the speed reducer <NUM>.

Claim 1:
An electric power steering system comprising:
a motor serving as a power supply target;
a main power source;
an auxiliary power supply device (<NUM>) provided in a power supply path extending from the main power source to the motor and including an auxiliary power source (<NUM>) and a booster circuit (<NUM>) for boosting an input voltage; and
an electronic control unit, characterized in that:
the electronic control unit is configured to
i) switch the auxiliary power source (<NUM>) between a serial connection mode and a parallel connection mode, the serial connection mode being a mode in which a first terminal (T1) is connected to the main power source and a second terminal (T2) is connected to the motor, and the auxiliary power source (<NUM>) is connected in series between the motor and the main power source, the parallel connection mode being a mode in which the first terminal (T1) is connected to a part between the main power source and the power supply target, at which the main power source and the power supply target are connected in series,
ii) switch among a charging state for charging the auxiliary power source (<NUM>), a holding state for holding a charge voltage of the auxiliary power source (<NUM>), and a discharging state for discharging power from the auxiliary power source (<NUM>), and
iii) switch the auxiliary power source (<NUM>) between the serial connection mode and the parallel connection mode,
the auxiliary power source (<NUM>) is configured to boost a voltage supplied from the main power source by a discharge voltage of the auxiliary power source (<NUM>) to supply power to the motor when the electronic control unit switches the auxiliary power source (<NUM>) to the serial connection mode; and
the auxiliary power source (<NUM>) is configured such that the second terminal (T2) of the auxiliary power source (<NUM>) is connected to an input terminal of the booster circuit (<NUM>), the discharge voltage of the auxiliary power source (<NUM>), which is an input voltage of the booster circuit (<NUM>), is boosted, and backup to supply the boosted discharge voltage to the motor is performed when the electronic control unit switches the auxiliary power source (<NUM>) to the parallel connection mode.