JP5471207B2 - Electric power steering device - Google Patents

Description

  The present invention relates to an electric power steering apparatus that applies an assist force (steering assist force) by a motor to a steering system of a vehicle such as an automobile.

  2. Description of the Related Art Conventionally, as this type of electric power steering apparatus, one that drives and controls a motor based on a vehicle speed detected through a vehicle speed sensor and a steering torque detected through a torque sensor is well known. Actually, the motor control is executed in consideration of not only the vehicle speed and the steering torque but also the steering angle detected through a rotation angle sensor such as a resolver. The driving force (rotational force) of this motor is transmitted as a steering assisting force to the steering shaft or the rack shaft via a speed reduction mechanism composed of gears or the like, thereby assisting the steering operation.

  In such an electric power steering apparatus, an in-vehicle battery is used as an operating power source. The voltage of this in-vehicle battery fluctuates due to various causes. When the voltage drops, the electronic control device of the electric power steering device is reset or the waveform of the signal generated in the rotation angle sensor is disturbed. May occur. Incidentally, the resetting of the electronic control device means that the electronic control device is forcibly restarted when the electronic control device does not operate normally.

  Therefore, a decrease in the voltage of the in-vehicle battery is detected in order to suppress a reset of the electronic control device due to a decrease in the voltage of the in-vehicle battery or a malfunction due to disturbance of the signal waveform generated in the rotation angle sensor. In the past, it has been proposed to limit the current command value that is set in accordance with the current supplied to the motor (motor current), more precisely, depending on the operating state of the steering. That is, by reducing the current drawn from the in-vehicle battery, further voltage drop of the in-vehicle battery is suppressed (see, for example, Patent Document 1).

  For example, as shown by a point A1 in the graph of FIG. 7, when steering assist is executed near the maximum value of the motor characteristics, a case where the motor current limit control accompanying the battery voltage drop is executed will be described. . In the graph, the horizontal axis represents motor current, and the vertical axis represents motor rotation speed (motor voltage). The graph also shows a motor characteristic map showing the motor characteristics (more precisely, power consumption of the motor) and a drawn current map showing the drawn current characteristics.

  In this case, as shown by the arrow X1 in the graph of FIG. 7, the motor current (pull-in current) limiting control is executed, and the motor current has a value smaller than the point A1 from the region indicated by the point A1. It is suppressed to the area indicated by the point A1 ′. As a result, the motor power (power consumption of the motor) is suppressed, and the pull-in current is reduced. This is because the drawn current depends on the power consumption of the motor. Therefore, further voltage drop of the in-vehicle battery is suppressed. Note that the current value (current command value) set in the current limiting control changes linearly according to the voltage state of the in-vehicle battery.

JP 2005-297748 A

  However, the conventional electric power steering apparatus has the following problems. That is, in the graph of FIG. 7, even when the steering assist is performed near the maximum value of the motor characteristic, the region indicated by the point A2 where the motor current value is smaller than the region indicated by the point A1 described above. When steering assist is being performed, there is a risk that it will be difficult to further suppress the voltage drop of the in-vehicle battery. That is, even if the motor current is limited as indicated by the arrow X2 due to the voltage drop of the in-vehicle battery, power is still consumed near the maximum value of the motor characteristics. Therefore, in this case, the pull-in current cannot be reduced, and as a result, it becomes difficult to further suppress the voltage drop of the in-vehicle battery.

  In the graph of FIG. 7, when steering assist is executed in the region indicated by the point A3, that is, the region where the maximum value of the motor characteristics has not been reached, the region where the pull-in current is small (that is, no large power is required). Despite this, when the voltage of the in-vehicle battery decreases, the motor current is limited. For this reason, there is a possibility that sufficient assist force may not be obtained. In other words, there is a concern that a situation where the assist area with a small required power as shown by the above point A3 cannot be used effectively will be generated.

  The present invention has been made to solve the above-described problems, and its object is to more reliably suppress a decrease in the voltage when the power supply voltage is decreased, and to suitably maintain the assistable region. An object of the present invention is to provide an electric power steering device that can perform the above-described operation.

The invention according to claim 1 assists a steering operation in the steering system by applying a power supply voltage boosted through the boost control to a motor operatively connected to the steering system of the vehicle through switching control of a plurality of switching elements. In an electric power steering apparatus comprising a control means for applying a steering assist force, a boost circuit that boosts the power supply voltage through boost control by the control means, and a first power supply path provided with an ignition switch for the vehicle, , the branch from the power supply side of the ignition switch at the first power supply path and a second power supply path connected to the booster circuit, wherein, the value of the supply voltage is specified threshold If it is determined that the drops below, to obtain the voltage value of the first and second power supply path, wherein Restriction for suppressing further reduction of the power supply voltage when the power supply voltage is lowered based on the value of the high-select voltage that is the larger voltage value of the voltage values of the first and second power supply paths The power is obtained, and based on the voltage value of the second power supply path, the maximum permissible power that is the maximum permissible value of the output power of the booster circuit is obtained, and the smaller one of the limited power and the maximum permissible power The gist is to limit the value of the boosted voltage to a value smaller than normal through the boost control of the booster circuit in order to apply the final allowable power as a value to the motor .

  According to the present invention, even when steering assist is being performed in the vicinity of the maximum value of the motor characteristics, when it is determined that the power supply voltage has decreased, the motor power is set to a value smaller than that during normal operation through the boost control of the power supply voltage. Limited to For this reason, the further fall of a power supply voltage is suppressed more reliably. Incidentally, it is conceivable to suppress the power supply voltage drop by limiting the motor current. In this case, depending on the execution range of the steering assist for the motor characteristics, the motor characteristics may be near the maximum value before and after the motor current limit. It is assumed that there is no change in consuming electricity. According to the present invention, such a situation does not occur. Further, as described above, unlike the case where the motor current is limited, the steering assist can be continuously executed in the steering assist region that does not require a large amount of electric power. Therefore, when the power supply voltage is lowered, it is possible to more reliably suppress the drop in the voltage and to suitably maintain the steering assistable region (assistable region).

In addition, as described above, the present invention includes a booster circuit that boosts the power supply voltage through the boosting control by the control means, and the control means determines that the value of the power supply voltage has fallen below a specific threshold value. If, by limiting the value of the boost voltage which is the output voltage the boost circuit to a normal value smaller than the time, that limits to a value smaller than the normal value of the power applied to the motor.

For this reason, by limiting the value of the boosted voltage, which is the power supply voltage boosted by the booster circuit, to a value smaller than normal, the value of power applied to the motor can be limited to a value smaller than the normal value. it can.

The present invention also provides a first power supply path in which an ignition switch of a vehicle is provided, and a second power that is branched from the power supply side of the ignition switch in the first power supply path and connected to the booster circuit. And the control means acquires the voltage values of the first and second power supply paths, and based on the value of the high-select voltage that is the larger voltage value of these voltage values The value of the allowable power allowed to be applied to the motor is determined as a value smaller than normal, and the value of the boosted voltage is normally set through the boost control of the booster circuit to apply the required allowable power to the motor. that limits to a value smaller than time.

According to this configuration , the allowable power value allowed to be applied to the motor is determined based on the value of the high-select voltage that is the larger voltage value of the voltage values of the first and second power supply paths. By determining, a decrease in power supply voltage is more preferably suppressed.

  According to the present invention, when the power supply voltage is lowered, it is possible to more reliably suppress the drop in the voltage and to suitably maintain the assistable region.

The schematic block diagram of an electric power steering apparatus. The block diagram which shows the electric constitution of an electric power steering device. The circuit diagram of a booster circuit. (A) is a functional block diagram of a power supply voltage drop suppression control part, (b) is a characteristic map for limiting power calculation. The flowchart which shows the procedure of an electric power restriction process. The graph which shows the restriction | limiting aspect of motor electric power. The graph which shows the restriction | limiting aspect of the conventional motor current.

  Hereinafter, an embodiment in which the present invention is embodied in a so-called column-type electric power steering device (EPS) in which a power assist unit and its control device are provided in a steering column will be described.

<Overall configuration>
First, a schematic configuration of the electric power steering apparatus will be described. As shown in FIG. 1, in the electric power steering apparatus 1, a steering shaft 3 that rotates integrally with the steering wheel 2 is connected in order of a column shaft 8, an intermediate shaft 9, and a pinion shaft 10 from the steering wheel 2 side. The pinion shaft 10 is meshed with a rack portion 5a of a rack shaft 5 provided orthogonal to the pinion shaft 10. The rotation of the steering shaft 3 accompanying the steering operation is converted into a reciprocating linear motion of the rack shaft 5 by the rack and pinion mechanism 4 including the pinion shaft 10 and the rack portion 5a. The reciprocating linear motion is transmitted to a knuckle arm (not shown) via tie rods 11 connected to both ends of the rack shaft 5, thereby changing the steering angle of the steered wheels 12.

  In addition, the electric power steering device 1 includes a steering force assist device (power assist unit) 13 that applies an assist force for assisting a steering operation to the steering system, and an electronic control device that controls the operation of the steering force assist device 13 ( ECU) 14.

  A motor 15 that is a driving source of the steering force assisting device 13 is operatively connected to the column shaft 8 via a speed reduction mechanism 16 formed of a gear or the like. The rotational force of the motor 15 is decelerated by the speed reduction mechanism 16 and this is transmitted as an assist force to the steering system, more precisely to the column shaft 8. The electronic control unit 14 controls the assist force as follows. That is, the electronic control unit 14 acquires the vehicle speed V through the vehicle speed sensor 17 provided on the steered wheels 12 and the like. Further, the electronic control unit 14 acquires the steering torque τ applied to the steering wheel 2 and the steering angle (rotation angle) θs of the steering wheel 2 through the torque sensor 18 and the steering angle sensor 19 provided on the column shaft 8. . The electronic control unit 14 calculates a target assist force based on the vehicle speed V, the steering torque τ, and the steering angle θs, and performs power supply control of the motor 15 to generate the calculated target assist force. The assist force applied to the steering system is controlled through the power supply control of the motor 15. In this example, a brushless motor is employed as the motor 15.

<Electrical configuration>
Next, the electrical configuration of the electric power steering device will be described.
As shown in FIG. 2, the electronic control unit 14 supplies a three-phase drive power to the motor 15 based on the microcomputer 31 that generates the motor control signal and the motor control signal generated by the microcomputer 31. And a drive circuit 32. In addition, the electronic control device 14 includes a booster circuit 42 that boosts the DC voltage of the battery 41 that is a vehicle-mounted power source based on a command signal (boost control signal) from the microcomputer 31 and applies the boosted voltage to the drive circuit 32. . The booster circuit 42 is provided on a power supply path 43 between the battery 41 and the drive circuit 32. The booster circuit 42 will be described in detail later.

  The drive circuit 32 is a known PWM inverter in which three arms corresponding to each phase are connected in parallel with a switching unit such as a pair of field effect transistors (FET) connected in series as a basic unit (arm). is there. The motor control signal generated by the microcomputer 31 defines the on-duty of each switching element constituting the drive circuit 32. A motor control signal is applied to the gate terminal of each switching element, and each switching element is turned on / off in response to the motor control signal, whereby the DC voltage of the battery 41 (more precisely, the voltage is boosted by the booster circuit 42). The boosted voltage) is converted into three-phase (U, V, W) driving power and supplied to the motor 15.

  The microcomputer 31 is connected to a current sensor 33 for detecting each phase current value Iu, Iv, Iw supplied to the motor 15 and a rotation angle sensor 34 for detecting the rotation angle θ (electrical angle) of the motor 15. . The microcomputer 31 determines a current command value corresponding to the target assist force based on the phase current values Iu, Iv, Iw and the rotation angle θ of the motor 15 detected through these sensors and the steering torque τ and the vehicle speed V described above. Is calculated. The microcomputer 31 performs current feedback control based on the current command value and each phase current value Iu, Iv, Iw of the motor 15 that is the actual current.

  An ignition switch 45 is provided on another power supply path 44 between the battery 41 and the electronic control unit 14. When the ignition switch 45 is turned on, the DC voltage of the battery 41 is supplied to each part of the electronic control unit 14.

<Boost circuit>
Here, the configuration of the booster circuit 42 will be described in detail. As shown in FIG. 3, the booster circuit 42 includes a booster coil 51, a rectifying capacitor 52, a booster capacitor 53, a first FET 54 and a second FET 55. The first and second FETs 54 and 55 are n-channel MOSFETs.

  On the power supply path 43 between the battery 41 and the drive circuit 32, the boosting coil 51 and the second coil 51 are connected between the input voltage application point P1 from the battery 41 and the output voltage application point P2 to the drive circuit 32. A series circuit with the FET 55 is connected. The source of the second FET 55 is connected to the boosting coil 51, and the drain is connected to the output voltage application point P2. The gate of the second FET 55 is connected to the microcomputer 31. A diode 56 is parasitic between the source and drain of the second FET 55.

  A connection point between the input voltage application point P 1 and the boosting coil 51 is grounded via a rectifying capacitor 52. A connection point P 3 between the output voltage application point P 2 and the second FET 55 is grounded via a boosting capacitor 53. The boosting capacitor 53 smoothes the boosted voltage generated by the boosting coil 51. The drain of the first FET 54 is connected to a connection point P4 between the boosting coil 51 and the second FET 55, and the source is also grounded. The gate of the first FET 54 is connected to the microcomputer 31. A diode 57 is parasitic between the source and drain of the first FET 54.

  The microcomputer 31 alternately turns on and off the first and second FETs 54 and 55 by applying a boost control signal (pulse signal) having a predetermined duty ratio to the gates of the first and second FETs 54 and 55. To do. That is, the microcomputer 31 sets the duty ratio of the boost control signal based on the voltage of the battery 41 (more precisely, the PIG voltage Vpig that is a pull-in voltage from the battery 41 described later) acquired through a voltage sensor (not shown). The microcomputer 31 controls the output voltage (boosted voltage) Vout of the booster circuit 42 by changing the on / off time of the first and second FETs 54 and 55 through setting the duty ratio of the boost control signal. The output voltage Vout increases as the duty ratio of the boost control signal increases, and decreases as the duty ratio decreases.

  The voltage at the connection point P4 is a voltage obtained by superimposing the back electromotive force generated in the boosting coil 51 on the voltage of the battery 41 (more precisely, the PIG voltage Vpig described later) when the first FET 54 is turned off. When the FET 54 is turned on, it becomes a ground potential. The voltage at the connection point P4 is transmitted to the connection point P3 when the second FET 55 is turned on. The voltage and current that change pulsatingly at the connection point P3 are smoothed by the boosting capacitor 53. In this way, the booster circuit 42 boosts the DC voltage of the battery 41 to generate the output voltage (boost voltage) Vout.

<Microcomputer>
Next, the configuration of the microcomputer 31 will be described in detail.
As shown in FIG. 2, the microcomputer 31 includes a steering angular velocity calculation unit 35, an assist control unit 36, a current control unit 37, a power supply voltage drop suppression control unit 38, and a boost control unit 39. Each part of the microcomputer 31 shows various arithmetic functions based on various control programs stored in a nonvolatile memory (not shown) of the microcomputer 31 as a block. The calculation result of each part is temporarily stored in a volatile memory (not shown). Hereinafter, an outline of arithmetic processing executed in each part of the microcomputer 31 will be described.

The steering angular velocity calculation unit 35 converts the rotation angle θ of the motor 15 acquired through the rotation angle sensor 34 into a steering angular velocity ω.
The assist control unit 36 calculates the current command value I * by adding the steering angular velocity ω to the steering torque τ and the vehicle speed V. The assist control unit 36 calculates a current command value I * having a larger absolute value as the steering torque τ (more precisely, its absolute value) is larger and as the vehicle speed V is smaller. The assist control unit 36 calculates a current command value I * based on an assist torque map stored in a storage device (not shown) of the microcomputer 31. The assist torque map is a map in which the vehicle speed V, the steering torque τ, and the target assist force are associated with each other, and is obtained in advance by experimental data using a vehicle model, a known theoretical calculation, or the like.

  The current control unit 37 has a predetermined value based on a current command value I * calculated by the assist control unit 36 and a difference value between each phase current value Iu, Iv, Iw of the motor 15 acquired through the current sensor 33. A motor control signal having a duty ratio is generated. The current control unit 37 controls the driving of the drive circuit 32 and, consequently, the supply of power to the motor 15 through the supply of the motor control signal to each switching element (more precisely, its gate terminal).

  The power supply voltage drop suppression control unit 38 is a voltage of the power supply path 43 (hereinafter referred to as “PIG voltage Vpig”), which is a drawing voltage from the battery 41, and a voltage on the load side of the ignition switch 45 in the power supply path 44 ( Hereinafter, it is referred to as “IG voltage Vig”), and whether or not the voltage of the battery 41 has decreased is determined based on these voltage values. When it is determined that the voltage of the battery 41 is decreasing, the power supply voltage decrease suppression control unit 38 limits the power supplied to the motor so as to suppress further decrease in the voltage of the battery 41. In other words, the power supply voltage drop suppression control unit 38 calculates the value of the output power of the booster circuit 42 based on the PIG voltage Vpig and the IG voltage Vig, in other words, the value of the power consumption in the motor 15. The power supply voltage drop suppression control unit 38 will be described in detail later.

  The boost control unit 39 sets the duty ratio of the boost control signal based on the steering angular velocity ω calculated through the steering angular velocity calculation unit 35 and the output power value calculated by the power supply voltage drop suppression control unit 38. The step-up control unit 39 controls the output voltage (step-up voltage) Vout and consequently the output power to the drive circuit 32 and the motor 15 through on / off control of the first and second FETs 54 and 55.

<Power supply voltage drop suppression control unit>
Next, the power supply voltage drop suppression control unit 38 will be described in detail. As shown in FIG. 4A, the power supply voltage drop suppression control unit 38 includes a first map calculation unit 61, a high selection unit 62, a second map calculation unit 63, and a minimum selection unit 64.

  The first map calculation unit 61 calculates the maximum allowable power Pmax, that is, the upper limit value of the output power Pout of the booster circuit 42, based on the acquired PIG voltage Vpig. The first map calculation unit 61 sets the maximum allowable power Pmax to a small value in accordance with the rate of decrease in the PIG voltage Vpig.

  In this example, the first map calculation unit 61 calculates a maximum allowable power Pmax corresponding to the PIG voltage Vpig with reference to a map for calculating a maximum allowable power stored in a storage device (not shown) of the microcomputer 31. The map for calculating the maximum allowable power is obtained by associating the PIG voltage Vpig with the maximum allowable power Pmax, and is obtained in advance by experimental data based on a vehicle model, a known theoretical calculation, or the like. Here, the output power Pout of the booster circuit 42 has a large correlation with the heat generation of the booster circuit 42. That is, when the output power Pout is large, there is a concern about the problem of heat generation in the booster circuit 42. In the map for calculating the maximum allowable power, the value of the output power Pout with respect to the PIG voltage Vpig is set based on the limit value of the output power Pout where the problem of heat generation of the booster circuit 42 becomes significant.

  The high select unit 62 compares the value of the captured PIG voltage Vpig with the value of the IG voltage Vig, and obtains the larger voltage value as the high select voltage value Vhi.

  The second map calculation unit 63 calculates the value of the limit power Plim based on the high select voltage value Vhi obtained by the high select unit 62. The second map calculation unit 63 sets the value of the limit power Plim to a larger value as the value of the high select voltage value Vhi is larger.

  In this example, the second map calculation unit 63 refers to a characteristic map for limiting power calculation stored in a storage device (not shown) of the microcomputer 31 to calculate the limiting power Plim corresponding to the high select voltage value Vhi. . As shown in FIG. 4B, this limit power calculation characteristic map Mlim is obtained by associating the PIG voltage Vpig with the maximum allowable power Pmax, and includes experimental data based on a vehicle model and well-known theoretical calculations. Etc. in advance. As shown in the figure, the characteristic map Mlim for limiting power calculation shows the limit power Plim as the high select voltage value Vhi increases, where the horizontal axis is the high select voltage value Vhi and the vertical axis is the limit power Plim. The value is set to gradually increase. The limit power Plim is required to suppress further reduction of the voltage when the power supply voltage (voltage of the battery 41) is reduced.

  The minimum selection unit 64 compares the value of the maximum allowable power Pmax obtained by the first map computation unit 61 with the value of the limit power Plim obtained by the second map computation unit 63, and among these, the value is smaller. This power value is calculated as the final allowable power Pfin.

  The step-up control unit 39 described above controls the value of the output voltage Vout of the step-up circuit 42 so that the value of the output power Pout of the step-up circuit 42 becomes the value of the final allowable power Pfin obtained by the minimum select unit 64. That is, switching of each switching element (FET) of the booster circuit 42 is controlled so that “Vout = Pfin / Iout”. Here, Iout is the output current of the booster circuit 42.

<Power supply voltage drop suppression control>
As described above, the voltage of the battery 41 fluctuates due to various causes. When the voltage decreases, there is a concern that a situation unfavorable for the control of the electric power steering apparatus may occur. Therefore, in this example, when it is determined that the voltage of the battery 41 has decreased, the output power Pout of the booster circuit 42 is limited so as to suppress further decrease in the voltage of the battery 41.

  Next, the processing procedure of such power supply voltage drop suppression control will be described with reference to the flowchart of FIG. The flowchart is executed in accordance with a control program stored in a nonvolatile memory (not shown) of the microcomputer 31. In this example, as the battery 41, a battery of 12V (when fully charged) is adopted.

  Now, as shown in FIG. 5, the microcomputer 31 determines whether or not the voltage of the battery 41 has decreased (step S101). That is, the microcomputer 31 determines whether or not the high select voltage value Vhi obtained by the high select unit 62 is equal to or less than the first voltage determination threshold value V1. The first voltage determination threshold value V1 is a criterion for determining whether or not the voltage of the battery 41 is decreasing, and is set to a value smaller than the voltage of the battery 41 when fully charged. In the present example, the first voltage determination threshold value V1 is set to 9.7 V (volts).

  If it is determined that the high-select voltage value Vhi is equal to or lower than the first voltage determination threshold value V1 (YES in step S101), the microcomputer 31 executes a power limit calculation (step S102) and ends the process. To do. The power limit calculation corresponds to a calculation process of the limit power Plim by the second map calculation unit 63 of the power supply voltage drop suppression control unit 38 described above.

  On the other hand, if the microcomputer 31 determines that the high select voltage value Vhi is not equal to or less than the first voltage determination threshold value V1 (NO in step S101), the microcomputer 31 proceeds to step S103.

  In step S103, the microcomputer 31 determines whether or not the voltage of the battery 41 is sufficient. That is, the microcomputer 31 determines whether or not the high select voltage value Vhi is greater than or equal to the second voltage determination threshold value V2. This second voltage determination threshold V2 is a criterion for determining whether or not the voltage of the battery 41 is sufficient, and is smaller than the voltage of the battery 41 at the time of full charge (12 V in this example), and A value larger than the first voltage determination threshold value V1 is set. In this example, the second voltage determination threshold value V2 is set to 10.0V.

  If the microcomputer 31 determines that the high-select voltage value Vhi is equal to or higher than the second voltage determination threshold value V2 (YES in step S103), the microcomputer 31 cancels the power limitation, assuming that the voltage of the battery 41 has recovered. (Step S104), the process ends. In the state where the power restriction is released, the microcomputer 31 sets the duty ratio of the boost control signal without taking into account the final allowable power Pfin calculated by the power supply voltage drop suppression control unit 38.

  In this example, the microcomputer 31 sets the maximum allowable power Pmax obtained by the first map calculation unit 61 as the final allowable power Pfin in a state where the power restriction is released. The same applies to the normal time when the voltage of the battery 41 is not lowered. In this case, the microcomputer 31 controls the booster circuit 42 so that the value of the output power Pout of the booster circuit 42 does not exceed the final allowable power Pfin (here, the maximum allowable power Pmax). (Control of voltage Vout). As a result, the booster circuit 42 is protected from heat generation and the like during normal operation.

  On the other hand, when the microcomputer 31 determines that the high select voltage value Vhi is not equal to or higher than the second voltage determination threshold value V2 (NO in step S103), the microcomputer 31 determines that the voltage of the battery 41 has not recovered. Exit. That is, the power limit is continuously executed.

Thereafter, the microcomputer 31 repeats the processes of steps S101 to S104 at a predetermined control cycle.
<Power restriction mode>
Next, the process of limiting the output power Pout of the booster circuit 42 that is executed when the voltage of the battery 41 decreases will be described in detail with reference to the graph of FIG. The graph in FIG. 6 corresponds to the graph in FIG. 7 and is basically the same. That is, in the graph of FIG. 6, the horizontal axis represents the motor current, and the vertical axis represents the motor rotation speed (motor voltage). The graph also shows a motor characteristic map (motor current map) indicating motor characteristics (more precisely, motor power consumption), a drawn current map indicating drawn current characteristics, and a power limit map. Yes. The power limit map indicated by the broken line in FIG. 6 corresponds to the value of the previous maximum allowable power Pmax.

  Here, in the case where the motor current limit control is executed when the power supply voltage is lowered, the steering assist is performed in a region (a region indicated by a point A2 in FIG. 6) in which it is difficult to suppress the battery voltage drop. Will be described. As described above, the area indicated by the point A2 is an area near the maximum value of the motor characteristics.

  When the motor power limit, that is, the output power Pout limit process of the booster circuit 42 is executed as the battery voltage decreases, the power limit map is shown in the graph as indicated by the arrow Y1 in FIG. The state changes from the state indicated by the broken line to the state indicated by the solid line. That is, by limiting the motor power, the motor power used in the area indicated by the point A2 in the graph of FIG. 6 is suppressed to the area indicated by the point A2 ′. Note that the area indicated by the point A2 ′ is an area in which the values of the motor current and the motor voltage (motor rotation speed) are smaller than the area indicated by the point A2. The power limit map after the power limit corresponds to the value of the previous limit power Plim.

  Due to the limitation of the motor power, the pull-in current map changes so that the peak value of the pull-in current is limited as indicated by the arrow Y2 in FIG. That is, the voltage drop of the battery 41 is suppressed by reducing the drawing current. Further, in the conventional motor current limitation, the motor cannot be used when the battery voltage decreases, that is, the region indicated by the point A3 in FIG. 6 (the region where the pull-in current is small and no large power is required). By limiting the electric power, the steering assist can be continued. To be precise, by limiting the motor power when the voltage of the battery 41 drops, the area A4 indicated by hatching in FIG. 6 that could not be used in the conventional motor current limiting method can also be used.

<Effect of Embodiment>
Therefore, according to the present embodiment, the following effects can be obtained.
(1) When it is determined that the voltage value of the battery 41 has fallen below a specific threshold, the microcomputer 31 controls the boosting so as to limit the value of the power applied to the motor 15 to a value smaller than normal. To do. According to this configuration, even when steering assist is being performed in the vicinity of the maximum value of the motor characteristics, when it is determined that the voltage of the battery 41 has decreased, the motor power is increased from the normal time through the boost control of the boost circuit 42. Are also limited to small values. For this reason, the further fall of the voltage of the battery 41 is suppressed more reliably. Incidentally, as described above, it is conceivable to suppress a decrease in the power supply voltage by limiting the motor current. In this case, depending on the execution range of the steering assist for the motor characteristics, the motor characteristics may be before and after the motor current is limited. It is also assumed that there is no change in consuming power near the maximum value of. According to this example, such a situation does not occur. Also, as described above, unlike the case where the motor current is limited, the steering assist can be continuously executed in the steering assist region that does not require large electric power. Therefore, when the voltage of the battery 41 decreases, the decrease in the voltage can be more reliably suppressed, and the steering assistable region (assistable region) can be suitably maintained.

  (2) When it is determined that the value of the power supply voltage has fallen below a specific threshold, the microcomputer 31 limits the value of the boosted voltage, which is the output voltage of the booster circuit 42, to a value smaller than normal. The value of the power applied to the motor 15 can be limited to a value smaller than the normal value.

  (3) The microcomputer 31 allows the allowable power to be applied to the motor 15 based on the value of the high select voltage value Vhi which is the larger voltage value of the voltage values of both the power supply paths 43 and 44. The value of (Pfin) is obtained as a smaller value than normal. The microcomputer 31 limits the value of the output voltage (boosted voltage) Vout to a value smaller than normal through the boost control of the booster circuit 42 so as to apply the required allowable power to the motor 15.

  Thus, based on the value of the high select voltage value Vhi which is the larger voltage value of the voltage values of the both power supply paths 43 and 44, the value of the allowable power allowed to be applied to the motor 15 is obtained. As a result, it is possible to more suitably suppress a decrease in the power supply voltage.

  (4) The microcomputer 31 obtains the limit power Plim (and thus the final allowable power Pfin) with reference to the characteristic map Mlim for obtaining the value of the limit power Plim in accordance with the value of the high select voltage value Vhi. Unlike the case where the value of the limit power Plim is obtained by calculation each time, the calculation load of the microcomputer 31 is reduced.

  (5) The microcomputer 31 calculates the value of the maximum allowable power Pmax that is the maximum power allowed to be applied to the booster circuit 42 based on the voltage value of the power supply path 43. Then, the microcomputer 31 uses, as a final power that is allowed to be applied to the motor 15, the smaller one of the limited power Plim obtained based on the value of the maximum allowable power Pmax and the high select voltage value Vhi. It is calculated as a certain final allowable power Pfin. Thus, the voltage drop of the battery 41 is suitably suppressed by setting the motor power to a power having a smaller value. Further, at normal times, by controlling the output voltage Vout of the booster circuit 42 so that the motor power does not exceed the maximum allowable power Pmax, the booster circuit 42 can be protected from heat generation or the like.

<Other embodiments>
The embodiment described above may be modified as follows.
The configuration of the booster circuit 42 is not limited to the configuration shown in FIG. 3, and may be changed as appropriate.

  The calculation process of the maximum allowable power Pmax may be omitted. In this case, the first map calculation unit 61 and the minimum selection unit 64 shown in FIG. 4A can be omitted. That is, the value of the limit power Plim obtained by the second map calculation unit 63 is always set as the final allowable power Pfin. Even if it does in this way, when the voltage of the battery 41 falls, it is possible to suppress the further fall.

<Other technical ideas>
Next, the technical idea that can be grasped from the above embodiment will be added below.
(A) In the electric power steering apparatus according to claim 3, the control means is an electric motor for obtaining the permissible power with reference to a characteristic map for obtaining the permissible power value according to the value of the high-select voltage. Power steering device.

  (B) In claim 3 or (a), the control means is configured to provide a maximum allowable power that is a maximum power allowed to be applied to the booster circuit based on a voltage value of the second power supply path. Electric power which calculates a value and calculates the electric power with the smaller value out of the allowable electric power determined based on the value of the maximum allowable electric power and the high select voltage as the final electric power allowed to be applied to the motor Steering device.

DESCRIPTION OF SYMBOLS 1 ... Electric power steering apparatus, 15 ... Motor, 31 ... Microcomputer (control means),
DESCRIPTION OF SYMBOLS 41 ... Ignition switch, 42 ... Booster circuit, 44 ... Power supply path (1st power supply path), 43 ... Power supply path (2nd power supply path)

Claims (1)

Control means for applying a steering assist force for assisting a steering operation to the steering system by applying a power supply voltage boosted through the boost control to a motor operatively connected to the steering system of the vehicle through switching control of a plurality of switching elements. In an electric power steering apparatus comprising:
A boosting circuit that boosts the power supply voltage through boosting control by the control means;
A first power supply path in which a vehicle ignition switch is provided;
A second power supply path branched from the power supply side of the ignition switch in the first power supply path and connected to the booster circuit;
The control means includes
When it is determined that the value of the power supply voltage has dropped below a specific threshold,
Obtaining voltage values of the first and second power supply paths;
In order to suppress a further decrease in the power supply voltage when the power supply voltage decreases based on the value of the high-select voltage that is the larger voltage value of the voltage values of the first and second power supply paths. Find the power limit of
Based on the voltage value of the second power supply path, obtain the maximum allowable power that is the maximum allowable value of the output power of the booster circuit,
In order to apply the final allowable power, which is the smaller value of the limit power and the maximum allowable power, to the motor, the boost voltage value is limited to a value smaller than normal through the boost control of the boost circuit.
Electric power steering device.

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