JP2006262669A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device which can perform the assist control of steering force while operating a motor appropriately, according to the charge state of a battery. <P>SOLUTION: A two-axis three-phase conversion means 29 generates three-phase voltage by the D-axis voltage from a D-axis PI control means 28 and the Q-axis voltage orthogonal to the axis D from the Q-axis PI control means 27, and a motor driving means 13 vector-controls a motor 8 via a PWM conversion means 30. At this time, a Q-axis voltage limiting means 32 sets the maximum value of the voltage to be applied to the axis Q, according to the voltage of a battery 19 which supplies the motor 8 with power. The Q-axis voltage limiting means 32 raises the maximum value of the Q-axis voltage when the voltage of the battery 19 is high, and supplies the motor 8 with a large Q-axis current thereby generating high torque. Moreover, when the voltage of the battery 19 is low, it lowers the maximum value of the Q-axis voltage and supplies the motor 8 with a small Q-axis current so that the battery voltage may not drop more. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an electric power steering apparatus that reduces the steering force burden on a driver by applying power of an electric motor to a steering apparatus of an automobile, and more particularly to an electric power steering apparatus that controls a steering force using a brushless motor.

  The electric power steering apparatus generates an auxiliary torque corresponding to the steering torque with an electric motor, and transmits the auxiliary torque to a steering system to reduce the steering torque. That is, an auxiliary torque corresponding to the magnitude of the steering torque is generated by the control of the control device, and this auxiliary torque is transmitted to the steering system to reduce the steering force of the steering wheel. As described above, various techniques for realizing an electric power steering device by controlling an electric motor have been reported (for example, see Patent Document 1).

  A technique for assisting steering torque by generating auxiliary torque using a three-phase brushless motor is also known. According to this technology, an arbitrary two-phase current is detected from a three-phase current of a three-phase brushless motor, and a d-axis current (field current) and a q-axis current (torque) based on the rotation angle signal of the three-phase brushless motor. The shaft current is fed back by dq conversion (3-phase 2-axis conversion). Then, the feedback d-axis current and q-axis current are individually controlled and compared with the command current, respectively, and d-axis PI control and q-axis PI control are performed, respectively, and dq reverse conversion (two-axis three-phase conversion) is performed. It is converted into a three-phase PWM signal to control the duty of the three-phase brushless motor. By using such a dq conversion method, vector control of a three-phase brushless motor can be performed by detecting an arbitrary two-phase current among the three-phase currents, so that the control system is greatly simplified (for example, , See Patent Document 2).

Furthermore, a technique for correcting the torque current by limiting the q-axis current when the value of the q-axis current is a predetermined value or more is also disclosed (for example, see Patent Document 3). According to this technique, since the q-axis current is limited so as to weaken the field at the time of quick steering in which the output torque of the electric motor (motor) is reduced, further reduction in the electric motor output torque can be reduced.
JP 2000-279000 A (see paragraph numbers 0010 to 0023, FIGS. 1 to 5) Japanese Patent Laying-Open No. 2004-40883 (see paragraph numbers 0027 to 0038, FIG. 2) Japanese Patent No. 3559258 (see paragraph numbers 0045 to 0058 and FIGS. 1 to 3)

  However, since all of the electric power steering devices disclosed in Patent Literature 1 to Patent Literature 3 are driven by using a battery as a power source, in particular, as in Patent Literature 2, a three-phase brushless motor is used as an electric motor. If this is the case, a relatively large current is consumed, so that the limited energy of the battery may be excessively consumed. However, the electric power steering device disclosed in Patent Document 2 controls the electric motor so that the current of the q-axis component becomes equal to the target value of the output torque regardless of the state of charge of the battery. Therefore, when the battery is in a charged state where it is not sufficiently charged, the battery voltage further decreases due to excessive output current to the electric motor, resulting in problems such as a decrease in light intensity of illumination lamps and room lights.

  In the electric power steering device disclosed in Patent Document 3, when the battery is in a good charge state, the q-axis current is limited when the q-axis current value is equal to or greater than a predetermined value during quick steering. Thus, the torque current can be corrected to prevent the motor output torque from decreasing. However, since the value of the q-axis current does not exceed a predetermined value when the battery voltage is lowered during quick steering, the q-axis current limiting operation is not performed, so that the control for suppressing the decrease in the motor output torque is normal. Sometimes it is not done. Furthermore, when the battery is in a charged state that is not fully charged, the battery voltage is further reduced due to excessive current from the battery to the electric power steering device, causing the system of the electric power steering device to be reset, There is also a risk that the driver may feel uncomfortable.

  The present invention has been made in view of the above problems, and provides an electric power steering apparatus capable of controlling a steering force while appropriately operating an electric motor according to a state of charge of a battery. With the goal.

  The electric power steering apparatus according to the present invention was created to achieve the above-described object, and calculates a target current based on the magnitude of the steering input, and in accordance with the calculated target current, Two-phase rotation having a control means for driving and controlling an electric motor for applying a steering force, wherein the control means uses the vector direction of the field current as the D axis and the vector direction orthogonal to the D axis as the Q axis An electric power steering apparatus that controls the electric motor by vector control represented by magnetic flux coordinates, and sets a maximum value of a voltage to be applied to the Q axis according to a voltage value of a power supply voltage that supplies electric power to the electric motor. A configuration including Q-axis voltage limiting means is employed.

  According to such a configuration, the Q-axis voltage limiting means increases the maximum value of the voltage (Q-axis voltage) applied to the Q-axis when the power supply voltage for supplying electric power to the motor, that is, the battery voltage is high, so The shaft current is supplied to the electric motor, and a high torque is generated in the electric motor. Further, when the battery voltage is low, the maximum value of the voltage (Q-axis voltage) applied to the Q-axis is lowered to supply a small Q-axis current to the motor so that the battery voltage does not further decrease. In this way, by limiting the maximum value of the Q-axis current according to the state of charge of the battery, it is possible to properly carry out power from the battery, and as a result, it is possible to maintain a good power state. .

  In addition, as the rotational speed increases, the electric motor generates a large counter electromotive force, and the rotational speed is difficult to increase. Therefore, the Q-axis voltage limiting means relaxes the limitation on the maximum value of the Q-axis voltage as the rotation speed of the motor increases, or the maximum value of the Q-axis voltage when the motor becomes higher than the predetermined rotation speed. I am trying not to restrict it.

  According to the electric power steering device of the present invention, by limiting the maximum value of the Q-axis voltage according to the state of charge of the battery that drives the electric power steering device, the electric power supplied from the battery to the electric power steering device (that is, The Q-axis current supplied from the battery to the electric motor) can be properly taken out, and as a result, the power supply state of the battery can be kept good. For example, when the battery voltage is low, the Q-axis voltage is lowered to limit the Q-axis current supplied to the motor, so that the battery voltage does not further decrease. As a result, the light intensity of the illuminating lamp and the interior lamp is not lowered during the operation of the electric power steering apparatus, and the system of the electric power steering apparatus is reset, which may cause the driver to feel uncomfortable.

<< Summary of Invention >>
The electric power steering apparatus of the present invention is provided with Q-axis voltage limiting means for constantly monitoring the battery voltage and varying the limit value (maximum value) of the Q-axis voltage that causes the motor to generate torque according to the battery voltage. For example, the Q-axis voltage limiting means increases the voltage limit to increase the limit value of the Q-axis voltage when the battery voltage increases, and decreases the voltage limit to decrease the limit value of the Q-axis voltage when the battery voltage decreases. . As a result, since the current supplied to the electric motor is limited when the battery voltage is low, there is no possibility that the brushless motor takes too much current and the battery voltage further decreases. Therefore, even if the electric power steering apparatus operates when the battery voltage is low, the light intensity of the automobile illumination lamp, the interior lamp, and the like will not decrease.

  In the electric power steering device of the present invention, the d-axis and the q-axis expressed in the conventional electric power steering device are represented by D in the electric power steering device of the present invention in order to be distinguished from the conventional electric power steering device. The axis and Q axis are expressed in capital letters to distinguish them. In the electric power steering apparatus of the present invention, other alphabets are also expressed in capital letters. However, the lowercase letters of the alphabet are used for the numerical subscripts.

<< Embodiment of the Invention >>
Embodiments of an electric power steering apparatus according to the present invention will be described below in detail with reference to the drawings. FIG. 1 is a configuration diagram of an electric power steering apparatus applied to an embodiment of the present invention. First, the overall configuration of the electric power steering apparatus 1 shown in FIG. 1 will be described. In FIG. 1, a steering shaft 4 provided integrally with a steering wheel 3 is connected to a pinion 7a of a rack and pinion mechanism 7 in a steering gear box 6 via a connecting shaft 5 having universal joints 5a and 5b. It is connected and constitutes a manual steering force generating means 2. Further, the rack teeth 7b that mesh with the pinion 7a and the rack shaft 9 that reciprocates by the meshing are connected to the left and right front wheels W and W as rolling wheels at both ends via tie rods 10 and 10, respectively. It is configured such that the direction of the vehicle can be changed by rolling the front wheels W through a & pinion type steering system. In order to reduce the steering force by the manual steering force generating means 2, an electric motor 8 for supplying an auxiliary steering force is provided, and an auxiliary torque corresponding to the steering torque is generated by the control of the control means 12, and this auxiliary torque is generated by the rack shaft 9 And the steering force of the steering wheel 3 is reduced.

  That is, the electric power steering apparatus 1 of the present embodiment is provided with a steering system S from the steering wheel 3 to the front wheels W, W, and assists the steering force by the manual steering force generation means 2. Therefore, the electric power steering apparatus 1 generates an electric motor voltage VM by the electric motor driving unit 13 based on the electric motor control signal VO from the control unit 12, and drives the electric motor 8 by the electric motor voltage VM to thereby generate an auxiliary torque (auxiliary steering force). ) To assist the manual steering force by the manual steering force generation means 2. In the present embodiment, a three-phase brushless motor is used as the electric motor 8, and DQ control for controlling the D axis (field current axis) and the Q axis (torque axis) is performed as drive control of the electric motor 8.

  In the manual steering force generating means 2, a pinion 7 a of a rack and pinion mechanism 7 provided in the steering gear box 6 is connected to a steering shaft 4 provided integrally with the steering wheel 3 via a connecting shaft 5. The connecting shaft 5 includes universal joints 5a and 5b at both ends thereof. In the rack and pinion mechanism 7, rack teeth 7b that mesh with the pinions 7a are formed on the rack shaft 9, and the rotational movement of the pinions 7a is reciprocated in the lateral direction (vehicle width direction) of the rack shaft 9 by the engagement of the pinions 7a and 7b. I am in motion. Furthermore, left and right front wheels W, W as rolling wheels are connected to the rack shaft 9 via tie rods 10, 10 at both ends thereof.

  In the electric power steering apparatus 1, the electric motor 8 is disposed coaxially with the rack shaft 9 in order to generate an auxiliary steering force (auxiliary torque). Then, the electric power steering apparatus 1 converts the rotation of the electric motor 8 into a thrust through a ball screw mechanism 11 provided coaxially with the rack shaft 9, and this thrust is applied to the rack shaft 9 (ball screw shaft 11a). ing.

  The detection means V, T, and IMO of the vehicle speed sensor VS, the steering torque sensor TS, and the motor current detection means 14 are input to the control means 12. The control means 12 determines the magnitude and direction of the motor current IM applied to the motor 8 based on these detection signals V, T, and IMO, and outputs the motor control signal VO to the motor driving means 13. Furthermore, the control means 12 determines the assist in the electric power steering device 1 based on the steering torque signal T and the electric motor current signal IMO, and controls the driving of the electric motor 8. The control means 12 includes a CPU that performs various calculations and processes, an input signal conversion means, a signal generation means, and a storage means (not shown). Incidentally, the control means 12 includes a CPU, which performs main control in the electric power steering apparatus 1.

  The vehicle speed sensor VS detects the vehicle speed as the number of pulses per unit time, and transmits an analog electrical signal corresponding to the detected number of pulses to the control means 12 as the vehicle speed signal V. The vehicle speed sensor VS may be a dedicated sensor for the electric power steering device 1 or may use a vehicle speed sensor of another system.

  The steering torque sensor TS is disposed in the steering gear box 6 and detects the magnitude and direction of the manual steering torque by the driver. Then, the steering torque sensor TS transmits an analog electrical signal corresponding to the detected steering torque to the control means 12 as a steering torque signal T. The steering torque signal T includes information on the steering torque indicating the magnitude and the torque direction indicating the direction of the torque. The torque direction is represented by a plus value / minus value of the steering torque. The plus value is the steering torque direction to the right direction, and the minus value is the steering torque direction to the left direction.

  The motor current detection means 14 is formed of, for example, a current transformer (CT) provided for each winding of the motor 8, and detects the magnitude and direction of the motor current IM that actually flows through the motor 8. The motor current detection means 14 feeds back (negative feedback) the motor current signal IMO corresponding to the motor current IM to the control means 12.

  The motor driving means 13 drives the motor 8 by applying the motor voltage VM to the motor 8 based on the motor control signal VO. For example, the motor driving means 13 applies a sine wave current to each winding of the motor 8 via a pre-drive circuit and an FET bridge in the motor driving means 13 in accordance with the duty of a PWM (Pulse Width Modulation) signal, for example. Take control.

  Next, the control means 12 of FIG. 1 will be described in detail. FIG. 2 is a block diagram showing the configuration of the control means 12 and its surroundings in the electric power steering apparatus 1 shown in FIG. The control means 12 includes a phase current detection means 21, a three-phase two-axis conversion means 22, a resolver 23, an angle calculation means 24, a motor speed calculation means 25, a field current means 26, a Q-axis PI control means 27, and a D-axis PI control. The unit 28 includes a two-axis three-phase conversion unit 29, a PWM conversion unit 30, a non-interference control unit 31, and a Q-axis voltage limiting unit 32. Further, as peripheral devices, the motor 8, the motor driving means 13, the motor current detecting means 14 having current sensors 14 a and 14 b, and a battery 19 are provided. The control means 12 operates in response to a command signal from the CPU, but the CPU is not shown in this figure.

  Such a control means 12 performs vector control of the electric motor 8 according to the command torque by vector control represented by a two-phase rotating magnetic flux coordinate system (hereinafter referred to as DQ coordinate system). That is, the steering torque applied to the steering wheel of the steering system S (see FIG. 1) is detected by the steering torque sensor TS, and the motor 8 is steered by vector control so that the assist torque corresponding to the detected steering torque is obtained. Is assisting.

  First, in a CPU (not shown), a command torque is obtained from the steering torque signal T detected and output by the steering torque sensor TS, the steering angular velocity signal, the vehicle speed signal V detected and output by the vehicle speed sensor VS, and the like. This command torque is converted into a Q-axis current command value by torque current conversion, and is input to the Q-axis PI control means 27 of the control means 12. A command signal from the CPU and a rotation speed signal MVEL from the motor speed calculation means 25 are input to the field current means 26, converted into a D-axis current command value, and input to the D-axis PI control means 28.

  Further, each phase current (for example, U phase current IU, W phase current IW) of the electric motor 8 is detected by the current sensors (CT) 14a, 14b of the electric motor current detecting means 14, and the phase current detecting means 21 at a predetermined cycle. Sampled. Each phase current (IU, IW) output from the phase current detection means 21 is input to the three-phase biaxial conversion means 22. On the other hand, the rotational position of the electric motor 8 detected by the resolver 23 is converted into an angle signal ANGLE by the angle calculation unit 24, and the angle signal is input to the three-phase two-axis conversion unit 22. The three-phase biaxial conversion means 22 performs DQ conversion on each phase current (IU, IW) based on the input angle signal, and outputs a D-axis current (ID) and a Q-axis current (IQ).

  Next, the Q-axis PI control means 27 performs a P (Proportional) control process and I ((proportional) control processing based on the Q-axis current command value input from the CPU and the Q-axis current (IQ) input by the feedback system. Integral (integral) control processing is executed, and as a result, a command voltage VQ for the Q axis is generated, and this command voltage VQ is input to the two-axis three-phase conversion means 29. Further, the D-axis PI control means 28 performs P (proportional) control processing and I based on the D-axis current command value input from the field current means 26 and the D-axis current (ID) input by the feedback system. (Integration) control processing is executed, and as a result, a command voltage VD for the D axis is generated, and this command voltage VD is input to the biaxial three-phase conversion means 29.

  Then, the two-axis three-phase conversion means 29 performs DQ inverse conversion on these command voltages VQ and VD to convert the command voltages VU, VV and VW for the U, V and W phases of the electric motor 8. To do. These command voltages VU, VV, and VW are converted into PWM duty signals by the PWM conversion means 30, and this PWM duty signal controls a pre-driving circuit and an FET bridge circuit (not shown) in the motor driving means 13. As a result, a PWM-controlled sinusoidal current is applied to each phase winding of the electric motor 8 such as a brushless motor, and predetermined vector control is performed on the electric motor 8.

  It should be noted that the non-interference control means 31 has one control input when there is mutual interference between a plurality of control inputs and a plurality of control amounts as in the Q-axis PI control means 27 and the D-axis PI control means 28. It works to eliminate mutual interference so that the influence reaches only one control amount. In the example of FIG. 2, the non-interference control means 31 is used to reduce the feedback loop of the motor angular velocity and the motor current (that is, increase the response speed of the feedback loop).

  Incidentally, as shown in FIG. 2, the electric power steering apparatus 1 of the present embodiment is provided with the Q-axis voltage limiting means 32 at the subsequent stage of the Q-axis PI control means 27. The Q-axis voltage limiting means 32 constantly monitors the voltage of the battery 19 and varies the limit value of the Q-axis voltage that causes the electric motor 8 to generate torque according to the voltage of the battery 19. That is, the Q-axis voltage limiting unit 32 increases the voltage limit so that the upper limit value of the Q-axis voltage output from the Q-axis PI control unit 27 is increased when the voltage of the battery 19 increases. The Q-axis voltage limiting means 32 lowers the voltage limit so that the upper limit value of the Q-axis voltage output from the Q-axis PI control means 27 is lowered when the voltage of the battery 19 decreases.

  FIG. 3 is a control characteristic diagram of the Q-axis voltage limiting means 32 in the electric power steering apparatus shown in FIG. 2, where the horizontal axis represents the motor rotation speed (rpm), and the vertical axis represents the ratio (%) to the rating of the Q-axis voltage. ). With the battery voltage as a parameter, the chain line (1) represents the characteristics when the battery voltage is high, the solid line (2) represents the characteristics when the battery voltage is rated, and the broken line (3) represents the characteristics when the battery voltage is low.

  When the battery voltage of the solid line (2) is rated, the Q-axis voltage increases in proportion to the increase in the motor rotation speed. However, when the predetermined rotation speed N1 is reached, the Q-axis voltage limiting means 32 determines the maximum Q-axis voltage. Suppress the value. Accordingly, it is possible to suppress a large current from being output from the battery 19 to the electric motor 8 when the electric motor 8 rotates at a high speed.

  When the battery voltage of the chain line (1) is high, the Q-axis voltage increases in proportion to the increase in the motor rotation speed, but when the predetermined rotation speed N1 is reached, the Q-axis voltage limiting means 32 causes the maximum value of the Q-axis voltage. Suppress. At this time, for example, if the battery voltage is 10% higher than the rating, the Q-axis voltage limiting means 32 sets the voltage limit at a Q-axis voltage upper limit value that is 10% higher than the Q-axis voltage upper limit value when the battery voltage is rated. As a result, when the battery voltage is high, a large Q-axis current corresponding to the high Q-axis voltage can be supplied to the electric motor 8 to generate a large torque.

  When the battery voltage indicated by the broken line (3) is low, the Q-axis voltage increases in proportion to the increase in the motor rotation speed. However, when the predetermined rotation speed N1 is reached, the Q-axis voltage limiting means 32 determines the maximum value of the Q-axis voltage. Suppress. At this time, for example, if the battery voltage is 10% lower than the rating, the Q-axis voltage limiting means 32 sets the voltage limit at the Q-axis voltage upper limit that is 10% lower than the Q-axis voltage upper limit when the battery voltage is rated. As a result, when the battery voltage is low, it is possible to prevent a further decrease in the battery voltage by preventing a large current from being supplied to the electric motor 8.

  That is, when the voltage of the battery 19 is high, the upper limit value of the Q-axis voltage becomes high, so that a large Q-axis current is supplied to the electric motor 8 according to the voltage, and as a result, the electric motor 8 can generate a large torque. . In addition, when the voltage of the battery 19 is low, the upper limit value of the Q-axis voltage is low, so that a small Q-axis current is supplied to the electric motor 8 according to the voltage. As a result, there is no possibility that the electric motor 8 takes too much current and the voltage of the battery 19 further decreases.

  FIG. 4 is a control characteristic diagram of another form of the Q-axis voltage limiting means 32 in the electric power steering apparatus shown in FIG. 2, wherein the horizontal axis represents the motor rotation speed (rpm), and the vertical axis represents the Q-axis voltage rating. The ratio (%) is expressed. That is, as shown in the control characteristics of FIG. 3, the Q-axis voltage limiting means 32 limits the increase of the Q-axis voltage when the rotational speed of the motor 8 increases to N1, but the control characteristics of FIG. As shown, even if the rotational speed of the electric motor 8 is increased, it is possible to prevent the Q-axis voltage from being increased. However, the Q-axis voltage limiting means 32 varies the Q-axis voltage increase characteristic according to the value of the battery voltage.

  For example, the Q-axis voltage limiting means 32 has a Q-axis voltage increase characteristic when the battery voltage of the chain line (1) is high, as opposed to the Q-axis voltage increase characteristic when the battery voltage of the solid line (2) is rated. When the battery voltage shown by the broken line (3) is low, the Q-axis voltage rise characteristic can be controlled to be 10% lower. Thus, since the Q-axis current corresponding to the battery voltage is supplied to the electric motor 8, there is no possibility of unnecessarily reducing the battery voltage.

  The electric motor 8 has a characteristic that a large counter electromotive force is generated as the rotational speed is increased, and the rotational speed is hardly increased. Therefore, the Q-axis voltage limiting means 32 relaxes the limitation on the maximum value of the Q-axis voltage as the rotational speed of the electric motor 8 increases. That is, the upper limit voltage of the Q axis voltage is gradually increased as the rotational speed of the electric motor 8 increases. Alternatively, the maximum value of the Q-axis voltage is not limited when the electric motor 8 rotates at a higher speed than a predetermined rotational speed. Thereby, the rotational speed of the electric motor 8 can be smoothly increased to a desired rotational speed even in the high rotational speed range.

  FIG. 5 is a flowchart showing the flow of determining the limit of the Q axis voltage performed by the Q axis voltage limiting means 32 of the electric power steering apparatus 1 shown in FIG. The Q-axis voltage limiting means 32 determines whether or not the battery voltage is 12 V or higher in FIG. 5 (step S1). Here, if the battery voltage is 12V or more (Yes in step S1), there is no need to limit the upper limit value of the Q-axis voltage, so the limit offset (that is, the limit level of the upper limit value of the Q-axis voltage is lowered). The margin is set to 0 (step S2). That is, if the battery voltage is 12 V or more, the Q-axis voltage limiting means 32 is in the best charged state, so the upper limit value of the Q-axis voltage is brought to the highest level and a large Q-axis current is supplied to the motor 8. To generate large torque.

On the other hand, when the battery voltage is not 12 V or higher (in the case of No in step S1), it is determined whether or not the battery voltage is 10 V or lower (step S3). If the battery voltage is 10 V or less (Yes in step S3), the limit offset is maximized. In other words, the margin for lowering the limit level of the upper limit value is maximized and the upper limit level of the Q-axis voltage is lowered to the minimum (step S4). That is, if the battery voltage is 10V or less, the Q-axis voltage limiting means 32 is in the worst state of charge, so the upper limit value of the Q-axis voltage is brought to the lowest level, and the Q-axis current that the battery can withstand is 8 to prevent further reduction of the battery voltage. Further, since the required torque tends to decrease as the vehicle speed increases, for example, when the battery voltage is low (when the upper limit value of the Q-axis voltage is the lowest level), the upper limit of the Q-axis voltage is increased as the vehicle speed increases. Processing for lowering the upper limit value further than the maximum value level may be performed to further prevent the battery voltage from decreasing.
Conversely, when the battery voltage is high, as the vehicle speed decreases, the upper limit value is further increased from the upper limit value maximum level of the Q-axis voltage, and a large torque such as garage entry is required. It may be possible to cope.

  Next, if the battery voltage is not less than 10V in step S3 (Yes in step S3), the battery voltage is in the range of 10V to 12V, so the limit offset is set to the gain of the Q-axis voltage limiting means × (12V−current Battery voltage) (step S5). For example, if the battery voltage is 10.5V, the limit offset is set to gain × 1.5V. That is, when the battery voltage is between 10V and 12V, the upper limit value of the Q-axis voltage is lowered by a limit offset corresponding to the differential voltage between 12V and the current voltage. Next, when the limit offset corresponding to the battery voltage is determined in each flow described above, the reference value (base) of the Q-axis voltage is determined according to the rotation speed of the electric motor (step S6).

  That is, when the battery voltage is 12 V or higher, the maximum Q-axis current is supplied to the electric motor 8 without applying an upper limit to the Q-axis voltage. When the battery voltage is in the range of 12V to 10V, the value obtained by multiplying the difference voltage between 12V and the current battery voltage by the gain is set as the limit offset, and the upper limit value of the Q-axis voltage is lowered to the limit offset level. The Q-axis current is supplied to the electric motor 8. In other words, when the battery voltage is in the range of 12V to 10V, if the battery voltage increases, the upper limit value of the Q-axis voltage is increased to increase the Q-axis current, and if the battery voltage decreases, the upper limit value of the Q-axis voltage is decreased. Reduce the Q-axis current. Further, when the battery voltage is 10 V or less, the upper limit value of the Q-axis voltage is reduced to the maximum and the Q-axis current is supplied to the electric motor 8. This can prevent the battery voltage from further decreasing. The present invention can also be applied to a steer-by-wire (Steer # By # Wire) in which the steering wheel 3 and the front wheel (rolling wheel) W in the steering system S are mechanically separated.

It is a lineblock diagram of an electric power steering device applied to an embodiment of the invention. It is a block diagram which shows the control apparatus in the electric power steering apparatus shown in FIG. 1, and the structure of the periphery. FIG. 3 is a control characteristic diagram of Q-axis voltage limiting means in the electric power steering apparatus shown in FIG. 2. It is a control characteristic figure of another form of the Q-axis voltage limiting means in the electric power steering device shown in FIG. 3 is a flowchart showing a flow of determining a limit of a Q-axis voltage performed by a Q-axis voltage limiting unit in the electric power steering apparatus shown in FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Electric power steering apparatus 2 ... Manual steering force generation means 3 ... Steering wheel 4 ... Steering shaft 5 ... Connection shaft 5a, 5b ... Universal joint 6 ... Steering gear box 7 ... Rack & pinion mechanism 7a ... Pinion 8 ... Electric motor 9 ... rack shaft 10 ... tie rod 11 ... ball screw mechanism 12 ... control means 13 ... motor drive means 14 ... motor current detection means 14a, 14b ... current sensor 15 ... motor voltage detection means 19 ... battery 21 ... phase current detection means 22 ... 3 Phase 2-axis conversion means 23 ... resolver 24 ... angle calculation means 25 ... motor speed calculation means 26 ... field current means 27 ... Q-axis PI control means 28 ... D-axis PI control means 29 ... 2-axis three-phase conversion means 30 ... PWM Conversion means 31 ... non-interference control means 32 ... Q-axis voltage limiting means S ... steering system TS ... steering torque sensor VS ... car Sensor front wheel W ...

Claims (3)

A control unit that calculates a target current based on the magnitude of the steering input, and drives and controls an electric motor that applies a steering force to the steering system according to the calculated target current;
Electric power for controlling the electric motor by vector control represented by two-phase rotating magnetic flux coordinates with the vector direction orthogonal to the D axis as the Q axis when the control means uses the vector direction of the field current as the D axis. A steering device,
An electric power steering apparatus comprising: Q-axis voltage limiting means for setting a maximum value of a voltage to be applied to the Q-axis according to a voltage value of a power supply voltage that supplies electric power to the electric motor.   2. The electric power steering apparatus according to claim 1, wherein the Q-axis voltage limiting unit relaxes a limitation on a maximum value of a voltage applied to the Q-axis as the rotation speed of the electric motor increases.   3. The Q-axis voltage limiting means does not limit the maximum value of the voltage applied to the Q-axis when the electric motor rotates at a higher speed than a predetermined rotation speed. The electric power steering device described in 1.

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