Electric power steering device

A microcomputer calculates an internal resistance value of a motor, and subsequently calculates an internal resistance value of a switching element of a motor drive circuit. When the internal resistance value of the motor is a semi-abnormal value, the microcomputer sets an upper limit current. When the internal resistance value of the motor drive circuit is a semi-abnormal value, the microcomputer sets an upper limit current. The microcomputer sets a smaller one of the upper limit current and the upper limit current as an upper limit current of the motor. In this manner, the progress of degradation of the motor and the motor drive circuit can be suppressed.

TECHNICAL FIELD

The present invention relates to an electric power steering device for generating a steering assist torque by driving a motor based on a driver's steering operation.

BACKGROUND ART

An electric power steering device includes a motor for generating a steering assist torque that assists a steering operation of a steering wheel, and an electronic control unit (hereinafter referred to as “ECU”) for controlling the energization of the motor. The ECU includes a motor drive circuit formed of switching elements, a microcomputer for calculating a control amount of the motor so as to generate a target assist torque corresponding to a steering torque, and a switch drive circuit for outputting a PWM-controlled gate signal to the switching elements of the motor drive circuit in accordance with the control amount calculated by the microcomputer.

This kind of electric power steering device prevents the motor and the motor drive circuit from being damaged by heat generation in a manner that detects temperatures thereof and limits a current to be supplied to the motor when the detected temperature exceeds a threshold set for overheat prevention. In an electric power steering device proposed in Patent Literature 1, the temperature of the switch drive circuit is detected as well, and the current to be supplied to the motor is limited when any one of the temperatures detected from the motor drive circuit and the switch drive circuit exceeds a threshold set for each circuit.

CITATION LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

In the conventional electric power steering device, however, although the heat-generating states of the motor and the motor drive circuit can be detected, it cannot be determined whether or not the heat generation is caused by increased internal resistances of the motor and the motor drive circuit. The current to be supplied to the motor is limited based merely on the phenomenon of the temperature increase. For example, the amount of heat generation increases when a contact resistance of a brush portion of the motor or an internal resistance of the switching element of the motor drive circuit increases, but the heat generation in this case is not discriminated from heat generation occurring when the internal resistance is normal.

Accordingly, the steering assist suddenly stops when the internal resistance of the motor or the motor drive circuit has increased to cause an abnormality (for example, the breakdown of the switching element). This is because the conventional electric power steering device takes no consideration of control in a semi-abnormal state of the internal resistances of the motor and the motor drive circuit, which is an intermediate process in which the internal resistances progress from a normal state to an abnormal state.

It is an object of the present invention to deal with the above-mentioned problem by detecting the semi-abnormal states of the motor and the motor drive circuit to perform steering assist control suitable for the semi-abnormal state.

In order to achieve the above-mentioned object, a feature of the present invention is to provide an electric power steering device, including: steering torque detection means (21) for detecting a steering torque input from a steering wheel; a motor (20) provided in a steering mechanism; a motor drive circuit (40) including a switching element for controlling energization of the motor; control amount calculation means (60) for calculating a control amount of the motor based on the steering torque detected by the steering torque detection means; switch control means (80) for controlling the switching element of the motor drive circuit in accordance with the control amount calculated by the control amount calculation means, thereby generating a steering assist torque from the motor; semi-abnormal state detection means (S40, S42) for detecting a semi-abnormal state in which an internal resistance value of an energization path through which a current flows to the motor via the motor drive circuit exceeds a normal range and is out of an abnormal range in which the motor is intended to be stopped; and semi-abnormal occasion control amount limiting means (S41to S46) for limiting, when the semi-abnormal state is detected, the control amount of the motor so that the energization of the motor is limited as compared to a case where the internal resistance value falls within the normal range.

According to the present invention, the steering torque detection means detects the steering torque input from the steering wheel. The control amount calculation means calculates the control amount of the motor based on the steering torque. For example, the control amount calculation means calculates a target current based on the steering torque, and calculates a voltage control value to be applied to the motor so that the target current may flow through the motor. The switch control means controls the switching element of the motor drive circuit in accordance with the control amount calculated by the control amount calculation means. For example, the switch control means outputs a PWM control signal to the switching element of the motor drive circuit to drive the switching element. In this manner, a current flows through the motor, and a steering assist torque is generated from the motor.

If the internal resistance (electric resistance) of the motor or the motor drive circuit increases, the amount of heat generation may increase to break down a component. If the component is broken down, the motor can no longer generate the steering assist torque. To deal with this problem, the present invention includes the semi-abnormal state detection means and the semi-abnormal occasion control amount limiting means.

The semi-abnormal state detection means detects the semi-abnormal state in which the internal resistance value of the energization path through which a current flows to the motor via the motor drive circuit exceeds the normal range and is out of the abnormal range in which the motor is intended to be stopped. The energization path means a circuit through which a current flows, including the motor drive circuit and the motor. The semi-abnormal occasion control amount limiting means limits, when the semi-abnormal state is detected, the control amount of the motor so that the energization of the motor is limited as compared to the case where the internal resistance value falls within the normal range. Therefore, the heat generation of the motor and the motor drive circuit is suppressed. In this manner, the progress of degradation of the motor and the motor drive circuit can be suppressed to prolong the lifetime of the electric power steering device. Even if a failure has occurred in the motor or the motor drive circuit, the energization of the motor has already been limited before the occurrence of the failure, which can suppress a trouble that the steering assist suddenly stops to cause an abrupt change in steering force.

Another feature of the present invention is to provide an electric power steering device further including internal resistance value measurement means (S20, S30) for measuring an internal resistance value of the motor and an internal resistance value of the motor drive circuit, in which the semi-abnormal state detection means (S40, S42) is configured to detect a state in which at least one of the internal resistance value of the motor and the internal resistance value of the motor drive circuit, which are measured by the internal resistance value measurement means, becomes a semi-abnormal value that exceeds the normal range and is out of the abnormal range.

According to the present invention, the internal resistance value measurement means measures the internal resistance value of the motor and the internal resistance value of the motor drive circuit. The semi-abnormal state detection means detects, as the semi-abnormal state, the state in which at least one of the internal resistance value of the motor and the internal resistance value of the motor drive circuit, which are measured by the internal resistance value measurement means, becomes the semi-abnormal value that exceeds the normal range and is out of the abnormal range. For example, the semi-abnormal state detection means stores a first threshold that sets a maximum value of the internal resistance value in the normal range, and a second threshold that sets a minimum value of the internal resistance value in the abnormal range, and determines that the internal resistance value is in the semi-abnormal state when the measured internal resistance value is a semi-abnormal value between the first threshold and the second threshold. Therefore, the semi-abnormal state can be detected before the components of the motor and the motor drive circuit are damaged.

Another feature of the present invention is to provide an electric power steering device further including limiting degree independently setting means (S41, S43, S84, S85, S88, S89, S91, and S93) for setting a limiting degree of the energization of the motor independently for a case where the internal resistance value of the motor becomes the semi-abnormal value and a case where the internal resistance value of the motor drive circuit becomes the semi-abnormal value.

The energization limiting degree appropriate for the motor in the case where the semi-abnormal state of the motor is detected and the energization limiting degree appropriate for the motor in the case where the semi-abnormal state of the motor drive circuit is detected do not always match with each other. According to the present invention, the case where the internal resistance value of the motor becomes a semi-abnormal value and the case where the internal resistance value of the motor drive circuit becomes a semi-abnormal value can be discriminated based on the internal resistance value of the motor and the internal resistance value of the motor drive circuit. To achieve this discrimination, the limiting degree independently setting means sets the limiting degree of the energization of the motor independently for the case where the internal resistance value of the motor becomes the semi-abnormal value and the case where the internal resistance value of the motor drive circuit becomes the semi-abnormal value. In this manner, the energization of the motor can be limited appropriately for the semi-abnormal region.

Another feature of the present invention resides in that the motor is a brush DC motor, that the motor drive circuit is an H bridge circuit including: a forward rotation energization path in which a current flows when the motor is driven in a forward rotation direction; and a backward rotation energization path in which a current flows when the motor is driven in a backward rotation direction, and that the internal resistance value measurement means is configured to measure an internal resistance value of a forward rotation switching element (Q1, Q4) provided in the forward rotation energization path, an internal resistance value of a backward rotation switching element (Q2, Q3) provided in the backward rotation energization path, and the internal resistance value of the motor.

According to the present invention, the brush DC motor is driven by the H bridge circuit to generate a steering assist torque. In the case of the brush DC motor, if the contact state between a brush and a commutator segment is poor, the contact resistance increases to increase the amount of heat generation in this region, and the degradation progresses. To deal with this problem, according to the present invention, the internal resistance value measurement means measures the internal resistance value of the forward rotation switching element of the H bridge circuit, the internal resistance value of the backward rotation switching element of the H bridge circuit, and the internal resistance value of the motor. In this manner, the degradation of a brush portion of the motor and the degradation of the switching elements of the motor drive circuit can be properly detected.

Another feature of the present invention resides in that the internal resistance value measurement means (S30) is configured to alternately turn ON the forward rotation switching element and the backward rotation switching element, to measure the internal resistance value of the forward rotation switching element when the forward rotation switching element is turned ON, and measure the internal resistance value of the backward rotation switching element when the backward rotation switching element is turned ON.

When the forward rotation switching element is turned ON, the motor is driven in the forward rotation direction. When the backward rotation switching element is turned ON, the motor is driven in the backward rotation direction. It is necessary to energize the motor in order to measure the internal resistance value, but when the motor rotates, the steering wheel rotates as well. To deal with this problem, according to the present invention, the internal resistance value measurement means alternately turns ON the forward rotation switching element and the backward rotation switching element, to measure the internal resistance value of the forward rotation switching element when the forward rotation switching element is turned ON, and measure the internal resistance value of the backward rotation switching element when the backward rotation switching element is turned ON. In this manner, the steering wheel can be prevented from being rotated. Note that, the cycle of alternately turning ON the forward rotation switching element and the backward rotation switching element is set short enough not to rotate the steering wheel.

Another feature of the present invention resides in that the semi-abnormal occasion control amount limiting means (S41, S43) is configured to set an upper limit value of the current to be supplied to the motor so that the upper limit value becomes smaller as the internal resistance value measured by the internal resistance value measurement means becomes larger.

According to the present invention, the upper limit value of the current to be supplied to the motor is set to be smaller as the internal resistance value of the motor or the motor drive circuit becomes larger. Thus, the heat generation in the semi-abnormal region can be appropriately suppressed to suppress the progress of degradation.

Another feature of the present invention resides in that the semi-abnormal occasion control amount limiting means (S91, S93) is configured to set an upper limit value of a voltage to be applied to the motor so that the upper limit value becomes smaller as the internal resistance value measured by the internal resistance value measurement means becomes larger.

According to the present invention, the upper limit value of the voltage to be applied to the motor is set to be smaller as the internal resistance value of the motor or the motor drive circuit becomes larger. Thus, the heat generation in the semi-abnormal region can be appropriately suppressed to suppress the progress of degradation.

Another feature of the present invention resides in that the semi-abnormal occasion control amount limiting means (S63, S84, S88) is configured to set an upper limit value of the current to be supplied to the motor so that the upper limit value gradually decreases with time in a case where the semi-abnormal state detection means detects the semi-abnormal state.

According to the present invention, in the case where the semi-abnormal state of the motor or the motor drive circuit is detected, the upper limit value of the current to be supplied to the motor is set to gradually decrease with time. Thus, the heat generation in the semi-abnormal region can be appropriately suppressed to suppress the progress of degradation.

Another feature of the present invention resides in that the semi-abnormal occasion control amount limiting means is configured to set an upper limit value of a voltage to be applied to the motor so that the upper limit value gradually decreases with time in a case where the semi-abnormal state detection means detects the semi-abnormal state.

According to the present invention, in the case where the semi-abnormal state of the motor or the motor drive circuit is detected, the upper limit value of the voltage to be applied to the motor is set to gradually decrease with time. Thus, the heat generation in the semi-abnormal region can be appropriately suppressed to suppress the progress of degradation.

Another feature of the present invention resides in that the electric power steering device further includes time limiting means (S64, S65) for inhibiting the energization of the motor when an elapsed time from the detection of the semi-abnormal state by the semi-abnormal state detection means reaches a preset stop time.

According to the present invention, in the case where the semi-abnormal state of the motor or the motor drive circuit is detected, the energization of the motor is inhibited when the elapsed time from the detection of the semi-abnormal state reaches the preset stop time. Therefore, even when the degradation of the motor or the motor drive circuit progresses, the steering assist can be appropriately stopped.

Another feature of the present invention resides in that the electric power steering device further includes stop time setting means (S85, S89) for setting the stop time independently for a case where the internal resistance value of the motor becomes the semi-abnormal value and a case where the internal resistance value of the motor drive circuit becomes the semi-abnormal value.

The remaining lifetime from the detection of the semi-abnormal state of the motor until the motor reaches the abnormal state and the remaining lifetime from the detection of the semi-abnormal state of the motor drive circuit until the motor drive circuit reaches the abnormal state do not always match with each other. In view of this, according to the present invention, the stop time setting means sets the stop time independently for the case where the internal resistance value of the motor becomes the semi-abnormal value and the case where the internal resistance value of the motor drive circuit becomes the semi-abnormal value. Therefore, both in the case where the motor becomes the semi-abnormal state and in the case where the motor drive circuit becomes the semi-abnormal state, an appropriate stop time corresponding to each remaining lifetime can be set. In this manner, the lifetime of the electric power steering device can be appropriately prolonged.

Another feature of the present invention resides in that the internal resistance value measurement means is configured to start measuring the internal resistance value based on a door open/close signal indicating an open/close state of a vehicle door or a seating signal indicating a seating state of a driver on a driver's seat (S11).

Measuring the internal resistance value of the motor or the motor drive circuit needs to energize the motor. However, it is not preferred that the driver be aware of this operation of the motor. To deal with this problem, according to the present invention, the measurement of the internal resistance value is started based on the door open/close signal indicating the open/close state of the vehicle door or the seating signal indicating the seating state of the driver on the driver's seat. For example, the internal resistance value is measured immediately after the vehicle door is open or immediately after the driver is seated on the driver's seat. In this manner, the driver is prevented from being aware of the operation of the motor relating to the measurement of the internal resistance value.

Another feature of the present invention resides in that the internal resistance value measurement means (S30) is configured to determine an internal resistance value of the switching element by calculation based on a power supply voltage to be applied to the motor drive circuit, a motor current flowing through the motor, and respective terminal voltages of the motor or an inter-terminal voltage of the motor.

According to the present invention, the internal resistance value of the switching element is determined by calculation based on the power supply voltage to be applied to the motor drive circuit, the motor current, and the respective terminal voltages of the motor or the inter-terminal voltage of the motor. For example, the power supply voltage, the motor current, and the respective terminal voltages of the motor or the inter-terminal voltage of the motor are detected, and the detected values are used to calculate the internal resistance value of the switching element. In this case, if the power supply voltage is known, the known value may be used. Therefore, the internal resistance value of the switching element can be measured easily.

Another feature of the present invention resides in that the internal resistance value measurement means (S20) is configured to determine the internal resistance value of the motor by calculation based on a motor current flowing through the motor and an inter-terminal voltage of the motor.

According to the present invention, the internal resistance value of the motor is determined by calculation based on the motor current and the inter-terminal voltage of the motor. For example, the motor current and the inter-terminal voltage of the motor are detected, and the detected values are used to calculate the internal resistance value of the motor. In this case, the inter-terminal voltage of the motor may be detected based on a voltage difference obtained by detecting respective terminal voltages of the motor, or the inter-terminal voltage of the motor may be detected directly. Therefore, the internal resistance value of the motor can be measured easily.

For facilitating the understanding of the invention, in the above description, the configurations of the invention corresponding to the embodiment are suffixed in parentheses with symbols used in the embodiment. However, the components of the invention are not intended to be limited to the embodiment as defined by the symbols.

DESCRIPTION OF EMBODIMENTS

Now, an electric power steering device according to an embodiment of the present invention is described below with reference to the accompanying drawings.FIG. 1illustrates a schematic configuration of a vehicle electric power steering device1according to the embodiment of the present invention.

The electric power steering device1includes, as main components, a steering mechanism10for steering a steered wheel by a steering operation of a steering wheel11, a motor20that is incorporated in the steering mechanism10to generate a steering assist torque, and an electronic control unit100for controlling the operation of the motor20in accordance with an operating state of the steering wheel11. The electronic control unit100is hereinafter referred to as “assist ECU100”.

The steering mechanism10is a mechanism for steering front-left and front-right wheels FW1and FW2by a turning operation of the steering wheel11, and includes a steering shaft12that is connected to the steering wheel11so as to be integrally rotatable, with its upper end facing the steering wheel11. A pinion gear13is connected to a lower end of the steering shaft12so as to be integrally rotatable. The pinion gear13is engaged with a gear portion14aformed in a rack bar14, thereby constituting a rack-and-pinion mechanism together with the rack bar14.

The gear portion14aof the rack bar14is housed in a rack housing16, and both left and right ends of the rack bar14are exposed from the rack housing16to be coupled to tie rods17. A stopper18constituting a stroke end is formed at each coupling portion of the rack bar14with the tie rod17. The horizontal stoke of the rack bar14is mechanically regulated by abutment between the stopper18and an end portion of the rack housing16. The other ends of the left and right tie rods17are connected to knuckles19provided to the front-left and front-right wheels FW1and FW2, respectively. With this configuration, the front-left and front-right wheels FW1and FW2are steered to the left or right in accordance with a displacement of the rack bar14in the direction of its axis along with the rotation of the steering shaft12about the direction of its axis.

The steering shaft12is incorporated with the motor20via a reduction gear25. The motor20rotates to drive the steering shaft12to rotate about the center of the shaft via the reduction gear25, to thereby apply an assist force to the turning operation of the steering wheel11. The motor20is a brush DC motor.

The steering shaft12is incorporated with a steering torque sensor21at an intermediate position between the steering wheel11and the reduction gear25. For example, the steering torque sensor21detects a torsion angle of a torsion bar (not shown) that is interposed at an intermediate portion of the steering shaft12with the use of a resolver or the like, and detects a steering torque tr acting on the steering shaft12based on the torsion angle. The operating direction of the steering wheel11is identified based on whether the steering torque tr is a positive value or a negative value. For example, the positive value indicates a steering torque tr when the steering wheel11is steered in the left direction, and the negative value indicates a steering torque tr when the steering wheel11is steered in the right direction. Note that, the torsion angle of the torsion bar is detected by the resolver in the embodiment of the present invention, but may be detected by another rotation angle sensor such as an encoder.

Next, the assist ECU100is described with reference toFIG. 2. The assist ECU100includes an electronic control circuit50for calculating a target control amount of the motor20and outputting a switch drive signal corresponding to the calculated target control amount, and a motor drive circuit40for energizing the motor20in accordance with the switch drive signal output from the electronic control circuit50.

The electronic control circuit50includes a microcomputer60formed of a CPU, a ROM, a RAM, or the like, an input interface70for inputting various kinds of sensor signals and converting the input signals into signals readable by the microcomputer60, and a switch drive circuit80for amplifying a switch control signal output from the microcomputer60and supplying the amplified switch control signal to the motor drive circuit40.

The assist ECU100is supplied with electric power from a power supply device200. The power supply device200includes a battery (not shown) and an alternator (not shown) for generating power by rotation of an engine. The rated output voltage of the power supply device200is set to, for example, 12 V. Note that,FIG. 2illustrates only a power line210as a power supply line from the power supply device200to the motor drive circuit40, but operating power of the electronic control circuit50is also supplied from the power supply device200.

The motor drive circuit40is provided between the power line210and a ground line220, and is formed of an H bridge circuit. In the H bridge circuit, an upper arm circuit45H including a switching element Q1and a switching element Q3connected in parallel and a lower arm circuit45L including a switching element Q2and a switching element Q4connected in parallel are connected in series. Energization lines47aand47bfor supplying electric power to the motor20are drawn from nodes A1and A2between the upper arm circuit45H and the lower arm circuit45L, respectively. Therefore, one energization terminal20aof the motor20is connected to the power line210via the switching element Q1, and is connected to the ground line220via the switching element Q2. The other energization terminal20bof the motor20is connected to the power line210via the switching element Q3, and is connected to the ground line220via the switching element Q4.

As the switching elements Q1, Q2, Q3, and Q4provided in the motor drive circuit40, metal oxide semiconductor field effect transistors (MOS-FETs) are used. The switching elements Q1, Q2, Q3, and Q4are provided in the upper and lower arm circuits45H and45L so that a power supply voltage may be applied between the source and the drain. The gates of the switching elements Q1, Q2, Q3, and Q4are connected to the switch drive circuit80of the electronic control circuit50.

As illustrated by circuit symbol inFIG. 2, a diode is parasitically formed in the MOS-FET in terms of structure. This diode is referred to as “parasitic diode”. Each of the parasitic diodes of the switching elements Q1, Q2, Q3, and Q4is a reverse conducting diode for interrupting the flow of a current from the power line210to the ground line220while allowing a current to flow only from the ground line220to the power line210. In the motor drive circuit40, another reverse conducting diode than the parasitic diode (a diode for interrupting the current in the same direction as in the parasitic diode while becoming conductive in a direction reverse to the power supply voltage direction) may be connected in parallel to each of the switching elements Q1, Q2, Q3, and Q4.

The microcomputer60outputs an independent drive signal to each gate of the switching elements Q1, Q2, Q3, and Q4of the motor drive circuit40via the switch drive circuit80. The ON state and the OFF state of each of the switching elements Q1, Q2, Q3, and Q4are switched by the drive signal.

In the motor drive circuit40, when the switching element Q1and the switching element Q4are turned ON in a state where the switching element Q2and the switching element Q3are kept being turned OFF, a current I1flows in the direction (+) ofFIG. 2. In this manner, the motor20generates torque in the forward rotation direction. On the other hand, when the switching element Q2and the switching element Q3are turned ON in a state where the switching element Q1and the switching element Q4are kept being turned OFF, a current I2flows in the direction (−) ofFIG. 2. In this manner, the motor20generates torque in the backward rotation direction.

The assist ECU100includes a current sensor31for detecting the current flowing through the motor20. The current sensor31is provided on the ground line220that connects the lower arm circuit45L and the ground. The current sensor31is formed of, for example, a shunt resistor (not shown) provided on the ground line220, and supplies a voltage signal obtained by amplifying a voltage generated across the shunt resistor by an amplifier (not shown) or a digital signal obtained by converting the voltage signal to the input interface70of the electronic control circuit50. The value of the current flowing through the motor20, which is detected by the current sensor31, is hereinafter referred to as “motor actual current Im”.

The assist ECU100further includes a first voltage sensor32and a second voltage sensor33for detecting terminal voltages of the motor20. The first voltage sensor32supplies a signal indicating a voltage at the one energization terminal20aof the motor20to the input interface70. The value of the voltage detected by the first voltage sensor32is referred to as “first motor terminal voltage V1”. The energization terminal20ais referred to as “first motor terminal20a”. The first motor terminal voltage V1indicates a potential of the node A1between the switching element Q1and the switching element Q2with respect to the ground.

The second voltage sensor33supplies a signal indicating a voltage at the other energization terminal20bof the motor20to the input interface70. The value of the voltage detected by the second voltage sensor33is referred to as “second motor terminal voltage V2”. The energization terminal20bis referred to as “second motor terminal20b”. The second motor terminal voltage V2indicates a potential of the node A2between the switching element Q3and the switching element Q4with respect to the ground.

The assist ECU100further includes a power supply voltage sensor34for detecting the power supply voltage to be supplied to the motor drive circuit40, in other words, an output voltage of the power supply device200. The power supply voltage sensor34supplies a signal indicating a voltage of the power line210to the input interface70. The value of the voltage detected by the power supply voltage sensor34is referred to as “power supply voltage Vcc”.

The assist ECU100further includes a substrate temperature sensor35for detecting the temperature of the motor drive circuit40. The substrate temperature sensor35supplies a signal indicating the temperature of a substrate of the motor drive circuit40having the switching elements Q1, Q2, Q3, and Q4provided thereon to the input interface70. The value of the temperature detected by the substrate temperature sensor35is referred to as “substrate temperature Tb”. The substrate temperature Tb indicates a temperature corresponding to the heat generating states of the switching elements Q1, Q2, Q3, and Q4.

The assist ECU100is connected to the steering torque sensor21, a vehicle speed sensor91, and a courtesy switch92. The steering torque sensor21supplies to the input interface70a detection signal indicating the steering torque tr input from the steering wheel11. The vehicle sensor91supplies a detection signal indicating a vehicle speed vx to the input interface70. The courtesy switch92supplies a detection signal indicating an open/close state S of a vehicle door to the input interface70. For example, the courtesy switch92outputs an OFF signal when the door is closed and an ON signal when the door is open.

Description is given next of control processing of the microcomputer60. First, steering assist control processing executed by the microcomputer60is described.FIG. 3illustrates a steering assist control routine executed by the microcomputer60. The steering assist control routine is executed repeatedly at predetermined short cycles while an ignition switch is turned ON.

Upon the start of this control routine, in Step S1, the microcomputer60reads the vehicle speed vx detected by the vehicle speed sensor91and the steering torque tr detected by the steering torque sensor21. Subsequently, in Step S2, the microcomputer60refers to an assist map shown inFIG. 4to calculate a target assist torque tr* that is set in accordance with the input vehicle speed vx and the input steering torque tr. The assist map is correlation data in which the relationship between the steering torque tr and the target assist torque tr* is set for each plurality of representative vehicle speeds vx. The assist map has such characteristics that the target assist torque tr* increases as the steering torque tr becomes larger and the target assist torque tr* increases as the vehicle speed becomes lower.FIG. 4is an assist map for steering in the left direction. An assist map for steering in the right direction is obtained by reversing the signs of the steering torque tr and the target assist torque tr* from those in the left direction (in other words, negative).

Subsequently, in Step S3, the microcomputer60calculates a necessary current I* that is necessary for generating the target assist torque tr*. The necessary current I* is obtained by dividing the target torque assist tr* by a torque constant. Subsequently, in Step S4, the microcomputer60reads an upper limit current Imax. The upper limit current Imax indicates an upper limit value of the current to be supplied to the motor20. In Step S4, the microcomputer60reads the latest upper limit current Imax calculated by a motor limit value setting routine to be described later.

Subsequently, in Step S5, the microcomputer60determines whether or not the necessary current I* is larger than the upper limit current Imax. When the necessary current I* is larger than the upper limit current Imax, in Step S6, the microcomputer60sets the upper limit current Imax as a target current Im* (Im*←Imax). When the necessary current I* is equal to or smaller than the upper limit current Imax, in Step S7, the microcomputer60sets the necessary current I* as a target current Im* (Im*←I*). The absolute value is herein used to discuss the magnitude of a detection value with a direction (sign). Therefore, the comparison in this case is made between absolute values that are irrelevant to the flowing direction of the current.

Subsequently, in Step S8, the microcomputer60calculates a deviation ΔI by subtracting the motor actual current Im detected by the current sensor31from the target current Im*, and performs proportional-integral control (PI control) using the deviation ΔI to calculate a target command voltage V* so that the motor actual current Im may follow the target current Im*. For example, the target command voltage V* is calculated by the following expression.
V*=Kp·ΔI+Ki·∫ΔIdt
where Kp represents a control gain of the proportional term in the PI control, and Ki represents a control gain of the integral term in the PI control.

Subsequently, in Step S9, the microcomputer60outputs a pulse width modulation (PWM) control signal corresponding to the target command voltage V* to the switch drive circuit80. The switch drive circuit80amplifies the input control signal and outputs the amplified control signal to the motor drive circuit40. In this manner, a pulse signal train with a duty cycle corresponding to the target command voltage V* is output to the motor drive circuit40as a PWM control signal. The PWM control signal controls the duty cycle of each of the switching elements Q1, Q2, Q3, and Q4to adjust the drive voltage of the motor20to the target command voltage V*. In this manner, the target current Im* flows through the motor20in the direction for turning in the steering operation direction. As a result, the motor20assists the driver's steering operation.

After outputting the PWM control signal in Step S9, the microcomputer finishes the steering assist control routine once. Then, the microcomputer repeats the above-mentioned processing at predetermined cycles.

The internal resistance of the motor drive circuit40increases due to the deterioration of the switching elements Q1, Q2, Q3, and Q4, the deterioration of soldered parts, or other such causes. The internal resistance of the motor20increases due to the increase in contact resistance of a brush portion or other such causes. The increased internal resistance (electric resistance) may increase the amount of heat generation to break down a component. Once the component is broken down, the steering assist can no longer be executed. To deal with this problem, in the embodiment of the present invention, the internal resistance values of the motor20and the motor drive circuit40are measured (calculated), and, when the measured internal resistance values are out of a normal range, the energization of the motor20is limited in accordance with the internal resistance values. For example, a severer limitation is imposed on the energization of the motor20along with the increase in internal resistance value.

First, a method of measuring the internal resistance values of the motor drive circuit40and the motor20is described.

As illustrated inFIG. 5, an internal resistance value of the switching element Q1is represented by Rq1; an internal resistance value of the switching element Q2, Rq2; an internal resistance value of the switching element Q3, Rq3; an internal resistance value of the switching element Q4, Rq4; an internal resistance value of the motor20, Rm; and an inductance of the motor, Lm. A current that flows in a direction of rotating the motor20forward when the switching elements Q2and Q3are turned OFF and the switching elements Q1and Q4are turned ON is represented by I1. A current that flows in a direction of rotating the motor20backward when the switching elements Q1and Q4are turned OFF and the switching elements Q2and Q3are turned ON is represented by I2.

A voltage equation when the current I1flows is expressed by Expression (1) below.

A voltage equation when the current I2flows is expressed by Expression (2) below.

When the rotation speed of the motor20is represented by ω, an induced voltage constant of the motor20is represented by φ, and the current flowing through the motor20is represented by I (=I1or I2), an inter-terminal voltage of the motor20is expressed by Expression (3) below.
V1−V2=Rm·I+Lm·dl/dt+φ·ω(3)

In the embodiment of the present invention, the internal resistance value Rm of the motor20is calculated in a manner that a DC current I1small enough not to rotate the steering wheel11is caused to flow through the motor20, and the terminal voltages V1and V2of the motor20at that time are measured. For example, the switching elements Q1and Q4are operated at a predetermined duty cycle to energize the motor20in a state where the switching elements Q2and Q3are kept being turned OFF. It should be understood that the switching elements Q2and Q3may be operated at a predetermined duty cycle to energize the motor20in a state where the switching elements Q1and Q4are kept being turned OFF.

The motor internal resistance value Rm can be obtained by Expression (4) below by substituting dl/dt=0 and ω=0 in Expression (3).
Rm=(V1−V2)/I(4)

In this case, the duty cycles of the switching elements Q1and Q4are controlled so that the motor current I detected by the current sensor31may become I1, and the terminal voltages V1and V2of the motor20in this state are measured. Because the current I1flowing through the motor20is set to a small value, the duty cycles of the switching elements Q1and Q4are also small.

Note that, in the embodiment of the present invention, the internal resistance value is obtained based on the motor inter-terminal voltage (V1−V2) obtained by subtracting the second motor terminal voltage V2detected by the second voltage sensor33from the first motor terminal voltage V1detected by the first voltage sensor32, but, instead of the voltage sensors32and33, a voltage sensor for directly detecting the voltage between the first motor terminal20aand the second motor terminal20b(inter-terminal voltage) may be provided.

The internal resistance values Rq1, Rq2, Rq3, and Rq4of the switching elements Q1, Q2, Q3, and Q4are calculated in a manner that the current is caused to flow alternately in the direction of rotating the motor20forward and in the direction of rotating the motor20backward at predetermined cycles. In this case, the motor current I1, the motor terminal voltages V1and V2, and the power supply voltage Vcc are measured in a state where the switching elements Q2and Q3are turned OFF while the switching elements Q1and Q4are turned ON for a given period of time (for example, about several milliseconds), and the motor current I2, the motor terminal voltages V1and V2, and the power supply voltage Vcc are thereafter measured in a state where the switching elements Q1and Q4are turned OFF while the switching elements Q2and Q3are turned ON for a given period of time (for example, about several milliseconds). In this manner, the current flows alternately in the direction of rotating the motor20forward and in the direction of rotating the motor20backward, and hence the rotation of the steering wheel11is prevented.

The internal resistance values of the switching elements Q1and Q4can be calculated by Expression (5) below based on Expression (1).
(Rq1+Rq4)={Vcc−(V1−V2)}/I1  5)

The internal resistance values of the switching elements Q3and Q2can be calculated by Expression (6) below based on Expression (2).
(Rq3+Rq2)={Vcc+(V1−V2)}/I2  (6)

Description is given next of processing of limiting the operation of the motor20based on the measured internal resistance values.FIG. 6illustrates a motor limit value setting routine executed by the microcomputer60. The motor limit value setting routine is executed repeatedly at predetermined cycles.

Upon the start of this routine, in Step S11, the microcomputer60first reads the detection signal S of the courtesy switch92, and determines whether or not the detection signal S has changed from the OFF state to the ON state. In other words, the microcomputer60determines whether or not the vehicle door has been opened (closed→open). When the detection signal S has not changed from the OFF state to the ON state, the microcomputer60finishes this routine once.

In the embodiment of the present invention, when the vehicle is not activated, a current is caused to flow through the motor20without being noticed by a driver, and the internal resistance values of the motor20and the motor drive circuit40are measured. Therefore, Step S11is a step of detecting a timing when the driver opens the door to ride on the vehicle or a timing when the driver gets out of the vehicle. Note that, in Step S11, “No” is determined when the system of the electric power steering device1has already started up and the steering assist control is in operation. In this example, “Yes” is determined both when the driver opens the door to ride on the vehicle and when the driver opens the door to get out of the vehicle, but only one of the timings may be detected.

The microcomputer60repeats the determination of Step S11. When detecting that the detection signal S of the courtesy switch92has changed from the OFF state to the ON state, in Step S12, the microcomputer60subsequently starts up the system of the electric power steering device1. Subsequently, in Step S13, the microcomputer60determines whether or not the system has started up normally. When the system has failed to start up normally, in Step S14, the microcomputer60stops the steering assist and finishes this routine.

When the system of the electric power steering device1has started up normally, in Step S20, the microcomputer60executes processing of calculating the internal resistance value of the motor20.FIG. 7is a flowchart illustrating the calculating processing, namely a motor internal resistance value calculating routine (subroutine).

Upon the start of the motor internal resistance value calculating routine, in Step S21, the microcomputer60clears a counter value i to zero. Subsequently, in Step S22, the microcomputer60increments the counter value i by 1. Subsequently, in Step S23, the current I1is caused to flow through the motor20. In this case, the current I1is caused to flow through the motor20by controlling the duty cycles of the switching elements Q1and Q4in a state where the switching elements Q2and Q3are kept being turned OFF. Subsequently, in Step S24, the microcomputer60measures the motor terminal voltage V1, the second motor terminal voltage V2, and the motor current I (motor actual current Im) in this energization state, and calculates a motor internal resistance value Rmi by using Expression (4). The calculated motor internal resistance value Rmi is temporarily stored in a memory such as a RAM.

Subsequently, in Step S25, the microcomputer60determines whether or not the counter value i is equal to or larger than a predetermined value N (for example, N=10). When the counter value i is smaller than the predetermined value N, the processing returns to Step S22to perform the same processing. When the microcomputer60repeats the energization of the motor20and the calculation of the motor internal resistance value Rmi N times (S25: Yes), in Step S26, the microcomputer60calculates an average value of the motor internal resistance values Rmi calculated N times. The microcomputer60sets the average value of the motor internal resistance values Rmi as the motor internal resistance value Rm, which is a final calculation result. After calculating the motor internal resistance value Rm, the microcomputer60finishes the motor internal resistance value calculating routine, and starts the processing from Step S30of the main routine ofFIG. 6.

The motor internal resistance value fluctuates depending on a contact position between a brush and a commutator segment. In order to deal with this problem, in the internal resistance value calculating routine, a moving average of the motor internal resistance values Rmi is used to calculate the final motor internal resistance value Rm. In this manner, the calculated motor internal resistance value Rm is not affected by the influence of the contact position between the brush and the commutator segment.

After calculating the motor internal resistance value Rm, in Step S30of the main routine (FIG. 6), the microcomputer60subsequently executes processing of calculating the internal resistance value of the motor drive circuit40.FIG. 8is a flowchart illustrating the calculating processing, namely a drive circuit internal resistance value calculating routine (subroutine).

Upon the start of the drive circuit internal resistance value calculating routine, in Step S31, the microcomputer60clears a counter value j to zero. Subsequently, in Step S32, the microcomputer60increments the counter value j by 1. Subsequently, in Step S33, the microcomputer60turns ON the switching elements Q1and Q4for a given period of time (for example, several milliseconds) in a state where the switching elements Q2and Q3are kept being turned OFF. Then, in Step S34, the microcomputer60measures the first motor terminal voltage V1, the second motor terminal voltage V2, the power supply voltage Vcc, and the motor current I (motor actual current Im) in this energization state, and calculates internal resistance values Rq1jand Rq4jof the switching elements Q1and Q4by using Expressions (7) and (10). The calculated internal resistance values Rq1jand Rq4jare temporarily stored in a memory such as a RAM.

Subsequently, in Step S35, the microcomputer60turns ON the switching elements Q2and Q3for a given period of time (for example, several milliseconds) in a state where the switching elements Q1and Q4are kept being turned OFF. Then, in Step S36, the microcomputer60measures the first motor terminal voltage V1, the second motor terminal voltage V2, and the power supply voltage Vcc in this energization state, and calculates internal resistance values Rq2jand Rq3jof the switching elements Q2and Q3by using Expressions (8) and (9). The calculated internal resistance values Rq2jand Rq3jare temporarily stored in a memory such as a RAM.

Subsequently, in Step S37, the microcomputer60determines whether or not the counter value j is equal to or larger than a predetermined value N (for example, N=10). When the counter value j is smaller than the predetermined value N, the processing returns to Step S32to perform the same processing. In this manner, the switching elements Q1and Q4for forward rotation and the switching elements Q2and Q3for backward rotation are alternately turned ON at predetermined cycles. The cycles are set short enough not to rotate the steering wheel11.

When the microcomputer60repeats the energization of the motor20and the calculation of the internal resistance values Rq1j, Rq2j, Rq3j, and Rq4jN times (S37: Yes), in Step S38, the microcomputer60calculates an average value of each of the internal resistance values Rq1j, Rq2j, Rq3j, and Rq4jcalculated N times. The microcomputer60sets the average values of the internal resistance values Rq1j, Rq2j, Rq3j, and Rq4jas the internal resistance values Rq1, Rq2, Rq3, and Rq4, respectively, which are final calculation results. After calculating the internal resistance values Rq1, Rq2, Rq3, and Rq4, the microcomputer60finishes the drive circuit internal resistance value calculating routine, and starts the processing from Step S40of the main routine ofFIG. 6.

In Step S40, the microcomputer60determines whether the internal resistance of the motor20is normal, abnormal, or semi-abnormal based on the motor internal resistance value Rm. For example, the microcomputer60stores a first threshold Rref_m1and a second threshold Rref_m2(>Rref_m1) in advance as thresholds for discriminating among a normal range, an abnormal range, and a semi-abnormal range of the motor internal resistance value Rm. The first threshold Rref_m1indicates a maximum resistance value in the normal range, and the second threshold Rref_m2indicates a minimum resistance value in the abnormal range. When the motor internal resistance value Rm is smaller than the first threshold Rref_m1, the microcomputer60determines that the internal resistance of the motor20is normal. When the motor internal resistance value Rm is larger than the second threshold Rref_m1, the microcomputer60determines that the internal resistance of the motor20is abnormal. When the motor internal resistance value Rm falls between the first threshold Rref_m1and the second threshold Rref_m2(Rref_m1≦Rm≦Rref_m2), the microcomputer60determines that the internal resistance of the motor20is semi-abnormal. Note that, a lower limit value may be set in the normal range so as to detect a short-circuit abnormality.

When determining in Step S40that the motor20is abnormal, the microcomputer60proceeds the processing to Step S14to stop the steering assist, and finishes this routine. When determining that the motor20is semi-abnormal, on the other hand, the microcomputer60sets an upper limit current Imax_m for motor resistance in Step S41. For example, the microcomputer60stores a motor resistance upper limit current map shown inFIG. 9. The microcomputer60refers to this map to set the upper limit current Imax_m for motor resistance. The motor resistance upper limit current map has such characteristics that the upper limit current Imax_m for motor resistance decreases as the motor internal resistance value Rm becomes larger.

When determining in Step S40that the motor20is normal or when determining in Step S40that the motor20is semi-abnormal and setting the upper limit current Imax_m for motor resistance, in Step S42, the microcomputer60subsequently determines whether the internal resistance of the motor drive circuit40is normal, abnormal, or semi-abnormal based on the internal resistance values Rq1, Rq2, Rq3, and Rq4of the switching elements Q1, Q2, Q3, and Q4.

For example, the microcomputer60extracts a largest internal resistance value Rq among the internal resistance values Rq1, Rq2, Rq3, and Rq4, and uses the internal resistance value Rq to determine whether the internal resistance of the motor drive circuit40is normal, abnormal, or semi-abnormal. The maximum value of the internal resistance values Rq1, Rq2, Rq3, and Rq4is hereinafter referred to as “circuit internal resistance value Rq”.

The microcomputer stores a first threshold Rref_q1and a second threshold Rref_q2(>Rref_q1) in advance as thresholds for discriminating among a normal range, an abnormal range, and a semi-abnormal range of the circuit internal resistance value Rq. The first threshold Rref_q1indicates a maximum resistance value in the normal range, and the second threshold Rref_q2indicates a minimum resistance value in the abnormal range. When the circuit internal resistance value Rq is smaller than the first threshold Rref_q1, the microcomputer60determines that the internal resistance of the motor drive circuit40is normal. When the circuit internal resistance value Rq is larger than the second threshold Rref_q1, the microcomputer60determines that the internal resistance of the motor drive circuit40is abnormal. When the circuit internal resistance value Rq falls between the first threshold Rref_q1and the second threshold Rref_q2(Rref_q1≦Rq≦Rref_q2), the microcomputer60determines that the internal resistance of the motor drive circuit40is semi-abnormal. Note that, a lower limit value may be set in the normal range so as to detect a short-circuit abnormality.

When determining in Step S42that the motor drive circuit40is abnormal, the microcomputer60proceeds the processing to Step S14to stop the steering assist, and finishes this routine. When determining that the motor drive circuit40is semi-abnormal, on the other hand, the microcomputer60sets an upper limit current Imax_q for circuit resistance in Step S43. For example, the microcomputer60stores a circuit resistance upper limit current map shown inFIG. 10. The microcomputer60refers to this map to set the upper limit current Imax_q for circuit resistance. The circuit resistance upper limit current map has such characteristics that the upper limit current Imax_q for circuit resistance decreases as the circuit internal resistance value Rq becomes larger.

The motor resistance upper limit current map and the circuit resistance upper limit current map are set individually. Therefore, it is possible to separately set an upper limit current value (upper limit current Imax_m for motor resistance) suitable for the semi-abnormal state of the motor20and an upper limit current value (upper limit current Imax_q for circuit resistance) suitable for the semi-abnormal state of the motor drive circuit40.

When determining in Step S42that the motor drive circuit40is normal or when determining in Step S42that the motor drive circuit40is semi-abnormal and setting the upper limit current Imax_q for circuit resistance, in Step S44, the microcomputer60subsequently determines whether or not the upper limit current Imax_m for motor resistance is larger than the upper limit current Imax_q for circuit resistance.

When the upper limit current Imax_m for motor resistance is larger than the upper limit current Imax_q for circuit resistance (S44: Yes), in Step S45, the microcomputer60sets the upper limit current Imax_q for circuit resistance as the upper limit current Imax to be used in the above-mentioned steering assist control routine (Imax←Imax_q). On the other hand, when the upper limit current Imax_m for motor resistance is equal to or smaller than the upper limit current Imax_q for circuit resistance (S44: No), in Step S46, the microcomputer60sets the upper limit current Imax_m for motor resistance as the upper limit current Imax (Imax←Imax_m). In this manner, a smaller one of the upper limit current Imax_m for motor resistance and the upper limit current Imax_q for circuit resistance is set as the upper limit current Imax. Note that, when the determination results in Steps S40and S42are both normal, the upper limit current Imax is set to be a value for the normal case. The upper limit current Imax for the normal case may be a preset fixed value, or a variable value that is set in accordance with a motor estimated temperature Tm and the substrate temperature Tb as described in a modified example later.

After setting the upper limit current Imax, the microcomputer60finishes the motor limit value setting routine once.

The electric power steering device1according to the embodiment of the present invention described above has the following functions and effects.

1. The internal resistance values of the motor20and the motor drive circuit40are measured, and hence the semi-abnormal states thereof can be reliably detected. For example, if the internal resistance of the motor energization path increases due to the increase in internal resistance of the switching elements Q1, Q2, Q3, and Q4, the deterioration of soldered parts, the increase in contact resistance of the brush of the motor20, or other such causes, the amount of heat generation may increase to damage the motor energization path. In the embodiment of the present invention, however, the semi-abnormal state before the damage occurs can be detected.

2. In the case where the semi-abnormal state is detected, the upper limit current Imax of the motor20is set to be lower than in the normal case, and hence the heat generation of the motor20and the motor drive circuit40can be suppressed. In this manner, the progress of degradation of the motor20and the motor drive circuit40can be suppressed. The upper limit current Imax of the motor20is set to be smaller as the internal resistance value of the motor20or the motor drive circuit40becomes larger, and hence an appropriate upper limit current Imax corresponding to the degree of semi-abnormality can be set. In this manner, the lifetime of the electric power steering device1can be appropriately prolonged.

3. Even if a failure has occurred in the motor20or the motor drive circuit40, the operation of the motor20has already been limited before the occurrence of the failure based on the increase in internal resistance value, and hence the steering assist is prevented from being stopped suddenly to cause an abrupt change in steering force. Therefore, the load on the driver can be reduced.

4. The internal resistance values of the motor20and the motor drive circuit40are measured at the timing when the courtesy switch92is turned ON, and hence the driver is prevented from being aware of the measurement.

5. The internal resistances of the motor20and the motor drive circuit40show a gradual change, but the resistance values are measured at every arrival of a preset timing, and hence the progress of the increase in internal resistance can be reliably detected.

6. In measuring the internal resistance value of the motor drive circuit40, the energization in the forward rotation direction and the energization in the reverse rotation direction are alternately performed, and hence the steering wheel11is prevented from being rotated. In measuring the internal resistance value of the motor20, on the other hand, a small current is caused to flow through the motor20, and hence the steering wheel11is prevented from being rotated.

7. The internal resistance value is measured with the use of a moving average value through a plurality of measurements, and hence the calculated value is not affected by the influence of the contact position between the brush and the commutator segment, and the accuracy is therefore improved.

8. The upper limit current Imax_m for motor resistance, which is set based on the internal resistance value Rm of the motor20, and the upper limit current Imax_q for circuit resistance, which is set based on the internal resistance value Rq of the motor drive circuit40, are both calculated, and a smaller one of the calculated upper limit currents is set as the upper limit current Imax, and hence both the motor20and the motor drive circuit40can be properly protected.

Next, modified examples of the embodiment of the present invention are described.

Modified Example 1

Timing of Measuring Internal Resistance Value

In the above-mentioned embodiment, the detection signal S of the courtesy switch92is used to start measuring the internal resistance value (see S11). However, the timing of measuring the internal resistance value can be set in various ways. For example, instead of the courtesy switch92, a detection signal S of a seating sensor93(illustrated by the broken line ofFIG. 1) may be used to set the timing of measuring the internal resistance value. The seating sensor93outputs the detection signal S indicating a seating state of a driver on a driver's seat. Therefore, the internal resistance value may be measured at a timing when the seating sensor93detects that the driver was seated on the drivers seat or at a timing when the seating sensor93detects that the driver got out of the drivers seat.

Alternatively, the internal resistance value may be measured during the night. In this case, for example, a clock function provided in the microcomputer60is used to measure the internal resistance value at a preset given time during the night.

Alternatively, the internal resistance value may be measured at preset given time intervals.

Modified Example 2

Limitation on Operation of Motor

In the above-mentioned embodiment, when the semi-abnormal state of the motor20or the motor drive circuit40is detected, the upper limit current Imax corresponding to the internal resistance value Rm or Rq is set to limit the operation of the motor20. However, various methods can also be employed to limit the operation of the motor20.

Modified Example 2-1

Current Limitation Depending on Elapsed Time

For example, the upper limit current Imax may be decreased gradually with time. As shown inFIG. 11, an internal resistance value R of the motor20or the motor drive circuit40increases gradually with time t. In view of this, according to this modified example, as shown inFIG. 12, an elapsed time t is measured from the first detection of the semi-abnormal state, and the upper limit current Imax is decreased gradually from an initial value Imax0with the elapsed time t, the initial value Imax0corresponding to an upper limit current Imax0measured in the normal case. In this case, the elapsed time t is set as an accumulated value of time during which the system of the electric power steering device1is in operation, that is, an accumulated value of time during which the steering assist control is performed. In this example, when the elapsed time t reaches a stop setting time t1, the steering assist is stopped. Therefore, the stop setting time t1is a steering assist executable period starting from the detection of the semi-abnormal state.

FIGS. 13 and 14illustrate processing of the microcomputer60according to Modified Example 2-1. This processing is alternative to the motor limit value setting routine (FIG. 6) according to the embodiment.FIG. 13illustrates an internal resistance value determining routine, andFIG. 14illustrates a motor limit value setting routine. In the following, the same processing inFIGS. 13 and 14as in the motor limit value setting routine (FIG. 6) according to the embodiment is denoted by the same step number as in the embodiment, and description thereof is omitted. The internal resistance value determining routine and the motor limit value setting routine are executed in parallel at predetermined cycles.

Upon the start of the internal resistance value determining routine (FIG. 13), in Step S51, the microcomputer60first determines whether or not a semi-abnormality determination flag F is “0”. The semi-abnormality determination flag F of “1” indicates that the semi-abnormal state of the motor20or the motor drive circuit40has been detected, and the semi-abnormality determination flag F of “0” indicates that the semi-abnormal state has not been detected. The initial value of the semi-abnormality determination flag F is set to “0”. When the semi-abnormality determination flag F is “0” (F=0), the microcomputer60performs the processing from Step S11.

When detecting the semi-abnormal state of the motor20in Step S40, or when detecting the semi-abnormal state of the motor drive circuit40in Step S42, the microcomputer60sets the semi-abnormality determination flag F to “1” in Step S52to finish the internal resistance value determining routine once.

On the other hand, when determining in Steps S40and S42that the motor20and the motor drive circuit40are normal, the microcomputer60finishes the internal resistance value determining routine once without any further processing. In this case, the upper limit current Imax is set to a value used for the normal case.

The internal resistance value determining routine is repeated at predetermined cycles. Once the semi-abnormality determination flag F is set to “1”, the determination of Step S51becomes “No”. In this case, the internal resistance values of the motor20and the motor drive circuit40are no longer detected, and instead, the upper limit current Imax is calculated in the motor limit value setting routine illustrated inFIG. 14.

In Step S61of the motor limit value setting routine (FIG. 14), the microcomputer60determines whether or not the semi-abnormality determination flag F is set to “1”. The microcomputer60repeats this determination until the semi-abnormality determination flag F is set to “1”. In the period until the semi-abnormality determination flag F is set to “1”, the abnormality determination (S40, S42) is performed based on the internal resistance values in the above-mentioned internal resistance value determining routine.

When the semi-abnormality determination flag F is set to “1” (S61: Yes), in Step S62, the microcomputer60increments a timer value t by 1. The timer value t indicates an elapsed time since the semi-abnormality determination flag F was set to “1”, that is, an elapsed time since the semi-abnormal state of the motor20or the motor drive circuit40was detected. The initial value of the timer value t is set to zero.

Subsequently, in Step S63, the microcomputer60calculates an upper limit current Imax corresponding to the timer value t. The microcomputer stores an upper limit current map as shown inFIG. 12. The microcomputer refers to the upper limit current map to set the upper limit current Imax corresponding to the timer value t (elapsed time). The upper limit current map has such characteristics that the upper limit current Imax decreases gradually from the initial value Imax0with time. The upper limit current Imax is a current limit value of the motor20to be used in Step S4of the steering assist control routine. Subsequently, in Step S64, the microcomputer60determines whether or not the timer value t has reached the preset stop setting time t1. When the timer value t has not reached the stop setting time t1, the microcomputer60finishes the motor limit value setting routine once.

The microcomputer60repeats the processing described above at predetermined cycles. In this manner, the upper limit current Imax is set so as to be decreased gradually along with the increase in the elapsed time t. Therefore, the limitation on the operation of the motor20becomes higher (severer) with time. Then, when the elapsed time t has reached the stop setting time t1(S64: Yes), in Step S65, the microcomputer60stops the steering assist and inhibits the further steering assist control. The microcomputer60finishes the internal resistance value determining routine and the motor limit value setting routine.

Note that, the upper limit current map is set in advance based on a possible change in the internal resistance value of the motor20or the motor drive circuit40(seeFIG. 11). Therefore, the stop setting time t1assumes a very long time. Accordingly, in order to store and hold the timer value t even when the ignition switch is turned OFF, the microcomputer60stores the timer value t in a non-volatile memory (not shown) each time the motor limit value setting routine is finished. The microcomputer60reads the timer value t stored in the non-volatile memory each time the motor limit value setting routine is restarted, and increments the timer value t to count an accumulated time.

According to Modified Example 2-1, in the case where the semi-abnormal state of the motor20or the motor drive circuit40is detected, the upper limit current Imax corresponding to a temporal change of the internal resistance value is set at the time of this detection, and hence the operation of the motor20can be appropriately limited.

Modified Example 2-2

Current Limitation Depending on Region where Semi-Abnormality has Occurred

The time (remaining lifetime) from the detection of the semi-abnormal state of the motor20until the motor20reaches the abnormal state and the time (remaining lifetime) from the detection of the semi-abnormal state of the motor drive circuit40until the motor drive circuit40reaches the abnormal state do not always match with each other. In view of this, in Modified Example 2-2, the upper limit current Imax is calculated with the use of an upper limit current map in which different stop setting times t1are set for the case where the semi-abnormal state of the motor20is detected and the case where the semi-abnormal state of the motor drive circuit40is detected. In this manner, an appropriate steering assist executable period corresponding to the remaining lifetime of the semi-abnormal region can be set.

FIGS. 15 and 16illustrate processing of the microcomputer60according to Modified Example 2-2. This processing is alternative to the motor limit value setting routine (FIG. 6) according to the embodiment.FIG. 15illustrates an internal resistance value determining routine, andFIG. 16illustrates a motor limit value setting routine. In the following, the same processing inFIGS. 15 and 16as in the motor limit value setting routine (FIG. 6) according to the embodiment is denoted by the same step number as in the embodiment, and description thereof is omitted. The internal resistance value determining routine and the motor limit value setting routine are executed in parallel at predetermined cycles.

Upon the start of the internal resistance value determining routine (FIG. 15), in Step S71, the microcomputer60first determines whether or not a motor semi-abnormality determination flag Fm is “0”. The motor semi-abnormality determination flag Fm of “1” indicates that the semi-abnormal state of the motor20has been detected, and the motor semi-abnormality determination flag Fm of “0” indicates that the semi-abnormal state has not been detected. An initial value of the motor semi-abnormality determination flag Fm is set to “0”. When the motor semi-abnormality determination flag Fm is “0” (Fm=0), the microcomputer60determines whether or not a circuit semi-abnormality determination flag Fq is “0” in Step S72. The circuit semi-abnormality determination flag Fq of “1” indicates that the semi-abnormal state of the motor drive circuit40has been detected, and the circuit semi-abnormality determination flag Fq of “0” indicates that the semi-abnormal state has not been detected. The initial value of the circuit semi-abnormality determination flag Fq is set to “0”.

When the circuit semi-abnormality determination flag Fq is “0” (Fq=0), the microcomputer60performs the processing from Step S11.

When detecting the semi-abnormal state of the motor20in Step S40, the microcomputer60sets the motor semi-abnormality determination flag Fm to “1” in Step S73to finish the internal resistance value determining routine once. On the other hand, when detecting the semi-abnormal state of the motor drive circuit40in Step S42, the microcomputer60sets the circuit semi-abnormality determination flag Fq to “1” in Step S74to finish the internal resistance value determining routine once.

On the other hand, when determining in Steps S40and S42that the motor20and the motor drive circuit40are normal, the microcomputer60finishes the internal resistance value determining routine once without any further processing. In this case, the upper limit current Imax is set to a value used for the normal case.

The internal resistance value determining routine is repeated at predetermined cycles. Once the motor semi-abnormality determination flag Fm or the circuit semi-abnormality determination flag Fq is set to “1”, the determination of Step S71or Step S72becomes “No”. In this case, the internal resistance values of the motor20and the motor drive circuit40are no longer detected, and instead, the upper limit current Imax is calculated in the motor limit value setting routine illustrated inFIG. 16.

In Step S81of the motor limit value setting routine (FIG. 16), the microcomputer60determines whether or not the motor semi-abnormality determination flag Fm is set to “1”. When the motor semi-abnormality determination flag Fm is “0” (Fm=0), in Step S82, the microcomputer60determines whether or not the circuit semi-abnormality determination flag Fq is set to “1”. The microcomputer60repeats those two determinations until the motor semi-abnormality determination flag Fm or the circuit semi-abnormality determination flag Fq is set to “1”. In the period until the motor semi-abnormality determination flag Fm or the circuit semi-abnormality determination flag Fq is set to “1”, the abnormality determination (S40, S42) is performed based on the internal resistance values in the above-mentioned internal resistance value determining routine.

The microcomputer60repeats the determination of the setting states of the semi-abnormality determination flags Fm and Fq. When the motor semi-abnormality determination flag Fm is set to “1” (S81: Yes), in Step S83, the microcomputer60increments a timer value t by 1. The timer value t indicates an elapsed time since the motor semi-abnormality determination flag Fm was set to “1”, that is, an elapsed time since the semi-abnormal state of the motor20was detected. The initial value of the timer value t is set to zero.

Subsequently, in Step S84, the microcomputer60calculates an upper limit current Imax corresponding to the timer value t. The microcomputer stores an upper limit current map as shown inFIG. 17. The microcomputer refers to the upper limit current map to set the upper limit current Imax corresponding to the timer value t (elapsed time). The upper limit current map has such characteristics that the upper limit current Imax decreases gradually from the initial value Imax0with time. The characteristics of the upper limit current map are set to be different between the case where the motor semi-abnormality determination flag Fm is set to “1”, that is, the case where the semi-abnormal state of the motor20is detected, and the case where the circuit semi-abnormality determination flag Fq is set to “1”, that is, the case where the semi-abnormal state of the motor drive circuit40is detected.

For example, in the case where the remaining lifetime from the detection of the semi-abnormal state of the motor20is shorter than the remaining lifetime from the detection of the semi-abnormal state of the motor drive circuit40, as shown inFIG. 17, a stop setting time t1mfor the case where the semi-abnormal state of the motor20is detected is set to be shorter than a stop setting time t1qfor the case where the semi-abnormal state of the motor drive circuit40is detected. Note that, in the case where the relationship of the remaining lifetime between the motor20and the motor drive circuit40is reverse, the characteristics of the upper limit current map are reversed so that the stop setting time t1mis set to be longer than the stop setting time t1q.

After referring to the upper limit current map to set the upper limit current Imax in Step S84, the microcomputer60subsequently determines in Step S85whether or not the timer value t has reached the preset stop setting time t1m. When the timer value t has not reached the stop setting time t1m, the microcomputer60finishes the motor limit value setting routine once.

The microcomputer60repeats the processing described above at predetermined cycles. In this manner, the upper limit current Imax is set so as to be decreased gradually along with the increase in the elapsed time t. Therefore, the limitation on the operation of the motor20becomes higher (severer) with time. Then, when the elapsed time t has reached the stop setting time t1m(S85: Yes), in Step S86, the microcomputer60stops the steering assist and inhibits the further steering assist control. The microcomputer60finishes the internal resistance value determining routine and the motor limit value setting routine.

On the other hand, when the circuit semi-abnormality determination flag Fq is set to “1” (S82: Yes), in Step S87, the microcomputer60increments a timer value t by 1. The timer value t indicates an elapsed time since the circuit semi-abnormality determination flag Fq was set to “1”, that is, an elapsed time since the semi-abnormal state of the motor drive circuit40was detected. The initial value of the timer value t is set to zero.

Subsequently, in Step S88, the microcomputer60refers to the upper limit current map (FIG. 17) to calculate an upper limit current Imax corresponding to the timer value t. Subsequently, in Step S89, the microcomputer60determines whether or not the timer value t has reached the preset stop setting time t1q. When the timer value t has not reached the stop setting time t1q, the microcomputer60finishes the motor limit value setting routine once.

The microcomputer60repeats the processing described above at predetermined cycles. In this manner, the upper limit current Imax is set so as to be decreased gradually along with the increase in the elapsed time t. Therefore, the limitation on the operation of the motor20becomes higher (severer) with time. Then, when the elapsed time t has reached the stop setting time t1q(S89: Yes), in Step S86, the microcomputer60stops the steering assist and inhibits the further steering assist control. The microcomputer60finishes the internal resistance value determining routine and the motor limit value setting routine.

According to Modified Example 2-2, the characteristics of the upper limit current Imax and the stop setting times t1are set independently for the case where the motor20becomes the semi-abnormal state and the case where the motor drive circuit40becomes the semi-abnormal state. In other words, the characteristics of the upper limit current Imax and the stop setting time t1are switched depending on the region where the semi-abnormality has occurred. In this manner, an appropriate steering assist corresponding to the degree of degradation of the semi-abnormal region can be executed. Further, an appropriate steering assist executable period corresponding to the remaining lifetime of the semi-abnormal region can be set. As a result, the operation of the motor20can be limited more appropriately.

Modified Example 2-3

Limitation on Operation Based on Maximum Voltage

The operation of the motor20may be limited by limiting a maximum voltage to be applied to the motor20. For example, an upper limit command voltage Vmax as an upper limit value of the target command voltage V* is set to a value corresponding to the internal resistance value. In this case, the microcomputer60stores a motor resistance upper limit command voltage map as shown inFIG. 19and a circuit resistance upper limit command voltage map as shown inFIG. 20. The motor resistance upper limit command voltage map has such characteristics that an upper limit command voltage Vmax_m for motor resistance decreases as the internal resistance value Rm becomes larger. The circuit resistance upper limit command voltage map has such characteristics that an upper limit command voltage Vmax_q for circuit resistance decreases as the internal resistance value Rq becomes larger.

The microcomputer60executes a motor limit value setting routine illustrated inFIG. 18. This motor limit value setting routine is obtained by partially changing the motor limit value setting routine (FIG. 6) according to the embodiment. In the following, the same processing inFIG. 18as in the embodiment is denoted by the same step number as in the embodiment, and description thereof is omitted.

When determining in Step S40that the motor20is semi-abnormal, in Step S91, the microcomputer60refers to the motor resistance upper limit command voltage map shown inFIG. 19to set the upper limit command voltage Vmax_m for motor resistance. When determining in Step S42that the motor drive circuit40is semi-abnormal, in Step S93, the microcomputer60refers to the circuit resistance upper limit command voltage map shown inFIG. 20to set the upper limit command voltage Vmax_q for circuit resistance.

When the upper limit command voltage Vmax_m for motor resistance is larger than the upper limit command voltage Vmax_q for circuit resistance (S94: Yes), in Step S95, the microcomputer60sets the upper limit command voltage Vmax as the upper limit command voltage Vmax_q for circuit resistance. On the other hand, when the upper limit command voltage Vmax_m for motor resistance is equal to or smaller than the upper limit command voltage Vmax_q for circuit resistance (S94: No), in Step S96, the microcomputer60sets the upper limit command voltage Vmax as the upper limit command voltage Vmax_m for motor resistance. In this manner, the upper limit command voltage Vmax is set to a smaller one of the upper limit command voltage Vmax_m for motor resistance and the upper limit command voltage Vmax_q for circuit resistance.

The resultant upper limit command voltage Vmax is used in the steering assist control routine.FIG. 21illustrates a modified example of the steering assist control routine. This steering assist control routine is obtained by partially changing the steering assist control routine (FIG. 3) according to the embodiment. In the following, the same processing inFIG. 21as in the embodiment is denoted by the same step number as in the embodiment, and description thereof is omitted.

After calculating the necessary current I* in Step S3, in Step S8, the microcomputer60sets the necessary current I* as a target current Im*, calculates a deviation M by subtracting a motor actual current Im detected by the current sensor ΔI from the target current Im*, and performs proportional-integral control (PI control) using the deviation ΔI to calculate a target command voltage V* so that the motor actual current Im may follow the target current Im*.

Subsequently, in Step S101, the microcomputer reads the upper limit command voltage Vmax calculated in the above-mentioned motor limit value setting routine. Subsequently, in Step S102, the microcomputer determines whether or not the target command voltage V* is larger than the upper limit command voltage Vmax. When the target command voltage V* is larger than the upper limit command voltage Vmax, the microcomputer sets the target command voltage V* as the upper limit command voltage Vmax in Step S103. On the other hand, when the target command voltage V* is equal to or smaller than the upper limit command voltage Vmax, the microcomputer skips the processing of Step S103. In other words, the microcomputer does not change the target command voltage V*.

According to Modified Example 2-3, in the case where the motor20or the motor drive circuit40becomes the semi-abnormal state, the upper limit command voltage Vmax corresponding to the degree of semi-abnormality is set, and hence the same function and effects as in the embodiment can be obtained.

Modified Example 2-4

Voltage Limitation Depending on Elapsed Time

In Modified Example 2-1, the upper limit current Imax is decreased gradually with time. In Modified Example 2-4, instead, the upper limit command voltage Vmax is decreased gradually with time as shown inFIG. 22. In this case, the microcomputer60executes the same processing as in the internal resistance value determining routine illustrated inFIG. 13, the motor limit value setting routine illustrated inFIG. 14, and the steering assist control routine illustrated inFIG. 21. Note that, the microcomputer60stores an upper limit command voltage map having the characteristics shown inFIG. 22, and, in Step S63of the motor limit value setting routine (FIG. 14), the microcomputer60sets the upper limit command voltage Vmax by referring to the upper limit command voltage map, instead of setting the upper limit current Imax.

Therefore, when detecting the semi-abnormal state of the motor20or the motor drive circuit40, the microcomputer60counts an elapsed time t from the first detection of the semi-abnormal state, and gradually decreases the upper limit command voltage Vmax from an initial value Vmax0with time, the initial value Vmax0corresponding to an upper limit command voltage Vmax0measured in the normal case. Then, when the elapsed time t reaches a stop setting time t1(S64: Yes), the microcomputer60stops the steering assist. Also in Modified Example 2-4, similarly to Modified Example 2-1, the microcomputer60stores and holds the timer value t in a non-volatile memory to count an accumulated time.

According to Modified Example 2-4, in the case where the semi-abnormal state of the motor20or the motor drive circuit40is detected, the upper limit command voltage Vmax corresponding to a temporal change of the internal resistance value is set at the time of this detection, and hence the operation of the motor20can be appropriately limited.

Modified Example 2-5

Voltage Limitation Depending on Region where Semi-Abnormality has Occurred

In Modified Example 2-2 described above, the upper limit current Imax is decreased gradually with time, and the upper limit current Imax is calculated with the use of the upper limit current map having characteristics different between the case where the semi-abnormal state of the motor20is detected and the case where the semi-abnormal state of the motor drive circuit40is detected. In Modified Example 2-5, instead of the upper limit current Imax, the upper limit command voltage Vmax is decreased gradually with time. In this case, the microcomputer60executes the same processing as in the internal resistance value determining routine illustrated inFIG. 15, the motor limit value setting routine illustrated inFIG. 16, and the steering assist control routine illustrated inFIG. 21.

Note that, the microcomputer60stores an upper limit command voltage map having the characteristics shown inFIG. 23, and, in Steps S84and S88of the motor limit value setting routine (FIG. 16), the microcomputer60refers to the upper limit command voltage map to set the upper limit command voltage Vmax. In Steps S85and S89, the microcomputer60stops the steering assist at a stop setting time t1mor a stop setting time t1qthat is set in the upper limit command voltage map.

The upper limit command voltage map has such characteristics that the upper limit command voltage Vmax decreases gradually from an initial value Vmax0with time. The characteristics of the upper limit command voltage map are set to be different between the case where the motor semi-abnormality determination flag Fm is set to “1”, that is, the case where the semi-abnormal state of the motor20is detected, and the case where the circuit semi-abnormality determination flag Fq is set to “1” that is, the case where the semi-abnormal state of the motor drive circuit40is detected.

FIG. 23shows a setting example of the upper limit command voltage Vmax in the case where the remaining lifetime from the detection of the semi-abnormal state of the motor20is shorter than the remaining lifetime from the detection of the semi-abnormal state of the motor drive circuit40. In this example, a stop setting time t1mis set when the semi-abnormal state of the motor20is detected, and a stop setting time t1qlonger than the stop setting time t1mis set when the semi-abnormal state of the motor drive circuit40is detected. Note that, in the case where the relationship of the remaining lifetime between the motor20and the motor drive circuit40is reverse, the characteristics of the upper limit command voltage map are reversed so that the stop setting time t1mis set to be longer than the stop setting time t1q.

According to Modified Example 2-5, the characteristics of the upper limit command voltage Vmax and the stop setting times t1are set independently for the case where the motor20becomes the semi-abnormal state and the case where the motor drive circuit40becomes the semi-abnormal state. In other words, the characteristics of the upper limit command voltage Vmax and the stop setting time t1are switched depending on the region where the semi-abnormality has occurred. In this manner, an appropriate steering assist corresponding to the degree of degradation of the semi-abnormal region can be executed. Further, an appropriate steering assist executable period corresponding to the remaining lifetime of the semi-abnormal region can be set. As a result, the operation of the motor20can be limited more appropriately.

Modified Example 2-6

Limitation on Operation Based on Target Assist Torque

The operation of the motor20may be limited in a manner that the target assist torque tr* is decreased as compared to the normal case when the semi-abnormal state of the motor20or the motor drive circuit40is detected. For example, in Step S2of the steering assist control routine (FIG. 3), the target assist torque tr* set based on the assist map may be corrected in a manner that the target assist torque tr* is multiplied by a limiting coefficient K (0≦K≦1) and the calculated value (tr*×K) is set as a final target assist torque tr*. In this case, the limiting coefficient K is set to have a value that decreases as the internal resistance value of the motor20or the motor drive circuit40becomes larger.

Alternatively, an elapsed time t may be measured from the first detection of the semi-abnormal state, and the limiting coefficient K may be gradually decreased from “1”, which is the value for the normal case, so that the steering assist is stopped when the elapsed time t reaches the stop setting time t1.

Alternatively, in the case where the semi-abnormal state of the motor20or the motor drive circuit40is detected, the target assist torque tr* may be set always by using the characteristics of an assist map for high-speed running regardless of the vehicle speed vx (seeFIG. 4). In this case, the operation of the motor20is limited in a manner that the target assist torque tr is set to be smaller than the one for the normal case.

Modified Example 2-7

Combination with Current Limitation Based on Estimated Temperature

In the above-mentioned embodiment, the upper limit current Imax is set based on the internal resistance value Rm of the motor20and the internal resistance value Rq of the motor drive circuit40. Alternatively, however, the upper limit current Imax may be set in consideration of the motor estimated temperature Tm and the substrate temperature Tb in addition to the internal resistance value Rm of the motor20and the internal resistance value Rq of the motor drive circuit40.FIG. 24illustrates a motor limit value setting routine executed by the microcomputer60at predetermined cycles. For executing the motor limit value setting routine, the microcomputer60stores a motor temperature upper limit current map that sets the relationship between the motor estimated temperature Tm and an upper limit current Imax_Tm for motor temperature as shown inFIG. 25(a), and a substrate temperature upper limit current map that sets the relationship between the substrate temperature Tb and an upper limit current Imax_Tb for substrate temperature as shown inFIG. 25(b). The motor temperature upper limit current map has such characteristics that the upper limit current Imax_Tm for motor temperature decreases as the motor estimated temperature Tm becomes higher. The substrate temperature upper limit current map has such characteristics that the upper limit current Imax_Tb for substrate temperature decreases as the substrate temperature Tb becomes higher.

The motor limit value setting routine (FIG. 24) is repeated at predetermined short cycles in parallel with the steering assist control routine. In Step S111, the microcomputer60reads the motor actual current Im detected by the current sensor31and the substrate temperature Tb detected by the substrate temperature sensor35. Subsequently, in Step S112, the microcomputer60calculates the motor estimated temperature Tm. The motor estimated temperature Tm indicates a temperature increase amount due to heat generation of the motor20, and can be calculated by using a square-integrated value of the motor actual current Im detected by the current sensor31. A current square-integrated value SUM for temperature estimation is calculated by Expression (11) below.
SUM(n)=SUM(n−1)+Ktm·(Im2−SUM(n−1))  (11)

In Expression (11), Ktm represents a predetermined coefficient indicating how the temperature of the motor20changes in accordance with the square value of the motor actual current Im. Further, (n) means a value that is calculated by the current processing in the upper limit current Imax setting routine repeatedly executed at predetermined short cycles. Therefore, SUM(n) is a current square-integrated value for temperature estimation that is to be obtained by the current calculation, and SUM(n−1) is a current square-integrated value for temperature estimation that is calculated in the previous calculation cycle.

As expressed by Expression (12) below, the microcomputer60calculates the motor estimated temperature Tm by multiplying the current square-integrated value SUM(n) for temperature estimation by a motor temperature gain Gm.
Tm=Gm·SUM(n)  (12)

In this calculation, an initial value of SUM(n−1) is necessary. For example, the initial value of SUM(n−1) may be obtained as follows. A current square-integrated value SUM(n) for temperature estimation measured at the end of the steering assist control routine is stored in a non-volatile memory, and, when the next steering assist control routine is started, a temperature change amount (ΔT) caused by heat dissipation is subtracted from the stored current square-integrated value SUM(n) for temperature estimation. For example, the temperature change amount (ΔT) can be calculated based on a temperature variation amount of the substrate temperature Tb. In the case where an operation stop period of the motor20is long, the initial value of SUM(n−1) can be set to zero.

Subsequently, in Step S113, the microcomputer60refers to the motor temperature upper limit current map to calculate the upper limit current Imax_Tm for motor temperature based on the motor estimated temperature Tm. Subsequently, in Step S114, the microcomputer60refers to the substrate temperature upper limit current map to calculate the upper limit current Imax_Tb for substrate temperature based on the substrate temperature Tb.

Subsequently, in Step S115, the microcomputer60compares the upper limit current Imax_Tm for motor temperature and the upper limit current Imax_Tb for substrate temperature, and sets a smaller one as the upper limit current Imax_T for temperature in Step S116or S117.

Subsequently, in Step S118, the microcomputer60determines whether or not the upper limit current Imax set in Step S45or S46of the motor limit value setting routine in the embodiment is larger than the upper limit current Imax_T for temperature. When the upper limit current Imax is larger than the upper limit current Imax_T for temperature (S118: Yes), in Step S119, the microcomputer60changes the upper limit current Imax to the value of the upper limit current Imax_T for temperature (Imax←Imax_T). On the other hand, when the upper limit current Imax is equal to or smaller than the upper limit current Imax_T for temperature (S118: No), the microcomputer60skips the processing of Step S119.

After setting the upper limit current Imax in this manner, the microcomputer60finishes the motor limit value setting routine once. In Step S4of the steering assist control routine, the microcomputer60reads the upper limit current Imax set in this motor limit value setting routine.

According to Modified Example 2-7, the upper limit current of the motor20is set based on the internal resistance value Rm of the motor20and the motor estimated temperature Tm while the upper limit current of the motor drive circuit40is set based on the internal resistance value Rq of the motor drive circuit40and the substrate temperature Tb, and the upper limit current of the motor20is set with the use of a smaller one of the upper limit currents. Therefore, the motor20and the motor drive circuit40can be protected more properly to prolong the lifetime.

Modified Example 2-8

Limitation on Operation Based on Temperature Gain

In the above-mentioned embodiment, when the semi-abnormal state of the motor20or the motor drive circuit40is detected, the upper limit current Imax corresponding to the internal resistance value is set to limit the operation of the motor20. According to Modified Example 2-8, however, the upper limit current Imax is changed by setting a temperature gain corresponding to the internal resistance value.

In Modified Example 2-8, the microcomputer60sets the upper limit current Imax to be used in Step S4of the steering assist control routine based on the motor estimated temperature Tm and the substrate temperature Tb. For example, the microcomputer60performs the same processing as in Steps S111to S117according to Modified Example 2-7 to calculate an upper limit current Imax_T for temperature, and sets the upper limit current Imax_T for temperature as the upper limit current Imax to be used in the steering assist control routine.

In this case, the microcomputer60calculates the motor estimated temperature Tm in Step S112by using a motor temperature gain Gm that is set in accordance with the internal resistance value Rm of the motor20. The motor estimated temperature Tm is calculated by Expression (12) below.
Tm=Gm·SUM(n)  (12)

Therefore, the motor estimated temperature Tm can be adjusted by varying the motor temperature gain Gm of Expression (12) in accordance with the internal resistance value Rm.

The microcomputer60stores a motor temperature gain map having the characteristics shown inFIG. 26(a). The microcomputer60refers to the motor temperature gain map to calculate the motor temperature gain Gm (≧1) based on the internal resistance value Rm. The motor temperature gain map has such characteristics that the motor temperature gain Gm increases as the internal resistance value Rm of the motor20becomes larger. Therefore, the motor estimated temperature Tm becomes higher as the internal resistance value Rm of the motor20becomes larger.

In the case where the upper limit current Imax_Tb for substrate temperature is set in Step S114, the microcomputer60corrects the substrate temperature Tb with a substrate temperature gain Gb. Specifically, as expressed by Expression (13) below, a value obtained by multiplying the substrate temperature Tb detected by the substrate temperature sensor35by the substrate temperature gain Gb is set as a final substrate temperature Tb.
Tb=Gb·Tb(13)

Therefore, the substrate temperature Tb can be adjusted by varying the substrate temperature gain Gb of Expression (13) in accordance with the internal resistance value Rq of the motor drive circuit40.

The microcomputer60stores a substrate temperature gain map having the characteristics shown inFIG. 26(b). The microcomputer60refers to the substrate temperature gain map to calculate the substrate temperature gain Gb (≧1) based on the internal resistance value Rq. The substrate temperature gain map has such characteristics that the substrate temperature gain Gb increases as the internal resistance value Rq of the motor drive circuit40becomes larger. Therefore, the substrate temperature Tb becomes higher as the internal resistance value Rq of the motor drive circuit40becomes larger.

According to Modified Example 2-8, in the case where the internal resistance value Rm of the motor20increases to become the semi-abnormal state, the motor estimated temperature Tm increases in calculation, and the upper limit current Imax decreases correspondingly (seeFIG. 25(a)). Therefore, the current flowing through the motor20can be limited as compared to the normal case. On the other hand, in the case where the internal resistance value Rq of the motor drive circuit40increases to become the semi-abnormal state, the substrate temperature Tb increases in calculation, and the upper limit current Imax decreases correspondingly (seeFIG. 25(b)). Therefore, the current flowing through the motor20can be limited as compared to the normal case.

The motor temperature gain map and the substrate temperature gain map can be set to have independent characteristics, and hence the operation of the motor20can be properly limited in accordance with the region where the semi-abnormal state has occurred.

Modified Example 3

Determination of Semi-Abnormal State

In the above-mentioned embodiment, the semi-abnormal state is determined by comparison between the measured internal resistance values Rm and Rq and the preset determination values ((Rref_m1, Rref_m2), (Rref_q1, Rref_q2)) (see S40and S42). Alternatively, however, various kinds of methods can be employed to determine the semi-abnormal state.

Modified Example 3-1

Determination Based on Initial Value of Internal Resistance Value

For example, an internal resistance value measured first may be set as an initial value, and the semi-abnormal state may be determined based on a fluctuation from the initial value. In this case, internal resistance values measured first are stored in a non-volatile memory (not shown) as initial values Rm0and Rq0, and after that, the measured internal resistance values Rm and Rq are compared against the initial values Rm0and Rq0in Step S40and Step S42, respectively, for every measurement of the internal resistance values.

The determination is made in a manner that, for example, an increase amount (Rm-Rm0, Rq-Rq0) or an increase rate (Rm/Rm0, Rq/Rq0) of the measured internal resistance value with respect to the initial value is calculated, and it is determined that the internal resistance value is in the semi-abnormal state when the increase amount or the increase rate exceeds a preset first reference value and that the internal resistance value is in the abnormal state when the increase amount or the increase rate exceeds a second reference value larger than the first reference value. Note that, the initial value Rq0of the internal resistance values of the switching elements Q1, Q2, Q3, and Q4may be, for example, an arbitrary value of the internal resistance values Rq1, Rq2, Rq3, and Rq4measured first, or may be an average value, a maximum value, or a minimum value thereof. Alternatively, increase amounts or increase rates of the internal resistance values Rq1, Rq2, Rq3, and Rq4of the individual switching elements Q1, Q2, Q3, and Q4may be calculated to perform the determination based on the increase amounts or the increase rates. In this case, a maximum value of the four increase amounts or increase rates is compared against the reference value.

Modified Example 3-2

Temperature Correction of Determination Value

The determination values ((Rref_m1, Rref_m2), (Rref_q1, Rref_q2)) for determining the measured internal resistance values Rm and Rq may be subjected to temperature correction. The microcomputer60is provided with the function of estimating the temperature Tm of the motor20based on the motor actual current Im and the function of detecting the substrate temperature Tb by the substrate temperature sensor35. Therefore, the determination values (Rref_m1and Rref_m2) for determining the semi-abnormal state of the motor20can be corrected based on the estimated temperature Tm of the motor20. Further, the determination values (Rref_q1and Rref_q2) for determining the semi-abnormal state of the motor drive circuit40can be corrected based on the substrate temperature Tb.

The relationship between the estimated temperature Tm of the motor20and the internal resistance value Rm and the relationship between the substrate temperature tb and the internal resistance value Rq can be obtained in advance by experiment. Therefore, a correction map showing the relationship between the estimated temperature Tm and a correction coefficient and a correction map showing the relationship between the substrate temperature Tb and a correction coefficient can be stored in the microcomputer60.

The microcomputer60reads the estimated temperature Tm of the motor20before determining in Step S40whether the motor20is normal, abnormal, or semi-abnormal based on the motor internal resistance value Rm. Then, the microcomputer60refers to the correction map to set a correction coefficient corresponding to the estimated temperature Tm, and multiplies the determination value (Rref_m1, Rref_m2) by the correction coefficient to obtain a new determination value (Rref_ml, Rref_m2). The microcomputer60uses the corrected determination value (Rref_m1, Rref_m2) to determine whether the motor20is normal, abnormal, or semi-abnormal. Note that, the estimated temperature Tm of the motor20can be calculated by the method described above in Modified Example 2-7.

Similarly, the microcomputer60reads the substrate temperature Tb detected by the substrate temperature sensor35before determining in Step S42whether the motor drive circuit40is normal, abnormal, or semi-abnormal based on the circuit internal resistance value Rq. Then, the microcomputer60refers to the correction map to set a correction coefficient corresponding to the substrate temperature Tb, and multiplies the determination value (Rref_q1, Rref_q2) by the correction coefficient to obtain a new determination value (Rref_q1, Rref_q2). The microcomputer60uses the corrected determination value (Rref_q1, Rref_q2) to determine whether the motor drive circuit40is normal, abnormal, or semi-abnormal.

According to Modified Example 3-2, the state (normal, abnormal, or semi-abnormal) relating to the internal resistances of the motor20and the motor drive circuit40can be determined more accurately.

Modified Example 3-3

Determination Based on Voltage Change

In the above-mentioned embodiment, the internal resistance value Rm of the motor20and the internal resistance value Rq of the motor drive circuit40are measured. However, it is not always necessary to measure the internal resistance values. For example, the semi-abnormal state can be detected based on a temporal change of the inter-terminal voltage of the motor20.

In Modified Example 3-3, the microcomputer60executes a motor limit value setting routine illustrated inFIG. 27. The motor limit value setting routine according to Modified Example 3-3 is obtained by changing the processing of Step S20to Step S46of the motor limit value setting routine (FIG. 6) according to the embodiment. In the following, the same processing inFIG. 27as in the embodiment is denoted by the same step number as in the embodiment, and description thereof is omitted.

When the system of the electric power steering device1has started up normally (S13: Yes), in Step S120, the microcomputer60subsequently operates the switching elements Q1and Q4to be turned ON and OFF at a preset duty cycle α. Subsequently, in Step S121, the microcomputer60measures the first motor terminal voltage V1, the second motor terminal voltage V2, and the power supply voltage Vcc at that time. Subsequently, in Step S122, the microcomputer60calculates a deviation ΔV1(=|(α×Vcc)−(V1−V2)|) between a value (α×Vcc) obtained by multiplying the power supply voltage Vcc by the duty cycle α and the inter-terminal voltage (V1−V2) of the motor20.

Subsequently, in Step S123, the microcomputer60operates the switching elements Q2and Q3to be turned ON and OFF at the duty cycle α. Subsequently, in Step S124, the microcomputer60measures the first motor terminal voltage V1, the second motor terminal voltage V2, and the power supply voltage Vcc at that time. Subsequently, in Step S125, the microcomputer60calculates a deviation ΔV2(=|(α×Vcc)−(V2−V1)|) between a value (α×Vcc) obtained by multiplying the power supply voltage Vcc by the duty cycle α and the inter-terminal voltage (V2−V1) of the motor20.

Subsequently, in Steps S126to S128, the microcomputer60sets a larger one of the deviation ΔV1and the deviation ΔV2as a deviation ΔV. Note that, the deviation ΔV1and the deviation ΔV2may be measured alternately a plurality of times to measure average values thereof.

When both the internal resistance value of the motor20and the internal resistance value of the motor drive circuit40are normal, the deviation ΔV is a small value. However, the deviation ΔV increases when the internal resistance value of the motor20or the internal resistance value of the motor drive circuit40increases. Utilizing this relationship, in Step S129, the microcomputer60determines whether a motor energization path formed of the motor20and the motor drive circuit40is normal, semi-abnormal, or abnormal based on the deviation ΔV. For example, a deviation ΔV measured first is stored in a non-volatile memory (not shown) as an initial value ΔV0, and after that, a deviation ΔV calculated in Step S122is compared against the initial value ΔV0for determination.

The determination is made in a manner that, for example, an increase amount (ΔV−ΔV0) or an increase rate (ΔV/ΔV0) of the deviation ΔV with respect to the initial value ΔV0is calculated, and it is determined that the motor energization path is in the semi-abnormal state when the increase amount or the increase rate exceeds a preset first reference value and that the motor energization path is in the abnormal state when the increase amount or the increase rate exceeds a second reference value larger than the first reference value. Note that, the determination may be made by comparing the deviation ΔV against a preset set value ΔV0, instead of the initial value ΔV0.

When detecting the semi-abnormal state in Step S129, the microcomputer sets the upper limit current Imax in Step S130. In this case, the upper limit current Imax is set to have a value that decreases as the increase amount (ΔV−ΔV0) or the increase rate (ΔV/ΔV0) becomes larger.

Modified Example 4

Alarming of Semi-abnormality

A warning lamp94(seeFIG. 1) may be connected to the assist ECU100so that the warning lamp94is turned ON when the semi-abnormal state of the motor20or the motor drive circuit40is detected. This configuration enables a driver to know the abnormality at a time when the motor20or the motor drive circuit40becomes semi-abnormal. Therefore, the repair can be arranged at an appropriate timing (before a failure occurs in a component).

While the electric power steering device1according to the embodiment and the modified examples has been described above, the present invention is not intended to be limited to the above-mentioned embodiment and modified examples, and various changes are possible within the range not departing from the object of the present invention.

For example, the embodiment of the present invention employs the configuration in which both the internal resistance values of the motor20and the motor drive circuit40are measured to limit the operation of the motor20in accordance with the measured internal resistance values. Alternatively, however, it is possible to employ another configuration in which any one of the internal resistance values of the motor20and the motor drive circuit40is measured to limit the operation of the motor20in accordance with the measured internal resistance value. For example, in the case where the motor drive circuit40is higher in reliability than the motor20, the operation of the motor20may be limited in accordance with the internal resistance value of the motor20while the measurement, the determination, and the motor operation limitation relating to the internal resistance value of the motor drive circuit40(S20, S40, and S41) are omitted, and, on the other hand, in the case where the motor20is higher in reliability than the motor drive circuit40, the operation of the motor20may be limited in accordance with the internal resistance value of the motor drive circuit40while the measurement, the determination, and the motor operation limitation relating to the internal resistance value of the motor20(S30, S42, and S43) are omitted.

In the embodiment of the present invention, MOS-FETs are used as the switching elements Q1, Q2, Q3, and Q4used in the motor drive circuit (H bridge circuit)40. However, the present invention is not limited thereto, and other switching semiconductor elements may be used.

In the embodiment of the present invention and the modified examples, various kinds of maps are used to derive variables. However, a calculation formula using a function or the like may be used instead of the maps.

In the embodiment of the present invention, the stop setting time is not provided. Alternatively, however, for example, as described in Modified Examples 2-1, 2-2, 2-4, 2-5, and 2-6, if the semi-abnormal state is detected, the steering assist may be stopped when an elapsed time from the detection timing has reached a stop setting time.

In Modified Example 3-2, the determination values ((Rref_m1, Rref_m2), (Rref_q1, Rref_q2)) to be compared against the measured internal resistance values Rm and Rq are subjected to temperature correction. Alternatively, however, the measured internal resistance values Rm and Rq may be subjected to temperature correction.

In the embodiment of the present invention described above, the electric power steering device1is a column-assist electric power steering device that applies torque generated by the motor20to the steering shaft12. Alternatively, however, a rack-assist electric power steering device that applies the torque generated by the motor to the rack bar14may be used.