JP2011178245A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device capable of exactly detecting an abnormality of a field effect transistor connected to a capacitor for smoothing in series. <P>SOLUTION: A motor driving circuit 13 for driving an electric motor generating steering auxiliary force in relation to a steering mechanism of a vehicle includes a bridge circuit 22 for driving the electric motor, power supply terminals tp and tn inputted with DC power from a DC power source for supplying DC power to the bridge circuit 22, the capacitor C1PC2 for smoothing interposed between a positive electrode line and a negative electrode line for connecting the power supply terminal and the bridge circuit 22, a field effect transistor FET2 connected between the capacitor C1PC2 and the positive electrode line so that a parasitic diode at the inside may have a direction for preventing accumulation of charge to the capacitor C1PC2, and an abnormality detection means 12 for detecting voltage between the capacitor C1PC2 and the field effect transistor to detect an abnormality of the field effect transistor. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electric power steering apparatus that transmits a steering assist force generated by an electric motor to a steering mechanism of a vehicle, and more particularly to an improvement of a motor drive circuit that drives the electric motor.

As this type of electric power steering device, for example, a relay circuit having a relay with a normally open contact configuration is inserted between a motor driving circuit for driving an electric motor and a battery, and the relay circuit is turned off when an abnormality occurs. As an example, an electric power steering device has been proposed in which the supply of DC power to an electric motor drive circuit is cut off (see, for example, Patent Document 1).
Further, instead of the relay circuit, two field effect transistors are connected in series, and one field effect transistor is arranged so that its parasitic diode is in the forward direction with respect to the power supply, and the other field effect transistor is connected to the field effect transistor. There has been proposed an electric power steering device using a switch in which a parasitic diode is disposed in the reverse direction with respect to a power supply (see, for example, Patent Document 2).

Japanese Patent No. 2506269 Japanese Patent No. 3375502

  Here, in the conventional example described in Patent Document 1, a relay circuit is applied to cut off the DC power supplied to the motor drive circuit when an abnormality occurs, and the power is cut off by the relay circuit when the system is stopped. Therefore, normally, the leakage current of the capacitor provided for smoothing the DC power supplied to the motor drive circuit can be prevented, and the dark current can be suppressed. However, since the relay circuit is applied, when the response is slow and a large current is required to drive the electric motor, the relay circuit itself becomes large and the operation noise is generated when the relay is turned on / off. There is a problem that the driver feels uncomfortable. Furthermore, since it is a mechanical contact, there is a problem that contact failure due to welding or foreign matter may occur.

  On the other hand, in the conventional example described in Patent Document 2, two field effect transistors are used in place of the relay circuit, and one of the field transistors has a parasitic diode in the forward direction with respect to the power supply. The switch is configured by arranging the other charge transistor so that its parasitic diode is in the opposite direction with respect to the power supply, so the response speed is increased and compared to a relay circuit for high power Thus, the power consumption can be reduced because the power is small and the power for controlling the on state may be small.

  However, in the conventional example described in Patent Document 2, the switch between the bridge circuit that drives the motor and the power source is configured by two field effect transistors in order to cope with the reverse polarity connection of the battery power source. Therefore, even when the battery power source is connected in reverse polarity, it is possible to prevent reverse current from flowing by the parasitic diode of the field effect transistor, and to protect the bridge circuit and the motor. Since a switch is formed by connecting two field effect transistors in series, it is necessary to prepare a field effect transistor with uniform characteristics, and a field effect transistor having a parasitic diode in the reverse direction with respect to the power source There is an unresolved problem that the power cannot be supplied to the bridge circuit when the off abnormality occurs and the reliability is lowered.

  In order to solve this unsolved problem, it is conceivable to omit a field effect transistor arranged so that the parasitic diode in the conventional example described in Patent Document 2 is in the opposite direction to the power supply. In this case, when a smoothing capacitor for smoothing battery power is connected in parallel with the bridge circuit, the forward diode is turned off when the field effect transistor that is forward with respect to the power supply is turned off when the system is stopped. Since a leakage current flows through the smoothing capacitor via the capacitor, a field effect transistor having a parasitic diode in the reverse direction to the power supply is inserted in series with the smoothing capacitor in order to cut off the leakage current. It is possible.

At this time, it is important whether or not the field effect transistor inserted in series with the smoothing capacitor operates normally, but no means for detecting an abnormality of the field effect transistor having the above configuration has been proposed.
Therefore, the present invention has been made paying attention to the unsolved problems of the above-described conventional example, and provides an electric power steering device capable of accurately detecting an abnormality of a field effect transistor connected in series with a smoothing capacitor. The purpose is to do.

  To achieve the above object, an electric power steering apparatus according to a first aspect of the present invention includes an electric motor that generates a steering assist force for a steering mechanism of a vehicle, a motor drive circuit that drives the electric motor, and the motor. An electric power steering apparatus including a control device that controls a drive circuit, wherein the motor drive circuit includes a bridge circuit that drives the electric motor, and a DC power from a DC power source that supplies DC power to the bridge circuit. Is input between the power supply terminal, a smoothing capacitor interposed between the positive electrode line and the negative electrode line connecting the power supply terminal and the bridge circuit, and between the smoothing capacitor and the positive electrode line, A field effect transistor connected so that an internal parasitic diode prevents the accumulation of charge in the smoothing capacitor; By sensing the voltage between the smoothing capacitor and said field effect transistor is characterized by comprising an abnormality detector for detecting an abnormality of the field effect transistor.

An electric power steering apparatus according to a second aspect of the present invention is the electric power steering apparatus according to the first aspect, wherein the abnormality detecting means includes a voltage detecting unit that detects a voltage between the smoothing capacitor and the field effect transistor; An abnormality that detects an abnormality of the field effect transistor based on a control state determination unit that detects a control state of the field effect transistor, a voltage that is detected by the voltage detection unit, and a control state that is determined by the control state determination unit And a detection unit.
Furthermore, the electric power steering apparatus according to a third aspect of the present invention is the electric power steering apparatus according to the first or second aspect, wherein a power supply field effect transistor is provided between the positive electrode line and a connection point between the power supply terminal and the field effect transistor. The internal parasitic diode is connected to the DC power supply in a forward direction.

According to the present invention, when a field effect transistor is connected between the smoothing capacitor and the positive electrode line so that the parasitic diode inside thereof is opposite to the power supply, the field effect transistor and the smoothing capacitor are connected. By detecting the voltage at the connection point between the two, a field effect transistor abnormality can be accurately detected.
In this case, the abnormality of the field effect transistor can be detected more accurately by detecting both the voltage at the connection point between the field effect transistor and the smoothing capacitor and the control state of the field effect transistor.

It is a figure showing the schematic structure of the electric power steering device to which the present invention is applied. It is a circuit diagram which shows the specific structure of a motor drive circuit. 3 is a flowchart showing an example of an abnormality detection processing procedure executed by a control unit 12.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic configuration diagram showing an embodiment of the present invention, in which 1 is a steering mechanism. The steering mechanism 1 includes a steering shaft 3 on which a steering wheel 2 is mounted, a rack and pinion mechanism 4 connected to the opposite side of the steering shaft 3 from the steering wheel 2, and a connection of a tie rod or the like to the rack and pinion mechanism 4. Left and right steered wheels (not shown) are connected via a mechanism 5.
The steering shaft 3 is connected to an electric motor 8 composed of, for example, a brushless motor via a speed reduction mechanism 7 composed of, for example, a worm gear.

The electric motor 8 operates as a steering assist force generation motor that generates a steering assist force of the electric power steering apparatus. The electric motor 8 is directly driven by a battery voltage output from a battery 9 mounted on the vehicle and is driven by a control device 11 that is input via an ignition switch 10.
The control device 11 receives a control unit 12 that calculates a steering assist command value and outputs a gate drive signal, and a gate drive signal output from the control unit 12, and an electric motor based on the gate drive signal. And a motor drive circuit 13 for driving the motor.

The control unit 12 includes a motor current detection value Imd detected by a motor current detection circuit 15 that detects a motor drive current supplied to the electric motor 8, and a steering detected by a steering torque sensor 16 disposed on the steering shaft 3. A steering torque T input to the wheel 2 is input, and a vehicle speed Vs detected by a vehicle speed sensor 17 as a vehicle speed detection unit is input.
Here, the steering torque sensor 16 detects the steering torque applied to the steering wheel 2 and transmitted to the steering shaft 3. For example, the torsion bar in which the steering torque is interposed between an input shaft and an output shaft (not shown). The torsional angular displacement is converted into an electrical signal, and the torsional angular displacement is detected with a magnetic signal and converted into an electrical signal.

  Then, the control unit 12 calculates a steering assist command value based on the steering torque T and the vehicle speed Vs, calculates a dq axis current command value based on the steering assist command value, and this dq axis current command. Two-phase to three-phase conversion is performed to calculate a three-phase current command value, and the current deviation between the calculated three-phase current command value and the motor current detection value detected by the motor current detection circuit 15 is proportional / integrated (PI) Control calculation is performed to calculate a three-phase voltage command value, inverter gate drive signals IG1 to IG6 are formed based on the three-phase voltage command value, and the formed gate drive signals IG1 to IG6 are output to the motor drive circuit 13. . In addition, the first and second field effect transistors FET1 and FET2, which will be described later as power switches of the motor drive circuit 13 when the power is turned on, are controlled to be turned on, and the gate drive signal is controlled to be turned off when the system is stopped or abnormal. SG1 and SG2 are output. Further, the field effect transistors FETu to FETw that are motor switches with the motor drive circuit 13 are controlled to be in an on state when the power is turned on, and gate drive signals SGu to SGw that are controlled to be in an off state when the system is stopped and above.

  As shown in FIG. 2, the motor drive circuit 13 has a power supply terminal tp to which the positive terminal of the battery 9 is connected and a power supply terminal tn to which the negative terminal is connected. The positive line Lp is connected to these power supply terminals tp and tn. And the negative electrode line Ln is derived | led-out. Between the positive electrode line Lp and the negative electrode line Ln, a noise filter 21 including a normal coil and a common coil for reducing noise on these lines is connected. Furthermore, the first field effect transistor FET1 for supplying power is connected to the positive line Lp on the output side of the noise filter 21 so that the parasitic diode thereof is in the forward direction with respect to the power of the positive line. On the output side of the first field effect transistor FET1, smoothing capacitors C1 and C2 are connected in parallel between the positive electrode line Lp and the negative electrode line Ln, and the positive electrode line Lp side of the smoothing capacitor and the positive electrode line Lp are connected to each other. In the meantime, the second field effect transistor FET2 is connected such that the parasitic diode is in a direction to prevent the charge from being accumulated in the smoothing capacitors C1 and C2.

Furthermore, an inverter circuit 22 as a bridge circuit is connected to the positive line Lp and the negative line Ln in parallel with the series circuit of the smoothing capacitors C1 and C2 and the second field effect transistor FET2.
This inverter circuit 22 has six field effect transistors FET11 to FET16. These field effect transistors FET11 and FET12 are connected in series to form a U-phase switching arm SAu, and the field effect transistors FET13 and FET14 are connected in series. A connected V-phase switching arm SAv is formed, field effect transistors FET15 and FET16 are connected in series to form a W-phase switching arm SAw, and the switching arms SAu to SAw are connected in parallel. The connection points of the field effect transistors FET11 and FET12, the connection points of the field effect transistors FET13 and FET14, and the connection points of the field effect transistors FET15 and FET16 of the switching arms SAu, SAv and SAw are respectively the U-phase AC output point Pu and V-phase AC. An output point Pv and a W-phase AC output point Pw are set. The field effect transistors FETu, FETv, and FETw serve as motor switches between the AC output points Pu, Pv, and Pw and the output terminals tu, tv, and tw. Connected as an allowable direction.

  The output terminals tu, tv, and tw are connected to the three-phase power feeding terminal of the electric motor 8. The gate drive signals IG1 to IG6 output from the control unit 12 are supplied to the gates of the field effect transistors FET11 to FET16 of the inverter circuit 22. The gate drive signals SG1 and SG2 output from the control unit 12 are supplied to the gates of the first and second field effect transistors FET1 and FET2. Further, gate drive signals SGu to SGw output from the control unit 12 are supplied to the gates of the field effect transistors FETu to FETw.

Further, in order to detect the abnormality of the field effect transistor FET2 connected between the smoothing capacitors C1 and C2 and the positive electrode line Lp, the connection point between the smoothing capacitors C1 and C2 and the field effect transistor FET2 is connected. A voltage detection unit 30 is connected to the point.
The voltage detection unit 30 includes voltage dividing resistors R1 and R2 connected between a connection point between the smoothing capacitors C1 and C2, a connection point between the field effect transistor FET2 and the ground, and the voltage dividing resistors R1 and R2 The detection voltage Vf is output during the interval. In the voltage detection unit 30, the voltage at the connection point between the smoothing capacitors C1 and C2 and the field effect transistor FET2 is shifted to the detection voltage Vf that can be handled by the control unit 12.

Then, the detected voltage Vf detected by the voltage detector 30 is input to the A / D conversion input terminal of the control unit 12 as shown in FIG.
In this control unit 12, the abnormality detection process shown in FIG. 3 for detecting an abnormality of the field effect transistor FET2 is executed as a timer interrupt process for the main program every predetermined time (for example, 1 msec). This abnormality detection process is started when the control unit 12 is powered on. First, in step S1, it is determined whether or not the gate drive signal SG2 for the field effect transistor FET2 is in an off state, and the gate drive signal. When SG2 is in the off state, the process proceeds to step S2.

  In step S2, the voltage Vf detected by the voltage detection unit 30 is converted into a digital signal by the A / D conversion unit and read. Then, the process proceeds to step S3, where the voltage Vf is equal to or lower than the low voltage side threshold Vthd close to zero. If it is determined that Vf> Vthd, it is determined that the field effect transistor FET2 is in an on-state abnormality that continues to be on, and the process proceeds to step S4.

In step S4, the variable N is incremented by “1”, and then the process proceeds to step S5 to determine whether or not the variable N has reached a set value Ns corresponding to a predetermined time set in advance. When Ns, the process returns to step S2, and when N = Ns, the process proceeds to step S6.
In this step S6, a warning process for displaying a flashing warning display element 31 formed on the instrument panel of the driver's seat, for example, an abnormality alarm indicating that the field effect transistor FET2 is abnormal on is terminated, and then the timer interruption process is terminated. Then, the program returns to the predetermined main program.

If the determination result in step S3 is Vf ≦ Vthd, it is determined that the field effect transistor FET2 is normal, the process proceeds to step S7, the variable N is cleared to “0”, and the timer interrupt process is terminated. Then, the program returns to the predetermined main program.
On the other hand, if the determination result of step S1 is that the gate drive signal SG2 is in the on state, the process proceeds to step S8, where the voltage Vf detected by the voltage detector 30 is converted into a digital signal by the A / D converter. After reading, the process proceeds to step S9.

In this step S9, it is determined whether or not the voltage Vf read in step S8 is equal to or higher than the high voltage side threshold Vthh close to the voltage Vb of the battery 9, and when Vf <Vthh, the field effect transistor FET2 is turned off. It is determined that the off abnormality continues, and the process proceeds to step S10.
In step S10, the variable M is incremented by “1”, and then the process proceeds to step S11 to determine whether or not the variable M has reached a set value Ms corresponding to a predetermined time set in advance. If Ms, the process returns to step S8. If M = Ms, the process proceeds to step S12.

In this step S12, after performing warning processing for turning on and displaying the warning display element 31 formed on the instrument panel of the driver's seat, for example, an abnormality alarm indicating that the field effect transistor FET2 is abnormally off, the timer interruption processing is terminated. Then, the program returns to the predetermined main program.
If the determination result in step S9 is Vf ≧ Vthh, it is determined that the field effect transistor FET2 is normal, the process proceeds to step S12, the variable M is cleared to “0”, and the timer interrupt process is performed. End and return to a predetermined main program.
The process of FIG. 3 and the voltage detection unit 30 constitute an abnormality detection means, among which the process of step S1 corresponds to the control state determination unit, and the processes of steps S2 to S7 and steps S8 to S13 are the abnormality detection unit. It corresponds to.

Next, the operation of the above embodiment will be described.
Now, when the ignition switch 10 is turned on while the vehicle is stopped, the battery voltage of the battery 9 is input to the control unit 12 in response to this, whereby the control unit is powered on. In this state, the control unit 12 performs initialization processing such as a predetermined initial diagnosis, and when the diagnosis result is normal, the first and second field effect transistors FET1 and FET2 of the motor drive circuit 13 Gate drive signals SG1, SG2, SGu, SGv, and SGw that turn on the effect transistors FETu, FETv, and FETw, respectively, are output to the motor drive circuit 13.

  As a result, the field effect transistors FET1, FET2, and FETu to FETw of the motor drive circuit 13 are turned on, and the DC power of the battery 9 is further smoothed by the smoothing capacitors C1 and C2 via the noise filter 21, and the inverter circuit. 22 and the output of the inverter circuit 22 is supplied to the electric motor 8 via the field effect transistors FETu to FETw.

  At this time, in a non-steering state in which the steering wheel 2 is not steered, the steering torque detected by the torque sensor 16 is maintained at zero, and thus the steering assist command value calculated by the control unit 12 becomes zero. The d-axis current command value and the q-axis current command value obtained by converting the auxiliary command value by dq axis are also zero, and the three-phase current command value obtained by converting these two-phase to three-phase is also zero. Further, since the electric motor 8 is maintained in a stopped state, and the motor current detection value detected by the motor current detection circuit 15 in the inverter circuit 22 is also zero, the current deviation between the current command value and the motor current detection value is also zero. Thus, the three-phase voltage command value of the PI control calculation result is also zero, and all of the inverter gate drive signals IG1 to IG6 have a 50% duty. For this reason, all the field effect transistors FET11 to FET16 of the inverter circuit 22 of the motor drive circuit 13 maintain the 50% duty state.

  When the driver performs a so-called stationary operation in which the steering wheel 2 is steered while the vehicle is stopped, a large steering torque is detected by the torque sensor 16 and is input to the control unit 12. For this reason, a large steering assist command value is calculated by the control unit 12, and the d-axis current command value and the q-axis current command value converted by the dq axis are increased accordingly, and this is converted into a two-phase to three-phase conversion The three-phase current command value also becomes a large value. Since the deviation between the three-phase current command value and the detected motor current value for maintaining zero is PI-controlled, the three-phase voltage command value is also a large value. -IG6 is output to the inverter circuit 22 of the motor drive circuit 13.

In response to this, a large three-phase drive current is supplied from the inverter circuit 22 to the three-phase coil of the electric motor 8, and the electric motor 8 is rotationally driven to generate a steering assist force corresponding to the steering torque. The steering assist force generated by the electric motor 8 is transmitted to the steering shaft 3 via the reduction gear mechanism 7 so that the steering wheel 2 can be lightly steered.
Thereafter, when the vehicle starts, the road resistance decreases accordingly, and the steering torque when the steering wheel 2 is steered becomes small. Therefore, the steering assist force command value corresponding to this is calculated, and the electric motor At 8, the auxiliary assist force corresponding to the steering torque is generated.

  Thereafter, when the ignition switch 10 is turned off after the vehicle is stopped, the power supply path input to the control unit 12 via the ignition switch 10 is cut off, but is directly supplied from the battery 9 to the control unit 12. The power supply path is held by the control unit 12 until a predetermined time elapses. During this time, the steering angle information and the failure information can be written to the nonvolatile memory, or the ignition switch 10 can be prepared for restart.

When the predetermined time has elapsed, the control unit 12 is released from the self-holding state, so that the supply of battery power to the control unit 12 is stopped, and the control unit 12 shifts to the non-operating state. For this reason, the gate drive signals IG1 to IG6, SG1, SG2, and SGu to SGw output from the control unit 12 are turned off.
At this time, in the motor drive circuit 13, as shown in FIG. 2, the field effect transistor FET1 is connected so that its parasitic diode is in the forward direction with respect to the DC power of the battery 9, so that the gate drive signal SG1 is Even when the field effect transistor FET1 is turned off due to the off state, the DC power of the battery 9 can be output to the rear stage side of the positive line Lp through the parasitic diode.

  However, the field effect transistor FET2 is connected between the smoothing capacitors C1 and C2 and the positive electrode line Lp so that the parasitic diode prevents the charge from being accumulated in the smoothing capacitors C1 and C2. In addition, the gate drive signal SG2 is turned off and the field effect transistor FET2 is turned off. For this reason, the battery power supplied through the parasitic diode of the field effect transistor FET1 is blocked by the field effect transistor FET2 and the parasitic diode, and the accumulation of charges in the smoothing capacitors C1 and C2 is surely prevented, thereby preventing dark current. Can be reliably prevented from flowing.

Further, the battery power output from the parasitic diode of the field effect transistor FET1 is also supplied to the inverter circuit 22, but the gate drive signals IG1 to IG6 supplied to the field effect transistors FET11 to FET16 are in an off state, and Since the parasitic diodes of the field effect transistors FET11 to FET16 are connected in the direction of blocking the battery power, the battery power is cut off by the inverter circuit 22.
In other words, the parasitic diode of the field effect transistor FET1 is in the forward direction with respect to the battery power, but is cut off by the field effect transistor FET2 and the inverter circuit 22 on the subsequent stage side, so that the battery power is not consumed. Thus, it is possible to reliably prevent the dark current from flowing.

  Similarly, when the self-diagnosis result of the control unit 12 is abnormal or when an abnormality occurs such as when the drive current to the electric motor 8 is in an overcurrent state, each output from the control unit 12 is performed. The gate drive signals IG1 to IG6, SG1, SG2, and SGu to SGw are controlled to be in an off state, and in this case as well, the battery power consumption is stopped in the same manner as described above to reliably prevent the dark current from flowing. it can.

  Further, when a new battery 9 is installed or when an old battery 9 is removed and replaced with a new battery 9, the connection polarity of the battery 9 is wrong, and the negative side is connected to the power supply terminal tp and the positive side is connected to the power supply terminal tn. Is connected to the positive line Lp from the negative line Ln through the smoothing capacitors C1 and C2 and the parasitic diode of the field effect transistor FET2, and the parasitic diodes of the field effect transistors FET11 to FET16 of the inverter circuit 22 The parasitic diode of the field effect transistor FET1 inserted in the positive line Lp is in the reverse direction, so that the battery power toward the negative side of the battery 9 is surely cut off by the parasitic diode. Due to incorrect connection of the battery 9 It is possible to reliably prevent the current path is formed.

On the other hand, in the control unit 12, when the ignition switch 10 is turned on to start supplying battery power from the battery 9 and to turn on the power, the abnormality detection process shown in FIG. 3 is started.
Therefore, the control unit 12 determines whether or not the gate drive signal SG2 for the field effect transistor FET2 of the motor drive circuit 13 is on (step S1).

  In this case, the control unit 12 maintains the gate drive signals SG1, SG2, SGu to SGw and IG1 to IG6 for the motor drive circuit 13 in the off state until the self-diagnosis process is completed when the power is turned on. Therefore, in the abnormality detection process of FIG. 3, the process proceeds from step S1 to step S2, the voltage Vf detected by the current detection unit electricity 30 is converted into a digital signal by the A / D conversion unit and read, and the read voltage It is determined whether or not Vf is equal to or lower than the low voltage side threshold value Vthd (step S3).

  At this time, if the field effect transistor FET is normal, the field effect transistor FET2 is turned off, and the parasitic diode is in the reverse direction to the power supply, so that the smoothing capacitors C1 and C2 have a charge. Is not accumulated, and the smoothing capacitors C1 and C2 are discharged through the voltage dividing resistors R1 and R2 of the power supply detection unit 30. For this reason, the voltage at the connection point between the smoothing capacitors C1 and C2 and the field effect transistor FET2 is substantially zero, and the detection voltage Vf detected by the voltage detection unit 30 is also substantially zero. Therefore, since Vf <Vthd in the abnormality detection process of FIG. 3, the field effect transistor FET2 is determined to be normal, the process proceeds to step S7, the variable N is cleared to “0”, and the timer interrupt process is performed. End and return to a predetermined main program. Therefore, the alarm display element 31 continues to be turned off.

  However, when the gate drive signal SG2 continues to be in the off state and the field effect transistor FET2 is in an on abnormality that continues to be in the on state, the current of the battery 9 is applied to the field effect via the noise filter 21. The voltage is supplied to the smoothing capacitors C1 and C2 via the field effect transistor FET2 via the parasitic diode of the transistor FET1, and charges are accumulated in the smoothing capacitors C1 and C2. Therefore, the voltage at the connection point between the smoothing capacitors C1 and C2 and the field effect transistor FET2 is close to the battery voltage Vb, and the detection voltage Vf is higher than the low voltage side threshold Vthd.

  Therefore, in the abnormality detection process of FIG. 3, it is determined that the field effect transistor is on abnormal, the process proceeds from step S3 to step S4, the variable N is incremented by “1”, and the variable N reaches the set value Ns. The process returns to step S3 until the predetermined time elapses. Then, when the state of Vf> Vthd continues for a predetermined time, it is determined that the abnormality is off, and the process proceeds to step S6 where the alarm display element 31 blinks to alert the driver of the on-abnormality of the field effect transistor.

Even if it is determined in step S3 that Vf> Vthd, when the state returns to Vf ≦ Vthd before N = Ns, the process proceeds to step S7 and the variable N is cleared to “0”.
Thereafter, when the self-diagnosis ends normally, the control unit 12 outputs the gate drive signals SG1, SG2, and SGu to SGw in the on state to the motor drive circuit 13, and the field effect transistors FET1, FET2, and FETu to the motor drive circuit 13 are output. The FETw is controlled to be in an ON state, and simultaneously, gate drive signals IG1 to IG6 for the inverter are output.
Therefore, in the abnormality detection processing of FIG. 3, the process proceeds from step S1 to step S8, the voltage Vf detected by the voltage detection unit 30 is converted into a digital signal by the A / D conversion unit and read, and the read detection voltage is read. It is determined whether Vf exceeds the high voltage side threshold value Vthh (step S9).

At this time, when the field effect transistor FET2 is normal, when the gate drive signal SG2 is turned on, the field effect transistor FET2 shifts from the off state to the on state. For this reason, the battery current from the battery 9 is noise-removed by the noise filter 21, and further flows through the field effect transistor FET1 and further through the field effect transistor FET2 to the smoothing capacitors C1 and C2, and the smoothing capacitors C1 and C2 are charged. Is accumulated and the battery is charged. Accordingly, the voltage between the field effect transistor FET2 and the smoothing capacitors C1 and C2 becomes a voltage close to the battery voltage Vb, and this voltage is detected by the voltage detection unit 30 as the detection voltage Vf.
Therefore, the detection voltage Vf exceeds the high voltage side threshold value Vthh, it is determined that the field effect transistor FET2 is normal, the process proceeds to step S13, the variable M is cleared to “0”, and the timer interrupt process is performed. To return to a predetermined main program. For this reason, the alarm display element 31 maintains an extinguished state.

However, in the state where the gate drive signal SG2 is in the on state, when an off abnormality occurs in which the field effect transistor FET2 continues to be in the off state, charges are accumulated in the smoothing capacitors C1 and C2 through the field effect transistor FET2. Since the charges accumulated in the smoothing capacitors C1 and C2 continue to be discharged through the voltage dividing resistors R1 and R2 of the voltage detection unit 30, the smoothing capacitors C1 and C2 and the field effect transistor FET2 Is substantially zero, and the detection voltage Vf output from the voltage detection unit 30 is also substantially zero. For this reason, it is determined that the field effect transistor FET2 is off abnormally, the process proceeds to step S10, and the variable M is incremented by "1" repeatedly. When the variable M reaches the set value Ms, it is determined that the off abnormality is present. Then, the process proceeds from step S11 to step S12, the alarm display element 31 is controlled to be in a lighting state, and the driver is warned that the field effect transistor FET2 is off abnormally.
Even if it is determined in step S9 that Vf <Vthh, when the state returns to the state of Vf ≧ Vthh before M = Ms, the process proceeds to step S13 and the variable M is cleared to “0”.

  As described above, according to the embodiment, the first line-effect transistor FET1 as the power switch is connected to the positive line Lp connected to the battery 9 so that the parasitic diode is in the forward direction with respect to the battery power. It is possible to simplify the configuration of the power switch, and it is possible to reliably prevent a current path in the opposite direction from being formed when the battery 9 is erroneously connected. Further, unlike the case where a relay circuit is applied as a power switch, the configuration of the relay circuit is not increased in size and an unpleasant operating noise is not generated.

  Further, with respect to the smoothing capacitor connected between the positive electrode line Lp and the negative electrode line Ln, the second field effect transistor FET2 is connected such that its parasitic diode prevents accumulation of charges in the smoothing capacitors C1 and C2. As a result, it is possible to reliably prevent charges from being accumulated in the smoothing capacitors C1 and C2 through the parasitic diode of the first field effect transistor FET1, and to reliably prevent dark current.

Furthermore, even when the first field effect transistor FET1 serving as a power switch is turned off, battery power can be supplied to the inverter circuit 22 via a parasitic diode. The driving assist control can be continued and the reliability of the motor drive circuit 13 can be improved.
Moreover, the on-state abnormality and the off-state abnormality of the field effect transistor FET2 in which the internal parasitic diode inserted between the smoothing capacitors C1 and C2 and the positive electrode line Lp is in the opposite direction to the power supply are indicated as the smoothing capacitors C1 and C2. And the field effect transistor FET2 can be detected by the voltage detector 30, and can be accurately detected by the abnormality detection process shown in FIG. 3 based on the detected voltage Vf and the control state of the field effect transistor FET2. .

  In the above-described embodiment, the case has been described in the abnormality determination process of FIG. 3 where it is determined whether the gate drive signal SG2 by the control unit 12 is in the on state or the off state, but the present invention is not limited to this. Instead, for example, the abnormality of the field effect transistor FET2 may be detected by controlling the gate drive signal SG2 between the off state and the on state for a predetermined period in the abnormality determination process during the self-diagnosis in the initial state.

  In the above embodiment, the case where the abnormality detection process is performed by the control unit 12 to detect the abnormality of the field effect transistor FET2 has been described, or the present invention is not limited to this. The voltage Vf is supplied to the first comparison circuit supplied with the low voltage corresponding to the low voltage side threshold Vthd and the second comparison circuit supplied with the high voltage corresponding to the high voltage side threshold Vthh, and the first comparison The circuit outputs a comparison output that is turned on when Vf> Vthd, and supplies the comparison output and the gate drive signal SG2 to the AND gate to detect the on abnormality, and Vf <Vthh from the second comparison circuit. A comparison output that is in an on state is output at this time, and this comparison output and an inverted signal of the gate drive signal SG2 are supplied to the AND gate to detect an off abnormality. May be so, it is possible to perform abnormality detection by applying any other logic circuit.

In the above embodiment, the case where two smoothing capacitors C1 and C2 having a small capacity are connected in parallel has been described. However, the present invention is not limited to this, and one smoothing capacitor having a large capacity is applied. You may do it.
Further, in the above-described embodiment, the case where the inverter circuit is applied as the bridge circuit has been described. However, the present invention is not limited to this, and when the DC motor is applied as the electric motor 8, four electric fields are used as the bridge circuit. An H-bridge circuit composed of effect transistors can be applied.
Further, in the above embodiment, the case where the present invention is applied to a column assist type electric power steering device has been described, but the present invention is not limited to this, and the present invention is not limited to a pinion assist type or rack assist type electric power steering device. Even if the present invention is applied, the same effect as described above can be obtained.

  DESCRIPTION OF SYMBOLS 1 ... Steering mechanism, 2 ... Steering wheel, 3 ... Steering shaft, 7 ... Reduction gear mechanism, 8 ... Electric motor, 9 ... Battery, 10 ... Ignition switch, 11 ... Control device, 12 ... Control unit, 13 ... Motor drive circuit , Lp ... positive line, Ln ... negative line, 21 ... noise filter, 22 ... inverter circuit, FET1 ... first field effect transistor, FET2 ... second field effect transistor, FET11-FWT16, FETu-FETw ... field effect transistor 30 ... Abnormality detection unit, 31 ... Alarm display element

Claims (3)

An electric power steering apparatus comprising: an electric motor that generates a steering assist force for a steering mechanism of a vehicle; a motor drive circuit that drives the electric motor; and a control device that controls the motor drive circuit,
The motor drive circuit includes: a bridge circuit that drives the electric motor; a power supply terminal that receives DC power from a DC power supply that supplies DC power to the bridge circuit; and a space between the power supply terminal and the bridge circuit. A smoothing capacitor inserted between the positive and negative lines to be connected, and an internal parasitic diode between the smoothing capacitor and the positive line prevents the charge from accumulating in the smoothing capacitor. Electric power steering comprising: a field effect transistor connected in this manner; and an abnormality detection means for detecting a voltage between the smoothing capacitor and the field effect transistor to detect an abnormality of the field effect transistor apparatus.   The abnormality detection means includes a voltage detection unit that detects a voltage between the smoothing capacitor and the field effect transistor, a control state determination unit that determines a control state of the field effect transistor, and a voltage detected by the voltage detection unit. The electric power steering apparatus according to claim 1, further comprising: an abnormality detection unit that detects an abnormality of the field effect transistor based on the control state detected by the control state determination unit.   A power supply field effect transistor is connected between a power supply terminal of the positive electrode line and the field effect transistor so that a parasitic diode therein is in a forward direction with respect to the DC power supply. The electric power steering apparatus according to claim 1 or 2.

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