JP2004098988A - Electric power steering device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electric power steering device not requiring too much force for steering when in a short circuit failure. <P>SOLUTION: This electric power steering device assists steering force of a steering member by a direct-current motor Ma having a field winding L1 and being driven by drive circuits Q1-Q4. It comprises detection means 17, 2 detecting a short circuit failure of the drive circuits Q1-Q4 and wires connecting the drive circuits Q1-Q4 and interruption means 12, 13, 13a interrupting current to the field winding L1 when the detection means 17, 2 detect a short circuit failure. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
TECHNICAL FIELD The present invention relates to an electric power steering device configured to assist a steering force of a steering member by a DC motor.
[0002]
[Prior art]
In a conventional electric power steering apparatus using a DC motor, a drive circuit of the DC motor is configured to bridge the DC motor M by an H-type bridge circuit, as schematically shown in FIG. In this drive circuit, the switching direction of the DC motor M is changed by turning on / off the switching elements Q1 to Q4 provided on each branch of the H-type bridge circuit to change the direction of the current flowing through the bridge. By controlling the magnitude of the current, the magnitude of the steering torque is controlled.
Some electric power steering devices using a DC motor use a DC motor using a field winding instead of a magnet, and are disclosed in Patent Documents 1 and 2.
[0003]
[Patent Document 1]
Japanese Patent No. 2711686 [Patent Document 2]
Japanese Patent Publication No. 6-24942
[Problems to be solved by the invention]
In the above-described conventional electric power steering apparatus, when a closed loop occurs due to a short-circuit failure of the switching elements Q1 to Q4 and the like of the drive circuit, the DC motor M is used as a generator even when the power-cutoff relay contact 13a is turned off. Operate. For example, as shown in FIG. 3, when a short circuit occurs in the switching element Q4, a closed loop is formed by the switching elements Q4 and Q3 and the DC motor M. In this state, when the steering wheel (steering member) is turned, the DC motor M also rotates, and an induced current flows as shown by a broken line. As a result, there is a problem that a braking force is generated in the DC motor M by the same operation as the dynamic braking and the regenerative braking, and the force required for steering is further increased as compared with a case where the electric power steering device is not operated.
[0005]
Conventionally, as a technique for solving this problem, a relay contact is connected in series to the DC motor M, and in the event of a short-circuit failure, the relay contact is turned off so that a closed loop does not occur. It is.
SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to provide an electric power steering device in which a force required for steering does not become too large in the case of a short circuit failure.
[0006]
[Means for Solving the Problems]
An electric power steering device according to a first aspect of the present invention is an electric power steering device which assists a steering force of a steering member by a DC motor having a field winding and driven by a drive circuit. It is characterized by comprising detection means for detecting a short-circuit failure of a wiring connected to a drive circuit, and interruption means for interrupting a current to the field winding when the detection means detects a short-circuit failure.
[0007]
In this electric power steering device, the steering force of the steering member is assisted by a DC motor having a field winding and driven by a drive circuit. The detecting means detects a short-circuit fault in the drive circuit and the wiring connected to the drive circuit, and when the detecting means detects the short-circuit fault, the cut-off means cuts off the current to the field winding.
As a result, even if a closed loop occurs in the event of a short-circuit fault, the magnet does not exist, and the electromagnet due to the field winding disappears, so that no induced current is generated. It is possible to realize an electric power steering device without any. Further, the relay conventionally provided for preventing the occurrence of the closed loop becomes unnecessary, and the cost of parts can be reduced.
[0008]
An electric power steering device according to a second aspect is characterized in that the drive circuit bridges the DC motor by a bridge circuit.
it can.
[0009]
In this electric power steering device, the drive circuit bridges the DC motor by the bridge circuit. Therefore, in the event of a short-circuit failure, even if a closed loop occurs, there is no magnet, and the drive circuit is driven by the field winding. Since the electromagnet also disappears, the DC motor does not generate power, and an electric power steering device in which the force required for steering does not become too large can be realized. Further, the relay conventionally provided for preventing the occurrence of the closed loop becomes unnecessary, and the cost of parts can be reduced.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described with reference to the drawings showing the embodiments.
FIG. 1 is a block diagram showing a main configuration of an electric power steering apparatus according to an embodiment of the present invention. In this electric power steering apparatus, the power transistors Q1, Q2, Q3, and Q4 form a bridge circuit between the power supply P and the ground terminal.
[0011]
This bridge circuit includes a DC motor Ma having a field winding L1 so as to bridge a connection point between the power transistors Q1 and Q3 connected in series and a connection point between the power transistors Q2 and Q4 connected in series. Are connected in series. Diodes D1, D2, D3, and D4 are connected in parallel to power transistors Q1, Q2, Q3, and Q4, respectively, in a direction opposite to the direction in which current flows from power supply P. A normally closed contact 13a (cutoff means) of a fail-safe relay and a resistor R1 for current detection are connected in series between the power supply P and the bridge circuit.
The field winding L1 is connected via a resistor R2 between a connection node of a normally closed contact 13a and a resistor R1 described later and a ground terminal.
[0012]
A torque detection signal of a torque sensor 10 that detects a torque applied to a steering shaft (not shown) is provided to a CPU 12 via an interface circuit 11.
Further, a vehicle speed signal from a vehicle speed sensor 14 for detecting a vehicle speed is provided to the CPU 12 via an interface circuit 15.
The CPU 12 refers to the built-in torque / current table 12a and creates a motor current command value according to the applied torque detection signal and vehicle speed signal. The motor current command value includes the magnitude and direction of the motor current and is input to the PWM control unit 21.
[0013]
The PWM control unit 21 creates a PWM signal having a duty ratio according to the motor current command value. For example, when the DC motor Ma is rotated in the positive direction, the power transistor Q4 is turned on, and the PWM signal is The power transistor Q1 is switched. When the DC motor Ma is driven to rotate in the reverse direction, the power transistor Q3 is turned on and the power transistor Q2 is similarly switched.
[0014]
The voltage across the resistor R1 is supplied to the motor current detection circuit 17, and the detection signal of the motor current detection circuit 17 (detection means) is supplied to the CPU 12 (detection means) as a feedback signal.
The relay control signal output from the CPU 12 is supplied to the relay drive circuit 13, and the relay drive circuit 13 turns on or off the normally closed contact 13a of the fail-safe relay according to the relay control signal.
The CPU 12 outputs the relay control signal to the relay drive circuit 13 when the motor current detected by the motor current detection circuit 17 exceeds a preset upper limit value or when the torque sensor 10 detects an excessive torque value. Then, the normally closed contact 13a is turned off, and the power supply from the power supply P to the bridge circuit is stopped, and at the same time, the power supply to the field winding L1 is stopped.
[0015]
Hereinafter, the operation of the electric power steering apparatus having such a configuration will be described with reference to the flowchart of FIG.
In the steering assist operation, the CPU 12 first reads the torque detection signal detected by the torque sensor 10 via the interface circuit 11 (S2), and then reads the vehicle speed signal detected by the vehicle speed sensor 14 via the interface circuit 15. And read it (S4).
The CPU 12 determines the target motor current from the read (S4) vehicle speed signal and the read (S2) torque detection signal with reference to the torque / current table 12a (S6).
[0016]
Next, the CPU 12 reads the motor current signal from the motor current detection circuit 17 (S8), determines whether or not the motor current exceeds a predetermined value (S9). It is determined whether or not a short-circuit failure has occurred based on whether or not the predetermined time has elapsed (S18).
When the CPU 12 determines that the motor current has exceeded the predetermined value for a predetermined period of time and that a short-circuit failure has occurred (S18), the CPU 12 inputs a relay control signal to the relay drive circuit 13 and outputs the normally closed contact 13a. When the power is turned off (S20), the power supply from the power supply P to the bridge circuit is stopped, and at the same time, the power supply to the field winding L1 is stopped and the process returns.
[0017]
When the motor current does not exceed the predetermined value (S9) or when there is no short-circuit failure (S18), the CPU 12 calculates a difference between the determined (S6) target motor current and the read (S8) motor current signal. Then, based on the calculated difference, the motor current command value is determined so that the target motor current flows through the DC motor Ma (S12).
[0018]
Next, the CPU 12 determines a PWM command value and a rotation direction according to the determined (S12) motor current command value (S14), and outputs the determined PWM command value and an instruction signal of the rotation direction to output the PWM control unit 21. (S16), and returns to move to another process.
The PWM control unit 21 drives the DC motor Ma to rotate based on the given PWM command value and rotation direction instruction signal.
[0019]
【The invention's effect】
According to the electric power steering apparatus according to the first and second aspects of the present invention, in the event of a short-circuit failure, even if a closed loop occurs, no magnet exists, and the electromagnet formed by the field winding disappears, so that the DC motor Since no power is generated, it is possible to realize an electric power steering device in which the force required for steering does not become too large. Further, the relay conventionally provided for preventing the occurrence of the closed loop becomes unnecessary, and the cost of parts can be reduced.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a main configuration of an electric power steering apparatus according to an embodiment of the present invention.
FIG. 2 is a flowchart showing the operation of the electric power steering device according to the present invention.
FIG. 3 is a schematic diagram showing a main configuration of a drive circuit of a conventional electric power steering device.
[Explanation of symbols]
12 CPU (detection means, interruption means)
13 Relay drive circuit (interruption means)
13a normally closed contact (cutoff means)
17 Motor current detection circuit (detection means)
21 PWM control unit L1 Field winding Ma DC motor P Power supply Q1, Q2, Q3, Q4 Power transistor R1, R2 Resistance

Claims (2)

In an electric power steering device which assists a steering force of a steering member by a DC motor having a field winding and driven by a drive circuit,
Detecting means for detecting a short-circuit fault in the drive circuit and wiring connected to the drive circuit; and interrupting means for interrupting a current to the field winding when the detecting means detects a short-circuit fault. An electric power steering device characterized by the above-mentioned. The electric power steering apparatus according to claim 1, wherein the drive circuit bridges the DC motor by a bridge circuit.

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