JP2729812B2 - Electric power steering device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electric power steering device for an automobile or the like.

2. Related Art and Problems of the Invention As an electric power steering device for an automobile or the like,
A motor drive circuit composed of a bridge circuit composed of four FETs, a means for outputting a steering direction command signal corresponding to the steering direction, a means for outputting a motor current command signal corresponding to the steering force, and a motor current command signal And a switching means for controlling a FET of a motor drive circuit based on a steering direction command signal and a PWM signal.

The motor drive circuit includes a first FE for forward rotation provided between a first terminal of the electric motor for assisting a steering force and a power supply side.
T, a second FET for forward rotation provided between the second terminal of the electric motor and the ground side, a first FET for reverse rotation provided between the second terminal of the electric motor and the power supply, and a first terminal of the electric motor 2nd FET for reverse rotation provided between
And a diode connected in parallel.

The switching means turns off the two reverse rotation FETs when the steering direction command signal is in the forward rotation direction, and switches the two forward rotation F
ET is turned on / off based on the PWM signal, and when the steering direction command signal is in the reverse direction, the two forward rotation FETs are turned off and the two reverse rotation FETs are turned on / off based on the PWM signal. .

However, in such a device, depending on the magnitude of the motor current command signal, the load time ratio (d
uty factor) may be close to 100%. At this time, the PWM frequency becomes unstable, and the generation of motor control noise becomes a problem.

An object of the present invention is to provide an electric power steering device that solves the above problem.

Means for Solving the Problems An electric power steering apparatus according to the present invention comprises: a first FET for normal rotation provided between a first terminal of an electric motor for assisting a steering force and a power supply side;
A second FET for forward rotation provided between the terminal and the ground side, a first F for reverse rotation provided between the second terminal of the electric motor and the power supply side
ET, a motor drive circuit composed of a second reverse FET provided between the first terminal of the electric motor and the ground side and a bridge circuit composed of a diode connected in parallel to each of these FETs, corresponding to the steering direction Means for outputting a steering direction command signal, means for outputting a motor current command signal corresponding to the steering force, and PWM for outputting a PWM signal corresponding to the motor current command signal
In an electric power steering device including a modulating unit and a switching unit that controls a FET of a motor drive circuit based on a steering direction command signal and a PWM signal, the PWM modulating unit forcibly applies a PWM signal in synchronization with a PWM frequency. Means for turning off the two FETs for reverse rotation and turning on the second FET for normal rotation to turn off the second FET for normal rotation when the steering direction command signal is in the forward direction. If one FET is controlled based on the PWM signal and the steering direction command signal is in the reverse direction, the two FETs for normal rotation are turned off, the second FET for reverse rotation is turned on, and the first FET for reverse rotation is controlled based on the PWM signal. And a control means.

In this specification, the term FET is used as an abbreviation for a field effect transistor.

The time for forcibly turning off the PWM signal is desirably the shortest time during which the load time ratio of the FET does not become close to 100%.

Function The PWM signal is forcibly turned off for a predetermined time in synchronization with the PWM frequency. During that time, the first FET for normal rotation (during normal rotation) or the first FET for reverse rotation (during reverse rotation) is turned off, and the load time ratio of the FET is reduced. Can never be close to 100%. Therefore, the PWM frequency does not become unstable, and the generation of the motor control sound is prevented.

Also, since the second FET for forward rotation is always on at the time of forward rotation and the second FET for reverse rotation is always on at the time of reverse rotation, one of these FETs that are on and the other FET that is off are connected in parallel. Flywheel current flows to the motor through the connected diode, which reduces the current ripple and increases the current flowing to the motor,
The reduction in motor current due to forcing the FET load time ratio below 100% is compensated to some extent.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic diagram of a main part of an electric circuit of an electric power steering apparatus, that is, an electric motor (1) for assisting a steering force, four FETs (2), (3), (4), and (5).
A motor drive circuit (6) constituted by a bridge circuit comprising: a PWM modulation circuit (7) for outputting a PWM signal B corresponding to a motor current command signal A from a command circuit (not shown);
Based on the steering direction command signal from the command circuit and the PWM signal B, the FETs (2), (3), and (4) of the motor drive circuit (6)
Switching circuit for controlling on / off of (5) (8)
Is shown.

Although not shown, a torque sensor for detecting an input torque of a steering shaft, a vehicle speed sensor for detecting a vehicle speed, and the like are connected to the command circuit. The command circuit determines the rotation direction of the motor (1), that is, the steering direction, from the direction of the input torque, and determines the necessary steering assist force and the corresponding motor current value based on the magnitude of the input torque and the vehicle speed. Then, the motor current command signal A corresponding to the motor current value is output to the PWM modulation circuit (7), and the steering direction command signal, ie, the normal rotation signal C1 and the reverse rotation signal C2, are output to the switching circuit (8). The forward rotation signal C1 is at the H level (ON) during forward rotation of the motor, is at the L level (OFF) during reverse rotation and stoppage, and the reverse rotation signal C2 is at the H level during reverse rotation and L level during forward rotation and stoppage. Become.
The motor (1) is connected to an appropriate portion such as an output shaft of a steering shaft or a steering gear via a speed reducer, a clutch, or the like.

Four FETs (2) (3) of motor drive circuit (6)
(4) and (5) are connected as follows. That is, the source terminal (S) of the first FET for forward rotation (2) is connected to the first terminal (forward rotation terminal) (Ma) of the motor (1), and the drain terminal (D) is connected to the power supply (BATT). I have. The drain terminal (D) of the second FET (3) for forward rotation is the motor (1)
(Mb), and the source terminal (S) is grounded via a current detection resistor (9). The source terminal (S) of the first FET for reverse rotation (4) is connected to the second terminal (Mb) of the motor (1), and the drain terminal (D) is connected to the power supply (BATT). 2nd FET for reverse rotation
The drain terminal (D) of (5) is connected to the first terminal (Ma) of the motor (1), and the source terminal (S) is grounded via a current detecting resistor (9). And each FET (2)
(3) (4) and (5) have diodes (10) (1
1) (12) and (13) are connected in parallel.

The PWM modulation circuit (7) includes an oscillation circuit (14) and a comparison circuit (1
5) and a one-shot circuit (16). The oscillation circuit (14) has a triangular wave signal E as shown in FIG.
And the trapezoidal wave signal F, and the triangular wave signal E is compared with the comparison circuit (1
The trapezoidal wave signal F is input to the inverting input terminal of 5), respectively, to the one-shot circuit (16). The motor current command signal A is input to the non-inverting input terminal of the comparison circuit (15), and the output signal of the comparison circuit (15), that is, the PWM signal B, usually has a triangular wave as shown in FIG. The pulse signal becomes H level only during the period larger than the signal E. Then, as described later, the FETs (2), (3), and (4) of the motor drive circuit (6) are based on the PWM signal B and the steering direction command signal.
(5) is controlled.

Assuming that there is no one-shot circuit (16), while the motor current command signal A is larger than the peak value of the triangular wave signal E, the PWM signal B does not become a pulse signal, but always becomes H level, and the load time of the FET is reduced. The rate becomes 100%. When the motor current command signal A approaches the peak value of the triangular wave signal E, the load time ratio approaches 100%, the frequency of the PWM signal B (PWM frequency) becomes unstable, and a motor control sound is generated. . In order to prevent this, the PWM signal B is forcibly turned off for a predetermined time in synchronization with the triangular wave signal E by the one-shot circuit (16) so that the load time ratio does not become close to 100%.

In the one-shot circuit (16), the trapezoidal wave signal F from the oscillation circuit (14) passes through an amplifier (17), is inverted by an inverter circuit (18), and sent to a differentiating circuit (19). From the differentiating circuit (19), a positive trigger pulse is output at the rising timing of the input signal, ie, the falling timing of the trapezoidal wave signal F, and a second trigger pulse is output at the falling timing of the input signal, ie, the rising timing of the trapezoidal wave signal. (Indicated by I in FIG. 2). The output signal I of the differentiating circuit (19) is sent to a rectifier diode (20) from which only the positive trigger pulse of the signal I is output (indicated by J in FIG. 2). The output signal J of the diode (20) is divided by two resistors (21) and (22) and sent to the base of the switching transistor (23). The collector of the transistor (23) is connected to the output terminal of the comparison circuit (15), and the emitter is grounded.

While the trigger pulse is not output from the diode (20), the transistor (23) is off, and the PWM signal B is the triangular wave signal E and the motor current command signal as in the case without the one-shot circuit (16). Take the value determined by A. When a trigger pulse is output from the diode (20), the transistor (23) is turned on and the collector of the transistor (23) is grounded.
The signal B becomes L level. The trigger pulse is trapezoidal wave signal F
, And the falling timing of the trapezoidal wave signal F is synchronized with the falling timing of the triangular wave signal E. Therefore, regardless of the value of the motor current command signal A, that is, when the motor current command signal A is smaller than the peak value of the triangular wave signal E as in the first half (left side) of FIG. Even when the motor current command signal A is larger than the peak value of the triangular wave signal E as shown on the right), the time (T) during which the trigger pulse is output at least in synchronization with the falling timing of the triangular wave signal E
During this time, the PWM signal B is at L level and the load time ratio is 1
It will not be close to 00%.

The switching circuit (8) is configured as follows.

The normal rotation signal C1 from the command circuit is input to the first NAND circuit (24), and the reverse rotation signal C2 is input to the second NAND circuit (25). In addition, these two NAND circuits (24) and (25) have a PWM signal B
Enter.

The output terminal of the first NAND circuit (24) is connected to the first switching F
It is connected to the gate terminal (G) of the ET (26). The drain terminal (D) of the first switching FET (26) is connected to a resistor (2
The source terminal (S) is connected to the gate terminal (G) of the first non-inverting FET (2) via the switch 7), and the source terminal (S) is grounded. Also,
A booster circuit (28) is provided between the drain terminal (D) of the first switching FET (26) and the first terminal (Ma) of the motor (1). The booster circuit (28) is provided with a diode (29), a capacitor (30), and a resistor (31) in this order from the power supply (BATT) side, and the resistor (31) is grounded. The portion between the resistor (31) and the capacitor (30) is connected to the first terminal (Ma) of the motor (1), and the portion between the capacitor (30) and the diode (29) connects the resistor (32). It is connected to the drain terminal (D) of the first switching FET (26) via the first switching FET (26).

The output terminal of the second NAND circuit (25) is connected to the second switching F
It is connected to the gate terminal (G) of ET (33). The drain terminal (D) of the second switching FET (33) is connected to a resistor (3
It is connected to the gate terminal (G) of the first FET for reverse rotation (4) via 4), and the source terminal (S) is grounded. Also,
A booster circuit (35) similar to the above is provided between the drain terminal (D) of the second switching FET (33) and the second terminal (Mb) of the motor (1).

The normal rotation signal C1 is sent to the inverter circuit (36), and the output signal of the inverter circuit (36) is divided by two resistors (37) and (38) to form a first switching transistor (39).
Sent to the base. The collector of the first switching transistor (39) is the second FET for normal rotation via the resistor (40).
It is connected to the gate terminal (G) of (3), and the emitter is grounded.

The reverse rotation signal C2 is sent to the inverter circuit (41), and the output signal of the inverter circuit (41) is divided by two resistors (42) and (43), and the second switching transistor (44)
Sent to the base. The collector of the second switching transistor (44) is the second FET for reverse rotation via the resistor (45).
It is connected to the gate terminal (G) of (5), and the emitter is grounded.

Next, the operation of the motor drive circuit (6) and the switching circuit (8) will be described.

When both the forward rotation signal C1 and the reverse rotation signal C2 are at the L level (during stop), the motor drive circuit (6)
The two FETs (2) (3) (4) (5) are all turned off,
No current flows through the motor (1).

That is, since the non-inversion signal C1 is at the L level, the base voltage of the first switching transistor (39) inverted by the inverter circuit (36) becomes the H level,
This transistor (39) turns on. For this reason, the gate terminal (G) of the second FET for forward rotation (3) becomes L level,
This FET (3) is turned off. Similarly, when the reverse rotation signal C2 is L
Since the level is at the level, the second reverse FET (5) is also turned off.
Further, since the normal rotation signal C1 is at the L level, the output signal of the first NAND circuit (24) becomes H level regardless of the value of the PWM signal B. Therefore, the gate terminal (G) of the first switching FET (26) becomes H level, and this FET (26) is turned on. When the first switching FET (26) is turned on, the gate terminal (G) of the first FET for normal rotation (2) becomes L level, and this FET (2) is turned off. Similarly, since the reverse rotation signal C2 is at the L level, the first reverse rotation FET (4) is also turned off.

When the forward rotation signal C1 is at the H level and the reverse rotation signal C2 is at the L level (during forward rotation), the first FET 4 for reverse rotation and the second FET 5 for reverse rotation are turned off and the second FET for forward rotation as follows. (3) is turned on and the first FET for normal rotation only while the PWM signal B is at the H level.
(2) is turned on, and during this time, a current in the forward direction flows through the motor (1).

That is, since the reverse rotation signal C2 is at the L level, the first reverse rotation FET (4) and the second reverse rotation FET
(5) turns off. Since the normal rotation signal C1 is at the H level, the first signal which is inverted by the inverter circuit (36)
The base voltage of the switching transistor (39) becomes L level, and this transistor (39) is turned off. Therefore, the gate terminal (G) of the second non-inverting FET (3) becomes H level, and this FET (3) is turned on. Since the forward rotation signal C1 is at the H level, the output signal of the first NAND circuit (24) is at the H level while the PWM signal B is at the L level.
When the WM signal B goes high, the output signal of the first NAND circuit (24) goes low. 1st NAND when PWM signal B is L level
The output signal of the circuit (24), that is, the first switching FET (2
While the gate terminal (G) voltage of 6) is at the H level, the gate terminal (G) of the first non-inverting FET (2) is at the L level,
The FET (2) is turned off, and no current flows to the motor (1). For this reason, the voltage at the point K of the booster circuit (28) is almost 0 V, whereas the voltage at the point N is almost the power supply (BA
TT) The voltage approaches and the capacitor (30) is charged. In such a state, the PWM signal B becomes H level and the output signal of the first NAND circuit (24), that is, the first switching FET (2
6) When the gate terminal (G) voltage goes low, this F
The ET (26) is turned off, the voltage at point P rises, and the gate terminal (G) of the first non-inverting FET (2) goes high. As a result, the first FET for normal rotation (2) is turned on, and the first FET for normal rotation is turned on.
A forward current flows through the ET (2), the motor (1), and the second FET for forward rotation (3). As a result, the voltage at the point K rises and almost reaches the power supply (BATT) voltage level. At this time, the capacitor (30) is charged as described above,
Since the gate current of the first FET for forward rotation (2) is small, the voltage at point P rises from the power supply (BATT) voltage to (power supply (BATT) voltage + charge voltage) in accordance with the voltage rise at point K. Therefore, the gate voltage is maintained at a level sufficiently higher than the source voltage of the first FET for forward rotation (2), and the first FET for forward rotation is maintained.
(2) can be kept on.

Here, assuming that the load time ratio of the first FET for forward rotation (2) is 100%, that is, if the first FET for forward rotation (2) is always on and the first switching FET (26) is always off,
The electric charge charged in the capacitor (30) between the point K and the point N is gradually discharged, and the voltage at the point P cannot be maintained at a sufficiently high level with respect to the voltage at the point K.
(2) cannot be maintained in the ON state. However, as described above, the one-shot circuit (16) of the PWM modulation circuit (7) prohibits the load time ratio from becoming close to 100%, and the PWM signal B is always synchronized with the triangular wave signal E to make the L signal L.
Since the level is set to the level, the first switching FET (26) is always turned off in synchronization with the triangular wave signal E. Therefore, there is no problem as described above, and the first FET for normal rotation (2) can be driven by the booster circuit (28) which is very simple as compared with the conventional case.

When the reverse rotation signal C2 is at the H level and the normal rotation signal C1 is at the L level (during reverse rotation), the first FET for normal rotation (2) and the second FET for normal rotation (3) are turned off, and the reverse rotation is used. The second FET (5) is turned on, and the first FE for reverse rotation only while the PWM signal B is at the H level.
T (4) is turned on, and during this period, a current in the reverse direction flows through the motor (1).

In the case of the above embodiment, since the FET is used in the motor drive circuit (6), high-speed switching is possible. In addition, at the time of forward rotation, since the second FET for forward rotation (3) is always on and only the first FET for forward rotation (2) is controlled by the PWM signal, even while the first FET for forward rotation (2) is off. Diode (13) connected in parallel with the forward second FET (3) that is on and the reverse second FET (5) that is off
Flywheel current flows to the motor (1) through the motor.
Similarly, at the time of reverse rotation, the second FET for reverse rotation that is on
The flywheel current flows through the motor (1) through the diode (11) connected in parallel with the (5) and the forward second FET (3) that is turned off. For this reason, the current ripple is reduced, and radio noise and motor control sound when the vehicle is mounted on an actual vehicle are reduced. In addition, when the flywheel current flows, the current flowing to the motor (1) increases, and
The current flowing through the ET (2) or the first FET for reverse rotation (4) is reduced. Therefore, the decrease in motor current due to forcibly reducing the load time ratio to less than 100% is compensated to some extent, and the current flowing through the FET is reduced, so that heat generation is also reduced. The second FET for forward rotation (3) and the second FET for reverse rotation
Since (5) is always on, it is not necessary to switch these at high speed, and transistors (39) and (44) are used for these switching.

In the above embodiment, since the load time ratio does not become 100%, the simple booster circuits (28) and (35) are used.
Instead, a conventional booster circuit can be used.

According to the electric power steering device of the present invention,
As described above, the load time ratio of the FET does not become close to 100%, so that the PWM frequency does not become unstable, and the generation of the motor control sound can be prevented.

[Brief description of the drawings]

FIG. 1 is an electric circuit diagram of a main part of an electric power steering apparatus showing one embodiment of the present invention, and FIG. 2 is a time chart showing signals of respective parts in FIG. (1) ... electric motor, (2) ... first FET for forward rotation,
(3)… second FET for forward rotation, (4)… first FET for reverse rotation,
(5): 2nd FET for reverse rotation, (6): motor drive circuit,
(7) PWM modulation circuit (8) Switching circuit (10) (11) (12) (13) Diode (14)
…… oscillator circuit, (16) …… one-shot circuit.

Claims (1)

(57) [Claims] A first electric motor for assisting a steering force.
The first FET for forward rotation provided between the terminal and the power supply side, the second F for forward rotation provided between the second terminal of the electric motor and the ground side
ET, a first FET for reverse rotation provided between the second terminal of the electric motor and the power supply side, a second FET for reverse rotation provided between the first terminal of the electric motor and the ground side, and in parallel with each of these FETs. A motor drive circuit composed of a bridge circuit composed of connected diodes, a means for outputting a steering direction command signal corresponding to the steering direction, a means for outputting a motor current command signal corresponding to the steering force, and a motor current command A PWM modulating means for outputting a PWM signal corresponding to the signal; a motor driving circuit based on the steering direction command signal and the PWM signal;
An electric power steering apparatus comprising: a switching unit for controlling an ET; wherein the PWM modulation unit includes a unit for forcibly turning off a PWM signal for a predetermined time in synchronization with a PWM frequency; If the signal is in the forward direction, the two FETs for reverse rotation are turned off, the second FET for normal rotation is turned on, and the first FET for normal rotation is controlled based on the PWM signal. In this case, the electric power steering device is provided with means for turning off the two FETs for normal rotation, turning on the second FET for reverse rotation, and controlling the first FET for reverse rotation based on the PWM signal. .