JPH08119132A - Motor-operated power steering device - Google Patents

Abstract

PURPOSE: To make it possible to correct not only an offset of a zero point of a three-phase AC motor due to a temperature change and a change with the passage of time but also an offset of the zero point due to irregularity of switching elements, motor coils, and so on. CONSTITUTION: Steering is assisted by a three-phase AC motor 12 which is PWM-driven based on a target current value corresponding to torque detected by a torque sensor 7. A current value of each phase of the motor 12 is respectively detected by current detection devices 42 and duty of each phase is corrected by a duty-correction means 49 based on detected values of current flowing in each phase when each phase is driven with duty of 50% in the case of torque which is detected within a dead zone of the target value of current and falls within a prescribed range in the vicinity of zero.

Description

Detailed Description of the Invention

[0001]

This invention relates to a three-phase AC motor as a P
The present invention relates to an electric power steering device that assists steering by WM driving.

[0002]

2. Description of the Related Art In this type of power steering apparatus, the current of each phase of a three-phase AC motor is feedback-controlled, but the zero point offset (shift) of each phase.
Greatly affects the torque ripple of the motor, and the torque ripple causes vibration of the handle shaft.

Therefore, a method has been proposed in which the PWM output is turned off at the time of initial diagnosis, the offset amount at the zero point is obtained, and the current value of each phase is corrected based on the offset amount (JP-A-3-
186477).

[0004]

In the above-mentioned conventional device,
Although it is possible to correct current detection elements and changes over time, it is not possible to correct variations in PWM switching elements and motor coils.

The object of the present invention is to solve the above problems,
An object of the present invention is to provide an electric power steering apparatus that can correct not only the offset of the zero point due to temperature change and the change with time but also the offset of the zero point due to variations in switching elements, motor coils, and the like.

[0006]

An electric power steering apparatus according to the present invention PWMs a three-phase AC motor based on a target current value corresponding to a torque detected by a torque sensor.
In an electric power steering apparatus for driving and assisting steering, a current detecting means for detecting a current value of each phase of the motor, and a detected torque within a predetermined range near 0 within a dead zone of a target current value. In addition, a duty correction means for correcting the duty of each phase based on the detection result of the current value flowing in each phase when each phase is driven at a duty of 50% is provided.

[0007]

The duty is corrected as needed with respect to the offset of the zero point of each phase of the motor, so that it is possible to suppress torque ripples and vibrations that occur due to variations in the zero point due to temperature changes and changes over time.

Further, since the duty is corrected in the state where the drive is performed at the duty of 50% for all three phases, that is, the PWM is on, the offset of the zero point due to the variation of the switching element, the motor coil and the like can also be corrected.

[0009]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of a mechanical portion of an electric power steering apparatus for an automobile, and FIG. 2 shows the details of its main portion. In the description of FIGS. 1 and 2,
The left and right sides of the drawing are left and right.

A housing that is long in the left-right direction on the body of the automobile
The (1) is fixed, and the rack shaft (2) extending in the left-right direction is supported in the housing (1) so as to be movable in the left-right direction (axial direction) without rotating. housing
An input shaft (4) and an output shaft (5) connected by a torsion bar (not shown) are rotatably supported in a gear box (3) formed near the left end of (1). The output shaft (5) is integrally formed with a pinion (6) which meshes with a rack (not shown) of the rack shaft (2), and the input shaft (4) is connected to a steering wheel (not shown). The gear box (3) is provided with a torque sensor (7) for detecting steering torque from the steering wheel by detecting twist of the torsion bar between the input shaft (4) and the output shaft (5). Rack axle protruding from housing (1)
Tie rods (10) and (11) are connected to both left and right ends of (2) via ball joints (8) and (9).

A cylindrical outer rotating shaft (13) rotated by a brushless motor (12), which is a three-phase AC motor, is axially disposed in the middle of the housing (1) via bearings (14) and (15). Is rotatably supported so as not to move. Motor (12)
Is composed of a stator (16) fixed in the housing (1) and a rotor (17) provided directly on the outer peripheral portion of the outer rotary shaft (13).

A rotary position sensor (35) using a rotary encoder or the like is attached to an appropriate position of the housing (1) around the rack shaft (2), and is fixed to the input shaft of this sensor (35). The gear (36) is meshed with the gear (37) fixed to the outer periphery of the right end portion of the outer rotary shaft (13). The rotation of the outer rotary shaft (13) is transmitted to the sensor (35) by being accelerated by the gears (37) (36), and the sensor (35) rotates the outer rotary shaft (13), that is, the rotational position of the rotor (17). (Rotor angle) is detected.

Inside the outer rotary shaft (13), a cylindrical inner rotary shaft (18) is attached via a spline (19) and the like so that they can move in the axial direction but do not rotate relative to each other. axis
The rack shaft (2) is passed through the bush (20) inside the (18) so that the rack shaft (2) can rotate and move in the axial direction.

A part of the inner rotary shaft (18) always projects to the left of the outer rotary shaft (13) even if it moves with respect to the outer rotary shaft (13). , The first ball screw (21) constituting the first screw coupling means as follows:
It is connected to the housing (1) via. That is,
A threaded portion (22) is formed on the outer peripheral surface of the inner rotating shaft (18), and this threaded portion (22) is inserted into a ball nut (23) fixed to the housing (1) through a number of circulating balls (24). Screwed.

The rack shaft (2) is connected to the left end portion of the inner rotary shaft (18) through the second ball screw (25) constituting the second screw coupling means as follows. That is, the rack axis
A screw part (26) is formed on the outer peripheral surface of (2), and this screw part (26)
Is a ball nut fixed to the left end of the inner rotary shaft (18).
It is screwed into (27) via a number of circulating balls (28).

In this embodiment, two ball screws (21) (2
The direction of the screw of 5) is opposite, and the lead of the screw of the second ball screw (25) is larger than that of the first ball screw (21).

When the driver rotates the steering wheel, the rotation is transmitted to the pinion (6) via the input shaft (4), the torsion bar and the output shaft (5), and the rotation of the pinion (6) causes the rack shaft ( 2) is moved left and right to steer the wheels. At this time, the motor (12) is driven based on the direction and magnitude of the steering torque detected by the torque sensor (7), which moves the rack shaft (2) in the same direction as that by the pinion (6). To be

For example, the rack shaft can be operated by operating the handle.
When moving (2) to the left, the rotor (17) of the motor (12)
Is rotated to the right (clockwise) when viewed from the right side of the drawing. As a result, the outer rotary shaft (13) and the inner rotary shaft (18) are also rotated in the same direction by the same amount. Inner rotating shaft (18)
When is rotated in the above direction, the inner rotation shaft (18) moves to the left with respect to the housing (1) by the action of the first ball screw (21) and the rack shaft by the action of the second ball screw (25). (2) moves to the left with respect to the inner rotation axis (18).
As a result, the rack shaft (2) moves to the left with respect to the housing (1), and the amount of movement is the amount of movement of the inner rotary shaft (18) with respect to the housing (1) and the rack shaft with respect to the inner rotary shaft (18).
It is equal to the sum of the movement amount of (2). The amount of movement of the inner rotating shaft (18) with respect to the housing (1) is proportional to the amount of rotation of the inner rotating shaft (18) and the lead of the screw of the first ball screw (21), and is the rack shaft relative to the inner rotating shaft (18). The movement amount of (2) is the inner rotation axis
It is proportional to the rotation amount of (18) and the lead of the screw of the second ball screw (25). Therefore, the amount of movement of the rack shaft (2) with respect to the housing (1) is proportional to the sum of the screw leads of the two ball screws (21) and (25) and the amount of rotation of the inner rotating shaft (18).

The same applies when the rack shaft (2) is moved to the right by operating the handle.

When the two ball screws (21) (25) have the same screw direction, the inner rotary shaft (18) with respect to the housing (1)
The direction of movement of the rack shaft (2) with respect to the inner rotating shaft (18) is opposite, so the direction of movement of the rack shaft (2) with respect to the housing (1) is the same as the rotating direction of the inner rotating shaft (18). The amount of movement is determined by the size relationship of the leads of the two ball screws (21, 25), and the amount of movement is proportional to the difference between the leads of these screws and the amount of rotation of the inner rotary shaft (18).

In this way, the inner rotary shaft by the motor (12)
Rack shaft (2) housing per unit rotation of (18)
The movement amount, that is, the reduction ratio, to (1) is two ball screws
Since it is determined by the screw leads of (21) and (25), the reduction ratio can be set arbitrarily by appropriately determining these.

Since the rotation of the inner rotary shaft (18) by the motor (12) is transmitted to the rack shaft (2) by the ball screws (21) and (25) having a very small transmission loss, the total transmission loss is very small. It is small and therefore the capacity of the motor (12) can be small.

For example, when transmitting the rotation of the motor to the rack shaft by the worm and the rack and pinion as in the conventional case,
The overall transmission efficiency is about 0.6, and when the rotation of the motor is transmitted to the rack shaft by the two ball screws as in the above embodiment, the overall transmission efficiency is about 0.9. Therefore,
According to the above-mentioned embodiment, the same driving force can be obtained with the motor having the capacity of about 2/3 of the conventional one.

Since the brushless motor (12) has the motor characteristics of low rotation and high torque, the reduction ratio may be small and the speed reduction mechanism can be simplified, and the brushless motor (12) and the ball screws (21) (25) By combining the speed reduction mechanism using
The motor characteristics of low rotation and high torque can be fully utilized, and the performance such as steering wheel return is greatly improved by reducing the motor inertia. In addition, brushless motor (1
Since 2) is used, a rotational position sensor is required, but
Since the rotational position sensor (35) is provided around the rack shaft (2), the degree of freedom in mounting is high.

The above motor (12), as shown in FIG.
It is controlled by the motor control device (40) of the power steering device.

In FIG. 3, a motor control device (40) includes a motor drive circuit (41) and a current detection device as current detection means.
(42) and a PWM control device (43).

The motor drive circuit (41) is U of the motor (12),
A total of 6 power switching elements, for example, FETs (41u1) (41u2) (41v1) (41v2) (4
It is equipped with 1w1) (41w2). Each FET (41u1) ~ (41w2),
As will be described later, the PWM control device (43) controls the opening and closing, and thereby controls the current value flowing in each phase of the motor (12).

The current detecting device (42) includes three current detectors (42) for detecting the values of the currents flowing in the respective phases of the motor (12).
u) (42v) (42w). Current detector (42u) ~ (42w)
Is composed of, for example, a Hall CT.
The outputs of the current detectors (42u) to (42w) are input to the PWM controller (43) through the amplifiers (44u) (44v) (44w).

The PWM control device (43) includes a torque sensor (7)
Also, the FETs (41u1) to (41w2) of the motor drive circuit (41) are controlled based on the output of the current detection device (42), whereby the motor (12) is PWM-controlled. The PWM control device (43) is composed of, for example, a microcomputer. The functional representation of the structure is as shown in FIG. That is, the PWM control device (43) is a torque sensor.
Target current value calculation means (45) for calculating the target current value of the motor (12) based on the output of (7), and three-phase phase separation processing based on the target current value and the output of the rotational position sensor (35). Do 3
Phase separation processing means (46), PI for performing PI calculation of each phase based on the output of the three phase separation by this and the output of the current detection device (42)
It has a calculation means (47) and a PWM drive means (48) for PWM-driving the FETs (41u1) to (41w2) of each phase based on the PI calculation result of each phase. Note that these functions are known in the conventional power steering device. PW
The M control device (43) also includes duty correction means (49) for correcting the duty of each phase at the zero point.

Next, referring to the flowchart of FIG.
An example of the operation of the motor control device (40) will be described.

First, when the ignition key of the vehicle is turned on, predetermined initialization (step 1) and initial diagnosis (step 2) are performed, the failsafe relay is turned on (step 3), and the motor (12) ) Is connected to the power supply. Then, the initial duty correction process is performed by the duty correction means (49) (step 4). Details of this process are shown in the flowchart of FIG. Figure 5
First, PWM output is performed to the motor drive circuit (41) so that the duty of each of the three phases becomes 50% (step 41). Next, each current detector (42u) ~ (4u
2w) output detects the current value of each phase (Step 4
2) It is checked whether the current values of all the phases are 0 (step 43). Then, if there is even one phase whose current value is not 0, the process proceeds to step 44, the offset value of the phase whose current direction is opposite to that of the other two phases is changed by a predetermined amount, and the process proceeds to step 42. Return. The offset value of each phase is for a duty of 50% and is set to 0 in the initial setting of step 1. In a three-phase AC motor, if the duty is 50% for all three phases,
All phases should have the same voltage, and no current should flow in each phase, but in reality, there is an offset at the zero point of each phase, so current flows in each phase. This current flows from a phase having a high voltage to a phase having a low voltage, and the direction of the current of any one of the three phases is opposite to the direction of the current of the remaining two phases. And step 43
At 44 and 44, the offset value of one phase whose current direction is opposite to that of the other two phases is changed until no current flows in each phase. When the current values of all the phases become 0 in step 43, the process proceeds to step 45, the offset value of each phase is stored, and the process of step 4 ends. At this time, the offset values are corrected so that the offset values of the three phases are around 0, that is, the duty of the three phases is around 50%.
This is stored as the initial offset value. As a result, the offset value of each phase when the total flow value of all phases becomes 0 is stored as the initial offset value. In FIG. 4, when the processing of step 4 is completed, the rotor angle is input from the rotational position sensor (35) (step 5), and the current value is input from each of the current detectors (42u) to (42w) (step 5). 6)
The steering torque is input from the torque sensor (7) (step 7), and the duty correction process during operation is performed by the duty correction means (49) (step 8). Details of this process are shown in the flowchart of FIG. In FIG. 6, first, the steering torque is the dead zone A in FIG.
It is checked whether or not it is in the zero region B (step 8).
1) If not in the zero area B, the process ends. FIG. 7 shows the relationship between the steering torque (input value) and the target current value (output value). As is clear from FIG. 7, in this motor control device (40), a constant dead zone A is provided for the steering torque, and when the steering torque is in this dead zone A, the target current value is 0.
I am trying to be. Further, in order to determine whether to update the offset value for the duty of 50% of each phase, which will be described later, a constant zero area B is set in the dead zone A, and when the steering torque is not in this zero area B, the offset is set. The process ends without updating the value. If the torque is in the zero region B in step 81, the process proceeds to step 82, the vehicle speed is detected by a vehicle speed sensor (not shown), and it is checked whether or not the vehicle speed is 0.
If not 0, the process ends. If the vehicle speed is 0 in step 82, the process proceeds to step 83, and PWM output is performed to the motor drive circuit (41) so that the duty of each of the three phases becomes 50% plus each offset value. Be seen. Note that initially, the initial offset value previously stored in step 4 is used, and from the second time onward, a new offset value updated in step 87 later is used. Next, each current detector (42u) ~ (42w)
The current value of each phase is detected from the output of (8), and it is checked whether the current value of all phases is 0 (step 85).
Then, if there is even one phase in which the current value is not 0, the process proceeds to step 86, and as in the case of step 44, the other 2
The offset value of the phase in which the directions of the current and the phase are opposite is changed by a predetermined amount, and the process returns to step 84. When the current values of all the phases become 0 in step 85, the process proceeds to step 87, the offset value of each phase is updated and stored, and the process of step 8 ends. Also in this case, the offset values of the three phases are 0
Is corrected, that is, the duty of the three phases is centered at 50%, and the offset value is corrected and updated as an offset value. As a result, the offset value of each phase when the total flow value of all phases becomes 0 is stored as a new offset value. In FIG. 4, when the process of step 8 is completed, calculation of the target current value (step 9), 3 of the target current value
Phase separation processing (step 10), PI calculation for each phase (step
11) and PWM output of each phase (step 12). In step 9, the target current value calculating means (45) obtains the target current value based on the relationship shown in FIG. In step 10, three-phase separation processing means
By (46), the current value of each phase corresponding to the rotor angle is obtained with respect to the target current value obtained in step 9. In step 11, the PI calculation means (47) calculates P (proportional) and I (integral) for each phase to obtain the control output of each phase. In step 12, P
The WM drive means (48) outputs a pulse width (duty) corresponding to the sum of the control output of each phase and the output of the duty correction means (49) of each phase, that is, the offset value of each phase, that is, a PWM control signal. Is output. Then, it is checked whether or not the ignition key is off (step 13), and if it is not off, the process returns to step 5 and the above operation is repeated. If the ignition key is turned off in step 13, the process ends.

[0033]

According to the electric power steering apparatus of the present invention, as described above, the duty is corrected as needed with respect to the offset of the zero point of each phase of the motor. It is possible to suppress torque ripples and vibrations that occur due to variations in points,
Further, since the duty is corrected in the PWM on state for all three phases, it is possible to correct the zero point offset due to variations in the switching element, the motor coil, and the like.

[Brief description of drawings]

FIG. 1 is a partially cutaway rear view of a mechanical portion of an electric power steering apparatus according to an embodiment of the present invention.

FIG. 2 is a partially cutaway rear view showing an enlarged main part of FIG.

FIG. 3 is a block diagram showing an example of a configuration of a motor control device of the electric power steering device.

FIG. 4 is a flowchart showing an example of processing of a motor control device of the electric power steering device.

5 is a flowchart showing an example of an initial duty correction process in the flowchart of FIG.

FIG. 6 is a flowchart showing an example of duty correction processing during operation in the flowchart of FIG.

FIG. 7 is a graph showing a relationship between a steering torque and a target current value of a motor.

[Explanation of symbols]

(7) Torque sensor (12) Brushless motor (3-phase AC motor) (40) Motor control device (41) Motor drive circuit (42) Current detection device (current detection means) (43) PWM control device (45) Target current Value calculation means (46) Three-phase phase separation processing means (47) PI calculation means (48) PWM drive means (49) Duty correction means

Claims (1)

[Claims] 1. A three-phase AC motor is PWM-driven based on a target current value corresponding to a torque detected by a torque sensor,
In an electric power steering apparatus for assisting steering, current detection means for detecting a current value of each phase of the motor, and detection torque of 0 in a dead zone of a target current value.
50% duty for each phase when it is within the prescribed range in the vicinity
An electric power steering apparatus comprising: a duty correction unit that corrects a duty of each phase based on a detection result of a current value flowing in each phase when driven by.