JP2012165503A - Pwm control method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a control method of PWM control capable of smooth control not only in an operating state but also in the case of starting to move at a low speed from a stop state.SOLUTION: A body to be driven, a motor for driving the body to be driven and motor control means for PWM controlling the motor are provided. In one cycle of the PWM control, a first duty comprising a pulse group of frequencies corresponding to a resonance frequency of the body to be driven and a second duty comprising a pulse group of frequencies higher than the frequencies of the pulse group of the first duty are provided.

Description

  The present invention relates to a PWM control method of a PWM control device used for an electric power steering device or the like.

Conventionally, PWM (Pulse Width Modulation) control for inductive loads such as motors and coils is controlled at a frequency where the current flowing through the load is continuous and the ripple amount of the current becomes an acceptable value. It is carried out. For example, in an electric power steering device that uses a motor to provide steering assist force to a steering mechanism of a vehicle such as an automobile to assist a steering operator, a relatively high frequency is controlled in order to ensure smooth steering performance. Used for.
Further, the motor of the electric power steering apparatus steers the wheels via mechanical gears. This mechanical structure has a physical mechanism in which the coefficient of friction is large due to static friction when not in operation, and the coefficient of friction is small due to dynamic friction after operation.
In order to cope with the above smooth steering performance and lack of assist at the start of turning at the time of steering, various devices are required, for example, there is Patent Document 1.

JP 2003-154953 A

However, the conventional current control selects the frequency so that the ripple (Ripple) current is small, so it is smooth in the operating state, but has a large friction coefficient when moving slightly from the stopped state (static friction). Since the movement from a stationary state to a dynamic state with a small friction coefficient (dynamic friction), the movement may suddenly occur. That is, since it changes to dynamic friction with a small friction coefficient from the moment of movement, there is a possibility that unintended steering acceleration will be performed after the movement starts.
In addition, when a drive torque is applied to the motor of the electric power steering apparatus when shifting from the stop state to the operation state, a relatively large pulse width of PWM control is input, but the drive frequency suitable for the mechanical system is a ripple. The current is large. Therefore, if the frequency is always adjusted to the drive frequency of the mechanical system in the PWM control cycle, a large ripple current always flows even in the operating state, and vibration and sound are generated. This lowers the merchantability as an automobile.
Also in Patent Document 1, a higher level of smooth steering performance and a strong assist force at the start of turning at the time of steering are required.

  Therefore, the present invention solves such problems, and an object of the present invention is to provide a PWM control method capable of smooth control not only in an operating state but also in a case of starting at a low speed from a stopped state. It is to be.

In order to solve the above-described problems and achieve the object of the present invention, the present invention is configured as follows.
That is, a driven body, a motor that drives the driven body, and motor control means that performs PWM control of the motor, and corresponds to the resonance frequency of the driven body in one cycle of the PWM control. And a second duty consisting of a pulse group having a higher frequency than the frequency of the first duty pulse group.

  With such a configuration, a sufficient assist force is generated at the start of turning at the time of steering by a pulse group having a frequency corresponding to the resonance frequency of the driven body having the first duty. Further, when the operation of the driven body changes from static friction to dynamic friction, a driving force for smooth steering with less current ripple, vibration, and noise is generated by the high-frequency pulse group of the second duty. .

  According to the present invention, it is possible to provide a control method of PWM control capable of performing smooth control not only in the operating state but also in the case of starting at a low speed from the stopped state.

It is a figure which shows the PWM control method of 1st Embodiment of this invention, and the control state of the electric current at that time. It is a figure which shows an example of the method of forming the double PWM signal of 1st Embodiment of this invention. It is a figure which shows the circuit example which implement | achieves the method of forming the double PWM signal of 1st Embodiment of this invention with a logic circuit. It is a figure which shows the frequency used for the PWM control of 1st Embodiment of this invention, and the relationship between the vibration and sound of a mechanical system which arise by it. It is the figure which showed the circuit example which controls the coil (solenoid) of the motor of 1st Embodiment of this invention by composite PWM. In 2nd Embodiment of this invention, it is the figure which showed a mode that the ratio of the 1st duty of a double PWM signal and a 2nd duty was changed with the electric current value of a motor, (a) is a current value 5% or less, (B) is more than 5% to 10% or less, (c) is more than 10% to less than 15%, and (d) is 15% or more.

  Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a diagram showing a PWM control method according to the first embodiment of the present invention and a current control state at that time.
In the PWM control method of the present invention, PWM control is performed by combining a plurality of duties (pulse groups) composed of different frequencies. When the number of duties is N (N is a positive integer), the case of N = 2 is appropriately expressed as double PWM. For general N (N ≧ 2, N is an integer), it is appropriately expressed as composite PWM.
First, the first duty (11A, 11B) and the second duty (12A, 12B), which are signals used in the composite PWM or the double PWM, will be described.

≪About the first duty and the second duty of the double PWM signal≫
In FIG. 1, the first duty (11A, 11B) corresponds to the response speed of the mechanical system of the electric power steering device (not shown) which is the driven body of the motor (driving body, not shown) (resonance of the mechanical system). A group of low frequency pulses (close to frequency).
In other words, since the frequency is lower than the pulse group of the second duty (12A, 12B) described later, the pulse width can be increased (widened) and the torque can be increased easily. In other words, the pulse has a frequency suitable for the mechanical system. Is a group.

The pulse group of the first duty (11A, 11B) is continuously generated at a frequency of 500 Hz, which is the first frequency, and each pulse width of the pulse group is increased when the load current amount is increased by a command of the control circuit. When the load current amount is small, it becomes narrow.
Details of why the frequency of the first duty (11A, 11B) pulse group is selected to be 500 Hz will be described later.

The second duty (12A, 12B) is a pulse group having a frequency suitable for the electric system, that is, to easily reduce ripples. The pulse group of the second duty (12A, 12B) is continuously generated at a frequency of 20 KHz which is the second frequency, and each pulse width of the pulse group is wide when the amount of current is increased by a command of the control circuit. In addition, when the current amount is reduced, it becomes narrower.
Details of why the frequency of the pulse group of the second duty (12A, 12B) is selected as the frequency 20 KHz will be described later.

In FIG. 1, the title indicated by the block in the column indicating the pulse group of the first duty (11A, 11B) and the second duty (12A, 12B) is “duty control state”.
Corresponding to the “duty control state”, the title indicated by the block in the column describing the current flowing through the load at that time is “current control state”.

  The first duty (11A, 11B) and the second duty (12A, 12B) are combined and repeated every cycle (13A, 13B). One period (13A, 13B) is 50 Hz in FIG. First, the first duty 11A is generated at the beginning of the period 13A, and then the second duty 12A is generated. Further, the first duty 11B is generated at the beginning of the cycle 13B, and the second duty 12B is generated thereafter. It occurs in the same order in the subsequent cycles.

Since the frequency of the first duty (11A, 11B) is 500 Hz as described above, the frequency is lower than the frequency of 20 KHz of the second duty (12A, 12B), and the pulse width can be easily increased. That is, it is suitable when generating a large torque.
However, since the frequency is low, the pulse interval is long, the load current tends to fluctuate during that time, and the current ripple tends to increase.

  Since the frequency of the second duty (12A, 12B) is 20 KHz as described above, the frequency is higher than that of the first duty (11A, 11B), the pulse interval is short, and the load current fluctuates during that time. Less current ripple. However, since the pulse width is narrow, it is not suitable for generating a large torque.

In FIG. 1, in the control state of the first duty (11A, 11B) and the second duty (12A, 12B), the current flowing through the load corresponds to the column of “Current control state” as described above. As shown.
When controlled at the first duty (11A, 11B) of 500 Hz, the ripple is relatively large as shown by the current waveform surrounded by the broken line 14. On the other hand, when controlled by the second duty (12A, 12B), the ripple is relatively small.

For each of the first duty 11A and the second duty 12A, a pulse width optimum for the situation at that time is selected by a PWM control means (motor control means, not shown). These optimum pulse widths are kept constant for 0.02 sec (1/50 Hz) in the period 13A which is one period. During 0.02 sec of period 13B, which is the next one period, the first duty 11B and the second duty 12B are each selected with the optimum pulse width for the situation at that time, so the first duty in period 13A The pulse width of 11A and the second duty 12A may be the same or may change.
However, one period is 50 Hz, and the time of periods 13A and 13B is a constant 0.02 sec (1/50 Hz).

Even after the period 13B, one period is unchanged at 50 Hz, the first duty is a 500 Hz pulse group, and the second duty is a 20 KHz pulse group. However, in each cycle, the optimum pulse width is selected as the first duty and the second duty for the situation at that time.
By combining the first duty (11A, 11B) and the second duty (12A, 12B), and every 0.02 sec (1/50 Hz) which is the reciprocal of one cycle, the PWM control means (motor control means, By monitoring and controlling (not shown), optimal motor control is possible.

≪Example of double PWM signal formation≫
FIG. 2 is a diagram illustrating an example of a method for forming a double PWM signal.
FIG. 2A shows a process in which a continuous pulse group of 500 Hz and a continuous pulse group of 20 KHz form a pulse group corresponding to the first duty and the second duty.
FIG. 2B shows a time chart of the waveform of the pulse group corresponding to the formed first duty and second duty.
FIG. 2C shows a time chart of a waveform in which pulse groups corresponding to the first duty and the second duty are combined.

In FIG. 2A, the first pulse width signal sequence 210 is a continuous pulse group of 500 Hz. Further, the second pulse width signal train 220 is a continuous pulse group of 20 KHz. The first selection signal 251 is a 50 Hz repetitive signal. The second selection signal 252 is an inverted signal of the first selection signal 251.
By passing the first pulse width signal train 210 of 500 Hz when the first selection signal 251 is High (high potential, positive potential, 1), the first pulse group 211 of 500 Hz is formed. Further, by passing the second pulse width signal train 220 of 20 KHz when the second selection signal 252 is High (high potential, positive potential, 1), a second pulse group 221 of 20 KHz is formed.

FIG. 2B shows waveforms of a first pulse group 211 of 500 Hz and a second pulse group 221 of 20 KHz.
The 500 Hz first pulse group 211 corresponds to the first duties 11A and 11B (FIG. 1), and the 20 kHz second pulse group 221 corresponds to the second duties 12A and 12B (FIG. 1).
Further, since the first selection signal 251 and the second selection signal 252 are in an inverted relationship with each other, the first pulse group 211 of 500 Hz and the second pulse group 221 of 20 KHz do not overlap each other.

FIG. 2C shows a waveform of a combined pulse group 212 in which a first pulse group 211 of 500 Hz and a second pulse group 221 of 20 KHz are combined.
The combined pulse group 212 corresponds to a signal that combines the first duty 11A and the second duty 12A for one period in FIG.

≪Circuit example for forming double PWM signal≫
FIG. 3 is a diagram showing a circuit example for realizing the method for forming the double PWM signal described in FIG. 2 with a logic circuit.
In FIG. 3, a first pulse width signal sequence 210 (FIG. 2) of 500 Hz is input to a first input terminal 310 of an AND (logical product) circuit 331. Further, the first selection signal 251 (FIG. 2) is input to the second input terminal 351.
The second pulse width signal train 220 (FIG. 2) of 20 KHz is input to the first input terminal 320 of the AND circuit 332. The second input terminal 352 receives a second selection signal 252 (FIG. 2) that is a signal obtained by inverting the first selection signal 251 (FIG. 2) by the inverter (inversion) circuit 334.
The output 311 of the AND circuit 331 and the output 321 of the AND circuit 332 are input to the first input terminal and the second input terminal of the OR (logical sum) circuit 333, respectively.

With the above configuration, the output 311 of the AND circuit 331 includes the first pulse group 211 (only the High section of the first selection signal 251 (FIG. 2) output from the first pulse width signal sequence 210 (FIG. 2) of 500 Hz. The signal of FIG. 2) is obtained.
Further, the second pulse group 221 (FIG. 2) in which the 20 kHz second pulse width signal train 220 (FIG. 2) outputs only the high section of the second selection signal 252 (FIG. 2) is output to the output 321 of the AND circuit 332. Is obtained.
An output 312 of the OR circuit 333 forms a combined pulse group 212 (FIG. 2) in which the 500 Hz first pulse group 211 (FIG. 2) and the 20 KHz second pulse group 221 (FIG. 2) are combined. Output.
As described above, the composite pulse group 212 corresponds to a signal that combines the first duty 11A and the second duty 12A for one period in FIG.

<Relationship between frequency and mechanical vibration and sound>
FIG. 4 is a diagram showing the relationship between the frequency used for PWM control and the vibration and sound of the mechanical system caused thereby.
The frequency included in the PWM control has various effects on the mechanical system of the electric power steering apparatus. The various components and structures used in the motor of the electric power steering device, the gear box (driven body) driven by this motor, and the mechanical mechanism are resonated with mechanical vibration. Has a frequency.

The closer the frequency (500 Hz) of the first duty corresponding to the response speed of the mechanical system included in the PWM control of the motor that is the driving body and the resonance frequency of the driven body (gearbox) are, the closer the motor that is the driving body is The driven body can be driven efficiently with less energy.
However, when the frequency of the first duty of the mechanical system approaches the resonance frequency of other various parts and structures, it may resonate and appear as unwanted vibrations and sounds.

In FIG. 4, a region having a frequency of 500 Hz or less is a resonance region in which various components and structures used in the mechanical mechanism are likely to resonate. And resonance of 500 Hz or less often appears as “vibration”. Therefore, the region of 0 to 500 Hz is a resonance region as mechanical system vibration.
Moreover, it often appears as “sound” above 500 Hz. Since the human audible range is generally 20 Hz to 20 KHz, even if 20 KHz or higher is generated, it is an area that cannot be heard by humans as sound.
Therefore, the region of 500 Hz to 20 KHz is a resonance region recognized as sound.
In this way, the resonance phenomenon generated by PWM control may appear as “vibration” or “sound”, but from experience, “vibration” is recognized as a relatively large problem, and “sound” is relatively minor. It is recognized as a problem.

From the above, the resonance phenomenon as "vibration" should not occur as much as possible.
The resonance region where various components used in the mechanical mechanism are likely to resonate is a region of 0 to 500 Hz.
Also, the lower the frequency, the easier it is to generate a large torque in PWM control,
In addition, the resonance frequency of the gear box that is the driven body of the electric power steering apparatus that is the object for the time being is approximately 500 Hz,
In view of the above, 500 Hz is selected as the frequency of the first duty (11A, 11B, FIG. 1).

Also, since the human audible range is generally 20 Hz to 20 KHz, if it exceeds 20 KHz, the “sound” generated in the electrical system will not be an obstacle.
Also, the higher the frequency, the less current ripple,
On the other hand, even if the region where the dynamic friction coefficient having a small friction coefficient dominates is targeted, if the frequency becomes too high, a predetermined pulse width for generating a predetermined torque cannot be secured,
20KHz is selected as the frequency of the second duty (12A, 12B, FIG. 1).

<Usage example in PWM control solenoid circuit>
FIG. 5 is a diagram showing an example of a circuit (solenoid circuit) in which a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) which is a switching element is configured by an H bridge, and a coil (solenoid) of a DC motor is controlled by a composite PWM. .
In FIG. 5, a DC voltage (DC power) having a positive potential E and a negative potential 0 is supplied from a DC power source 56.

The source of the P-type MOSFET 52 is connected to the positive potential E, and the drain of the P-type MOSFET 52 is connected to the drain of the N-type MOSFET 53. The source of the N-type MOSFET 53 is connected to the negative potential 0.
The source of the P-type MOSFET 54 is connected to the positive potential E, and the drain of the P-type MOSFET 54 is connected to the drain of the N-type MOSFET 55. The source of the N-type MOSFET 55 is connected to the negative potential 0.
A solenoid (coil) 51 is connected between a connection point between the drains of the P-type MOSFET 52 and the N-type MOSFET 53 and a connection point between the drains of the P-type MOSFET 54 and the N-type MOSFET 55.

In the circuit of FIG. 5, for example, if the P-type MOSFET 52 and the N-type MOSFET 55 are turned on while the P-type MOSFET 54 and the N-type MOSFET 53 are turned off, a current flows through the solenoid 51 from the left to the right in FIG. If the gate potentials of the P-type MOSFET 52 and the N-type MOSFET 55 are PWM-controlled, the average amount of current flowing through the solenoid 51 can be controlled.
If the P-type MOSFET 54 and the N-type MOSFET 53 are turned on while the P-type MOSFET 52 and the N-type MOSFET 55 are turned off, a current flows through the solenoid 51 from the right to the left in FIG. If the gate potentials of the P-type MOSFET 54 and the N-type MOSFET 53 are PWM-controlled, the average amount of current flowing through the solenoid 51 can be controlled.

Therefore, in the circuit of FIG. 5, the amount of current flowing through the solenoid 51 can be controlled and current can flow in both directions.
For example, an electric power steering device is a circuit suitable for controlling a DC motor (a motor for driving a driven body, not shown) provided in the electric power steering device (not shown).
At this time, the DC motor drives a gear box (a driven body, not shown) of the electric power steering apparatus.

(Second embodiment)
Next, a current control method using the PWM control method (combined PWM) according to the second embodiment of the present invention will be described.
In the second embodiment, the ratio between the first duty and the second duty of the double PWM signal is changed according to the magnitude of the current value flowing through the motor to be controlled.

FIG. 6 is a diagram showing how the ratio between the first duty and the second duty of the double PWM signal is changed by this current value for one period (50 Hz, 1/50 second). FIG. 6 shows (a) a current value of 5% or less, (b) a current value of 10% or less, (c) a current value of less than 15%, and (d) a current depending on the ratio of the current value flowing through the motor to be controlled to the maximum value. It shows that a different control method is used for each of the values of 15% or more.
In the pulse width control (PWM control), if the pulse potentials are all positive potentials (high potentials), the current flowing through the controlled object always flows, and thus takes the maximum current value. If the pulse takes not only a positive potential (high potential) but also a negative potential (low potential) at a certain rate, the current decreases accordingly. Therefore, for example, “current value of 5% or less” in (a) also means that the ratio of the pulse taking a positive potential (high potential) in one cycle is “5% or less”.

FIG. 6A is a diagram showing the first duty and the second duty when the current value flowing through the motor is small and the current value is 5% or less of the maximum value.
In FIG. 6A, the pulse having the first duty of 500 Hz is three times. Then, the second duty of 20 KHz is stopped.

This corresponds to a state in which the motor of the electric power steering apparatus starts to operate from a stationary state (current value 0). In the stationary state, static friction works and the friction coefficient is large. Therefore, a large torque is required to eliminate or alleviate the “sticking feeling” at the start of turning during steering.
It should be noted that at the beginning of movement, it is important to start movement, and since some vibrations do not cause a problem, a first duty of 500 Hz is used. In addition, it is the same that the first duty 500 Hz pulse is three times, but the pulse width is selected and controlled so as to be optimal at a current value of 0 to 5%.

FIG. 6B is a diagram showing the first duty and the second duty when the current value flowing through the motor is more than 5% to 10% or less of the maximum value. In FIG. 6B, the case where the current value is more than 5% to 10% or less of the maximum value is simply expressed as “current value 10% or less”.
In FIG. 6B, the pulse having the first duty of 500 Hz is twice. The two pulses that are the first duty are generated at the beginning of the cycle. Then, after the two pulses of the first duty, the second duty of 20 KHz operates.
This is because the electric power steering apparatus is already in operation, and the demand for smoothness of the second duty of 20 KHz is increased compared to the torque of the first duty of 500 Hz.
In the process in which the current value changes from more than 5% to 10% or less of the maximum value, the pulse widths of the first duty of 500 Hz and the second duty of 20 KHz are selected and controlled to be optimum respectively. .

FIG. 6C is a diagram showing the first duty and the second duty when the current value flowing through the motor is more than 10% to less than 15% of the maximum value. In FIG. 6C, the case where the current value is more than 10% to less than 15% of the maximum value is simply expressed as “less than 15% current value”.
In FIG.6 (c), the pulse which is a 1st duty of 500 Hz is 1 time. One pulse as the first duty is generated at the beginning of the cycle. Then, after one pulse of the first duty, the second duty of 20 KHz operates.
This is because the control is suitable for a state where the electric power steering apparatus is operating more strongly.
In the process in which the current value changes between more than 10% and less than 15% of the maximum value, the pulse widths of the first duty of 500 Hz and the second duty of 20 KHz are respectively selected and controlled to be optimal. .

FIG. 6D is a diagram showing the first duty and the second duty when the current value flowing through the motor is 15% or more of the maximum value.
In FIG. 6D, there is no first duty pulse of 500 Hz. Only the second duty of 20 KHz operates.
This is because the electric power steering device is operating more strongly, so that a large torque with a first duty of 500 Hz is not necessary and smooth control of 20 KHz is required.
In the process of changing the current value at 15% or more of the maximum value, the pulse width of the second duty of 20 KHz is selected and controlled to be optimum.

(Other embodiments)
The present invention is not limited to the embodiment described above. Here are some examples:
In FIG. 1 and FIG. 6, an example in which the first duty 11 is formed with a frequency of 500 Hz that is the first frequency and the second duty 12 is 20 kHz that is the second frequency is given, but this is only an example. Other frequency combinations may be used.
In particular, 500 Hz, which is the first frequency of the first duty 11, is set depending on the characteristics of the mechanical system. Therefore, if the motor or the driven body changes, the first frequency of the first duty 11 also changes. It is possible.
In addition, although 50 Hz is given as an example of a cycle in which the first duty 11 and the second duty 12 are repeated as a set, a cycle with another frequency may be used.

Further, not only a combination of the first frequency and the second frequency, but also a combination of N types of frequencies having N (N ≧ 3) frequencies may be used.
The optimum frequency of the control varies depending on the type of mechanical system to be operated. Therefore, if there are many parts and components having different characteristics of the mechanical system, the first duty to the Nth duty composed of N (N ≧ 3) frequencies may be used in combination depending on the type.

  In the second embodiment, when the value of the current flowing through the motor is 5% or less, 5% to 10%, 10% to less than 15%, 15% or more, the first duty 11 and the second duty 12 Although the ratio was changed, the current value for changing the above setting may be another value. A predetermined value suitable for each motor or electric power steering device may be set.

  In the second embodiment, the number of pulses of the first duty is determined when the value of the current flowing through the motor is 5% or less, 5% to 10%, 10% to less than 15%, 15% or more. However, another number may be set. A predetermined value suitable for each motor or electric power steering device may be set.

  Further, when the value of the current flowing through the motor increases and decreases, the predetermined value of the current value that changes the above setting may be different. As a result, the control may be switched more smoothly at the boundary between the set values.

  Further, the number of pulses of the first duty may be different when the value of the current flowing through the motor increases and decreases. As a result, the control may be switched more smoothly at the boundary between the set values.

  In the above description, the motor of the electric power steering apparatus has been described as an example. However, the present invention may be applied to PWM control of motors in other fields.

  Further, in the above description, the embodiment of the system in which the current is zero and stationary is described. However, even in a system that is stationary in a state where current is flowing, such as a linear solenoid, the first duty and the first duty having different frequencies are the same. You may apply the current control by the composite PWM using 2 duties.

In the circuit diagram of FIG. 5, a circuit in which an H bridge is configured by a MOSFET switching element is shown. However, in an application where it is not necessary to switch the direction in which a direct current flows, one N-type MOSFET is used as a switching element. On / off (ON / OFF) of the current flowing through the (coil) and the amount of current may be controlled.
Further, a P-type MOSFET may be used instead of the N-type MOSFET. However, when a P-type MOSFET is used, a signal having a polarity opposite to that when an N-type MOSFET is used is used. That is, control is performed using a signal obtained by inverting the PWM control signal shown in FIG. 1 or FIG.

  In addition, although an example in which a MOSFET is used as a switching element is shown in FIG. 5, it may be other than the MOSFET because it only has to fulfill the function of the switching element. For example, an IGBT (Insulated Gate Bipolar Transistor), a BJT (Bipolar Junction Transistor), or other appropriate switching element may be used.

(Supplement of the present invention and this embodiment)
In an electric power steering system, it is necessary to ensure smooth steering performance and sufficient assist force at the start of turning at the time of steering.
For this purpose, the present system performs current control by using at least two PWM control frequencies: a second frequency with a small ripple current and two frequencies based on the first frequency to which the mechanical system responds.
The second duty formed by the second frequency of the electric system reduces the current ripple, and realizes fine and smooth steering performance, low vibration and low noise.
In addition, the first duty formed by the first frequency of the mechanical system ensures a sufficient assist force at the start of turning at the time of steering, and alleviates the “stickiness”.
More specifically, the movement of a machine having a large frictional force (for example, coaxial EPS: Electric Power Steering) is smoothed.
Further, in an automobile, it is effective for correcting rudder on a highway, cutting out from a steering wheel neutral position, and the like.
As described above, by adopting the method of the present invention in an electric power steering system, it is possible to realize performance with a balance between merchantability and function.

11A, 11B First duty 12A, 12B Second duty 13A, 13B Period 210 First pulse width signal train 211 First pulse group 212 Synthetic pulse group 220 Second pulse width signal train 221 Second pulse group 251 First selection signal 252 Second selection signal 310, 320 First input terminal 311, 312, 321 Output 331, 332 AND circuit 333 OR circuit 334 Inverter circuit 351, 352 Second input terminal 51 Solenoid (coil)
52, 54 P-type MOSFET
53, 55 N-type MOSFET
56 DC power supply

Claims (6)

A driven body, a motor for driving the driven body, and motor control means for PWM controlling the motor,
In one period of the PWM control,
A first duty consisting of a pulse group having a frequency corresponding to the resonance frequency of the driven body;
A second duty comprising a pulse group having a frequency higher than the frequency of the first duty pulse group;
A PWM control method comprising:   2. The PWM control method according to claim 1, wherein the motor control unit changes a ratio of the first duty and the second duty appearing in the one cycle based on a value of a current flowing through the motor.   3. The PWM control method according to claim 1, wherein as the current value increases, a ratio of the second duty appearing in the one cycle is increased.   4. The PWM control method according to claim 1, wherein when the current value is equal to or greater than a predetermined value, driving is performed only with the second duty. 5.   4. The PWM control method according to claim 1, wherein when the current value is equal to or less than a predetermined value, driving is performed only with the first duty. 5.   6. The PWM control method according to claim 1, wherein the driven body is a gear box of an electric power steering apparatus.

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