JP2000184776A - Method and device for controlling brushless dc motor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To increase a capacitor charge voltage and achieve smooth activation regardless of the input of a large-duty signal by outputting a pseudo pulse width modulation control signal corresponding to a rotary angle until the rotation start of a rotor is detected and then outputting a control signal based on a torque command after rotation is started. SOLUTION: The output of an encoder EN is inputted to a CPU 30 and enters a motor speed detection part 36 for detecting a bicycle speed and a motor rotation logic 38 for determining which of three-phase armature coil winding an excitation current is supplied from the rotary angle of the rotor. A main circuit current corresponding to the drive torque of a motor 12 is detected by a current detection part 40, is amplified by an amplifier 43, is further binarized, and then is inputted to the CPU 30. A butt part 44 obtains the difference between a torque command value Tp and a drive torque TD, performs correction with a motor speed N, and obtains a torque target value T0. A main PWM logic part 48 operates after the motor is started and outputs a PWM control signal P1 with a duty corresponding to the torque target value. A pseudo PWM logic part 50 outputs P2 when the motor is started and controls six power FETs with a PWM control signal P1.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a brushless DC
The present invention relates to a control method and a control device for performing PWM control of an exciting current of an armature serving as a stator of a motor by a voltage-type PWM inverter using a bootstrap drive circuit.

[0002]

2. Description of the Related Art A rotating angle of a permanent magnet type rotor is detected by an encoder, and an exciting current (armature current) supplied to an armature winding of a stator is switched to form a rotating magnetic field on the stator. Brushless DC motors for rotating a child are widely known. Here, the exciting current of the armature winding is converted into a duty (on time /
(ON time + OFF time)) and PWM (Pulse Width Modu)
A voltage-type PWM inverter that controls (lation, pulse width modulation) is also known.

A plurality of switching elements (for example, power MOSF) forming a bridge circuit of the PWM inverter
ET) is also known in which a bootstrap drive circuit (gate circuit) controls on / off. Here, the bootstrap drive circuit includes a bootstrap capacitor for supplying a drive current (gate current) charged at a predetermined voltage, and a PW having a duty corresponding to the torque command.
When the M control signal is turned on, a drive current (gate current) is sent from this capacitor to the control terminal (gate) of the switching element to turn on the switching element. This is for turning off the switching element.

[0004]

In such a bootstrap type driving circuit (gate circuit), PWM is used.
When the control signal is off, that is, when the switching element is off, the bootstrap capacitor is charged. However, if the charge of the capacitor is insufficient, it becomes impossible to supply a sufficient drive current (gate current) to the control terminal (gate) of the switching element (power MOSFET) when the PWM control signal is turned on.

[0005] In this case, there is a possibility that the ON operation of the switching element is not performed, the operation becomes unstable, or the element is damaged. For example, when a PWM control signal for turning on and off at a high speed is input immediately after the start of the motor, the capacitor is discharged without being sufficiently charged, so that the charging voltage does not sufficiently rise. In this case, as the duty of the PWM control signal increases, the proportion of the charging time of the capacitor decreases, so that the recovery of the charging voltage is delayed.

[0006] Such a state should be eliminated if the switching elements that perform on / off operations change as the rotor rotates. However, in an actual brushless DC motor, if the load or friction torque applied to the rotor at the time of startup is large, the rotor may not be able to start rotating immediately but may start rotating with a delay. In this case, if a PWM control signal having a large duty is input, a state in which the capacitor is not charged in the drive circuit of the specific switching element continues, and a state in which the motor cannot be started occurs.

The charge voltage of the bootstrap capacitor is always monitored by a normal monitoring circuit. When the charge voltage of the capacitor is lower than a specified value, the operation of the switching element is stopped (turned off) to protect the switching element. . Therefore, even when such a monitoring circuit is provided, if a PWM control signal having a large duty is input immediately after starting and if the start of rotation of the rotor is delayed, the capacitor charging voltage is kept below a specified value and the motor cannot be started. Occurs.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and a PWM control signal having a large duty is used when the start of rotation of a rotor is delayed or rotation cannot be performed immediately after the start of a motor using a PWM inverter. It is a first object of the present invention to provide a control method of a brushless DC motor which can increase a charging voltage of a bootstrap capacitor and start a motor smoothly even if it is input. It is a second object to provide a control device that can be used directly to implement this method.

[0009]

According to the present invention, a first object is to detect a rotation angle of a permanent magnet type rotor by an encoder and to switch a stator armature winding to be excited by a bridge circuit based on the detected rotation angle. On the other hand, in a method of controlling a brushless DC motor in which a switching element of the bridge circuit is turned on / off by a bootstrap type driving circuit based on a PWM control signal having a duty corresponding to a torque command, the driving circuit includes: Until the encoder detects the start of rotation of the rotor when is turned on from off, the switching element is turned on / off based on a pseudo PWM control signal having a fixed duty regardless of the magnitude of the torque command. And a control method of the brushless DC motor.

Here, it is desirable to minimize the duty of the pseudo PWM control signal used at the time of starting the motor so as to increase the charging voltage of the bootstrap capacitor as quickly as possible. However, if this duty is too small, the starting of the motor is delayed. Therefore, it is desirable to set the duty of the pseudo PWM control signal to about 1/6.

A second object is to detect a rotation angle of a permanent magnet type rotor by an encoder and to switch a stator armature winding to be excited based on the detected rotation angle by a bridge circuit, while responding to a torque command. PWM with duty
A control device for a brushless DC motor that controls on / off of a switching element of the bridge circuit by a bootstrap type drive circuit based on a control signal; a motor rotation logic unit that detects the rotation angle based on an output of the encoder; When the torque command is turned on from off, the armature winding corresponding to the rotation angle is selected until the motor rotation logic unit detects the start of rotation of the rotor, and a pseudo PWM control signal having a fixed duty is output. And a PWM logic unit that outputs a PWM control signal based on the torque command after the start of rotation.

It is desirable that the encoder detects the position of the permanent magnet fixed to the rotor with an alternating magnetic field type Hall element. Here, the alternating magnetic field type Hall element can detect the presence or absence of a magnet even when the direction of the magnetic field incident on the element is reversed, and can detect the stop range even when the rotor is stopped. is there. Further, the brushless DC motor can be a three-phase AC synchronous motor.

The positive-side (high-side) switching element group and the negative-side (low-side) switching element group that form a bridge circuit turn on one of the switching elements continuously within a predetermined operation period, and turn on the other group. Can be controlled so as to be turned on / off by a PWM control signal within a predetermined operation period. In this way, the control circuit can be simplified. However, it is a matter of course that a general control method in which both groups of switching elements are synchronized and turned on / off by a PWM control signal may be used.

The switching element of the bridge circuit has a large input impedance and a power M suitable for high-speed operation.
OSFETs are preferred. A gate circuit as a bootstrap drive circuit for controlling ON / OFF of the power MOSFET is constituted by connecting two MOSFETs in series and
It can be formed by a totem-pole circuit that switches the direction of the drive current (gate current) by alternately turning on and off the SFET.

In this case, by turning on one MOSFET, a gate current is sent from the bootstrap capacitor to the power MOSFET to turn on the power MOSFET, and by turning on the other MOSFET, the gate current flows in the opposite direction to turn on the power MOSFET. Turn off. Also, the capacitor charging circuit can be formed by a bootstrap diode that supplies a charging current from a reference power supply voltage.

[0016]

FIG. 1 is a block diagram showing an embodiment of the present invention.
2 is a diagram showing a main circuit configuration, FIG. 3 is a diagram showing a bootstrap type driving circuit (gate circuit), and FIG. 4 is an induced voltage (A) applied to an armature winding and an armature current (B). FIG. 5 is a gate sequence diagram showing the on / off operation of the switching element of the bridge circuit.

In this embodiment, the present invention is applied to a three-phase AC synchronous motor, and FIGS.
This shows a case where current is applied by 20 ° (always two-phase energization). FIG. 1 shows an example in which the actual motor torque (indicated by the current) with respect to the torque command is changed by the motor speed. For example, as the motor speed increases, the ratio of the motor output to the torque command ( Motor torque / torque command) can be reduced. Such a control device controls the motor output using, for example, a human-powered driving force as a torque command, and has an electric assist power that reduces an assist ratio (assistance ratio of the driving force) by the motor as the vehicle speed increases. Suitable for bicycles.

1 and 2, reference numeral 10 denotes a bridge circuit, which includes six power MOSs as switching elements.
FETs (U, V, W, X, Y, Z). That is, three power MOs forming one switching element group
The SFETs (U, V, W) are connected to the positive terminal of the battery _Vb of the DC power supply, and the other three power MOSFETs (X, Y, Z) forming the other switching element group are connected to the negative terminal of the battery _Vb. ing.

Each power MOSFET (U, V, W,
X, Y, and Z) each include a parasitic diode. Further, a smoothing electrolytic capacitor C _O is connected in parallel to the battery _Vb, and the output impedance on the power supply side as viewed from the bridge 10 side is lowered so that the battery _Vb can be regarded as a voltage source to form a voltage type inverter. The negative electrode side of the battery V _b is interposed shunt resistor R _O, the motor current from the voltage drop of the resistor R _O is detected.

Reference numeral 12 denotes a three-phase blancless DC motor,
It has a permanent magnet type rotor, a stator having an armature winding, and an encoder EN (FIG. 1) for detecting a rotation angle of the rotor. The encoder EN detects a plurality of permanent magnets fixed to the rotor with a plurality of Hall elements. The Hall element is an alternating magnetic field type Hall element capable of detecting the magnet even when the direction of the magnetic field is reversed.

The three-phase armature windings of the stator are star-connected to the bridge circuit 10. That is, the neutral points of the three-phase armature windings are connected to each other, and the other end (non-neutral point side) is connected between the power MOSFETs (U, X) of the bridge circuit 10, (V,
Y) and between (W, Z). As a result, U-phase, V-phase, and W-phase currents iu, iv, and iw flow through the three-phase armature windings (see FIG. 4B).

The power MOSFETs (U to Z) are on / off controlled by a gate driver 14. The gate driver 14 has a built-in bootstrap type driving circuit 16 (FIG. 3) provided separately for each power MOSFET (U to Z), and controls each power MOSFET (U
To Z) are turned on and off separately. Here, the drive circuit 16 controls the current ig supplied to the gates G of the power MOSFETs (U to Z).

As shown in FIG. 3, the gate circuit 16 supplies an ON signal bON and an OFF signal bOFF based on the control signal a.
, A totem-pole circuit 20 and a bootstrap capacitor 22
And a bootstrap diode 24. The pulse generation circuit 18 generates a signal based on a control signal a described later.
An on signal bON and an off signal bOFF are generated alternately. Here, the time T (bON) of the ON signal bON with respect to the total time T (bON + bOFF) of the ON signal bON and the OFF signal bOFF, that is, [T (bON) / T (bON
+ BOFF)] is the duty.

The totem pole circuit 20 has two MOS transistors.
The FETs 26 and 28 are connected in series, and the drain D of the FET 26, that is, the source S of the FET 28 is a power MO.
It is connected to the gate G of the SFET. Capacitor 22
Is connected between the source S of the FET 26 and the drain D of the FET 28. Diode 24 to charge the polarity illustrating a capacitor 22 in FIG. 3 by i ₁ indicated by a broken line by the current supplied from the reference power supply voltage V _CC. In FIG. 3, dotted lines i ₁ and i ₂ show the current when the power FET is off, that is, the current when the capacitor 22 is charged, and the solid line i ₃ shows the power FE.
The current when T is on, that is, when the capacitor 22 is discharged.

When the ON signal bON is input while the capacitor 22 is charged to the specified voltage, the FET 26 is turned on and the FET 28 is turned off, and the capacitor 22 includes the FET 26, the gate G of the power MOSFET, and the drain D. Discharge through the path. That is, the gate current i ₃ flows through the power FET, and the power FET is turned on. Next, when the off signal bOFF is input, FE
T28 is turned on and FET 26 is turned off.
Therefore, the gate current i ₃ disappears and the current i _{2 in the} opposite direction
Flows from the gate G to the drain D through the FET 28.
Therefore, the power FET is forcibly turned off. By alternately inputting the ON signal bON and the OFF signal bOFF in this manner, the FETs 26 and 28 are alternately turned on, and the power FET is turned on and off.

If an ON signal bON having a large duty is input immediately after the motor is started, the capacitor 22 starts discharging with insufficient charge, and the power F
A gate current i ₃ sufficient to turn on ET cannot be generated. At this time, the rotor is locked and not rotating, the rotation rise is slow, and the on-time T
(BON) is longer than the off time T (bOFF),
The charging voltage of the capacitor 22 does not rise forever, and the motor cannot start. This is as described above. In the present invention, when the motor is started, the on-time T (bON) is made sufficiently short by using a pseudo PWM control signal having a small duty so that the charging voltage of the capacitor 22 can be increased sufficiently and quickly.

Next, a control circuit for generating the pseudo PWM control signal will be described with reference to FIG. This circuit is formed by software using the CPU 30. In FIG. 1, functions formed by the software are represented by blocks. Reference numeral 32 denotes an accelerator potentiometer. In this embodiment applied to a bicycle with an electric assist power, this potentiometer 32 has a pedal depression force F _P.
Is detected. The pedal pressing force F _P is input to CPU30 is binarized through an input interface (not shown).

[0028] obtaining the torque command value T _P In CPU30 from the pedal pressing force F _P in the torque command unit 34. The output of the encoder EN is input to the CPU 30 and enters the motor speed detection unit 36 and the motor rotation logic unit 38. The motor speed detector 36 detects the motor rotation speed N, that is, the vehicle speed of the bicycle. Motor rotation logic section 38
Outputs a logic signal L for determining which of the three-phase armature windings to supply the exciting current from the rotation angle of the rotor. That is, the logic signal L determines the timing of supplying U, V, and W phase currents with respect to the change in the rotation angle θ of the rotor in FIG. In this embodiment, since two-phase current is always applied, each layer flows current in the forward and reverse directions at 120 ° intervals at every 60 ° current stop interval.

The current I of the main circuit is equal to the shunt resistance R_OTo
The current is detected by the used current detection unit 40. Its output is an amplifier
Amplified at 42 and further binarized and input to the CPU 70
It is. This current I is the driving torque T of the motor 12_DCompatible with
I do. Reference numeral 44 denotes an abutting portion, which is a torque command value T_PWhen
Drive torque T_DDifference (T_P-T_D), And the difference (T _P
-T_D) Is corrected by the motor speed N and the torque target
Value T_OAsk for. That is, it decreases with an increase in the motor speed N.
The correction coefficient that becomes 0 at a speed slightly higher than a certain speed is determined by this difference (T_P−
T_D) To obtain the torque target value T_OAsk for.

Reference numeral 46 denotes a PWM logic unit, which is a main PWM.
It has a logic unit 48 and a pseudo PWM logic unit 50. The main PWM logic unit 48 operates after starting the motor 12, the duty of PWM corresponding to the torque target value T _O
Outputs a control signal P _1. Pseudo PWM logic unit 50 outputs a pseudo PWM control signal P ₂ small fixed duty example duty 1/6 actuated when starting the motor 12. In the pseudo-PWM logic unit 50 here, the activation of the motor 12, and the torque command value T _P is output, it is determined from the output L of the motor rotation logic unit 38 is changed.

Reference numeral 52 denotes a PWM interface, which receives a PWM control signal P ₁ after starting the motor, a pseudo PWM control signal P ₂ before starting the motor, and a logic signal L.
Is transmitted to the gate driver 14 at a timing corresponding to the six power FETs (U to Z) based on. Here P
The WM interface 52 is connected to a battery V as shown in FIG.
Power FET (U, V, W) connected to the positive side of _b
Is continuously turned on during the operation period based on the logic signal L output from the motor rotation logic unit 38. The power FET connected to a negative electrode side of the battery V _b (X, Y,
Z) during the operation period based on the logic signal L, PW
PWM control by M control signal P _1. Thus, by performing PWM control on only one of the power FETs (X, Y, Z), the circuit configuration of the gate driver 14 can be simplified.

Next, the operation of this embodiment will be described. In a bicycle with an electric auxiliary power, if the main key SW is turned on and left for a certain period of time, the power is turned off in order to reduce energy consumption, and the bicycle is restarted when the pedal is started to be depressed. In this case, for example, if the pedal is strongly depressed while the brake is being applied, or if the pedal is strongly depressed when the vehicle does not immediately start on a slope, a large torque command value T _P is output while the bootstrap capacitor 22 is not sufficiently charged. Will be. A similar situation can occur when the main key SW is turned on with the pedaling force applied.

However, at this time, the PWM interface 52 is connected to the pseudo PWM generated by the pseudo PWM logic unit 50.
A control signal a is generated based on the M control signal P ₂ and sent to the gate driver 14. Therefore, the power FETs (U to Z) are PWM-controlled with a small fixed duty, and the operation of the power FETs (U to Z) can be reliably performed by increasing the charging voltage of the bootstrap capacitor 22. become. Therefore, even if the rotor of the motor is locked or does not immediately start rotating, the armature current flows and the motor driving torque can be generated.

When the rotator slightly rotates and the logic signal L changes, the signal is switched to the normal PWM control signal P ₁ output from the main PWM logic 48. When the logic signal L changes, power FETs (U to
Since the combination of Z) changes, the bootstrap capacitor 22 of the power FETs (U to Z) which start to turn on / off is already sufficiently charged before starting the on / off operation. It is possible to operate reliably even if it becomes large.

[0035]

As described above, according to the first aspect of the present invention, when the motor is started, the predetermined switching element of the bridge circuit is PWM-controlled by the pseudo PWM control signal having a small duty until the rotor starts rotating. Because
Even when the rotor is locked or the start of rotation is delayed, the charging voltage of the bootstrap capacitor can be sufficiently increased and the motor can be started smoothly.

Here, it is desirable to set the duty of the pseudo PWM control signal to about 1/6 (claim 2). According to the second aspect of the present invention, there is provided a control device directly used for implementing the control method (claim 3). The encoder used here is optimally one that detects the position of the permanent magnet fixed to the rotor with an alternating magnetic field type Hall element. Also, a three-phase AC synchronous motor is suitable for the blanless DC motor (claim 5).

In the bridge circuit, one of a positive side (high side) switching element group and a negative side (low side) switching element group is turned on continuously during an operation period, and the other group of switching elements is turned on. May be configured to perform PWM control within the operation period (claim 6). In this case, the gate drive circuit of the switching element group that is continuously turned on can be simplified.

The power MOSFET is optimal for the bridge circuit, and the drive circuit (gate circuit) is composed of two MOSFETs.
, A bootstrap capacitor, and a capacitor charging circuit. Here, the capacitor charging circuit can be constituted by a circuit for supplying a charging current from a reference voltage power supply via a bootstrap diode.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention.

FIG. 2 is a diagram showing the configuration of the main circuit.

FIG. 3 is a diagram showing a bootstrap type driving circuit;

FIG. 4 shows an armature applied voltage (A) and an armature current (B)
Figure showing

FIG. 5 is a gate sequence diagram of a bridge circuit.

[Explanation of symbols]

Reference Signs List 10 bridge circuit 12 motor 14 gate driver 16 drive circuit (gate circuit) 18 pulse generation circuit 20 totem pole circuit 22 bootstrap capacitor 24 bootstrap diode 26, 28 MOSFET 30 CPU 34 torque command unit U, V, W Positive Side (high side) power MOSFET X, Y, Z Negative side (low side) power MOSFET 46 PWM logic section 48 Main PWM logic section 50 Pseudo PWM logic section P ₁ PWM control signal P ₂ Pseudo PWM control signal

Claims (7)

[Claims] An encoder detects a rotation angle of a permanent magnet type rotor, and switches a stator armature winding to be excited based on the detected rotation angle by a bridge circuit, while having a duty corresponding to a torque command. A control method of a brushless DC motor that controls on / off of a switching element of the bridge circuit by a bootstrap type drive circuit based on a control signal, wherein the drive circuit includes: A control of the brushless DC motor, wherein the switching element is turned on / off based on a pseudo PWM control signal having a fixed duty cycle until the start of rotation of the rotor is detected, regardless of the magnitude of the torque command. Method. 2. The duty of the pseudo PWM control signal is about 1
The method of controlling a brushless DC motor according to claim 1, wherein 3. A PWM having a duty corresponding to a torque command while detecting a rotation angle of a permanent magnet type rotor by an encoder and switching a stator armature winding to be excited based on the detected rotation angle by a bridge circuit. A control device for a brushless DC motor that controls on / off of a switching element of the bridge circuit by a bootstrap type drive circuit based on a control signal, a motor rotation logic unit that detects the rotation angle based on an output of the encoder, When the torque command is turned on from off, the armature winding corresponding to the rotation angle is selected until the motor rotation logic unit detects the start of rotation of the rotor, and a pseudo PWM control signal having a fixed duty is output. After the start of rotation, a PWM logic unit that outputs a PWM control signal based on the torque command is provided. Control device for a brushless DC motor, wherein the obtaining. 4. The control device for a brushless DC motor according to claim 3, wherein the encoder detects the position of the permanent magnet fixed to the rotor with an alternating magnetic field type Hall element. 5. The control device for a brushless DC motor according to claim 3, wherein the brushless DC motor is a three-phase AC synchronous motor. 6. The bridge circuit includes a positive switching element group having a current inflow end connected to the positive polarity side of the DC power supply, and a negative switching side element group having a current outflow end connected to the negative polarity side of the DC power supply. The switching elements included in one of the switching element groups are continuously turned on within an operation period determined by the encoder output, and the switching elements included in the other switching element group are controlled by the PWM control within an operation period determined by the encoder output. The control device for a brushless DC motor according to any one of claims 3 to 5, wherein on / off control is performed by a signal. 7. The switching element of the bridge circuit is formed of a power MOSFET, and the drive circuit is composed of two MOSFs.
7. The brushless DC motor control device according to claim 6, comprising a totem-pole circuit in which ETs are connected in series, a bootstrap capacitor for supplying a gate current, and a charging circuit for the capacitor.