JPS6233839B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION It is an object of the present invention to provide a drive circuit for a brushless DC motor in which the adverse effects of the DC offset voltage of a magnetoelectric transducer for position detection and an amplifier are reduced.

Hereinafter, a balanced two-phase sine wave drive brushless DC motor to which the present invention can be applied will be described. In FIG. 1 showing a longitudinal section of this brushless DC motor, 1 is a rotating shaft, 2 is a rotor magnet, and 3 is a rotor yoke. The rotor magnet 2 is magnetized into eight poles, for example, as shown in FIG. in this case,
The rotor magnet 2 is magnetized so that the magnetic flux distribution becomes sinusoidal. The stator coil is provided with two phases, and as shown in Fig. 3, winding blocks _C1 and _C2 , which are placed in the same phase with respect to the magnetic field from the rotor magnet 2, are connected in series. A first stator coil _L1 is formed, and similarly arranged winding blocks _C3 and _C4 are connected in series to form a second stator coil _L2 . These first and second stator coils L ₁ and L ₂ are arranged to face the rotor magnet 2 and are arranged to differ from each other by an odd multiple of 90 degrees in electrical angle.

Further, two position detecting elements such as Hall elements _H1 and _H2 for detecting the magnetic field of the rotor magnet 2 are provided corresponding to the two stator coils _L1 and _L2 . As shown in FIG. 3, the two Hall elements H ₁ and H ₂ are arranged at positions where the magnetic flux from the rotor magnet 2 is detected, and at positions different from each other by 90 degrees in electrical angle.

FIG. 4 shows the basic configuration of a drive circuit for the above-mentioned brushless DC motor. A DC input signal from an input terminal indicated by 4 in FIG. 4 is supplied to excitation terminals of Hall elements H ₁ and H ₂ via an operational amplifier (hereinafter simply referred to as amplifier) 5. When the input signal is a constant voltage of constant polarity, the rotor magnet 2 causes a sinusoidal magnetic flux of constant amplitude and constant frequency to act on the Hall elements H ₁ and H ₂ . Since both are arranged with a 90° electrical angle difference between them, two-phase AC voltages whose phases are shifted by 90° from each other are generated at the output end of the Hall element in response to the rotation of the rotor magnet 2. This two-phase AC voltage is amplified by amplifiers 6 and 7, respectively, and applied to stator coils _L1 and _L2 .

Letting the magnetic fluxes acting on the Hall elements H ₁ and H ₂ mentioned above be (Bsinθ) and (Bcosθ), respectively,
Currents (Isinθ) and (Icosθ) flow through the stator coils _L1 and _L2 , respectively. Therefore, a constant torque of BIsin ² θ+BIcos ² θ=BI is generated. However, the output voltages of the Hall elements H ₁ and H ₂ include a DC offset voltage, and the amplifiers 6 and 7 also generate a DC offset voltage, so the stator coils L ₁ and L ₂
The currents flowing in are I(x+sinθ) and I(y+
cos θ) and offset components x and y. Therefore, the generated torque is BI×{1+√ ² + ² sin (θ+δ)} (where δ=tan ⁻¹ y/x: constant), which includes a component of the rotation angle θ. Therefore, when the rotor 2 is magnetized with eight poles, a torque ripple of four times the rotation period will occur, and vibrations or abnormal noises will also occur.

Therefore, in the past, a semi-fixed variable resistor was provided to which positive and negative power supply voltages were applied to both ends of the stator, and the output terminal of the slider Hall element was connected to cancel the offset voltage of the Hall element and amplifier. Ta. However, this conventional configuration has the disadvantage that it is susceptible to fluctuations in power supply voltage. Furthermore, when controlling the rotational speed of the motor at a constant speed by changing the DC input signal applied to the input terminal 4 in FIG. 4, the value of the offset voltage of the Hall element is approximately proportional to the input current applied to it. In order to change
It was not possible to always cancel the offset voltage. Furthermore, since the variable resistor needs to be adjusted, the cost increases.

The present invention seeks to eliminate these conventional drawbacks.

An embodiment of the present invention will be described below with reference to FIG. The input DC voltage Vi is amplified by the amplifier 8 and applied to the excitation terminal of the Hall elements _H1 and _H2 . One of the output terminals of the Hall element _H1 is connected to the in-phase input terminal of an amplifier 12 as a first amplifier via a resistor 9, and the other is connected to the amplifier 1 via a resistor 10 and a capacitor 11.
Connected to the negative phase input terminal of No.2. The output terminal of the amplifier 12 is connected to the second amplifier via the resistor 13.
It is connected to the bases of the NPN transistor 14a and the PNP transistor 14b. Both transistors 14a and 14b are complementary connected, and a stator coil _L1 is inserted between the common emitter connection point and ground. The amplifier 12 is connected between this emitter common connection point and the input terminal of the amplifier 12.
A feedback resistor 15 is inserted for setting the gain. Further, a resistor 16 of the same size as the resistor 15 is inserted between the input terminal of the amplifier 12 and the ground, thereby improving the offset voltage of the amplifier 12 to be small. And capacitor 1 connected to the input side of amplifier 12
An NPN transistor 17 having a collector and an emitter connected to both ends of the transistor 1 is provided.
This transistor 17 constitutes short-circuit means for short-circuiting the capacitor 11 when starting the motor.

The same configuration as the one described above from the output terminal of Hall element H ₁ to stator coil L ₁ is provided between the output terminal of Hall element H ₂ and stator coil L ₂ , and the corresponding component is The same reference numerals are given to the corresponding parts.

The input voltage Vi supplied to the amplifier 8 is generated from a frequency-voltage converter 18. stator coil
At one end of L ₁ and L ₂ that is not grounded, a sin waveform or cos waveform voltage is generated when the motor rotates. On the other hand, for example, the terminal voltage of the stator coil _L1 is supplied to the frequency-voltage converter 18, and a DC voltage Vi corresponding to the frequency f of this terminal voltage, that is, the rotational frequency of the motor is generated. FIG. 6 shows the relationship between this frequency f and the DC voltage Vi, and the higher the frequency f, the smaller the DC voltage Vi. A feedback control loop including such a frequency-to-voltage converter 18 allows the rotational speed of the motor to be maintained at a constant speed. As shown in FIG. 6, the frequency-voltage converter 18 is configured to generate a maximum DC voltage when the frequency f=0, that is, when the motor is not rotating.

A Zener diode 19 is inserted between the output terminal of the frequency-voltage converter 18 and the base of the transistor 17. zener diode 19
The Zener voltage Vz is selected to be slightly smaller than the DC voltage of the frequency-voltage converter 18 when (f=0), as shown in FIG. Therefore, when starting the motor, Zener diode 1
9 is turned on, transistor 17 is turned on, and capacitor 11 is shorted. When the motor is started and its rotational frequency approaches a steady state, the Zener diode 19 is turned off and therefore the transistor 17 is turned off.

In one embodiment of the present invention described above, the Hall element
When an offset voltage Vh ₀ exists at H ₁ (or H ₂ ), the output voltage becomes (Vhsinθ+Vh ₀ ) as shown in FIG. 7A. Since the frequency of the output voltage of this Hall element is low, such as 120 [Hz], assuming that the capacitor 11 is not connected, the DC gain and AC gain of the amplifier 12 are approximately equal. When the offset voltage of amplifier 12 is V ₀ , transistor 1
As shown in FIG. 7B, the voltage supplied from the amplifier 12 to the drive amplifier composed of the drive amplifiers 4a and 14b is A(Vhsinθ+Vh ₀ +V ₀ ), and the offset voltage is A(Vh ₀ +V ₀ ). becomes.

In the present invention, since the capacitor 11 is inserted, the amplifier 12 has (DC gain=1, AC gain≈R ₁₅ /R ₁₀ ). R ₁₀ is the value of resistor 10 and R ₁₅ is the value of resistor 15. Therefore, the voltage supplied from the amplifier 12 to the driving amplifier becomes (AVhsinθ+Vh ₀ +V ₀ ) as shown in FIG. 7C.
That is, since the DC offset voltage is not amplified by the amplifier 12, the adverse effects such as torque ripple caused by this can be reduced.

This improvement can be calculated by changing the voltage applied to the drive amplifier.
It is expressed by the ratio of AC voltage V _H ' to DC voltage V ₀ ' (V _H '/V ₀ '). As an example (Vh = 40 [mV]) (Vh ₀ = 8
[mV]) (V ₀ =6 [mV]) (A=100). First, when the capacitor 11 is not connected, _VH ′/V ₀ ′ = AVh/A (Vh ₀ +V ₀ )=Vh/Vh ₀ +V ₀
≒2.86 In the case of the present invention where the capacitor 11 is connected, V _H ′/V ₀ ′=AVh/Vh ₀ +V ₀ ≒285.
It becomes 7. In this way, the gain is improved by a factor of 100, which corresponds to the gain of the amplifier 12.

Further, even if the DC voltage Vi changes by the frequency-voltage converter 18 and the offset voltage of the Hall element H ₁ H ₂ changes accordingly, according to the present invention, the amount of the change is converted to the amplifier 12. Since the signal is not amplified, the gain is still improved by the gain of the amplifier 12.

Further, a capacitor may be inserted in series on the common-mode input terminal side of the amplifier 12 as well. Furthermore, amplifier 1
A capacitor may be inserted between the output terminal of 2 and the base of the transistor of the drive amplifier. In this way, transmission of the offset voltage (Vh ₀ +V ₀ ) can be eliminated. Similarly, a capacitor may be inserted between the stator coils _L1 and _L2 and the ground. In either case, it is necessary to short-circuit the capacitor at start-up. As this short circuit means,
In addition to using the output of the frequency-voltage converter 18 for servo, an operation signal or the like may be used. Furthermore, as the magnetoelectric transducer for position detection, a magnetoresistive element or the like other than the Hall element may be used.

As described above, according to the present invention, since the offset voltage is not canceled by a predetermined DC voltage formed by a variable resistor, it is not affected by fluctuations in the power supply voltage and does not require adjustment man-hours. , it is possible to prevent a significant increase in the offset voltage when the DC voltage Vi changes due to the servo.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of a brushless DC motor according to an embodiment of the present invention, FIGS. 2 and 3 are plan views showing the rotor magnet and stator coil,
Fig. 4 is a basic connection diagram of the drive circuit, Fig. 5 is a connection diagram of an embodiment of the present invention, Fig. 6 is a diagram showing the characteristics of the frequency-voltage converter, and Fig. 7 is an embodiment of the invention. FIG. 3 is a waveform diagram used to explain an example. L ₁ and L ₂ are stator coils, H ₁ and H ₂ are Hall elements, 2 is a rotor magnet, 11 is a capacitor, 12 is an amplifier, and 18 is a frequency-voltage converter.

Claims (1)

[Scope of Claims] 1. A magnetoelectric transducer to which a change in magnetic flux is applied in accordance with the rotation of the rotor magnet, a first amplifier to which an output signal of the magnetoelectric transducer is supplied, and a first amplifier to which an output signal of the magnetoelectric transducer is supplied.
and a second amplifier that supplies a drive current to the stator coil based on the output signal of the amplifier, wherein the output signal of the magnetoelectric conversion element is sent to the first amplifier via a capacitor. Alternatively, the output signal of the first amplifier is supplied to the second amplifier via a capacitor, and short-circuiting means is provided for short-circuiting the capacitor, and the short-circuiting means is connected to the second amplifier via a capacitor. A drive circuit for a brushless direct current motor, characterized in that the capacitor is short-circuited when the motor is started by controlling in accordance with the frequency of the drive current output from a second amplifier.