Brushless Motor Control Circuit

The present invention relates to a brushless motor control circuit for controlling rotation of a brushless motor.

FIG. 1 shows an example of a conventional drive circuit of a brushless motor. Signals H.sub.a, H.sub.b, and H.sub.c of three Hall elements used as rotor position detecting elements of a three-phase brushless motor are amplified by amplifiers 1.sub.1, 1.sub.2, and 1.sub.3. To reduce third harmonic component, a difference "H.sub.a -H.sub.b ", a difference "H.sub.c -H.sub.a ", and a difference "H.sub.c -H.sub.a " are obtained respectively by amplifiers 2.sub.1, 2.sub.2, and 2.sub.3. Output signals from the amplifiers 2.sub.1, 2.sub.2, and 2.sub.3 are amplified by amplifiers 3.sub.1, 3.sub.2, and 3.sub.3 to obtain output signals L.sub.a, L.sub.b, and L.sub.c, thereby driving a motor coil (stationary) 4.

Another circuit example is also conventionally employed as shown in FIG. 2 in which the output signals from two Hall elements are synthesized by an amplifier 5 to obtain a signal H.sub.c ' which is equivalent to the third output signal of FIG. 1. Signals H.sub.a and H.sub.b, and the signal H.sub.c ' are processed in the same manner as in FIG. 1, thereby driving a motor coil 4 (not shown in FIG. 2).

However, with the drive circuit shown in FIGS. 1 or 2, when the motor position detecting elements, i.e., the Hall elements, are incorporated in a motor, a variation in the Hall elements or imbalance during motor assembly may occur. Hence, it is difficult to obtain three or two Hall element signals that are amplified in a similar manner.

This leads to the following problem. The outputs L.sub.a, L.sub.b, and L.sub.c that finally drive the motor coil 4 are adversely influenced by variations in Hall element output signals, amplifiers, and motor coils and become unbalanced with respect to ideal output signals (motor coil drive signals in which only the phases are shifted from each other by 120.degree. and which have no imbalance regarding other conditions, e.g., amplitudes.) Then, the motor coil windings of the respective phases are driven in an unbalanced manner to increase a torque ripple, thus, reducing rotational precision.

It is, therefore, an object of the present invention to perform well-balanced motor driving which is free from an adverse influence caused by variation in Hall elements as the position detecting elements or in motor assembly.

A brushless motor control circuit according to the present invention is characterized by comprising a three-phase brushless motor, amplifiers, commonly provided to three-phase input lines of the motor, for receiving rotor position detection signals of the motor, and a control circuit for comparing, with a constant value, one of a difference and a sum of a product of two phase signals, among three phase outputs of the amplifiers, and a signal as a square of the remaining phase signal, and feeding back a comparison result signal to the amplifiers of common stages of the three-phase input lines, thereby controlling rotation of the motor.

More specifically, three Hall elements as rotor position detecting elements are used, and are arranged on the rotor side such that their phases are shifted from each other by 120.degree.. Then, the signals input to the three-phase motor coil are shifted from each other by 120.degree.. Similarly, when two Hall elements are used, they are also arranged such that their phases are shifted from each other by 120.degree.. Therefore, the third signal formed by using the two signals is also phase-shifted from the two signals by 120.degree.. Assuming that signals whose phases are shifted from each other by 120.degree. are "H.sub.a =K.times.sin.alpha., H.sub.b =K.times.sin(.alpha.+120.degree.), H.sub.c =K.times.sin(.alpha.+240.degree.)", it is possible to prove that the following relation:

or

in numeric operation is established for all the values .alpha. can take. Ideally, since the outputs L.sub.a, L.sub.b, and L.sub.c become signals that are shifted from each other by 120.degree., a following relation is established:

The fact that the three phase signals have a constant relationship with each other, in this manner, is utilized. A difference or sum between a signal as a product of a certain signal to one phase of a three-phase motor coil and another signal to another phase of the three-phase motor coil, and a square signal of a signal to still another phase is calculated. The calculated value (constant value) is compared with an arbitrary value. The comparison result is fed back to the bias current source or the like of the input amplifier for receiving the three phase currents. As a result, even when a certain one of the three phases is unbalanced, balanced three phase inputs can be obtained for the three-phase motor. In other words, control is performed in such a manner that the difference between two phase inputs among the three phase inputs is constant.

FIG. 1 is a circuit diagram of a conventional brushless motor control circuit,

FIG. 2 is a circuit diagram showing a modification of a first-stage differential amplifier of the circuit shown in FIG. 1,

FIG. 3 is a circuit diagram of a brushless motor control circuit according to an embodiment of the present invention,

FIG. 4 is a signal waveform chart showing the operation of the circuit shown in FIG. 3, and

FIGS. 5 to 9 are circuit diagrams, respectively, of brushless motor control circuits according to different embodiments of the present invention.

The present invention will be described in detail with reference to the accompanying drawings. FIG. 3 is a circuit diagram of a brushless motor control circuit according to an embodiment of the present invention. Since this embodiment corresponds to the conventional example of FIG. 3, the same reference numerals are used to denote the corresponding parts. More specifically, three Hall elements are arranged on the stator side of a three-phase brushless motor such that their phases are shifted from each other by 120.degree.. The signals of the respective Hall elements are amplified by corresponding first-stage differential amplifiers 1.sub.1, 1.sub.2, and 1.sub.3 to become signals H.sub.a, H.sub.b, and H.sub.c. For example, the outputs H.sub.a and H.sub.b are input to the non-inverting and inverting terminals, respectively, of a second-stage amplifier 2.sub.1 so that "H.sub.a -H.sub.b ", "H.sub.b -H.sub.c ", and "H.sub.c -H.sub.a " are obtained by second-stage amplifiers 2.sub.1, 2.sub.2, and 2.sub.3, respectively. The same applies to the inputs to the amplifiers 2.sub.2 and 2.sub.3. Outputs from the second-stage amplifiers 2.sub.1 to 2.sub.3 are amplified by final-stage amplifiers 3.sub.1 to 3.sub.3, respectively. Outputs L.sub.a to L.sub.c of the amplifiers 3.sub.1 to 3.sub.3 drive a motor coil 4. At the same time, the outputs L.sub.a and L.sub.b from the amplifiers 3.sub.1 and 3.sub.2 are input to a multiplier 11 to obtain a product signal P thereof. The output L.sub.c of the amplifier 3.sub.3 is input to a square multiplier 12 to obtain a square signal Q of the signal L.sub.c. The difference between the signals P and Q is obtained by an arithmetic unit 13 as a signal R. The difference signal R is compared with an arbitrary constant value V.sub.r by a comparator 4, and the comparison result is output as a current output .beta.. The current output .beta. is supplied to terminals 15.sub.1, 15.sub.2, and 15.sub.3 through a common bus so that it serves as a bias current source of the amplifiers 1.sub.1, 1.sub.2, and 1.sub.3.

FIG. 4 shows the operation of the circuit shown in FIG. 3. In this case, only the output H.sub.b from the amplifier 1.sub.2 becomes higher than the normal outputs from the amplifiers 1.sub.1 and 1.sub.3 due to the imbalance of the second-phase Hall element signal. The current source output .beta. from the comparator 14 is commonly supplied to the amplifiers 1.sub.1 to 1.sub.3. Namely, feedback is performed in such a manner that the above relation "L.sub.a .times.L.sub.b -L.sub.c.sup.2 =constant" is established. A common bias current is supplied to the amplifiers 1.sub.1 to 1.sub.3 to perform amplitude adjustment. More specifically, control is performed such that the amplitude of the signal H.sub.b in the positive direction is reduced, that a crossing point x of the signals H.sub.a and H.sub.c is moved upward in the positive direction as shown in FIG. 4, and that a constant amplitude l is obtained accordingly. As a result, the amplitudes of the respective phases become the same, thus obtaining three phase motor coil control signals capable of performing well-balanced motor control. With this control, the difference between any two given phase inputs among the three phases inputs becomes constant at the respective portions of the signal waveforms, as indicated by a large number of l in FIG. 4.

In this embodiment, the imbalance in amplitude of a Hall element signal as a rotor position detection signal is absorbed. However, the present invention is not limited to this but an imbalance such as a distortion in the three phases of the motor coil or in the amplifiers, i.e., the imbalance in the circuit elements to be used can also be absorbed. In the present invention, it is possible to use only two Hall element signals as signals to be input to the second-stage amplifier, as in FIG. 2. These signals are synthesized to obtain a signal H.sub.c ' equivalent to the third Hall element signal, and the synthesis signal is used in place of the signal H.sub.c. In the present invention, the signals L.sub.b and L.sub.c, or the signals L.sub.c and L.sub.a may be input to the multiplier 11. In this case, the input to the square multiplier 12 is the signal L.sub.a or L.sub.b. The signals to be input to the multiplier 11 and the square multiplier 12 may be outputs from the amplifiers 2.sub.1 to 2.sub.3 or 1.sub.1 to 1.sub.3. This is because any signals can be used as these input signals as far as they are phase-shifted from each other by 120.degree.. FIG. 5 shows a case wherein the outputs from the amplifiers 2.sub.1 to 2.sub.3 are used as these input signals, and FIG. 6 shows a case wherein the outputs from the amplifiers 1.sub.1 to 1.sub.3 are used as these input signals. In the present invention, as shown in FIG. 7, the output from the comparator 14 may be fed back to terminals 16.sub.1 to 16.sub.3 for supplying a common bias current to the amplifiers 2.sub.1 to 2.sub.3. Alternatively, as shown in FIG. 8, the output from the comparator 14 may be fed back to terminals 17.sub.1 to 17.sub.3 to supply a common bias current to the amplifiers 3.sub.1 to 3.sub.3. This is because the imbalance between the signals can be corrected where desired. In the present invention, as shown in FIG. 9, a sum of a signal as a product of two arbitrary phase signals, among the three phase signals, and a signal obtained by squaring the remaining phase signal may be obtained by an arithmetic unit 13.sub.1, and the obtained sum may be compared with a constant value. This is because the relationships of equations (2) and (3) are established in any of these cases. Various other changes and modifications can be made within the spirit and scope of the invention.

The brushless motor control circuit according to the present invention is effective when it is used as a Hall motor control circuit packaged in an integrated circuit.