The present invention relates to a brushless motor which dispenses with sensing devices to detect the position of a rotor, such a brushless motor will be referred hereinafter to as a sensorless-type brushless motor.
One example of conventional brushless motors such as disclosed in Japanese Patent Provisional Publication No. 58-119794 will be described hereinbelow with reference to FIGS. 1 and 2. In FIG. 1, the brushless motor includes a rotor (not shown) having on its surface a disc-like drive magnet 2 permanently magnetized with magnetic fields so as to form four poles 2a indicated alternately by N and S and a stator (not shown) having therein four-phase drive coils 3 disposed to be opposed relation to the poles 2a thereof. On the circumference of the drive magnet 2 is disposed an annular FG magnet 4 having FG (frequency generator) poles 4a whose number is 128. On the stator is provided a FG head 5 which serves as an electro-magnetic converting device. The FG head 5 is disposed to be in opposed relation to the FG poles so as to make up a frequency generator (FG) 6 which generates a detection signal indicative of the rotational speed of the rotor. Further, at the vicinity of the circumference of the rotor is disposed a PG (pulse generator) magnet 32 which has a single PG pole 32a and in the stator is provided a PG head 33 which acts as an electro-magnetic converting device. The PG head 33, together with the PG magnet 32, makes up a pulse generator (PG) 34 which generates a detection signal indicative of the rotational position of the rotor.
In the response to rotation of the rotor, the PG head 33 produces a PG output l (C-1 of FIG. 2) at every one revolution of the rotor, i.e., every time the PG head 33 faces the PG pole 32a of the PG magnet 32. The PG output l is supplied to a waveform shaping circuit 7-2 so as to be wave-shaped to be a rectangular signal which is in turn supplied as an index signal m (C-2 of FIG. 2) to an FG frequency divider 39. On the other hand, the FG head 5 produces an FG output a (A of FIG. 2) whose frequency is proportional to 64-cycle rotational speed per one revolution of the rotor, the FG output a being supplied to a waveform shaping circuit 7-1 so as to be fed as an FG signal b (B of FIG. 2) having a rectangular configuration to a start confirmation circuit 8, the FG frequency divider 39 and a constant-speed control circuit 14.
During the stop of the rotor, the start confirmation circuit 8 is operated so that an electronic switch 12 is switched to the left side in FIG. 1 to transmit a start signal generated by an oscillation start circuit 11 to a brushless motor drive circuit 13-2. In response to the start signal, the drive circuit 13-2 feeds drive signals in shifts to the above-mentioned four-phase drive coils 3 whereby the rotor is rotationally driven. Further, in response to the FG signal b, the start confirmation circuit 8 confirms that the rotational speed of the rotor reaches a predetermined speed and switches the movable contact of the electronic switch 12 to the right side fixed contact thereof in FIG. 1 whereby the output (d-2) of the FG frequency divider 39 is supplied to the drive circuit 13-2. The FG frequency divider 39 divides the FG signal b with a predetermined division ratio (1/8 in this case) and is reset by the index signal m so as to produce drive pulses d-2 (D of FIG. 2). On the basis of the drive pulses d-2, the drive circuit 13-2 produces four-phase current signals e-2, f-2, g-2 and h-2 (E to H of FIG. 2), and supplies drive signals in shifts to the drive coils 3 in accordance with the four-phase current signals e-2, f-2, g-2 and h-2 whereby the rotor is rotationally driven. Moreover, the constant-speed control circuit 14 produces, on the basis of the FG signal b, a constant-speed control signal which is supplied to the drive circuit 13-2. The drive circuit 13-2 controls the drive signals to the drive coils 3 on the basis of the constant-speed control signal to form a negative-feedback loop for the brushless motor whereby the rotor is controlled to be driven at a constant speed.
There is a problem which arises with the above-described brushless motor, however, in that difficulty is encountered to meet the requirement of size-reduction because the pulse generator 34 is disposed at a side portion of the motor output section (comprising the drive magnet 2 and drive coils 3). Another problem is to require troublesome position adjustment of the PG head 33 to increase its cost because of phase adjustment between the PG signal m and the FG signal b. Particularly, in the case that the pulse number of the FG signal b is large, a precise phase adjustment between the PG signal m and the FG signal b can become difficult. Further, although in the conventional brushless motor the drive poles 2a, FG poles 4a and PG pole 32a are arranged to be in predetermined position relationship to each other so that the timings of the four-phase signals e-2, f-2, g-2 and h-2 become appropriate with respect to the back-electromotive force waveforms induced in the four-phase drive coils 3 due to the rotation of the drive poles 2a when the brushless motor is under the rated load operation, when the load varies, due to variation of the armature-reaction occurring in response to the load variation, the timing of generation of the drive signals is shifted (so-called, neutral-point shift) so as to reduce the generation torque of the brushless motor.