Brushless motor

Abstract: A brushless motor including a delay circuit for improving motor start-up performance. the delay circuit increases the BW of a magnetic sensor, which control power switching operation to a motor coil, during motor start-up.

Description: The present invention relates to a brushless motor which controls the rotation of a rotor on a single-phase basis. Figs. 1 to 3 show an arrangement of a prior art brushless motor of the type referred to. More specifically, Fig. 1 shows a cross-sectional view of the prior art brushless motor showing an array of parts in the motor, Fig. 2 is an exploded perspective view of the parts of the motor, and Fig. 3 is a block diagram of the brushless motor. In the drawings, a casing 1 comprises a lower casing 1A and an upper casing 1B. Rotatably supported within the casing 1 is a rotor 2. Mounted on the bottom of the rotor 2 are permanent magnets 3 and 4 at positions symmetrical with respect to the centre of the rotor 2. A coil 5 is mounted on the lower casing 1A. When a current is passed through the coil 5, this causes a magnetic field to be generated so that the permanent magnets 3 and 4 are attracted and therefore the rotor 2 is turned. A magnet 6 is disposed in the lower casing 1A to position the rotor 2 at its initial position between the permanent magnets 3 and 4. The magnet 6 for positioning the rotor 2 at its initial position has the same polarity as that of the permanent magnets 3 and 4, and when a power source is turned off this results in the rotor 2 stopping at its balanced position because the magnets repel each other. A magnetic sensor 7 such as a Hall effect element is mounted together with another electronic part 8 on a substrate 9 which in turn is disposed in the lower casing 1A. In more detail, the magnetic sensor 7 is located at such a position as to be subjected to the influence of the magnetic force of the permanent magnets 3 and 4 of the rotor 2. The substrate 9 having such parts mounted thereon is disposed in the lower casing 1A and then moulded with resin material 10. Referring to Fig. 3, between the coil 5 and the power source, a switching element 11 such as a transistor is inserted. The switching element 11 is connected to a current limiting element 12 such as a resistor which in turn is connected to an output terminal of the magnetic sensor 7. That is, the magnetic sensor 7 detects the positions of the permanent magnets 3 and 4 and generates an output signal that is sent to the switching element 11. The switching element 11 controls current supply to the coil 5 on the basis of the output signal received from the magnetic sensor 7. In Fig. 3, reference numeral 13 denotes a constant-voltage diode, 14 a resistor. In the operation of such an arrangement, when the permanent magnet 3 is positioned above the magnetic sensor 7, the magnetic sensor 7 generates an on-state output and thus the switching element 11 is turned on so that a current flows through the coil 5, thus resulting in generation of a torque in the coil 5 for attracting the permanent magnet 4 and the rotor 2 is rotated. As the permanent magnet 4 moves toward the vicinity of the coil 5, the permanent magnet 3 moves away from the vicinity of the magnetic sensor 7, whereby the magnetic sensor 7 generates an off-state output and thus the switching element 11 is turned off. This causes the current flowing through the coil 5 to be stopped so that the attractive force between the permanent magnet 4 and the coil 5 disappears and the rotor 2 continuously coasts. Thereafter, the permanent magnets 3 and 4 change roles but operate in a similar manner to that mentioned above. A force resisting the movement of the rotor 2 trying to continuously coast exists as a repulsive force between the magnet 6 for positioning of the initial position of the rotor 2 and the permanent magnets 3 and 4 of the rotor 2. The magnetic sensor 7 has a hysteresis width (which will be referred to as the BW, hereinafter) between a detection level (which will be referred to as the BH-L, hereinafter) changing from its detection off to detection on and a detection level (which will be referred to as the BL-H, hereinafter) changing from its detection on to detection off. For this reason, in the case where the rotor 2 loses an attractive force generated by the coil 5 at the BL-H and is subjected to the influence of the repulsive force of the magnet 6 to thereby start to reversibly rotate, the rotor 2 returns to the BH-L and again normally rotates under the influence of the attractive force of the coil 5, which is repeated several times. After this, acceleration obtained during a period between the BH-L and BL-H causes the rotor 2 to overcome the repulsive force of the magnet 6. Such a prior art brushless motor, however has a problem in that variations in the BW of the magnetic sensor cause the motor not to be able to rotate under the worst conditions (when BW is narrow). The current-level technique of suppressing variations in the BW of the magnetic sensor has its limits and thus it has been impossible to solve this problem. EP-A-0150324 discloses a brushless motor comprising a rotor having permanent magnets; a coil for rotating said rotor; a switching element inserted between said coil and a power source; a magnetic sensor for detecting a position of one of the permanent magnets of said rotor to control switching operation of said switching element; a current limiting element inserted between said magnetic sensor and said switching element; and a delay circuit inserted between said current limiting element and said magnetic sensor. The delay circuit in this motor acts only to delay the signals delivered by the magnetic sensor. It is, however, an object of the present invention to provide a brushless motor which is immune to the influence of variations in the BW of the magnetic sensor and in which the BW of the magnetic sensor is effectively increased when the rotor is vibrated on start up as a result of a repulsion force exerted on it by an initial positioning magnet. The above object is attained by providing a brushless motor which comprises a rotor having permanent magnets; a coil for rotating said rotor; a switching element inserted between said coil and a power source; a magnetic sensor for detecting a position of one of the permanent magnets of said rotor to control switching operation of said switching element; a current limiting element inserted between said magnetic sensor and said switching element; and a delay circuit inserted between said current limiting element and said magnetic sensor or between said current limiting element and said switching element; characterised in that said motor includes a magnet for holding said rotor at an initial position when said coil is deenergised; and said delay circuit extends a hysteresis width of said magnetic sensor and thus an on-period of said coil is increased when said rotor is vibrated as a result of a repulsion force of said initial positioning magnet. Further, the present invention is provided with a timer circuit which conducts the switching element for a constant time at the time of turning on the power source. With such an arrangement, in the case where the rotor yields to the repulsive force of the initial-position positioning magnet, the rotor cannot normally rotate and vibrates. The delay circuit causes the switching element to be turned on even after the turning off of the magnetic sensor at the BL-H; whereas, the delay circuit causes the switching element to be turned off even after the turning on of the magnetic sensor at the BH-L. This means that the apparent hysteresis width BW of the magnetic sensor is broadened so that an acceleration acquired during the apparent hysteresis width BW enables the rotor to overcome the repulsive force and rotate normally, thereby eliminating the influence of variations in the BW of the magnetic sensor. Further, the timer circuit is provided for conducting the switching element for the constant time at the time of turning on the power source, so that, even when one of the permanent magnets of the rotor is stopped immediately above the initial-position positioning magnet of the rotor though this is very rare, the conduction of the switching element for the constant time at the time of the turning on the power source causes the rotor to be moved by the attractive force of the coil, thereby ensuring that the rotor is returned to its initial position determined by the repulsive force of the initial-position positioning magnet of the rotor and thus any failure of starting the rotor can be avoided. The features of the invention will be more readily understood from the following description with reference to the accompanying drawings, of which:- Fig. 1 is a cross-sectional view of a prior art brushless motor showing an array of parts in the motor; Fig. 2 is an exploded perspective view of the parts of the motor shown in Fig. 1; Fig. 3 is a circuit diagram of the motor of Fig. 1; Fig. 4 is a circuit diagram of a brushless motor in accordance with an embodiment of the present invention; Fig. 5 is a diagram for explaining the principle of operation of the motor shown in Fig. 4; Fig. 6 is a specific circuit diagram of the motor of Fig. 4; An embodiment of the present invention will be described with reference to Figs. 4 to 6. More specifically, Fig. 4 shows a circuit diagram of a brushless motor in accordance with an embodiment of the present invention, Fig. 5 is a diagram for explaining the principle of operation of the motor, and Fig. 6 is a specific circuit diagram of the motor. In Figs. 4 to 6, the same parts as those in the prior art motor of Figs. 1 to 3 are denoted by the same reference numerals. In the drawings, a delay circuit 15, which is inserted between the current limiting element 12 and the magnetic sensor 7, acts to prolong, in addition to the BW of the magnetic sensor 7, the on-period of the switching element 11 to lengthen the energisation period of the coil 5, only when the rotor 2 is vibrating when overcome by the repulsive force of the magnet for initial positioning of the rotor. In more detail, the delay circuit 15, as shown in Fig. 6, comprises transistors Q151 and Q152, diodes D151 and D152, resistors R151 and R152 and a capacitor C151. In the circuit of Fig. 6, when a magnetic sensor 7 such as a Hall effect element is switched from its on state to off, a potential at a point A gradually increases to a level corresponding to diode forward voltage plus base/emitter voltage of transistor Q151, at which time the transistor Q151 is turned on and the transistor Q152 is turned off so that the switching element 11 is turned off as delayed by a predetermined time from the turning off of the magnetic sensor 7. Conversely, when the magnetic sensor 7 is switched from its off state to on, the transistor Q151 is turned off as delayed by a time corresponding to the discharge time during which the capacitor C151 is discharged through the diode D152, so that the switching element 11 is turned on as delayed by a predetermined time from the turning on of the magnetic sensor. The delay circuit 15 may be inserted between the current limiting element 12 and the switching element 11. A timer circuit 16 is provided for causing the current limiting element 12 to supply a current to the switching element 11 for a predetermined time when the power source is turned on. With such an arrangement, when the permanent magnet 4 is positioned above the magnetic sensor 7 as shown in Fig. 5, the magnetic sensor 7 generates an on-state output and the switching element 11 is turned on as delayed by a predetermined time, so that a current flows through the coil 5 which develops a torque for attracting the permanent magnet 3, whereby the rotor 2 is rotated. As the permanent magnet 3 is moved closer to the coil 5, the permanent magnet 4 is moved away from the magnetic sensor 7, so that the magnetic sensor 7 generates an off-state output and the switching element 11 is turned off as delayed by a predetermined time therefrom. This results in that the supply of current to the coil 5 is stopped so that an attractive force between the permanent magnet 3 and the coil 5 disappears, whereafter the rotor 2 is coasting. Thereafter, the permanent magnets 3 and 4 occupy mutually opposite positions to the above and continuously move in a similar manner to the above. A force resisting the movement of the rotor 2 trying to coast continuously exists as a repulsive force between the magnet 6 and the permanent magnets 3 and 4. In the present embodiment, even when the rotor 2 is reversely rotated and vibrated, the delay circuit 15 provides a delay time in addition to the hysteresis width BW of the magnetic sensor 7 between its detection level BH-L changing from its detection off to detection on and its detection level BL-H changing from its detection on to detection off. As a result, the coil 5 is energised for a long time when compared with the prior art, so that, when the rotor 2 loses the attractive force of the coil 5 at the detection level BL-H and yields to the repulsive force of the magnet 6 to thereby start to reversely rotate, the rotor 2 returns to the detection level BH-L and again starts to normally rotate under the influence of the attractive force of the coil 5, which is repeated several times. Thereafter, acceleration obtained during a period corresponding to a sum of a duration from the BH-L to the BL-H and the delay time of the delay circuit 15 causes the rotor 2 to overcome the repulsive force of the magnet 6. Since the delay time of the delay circuit 15 is set to be constant, the vibration of the rotor 2 increases in amplitude with the gradual increase of acceleration and eventually is able to overcome the repulsive force of the magnet 6. When the rotor 2 continues to normally rotate, the delay time based on the BH-L and BL-H takes place in the same rotational direction as the rotor 2, whereby this does not substantially affect the energisation period of the coil 5 and therefore the motor can be designed according to the conventional design specifications. When the permanent magnet 3 or 4 of the rotor 2 stops immediately above the initial-position positioning magnet 6, though this is very rare, the turning on of the power source causes the transistor Q161 to be turned on for a constant time (about 10ms) determined by a resistor R161 and a capacitor C161 making up the timer circuit 16, whereby the switching element 11 is turned on so that the coil 5 is put in its energised state to as to attract the permanent magnet 3 or 4 of the rotor 2, thus resulting in movement of the rotor 2 from its position immediately above the initial-position positioning magnet 6, thereby avoiding disabling rotation of the motor. In Fig. 6, reference symbol D161 denotes a diode, C a capacitor, R a resistor, D1 and D2 are diodes respectively. As has been disclosed in the foregoing, in accordance with the present invention, the current on period of the coil during which a current flows therethrough can be prolonged by an amount corresponding to a sum of the delay time of the delay circuit and the hysteresis width BW determined by the performance of the magnetic sensor, only when the rotor is vibrating and it is necessary for the rotor to overcome the repulsive force of the initial-position positioning magnet of the rotor. As a result, there can be provided a brushless motor wherein the rotor can be rotated without yielding to the repulsive force of the initial-position positioning magnet of the rotor and which can also be immune to the influence of variations in the BW of the magnetic sensor.

Claims: A brushless motor comprising: a rotor (2) having permanent magnets (3,4); a coil (5) for rotating said rotor (2); a switching element (11) inserted between said coil (5) and a power source; a magnetic sensor (7) for detecting a position of one of the permanent magnets of said rotor (2) to control switching operation of said switching element (11); a current limiting element (12) inserted between said magnetic sensor (7) and said switching element (11); and a delay circuit (15) inserted between said current limiting element (12) and said magnetic sensor (7) or between said current limiting element (12) and said switching element (11); characterised in that said motor includes a magnet (6) for holding said rotor (2) at an initial position when said coil (5) is deenergised; and said delay circuit (15) extends a hysteresis width of said magnetic sensor (7) and thus an on-period of said coil (5) is increased when said rotor (2) is vibrated as a result of a repulsion force of said initial positioning magnet (6). A brushless motor as claimed in claim 1, characterised in that it further comprises a timer circuit (16) for conducting said switching element (11) for a predetermined time at the time of turning on the power source.

Inventors: HARA TAKASHI, KINOSHITA TSUTOMU, MATSUMOTO SUEO, SHIMMURA TETSURO, HARA, TAKASHI, KINOSHITA, TSUTOMU, MATSUMOTO, SUEO, SHIMMURA, TETSURO
Applicants: ASMO CO LTD, MATSUSHITA ELECTRIC IND CO LTD, ASMO CO., LTD., MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: H02K29
IPC codes: H02P6, H02K21, H02K11, H02K29