Brushless motor

Abstract: A brushless motor which comprises a rotor (2) having permanent magnets (3, 4) and a coil (5) for rotating the rotor (2). In the motor, a switching element (11) is connected between the coil (5) and a power source, a magnetic sensor (7) is provided for detecting the position of one of the permanent magnets (3, 4) of the rotor to control the switching operation of the switching element (11), and a delay circuit (15) is provided for delaying a signal from the magnetic sensor (7) and then applying the delayed signal to the switching element (11). With this arrangement, the apparent hysteresis width of the magnetic sensor can be broadened and such a problem can be solved that variations in the hysteresis width of the magnetic sensor results in the disabled rotation of the rotor.

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 lA. 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. The rotor in such a prior art brushless motor may, on occasions, stop with one of the permanent magnets 3 or 4 at rest immediately above the initial position positioning magnet 6. A brushless motor is disclosed in United Kingdom Patent Specification No. 2 014 810 A. This brushless motor includes a magnetic sensor for detecting a position of a magnetic disc to control switching operation of at least one switching element and a start control circuit which causes a switching element to be turned on for a predetermined time after a power source is turned on to cause the rotor of the motor to reversely rotate away from a position where it may have previously come to rest with its neutral axis in a position between one coinciding with a switching element switch over point and the neutral axis of the motor stator. The start control circuit comprises a timing circuit and a signal blocking circuit for blocking control signals from the magnetic sensor and then releasing them after the predetermined time. The present invention, however, includes a magnetic sensor and a start control circuit, wherein the start control circuit causes a switching element to be firstly turned on for a predetermined time irrespective of control signals from the magnetic sensor, and then causes the switching element to be turned off for a further predetermined time. According to the present invention, there is provided 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; and a start control circuit which, at the time of turning on the power source, causes said switching element to be turned on for a predetermined time irrespective of control by said magnetic sensor, characterised in that after said predetermined time the start control circuit causes said switching element to be turned off for a further predetermined time. With this arrangement, even in the case where the power source is turned on during the reverse rotation of the rotor between a position where the coil loses its attractive force at the BL-H and the initial-position positioning magnet of the rotor or where the power source is turned on during stoppage of one of the permanent magnets of the rotor immediate above the initial-position positioning magnet, the switching element is first conducted for a constant time (about 10 seconds) so that, even when the rotor is previously rotated, the rotor is subjected to a braking by the attractive force of the coil and to a complete stoppage. Thereafter, since the switching element is put in its non-conductive state for a constant time (about 2 seconds), the attractive force of the coil disappears and the rotor is reversely rotated by the repulsive force of the initial-position positioning magnet of the rotor and is completely stopped at its balanced point. Further thereafter, since the power of the power source is applied to the motor for its normal operation, the influence of variations in the hysteresis width BW of the magnetic sensor can be eliminated and the motor can be positively started. 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 operation of the motor of Fig. 4; and Fig. 6 is a specific circuit diagram of the motor of Fig. 4. Explanation will be made of an embodiment of the present invention by referring to Figs. 4 to 6. 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 operation of the motor, and Fig. 6 is a specific circuit diagram for the motor. In the drawings, the same parts as those in Figs. 1 to 3 are denoted by the same reference numerals and symbols. In the drawings, a start control circuit 18 functions to turn on and off a switching element 19 for controlling the switching element 11 and also to turn on and off a switching element 20 for controlling the operation of the magnetic sensor 7. More specifically, the start control circuit 18, as shown in Fig. 6, comprises comparators IC181 and IC182, diodes D181, D182, D183 and D184, resistors R181 to R188 and capacitors C181 and C182. In Fig. 6, when a power source is turned on, a charging current flows through the capacitor C181, the resistor R186, the diode D183 and the resistor R187 to thereby increase gradually a voltage across the capacitor C181. Applied to respective plus terminals of the comparators IC181 and IC182 is a reference voltage which is determined by the voltage division ratio of the resistors R183 and R184. Voltages at respective minus terminals of the comparators IC181 and IC182 are increased with the voltage across the capacitor C181. When the voltage at the minus terminal of the comparator IC182 first exceeds the above reference voltage, an output at the output terminal of the comparator IC182 is changed from its high level to low. At this time, the switching elements 19 and 20 are switched form the on state to off. When the switching element 19 is put in its on state, the switching element 11 is forcingly turned on regardless of the magnetic sensor 7 because a driving current flows therethrough via the current limiting .element 12. When the switching elements 19 and 20 are in the off state, the switching element 11 is forcingly turned off regardless of the magnetic sensor 7. Accordingly, the switching element 11 is put in the forcing on state until the power source is turned on and the voltage at the minus terminal of the comparator IC182 exceeds the aforementioned reference voltage. As soon as the voltage at the minus terminal of the comparator IC182 exceeds the aforementioned reference voltage, the switching element 11 is forcingly turned off. When the capacitor C181 is further charged to such an extent that the voltage at the minus terminal of the comparator IC181 exceeds the above reference voltage, the output of the comparator IC181 at its output terminal is changed form its high level to low. At this time, the comparator IC182 receives at its minus terminal a voltage which is determined by the voltage division ratio of the resistors R185 and R186 and which has previously been set to be smaller than the above reference voltage. As a result, the output of the comparator IC182 at the output terminal is changed from its low level to high. At this time, the switching element 20 is switched to the on state and the switching element 19 remains off because of its low level via the diode D181. Accordingly, the switching element 11 is released from its forcing on/off state and is operated according to the on/off operation of the magnetic sensor 7. In Fig. 4, a constant-voltage diode 13, which is applied with the voltage of the power source via a resistor 15, is provided for the stabilisation of operation of the start control circuit 18. Diodes 21 and 22 are provided for prevention of any surging current of the magnetic sensor 7, while a surge prevention circuit 23 is provided to protect circuits and elements other than the switching element 11. A current limiting resistor 14 is connected in a power source line for driving the magnetic sensor 7. More specifically, the surge prevention circuit 23 comprises transistors Q231 and Q232, and resistors R231, R232 and R233. With such an arrangement, when the permanent magnet 4 is located above the magnetic sensor 7 as shown in Fig. 5, the output of the magnetic sensor 7 is in its on state, so that a current flows through the coil 5 to develop 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 output of the magnetic sensor 7 is put in its off state and the switching element 11 is put in its off sate. As a result, the current flowing through the coil 5 is stopped and the attractive force between the permanent magnet 4 and the coil 5 disappears, whereby the rotor is coasting. Thereafter, the permanent magnets 3 and 4 occupy mutually opposing 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. Therefore, there is a danger that one of the permanent magnets 3 or 4 will stop immediately above the initial-position positioning magnet 6. The present embodiment, however, is arranged so that the turning on of the power source causes the switching element 11 to be forcingly turned on for a constant time (about 10 seconds). For this reason, even when one of the permanent magnets 3 or 4 is stopped immediately above the initial-position positioning magnet 6 or when the rotor 2 is reversely rotating under the influence of the repulsive force of the initial-position positioning magnet of the rotor 2, the permanent magnet 3 or 4 of the rotor 2 is attracted toward the coil 5 and stopped immediately above the coil 5. Thereafter, since the switching element 11 is forcingly turned off, the rotor 2 is reversely rotated under the influence of the repulsive force of the initial-position positioning magnet 6 of the rotor 2 so that the permanent magnet 3 or 4 of the rotor 2 is stopped immediately above the magnetic sensor 7. When the rotor 2 is stopped at its initial position and is to be rotated under control of the magnetic sensor 7 through the switching element 11, the magnetic sensor 7 is in the detection on mode at the initial position of the rotor 2 so that, while the rotor 2 is rotated and the magnetic sensor 7 is changed from its detection on to detection off, the rotor 2 can acquire a sufficient rotary torque, with the result that the rotor 2 can overcome the repulsive force of the initial-position positioning magnet 6 and can continuously rotate. With a start control circuit as aforesaid, since the rotation of the rotor can be started from its initial position after application of the power source voltage, the rotor can be rotated without yielding to the repulsive force of the initial-position positioning magnet of the rotor and without being subjected 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 to control switching operation of said switching element (11); and a start control circuit (18) which, at the time of turning on the power source, causes said switching element (11) to be turned on for a predetermined time irrespective of control by said magnetic sensor (7), characterised in that after said predetermined time the start control circuit causes said switching element (11) to be turned off for a further predetermined time. A brushless motor as claimed in claim 1, characterised in that the start control circuit (18) disables said magnetic sensor (7) during said further predetermined time. A brushless motor as claimed in claims 1 or 2, characterised in that the start control circuit (18) controls another switching element (19) to cause said switching element (11) to be turned on for the predetermined time. A brushless motor as claimed in claim 2 or 3, characterised in that the start control circuit (18) controls a further switching element (20) to cause said magnetic sensor (7) to be disabled during the further predetermined time. A brushless motor according to any preceding claim, characterised in that a current limiting element (12) is inserted between the switching element (11) and the magnetic sensor (7). A brushless motor as claimed in any preceding claim, characterised in that the motor includes a surge prevention circuit (23).

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, H02K29