The present invention relates to a device and a method for driving a sensorless motor, and, in particular, those for controlling the start of a sensorless motor under a forced commutation control.
In brushless motors, electric commutation is performed instead of mechanical commutation with brushes. Electric commutation requires a rotor position, in other words, a rotation angle of a rotor. Position sensors such as Hall devices are provided for conventional brushless motors, and are used to detect rotor positions. See, for example, Published Japanese patent application 2003-174789 and 2003-244983 gazettes.
In a sensorless motor, a voltage is detected, which is induced in a motor coil during the rotor rotation. The voltage is hereafter referred to as a BEMF (back electromotive force). Using the BEMF, the detection of the rotor position is performed without a position sensor. Since the sensorless motor has no position sensor, its component count is small and its wiring is comparatively simple. Accordingly, its miniaturization is easy, and hence, it is extensively used as, for example, a spindle motor of a FDD, a HDD, a MD/CD/DVD drive, and the like, and a cylinder motor of a VTR, a video camcorder, and the like.
FIG. 27 is a block diagram showing a conventional sensorless motor driving device. See, for example, Kusaka Satoru, “Feature Story, Design of Motor Control Circuit to Learn in Experiments, Chapter 4, Method for Driving Sensorless DC Motor,” in “Transistor Gijutsu” (written in Japanese), CQ Publishing, the February 2000 issue, Vol.37, No.425, pp.221–228. This sensorless motor driving device drives, for example, a three-phase sensorless motor M. A PWM control section 10 generates a PWM control signal P and a PWM mask signal MPWM based on a position signal PS. Here, the position signal PS shows an estimated position of the rotor (not shown) of the sensorless motor M. The PWM control signal P shows the timing of energizing of the motor coils Mu, Mv, and Mw of the sensorless motor M. The PWM mask signal MPWM stays active for a predetermined time from each of the rising and falling edges of the PWM control signal P.
According to an energization phase switching signal CP, a pre-drive circuit 20 selects, for example, one of three high side power transistors 31U, 31V, and 31W of an output circuit 3, and maintains it in the ON state. The pre-drive circuit 20 further selects one of three low side power transistors 32U, 32V, and 32W, and quickly turns it on and off according to the PWM control signal P. Thereby, two of three motor coils Mu, Mv, and Mw are energized. Here, there are six types of energization pattern, which correspond to separate energization phases. The motor coils energized generate magnetic fields and then produce a torque at the rotor.
A BEMF comparing section 4 includes three comparators 4U, 4V, and 4W, and compares each potential of three driving terminals U0, V0, and W0 of the sensorless motor M with the potential of the center point C of the three motor coils Mu, Mv, and Mw, which is hereafter referred to as a center tap voltage. A self-commutation circuit 50 detects agreement between each potential of the driving terminals U0, V0, and W0, and the center tap voltage of the motor coils, based on three output signals BCU, BCV, and BCW of the BEMF comparing section 4. Here, the output signals BCU, BCV, and BCW of the BEMF comparing section 4 are masked according to two types of mask signal, MZC and MPWM. Thereby, the agreement between the BEMF in the non-energized motor coil and the center tap voltage of the motor coils, which is hereafter referred to as a zero crossing, is precisely detected in each of the energization phases. When detecting the zero crossing, the self-commutation circuit 50 generates a self-commutation signal SC.
A forced commutation circuit 60 generates a fixed pulse signal, that is, a forced commutation signal FC at predetermined intervals. A count section 70 selects either the self-commutation signal SC or the forced commutation signal FC, and sends it as a commutation signal CS to an energization phase switching circuit 80. On the other hand, the count section 70 measures the intervals of the commutation signal CS and generates a position signal PS based on the intervals. Here, a commutation signal CS is adjusted to lag, for example, about 30 degrees behind either the self-commutation signal SC or the forced commutation signal FC, whichever is selected. The energization phase switching circuit 80 generates an energization phase switching signal CP at every entry of the commutation signal CS. The energization phase switching circuit 80 further maintains a position detection mask signal MZC active for a predetermined time from the entry of the commutation signal CS.
FIG. 28 is a waveform diagram showing, for the sensorless motor M during stable rotor rotation, three electric currents, or phase currents Iu, Iv, and Iw, and three BEMF Vu, Vv, and Vw of the respective motor coils Mu, Mv, and Mw; three potential VU0, VV0, and VW0 of the respective driving terminals U0, V0 and W0; and a position-detection mask signal MZC. In FIG. 28, the horizontal axis represents phases, which correspond to the rotor positions expressed in electrical angles.
The pre-drive circuit 20 turns on and off six power transistors of the output circuit 3 at every energization phase. Here, there are six types I–VI of pattern of the turning on/off and the following energization of the motor coils, shown in FIG. 28. In other words, the energization phases divide one period of the phase current into 60 degrees each. In each energization phase, among the three motor coils Mu, Mv, and Mw, the one allows a source current, that is, a current flowing in the direction from one driving terminal to the center point C (the direction of the arrow shown in FIG. 27) to flow, the other allows a sink current, that is, a current flowing in the reverse direction of the source current to flow, and the rest stays in the non-energized state. In FIG. 28, the sink currents are shown with hatched areas.
In the conventional sensorless motor driving device in each energization phase, the pre-drive circuit 20, for example, maintains the ON states of the high side power transistors 31U, 31V, and 31W in which the source currents flow, and performs a PWM control over the turning on and off of the low side power transistors 32U, 32V, and 32W, in which the sink currents flow. The pre-drive circuit 20 further performs a hard switching for the turning on and off of the power transistors involved in the switching between the energization phases. Thereby, the phase currents Iu, Iv, and Iw take rectangular waveforms. In particular, each of the motor coils Mu, Mv, and Mw repeats to alternately experience the energization period of 120 degrees and the non-energization period of 60 degrees. Furthermore, a phase difference between the phase currents Iu, Iv, and Iw is maintained at 120 degrees.
The rotor rotation induces the BEMF Vu, Vv, and Vw in the motor coils Mu, Mv, and Mw, respectively. The BEMF Vu, Vv, and Vw have a near-sinusoidal waveform. The potentials VU0, VV0, and VW0 of the three driving terminals U0, V0, and W0 of the sensorless motor M are equal to the driving voltages applied by the output circuit 3 with the BEMF Vu, Vv, and Vw overlaid, respectively. Here, fine ripples of the output voltages VU0, VV0, and VW0 caused by the PWM control are omitted in FIG. 28. In each energization phase, the BEMF Vu, Vv, or Vw causes a zero crossing at the non-energized motor coil, that is, the motor coil to which the output circuit 3 does not apply the driving voltage. See white circles ZC shown in FIG. 28. Accordingly, the instant when each of the potentials VU0, VV0, and VW0 of the driving terminals U0, V0, and W0 agrees with the center tap voltage of the motor coils corresponds to the zero crossing ZC of the BEMF Vu, Vv, and Vw. See the black circles ZC0 shown in FIG. 28.
Each level of the BEMF Vu, Vv, and Vw correspond to the angle between the magnetic pole center of the rotor and the magnetic pole center of the stator, that is, a part where each of the magnetic fields generated by the phase currents Iu, Iv, and Iw flowing through the motor coils Mu, Mv, and Mw, are especially concentrated. Especially at the zero crossing, the rotor position agrees with either of six positions predetermined at intervals of 60 degrees in electrical angles; 0, 60, 120, 180, 240, and 300 degrees in FIG. 28. Accordingly, the rotor position is estimated through the detection of the zero crossing ZC0 by each of the potentials VU0, VV0, and VW0 of the driving terminals U0, V0, and W0. Based on the rotor position estimated, the energization of the motor coils is controlled, and then, for example, the angle between the magnetic pole center of the stator and the magnetic pole center of the rotor is maintained within an appropriate range. Thereby, a torque is efficiently produced at the rotor, and maintained at a sufficiently high level. In FIG. 28, the energization phase is changed with a lag of about 30 degrees behind the zero crossing detected, since the commutation signal CS lags about 30 degrees behind the zero crossing detected. Thereby, the torque produced at the rotor is maintained at the maximum, since the angle between the magnetic pole center of the stator and the magnetic pole center of the rotor is maintained substantially equal to 90 degrees in electrical angles Accordingly, the stable rotor rotation is efficiently maintained and resistant to changes of load.
Changes of the potentials VU0, VV0, and VW0 of the driving terminals U0, V0, and W0, respectively, actually include noises caused by turning on and off of the power transistors. The noises are mainly the noises N (cf. FIG. 28) caused by the switching between the energization phases and fine ripples (not shown) caused by the PWM control. The noises provide errors for the zero-crossing detection by the BEMF comparing section 4. In the conventional sensorless motor driving device, the position-detection mask signal MZC is maintained active for the predetermined time at every switching between the energization phases, that is, every generation of the commutation signal CS. See FIG. 28. In that period, the output signals BCU, BCV, and BCW of the BEMF comparing section 4 are masked, and therefore, false detection of the zero crossing due to the noises N caused by the switching between the energization phases is avoided. Furthermore, the PWM mask signal MPWM is maintained active for a predetermined time at every rising/falling edge of the PWM control signal P. In that period, the output signals BCU, BCV, and BCW of the BEMF comparing section 4 are masked, and therefore, false detection of the zero crossing due to the ripples caused by the PWM control is avoided. Thus, the accurate detection of the zero crossing is realized.
The detection of the rotor position in the sensorless motor is based on the detection of the zero crossing through the BEMF in the motor coils, as described above. The above-described detection of the rotor position cannot be used for the start of the sensorless motor since the BEMF is detected only during when the rotor rotates at a speed more than a certain extent. At the start of the sensorless motor, the conventional sensorless motor driving device, for example, uses the forced commutation signal FC from the forced commutation circuit 60 instead of the self-commutation signal SC from the self-commutation circuit 50. See FIG. 27. Thereby, the conventional sensorless motor driving device changes the energization of the motor coils at a constant period, regardless of the actual position of the rotor. The energization control based on the forced commutation signal FC, which is hereafter referred to as a forced commutation control, continues from the start of the sensorless motor for a predetermined time or until the instant when the revolving speed of the rotor attains a fixed value. After that, the forced commutation control is changed into the energization control based on the self-commutation signal SC, which is hereafter referred to as a self-commutation control. Thus, the conventional sensorless motor driving device realizes the start of the sensorless motor, regardless of the detection of the rotor position.
Generally in the brushless motor, the switching between the energization phases periodically changes suction/repulsion forces between the magnetic pole of the stator and the magnetic pole of the rotor, and stress distributions inside both of the stator and the rotor. Thereby, the brushless motor generally causes noises, which are hereafter referred to as motor echo noises. In particular, the motor echo noises easily become excessive when the phase currents Iu, Iv, and Iw show abrupt changes as shown in FIG. 28. Accordingly, gentle changes of the phase currents are desirable for the suppression of the motor echo noises.
In brushless motors provided with position sensors, the detection of the rotor position does not require the non-energization period of the motor coils, in contrast to the sensorless motors. Accordingly, the suppression of the motor echo noises is comparatively easy since gentle changes of the phase currents can be easily realized. See, for example, Published Japanese patent application 2003-174789 and 2003-244983 gazettes. In particular, when the energization control of the motor coils is performed under a PWM control, the detection with the position sensor is maintained with high accuracy, regardless of the ripples caused by the PWM control.
In the sensorless motor, on the other hand, the detection of the zero crossing of the BEMF requires the non-energization period of the motor coils, as described above. However, gentle changes of the phase currents reduce the non-energization periods of the motor coils. In particular, the non-energization periods of the motor coils cannot be secured under the energization control similar to that of the brushless motor provided with position sensor. Furthermore, ripples caused by the PWM control have to be reliably masked since they reduce the accuracy of the zero crossing detection. However, gentle changes of the phase currents require the extension of the PWM control period. Then, the period required for the above-described mask is extended, and thereby, the detection period of the zero crossing is reduced. Thus, the suppression of the motor echo noises obstructs the accurate detection of the zero crossing in the sensorless motor.
The obstruction to the accurate detection of the zero crossing obstructs improvement in the reliability of the self-commutation control, and, in particular, obstructs an increase in the torque produced in the sensorless motor. In other words, the suppression of the motor echo noise is difficult to be compatible with the increase in the torque produced under the self-commutation control.
The obstruction to the accurate detection of the zero crossing, in addition, causes a difficulty of the prompt and reliable start of the sensorless motor as follows. The forced commutation control changes the energization of the motor coils, regardless of the actual rotor position. Accordingly, the angle between the magnetic pole center of the stator and the magnetic pole center of the rotor generally falls outside the optimum range, and thereby, the increase in the torque produced is generally difficult. As a result, the start of the sensorless motor under the forced commutation control has the difficulty of increasing the starting torque, and therefore, the reduction of the starting time is difficult. Furthermore, the starting control is susceptible to changes of load. For example, in the case of a sensorless motor used as a spindle motor of a CD/DVD combination drive, the moment of inertia varies between a CD and a DVD. Furthermore, in the case of a sensorless motor used as a spindle motor of a HDD, the number of the magnetic disks varies with capacities, and further, the disk radius varies with sizes. Stabilization of the start of the sensorless motor for any of such various loads is difficult under the forced commutation control. Resolution of these difficulties requires as prompt and reliable the switching from the forced commutation control to the self-commutation control as possible, at the start of the sensorless motor.
However, the conventional sensorless motor driving device continues the forced commutation control at the start of the sensorless motor, for example, for a predetermined time from the start or until the instant when the revolving speed of the rotor attains a constant value. In other words, the forced commutation control is not changed into the self-commutation control until a state is attained, in which the accurate detection of the zero crossing is considered to be possible. Reduction of the above-described predetermined time is difficult since it impairs the reliability of the start when the accurate detection of the zero crossing is obstructed. On the other hand, the forced commutation control has a difficulty of reducing the time required from the start of the sensorless motor until the instant when the revolving speed of the rotor attains the constant value. Thus, the conventional sensorless motor driving device has the difficulty of a prompt and reliable switching from the forced commutation control to the self-commutation control at the start of the sensorless motor. Therefore, a prompt and reliable start of the sensorless motor is difficult.