Electricity: motive power systems – Limitation of motor load – current – torque or force
Reexamination Certificate
1999-10-08
2001-09-04
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Limitation of motor load, current, torque or force
C318S254100
Reexamination Certificate
active
06285149
ABSTRACT:
BACKGROUND OF THE INVENTION
Hall-effect sensors are commonly used to monitor the radial position of the spindle motor of a hard disk drive. The radial position information is provided to a controller to operate a motor commutator at the correct time, to thereby maintain rotation of the motor and disks thereon at a predetermined rotational velocity. These Hall-effect sensors added additional cost and circuit board space to the hard drive system. Consequently a simpler and less expensive technique was developed using the back electromotive force (BEMF) generated by the motor with a six-state commutation. The six-state commutator operates a three-phase spindle motor to drive one winding high, one winding low, and one winding is left undriven. The BEMF voltage of the undriven winding provides radial motor position and speed information to the controller.
There are some drawbacks with the six-state commutation. Torque ripple leads to audible resonance and reduced efficiency, causing the motor speed to vary, which can also create a force on the disk drive assembly. If there is a mechanical resonance of the disk drive assembly that is close in frequency to the harmonics of this force, an audible noise results which can affect the acoustic performance of the drive. Driving the three windings of the motor with appropriate waveforms can minimize torque ripple. If the torque profile for each stator motor coil is sinusoidal, each motor terminal should be driven with a sinusoidal signal to create a flat torque waveform. A sinusoidal driving current can be accomplished by pulse width modulating the DC source voltage during each of the three phases. Sinusoidal pulse width modulation requires all three windings be driven simultaneously, with one winding being driven high and the other two windings being modulated by driving them high or low or not at all to shape the driving signal. See, for example, T. Kenjo,
Electric Motors and Their Controls
, Oxford Press, (1991), pp 143-136. However, with all three windings being driven simultaneously, it is not possible to detect the rotational position of the motor. Consequently, motor position information must be obtained from some other source.
One technique for detecting motor position with pulse width modulated driven motors is to sample the maximum motor current when only one of the windings is driven high and using that sample to detect the phase of the motor current in relation to the driving signal. To assure sampling is of the maximum motor current, motor current sampling is performed only when two windings are driven low. However, the sample signal carries a DC component which adversely affects operation of the apparatus. Consequently, there is a need for a phase detector that eliminates the DC component of the sample signal.
BRIEF SUMMARY OF THE INVENTION
In one form of the invention, a motor drive circuit for a three-phase motor includes a commutator that provides a driving voltage to the motor from a DC voltage source. A sequencer operates the commutator to pulse width modulate the source voltage so that the driving current has a continuously variable waveform, such as a sinusoidal waveform. The sequencer is responsive to an error signal from a phase detector to align the driving voltage waveform with a motor current waveform. The phase detector includes a sensor and first and second sample circuits. The sensor senses a voltage representative of the motor current and provides the voltage to the first and second sample circuits. The first sample circuit provides a plurality of first samples of respective instantaneous values of the sensed voltage during a single phase of the motor current, and the second sample circuit provides a second sample of a selected instantaneous value of the sensed voltage. A difference circuit subtracts the second sample from each of the plurality of first samples, and an error generation circuit provides the error signal based on the plurality of differences.
In the preferred embodiment, the commutator is a six-state commutator having three pairs of switches, each pair comprising an upper switch for connecting the DC voltage source to an individual one of three windings of the motor and a lower switch for connecting the winding to the sensor. The sequencer operates the commutator so that respective ones of the upper switches supply source voltage to a respective winding during a respective 120° rotational phase of the motor, the sequencer further selectively operating up to two lower switches and up to two upper switches from the pairs that do not include the operated upper switch to pulse width modulate the source voltage thereby generating the driving voltage, with only one switch from a pair being operated at any one time. The first sample circuit includes a first sample switch and a first capacitor, and the second sample circuit includes a second sample switch and a second capacitor. The first sample switch is operated a plurality of intervals during each phase of the motor to store a voltage in the first capacitor representative of successive instantaneous values of the sense voltage. The second sample switch is operated during a selected interval during the motor phase to store a voltage in the second capacitor representative of a selected instantaneous value of the sense voltage, which represents the DC value of the motor current. The difference circuit operates to subtract the voltage stored in the second capacitor from the voltage stored in the first capacitor. Preferably, the first and second sample switches are operated by the sequencer upon a selected simultaneous operation of two lower switches.
Another aspect of the present invention resides in a process of operating a three-winding motor in which a DC voltage is commutated to provide a continuously variable driving current to the motor. A voltage representative of motor current is sensed while providing the driving voltage to one winding of the motor. A plurality of instantaneous values of the sensed voltage are sampled and one of the sampled instantaneous values is selected. The selected value is subtracted from each of the plurality of values to derive an error signal. The voltage phase is adjusted based on the error signal to align the voltage phase with the current phase. Preferably, the selected instantaneous value is approximately representative of a DC component of the motor current, so the error signal is free of a DC offset.
REFERENCES:
patent: 4490661 (1984-12-01), Brown et al.
patent: 5187419 (1993-02-01), DeLange
patent: 5420492 (1995-05-01), Sood et al.
patent: 5448149 (1995-09-01), Ehsani et al.
patent: 5457375 (1995-10-01), Marcinkiewicz et al.
patent: 5821708 (1998-10-01), Williams et al.
patent: 5825145 (1998-10-01), Pham et al.
Kenjo, Tak, Electric Motors and Their Controls: An Introduction, 1991, pp. 134-136.
Brenden Jason P.
Peterson Michael J.
Agere Systems Guardian Corp.
Duda Rina I.
Kinney & Lange , P.A.
Ro Bentsu
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