Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Reexamination Certificate
1998-08-14
2001-03-27
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
Reexamination Certificate
active
06208160
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to motor control, and, more particularly, to a method for detecting the instantaneous rotor position of a DC brushless motor, a necessary requirement for its efficient driving.
BACKGROUND OF THE INVENTION
The brushless motor typically includes a permanent magnet rotor and a stator made up of a number of windings that may be connected in a star configuration or completely independent from each other. In the majority of cases, brushless motors have three-phase windings. In addition, the driving circuit may be integrated and have an output stage that drives the phase windings using a three-phase full-wave bridge circuit employing six MOS or bipolar power transistors.
FIG. 1
represents a driver stage and the electrical diagram of a DC brushless motor connected to the driver stage. A most typical mode of driving this type of motors is the so-called bipolar mode where at any instant two phase windings are powered and the third remains idle, that is, the output of the respective full-wave bridge is kept in a state of high impedance (Hi Z).
The phase windings are switchingly driven according to a cyclic sequence which must be synchronized with the rotor's instantaneous position. In a bipolar driving mode, the instantaneous position may be determined by monitoring the induced back-electromotive-force (BEMF) on the unpowered phase winding or detected by sensors (a more expensive approach that is seldom used).
Another driving mode for this type of motor is the so-called tripolar mode, or, more precisely and generally, the mode according to which all the motor's phase windings are constantly powered. This is a technique wherein detecting of the BEMF on a momentarily unpowered winding cannot be exploited and more sophisticated techniques must be resorted to.
To optimize the system's performance, the power supplied to the phase windings should be such as to operate the motor at its maximum efficiency. This is attained by driving the phase windings with a precise phase between the current forced through the phase winding and the relative BEMF induced thereon.
The torque in a permanent magnet synchronous motor, such as the brushless DC motor, is produced by the stator's current component that creates a magnetic field in quadrature with the rotor's magnetic field. By calling i
d
and i
q
the stator's current components that respectively generate a magnetic field in phase and in quadrature with the rotor's magnetic field, the maximum efficiency is obtained for i
d
=0. To ensure that the whole stator's current generates a field in quadrature it is necessary to force the current through each phase winding in phase with the relative BEMF.
Driving the motor in a bipolar or even in a unipolar mode implies a certain ripple in the produced torque characteristic. The bipolar or unipolar driving does permit an easy detection of the BEMF on the winding or on the windings that are not excited to derive the required synchronous information to effect the phase switchings in perfect phase with the rotor position.
As a matter of fact, systems capable of implementing a tripolar driving mode have been developed to reduce or eliminate the torque ripple. In a tripolar driving mode all the phase windings are constantly driven by alternating (sinusoidal) signals, mutually out of phase from one another. Hence, to be able to “read” the BEMF signal, a momentary interruption of the driving of at least one phase winding of the motor, is effected by placing the respective driving bridge in a condition of high impedance (tristate). This is done for a time interval sufficient to detect a zero-cross event of the BEMF signal. From this information a synchronization signal of the motor's phase switchings is reconstructed. Nevertheless, these techniques introduce a certain torque ripple as compared to a pure tripolar driving mode, that is, without any interruptions for detecting the BEMF signal.
Many driving systems are designed to switch from a unipolar and/or bipolar mode to a tripolar mode and vice-versa. In practice, the motor is started and brought to a steady state by driving it in a unipolar and/or bipolar mode and, once the steady state speed is reached, the system switches to a tripolar driving mode to reduce or eliminate the torque ripple of the motor operating in a steady state condition. If Hall effect sensors are not employed, an alternative technique is based on the use of relatively complex electronic circuits capable of reconstructing a BEMF signal once the motor's electrical parameters (resistance and inductance) are known. The reconstructed BEMF signals are resorted to so as maintain a correct synchronization of the phase switchings during the operation in a tripolar mode.
It is evident that there is a need or usefulness of a method and a system capable of driving the motor at a steady state speed in a pure tripolar mode, thus minimizing or nullifying the torque ripple. This would also be desirable without driving interruptions necessary to monitor the BEMF or implementing complex reconstructing systems of the BEMF signal while ensuring a perfect synchronization of the phase switchings with the rotor's instantaneous position and optimizing the efficiency.
SUMMARY OF THE INVENTION
Therefore, a main object of the invention is to provide a system for detecting any perturbation of the phase angle between a driving voltage applied to a respective motor's phase winding and the relative BEMF during a tripolar mode driving of the motor, and without effecting any interruption of the driving or requiring a reconstruction of the BEMF signal.
This object is fully met by the method and related system of the invention which are based upon the monitoring of the modulation caused by the driving current forced through the motor's phase windings on the voltage present at their terminals.
The monitoring system of the invention is perfectly “transparent” and does not cause any undesired torque ripple, being at the same time capable of promptly detecting any perturbation of the switching synchronization with the instantaneous position of the rotor. By providing information on an occurred perturbation, the driving system can carry out a realigning or resynchronizing. This can be done, for example, by switching from a pure tripolar driving mode (steady state) to a bipolar mode or by momentarily passing from a pure tripolar driving mode to a tripolar driving mode, with interruption of the driving of at least one phase winding for a certain time interval during which the BEMF signal is sensed until recovering a correct synchronization and/or corrected steady state speed.
As soon as the optimal steady state running operating conditions are recovered, the driving system switches again to a pure (uninterrupted) tripolar mode. This minimizes the torque ripple during the steady state operating condition, and relies upon the monitoring system of the invention to detect any eventual new perturbation that may occur.
REFERENCES:
patent: 3931556 (1976-01-01), Ban et al.
patent: 5172036 (1992-12-01), Cameron
patent: 5231338 (1993-07-01), Bulgarelli et al.
patent: 5731670 (1998-03-01), Galbiati et al.
patent: 5767654 (1998-06-01), Menegolli et al.
patent: 5808440 (1998-09-01), Bennett et al.
patent: 5969491 (1999-11-01), Vitti et al.
Boscolo Michele
Viti Marco
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Galanthay Theodore E.
Metjahic Safet
STMicroelectronics S.r.l.
Sundaram T. R.
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