Electricity: motive power systems – Switched reluctance motor commutation control
Reexamination Certificate
1999-07-29
2001-03-20
Masih, Karen (Department: 2837)
Electricity: motive power systems
Switched reluctance motor commutation control
C318S434000, C318S132000, C318S778000, C318S702000, C318S723000
Reexamination Certificate
active
06204617
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to a device for detecting an angle of rotation of a rotor with respect to a stator of a brushless multi-phase d.c. motor, the rotor comprising a permanent magnet and the stator comprising a plurality of electrical windings to which, in operation, electric drive signals are applied for driving the rotor, the device having drive means for applying electric test pulses to the windings while the rotor does not rotate.
The invention also relates to a driving system comprising a multi-phase d.c. motor and a device for the detection of the angle of rotation. The invention further relates to a disk drive and a tape streamer including a driving system in accordance with the invention.
Such a device is known inter alia from U.S. Pat. No. 5,117,165. In this known device current pulses of short duration and of mutually opposite polarities are applied to each winding of the stator during standstill of the motor. Moreover, the build-up of the current in the respective winding is measured by means of a series resistor, which acts as a current-voltage converter. In this way, 2n voltages are measured in the case of a n-phase d.c. motor. It has been found that each of the measured currents depends on the position of the rotor with respect to the stator windings. With the aid of the 2n measured currents through the windings the position of the rotor with respect to the stator windings can be determined with an accuracy of 180°
. Subsequently, information about the angle of rotation of the rotor with respect to the stator can be used, in a manner known per se, to start the motor in such a way that it is directly set into rotation in a predetermined direction. This is very important in the case where a multi-phase d.c. motor is used in disk drives of a certain type in order to preclude damage to the read and write heads. When multi-phase d.c. motors are used in tape streamers this is important for an optimum use of the storage capacity of a tape.
A drawback of the known device is that the build-up of currents in the windings is monitored by means of a current-voltage converter arranged in series with the respective windings. As a result of this, a comparatively large amount of power is dissipated in the current-voltage converter.
SUMMARY OF THE INVENTION
It is an object of the invention to provide a solution in order to overcome this drawback. To this end, the device in accordance with the invention is characterized in that the device further comprises a measurement circuit for detecting the flyback pulses generated by the motor in response to the test pulses and a processing unit which processes the durations of the detected flyback pulses in combination in order to determine said angle of rotation.
The detection of the flyback pulses can be effected without a current-voltage converter being arranged in series with the respective windings. It appears that the width of the flyback pulses, i.e. the duration of the flyback pulses, depends on the self-induction of the respective windings. In turn, the self-induction of a winding depends on the angle of rotation of the rotor with respect to the respective winding. As a result of this, said angle of rotation can be determined with the aid of the measured durations of the detected flyback pulses. In contradistinction to the situation in the known device, the detection of the flyback pulses implies that a measurement is carried out at an instant at which the test pulse is not present. In other words, in accordance with the invention the breakdown of the self-induction of a winding is measured, while in the known device the build-up of the self-induction in a winding is measured.
Another advantage of the device in accordance with the invention is that, in view of the nature of the signals to be processed by it, the processing unit can be very simple without use being made of a microprocessor.
In particular, it is determined whether each winding has a north or south orientation with respect to the magnetic stator. This information makes it possible to determine the angle of rotation of the rotor with respect to the stator, i.e. with respect to the stator windings.
In particular, the drive means are also adapted to recurrently apply drive signals to the windings of the motor in a given sequence in order to rotate the rotor.
Thus, the drive means preferably have a dual function. They can be used both for controlling the rotation of the motor and for determining the angle of rotation of the rotor with respect to the stator during standstill of the motor.
In accordance with a further embodiment of the invention the device is characterized in that the drive means include a plurality of switching elements for the generation of the test pulses and the drive signals, the drive means also including said measurement circuit for monitoring, during the period in which the drive means generate the drive signals, the voltages across the switching elements of the drive means when these switching elements are in an open state.
This means that in this case the measurement circuit also has a dual function. During rotation of the motor the measurement circuit is used for monitoring the voltages across the switching elements and during standstill of the motor the measurement circuit is also used for determining the angular position of the rotor with respect to the stator.
In particular, the drive means are adapted to apply test pulses to each of the windings. This makes it possible to determine the angle of rotation with an optimum accuracy. Preferably, two test pulses of mutually different polarities are applied to each of the windings. In the case of an n-phase d.c. motor, which is a d.c. motor having at least two windings which comprise a different motor phase during use, the angle of rotation can then be determined with an accuracy of 180°
.
In a very practical embodiment the device is adapted to a motor having three windings (n=3) arranged with a mutually different phase with respect to one another, the device for determining the angle of rotation performing, in operation, a measurement process comprising three cycles in order to detect six flyback pulses, a first test pulse of a first polarity being applied to a first pair of windings in a first cycle; a flyback pulse generated in response to the first test pulse being detected; a second test pulse of a second polarity opposite to the first polarity being applied to the first pair of windings; and a flyback pulse generated in response to the second test pulse being detected; the first cycle being repeated in a second cycle and in a third cycle for a second pair of windings and a third pair of windings, respectively, the first, the second and the third pair of windings being different from one another.
The processing unit, in particular, can measure the difference in duration of two flyback pulses measured in each step. This implies that eventually only three differences in duration are detected. These differences can be determined simply by means of an up-down counter. The up-down counter counts the pulses of a clock during the presence of a flyback pulse in response to the first test pulse and subsequently counts down the pulses of the clock signal during the presence of a flyback pulse in response to the second test pulse. In this way the counter eventually indicates the difference in duration between said two flyback pulses. The three differences in duration thus detected can be applied directly to the drive means in order to determine the condition in which the drive means can be started so as to cause the motor to rotate, namely in such a manner that the motor starts to rotate directly in a predetermined position.
Thus, the processing unit can be realized very simply by means of an up/down counter, without the use of a microprocessor being required.
REFERENCES:
patent: 5117165 (1992-05-01), Cassat et al.
patent: 5530326 (1996-06-01), Galvin et al.
patent: 5652525 (1997-07-01), Mullin et al.
Galema Catharinus T.
Van Hout Henricus M.
Franzblau Bernard
Masih Karen
U.S. Philips Corporation
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