Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-08-23
2001-12-04
Mullins, Burton S. (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C324S207120, C324S207260, C310S06800R
Reexamination Certificate
active
06326712
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic bearing device for levitating and supporting an object under electromagnetic forces generated by an electromagnet.
2. Description of the Related Art
Displacement sensors for detecting the displacement of an object levitated by a magnetic bearing device include an eddy-current sensor, an inductive sensor, an electrostatic capacitive sensor, and an optoelectronic sensor such as a laser sensor. Of these displacement sensors, the eddy-current sensor and the inductive sensor are mainly used in turbomolecular pumps.
For use in corrosive environments, magnetic bearings and displacement sensors need to be covered with a protective material. Magnetic bearing devices which employ eddy-current sensors, inductive sensors, and electrostatic capacitive sensors can be covered with a protective material which may be synthetic resin such as Teflon or ceramics. Optoelectronic sensors such as laser sensors are required to be covered with glass which allows a laser beam to pass therethrough. Eddy-current sensors, electrostatic capacitive sensors, and optoelectronic sensors cannot be used in situations where displacement sensors need to be covered with a metal material. Inductive sensors can be used if they are to be covered with a nonmagnetic metal.
Protecting magnetic bearings and displacement sensors with Teflon, ceramics, glass, etc. poses problems in terms of fabrication process, cost, and mechanical strength. In addition, these protective materials may not be used in special environments where gas contamination is problematic.
It is customary to employ inductive sensors protected by a nonmagnetic metal in such applications. However, the carrier frequency of an inductive sensor produces a magnetic field that generates an eddy current on the surface of a nonmagnetic metal partition, resulting in a reduction in the sensitivity of a detected signal from the inductive sensor, i.e., a reduction in the s
ratio thereof.
A magnetic bearing operates by passing a current through an electromagnet and levitating an object under electromagnetic forces generated by the electromagnet. If the magnetic bearing and an inductive sensor combined therewith are covered with a nonmagnetic metal, then since both the magnetic bearing and the inductive sensor are covered with one nonmagnetic metal partition, electromagnetic noise generated by the electromagnet and an eddy current produced on the surface of the nonmagnetic metal partition by the electromagnetic noise pass through the nonmagnetic metal partition, adversely affecting the inductive sensor. The nonmagnetic metal partition that protects the inductive sensor and the magnetic bearing is thus disadvantageous in that it makes magnetic levitation control difficult due to magnetic and electric noise applied to the inductive sensor.
FIG. 1
of the accompanying drawings shows a circuit arrangement of a control circuit for a conventional magnetic bearing device. As shown in
FIG. 1
, the control circuit includes an oscillator
1
whose output signal is supplied via operational amplifiers
2
-
1
,
2
-
2
, current-limiting resistors
3
-
1
,
3
-
2
, and a cable CB to a pair of series-connected displacement sensors Z
1
, Z
2
which detect the displacement, in an X-axis direction, for example, of an object
5
levitated by a magnetic bearing MC. A potential (sensor signal) Eg at the junction between the displacement sensors Z
1
, Z
2
is applied to a negative terminal of a differential amplifier
6
, and a reference potential Es that is set up by reference resistors Ra, Rb is applied to a positive terminal of the differential amplifier
6
. The differential amplifier
6
applies an output signal via a synchronous detector
7
and a phase compensating circuit
8
to a drive circuit
9
.
As shown in
FIG. 2
of the accompanying drawings, the drive circuit
9
comprises a controller
9
-
1
and a driver
9
-
2
. The controller
9
-
1
controls the driver
9
-
2
according to a PWM process. The driver
9
-
2
supplies an output signal to an electromagnet coil
10
of the magnetic bearing MC.
In
FIG. 1
, a capacitor
4
is connected parallel to the displacement sensors Z
1
, Z
2
to cause parallel resonance therewith.
If a protective plate of nonmagnetic metal is disposed between the displacement sensors Z
1
, Z
2
and the object
5
and each of the displacement sensors Z
1
, Z
2
comprises an inductive sensor, then the above problems arise, i.e., an eddy current generated by the protective plate of nonmagnetic metal causes a reduction in the sensitivity of a detected signal from the displacement sensors Z
1
, Z
2
, and the displacement sensors Z
1
, Z
2
are adversely affected by a magnetic field generated by a current that is supplied to energize the electromagnet coil
10
.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a magnetic bearing device which includes a magnetic bearing and a displacement sensor, at least the displacement sensor being protected by a nonmagnetic metal material, and which is capable of performing accurate levitation control without reducing the sensitivity of a detected signal from the displacement sensor, i.e., without reducing the s
ratio thereof, with an eddy current generated by the nonmagnetic metal material, and also without allowing the displacement sensor to be adversely affected by a magnetic field produced by a current that is supplied to energize an electromagnet coil of the magnetic bearing.
According to the present invention, there is provided a magnetic bearing device comprising an electromagnet for levitating an object under electromagnetic forces, an inductive displacement sensor for detecting the displacement of the levitated object, a controller for supplying a signal to the displacement sensor through a cable and a current to the electromagnet through a cable, and a protective plate of nonmagnetic metal material disposed between the displacement sensor and the levitated object, the controller including a levitation control system having a noise removing filter for preventing an abnormal signal caused by the protective plate from being applied to the displacement sensor.
Since the controller includes the levitation control system having the noise removing filter for preventing an abnormal signal caused by the protective plate from being applied to the displacement sensor, the s
ratio of a sensor signal of the displacement sensor is increased and the displacement sensor functions sufficiently even though the displacement sensor is of the inductive type and the protective plate of nonmagnetic metal material is disposed between the displacement sensor and the levitated object.
The noise removing filter may comprise either a filter or a filter and a phase compensator.
The displacement sensor may include a yoke extending through the protective plate, or the displacement sensor may include a yoke, and the magnetic bearing device may further comprise a magnetic member of a material which is identical or similar to the material of the yoke, the magnetic member being embedded in the protective plate adjacent to the yoke.
With the above arrangement, the distance between the yoke and the levitated object is reduced by the thickness of the protective plate, resulting in an increase in the sensitivity of the displacement sensor for detecting the displacement of the levitated object with high accuracy.
The controller may include a power driver for energizing a sensor element of the displacement sensor. The power driver is effective in increasing the rate of change of a detected signal of the displacement sensor with respect to a change in the displacement of the levitated object.
The controller may include a PWM driver for energizing the electromagnet and a common-mode coil, a normal-mode coil, or a common-mode coil and a normal-mode coil connected to an output terminal of the PWM driver.
The PWM driver and the common-mode coil, the normal-mode coil, or the common-mode coil and the normal-mode coil
Barada Toshimitsu
Inoue Masafumi
Nakazawa Toshiharu
Ooyama Atsushi
Sekiguchi Shinichi
Armstrong Westerman Hattori McLeland & Naughton LLP
Ebara Corporation
Mullins Burton S.
LandOfFree
Magnetic bearing device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Magnetic bearing device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Magnetic bearing device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2561594