Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2000-10-10
2003-02-04
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
Reexamination Certificate
active
06515388
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic levitation control apparatus, such as a magnetic bearing or a magnetic levitation damping apparatus, and more particularly to a control apparatus for holding a levitated body having a magnetic body in a predetermined position in a non-contact manner by controlling the magnetic attraction of electromagnets.
2. Description of the Related Art
In the magnetic levitation control apparatus, a detector for detecting the position of the levitated body and a feedback control unit for controlling the magnetic attraction of the electromagnets based on the detected positional information are generally required and indispensable as a mechanism for stably holding the levitated body in a non-contact manner. One example of the construction is shown in FIG.
1
. Position detection sensors
22
a
,
22
b
are a conventional sensor called a “magnetic induction system”, wherein the position of the levitated body is detected in a non-contact manner by a change in inductance, and, together with balancing resistors
23
a
,
23
b
and a differential amplifier
26
, constitute a bridge circuit. A carrier signal generated by an oscillator
25
and a buffer amplifier
24
is added to the bridge circuit. Upon the occurrence of unbalance of the inductance between the sensors
22
a
,
22
b
due to the displacement of the levitated body
21
from the reference position, a sensor signal having an amplitude substantially proportional to the displacement appears in the differential amplifier
26
. The sensor signal contains, in a superimposed state, a noise signal included by magnetic coupling between the sensor and the electromagnets or between the sensor and other electromagnetic actuator. Therefore, a band-pass filter
27
with the frequency of the carrier signal being the central frequency is generally provided to remove the noise component.
In
FIG. 1
, in order to remove the carrier frequency component to extract the amplitude component only, the sensor signal passed through the band pass filter
27
is processed in a detection circuit
28
, and sent as a displacement signal to a controller
29
. On the other hand, electromagnets
35
a
,
35
b
are excited by a voltage control type current source comprised of differential power amplifiers
32
a
,
32
b
and current detecting resistors
34
a
,
34
b
. In general, an adder
30
and a subtracter
31
are used to supply, to the electromagnets
35
a
,
35
b
, a superimposed current composed of a bias current corresponding to the voltage of a direct-current power supply
33
and a control current corresponding to the control signal of the controller
29
. This creates a difference in magnetic attraction between the electromagnets
35
a
,
35
b
. This difference cancelles the force of disturbance and the gravitational force acting on the levitated body
21
, whereby the levitated body
21
is held at a predetermined levitation position.
In the above-described feedback control unit, in recent years, a large number of examples of the construction of digital controller are also found in the field of the magnetic levitation control apparatus, from the viewpoints of easiness in implementation on hardware of the control law at the time of design of a control system and the flexibility of a change in control law. As compared with the analog controller, however, the digital controller is disadvantageous in that extra delay elements in frequency response, such as a hold element in the discretization of displacement signal and a waste time element dependent upon the operation time, are included in the feedback control unit. For this reason, when feedback control with quick response is required, for example, in magnetic bearings or magnetic levitation damping apparatuses, hardware having good frequency characteristics should be constructed in the detector for detecting the position of the levitated body and the section for controlling the attraction of the electromagnets.
In recent magnetic levitation control apparatuses, attention is being drawn to the application of a sensorless magnetic levitation system wherein the levitation position is detected using electromagnets per se aiming at the utilization of the apparatus within a vacuum environment by taking advantage of non-contact bearing and a reduction in cost and a reduction in size of the whole apparatus. In this sensorless magnetic levitation system, a carrier signal is directly supplied to the electromagnets
35
a
,
35
b
without use of the sensors
22
a
,
22
b
shown in
FIG. 1
in the detection of the position of the levitated body, whereby the position of the levitated body is detected based on a change in inductance of the electromagnets thereof.
This magnetic levitation system is particularly disadvantageous in that a deterioration in frequency characteristics in the position detector due to a lowering in the frequency of the carrier signal is unavoidable. This suggests that, in the magnetic levitation control apparatus utilized within the vacuum environment, from the viewpoint of vacuum contamination, a thin metallic pressure bulkhead is preferably provided between the sensor and the levitated body. In this case, however, as the frequency of the carrier signal increases, the loss in this portion increases. Therefore, the frequency of the carrier signal is preferably as low as possible. This necessarily requires the use of a carrier signal having a low frequency. The use of the carrier signal having a low frequency results in interference with the frequency band for controlling the position of the levitated body. Therefore, in order to further develop the application of the magnetic levitation technique while utilizing the advantage of the digital controller, it is necessary to take some measure for preventing the deterioration in frequency response of the feedback control unit due to a lowering in carrier frequency of the position detecting sensor.
SUMMARY OF THE INVENTION
Under the above circumferences, the present invention has been made, and it is an object of the present invention to provide a magnetic levitation control apparatus that applies self-sensing control, which utilizes electromagnets for controlling the levitation position of the levitated body, to a position detecting system for detecting the position of the levitated body and can achieve a lowering in carrier frequency without a deterioration in frequency response characteristics of the feedback control unit.
It is another object of the present invention to provide a magnetic levitation control apparatus which comprises a combination of the digital controller with the self-sensing control and can realize good controllability.
In order to attain the above objects, according to an aspect of the present invention, there is provided a magnetic levitation control apparatus comprising: a pair of electromagnets for holding a levitated body having a magnetic body in the levitated state, the pair of electromagnets being positioned opposite to each other in such a manner that the point of application of the electromagnetic attraction in the electromagnets conforms to the point of a position detected using the electromagnets as a position sensor. A signal source supplies a voltage signal of a frequency on a level such that enables the electromagnets to function as the position sensor, wherein a control voltage signal for controlling the magnetic attraction of the electromagnets is superimposed on the voltage signal. A circuit differentially supplies the voltage signal to the pair of electromagnets to form a position signal of the levitated body from an add signal of currents respectively from the electromagnets, and a circuit detects a control current of the electromagnets from a subtraction signal of currents respectively from the electromagnets. A controller generates a control voltage signal of the electromagnets from the position signal of the levitated body and, in addition, corrects the detected position signal from the detected control current of t
Haga Takahide
Moriyama Shinichi
Watanabe Katsuhide
Ebara Corporation
Perez Guillermo
Ramirez Nestor
Wenderoth , Lind & Ponack, L.L.P.
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