Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2002-01-03
2003-12-02
Lam, Thanh (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S051000
Reexamination Certificate
active
06657345
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a magnetic bearing controller, and more particularly to a magnetic bearing controller which controls a levitated rotating body actively by controlling magnetic force, which is generated by electromagnets by supplying controlled current thereto, whereby the current is controlled by pulse width modulation, and the rotating body is controlled in accordance with the status signals, such as detected displacement sensor signals thereof.
BACKGROUND ART
Recently, a magnetic bearing device is becoming to be widely used in the rotary machines in the various fields. The magnetic bearing device levitates and supports a rotating body without contact by magnetic force, which is generated by electromagnet. The advantages of the rotary machines equipped with the magnetic bearings are; abrasion dusts free, maintenance free because lubrication oil is not used, high-speed rotatability, and reduction of noises.
The magnetic bearings are also suitable when these are used in rotary machines, which are disposed in extreme clean atmospheres, such as clean rooms for semiconductor manufacturing. Because the magnetic bearings need no lubrication oil and do not generate abrasion dust, the semiconductor wafers are prevented from being contaminated. Therefore magnetic bearings are advantageous when these are used in clean space, vacuumed space, and so on. Especially in the vacuum space, friction coefficient of usual mechanical bearings becomes extremely large; therefore the magnetic bearings are suitable because there is no problem for the magnetic bearings which support the axes without contact.
FIG. 1
shows a general circuit configuration of a magnetic bearing controller, which controls a levitated rotating body actively by controlling current supply for the electromagnets. The electromagnets are disposed around the rotating body nearby for applying magnetic forces thereto. When the rotating body is levitated and supported by magnetic forces generated by electromagnets without contact thereto, the controlled object of the system is the rotating body, for example, a position thereof. The displacement X of the rotating body comparing the command position X
0
is detected by status detector unit, namely displacement sensor unit
11
in this case. The detected position X is compared with the command position X
0
in the deviation circuit, and a difference &Dgr;X between detected position X and command position X
0
is inputted to compensator unit
12
.
The compensator unit comprises of a control circuit, such as PID (Proportional plus Integral plus Derivative) control circuit, and generates an output signal so as to control that the difference &Dgr;X between detected position X and command position X
0
becomes zero. An output signal of the compensator unit
12
is inputted into signal amplifier unit
13
, where the signal is amplified and the amplitude of the signal is limited by a limiter circuit(limiter).
Power amplifier unit
14
generates a controlled current for supplying to electromagnet
15
corresponding with the output signal of the signal amplifier unit
13
. The power amplifier unit
14
includes a pulse width modulation circuit for supplying controlled current, which is pulse width modulated thereby into coils of electromagnets
15
. The electromagnet
15
generates a magnetic attractive force in accordance with amount of the controlled current. The magnetic attractive force is applied to the controlled object
16
, namely the rotating body in this case, and the controlled object is moved so as to be the difference &Dgr;X is decreased to be zero, namely the detected position X moves to the command position X
0
. By the above mentioned feedback control, the controlled object
16
(rotating body in this case) is controlled to be stably positioned at the command position X
0
, even if disturbance force is applied from outwards to the controlled object
15
so as to disturb the position thereof.
In the magnetic bearing controller, a displacement sensor is usually employed as the status detector unit
11
for detecting the status, which is displacement X comparing to the command position X
0
in this case. One of typical displacement sensor is an induction type displacement sensor, which has a core of magnetic material with coils wound thereof. According to the inductance type displacement sensor, the displacement of the controlled object which has a magnetic material fixed thereon is measured by the detection of variation of the inductance of the coils thereof.
FIG. 2
shows a detail of signal amplifier unit
13
and a portion of power amplifier unit
14
, which includes PWM (Pulse Width Modulation) circuits. The output of compensator unit
12
is inputted to the signal amplifier unit
13
which comprises of an amplitude limiter
25
, a deviation circuit
29
, a signal amplifier
28
, another amplitude limiter
29
and so on. An output of DC signal generator
26
for setting bias DC current is inputted to the deviation circuit
29
for adding the output signal thereto. The power amplifier unit
14
comprises of a PWM circuit having a comparator circuit
31
and a chopping wave generator
32
, and a power amplifier circuit
35
which amplifies the output signal of the comparator circuit
31
to the actual current to be supplied to the coils of electromagnets
21
.
The output current of the power amplifier
35
is supplied to the coils of electromagnets
21
as a controlled current, thereby controlled magnetic force is generated by electromagnets
21
, and applied to the controlled object (rotating body)
22
for controlling the position thereof. The levitated actual position of the controlled object
22
is detected by the status detector, namely the inductance type displacement sensor
23
in this case. The controlled current which is applied to the coils of the electromagnets is detected by a current sensor
36
, and lower frequency components of the output signal of the current sensor are returned to the deviation circuit
29
by a feedback loop through a low pass filter
39
for stopping higher frequency components which are corresponding to the frequency components of the PWM signals of the controlled current.
As above mentioned, the signal amplifier unit
13
is comprised of the signal amplifier
28
for amplifying an output signal of compensator unit
12
and the limiter
29
for limiting the amplitude of the amplified signal by the amplifier
28
. These circuits are inserted at the front end of the PWM circuit, which comprises of the comparator
31
and the chopping wave generator
32
. For driving the coils of electromagnets
21
which are inductive load, relatively large gain, for example 10 through 100 times amplification is required by the signal amplifier
28
which amplifies the output signal of the compensator unit
12
for inputting to the PWM circuit
31
of the power amplifier unit
14
.
However, there is a problem that output signal of the signal amplifier unit
13
is deformed to be a rectangular shaped waveform from input sine shaped waveform by passing through the deviation circuit
27
, the signal amplifier
28
, and the amplitude limiter
29
.
FIGS. 3A through 3C
show the deformation of waveform and frequency spectrum of the deformed waveform.
FIG. 3A
shows an input signal of sine waveform of 1 kHz, which is inputted to the signal amplifier unit
13
.
FIG. 3B
shows a waveform of controlled current of 1 kHz corresponding to
FIG. 3A
which flows in the coils of electromagnets showing a rectangular shape. The reason of such deformation of the waveform is estimated that a phase difference is generated in the deviation circuit
27
between input signal and feedback signal, and that the output signal of the deviation circuit is amplified in the signal amplifier
28
including phase differences therebetween and saturated therein by the power supply voltage (±15 V). Therefore, the input signal of the PWM circuit
31
becomes rectangular shaped waveform from original sine shaped waveform by passing through the signal ampl
Armstrong Westerman & Hattori, LLP
Ebara Corporation
Lam Thanh
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