Electricity: electrical systems and devices – Control circuits for electromagnetic devices – Systems for magnetizing – demagnetizing – or controlling the...
Reexamination Certificate
2000-03-08
2002-03-19
Leja, Ronald W. (Department: 2836)
Electricity: electrical systems and devices
Control circuits for electromagnetic devices
Systems for magnetizing, demagnetizing, or controlling the...
C267S140150
Reexamination Certificate
active
06359767
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic levitation system for levitating and supporting a magnetizable object as a target in a predetermined position in a contactless manner through magnetic attraction or repulsion forces caused by electromagnets. More particularly, the present invention pertains to a control apparatus for controlling such a magnetic levitation system as above, which is capable of depressing a transient response presented by a magnetizable object at the beginning and the ending of levitation control.
2. Description of the Related Art
FIG. 1
illustrates a schematic diagram showing an arrangement of a target or magnetizable object
15
to be levitated, electromagnets
11
and
13
, and distance detection sensors
12
and
14
in a conventional magnetic levitation system. As shown in
FIG. 1
, the object
15
is disposed between the electromagnets
11
and
13
and also between the distance detection sensors
12
and
14
.
The object
15
is levitated by magnetic attraction and/or repulsion forces generated by the electromagnets
11
and
13
. The distance detection sensors
12
and
14
detect distances from the object
15
to the sensors and generate distance signals, respectively. The generated signals are then provided to a control unit (shown in FIG.
2
), which controls phases and frequencies of AC currents flowing through the electromagnets
11
and
13
in response to the distance signals, so that the object
15
is levitated at a predetermined target position between the electromagnets
11
and
13
.
FIG. 2
shows a block diagram of a prior art magnetic levitation system including a control unit
20
connected to the electromagnets
11
and
13
(EMs) and the distance detection sensors (DDSs)
12
and
14
. The control unit
20
comprises a sensor signal processing circuit
21
, a comparator
22
, a phase compensation circuit
23
, a variable gain amplifier
24
, a main amplifier
25
and a target position (TP) signal generator
28
. The distance signals detected by the sensors
12
and
14
are inputted to the sensor signal processing circuit
21
, where a current levitation position of the object
15
is calculated in response to the distance signals. The obtained current position signal is compared at the comparator
22
with a target levitation position signal from the target position signal generator
28
to generate a signal representing a displacement between the current and target positions of the object
15
. The phase compensation circuit
23
determines compensation currents or compensation frequencies and phases of the AC currents flowing through the electromagnets
11
and
13
so that the displacement signal from the comparator
22
becomes zero. The electromagnets
11
and
13
receive the compensated AC currents through the variable gain amplifier
24
and the main amplifier
25
, and thus provide the object
15
with adequate magnetic attraction and/or repulsion forces to levitate the object
15
at the target position. Therefore, by such a feedback control as described above, the object
15
can be levitated and supported at the target position between the electromagnets
11
and
13
.
According to the aforementioned prior art control manner, a target position signal is already set at the target position signal generator
28
, before a levitation control procedure is carried out. Accordingly, when the levitation system initiates a levitation control procedure of an object levitation, since a displacement between a current position and a target position is relatively large, electromagnetic forces from the electromagnets
11
and
13
to the object
15
relatively large at the starting time of the procedure. Therefore, as is represented by A in
FIG. 3
, when the control starts at a time t
0
, the objects
15
rises from a seated position C to a position above the target position B and then falls to a position under the target position B, due to feedback response characteristics etc. of the magnetic levitation system. Thus, the object
15
swings around the target position B until the levitation control becomes stable condition at t
1
, as shown in FIG.
3
.
In other words, at the moment t
0
magnetic levitation of the object
15
is started, the sensor signal processing circuit
21
outputs a current position signal indicating that the object
15
is at the seated position C, and therefore, the comparator
22
outputs a differential signal indicating a difference between the target position signal from the generator
28
and the current position signal from the sensor signal processing circuit
21
. As a result, an output from the comparator
22
changes stepwise at t
0
as is indicated by D in FIG.
3
. Therefore, an output of the phase characteristic compensation circuit
23
also varies stepwise, which causes the object
15
to suddenly levitate to the portion above the target portion B. In response thereto, the system renders the object
15
to fall down by the feedback control, by which the object
15
fall down to the position under the target position B.
The up- and-down or fluctuation of the object
15
is gradually damped and, the position of thereof is finally stabilized at the target position B at t
1
.
It has been confirmed by experiments, etc., that such a fluctuation or oscillation of an object at the beginning of a levitation control procedure in a magnetic levitation system appears in such cases that a stationary position or seated position C of the object
15
is not constant with respect to a target position B, and start condition of levitation of the object
15
is not constant with respect to AC currents flowing through electromagnets
11
and
13
. In the worst case, the object
15
repeatedly comes into contact with the electromagnets
11
and
13
until the object
15
is levitated at the target position B in a stable manner. As a result thereof, the object
15
and/or the magnets
11
and
13
could be damaged and dust could be produced thereby. Such dust causes problems in particular when the magnetic levitation system is utilized in a semiconductor production system and so on.
Further, when levitation control procedure is stopped, the target position signal from the circuit
28
is suddenly lowered to indicate the seated position C, while the current position signal from the sensor signal processing circuit
21
is still indicate the target position B. As a result, the output from the comparator
22
and hence the output from the compensation circuit
23
change stepwise as shown by E in
FIG. 4
, when the control procedure is stopped at t
2
, the object
15
is steeply descending to the seated position C and it rebounds therefrom, resulting in damage to the object
15
and/or the magnets
11
and
13
and the production of dust.
SUMMARY OF THE INVENTION
The present invention has been accomplished in order to obviate the aforementioned problems of the prior arts. Thus, it is an object of the present invention to provide a control apparatus for controlling a magnetic levitation system, which is capable of preventing fluctuation or oscillation of a targeted object from occurring at the beginning and ending of a levitation control procedure, and thereby of preventing the object from coming into contact with electromagnets due to the oscillation.
In view of one aspect of the present invention, it provides a control apparatus in a magnetic levitation system, for controlling attraction and/or repulsion forces created from a pair of electromagnets to levitate an object therebetween at a predetermined target position in a contactless manner, in response to a displacement of the object from the target position, the control apparatus comprising: (a) a start/stop detector for detecting a start and stop of a levitation control procedure and generating a control signal having a predetermined time duration when either of the start and stop of the levitation control procedure is detected; (b) a compensation circuit for providing a compensation signal to compensate AC
Misu Keijiro
Ooyama Atsusi
Sekiguchi Shinichi
Armstrong Westerman & Hattori, LLP
Ebara Corporation
Leja Ronald W.
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