Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
--1--
The present invention relates to a device
for measuring the induction in the air yap of a magne-
tic bearing comprising a rotor armature made of ferro-
magnetic material, a stator constituted on the one
hand by a frame made of ferromagnetic material defining
a succession of poles and of notches in its part
located opposite the rotor armature and, on the other
hand, by electro-magnet coils disposed in the notches
of said frame, and an air gap located between the
rotor and the stator.
In an active magnetic bearing, the current
for energizing the stator electro-magnets is controlled
from the detection signals delivered by detectors
which measure at every moment the displacements of
the bearing rotor with respect to the stator, in
order to maintain the rotor in a pre~etermined position.
The lifting force of a magnetic bearing
is proportional to the square of the magnetic induction
in the air gap of the bearing and to the useful active
surface of the magnetic circuit of the bearing.
It is often useful to effect a precise
measurement of the force of a bearing. This is the
case, for example, when it is desired to measure
the force of traction of a metal sheet resting on
magnetic bearings.
In that case, the determination of the
magnetic induction necessary for determining the
lifting force is effected by a calculation which
takes into account the result of different measure-
ments, particularly of the air gap and the current
for energizing the electro-magnets of the bearing.
However, this method cannot take into account the
remanent field of the electro-magnets. The error
thus introduced remains relatively small, but becomes
~, i~
appreciable for certain applications.
It is an object of an aspect of the present
invention to remedy the above drawbacks and to effect a
direct measurement of the induction in the air gap of a
magnetic bearing, in order in particular ~o allow a
precise determination of the force of attraction of the
bearing.
It is an object of an aspect of the invention
to allow a measurement of magnetic induction without it
being necessary to employ measuring elements on the
rotor and therefore to use commutators for taking
signals which would be delivered by these measuring
elements.
An aspect of the invention is as follows:
~ device for measuring the induction in the
air gap of a magnetic bearing, comprising:
(a) a rotor armature made of ferromagnetic
material;
(b) a stator, including--
(i) a frame made of ferromagnetic material
and defining a succession of poles having end
faces,
(ii) notches formed opposite the rotor
armature, and
(iii) electro-magnet coils disposed in the
notches of said frame;
(c) an air gap located between the rotor armature
and the stator;
(d) a plurality of small notches of small width
made in the longitudinal direction of the rotor and
distributed on the periphery thereof;
(e) a plurality of small coils disposed in the end
faces of the poles opposite the rotor armature
correspondin~ to said notches: and
(f) means for detecting the alternating voltage
induced in each small coil due to the passage of the
small notches of the rotor opposite said small coils,
which detected alternating voltage can be used to derive
2a ~ 2~ 9~
values of the induction in the portions of the air gap
which are located opposite the end faces of the poles of
the stator frame which support said small coils.
According to a particular feature of the
invention, the small coils disposed in the poles of the
stator frame and the small notches made on the periphery
of the rotor are inclined with respect to the axis of
rotation of the rotor.
According to another feature, which makes it
possible to avoid a modulation of the induction
measured, the distance between two adjacent small
notches corresponds to the width of a pole of the
stator frame.
The width of the sma]l coils disposed in
the poles of the stator frame is substantially equal
to the width of the small notches made on the periphery
of the rotor.
In order not to affect the carrying capacity
of the bearing whilst conserving a good sensitivity
of measurement, the width of the small coils and
of the small notches advantageously corresponds to
about one tenth of the width of a pole of the stator
frame.
According to a particular embodiment, the
small coils are fixed by adhesion on the end faces
of the poles of the stator frame.
According to another embodiment, the small
coils are disposed in small notches made in the end
faces of the poles of the stator frame.
The invention will be more readily understood
on reading the following description with reference
to the accompanying drawings, in which:
Fig. 1 is a schematic partial view in section
perpendicular to the axis of an active, radial magnetic
bearing incorporating the induction measuring device
according to the invention.
Fig. 2 is a developed plan view of the
outer surface of the rotor of the bearing of Fig.
1.
Fig. 3 is a view of poles of the stator
of the bearing of Fig.l, along line III-III of Fig.
1.
Fig. 4 is a detailed view of a s-tator pole
in section along line IV-IV of Fig. 3, and
Fig. 5 is a variant embodiment of the stator
pole of Fig. 4.
--4--
Referriny now to the drawings, Fig. 1 shows
part of an active radial magnetic bearing cornprising
a stator 1 constituted by a frame 10 defining notches
11 and pole pieces 12, and electro-rrlagnet windings
13 disposed around the pole pieces 12 in the notches
11. The frame 10 is formed by a stack of ferromagnetic
plates perpendicularly to the axis of the bearing,
i.e. parallel to -the plane of Fig. 1. The stator
1 is coaxial to a rotor 2 constituted by an armature
20 made of laminated ferromagnetic material, which
defines with poles 12 an air gap of width E.
The basic structure of an active magnetic
bearing is well known and wi]l not be described in
detail. However, it will be noted that rotor 2 may
be disposed either inside stator 1, as shown in Fig.
1, the outer cylindrical surface 21 of rotor 2 lying
opposite the terminal surfaces 121 of the pole pieces
12 of the stator 1, or outside stator 1. In the latter
case, the pole pieces 12 of stator 1 face outwardly
and it is the inner cylindrical surface of the annular
rotor which lies opposite the terminal surfaces of
the pole pieces 12 of stator 1.
The carrying capacity of a magnetic bearing
is proportional to the square of the magnetic induction
in the air gap and to the useful active surface.
The useful active surface is constituted by the sum
of the surfaces of the terminal faces of poles 12
of the stator magnetic circuit and is therefore propor-
tional to diameter D of the rotor, to the air gap
and to the width L of the magnetic bearing. I'he useful
active surface being constant for a given magnetic
bearing, the force of attraction of the bearing,
which is the resultant of the forces of attraction
of the different electro-magnets constituted by pairs
--5--
of adjacent poles 12 of opposite polarities and the
corresponding windinys, is therefore proportional
to the magnetic induction in the air gap of the beariny.
In a magnetic bearing, the flux circuit
of the electro-magnets is such that the major part
of the energy of magnetization lies in the air gap.
If, furthermore, the relative permeabilities of the
magnetic plates of the rotor and of the stator are
high, it may be considered that, for a given air-gap,
the magnetic induction is proportional to the intensity
I of the current circulating in the windings 13 of
the stator electro-magnets. In that case, by measuring
the intensity I of the current for energization of
the stator electro-magnets, combined with a precise
measurement of the value of the air-gap, the force
of attraction of the bearing can be determined. Such
a measurement of force thus depends on an indirect
determination of the value of the magnetic induction
in the air gap. Despite its convenience, this method
is not entirely satisfactory for certain applications,
or necessitates the introduction of correction factors,
and does not take into account the remanent field
of the electro-magnets.
The magnetic bearing according to the inven-
tion is thus equipped with a device for directly
measuring the magnetic induction in the air-gap of
the bearing, and consequently makes it possible-to
determine the lifting force of the bearing in particu-
larly reliable manner.
This induction measuring device essentially
comprises an assembly of small coils 32 placed on
the stator 1 and cooperating with small notches 31
made on the cylindrical surface 21 of the armature
20 of the rotor which defines the air gap 4 of the
-6- ~56~9
bearing. Each terminal Eace 121 oE a pole piece 12
of the stator 1 comprises a small coil 32 and the
number of the small notches 31 made on the rotor
2 is equal to the total number of small coils 32
and therefore of the poles of stator 1.
The small coils 32 and the small notches
31 are disposed lengthwise of the bearing (Figs.
2 and 3), but are inclined by the same angle ~ with
respect to the axis of rotation X'X of the bearing,
similarly to the notches made in the rotor of an
induction motor which are inclined with respect to
the axis of rotation of this rotor.
The distance d which separates two successive
notches 31 of the rotor is equal to the width 1 of
a pole 12 of the stator 1. In this way, any phenomenon
of modulation of -the signal delivered by the small
coils 32 is avoided.
In operation, upon each passage of a small
notch 31 beneath a small coil 32, an alternating
voltage is induced in this small coil 32. Taking
into account the fact that the geometrical characteris-
tics of each small coil 32 are well determined, the
alternating voltage produced upon passage of a notch
31 beneath the coil 32 is proportional to the magnetic
induction in the air gap 4.
By taking the signals available at the
terminals of the different small coils 32 disposed
on the different poles 12 of the stator 1 when the
rotation of rotor 2 causes notches 31 to pass opposite
said small coils 32, a value proportional to -the
magnetic induction B may thus be obtained and the
value of the force of the beari.ng may be deduced
therefrom.
The width a of a notch 31 corresponds to
_7_ ~2t~9~
the width b of a coil 32. Notch and coil wid-ths a,
b are advantageous]y chosen which correspond to about
one tenth of the width 1 of a pole 12. This makes
it possible not to reduce substantially the carrying
capacity of the bearing whilst leading to voltages
induced at the terminals of the small coils 32 which
are sufficiently high to ensure a reliable and sensi-
tive measurement of induction.
Each small coil 32 may be inserted in small
notches 132 made in the terminal face 121 of the
pole pieces 12 of the stator (Fig. 4). However, accor-
ding to a variant embodiment, the small coils 32,
of which the thickness is much less than the width
E of the average air gap 4, are simply connected
to the terminal faces 121 of the pole pieces 12 and
are fixed on these faces 121 for example via a small
layer of adhesive 232. Fixation of the small measuring
coils 32 is facilitated by the fact that tne diameter
of a magnetic bearing at the level of the air gap
4 is generally relatively large.
It will be noted that the device described
hereinbefore requires no rotating contact insofar
as all the coils are placed on the stator 1, whether
it is question of the windings 13 of electro-magnets
of the bearing or of the small coils 32 for taking
signals~ Consequently, practical embodiment is particu-
larly simple and very reliable.
A magnetic bearing equipped with the induc-
tion measuring device according to the invention
may be used for example within the framework of the
application described in French Patent Application
No. 83 18435 filed on November 18, 1983 and entitled:
"Device for measuring the longitudinal voltage of
a strip of material". In this application, which
5Ei~
--8--
may concern for exarnple the measurement o:E the forces
of traction in a rolling mill, the rneasuremen-t o:f
the induction in the air gap of a bearing according
to the present invention may be replaced by the measure-
ment of the current in the windings of the activemagnetic bearing serving as force detector in order
to deliver a signal as a function of the effect of
traction exerted on the strip of material in abutment
on a roller mounted on said active magnetic bearing.
