Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
Device for the o2~ al
:E~At~K~ROUND OF T~IE INVE~TION
The invention re]a~es to the field o data
storage on supports or media which can be optical.ly
read by a concentrated beam and more specifi.cally
re].ates to a device for scanning a track carried by
a medium which can be recorded or read by means of
a beam of radiation from a radiation source~ The
combination of the advances which have occurred in
connection with lasersS thin layers and control systems
has made it possible to obtain a data medium, which
can be read by means of a beam of radiation on which
can be recorded 101 data bits. The medlum is in the
form of a disk with a diameter of about 30cm. Such a
medium can be used as a memory for video andlor audio
signals or as an informatic memory. In the latter
application, it is necessary to have a device giving
rapid access to the track either for recording the
information at a predetermined point on a previously
recorded smooth track, or for reading data recorded
at a random point on the track. By its very nature,
the invention is more particularly used in the field
of informatics.
The prior art scanning devices were constructed
so as to give access to data in a few seconds and
although they are adequate for data of the video signal
type9 they are not adequate when used in informatics~
In the prior art scanning devices, -the correct radio
positioning of the optical reading head is ensured
by mechanical means, whlch either bring about the
--1--
displacement of the disk or of the reading head.
~en the reading head and dis`k are correctly
positioned relative to one another, it is possible
to radially follow the track on which the data is
recorded by means of a galvano.~eter mirror, which
moves around an a~is parallel to the plane of the
disk, which reflects the radiation beam towards
the reading lens. The vertical control, so that the
beam of reading light îs correctly focused on the
disk, is provided by a linear motor of the electro-
dynamic type, which moves the lens.
BRIEF SUMMARY ~F THE INVENTI0~
The invention more particularly relates to the
arrangement of the motor relative to the lens.
The invention is remarkable in the sense that
it makes it possible to reduce the weight of the
magnetic pot used for creating the magnetic field
in which the moving coil of the motor is immersed.
This weight reduction leads to an easier and faster
positioning of the moving reading head in the radial
direction with respect to the support disk. Thus7 in
the prior art, it was virtually impossible to make
the reading head weight below about 100 grams~ DLIe
to the fact that the weight which has to be moved is
high, long access times correspond thereto. A reading
head according to the invention also makes it possible
to obtain a more compact shape of the motor -- lens
assembly~ so that the moving element is less sensitive
to undesired mechanical resonances~
The invention therefore specifically relates to
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a device for the optical scanning o~ a data medium
by optical radiation focused by Means of an objective,
whose displacement along its optical a~is is ensured
by an electrodynamic motor, whlch comprises a
magnet with an annular air gap and a moving coil
which is immersed in said air gap, wherein the
objective is mounted in a pod forming an extension
o~ the coil, said pod containing a deviating mirror
for deviating the said radiation and which can slide
along the optical axis in a sleeve fixed to the
outer pole part of the magnet.
~1~
The invention is described in greater detail
hereinater relative to non limitative embodiments
and the attached drawings, wherein showo
Fig 1 diagrammatically, a simplified optical reading
head according to the prior art.
Fig 2 diagrammatically, an optlcal reading head
according to the invention.0 Fig 3 a part sectional view of an embodiment of an
optical reading head according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIME~TS
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The scanning device is specifically described
in connection with the optical reading of supports
or media on which data is recorded. However, the
same scanning device can be used for recording data
on a medium on which a smooth track has been previously
recorded, the data being written on said track~
Fig 1 diagrammatically shows a prior art optical
reading device. Such a readin~ head 1 comprises an
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electrodynamic motor having a-l annular permanent
magnet ~, an outer pole piece 5 resting on the
upper face of magnet ~, as well as a central pole
piece 6, whereof part exactly covers the lower face
of magnet 4 and whereof another part forms a core
of the same height as magnet 4. ~ole pieces 5 and ~
define an annular air gap 7 in which is established
a radial magnetic field coniguration. The motor also
has a moving coil 3 placed coaxially around the
core of pole piece 6 and integral with objective 2.
The coil ~ objective assembly can move axially within
annular air gap 7. Under the influence of an
electric current traversing the winding of the moving
coil~ an electrodynamic force is created in the
direction of the optical axis of objective 2. Objective
2 slides by means of a sleeve 8 traversing the core
of pole piece 6. The frictional resistance to the
sliding movement is made as low as possible by
lining the sliding surfaces with a material formed
from graphite and lead. A light beam from a light
radiation source, after passing through different
optical systems which will not be described in
greater detail within the scope of the invention,
is received by objective 2~ which focuses it in such
a way as to form a scanning spot.
In Fig 1, the rays which emerge from objective
2 converge at one point at which must be locat~d
the surface of a data medium or support. The moving
part constituted mainly by the objective 2 of moving
coil 3 occupies the central part of the scanning device.
--4--
Thereore, the magnetic elements 49 5 and 6 are
placed around objective 2, which leads to large
dimensions and a by no means negligible total
weight, which must be displaced in order to reach
the separate regions of the data medium. It is
scarcely possible for the weight of such a system
to be below about 100 grams for acceptable focusing
performances within the scope of informatics
applications.
The present invention aims at reducing the
overall weight and this is achieved by the central
part of the magnetic assembly no longer containing
the reading objective. Therefore, the structure of
the magnet can be more compact, whilst supplying a
motive force which is as high as that achieved with
the arrangement of Fig 2.
Fig 2 diagrammatically shows a scanning device
according to the invention. Scanning device 1 comprises
a cylindrical permanent magnet 4' and a pole piece 5'
~0 which exactly covers the upper face of the cylindrical
magnet 4'. The lower face of magnet 4' carries another
pole piece 6', which also forms a ring around a magnet
4' and pole piece 5'. Piec-e 6' is radially separated
from core 4' and 5' by an air gap 7. A moving coil
3 is able to move axially about the core constituted
by pieces 4' and 5' described hereinbefore within
the annular air gap 7. An objective-carrying pod 9
is placed in the extension of moving coil 3. It has
a circular opening in its side wall to permit the0 passage of light radiation. This pod may contain
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different optical or electronic systems. Thedrawing shows in an e~emplified and non-limitative
manner a deviating mirror lO for deviating the
reading light radiation by an angle of 90. The
mirror can be supported by the bottom of moving
coil 3 or, when the latter has no bottom, by the
central pole 51. ODjective 2 is installed in the
upper part of pod 9, which ~an slide in the direction
of the optical axis in a sleeve ll attached to the
outer pole piece 6'. Pod 9 can be in two partsS one
serving as a spacer between the bottom of the moving
coil 3 and objective 2, whilst the other 8 forms
an anti-friction lining around the lens. Sleeve ll
may have a window for the passage of the optical
radiation, which faces another window provided in
pod 9. The configuration of pod 9 adopted in Fig 2
ensures that the motive force is applied to the lens
under good conditionsO This makes it possible to
reduce parasitic mechanical vibrations, which can
occur with the configuration of Fig l. These
mechanical vibrations are particularly disturbing
because they correspond to natural resonant frequencies
which may be in the operating band of the scanning
device.
Fig 3 shows a part sectional view of a con-
str~lction of the optical reading head l according to
the invention. The same references as in Fig 2 are
used. It is pointed out that the magnetic parts 4', 5'
and 6' may occupy the central part of the system, due
to the f~ct that objective 2 is located in the extension
--6--
p~
of coil system 3.
Mirror 10, to which reference has been made
hereinbefore, mainly serves to deviate the reading
light radiation. It may a:Lso be provided with an
electromechanical trans~ucer for correcting errors
in the following of a track by the focused spot.
Pod 9 serves as a lens carrier, but it may
also carry the winding of moving coil 3 on its
lower part, which then serves as a mandrel. A light-
weight elastic element may be provided for connectingpod 9 to sleeve 11 for opposing a rotation o the
latter, which can lead to an error in the orientation
of mirror 10. If the mirror is fi~ed to the central
pole 5', the elastic element must keep the two
transmission windows aligned, but this alignment
is not critical.
