Optically readable storage disc

An interchangeable optically readable storage disk has a substrate with a recording surface, and a cover thereon which may comprise a second substrate with a further recording layer. The disk includes a protecting arrangement at the circumference of the substrate, which includes a balancing arrangement. The balancing may be effected by apertures in the protecting arrangement, or balancing masses affixed thereto.

The invention relates to an interchangeable optically readable storage 
disc, comprising: 
a flat transparent substrate having flat sides and a circular periphery, 
a recording layer which can be modified locally by a radiation beam and 
which is situated on a flat side of the substrate, 
a cover which is secured to a flat side of the substrate near the 
circumference by annular means at a distance from at least that part of 
the recording layer which is intended for recording, in such a way that a 
space is enclosed between the substrate and the cover, and 
protective circumferential portions which form part of the annular means 
and which comprise an outer edge portion with an external diameter which 
is larger than that of the circumference of the substrate. 
Such a storage disc is disclosed in, for example, Japanese Patent 
Application No. 57-172541 (Patents Abstracts of Japan, Vol. 7, No. 
18(8-170), (1163), Jan. 25, 1983). The storage disc comprises two flat 
disc-members which are secured to each other by an adhesive with the aid 
of interposed concentric annular means in the form of annular spacers. 
These annular means are arranged near a centre hole and near the 
circumference of the storage disc. 
A storage disc which has been used most frequently until now comprises two 
glass substrates and interposed concentric annular spacers which are 
secured to each other by means of an adhesive, see U.S. Pat. No. 4,074,282 
(herewith incorporated by reference). On the side which faces the sealed 
space each of the glass substrates is provided with a recording layer. 
Thus a two-sided storage disc is obtained, one substrate serving as the 
cover for the other substrate. Another storage disc of the specified type 
is described in the applicant's previously filed patent applications Ser. 
No. 490,801 filed May 2, 1983 and Ser. No. 485,530 filed June 15, 1983. 
This storage disc comprises only one transparent substrate. The cover is 
made of a thin aluminium sheet material and the annular means are integral 
with the cover. The cover is hat-shaped, the brim of the hat having the 
function of said annular means and the other portion being spaced from the 
substrate. 
Especially when glass substrates are employed problems may arise due to the 
eccentric location of the mass centre of the storage disc. This may be 
attributed to various causes. Firstly, glass has a comparatively high 
specific mass, so that slight eccentricities are liable to give rise to 
comparatively large out-of-balance forces. A second cause is that it is 
difficult to manufacture a glass substrate having a perfectly plane 
parallel shape. The cross-section of the substrate will be substantially 
wedge-shaped, resulting in an eccentric mass distribution of the 
substrate. A further cause is the fact that storage discs comprising glass 
substrates may have diameters of up to substantially 30 cm and are rotated 
with comparatively high speeds up to 25 or 30 revolutions/sec. In the case 
of a one-sided storage disc, even if it is not made of glass but for 
example of a plastic, it is rather difficult to ensure that the mass 
centre coincides exactly with the geometrical centre. When a plurality of 
disc members, such as a substrate and a cover or two substrates are 
secured to each other, the problems multiply. 
The invention aims at providing an interchangeable optically readable 
storage disc of the type specified in the opening paragraph with a minimal 
eccentricity of the mass centre. The invention is characterized in that 
the protective circumferential means are also employed as balancing means 
for correcting any eccentric position of the mass centre by locally and 
selectively removing portions from the circumferential means or by 
securing balancing mass to the circumferential means. 
The advantage of the invention is that the optically readable storage disc 
is balanced by means of a part which is present anyway, namely the 
protective circumferential means. In many cases the substrate in itself is 
not suitable for balancing purposes, for example, for mounting an 
additional balancing mass on the substrate or, conversely, removing mass 
from the substrate. This applies in particular to a glass substrate. A 
part which is separate from the substrate and which is disposed on the 
circumference of the storage disc is better suited for this purpose. 
A further advantage is that balancing is effected at the periphery, where 
the effect of removing or adding mass is largest. In particular in the 
case of interchangeable storage discs, to which the scope of the invention 
is limited, this is an advantage, because these storage discs have no 
portions near the centre of the disc which may be used for this purpose. 
In the case of magnetic-disc memories with fixed storage discs balancing 
may be effected at the location of a central hub (see for example U.S. 
Pat. No. 3,838,464). 
An advantageous embodiment of the invention is characterized in that said 
annular means and the circumferential means are together constituted by a 
flat ring. This is the simplest embodiment of the invention. The 
circumferential means together with the annular means comprise merely a 
flat ring of minimal mass, so that they contribute only to a small extent 
to the mass inertia and to the eccentricity of the mass centre of the 
storage disc. 
Another embodiment of the invention is characterized in that the annular 
means are provided with outer circumferential edge portions which extend 
perpendicularly to the annular edge portions at some radial distance from 
the circumference of the substrate and balancing mass is situated against 
the outer edge portions. Said outer edge portions can take up the 
centrifugal faces produced by the balancing mass. In this respect an 
embodiment of the invention may be advantageous, which is characterized in 
that balancing mass is situated on those sides of the outer edge portions 
which face the centre of the storage disc. The centrifugal forces now urge 
the balancing mass against the outer edge portions. 
For mounting and securing balancing mass comparatively simple techniques 
may be used. If the outer edge portions have the shape of an uninterrupted 
circular outer edge, this provides total freedom as regards the location 
where balancing mass is removed or added. Preferably, the mass 
distribution of the storage disc remains substantially symmetrical despite 
the presence of the circumferential means. This has the advantage that if 
the storage disc should be balanced on one side only it is irrelevant 
which side faces upwards during balancing on the balancing machine. Some 
storage discs are centered on one side with centering means which differ 
from the centering means for the other side. In that case the storage disc 
cannot be balanced perfectly and a compromise must be found between the 
two balancing operations which are necessary. Therefore, it is important 
that balancing mass can be removed or added on each of the two sides of 
the storage disc.

The invention will now be described in more detail, by way of example, with 
reference to the drawings in which FIGS. 1 to 6 are perspective, and 
partly sectional views of six interchangeable optically readable storage 
discs embodying the invention. 
In the various FIGS. corresponding parts bear the same reference numerals 
to which the figure number has been prefixed. 
The storage disc shown in FIG. 1 comprises a transparent substrate 11 on 
which a recording layer 12 has been deposited, which layer can be modified 
by a radiation beam. The cover 13 is secured to the substrate 11 at some 
distance from the recording layer by annular means 14 and 15, so that a 
sealed space 16 is enclosed between the substrate and the cover disc. The 
cover 13 is identical to the transparent substrate 11 and has also been 
provided with a recording layer, not shown. Thus, a two-sided storage disc 
is obtained, one substrate forming a cover for the other substrate. The 
annular means 14 and 15 comprise concentric annular spacers as disclosed 
in the afore-mentioned U.S. Pat. No. 4,074,282. The annular spacer 14 
comprises a portion 17 which projects from the circumference of the two 
substrates 11 and 13 and which serves as a balancing means for correcting 
any eccentric position of the mass centre of the storage disc. For this 
purpose holes 18 may be formed locally in the portion 17, so that material 
is removed locally. In many cases the mass of the portion 17 is low in 
comparison with the total mass of the storage disc, so that the local 
removal of portions of the circumferential means has a comparatively small 
influence on the location of the mass centre of the storage disc. In such 
a case the embodiment shown in FIG. 1 may be modified slightly by securing 
balancing mass, for example lead, to the portion 17, if desired by use of 
the holes 18. The holes 18 may then be used for anchoring the balancing 
mass to the portion 17. 
The storage disc shown in FIG. 2 is a modification of the storage disc 
shown in FIG. 1. The difference is that the protective circumferential 
means 27 are provided with circumferential outer edge portion 27A which 
extends perpendicularly to the annular edge portion at some radial 
distance from the substrates 21 and 23, as known per se in a different 
version from the above-mentioned Japanese Patent Application No. 
57-172541. The balancing means comprise balancing mass in the form of 
separate balancing portions 28 arranged on the inner side against the 
outer circumferential portion 27A. The circumferential outer edge portion 
constitutes an uninterrupted cylindrical wall which extends equally far to 
both sides of the annular surface. In this way the storage disc has a 
symmetrical construction and balancing mass may be arranged on both sides 
of the storage disc. In practice this will often be necessary in the case 
of a storage disc which, like the storage disc shown in FIG. 2, comprises 
two separate substrates if the substrates are made of a comparatively 
heavy material such as glass. Balancing is effected on a balancing machine 
by first providing one side of the storage disc with balancing portions 28 
during this balancing operation. Subsequently, the storage disc is placed 
on the balancing machine in the turned-over position. The storage disc is 
always centred in the centre hole of the relevant substrate, so that after 
the storage disc has been turned over, it must be possible to correct the 
balancing already performed. This may be effected by mounting further 
balancing portions on the second side of the storage disc. 
In principle many modifications of the storage disc shown in FIGS. 1 and 2 
are possible. FIG. 3 shows an example of such a modification. The annular 
outer edge portion 37 now has a different shape. In cross-section the 
assembly comprising the annular means 34 and the outer edge portion 37 has 
the shape of a fork. The balancing mass may be arranged between the 
flanges 37A and 37B. 
FIGS. 4, 5 and 6 relate to embodiments of the invention which are based on 
an optically readable storage disc as desired in the Applicant's 
aforementioned previous patent application Ser. No. 490,801 and Ser. No. 
485,530. 
FIG. 4 shows a substrate 41 made of glass or a transparent plastic provided 
with a recording layer 42. A hat-shaped cover 43 made of thin aluminium 
sheet or another suitable material is secured to the substrate 41 so as to 
enclose a sealed space 46 between the substrate and the cover. The brim 44 
of the hat-shaped cover 43 constitutes an annular means and is secured to 
the substrate 41 by means of an adhesive. On its circumference the brim 44 
has circumferential means in the form of an annular edge portion 47 which 
is employed for balancing the storage disc. It comprises a crimped portion 
of the cover disc 43, which adjoins a portion 49 which is sprung over the 
edge of the substrate 41. For the purpose of balancing the storage disc 
holes 48 may be formed in the outer rim 47 by means of a suitable 
operation such as, for example, grinding or nibbling. 
In the modification shown in FIG. 5 the circumferential means 57 is 
provided with a circumferential cylindrical outer rim 57A. Between this 
outer rim and the substrate, centring mass in the form of centring 
portions 58 may be arranged in the same way as with the storage disc shown 
in FIG. 2. 
The modification shown in FIG. 6 uses annular means which are separate from 
the cover disc 63 and which clamp the substrate 61 and the cover disc 63 
onto each other at their circumference. There are provided circumferential 
means 67 with a cylindrical outer rim 67A, so that in the same way as the 
storage disc shown in FIG. 2 balancing mass in the form of balancing 
portions 68 may be arranged on the inner side of the outer rim. 
Within the scope of the invention many modifications are possible in which 
the protective circumferential means which are present anyway for securing 
the cover to the substrate may be used for balancing the storage disc. 
In order to avoid thermal stresses in the substrate or substrates or damage 
as a result of shocks or falling, it may often be desirable to construct 
the protective circumferential means in a manner as described in 
Applicant's patent application Ser. No. 623,048 filed June 21, 1984, 
(herewith incorporated by reference). According to said application care 
is taken that between the circumference of the substrate and the 
circumferential means a space is formed in which substantially no 
intermediate means are present which in the case of deformation of said 
outer edge can transmit a force which is directed towards the centre of 
the storage disc to the circumference of the substrate or substrates and 
which has such a magnitude that it may cause damage to the substrate. 
The presence of the balancing portions 28, 58 and 68 will generally be 
acceptable in this respect, because only over a very small part of the 
circumference of the storage disc these portions can give rise to forces 
which act on the circumference of the substrate. However, as is shown in 
FIG. 2, it is even better to provide some space between the balancing 
portions and the circumference of the substrate. The embodiment shown in 
FIG. 3 is also favourable in this respect.