Optical scanning device

An optical scanning device scans recording tracks on a record carrier with the aid of a beam of radiation, while an objective, which concentrates the beam of radiation to a scanning spot, performs automatic electrically controlled movements in a direction perpendicular to the plane of the record carrier with the aid of a focussing bearing arrangement, so as to ensure a continuous focussing of the scanning spot on the recording tracks. Moreover, in order to follow the oscillations of the recording track, the objective also performs electrically controlled movements which are such that the scanning spot moves at least substantially parallel to the plane of the record carrier.

The invention relates to an optical scanning device which is suitable for 
scanning recording tracks in a recording surface of a record carrier with 
the aid of a beam of radiation which is obtained from a radiation source, 
and more specifically for scanning video and/or audio recording tracks in 
a reflecting recording surface of a rotating video or audio disc with the 
aid of a light beam, comprising: 
A frame, 
An objective with a lens system and an optical axis, suitable for 
concentrating the beam of radiation to a scanning spot in a focussing 
plane, 
A focussing bearing-arrangement for enabling focussing movements of the 
objective relative to the frame in a direction which at least 
substantially coincides with the optical axis, so as to enable incidental 
deviations from a coarse position of the recording plane of the record 
carrier in a direction perpendicular to the recording plane to be followed 
with the focussing plane, and 
Electrically controllable focussing means for electrically effecting and 
controlling the focussing movements of the objective. 
Such an optical scanning device, intended for use in a video disc player, 
is known. In a series of articles concerning the Philips VLP video disc 
system in the magazine "Philips Technical Review", Vol. 33, 1973, No. 7, 
pages 178-193, a scanning device is described in which said electrically 
controllable focussing means for controlling the focussing movements 
comprise a magnetic circuit with an axially magnetized permanent magnet 
with a central opening having soft-iron end plates at the two axial ends, 
as well as a hollow cylindrical soft-iron core which is disposed in the 
central opening. A cylindrical coil arrangement which is coaxially 
disposed around the soft-iron core is rigidly connected to the objective 
and is axially movable in an annular air gap around the soft-iron core. 
The focussing bearing arrangement of the objective employs a number of 
leaf springs which each are connected to the objective with their one end 
and to the frame with their other end, so that a limited movability of the 
objective in an axial direction is obtained, but movements in a radial 
direction are substantially prevented. 
For playing a video disc it is necessary that in addition to means for 
focussing the beam of radiation onto the recording track means are 
provided for continuously following the track in a radial direction by 
means of the very small scanning spot which is projected onto the disc by 
the objective. Oscillations in a radial direction of the track are 
produced owing to irregularities in the recording process and owing to 
axial and radial movements of the disc during its rotation. Therefore, in 
addition to a focussing bearing-arrangement for the purpose of focussing, 
provisions are necessary for following the track of the disc. This may for 
example be effected, as is also described in the previously cited 
magazine, with the aid of a movable mirror which is disposed in the 
radiation path of a laser beam. The mirror can be controlled under the 
influence of a control signal which is related to the deviation of the 
scanning spot relative to the track on the disc. Such mirrors can deflect 
the laser beam through a small angle, so that the objective should also be 
capable of handling and focussing laser beams which do not enter the 
objective perfectly along the optical axis and which moreover may be 
slightly eccentric. This demands a lens system which is of good quality 
over a certain "field". For said known objective a circular field with a 
diameter of approx. 400 microns is needed. 
Realizing a field of such dimensions is a stringent optical requirement for 
a lens system which must be capable of focussing a laser beam to a read 
spot with a diameter of approx. 0.9 micron. It is an object of the 
invention to provide an optical scanning device, of the type mentioned in 
the preamble which imposes less stringent requirements on the lens system 
of the objective (in particular in respect of the field dimensions) and 
which, depending on the embodiment, also has further advantages. The 
invention is characterized in that the scanning apparatus furthermore 
comprises: 
a tracking bearing-arrangement for enabling tracking movements of the 
objective relative to the frame, so as to enable incidental deviations 
from a coarse position of the recording track in a direction normal to the 
recording track and disposed in the recording plane to be followed with 
the scanning spot, and 
electrically controllable tracking means for electrically effecting and 
controlling the tracking movements of the objective. 
In a scanning device in accordance with the invention it is possible to 
ensure by means of the said electrically controllable tracking means that 
the optical axis of the objective always follows the recording track. As a 
result of this a smaller field is needed with a diameter of approximately 
100 microns instead of a diameter of 400 microns. Such a field dimension 
remains always necessary in view of tolerances and play. Thus the cost of 
the objective is favorable influenced in two manners. In the first place 
the optical specifications can be made less stringent; in the second place 
the requirements imposed on the accurate alignment of the lenses in the 
lens system may be less stringent, though this will be at the expense of a 
part of the available field. 
The invention enables a more compact construction of the means required for 
scanning a record carrier to be obtained with the aid of an embodiment 
which is characterized in that in addition to the lens system further 
optical elements and/or radiation-sensitive electronic elements which are 
needed on the radiation path of the radiation beam are incorporated in the 
objective. For example, when a low-power radiation-source of small size 
(such as a laser diode) suffices, it is possible to include the radiation 
source itself in the objective. In the case of a radiation source of 
higher power and larger size (such as a gas laser) an embodiment may be 
used in which the radiation source is situated outside the objective and a 
filamentary flexible radiation guide or optic fiber is connected to the 
objective at one end and is disposed in the radiation beam from the 
radiation source at its other end. An advantage of this embodiment is that 
although the radiation source is disposed outside the objective and may 
rigidly be connected to the stationary parts of a video player or similar 
apparatus, no movable mirror, electrically controlled via a control 
circuit, is necessary and that furthermore the orientation of the 
radiation beam relative to the lens system is invariable. This results in 
minimal requirements with respect to the required field of the lens 
system. The optic fiber then serves as a flexible connection between the 
objective and the radiation source. Modern optic fibers of glass or 
plastic can combine a high efficiency with sufficient flexibility to 
ensure that no undesired forces are exerted on the moving objective. 
In view of the generally long narrow structure of the objective it is 
difficult in the last two embodiments to find a suitable solution for 
receiving the radiation beam which is reflected by the record carrier and 
aiming this beam at the appropriate radiation-sensitive electronic 
elements. It may be advantageous to employ an embodiment in which in the 
objective and outside the part of the radiation path which is disposed 
between the radiation source and the lens system one or more reflecting 
surfaces are provided for reflecting the reflected radiation beam (for 
example in the transverse direction). In this respect it is advantageous, 
in view of the small space which is generally available, to utilize an 
embodiment which is characterized in that the reflecting surface(s) 
consist(s) of the surface of a single mirror which has a 
radiation-transmitting portion (for example a hole) for transmitting the 
part of the radiation beam which is directed to the lens system. 
For the construction of a scanning apparatus in accordance with the 
invention an embodiment is of significance which is characterized in that 
the focussing bearing-arrangement consists of a parallel guide-arrangement 
and that the tracking bearing arrangement consists of a pivot-bearing 
arrangement which enables a limited pivoting movement of the objective 
relative to the frame about a pivoting axis which is transverse to its 
optical axis. 
In this embodiment the optical axis of the objective follows the positional 
deviations from the track by pivoting movements of the complete objective. 
It is apparent that a suitable location of the pivoting axis will have to 
be selected. The most suitable location depends on the design of the 
apparatus in which the scanning device is to be incorporated. However, 
unless the radiation beam has an orientation which is stable relative to 
the lens system such as when the radiation source is integrated in the 
objective or connected to it through an optic fibre, there is obviously no 
point in arranging the pivoting axis in the principal plane of the lens 
system. As is known from theoretical optics the principal plane is to be 
understood to means a plane which may be assumed to contain a single 
imaginary lens which has the same properties as the actual lens system. 
Limited pivoting movements of the lens system relative to a stationary 
radiation source about a pivoting axis in the principal plane do not 
result in movements of the scanning spot. 
As previously stated, a scanning device is known in which the electrically 
controllable focussing means for electrically effecting and controlling 
the focussing movements of the objective comprise: a permanent magnetic 
circuit which comprises an axially magnetized permanent-magnet with a 
central opening and with soft-iron end plates at the two axial ends, as 
well as a hollow cylindrical soft-iron core fitted in the central opening 
and a cylindrical coil construction which is rigidly connected to the 
objective and is disposed co-axially around the soft-iron core, which 
construction is axially movable in an annular air gap around the soft-iron 
core. In this respect an embodiment is of advantage which is characterized 
in that the focussing bearing-arrangement consists of a sleeve bearing 
arrangement comprising a first bearing bush which is connected to the 
frame and a second bearing bush which is connected to the objective and 
which is movable relative to the first bearing bush, and that the first 
bearing bush is pivotably connected to the frame by means of the tracking 
bearing-arrangement and the said coil construction comprises two tracking 
coils which are disposed symmetrically one on each side of the pivoting 
axis. Preferably, in view of a high efficiency of the coil construction, 
an embodiment is used which is characterized in that between each of the 
two soft-iron end plates and the cylindrical soft-iron core an annular air 
gap is formed and that of each tracking coil a part of the turns is 
disposed in the one air gap and an other part of the turns in the other 
air gap, in such a way that the said parts of the turns provide an equally 
directed contribution to an electrically generated pivoting torque. 
Another method of pivoting the objective is possible with an embodiment of 
the invention which is characterized in that the focussing 
bearing-arrangement consists of a parallel guide-arrangement and that the 
tracking bearing-arrangement consists of a parallel guide-arrangement and 
that the tracking bearing-arrangement consists of a pivot-bearing 
arrangement for enabling a limited pivoting movement of the objective 
relative to the frame about a pivoting axis which is disposed parallel to 
and at some distance from the optical axis. By means of this embodiment it 
is possible, as is shown in the drawing, to realize a scanning device in 
which the radiation source is rigidly connected to the frame and focussing 
and tracking are effected exclusively by movements of the objective 
without the help of other optical elements or an optical fiber. 
A suitable and simple construction can be obtained if the device comprises 
a movable objective support, the focussing bearing-arrangement movably 
connects the objective support to the frame, and the tracking 
bearing-arrangement movably connects the objective to the objective 
support. 
In particular when the bandwidth of the scanning device need not comply 
with excessively stringent requirements, for example when the device 
serves for scanning audio recording tracks of an optically readable audio 
disc, it may be advantageous to use an embodiment in which the focussing 
bearing-arrangement consists of a number of identical and parallel leaf 
springs which are spaced from each other, which springs with one of their 
ends are connected to the frame and with their other ends to the objective 
support, and in which furthermore the tracking bearing arrangement 
consists of a number of identical leaf springs which are spaced from each 
other in a crosswise fashion, which springs with one of their ends are 
connected to the objective support and with their other ends to the 
objective. These steps result in a simple robust construction which 
requires no maintenance. In order to avoid unnecessary loss of power and 
parasitic resonances it is favorable when the electrically controllable 
focussing means consist of a cylindrical axially magnetized permanent 
magnet which is connected to the frame, and an annular focussing coil 
which is connected to the objective support and which is disposed 
concentrically around said magnet, said magnet and coil both being coaxial 
with the objective. 
A high efficiency of the electrical tracking means is attainable with an 
embodiment which is characterized in that the electrically controllable 
tracking means consist of an assembly of two cylindrical axially 
magnetized permanent magnets, which assembly is connected to the objective 
support, which magnets are mounted against each other with ends of like 
polarity, and of an annular coil which is connected to the objective and 
which is concentrically arranged around said assembly, and that the axis 
of these tracking means crosses the optical axis of the objective at some 
distance and opposite the pivoting axis of the tracking 
bearing-arrangement.

In FIG. 1 an optical scanning device bears the general reference numeral 1. 
It comprises a frame 2, which frame includes all parts of the device which 
do not move along with an objective 3. The scanning device forms part of a 
video disc player which is suitable for playing a video disc 4 which is 
disposed on a rotating spindle 5. The video disc consists of a transparent 
part 6 as well as a protective coating 7 (which need not be transparent) 
between which a recording surface with an extremely thin reflecting layer 
8 which is provided with the video information in the form of small 
recesses and/or raised portions. The frame 2 of the scanning device is 
movably mounted on a base plate 9 of the video player. It is movable in a 
slot 10 with the aid of an electric motor 11 which via two conical gear 
wheels 12 and 13, of which the last-mentioned gear wheel contains an 
internal nut, moves a lead screw 14. The recording track on the video disc 
4 takes the form of a spiral, the consecutive turns of the spiral being 
situated very near to each other at a distance of the order of magnitude 
of a few microns. 
With the aid of a gas laser 15 a powerful radiation beam is produced which 
via a radiation guide 16 is led to the underside of the objective 3. Such 
radiation guides are generally known in optics; they consist of a 
transparent core surrounded by a transparent cladding of a material with a 
different refractive index than the core, so that the radiation once it is 
present in the core cannot emerge via the cladding. The part of the 
radiation beam which emerges from the objective is shown in the drawing 
and bears the reference numeral 17. It is one of the functions of the 
scanning device to focus this part of the radiation beam onto the layer 8, 
in other words to ensure that the focussing plane of the objective and the 
recording plane coincide as far as possible while the disc is being 
played. For this purpose the objective is movable to and fro in the 
direction of its optical axis. in the drawing this is symbolically 
represented by a double-headed arrow with the reference numeral 18. The 
objective is moreover pivotable about an axis which is perpendicular to 
the plane of drawing, which pivoting movement is indicated by the curved 
double-headed arrow 19. This pivoting movement serves to keep the scanning 
spot 20 of the radiation beam 17 constantly aimed at the recording track 
of the video disc. During the rotation of the video disc the track will 
oscillate in a direction transverse to the axis of rotation 22 of the 
spindle 5 owing to irregularities in the course of the track on the disc 
and owing to the eccentricity of the central opening 21 and the spindle 5. 
These oscillations are generally referred to as "radial oscillations", 
while following these oscillations with the radiation spot 20 is generally 
referred to by the term "radial tracking". The objective 3 is included in 
two control circuits that assist in effecting the necessary focussing and 
radial tracking. These circuits will not be described in more detail in 
the present Application, because they are irrelevant for the invention; 
however, for information in respect of focussing and radial-tracking 
techniques which may be used, reference is made to the article in the 
previously mentioned magazine. 
The objective 3 accommodates all the optical elements as well as the 
electronic radiation-sensitive elements which are necessary for detecting 
the position of the scanning spot and for scanning the video and audio 
information contained in the layer 8. The objective 3 and the frame 2 have 
electro-magnetic focussing and tracking means which co-operate with each 
other for moving the objective to and fro and pivoting it in an 
electrically controlled fashion. Via a multi-pole connector 23, the 
electronic elements in the objective and said the said electromagnetic 
means are connected to electronic circuits, which are accommodated in the 
video player and which are symbolically represented in a drawing as a box 
24. Via a multi-core electrical conductor 25 and a multi-pole connector 26 
the connector 23 is connected to the box 24. Via an electrical conductor 
27 and the connectors 28 and 29 for power supply and control the motor 11 
is also connected to the box 24. The motor 11 need not be controlled so 
that the focussing-tracking device 1 is constantly moved with a speed 
which corresponds to the average pitch of the track on the video disc 4. 
It is alternatively possible to move the focussing-tracking device 
intermittently, tracking being effected by means of the pivoting movement 
of the objective 3 during the time that said device is stationary. 
FIG. 2 shows how a scanning device may be constructed. The frame of this 
scanning device comprises an axially magnetized annular permanent magnet 
30, two soft-iron end-plates 31 and 32 at the two axial ends, one on each 
side of the magnet, as well as a hollow cylindrical soft-iron core 33 
which is fitted in the central opening 34 of the permanent magnet. Between 
the upper end plate 32 and the core 33 an annular air-gap 35 is formed. In 
this gap a cylindrical coil construction 37 is movable coil 37 is rigidly 
connected to the objective 36 and is coaxially disposed around the 
soft-iron core 33. The construction 37 is connected to a bearing bush 39 
which with the objective is movable in the direction of its optical axis 
38, which bush together with a bush 40 which is rigidly connected to the 
frame serves for the parallel guidance of the objective and constitutes 
the focussing bearing-arrangement. 
On top of the end plate 32 a plastic cover 41 is disposed to which a rubber 
cuff 42 is attached. This cuff has a number of concentric folds 43 and 44 
and is connected to the objective 36. The shape of the cuff is such that 
it ensures that movements in the direction of the optical axis 38 and 
slight pivoting movements are only slightly impeded. 
Around the coil construction 37 an annular disc 45 is mounted which at two 
oppodite sides, also see FIG. 3, has local notches 46. These notches 
cooperate with locally formed ridges 47 of the cover 41, thus preventing 
the objective 36 together with the bearing bush 39 as a whole from 
rotating about its optical axis 38. 
The objective is pivotable relative to the bearing bush 39 about a pivoting 
axis 48. For this purpose a pivot bearing-arrangement is provided which 
comprises two bearing pins 49 as well as two bearing bushes 50. For 
controlling the pivoting movements of the objective two annular tracking 
coils 51 are mounted underneath the bottom 53 of the objective with the 
aid of supports 52, also see FIG. 4. The coils 51 are arranged around two 
cylindrical axially magnetized permanent magnets 54 and 55 with play. The 
poles of these permanent magnets are located at their ends. They are 
mounted with like poles facing each other and against each other; their 
other ends are mounted in a soft iron ring 56. This method of mounting 
results in a radially directed permanent magnetic field at the location 
where the like poles of the magnets 54 and 55 adjoin each other, so that 
upon energization of the coils 51 these are subjected to a force in 
accordance with the axis of the cylindrical magnets. The direction of the 
current then determines the direction of pivoting. 
In the bottom 53 of the objective 36 a light guide 59 is clamped with the 
aid of a threaded cap 57 and a deformable plastic ring 58. This light 
guide is mounted in the center of the bottom 53 and directs a radiation 
beam towards the other end of the objective 36 via the optical axis 38. At 
this end a lens system is located which consists of a single aspherical 
lens 60, which by means of a threaded cap 61 is fitted in a lens mount 62. 
In addition to this lens a number of further optical elements are 
accommodated in the objective, namely a quarter-wavelength plate 63, a 
Wollaston prism 64, as well as a mirror 66 in which an opening 65 is 
formed. This mirror is provided with a reflecting surface 67. Furthermore, 
the objective accommodates a stop plate 68 which stops out half the 
radiation beam which emerges from the light guide 59, as well as two 
radiation-sensitive diodes 69 and 70 which serve for converting a 
light-beam modulation into high-frequency video and audio information and 
into information about the position of the focussing plate relative to the 
recording plane. 
The electrical connections of the coils 51, the diodes 69, 70 and the coil 
37 are not shown in the drawing. However, the connecting wires of the 
coils 51 and the diodes 69 and 70 are preferably led upwards through the 
wall of the bearing bush 39 near the bearing pins 49, so that they can be 
attached to mounting pins 71 (see FIG. 3) which are fitted in the cover 
41. 
The optical elements present in the objective 36, the diodes 69, 70 as well 
as the stop plate 68 accommodated in the objective 36 together constitute 
a device for reading optical information about which comprehensive data 
can be found in the Applicant's previously application Ser. No. 543,111 
filed Jan. 22, 1975, now U.S. Pat. No. 3,969,576, (herewith incorporated 
by reference). Said Application also describes how automatic focussing of 
the objective 36 is effected. For automatic tracking use is made of a 
system as described in the Netherlands patent application No. 7,401,470, 
which has been laid open for public inspection. This system employs a 
periodic small amplitude oscillation of the information track on the 
record carrier, as a result of which a signal of small amplitude is 
superimposed on the light-beam modulation. This superimposed signal 
contains information about the position of the scanning spot relative to 
the recording track. 
The objective of the scanning device in FIG. 5 bears the reference numeral 
72. The permanent magnetic circuit has an axially magnetized 
permanent-magnet 73 with a central opening 74 and two soft-iron end plates 
75 and 76 at the two axial ends. A hollow soft-iron core 77 is mounted in 
the central opening 74. A cylindrical coil construction 78 which is 
coaxially arranged around the soft-iron core 77 is fixed to the objective 
72 and is axially movable in the annular air gap 79 between the end plate 
75 and the core 77 and a second annular air gap 80 between the end plate 
76 and the core 77. The focussing bearing-arrangement is a sleeve bearing 
arrangement for the parallel guidance of the objective 72 and comprises a 
first bearing bush 81 which is connected to the objective and which is 
axially movable relative to the bearing bush 81. The bearing bush 81 is 
rigidly connected to the core 77 with the aid of two bearing pins 83. 
These bearing pins are rigidly mounted in the core 77, while two bearing 
bushes 84, which are rigidly mounted in the bearing bush 81, are pivotable 
on the pins. In the present embodiment the bearing bush 81, as previously 
stated, is rigidly connected to the objective 72 and movable in the 
bearing bush 81. Of course, the outer wall of the objective 72 itself may 
alternatively be used as a part of the sleeve bearing-arrangement. 
The major advantage of the construction in accordance with FIG. 5 is that 
the objective 72 is movable in the direction of its optical axis 85 and, 
for the tracking movement, about the pivoting axis 86 with the aid of a 
single electro-magnetic system only, the permanent magnet 73 performing a 
dual function. In comparison with the embodiment of FIG. 2 the mass to be 
moved in the direction of the optical axis 85 is thus reduced. The coil 
construction 78 is provided with two coils 87 which are symmetrically 
disposed one on each side of the pivoting axis 86, also see FIG. 6, which 
coils serve both for focussing and for tracking. With the aid of a 
mounting plate 88, see FIG. 5, on which the cylindrical soft-iron core 77 
is mounted, a construction is obtained which is such that between each of 
the end plates 75 and 76 and the core 77 an annular air gap is formed, 
namely the air gaps 79 and 80. Thus, a high efficiency of the 
electro-magnetic means for the axial movement and for pivoting the 
objective 72 can be obtained. Of each coil 87 a part 89 of the turns, see 
FIG. 6, extend into the air gap 79 and an other part 90 in the air gap 80. 
These parts 89 and 90 are situated so that they provide an equally 
directed contribution to an electrically generated torque about the 
pivoting axis 86. 
The electromagnetic forces which are exerted on the parts 89 and 90 of the 
coils 87 are directed axially. Focussing movements can be effected if the 
current directions through the corresponding parts 89 and 90 of the two 
coils are selected so that the axial forces have the same direction and 
are equal. If this is not the case a pivoting movement of the objective is 
obtained, which may be used for the purpose of tracking. 
FIGS. 7 to 10 relate to a scanning device in which an objective 91 is 
pivotable about a pivoting axis 92 parallel to the optical axis 93 of the 
objective. A movable objective support 94 is connected to a frame 99 by 
means of a focussing bearing-arrangement 95 consisting of two leaf springs 
96 which are arranged parallel to each other and with the aid of screws 97 
and clamping plates 98. This frame consists of a base plate 100 on which a 
support 101 is secured with bolts 102. 
The tracking bearing-arrangement 103 connects the objective 91 movably to 
the objective support 94 and consists of four identical leaf springs 104 
which are spaced from each other in a crosswise fashion, which springs are 
glued to the objective support 94 with one of their ends and are glued to 
the objective 91 with their other ends. 
The electrically controllable focussing means 105 consists of an axially 
magnetized permanent magnet 106 which is glued to the frame 99, and an 
annular focussing coil 107 which is arranged concentrically around said 
magnet and is glued to the objective support 94. Both the magnet and the 
coil are coaxial with the objective 91 and in the magnet 106 a hole 108 is 
formed for the passage of a radiation beam obtained from a stationary 
radiation source, not shown. The electrically controllable tracking means 
109 consists of an assembly of two cylindrical axially magnetized 
permanent magnets 110 connected to the objective support 94, which magnets 
are mounted against each other with ends of like polarity, and of an 
annular coil 111 which is concentrically arranged around this assembly and 
connected to the objective 91. The axis 112 of these tracking means 
crosses the optical axis 93 of the objective 91 at some distance and 
opposite the pivoting axis 92 of the tracking bearing-arrangement 103. 
For the fixation of the two permanent magnets 110 on the objective support 
94 this support is provided with two tabs 113. The two magnets have a bore 
through which a bolt 114 is passed which clamps the magnets between the 
tabs 113. 
The coil 111 is glued in a holder 115 which is soldered to a sleeve 116 in 
which the objective 91 is fitted. The leaf springs 104 are connected to 
the objective via this sleeve.