CA1188797A

CA1188797A - Opto-electronic apparatus for inscribing and/or reading recording tracks by means of a radiation beam

Info

Publication number: CA1188797A
Application number: CA000406874A
Authority: CA
Inventors: Peter J.M. Janssen; Gerard E. Van Rosmalen
Original assignee: Philips Gloeilampenfabrieken NV
Current assignee: Koninklijke Philips NV
Priority date: 1981-07-10
Filing date: 1982-07-08
Publication date: 1985-06-11
Also published as: EP0186218B1; DE3280200D1; AU8568982A; NL8103305A; JPH0680533B2; EP0070070A1; HK49394A; JPS5817552A; SG20587G; ATE23072T1; ES513804A0; DD206598A5; HK39287A; US4561081A; EP0186218A2; ATE54218T1; EP0070070B1; KR880002696B1; EP0186218A3; DE3273918D1

Abstract

ABSTRACT:

An opto-electronic apparatus for writing and/
or reading recording tracks on a record carrier by means of a radiation beam comprises as objective mounted in an objective holder which is movable in accordance with a number of desired degrees of freedom by means of a bear-ing arrangement and comprises electomagnetic actuator means for driving the objective holder in accordance with the desired directions of movement. The bearing arrangement of the objective holder on the frame com-prises electromagnetic bearing means for counteracting movement in accordance with at least one undesired degree of freedom and comprises an objective-position measuring device for supplying a positional-error sig-nal which represents the deviation of the objective holder in accordance with the undesired degree of free-dom relative to the frame and which is utilized by a levitation control circuit which counteracts movements of the objective holder relative to the frame in accord-ance with the undesired degree of freedom by means of electromagnetic levitation forces.

Description

PHN. 10.095 The invention relates to an opto-electronic appa-ratus for writing and/or reading recording tracks in a recording surface of a record carrier by means of a radia-tion beam, such as the scanning by means of a light beam of video and/or audio recording tracks or digital data recording tracks in a reflecting recording surface of a rotating video or audio disc or a digital data disc respec-tively, which apparatus comprises: a frame; an objective holder provided with an objective having an optical axis and having a lens system for concentrating the radiation beam so as to form a radiation spot in a focussing plane, a bearing arrangemen-t for the objective holder, which arrangement comprises bearing means which permit movements of the objective holder relative to the frame in accordance with a desired number of degrees of freedom out of six theoretically possible, independent degrees of freedom and which substantially counteract those movements of the objective holder relative to the frame which are in accord-ance with the other, undesired degrees of freedom; and actuator means comprising parts which are connected to the frame and to the objective holder respectively and which cooperate magnetically with each other via an air gap, for driving the objective holder relative to the frame with electromagnetic actuator forces in accordance with the said desired degrees of freedom, which actuator means com-prise at least one actuator coil for each desired degree of freedom.
The op-to-electronic apparatus comprises an opti-cal disc which rotates about an axis of rotation. The disc comprises a transparent substrate, on whose upper surface recording tracks are located. An objective scans the recording tracks of the optical disc by means of a light beam. The objective is mounted in an objective holder, which is supported b~ a frame. The origin of an orthogonal X-Y-Z system of axes is situated in the centre of the objec-PHN. 10.095 2 tive. The Z-axis is parallel to the axis of rotation of the optical disc and the X-axis in-tersects the axis of rotation perpendicularly, so that the X-axis is parallel to a radial line of the optica:L disc. The Y-axis is perpendicular to both the X-axis and the Z-axis and con-sequently extends in a tangential direction. The six independent degrees of freedom of the objective holder and thus of the objective which are theoretically possible are translational movements along the three axes and rotational movements about these axes. This yields a total of three directions of translational movement and three of rotational movement, so that six independent degrees of freedom are obtained.
In principle, any material body can move in accordance with six independent degrees of freedom. In principle, a bearing arrangement is a device which serves to ensure that a material body can perform movements only in accordance with some of the theoretically possible in-dependent degrees of freedom, namely the desired degrees of freedom, and which counteracts movements in accordance with all the other degrees of freedom. Most bearing arrangements allow only one or two movements, for example a single rotational or translational movement, a combina-tion of the two, or two rotational movements. Opto-electronic apparatuses for writing and/or reading recordingtracks on a rotating optical disc are known, in which the objective holder can move in accordance with two or three degrees of freedom. Movements along the Z-axis are required in order to focus the radiation beam on the recording tracks. Movements along the ~-axis may be required in order to follow oscillations of the recording tracks. Fina]ly, movements along the Y-axis may be required in order -to correct time errors resulting from an eccentrici-ty of the recording tracks (United States Patent Specification 4,135,206).
For a correct movement of the objective holder in accordance with the desired degrees of freedom electro-,~, ~ ,~

7''37 PHN. 10.095 3 nic control circuits are required which determine devia-tions of the radiation spot relative to the track along said axes and which ensure that the objective holder per-forms the appropriate compensating movements. ~he optical disc reflects the light beam and modulates it wi-th the in-formation contained in the recording tracks. The light beam modulation is converted into an electrical modulation, so that an electric signal is obtained which contains the informa-tion s-tored in the optical disc. This signal also contains information on the position of the light spot relative to the recording track to be followed, so that error signals for said control circuits can be derived from this signal. Thus, the position of the objective holder along said axes is determined dynamically by the control circuits and the objective holder is free to move in accordance with said desired degrees of freedom. The bearing arrangement of the objective holder is such that the objective holder is capable of performing the desired movements.
Alternative versions of opto-electronic apparatus-es are known in which, instead of a translational movement of the objective holder along the X-axis, a rotational movemen-t about the Y-axis is used. Ins-tead of a small transalational movement along the Y-axis it is possible to use a small rotational movement about the X-axis. Since the optical disc in the version shown is flat, small rotational movements about the X or the Y-axis will give rise to a certain focussing error of the light beam 6, but this error is so small that no significant interaction with the control of the translational movements along the Z-axis will occur. Moreover, opto-electronic devices are known, for example from United States Patent Specification ~,021,101, in which the bearing arrangement only permits movements alo~lg the Z-axis. For moving the radiation spo-t along the X-axis and the Y-axis, the objective is not moved but the light beam is moved relative to the objective by means of an electromagnetically actuated mirror. Such 7~3~

PHN 10.095 4 an electromagnetically actuated mirror is, for example, known from United States Patent Specification 4,129,930.
Generally, said compensation movements of the objective holder should be effected at high speed, which means tha-t the required control circuits should have a large dynamic bandwidth. Bandwidths of at least 3 kHz are desirable, but preferably they should be 5 kHz. It has been found that the bearing arrangements of objective holders and of pivoting mirrors used until now substan-tially limit the desired bandwidths. This is because thebearing arrangements comprise either surfaces which fric-tionally cooperate with each other or moving elastic parts, which introduce an amount of frictional damping or inter-nal damping into the control system which is difficult to control. Resonance phenomena in the bearing arrangemen-t also give rise to problems. In the case of bearing arrange ments with moving elastic parts the choice of the spring constant of the elastic parts is a difficult problem. If the stiffness is too low this gives rise to resonances at low frequencies and if the stiffness is too high the power required for driving is too high, giving rise to problems with respect to heat dissipation and the dimensioning of the control circuits.
It is an object of the invention to improve opto-electronic apparatus of the type mentioned in the opening paragraph in such a way that large bandwidths are at-tain-able with a low power consumption. To this end the invention is characterized in that the apparatus comprises an objective-position measuring device for continuously measuring the position of the objective holder relative to the fxame in accordance with at least one of the said undesired degrees of freedom and yenerating a positional-error signal; the bearing arrangement comprises bearing means comprising parts which are connected to the frame and to the objective holder respectively and which co-operate magnetically with each other via an 8~7 PHN 10OO95 -5- 2-2-19~2 air gap for counteracting undesired movements of the objec-tive holder in accordance with at least one of said undesired degrees of freedom by means of electro-magnetic levitation forces, which means comprise at least one levitation coil for the or each undesired degree of f~eedom in accordance with which an undesired movement of the objective holder is co~unteracted by electromagnetic levitation forces; and a levitation control circui-t is provided which compares the positional error signal from the objective -position measuring device with a reference signal of specific value and which comprises at least one output for applying an electric levitation current whose value depends on the error signal to a levitation coil f`or maintaining substantially constant the position of -the objective relative to -the frame in accordance with an undesired degree of freedom.
The word "levitation" in a narrower sense means balancing the force of gravi-ty. In the present text -the word is used in a broader sense in order to indicate that the objective holder is supported by the frame in a sub-stantially invariable position, by remotely exerted field forces, in relation to one or more degrees of freedom.
The opto-electronic apparatus in accordance with the invention employs two different types of control cir-cuit. The position of the light spo~relative to the record-ing -tracks on the record carrier is controlled by pressing and scanning control circuits, being actuator control circuits, which automatically con-trol the posi-tion of the objective and eventually also of other opticaL elements such as an articulated mirror rela-tive to the recording surface of -the disc. Deviations in the position of the recording surface at the location of the ligh-t spot owing to an oblique position of the disc or undulations in the recording surface, deviations of the position Or the recording -track owing to an eccen-tricity of the disc centre relat:ive to an axis of` rotation etc., are automatically compensated for by these actuator control circuits. ln addition there are provided one or more levi-tation con-trol circui-ts which do not control the position of the objective rela-tive to the recording sur-face and the recording track to be followed but relative to the frame of the apparatus. Thus, the levitation control circuits have a distinc-t other function and should be regarded as a part of an electromagnetic bearing arrangement of the objective on the frame. The absence of mechanical damping and resonance in the electromagnetic part of the bearing arrangement permits a larger bandwidth of the control eircui-ts which control the desired movementsof the objective.
In one embodiment of the inven-tion all bearing means for counteracting movements of the objective holder in accordance wi-th undesired degrees of freedom comprise only electromagnetic bearing means so that the objective holder is exclusively supported on the frame by electro-magnetic actuator forces and by electromagnetic levitation forces. In such an embodiment of the invention not a single kind of mechanical damping or resonance occurs, the objective holder is entirely free-floating in space and is subject o~ly to field forces, namely those as a result of the gravitational field of the earth and the magnetic fields produeed by the actuator and levitation coils.
Such an opto-electronic apparatus can be stable from the point of view of control technology. For the stabi~-ty a certain damping is necessary~ which c~n be provided in the electronic con-trol circuits and whose value is op-tional and easy to control.
When such a fully free-floa-ting objective holder is required, an embodiment of the invention may be used which is characteri~ed in tha-t: -the objec-tive holder is ferromagnetie and armular; at least three ferrornagnetie armatures are arranged on the objee-tive holder at equal radial distanees from the optical axis and uniformly spaced around the circumference of the holder; a radially magrleti~ed permanent magnet is arranged be-tween eaeh armature and the objeetive holder; each armature eomprises two teeth whieh extend in a substantially P1~ 1O.O95 -7- 2-2-1982 tangential direction and which are spaced from each other in a direction parallel to the optical ax~s of the objective; for each armature tooth cornbination coil having an elongate shape in a direction parallel to the optical axis is arranged on the frarne, which coil functions both as actuator coil and as levi-tation coil and comprises turns which are disposed substantially in plarles containing the op~cal axis of the objective and each have a central opening which receives the teeth with clearance;
and for translational movements along the optical axis there is provided an actuator coil whose centre lies on the optical axis. An opto-electronic appara-tus in this embodiment may comprise six combination coils and one further actuator coil in total. In the descrip-tion an embodiment will be described comprising eight combination coils and one further actuator coil (Figs. 2~
It is possible to employ an embodiment of the invention comprising an objective holder which may be fully free-floating and which is at least partly made of a ferromagnetic ma-terial, which embodiment is character-ized in that all actua-tor coils and levitation coils are rigidly mounted on the frame and cooperate magnetically with ferromagnetic par-ts of the objective holder In principle, it is favourable to use an embodiment without moving coils. A disadvantage of moving coils is that a movable connection is required between the coil and an electronic con-trol circuit, which connection ma~ give rise to problems because of its mobility.
An embodimen-t of the invention requiring neither moving coils nor permanen-t magne-ts is characterized in that: the objective holder comprises a ~erromagne-tic mounting ring which is concentric with the optical axis of -the objective; for movements in accordance with all the degrees of ~reedorn of the objective, excep-t for rotational movernents about the op-tical axis, these are arranged on the frame a plurality of coils, which are situated on axially opposite sides of the mounting PHN IO.O95 -~- 2~2 1~82 ring and which exert a~.ially opposite magnetic force components on the mounting ring, and a plurali-ty of coils which are situated on diametrically opposi-te sides of the mounting ring and which exer-t diametrically opposite magnetic force components on the mounting ring, which coils function as actuator coils, as levitation coils, or both as actuator coils and 1evitation coils.
A further embodiment of the invention is cha-racterized in that -the bearing arrangemen-t permits only movements of the objective holder relative to the ~rame in accordance with two desired degrees of freedom in a single plane of movement, namely a first degree of freedom along a first axis of translational movement which coincides with the optical axis of the objective, and a second degree of freedom along a second axis o~
translational movement which is perpendicular to the first axis of translational movement; and for elec-tromagnetically driving the objective holder along the second axis of translational movement and for electromagnetically counter-acting movements of the objective holder in accordance witha third, undesired, degree of ~eedom about an axis of rotation which is perpendicular to the first and second a~es of translational movement, there-are provided combined electromagnetic means comprising at least a first and a second combination coil which function both as actuator coils and levitation coils, which two combination coils are spaced ~rom each other, viewed in a direction parallel to the second axis of translational movemen-t.
Such an embodiment is especially suitable for 30 an objective holder which should move in the X-Z plane only. rhe m~ving mass, comprising the objective, the objective holder and -the associated moving coil, can then be minimi~ed. In this respect an embodiment may be of interes-t which is characterized in that: the objective 35 holder is movable relative to the frarne along the second axis of -transla-t:ional moverrlent o-ver a distance of not more than a few millirrletres; -two sets of combination coils are provided which are disposed on opposite sides 7~7 PHN 10.095 -9- 2-2-1982 of the objective holder, viewed in a direetion parallel with the first axis of translational movement, and an actuator coil is provided whose centre lies substantially on -the optical axis of the objective holder for exerting actuator forees along the optical axis of the objective.
This embodiment requires at least the four combination coils and one aetuator coil. In the usual manner the eombination coils at the sides of the objective holder and the actua-tor coil at the lower end of the objective holder rnay eooperate with permanent magnet on the frame, relative to whieh they perform only a limi-ted stroke. If sueh an opto-eleetronic apparatus is used for reading, for example, the recording traeks of a rotating video disc, the entire frame should be movable slowly along the X-axis. This may be referred to as a semi-stationary system. The use of such apparatus in video disc players is known. The slow movement along the X-axis is also controlled by means of a oon-trol circuit~ Thus, for ~ollowing the recording traek two eontrol eireuits are neeessary: one fast aetuator eontrol eircuit which causes the objeetive to move relative to the semi-stationa-ry frame in order to rapidly eliminate minor errors in the position of the recording traek during eaeh revolu-tion of the dise, and a slow eontrol eireuit for slowly moving the semi-stationary frame in sueh a way that the objective holder always performs its fast traeking movements with respect to its neutral position relative to the semi-stationary frame.
However, it may be attraetive to employ an opto-eleetronie apparatus in whieh the bearing arrangement Por the objeetive holder eornprises meehanieal means for eounterae-ting movements of the objee-tive holder relat-ive - to the frame in accordance with a-t least one or some of the undesired degrees of ~reedom.
Such an embodimen-t may lead to favourable dynamic proparties, especially if the levitation forces, the aetuator forces, the gravitational forces and the dynamic aeeeleration and decelera-tion forces impose only a small, 7~'7 P~ 1O.O95 -IO- 2-2-1982 negligible load on the mechanical bearing means. The less favourable properties of the mechanical bearing means then have a minor effec-t or are negligible.
An embodiment of the invention in which this principle is applied to an opto-electronic appa.ratus of the type mentioned in the foregoing, the ob~jective holder performing only desired translational movements in the X-Z plane, is characterized in that- the bearing means, in order to permit only movements of -the objec-tive holder in a single plane of movement in accordance with the two said desired degrees of :.-freedom~ comprise spaced guide surfaces which are parallel to said plane of movement, and movements of the objective holder which are not within said plane of movement (X-Z) and consequen-t ly are in accordanee with the three other degrees of freedom, are substantially counteracted by -the guide surfaces. Since.the gravitational force, the actuator forces, the levitation forces and the acceleration and deceleration forces do not exert components i.n the Y-direction, the guide surfaces, in principle, are not subjeeted -to any mechanical load. Thus, this embodiment combines the simplicity of the mechanical means with the absence~of any unfavourable influence on the dynamic bandwidth, It is possible to employ an embodiment of the invention which bears resemblance to the precading embodiment but which does no-t require the use of a movable semi-station~ry frame.
In this embodiment the obJec-tive holder can travel through a larger range in the X-direction relative to the frame, which can now be stationary~ This embodiment of the invention is charaeterized in that: -the objective holder is movable relative to -the frame along the second .. axis of translational movement over a distance which is sufficient for writing and/or reading recording tracl~s over t:he entire recording surface of a record carrier; sai~ first and second cornbina-tion coils are spaced from each other, viewed iIl directions parallel to 7~7 P~ 1O.O95 ~ 2-2-l982 said two axes o~ translational movement and also -to a third a~is of translational movement which is perpendicular to said two axes; the combination coils exer-t on the objective holder only elec-tromagnetic forces which are directed parallel to the second axis of translational movement; an actuator coil is connected to the objective holder for exerting actuator forces which are directed parallel to the first axis of translational movemen-t;
and the parts of the electromagnetic actuator means and the electromagnetic levitation means which are connected to the frame comprise a plurality of elongate stator portions which are parallel with the second axis of translational movement and which ha~e a length which is at least equal to said distance of movement of -the objecti-ve holder, said coils being movable rela-tive to said elonga-te stator portions in a direction parallel with the second axis of translational movement in such a way that an air gap is le~t.
Furthermore~ an embodiment can be used which is characteri~ed in that: said first and second combination coils belong to a first set and a second set of combination coils respecti-vely, which coils are disposed in first and second planes suhstantially parallel to the plane of movement of the objective holder, the coils in each se-t being arranged adjacent each other and overlapping each other in a direction parallel with the second axis of translational movement; said elongate stator portions comprise elongate permanent-magne-tic stators on the ~`rame, which stators have areas of alternate north and south polarity which extend in accordance wi-th a regular pattern in the longitudinal direction; and commutation Means are provided for commutating the elec-tric currents to be applied to the combination co:ils, depending on the position and the direction of movement of the objective holdor along the second QXiS of transla-tional mo~ement.
The use of cornmuta-ted cornbination coils has tho ad~antage that relatively small coils may be used in conjunction with elongate perrnanent magnets on the ~rame 79~

P~ 10.0~5 -l2- 2~2-1982 which are easy to manufacture. In principle, this permits the construction of an opto-electronic apparatus wi-th an almos-t unlimited range of movemen-t of the objective holder.
In practice t~is range need not be greater than a part of the radius of a rotating recording disc. In contra-distinction to the said opto-electronic equipmen-t using a semi-stationary frame, opto-electronic equipment in the last-mentioned embodimen-t permi-ts very fast movemen-ts over the entire recording and/or reading area and hence very short access times to be obtained~
It may also be advantageous to use an embodiment of the invention which combines the presence of an at least partly ferromagnetic objective holder and coils arranged only on -the frame with the fact that the ob-jective holder has a travel relati-ve to the frame of only a few millimetres, combination coils being arranged on both sides of the objective holder and an actuator coil, which is concentric with the optical axis of the objective being provided for the purpose of focussingO This embodi ment may be characterized in that: the objective holder is provided with ferromagnetic objective pole shoes with free ends, which shoes project ~rom the holder in directions parallel to the second axis of translational movement; a plurality of ferromagne-tic stator pole shoes is arranged on the frame, which pole shoes have free ends which are each disposed opposite the free end of an objective pole shoe to form an air gap; and said combinat-ion coils are arranged on the stator pole shoes. The use of pole shoes enables greater forces -to be imparted to the objective holder by the combination coils, the presence of the pole shoes on the objective perrnitting -the formation of a small air gap between the two types of pole shoes. F~rtherrnore, i-t is possible to use a fur-ther embodiment which is characterized in that the apparatus comprises at least one perrnanent Magnet for producing a permanent magne-tic field in the air gaps between the s-ta-tor pole shoes and -the objective pole shoes; ihe stator pole shoes an-l the objective pole shoes comprise teeth which Pl~ 10.095 -13- 2-2-1982 face each other and magnetically co-operate with each other via the air gap; and the frame comprises a ferro-magnetic yoke which magnetically coup:Les the permanent magnets to the stator pole shoesO
The use of one or more permanent magne-ts provides a higher rigidity of the magnetic bearing arrangement 9 especially for rotational movements about the optical axis and translational movements transverse to the optical axis.
For measuring the position of the objective relative to -the frame an embodiment of the invention may be used which is characterized in -that -the objective-position measuring device at least comprises: a~ two capacitive elements arranged in seriesj which elements comprise a facing stationary plates of an electrically conductive material on the frame and facing movable pla-tes of an electrically conduc-tive material on -the objec-tive holder; b) a high-frequency alternating-current source;
c~ a voltage di~ference circuit which is inductively coupled -to the high frequency alterna-ting-curren-t source and connected to the two capacitive elements; and d) means for applying the output signal of the voltage dif-ference circuit to the levitation control circuit.
The movable plates of an electrically conductive material on the objective holder may have a very low mass and may for example comprise -thin metal layers which are physically or chemically deposited. In addi-tion to a capacitive objective-position measuring device it may also be advantageous -to use an embodirnent of the invention which is charac-terized in -that the apparatus comprises an opto-electronic objective-posltion measuring device, which device compr:ises: a) a radiation source on the frame ~or emitting a radiation beam towards the objective holder; b) a radiation-sensitive detection system on -the frarrle~ which systern comprises a plurality of detectors, each div:Lded into at least two sub-detectors, -the separatin~ lines be-tween the sub-detectors being parallel to each other; c) a beam-split-ting op-tical elemen-t on 7~7 PHN. 10.095 14 the objective holder for splitting the radiation beam emitted by the radiation source in the direction of the radiation-sensitive detection system into sub-beams, the radiation distribution among the sub-detectors being determined by the position of the optical element and thus of the objective holder relative to the frame; and d) an electronic circuit for supplying posi-tion-error signals, which circuit comprises inputs which are each individually connected to a sub-detector of the detec-tion system.
The invention will now be described in moredetail with reference to the drawings, in which the prin-ciples of a number of embodiments of the invention are represented and in which:
Figure/illustrates an opto-electronic apparatus according to the in~ention, Figure 2 is a partly sectional view of an opto-electronic apparatus with permanent magnets, in which the objective holder is fully free-floating relative to a surrounding frame and comprises a plurality of tangenti-ally projecting pole shoes, Figure 3 is a side ~iew of the objective holder and objective of the apparatus of Figure 2, Figure 4 is a plan view of the apparatus of Figure 2, Figure 5 schematically represents two actuator control sys-tems for the apparatus of Figures 2-4, Figure 6 relates to an objective-position measuring dev:ice, 7~7 PHN 10.095 ~15 2-2~1982 Figure 7 is a circult diagrarn of an electronic circuit for generating positional-error signals, Figure 8 is a symbolic diagram of a levitation control circuit, Figure 9 is a sectional view of an other embodi-ment of the invention which concerns an opto-electronic apparatus without permanen-t magnets and with an objective whichis mounted in a ferromagnetic objective holder which is fully free-floating relative to a surrounding frame in the magnetic field of a number of combina-tion coils 9 Figure 10 is a plan view of the apparatus of Figure 9, some of the parts being omitted for the sake of clarity9 Fig~ure 11 schematically represents a levitation circuit for the apparatus of Figures 9 and 10, Figure 12 is a circuit diagram of a capacitive objective-position measuring device for use in conjunction with the levitation control circuit of Figure 11, Figure 13 shows an objective holder for use in 20 another embodiment of the invention which comprises optical-position de-tection means and which is movable in the X-Z plane only, Figure 14 is a perspective bottom view of a semi-sta-tionary frame for an objective holder in accordance 25 with Figure 13, Figure 15 is a perspective bottom view of an optical storage-disc apparatus employing the objective holder of Figure 13 and the frame of Figure 14, Figure 16 is a perspec-tive plan view of a part 30 of the apparatus of Figure l5, Figure 17 is a perspective view o* the serni-stationary rrame of F:igure 14 from a di*ferent direction and with the omission of a number of parts, Fi~ re 18 in perspective illustrates the principle 35 o* an other opto-electronic apparatus comprising an ob-jective holder with a large range o~ rnovement 7 Figure 19 is a section view of a further opto-electronic apparatus in which the objective holder is P~ 10.095 -16- 2-2-1982 provided with magnetic pole shoes which are disposed opposite pole shoes of a permanent mag~tic stator yoke, and Figure 20 is a plan view of the apparatus of Figure 190 All the embodiments of an opto-electronic appara-tus shown in the drawing are in principle intended for scanning video and/or audio recording tracks or digital data recording tracks in a reflecting recording surface of a rotating disc by means of a laser light beam. How-ever, in principle they may also be used for recording and/or reading the recording tracks in a recording surface of a different type of record carrier, for example a rectangular record carrier which is linearly reciprocated or record carriers which are s-tationary~
The opto-electronic apparatus shown in Figures

2 to 4 comprises a frame 10 and an objective holder 11 9 which carriesan objec-tive 12 having an optical axis 13 The objective 12 is only shown schematically and has a lens system which comprises a bi-aspherical lens 14. The objective 12 concentrates a light beam 15 so as to form a light spot 16 in a focussing plane 170 The desired degrees of freedom comprise translational movements along the Z-axis, that is, the optical axis 13, so-called "focussing"
movements, translational movements along an X-axis per-pendicular to the Z-axis, so-called "tracking" movements, and transla-tional movements along a Y-axis which is perpendicular to the X-axis as well as to the Z-axis, so-called "tirne-error correction" movements. The bearing arrangement for the objective holder 11 allows said movements relative to the frame in accordance with said desired degrees of freedom but subs-tantially counteracts movernents in accordance with the -three other degrees of freedom~ For driving the objective holcLer along the Z-axis there is provided an annular actua-tor coil 19. The ob-jective holder carries an annular and axially magrnetized permanent magnet 20, which is coaxially movable within the coil 19 with clearance, so that the actuator coil '7~7 P~N 10.095 -17- 2-2-1982 and the permanent magnet magnetically coopera-te with each other via an air gap formed between them. The actuator coil 19 is included in a focussing system which falls beyond the scope of the invention and which serves to ensure that the focussing plane 17 constantly coincides with the recording surface of a video disc. This will be explained later, with re~erence to Figure 5 which schematically represents the opto-~ectronic apparatus of Figures 2 to 4 when used in an apparatus for reading a video disc 21.
The round disc-shaped video record is sho~rn in a radial cross-section in Figure 5, so that the recording tracks 22 extend perpendicularly to the plane of drawing, that is in the Y~direction~ The light beam 15 produced by a light source 23, for example a gas laser or a semiconductor diode laser, is reflec-ted onto the video disc by a mirror 2~9 the beam being focussed by the oh-jective 12 to form the radiation spot 16 of minimal dimensions on the reflecting recording surface in which the recording tracks 22 are disposed. The video disc 21 is disposed on a spindle 27 and is rotated by a motor 28.
~ hen reading the video disc a beam 15R is used which is reflected by the recording tracks, This beam traverses the objective, is reflected by -the mirror 24 and is subsequently separated from the beam 15 emitted by the source 23, for example by means of a semitrans-parent mirror 25, which reflects the beam 15R onto a radiation-sensitive detection system 26. ~s -the video disc is rotated by the spindle 27 which is driven by the motor 28 the intensity of -the beam 15R varies with a high fre4uency in conformity with the information stored in the recor-ling track.
~ n eclulpmenti in which information is to be record~
od the intonsity of the beam 15 may be modulated in con-form:ity with t;he information to be recorded. ~or -this purpose a modulator 29, for example an electro-optical or an acousto-optical modulator, may be arranged in the radiation path. The signal to be recorded is applied to 7~
PI~ 10.095 -18- 2y2-1982 the input terminals 30A~ 30Bo If the ligh-t source is a diode laser, this laser may be modulated clirectly and no separate modulator is needed.
For reading the recording tracks the detection system 26 may in principle comprise one radiation~sensitive detector, which converts the intensi-ty modulation of the beam 15R into an electric signal. A tracking s~vstem is necessary to correct the position of the radiation spot 16 relative to a recording track. A deviation of the position of the cen-tre of the radiation spot relative to the record-ing track may be detected in, for example, the manner shown in ~ig~re 5~ The de-tection system 26 in this Figure comprises -two detectors 26A and 26B, which are arranged adjacent each other in the X-direction, that is in the direction transverse to the recording tracks. The output signals of -these detectors are applied to the inputs of a differential ampli~ier 31, on whose ou-tput a positional-error signal S2 is available. This signal is applied to a controller 32 which controls actuator means 33 by means of which the objective system can be moved -translationally along the X-axis. If the radiation spot 16 has shifted to the left or th-~ ~ght relative to the centre of a record-ing track, one of the detectors ,26A or 26B respectively, receives a greater radiation intensity than the other, so that the signal S2 increases or decreases respectively.
As a result of -this the objec-tive holder is moved to the right or to the left respec-tively.
By adding the signals from the detectors 26A and 26B in a summing device 3l~ a signal Siis ob-tained, which contains the information read~ This signal is applied to an electronic processing circuit 35, which malces the signal suitab:Le for reproduction by means of a television set, During read-out with the minute light spo-t 16 the correct ~ocussing of the radiation beam 15 at -the recording surface shoulcl be checked, cons-tantly. Figure 5 represents a ~ocussing servo-system which is known per se. By means of a serni-transparent mirror 36 a paxt of the reflected PHN 10.095 -19- 2-~-1982 beam 15R is directed into a seconcl radiation-sensi-tive system 37. By means of a lens 38 the directed beam 39 is focussed at a wedge 4OO This wedge splits -the 'beam into two sub-beams 39A and 39B, the directions of said beams being dependent on the degree of focussing of the beam 15 at the recording sur~ace. The detection system ~ com-prises four detectors 37A to 37D~ The signals from the detectors 37A to 37D are toge-ther applied to a first input of a dif~erential amplifier 41 and the signals from detectors 37B and 37C are app:Lied to a second input o~
said amplifier. The error signal Sf supplied by the differential amplifier ~s applied to a con-troller 42. The controller 42 controls the actuator current -through the actuator coils 19, If the 'beam 15 i5 focussed exactly at the recording surface~ the beam 39 is ~ocussed exactly at the apex of the wedge 4O and the respectively beam 39A
or 39B in symmetrically i~cident on the detectors 37A, 39B
or 37C, 37D respectively and the signal Sf is ~ero~ ~hen the light spot 16 is moved along the Z-axis the -two beams 39A and 39B move inward or outward, depending on the direction of movement of the light spot, and the actuator current through the actuator coil 19 is varied.
By means of the tracking system described in the foregoing the posi-tion of the light spot 16 can be correct-ed very accurate~ in the radial direction, that is in theX-direction~ This system is ineended for fast fine control and has a small range. In practice, -this fine control system will be combined wi-th a coarse control system. This is a second tracking system which provides coarse control' of the radial position of the light spot. This second tracking system comprises a system for controlling the position, in the X-direction, of a carriage on which the frame 'IO is mounted. This means that the frame in the video disc player is semi-stationary.
It is desirable to measure translational movements of tho o'bjcc-tive holder 11 along the X-axis relative to the semi-stationary frame 10, in order to ascertain whether during reading the fine control system is approaching the Pl~ 10.095 ~20- 2~2-l982 end o~ its control range~ which is a ~ew millime-tres at the most9 so that a signal :~or rendering -the coarse control system operative is obtained. ~l-Lrthermore, when the video disc is started, and does not yet supply a signal S2, it is desirable to measure -the position o~ the ob-jec-tive holder, so that it can be set to its centre position relative to the frame. In ac^ordc~nce with the invention, it is further necessary to have an objective-position measuring device for measuring the position of the objective holder 10 relat:ive to -the frarne -l1 wi-th respect to the three undesired rotational movements about the X9 Y and Z-axes and for generating positional-error signals For simultaneously measuring the said translation-al and rotational movements of the objec-tive holder relative to the frame a prism 43 is arranged on the oo-jective holder. This prism forms part of an objec-tive-position measuring device 9 which is schema-tically re-presented in Eigure 6. It is assumed that the re~racting edge 46 of -the prism should be disposed in tha ~-Z plane and should be parallel to the Z-axis. An auxiliary radiation source 44, for example a ~liode laser, emits a radiation beam 45, which is incident on the prism 43. I~
the beam is incident on the refracting edge 46 of the prism, two sub-beams 45A and 45B are formed, which are reflected to two de-tectors 47 and 48. Each o~ these detectors comprises four sub-detectors 47A to ~7D and 48A to 48D- respectively. The two separating lines of -the sub-detec-tors respectively extend transversely of the refracting edge 46 of -the prism 43 and in the longitudinal direction thereof. The detectors are arranged on a common support 49, in which a hole is formed for the passage o~ the bearn 45. A lens 50 converts the diverging beam 45 into a parallel beam. The radiation source 44 is also ar-ran~e~ on the support 49.
I~ the objective holder occupies i-ts centre posit-ion alollgr the X~-axis~ the centre of -the radiation beam 45 is exactly inoi~en-t on the re~racting edge 46 o~ the prism 7~
Pl~ l0.095 -2l- 2-2-1982 43. The radiation spots 51 and 52 formed on the de-tectors 4Z and 48 by the reflected beams 45A and 45B then have the same intensity. If the objective holder is out of its centre position, the beam L~5 is asymrnetrically incident on the prism and one o~ the re~lected 45A and 45B will convey more radiation energy than the other.
If the detectors are correctly aligned relative to the radiation source 44 and the refracting edge 46 of the prism 43 is parallel -to the Z-axis, the radiation 0 spots 51 and 52 will be disposed symme-trically relative to the respective detectors 47A to 47D or 48A to 48D, The arrangement sho~n enables translational movements along the X-and Y-axes and rotational movements about the three a~es X, Y and Z to be measured simultaneously.
lS If the positional-error signals for -the said translational movements are designated STx and Srry and those for the said undesired rotational movements are designated SRx, SRy and SRz~ the signals ~rom the detec-tors being designated S47A, SL~7B etc.~ the relationships between these signals are as follows4 47~ 47B 47C + S47D) ~ (S48A ~ S48B + S4 ~S
'rY ~ 47A + 47D + S48B + S48C) -- (S47B + S47C ~ SL~8A ~S48D) RX ( 47A 47B + S48A + S48B) ~ (S47C + S47D + S48C +S48D) SRY (S47A + S47B * S48C + S48D) ~ (S47C * S47D + S48A +S48B) SRZ=(S47A * S47D + S48A + S48D) ~ (S47B * S47C * S48B ~ 48C) Only the last three positional-error signals are o~ interest ~or -the present inven-tion, because they rela-te to movements in accordance with the three undesired degrees of freedom of the objective holder 11. For adding and subtrac-ting the detector signals a c:ircui-t 71 as shown in Fi~lre 7 may be used. The circuit cornprises a number of adder arrlpli~iers, represented by squares, and a number of subtraotor arnpl:ifiers, represented by triangles, and recluires no ~urther descript:ion.
~ ht actuator coils 53A,B to 56A,B are arranged orn the f`ratrle 10. These coils forrrl parts of electromagnetic actuator means~ which in addi-tion cornprise four ~erro-magnetic armatwres 57 to 60 which are disposed at equal 7~
P~ 10.095 -22- 2-2-1982 radial dis-tances from the optical axis 13 and which are uniformly spaced amouncL the circumference o~ -the objective holder 11. Each armature comprises two -teeth 57A,B to 60A,B ~hich extend in subs-tantially tangen-tial directions and which are spaced ~rom each other in a direction parallel to the optical axis 13 of the objectiveO The actuator coils 53A,B to 56A,B have an elongate shape in the axial d:irection, i.e. the direction parallel to the optical axis 13, so that the armature teeth have sorne axial freedom of movement in the actuator coils. The actuator coils have turns which are disposed substantially in planes containing -the optical axis l3 of the objective and each have a central opening in which the associated armature tooth is situated with a clearance such that -the objective holder 11 is movable to a limited extent in accordance with all the degrees of freedom. The obJective holder 11 ls annular and consists of a ferromagnetic material. Between the armatures 57 ~o 60 and the objective holder 11 radially magnetized permanent magnets 61 to 64 are arranged. Thus, some of the turns of each actuator coil are si-tuated be-tween an armature tooth and the annular ferromagnetic objeetive holder and consequently in a permanent magnetic field which extends radially from the arma-Sure teeth to the ferromagnetic objective holderO
The objective holder 11 can be subjected to electromagnetic actuator forces for driving the objective holder in the desired directions of movement, that is, translational movements a~ng the X-axis and the Y-axis.
For a rnovement of the objective holder 11, and thus of the objec-tive 12, along the X-axis, e:Lec-tric ac-tuator currents of equal magni-tude are simul-tc~eously applied to the actu-a-tor eoils 55A and 55B. The aetuator coils 53~ and 53B
rece:ive actuator currents of opposite polarity and equal cabsolute magnitude. In a similar way the objective holder can be moved a:long t~e Y-ax:is by means of the actuator eo:i:Ls 5~A,B and 56A,B~
In the opto-electronic app,aratus shown in Figures 2 to ~ the bear:ing arrangerrlent for the objective holder l1 Pl~ 10.095 -23- 2-2-l982 comprises a number of electromagnetic bearing means which are connected to the frame 10 and the objectivc holder 11 respectively and which electro-magnetically cooperate wi-th e~ch other via air gaps, for counteracting the undesired rotational movements about the X axis, the Y-axis and the Z-a~is by rneans of electromagnetic levitation ~orces~ The actuator coils 53A,B to 56A,B in this apparatus also serve as 3evitation coils, for which reason they are referred to hereinafter as l'combi-nation coils". All bearing means for coun-teracting move-ment of the objective holder 11 in accordance with the undesired degrees of freedorn comprise only electromagne-tic bearing means, so that the objective holder 11 is sup-ported on the frame 10 only by electromagne-tic actuator lS forces and electromagnetic levitation forces.
The aforesaid positional-error signals (SRx3 SRy and SR~) from the objective-position measuring device (see Figure 7) are each applied to a levitation control circuit, which compares the positional-error signal with a reference signal of predetermined value and which comprises an output for applying electric levitation currents whose values depend on the error signal to the relevant combination coils in order to maintain the position of the objective 12 relative to the frame 10 in accordance with the relevant undesired degree of freedom substan-tially constant.
~ ow the appropriate levitation coils can be exerted on the objective holder 11 by means of a levitation control circuit and by means of combination coils will be described~ by way of example, with reference to Figure 8.
The signal SRx is applied to a symbolically represented electronic controller CRx. The controller cornpares the signal SRx wi-th a signal FRX of preset value. Since it will ~enerally be clesirable -to maintain the objec-tive 35 holdor in a position which is as level possible relative to the ~rame, the signal F~X will have a value which corresponds to this level position, for exarnple a value 0.
l~le controller CRx comprises two outputs 65 and 66 and PHN. 10.095 24 generates levitation curxents of equal absolu-te value but g CRX and LcRx on the two ou-tputs 65 and 66 Each of these levitation currents is applied to -two com-bination coils. One of the two levitation currents is applied to the two combination coils 53A and 55A and the other levitation current is applied to the two combina-tion coils 53B and 55B. As a result of this the teeth 57A and 59A are subject to a tangential electromagnetic force which is opposite but equal in absolute magnitude to the electromagnetic forces exerted on the teeth 57B and 59B. Consequently, a torque is exerted on the objective holder 11 which counteracts the undesired rotational movement. Rotational movements about the Y-axis are counteracted in a similar manner. For rota-tional move-ments about the Z-axis a controller having only one output can be used, the levitation current being applied to all eight combination coils in equal proportion.
The use o~ the objective-position measuring device described in the foregoing with reference to Figures 5 to 7 in a levitation control circuit as des-cribed here is novel, some of the positional-error signals produced being employed in order to maintain an objective holder in a substantially invariable position relative to a frame by means of le~itation coils, in other words for use in an electromagnetic bearing arrangement of an objec-tive holder in a frame.
In Figures 5 and.6 the prism 43 is shown in such a posi-tion relative to the X-axis, the Y-axis and the Z-axis that, as already stated, the refracting edge 46 of the prism is disposed substantially in the Y-Z plane and parallel to the Z-axis~ As is shown in Figure 4, this is not readily possible with the opto-electronic apparatus described so far, because the X-axis and the Y-axis intersect with the coils 53A,B to 56A~B so that these coils form an obstruction. to the light beam 45. Therefore, PHN 10.O95 -25- 2-2-1982 the prism 43 is arranged at an angle ~ relative to -the X-axis. This gives rise to a complication as regards the control method. Viewed ~rom the support l~g~ the re~racting prism 4G 9 ~hen the objective holder 'I 1 moves along the s ~-axis, moves not only in the ~-direction but also in the Y-direction, the displacement in the Y-direction being related to the displacements in -the X-direction in accordance with a sine function. Since the angle ~ has a ~ixed value, compensating measures may be taken in the circuit o~ ~igure 7n As these measures ~all beyond -the scope of the present description and are obvious to those skilled in the art o~ control technology, they will not be described in more detail~
As can be seen in Figure L~ the ~rame lO has recesses 67 to 7O adjacen-t the pole shoes such that the objective holder 11 can be manualli~ positioned so that 9 by a small ro-tary movement about the Z-axis, the armature teeth no longer projec-t into the combination coils. The objective holder can then be removed ~rom the ~rame along the Z-axis. Since no electrical wiring is connected to the objective holder, this embodiment is highly suitably in those cases in which objectives should be rapidly and readily exchangeable, Another embodiment of` an opto-electronic apparatus in accordance with the invention will now be described with re~erence to Figures 9 to 13. This embodiment also concerns an apparatus in which all bearing means for counteracting movements o~ the objective holder in accord-ance with undesired degrees o~ ~reedom comprise only e~ctromagnetic bearing means. The objecti-ve holder 72 is supported on the ~rame 73 only by electromagnet:ic actuator ~orces and by electro-nagnetic levitation forces. The ob-jective comprises a single aspherical lens 74~ The undesired degrees o~` f'reedom comprise rotational movements 35 about the ~-ax:is and about the Y-axis. Rota-tional movements a~out the G~axis do not af'~ect the opt:ical operation o~
the apparatu~ and are theref'ore regarded nei-ther as des:ired nor as undesired movements. For this reason no 7~

P~ 10-095 _26- 2-2-1982 bearing means are provided which eounterac-t rotational movements about the Z-axis. The Z-axis coincides with the op-tieal axis of -the lens 74.
The objecti~e holder 72 comprises a ferromagnetic mounting ring which i~ concentric with -the optical axis of the objectiv~ 74. The lens is glued into the mounting ringO All actuator coils and levitation coils are rigidly mounted on the frame 73 and cooperate magnetically with the ferro~agnetic ring 72. In the bottom of the frame 73 an opening 75 is formed for the passage o~ a light beam. ~t the top of the frame is a cover 76 with an opening 77 for the light beam. Inside the assembly com-prising the frame 73 with the eover 76 three ferromagnetic yokes 78, 79 and 80 are arranged. T~e yoke 78 earries eight eombination coils 81 which serve as actuator coils and as levitation coils. On the axially opposite side of the mounting ring 72 from the coils 81 identical com-binations coils 8z are ar~anged on the yoke 80, The coils - 81 and 82 are arranged around limbs 83 and 84 respectively of the yokes 78 and 80 r~spectively. Said coils ex~rt -a~ially opposed magnetic force components on the mounting ring 72. The yoke 79 earries eight combination coils 8~
whieh are arranged around limbs 86 of the yoke 79. These eombination eoils exert radial ~orees on the mounting ring 72, eombination coils 85 situated on diametrically opposite sides of the mounting ring exerting diametrically opposed magnetie foree eomponents. The speeial feature of this embodiment of the invention is that no permanent magnets are used and tha-t the objeetive holder 72 with the objeetive 74 is kept floating wi-thin the frarne 73 by rneans of eleetromagne-tie forees only, whieh forees are exerted on the mounting ring 72 in opposite direetions.
For mo~ing -the objeetive holder in the d:ireetion of -the Z-axis eqwal eurrents must be applied to Qll the coils 81 ancl all the eoils 82 should also be energized with rnutual-ly eqwal eurrents. For tilting the objective holder about the X-axis di~erent ewrrents should be applied to those eoils 81 whieh are diametrieally opposed in the Y-direetion, 7~7 equal curren-ts 'being applied to coils which are diametrical-ly opposed in the X-direction. The same applies to the coils 82. Tilting abou-t -the Y-axis is possible in a similar way. Translational movements in the X-direction or in the s Y-direction are effected by applying mutually di~ferent currents to diame-trically opposite coils 85 on the yoke 79.
The position of the objective ring 72 relative to the frame 73 in accordance with all the degrees of freedom, e~cept for rotational movements about -the Z-axis, are measured by means of a capacitive objective-posi-tion measuring device. At the top of the objective holder an annular plate 87 of an electrically conductive material is arranged, an identical plate 88 being arranged at the bottom. A cylindrical sleeve 89 is arranged amound the entire circumference. Opposite the plate 87 four stationary plates 9O are arranged on the yoke 80 and opposite the pla-te 88 four identical stationary plates 91 are arranged on the yoke 780 These stationary plates are disposed bet~een the limbs 84 of the yoke 80 and between the limbs 83 of the yoke 78 respectively. Between the limbs 86 of the yoke 79 four s-ta-tionary plates 92 are disposed.
Each of these sta-tionary plates constitutes a capacitive element with the facing movable plate on the objective holder 72. The capacitance of each of these capacitive elements depends on -the distance between the plates. ~or each of the five degrees of freedom which are of impor-tance there is pro-vided an objective-posi-tion measuring device. Two of these objective-position measuring devices are only used for counteracting -the undesired rotational rnovements about -the X~axis and about '',bhe Y-ax:is and are included in levitation control circui,ts. The other objective-position measuring devices have functions in -the actuator control circuits for controlling the position of' a read spot relative to a recording track in an in-formation carrier. The electronic part of an objective position rneasuring de-vice will be described wit'h reference to Figures 11 and 12, For the sake of convenience identical P~ 100095 -28- 2-2-1982 parts in Fi~ure l1 which are situated -to the left and right of the Z-axis in the drawing are distinguished from each other by adding an L or an R respectively -to the re:ference numeral.
The stationary plates 90L and 9lL are connected to an objective-position measuring device 93L and the stationary plates 90R and 91R are connected to an identical objective-position measuring clevice 93R. Figure 12 shows an objective position measuring~device 93 in grea-ter detail. The objective mounting ring 72 may be regarded as being earthed because of its comparatively high capacitance relative to its surroundings. A voltage is induced into a coil T by a high-frequency voltage source a, which may be common to a plurality of objective-position measur-ing devices. The voltage induced in T causes equal currentsthrough the two capacitances constituted by -the capacitive elements 87-9OL and 8~-91L. Since the stationary plates 90L and g1L have equal surface areas the dielectric strain and thus the field strength in the two capacitive elements is the same. Therefore, the voltages across the capacitive elements are proportional to the distance between the relevant plates, their sum being constant, These v~ltages are rectified wi-th opposi-te sign by means of two diodes D1 and D2 and are added to each other a-t the junction U of a bridge comprising two identic-al resistors R. The voltage S on this junction is there-fore proportional to the difference of -the voltages across the capacitive elements and hence proportional to the po-sition of the objective holder 72 relative to the frame along the Z-a~is. The output signal S is zero i~ the ob-jective holder 72 is in a centre position.
The output signals SL and SR appearing on the respective OUtplltS UL and UR of the objective-position measurin~ devices 93L and 93R respectively are used as positional error signals in the levita-tion control circuit sho~L in Fi~lre 11 for co~lteracting ro-tational movements about -the X-a~is. The circuit of Figure l1 comprises two types of loops the loops partly designated A belong to 79~

PHN 10.095 29-- 2-2--l982 the levitation control circuit w~ich counteracts rotational movements about the X-axis and the loops with the designation B belong to a Z-a~is actuator control circuit which responds to a positional error signal Ez generated elsewhere (applied to an ampl-ifier 'V3). On explaining the operation it is assumed that the levitation control circuit is in a balanced condition. Assume that the ob-jective holder now rotates about the X-axis 9 for example in an anticloc~wise direction. The output signal SR increases and the output signal SL decreases to the same extent. The two signals are added to each other in an amplifier ~2 whose output signal does not vary; consequently, the loop does not become active. In a differential amp~ifier SR
and SL are subtracted from each other. A first output signal SR-SL'increases and a second output signal -(SR SL) decreases. An amplifier V4 raises the levi-tation current through the coil 81R and reduces the levition current through the coil 81L and raises the levitation current through the coil 82L.
This results in a downward levitation force at the right-hand side of the objective and an upward levitation force at the left-hand side. The torque exertec~ by these forces counteracts the original rotary movement and returns the objective holder to its neutral position rela-tive to the frame~
In a similar way an upward translational movement of the objective holder along the Z-axis resul-ts in,an equal increase of the two output signals SL and SR. The output signals of the differential amplifier V1 consequen-t-ly do not change. Added in amplifier ~2 and inver-ted iIl amplifier V4, they produce a signal ~2 ~ ~ (SL + SR) which causes an equal increase of the actuator currents in the coils 81L and 81R and an equal decrease of the actuator currents in the coils 82L and 82~. As a result of this, a downward actuator force is exerted, which correc-ts the po~:ition of the read spot relative -to the recording sur-face alon~,r thc X-axis. In order to obtain control circuits with suita'ble responses the use of appropriate fil-ter net-9~

P~ 10.095 -3- 2-2-1982 worl~s and suitable lineari~ation o~ the loops by means o~
bias currents in the coils are essential. These and other steps are obvious to those skilled in the art of control technology and are there~ore not discussed in more detail.
The opto electronic apparatus of Fi~gures 13 to 17 comprises an objective holder 94 with an objective 95.
For focussing a radiation beam the objective holder, with the objective, is movable along a 2 axis which coincides with the optical axis. The desired degrees o~ ~reedom are the translational movements along the Z-axis and -the translational movements along the X-axis for tracking pur-poses. The objective holder 94 is arranged in a semi-stationary frame 96~ By means of a number of rollers 97 in the form of ball-bearings said frame is movable in the X-direction on a stationary frame 98 of a small optical recorcler for writing and reading digital data recordin~
-tracks in the reflecting recording surface of a rotating digital data disc 99 by means of a laser beam. By means of a motor lO0 said disc can be rotated about an axis 10'1 parallel to the Z-axis. The semi~sta-tionary frame 96 is driven relative to the stationary frasne 98 in the X-direction by means of a cord or cable 102 and a servo-motor, not shownO
The objective holder 94 is only movable relative to the frame 96 along the X-and the Z-axis, so that all movements in accordance with the two desired degrees of freedom are performed in a single plane of movement 9 name-ly the X Z plane. For electromagnetically driving the objective holder 94 along the X-axis and for electro-magnetically counterac-ting movements in accordance with an undesired degree of ~reedom in said X-Z plane, narnely rotational movements abou-t the Y-axis, there are provided combined electromagne-tic means. These means cornprise two combination coils 103A and 103B on one side of the ob-jectiveholder 94 and two identical combination coils 10L~Aand IOl~B on the other side, which combination coils, viewed in a direction parallel to -the X-axis, are arranged in pairs and spaced from each other. The combination coil Pl~ 10.095 -31- 2-2-1982 10llB is not visible in the drawing but ls disposecl opposite the combination coil 103B in the X-direc-tion. For trans-lational mo~ements in the Z~direction there is provided an actuator coil 105 whose centre lies on the Z-axis and thus on the optical axis of the objective 95. The objective holder 94 has a range of movement relative to the frame in the X-direction of a few m:Lllimetres a-t the most.
The three other degrees of freedom not yet men-tioned, namely translational movements along the ~'-axis, rotational movements about the Z-axis and rotational mo-vements about the X-axis, are counteracted by mechanical bearing means. These bearing means only allow movemen-ts of the objective holder 94 in -the X-Z plane~ Said mechanical bearing means comprise two guide surfaces 1o6A ~nd 1o6B
on one side of the objective holder 94 and a guide sur-face 107 which, viewed in the Y-direction9 is disposed on the other side of the objective holderO Said guide surfaces are parallel to the X-Z plane. The frame 96 comprises guide pla-tes 108 and 109 which are coated with a low-friction material. The objective holder is movable betweenthese guide plates with a small clearance. 'rhe electro-magnetic bearing means serve only to counteract rotational movements about the Y-axis.
The objective-position measuring device comprises a prism 110 on the objective holder 94 and is, in principle, identical to that in the embodimen-t of the invention described with reference to Figures 2 to 8. Figure 17 shows a permanent magne-t 111 for cooperation with the actua-tor coil 105 and one of two permanent magnets 112 for cooperat-ion with the combination coils 103A and 103B via yokeportions 112~ and 112B respectivelyO For the combination coils 10l~ and 104B identical permanent magnets 117 and yoke por-tions are provided on the frame 96. The yoke por-tions fit into the combination coils with some clearance so that they do not irnpede the limited movements of -the ob~jective holder 91~ in the X-Z plane. The magnet 111 fits into the actuator coil 105 with clearance.
In the guide plate 108 an opening 11~ is formed.

8'~9~

PHN 10.o95 -32- 2-2~l982 This opening serves for -the passage/of a light ~eam which is directed towards -the prism 110 in the Y-direction and which is produced by an auxiliary l-ght source. ~ia said opening the two sub-beams reflected by the prism l10 return to the two light-sensitive cGlls of the objective-position measur-ing device. It will be evident from the foregoing that by means of the four combina-tion coils 103A -to 10L~B force components in the X-direction and moments o~ force about the Y-axis ~an be exerted on the objec-tive holder gL~ by a tracking control circuit and a levitation con-trol circuit respectively.
The opto-electronic apparatus which is schematical-~ly shown in Figure 18 bears some resemblance to the apparatus described in the foregoing. Again the objective holder 11L~ has a mechanical bearing arrangement which permits only movernents in the X-Z plane. ~Iowever, the ob~
jective holder is so suspended relative to a stationary frame 115 that it is movable along the X-axis over a range which is large enough for writing and/or reading recording tracks over the entire recording surface of a record car-rier. In principle, this range may be extended at option~
The objective holder 114 contains an objective 116 whose optical axis coincides with the Z-axis. A small light source, for example a semiconductor laser, is located in-side the objective holder, which also accommodates alloptical elements of the light path and the required opto-electronic elements. There are four sets of combination coils, namely 118A to 122A, 118B -to 122B, 123A to 127A
and 123B -to 127B. Not all these coils are visible ~n the drawing. The coils 118A to 122A, viewed in directions parallel to the Y-axis, are disposed opposite -the com-bination coils 123A to 127A. All these coils~ viewed in directions parallel to the Z-axis, are disposed opposite the combination coils 1l8B to 121B and l23B to 127B
re~pectively. ~oreover, all combination coils, viewed in directions paralle:l to the X-axis, are each spaced frorn the nearest cornbina-tion coil. T~e electromagnet:ic forces which are exerted on the objective holder 11L~ by all these '7~3~
PHN 100095 33~ 2-2-1982 combination coils are all directed parallel to -the X-axis.
For the movements of -the objective ho:Lder in the Z-direct-ion there is provided an actuator coil 128. The frame 115 comprises four elongate stator magne-ts 129A,B and 130A9 B, These magnets are arranged on ferromagnetic yokes, com-prising guide portions 131A and 131B and end pla-tes 132A
and 132B. At their sides which face the objective holder the guide portions 131A, B comprise guide surfaces which cooperate with the flat side faces of the o'bjective holder, so this holder is movable with a small clearance betwe0n the guide plates in the X-Z plane only. The cornbination coils, leaving an air gap, move along the stator magnets 129A,B and 130A,B in slo-ts '133A, B and 13L~A,B. The ac-tuator coil 128 for the focussing moves in a slot 135 in a separate stator comprising an elonga~ permanent stator magnet 136 and two yoke plates 137A and 137B.
The combination coils are arranged in four sets ''of five, with the coils in each set disposed adjacent and overlapping each other, the combination coils 118A to 122A and 118B to 122B being disposed substantially in a ~irst plane parallel to the X-Z plane and -the combina-tion coils 123A to 127A and 123A to 127B being disposed sub-stantially in a second plane parallel to the X-Z plane The elonga-te permanent stator magnets 129A,B and 130A,B
comprise a regular pattern of alternate areas of north and south polarity in the longitudinal direction, that is~
parallel to the X-axis. For driving the objective holder 111~ in the direction of the X-axis the combination coils are elec-trically connected to comrnutation means which, in a manner known per se~ commutate the electric currents to be applied to the combination coils, depending on the position and 1;he direction of the movement of -the objective holder along 1;he X-axis. In this way~ as is lunown~ an ef'~icient transla-tional drive is possible. On a side face of the objective holder 11L~ a prism 138 is mounted, which prism belon~s to an opto-electronic objective-position mea~1lring device o~ the type already described. The auxil:iary light beam is directed towards the prism 138 P~ 10.095 -3~- 2-2-19~2 along the X-axis. The reflecting faces of -the prism are arranged so that -the reflected sub-beams are returned at a small angle to the X-axis. Even in the case of a large travel of the objec-tive holder this prevents the reflec-t-ed sub-beam from impinging on the guide portions 131A and .
By means of the apparatus of Figure 18 very short access -times can be achieved. ~he objective holder 114 and the moving parts connected to it, such as the objective 116 and the combination coils, may have a very low mass.
The combination coils or the actuator coil 128 do not subject the objec-tive holder to force components acting in the Y-direction, so that extremely small frictional forces occur between the objective holder 114 and the guide sur~aces of the guide portions 131A~ Bo Therefore, this embodiment seen~s to be particularly suitable for use in optical storage equipment.
Figures 19 and 20 relate to an embodiment of an opto-electronic appara-tus in accordance with the invention, which again bears resemblance to the apparatus of ~igures 13 to 17. The objective holder 139 contains an objective 140 and is made of a ferromagnetic material. l`he optical axis of the objective i40 again coincides with the Z-axis.
The objective holder is movable in the X-Z plane, desired degrees of freedom being translational movements along the X-axis and the Z-axis and all the other degrees of freedom being undesired. The fr,ame 141 again carries four com-bination coils 142A, B and 143A, B. The objective holder 139 carries an ac+ua-tor coil 14L~ whose cen-tre lies vn the Z-axis. By means of the four combination coils the objecti-ve holder 139 can be moved -translationally in the X-direct-ion and rota-t:ional movements about the Y-axis can be counteracted electromagnetically. 'rhe actuator coil 144 serves only ~or translational movements along -the Z-axis.
t The objective holder 139 cornprises pole shoes 145 and 146 which projec-t from -the holder in directions parallel to the X-ax:is. They extend over substantially the ent:ire height of the objective holder. They are integral 79~

PI~N 10.095 ~35~ 2-2-1982 with the other parts of the objective holder and are con-sequently also ~erromag-ne-tic. On the ~rame 141 ferromagnet-ic s-tator pole shoes 147A, B and 148A, B are arrangedO They comprise ~ree ends which, leaving an air gap, are disposed opposite the ~ree ends o~ the pole shoes 11~5 and 146 o~ the objective holder 139. The combination coils are arranged on the stator pole shoes. These pole shoes are glued in pairs onto two permanen-t stator magnets 149 and 150, which are magnetized in the direction o~ the X-axis. The perma nent magnets 149 and 150 are mounted on two U-shaped iron ~rame members 151 and 1520 Owing to the presence of the permanent magnets 149 and 150 a permanent magnetic ~ield exists in the air gaps between the stator pole shoes and the objective pole ~hoes. The objective pole shoe 145 comprises teeth 153. The stator pole shoe 147A comprises teeth l54 which magnetically cooperate with the teeth 153 of -the objective pole shoe. The other pole shoes have similar teeth. This results in a comparatively high magnetic rigidity in the Y-direction, so that no mechanical or electromagnetic bearing means are needed ~or counter-acting translational movements of the objective holder 139 along the Y-axis or rotational movements abou-t the X and the Z-axis. The studs 155 on the inner side of th0 frame members 151 and 152 only serve as sa~ety stops.
A separate permanent magnetic stator ~56 is provided for cooperation with the actuator coil 144. Said stator comprises two annular permanent magnets 157 and 158 which are magnetized in opposite axial directions, an annular iron disc 159 glued be-tween these magne-ts, an iron housing 160 and a co-ver 161. The yoke 160 has a central cylindrical portion 162 and the actuator coil 1 L~l~ moves in the air gap between the disc 159 and the cylindrical portion 16~. The permanent magnetic ~ield extends radially via -this air gap.
'~le ~rarne 14l is mounted on the cover 161 o~ the stator 156 by rneans o~ two L-shaped brackets 163 and 164 and a number o~ screws.

7~7 ~l~ 10.095 -36- 2-2~1982 The objective-position measuring device is again of the capacitive type. On the pole shoes 147A9B and 140A, retaining rings 166A, B and l67A, B are ~itted~
On -these rings metal plates 168A,B and 169A,B are arranged opposite the pole shoes 1L~5 and 146 of` the objective holder. Each of the me-tal plates f`orms a capacitive elemen-t with the surf`ace o~ the ~acing pole shoe~ By means of` the ~our capacitive elernents it is possible to measure trans-lational movements along the X-axis and rotational move-men-ts about the Y-axis~ The error signals rela-ting to the rotational movements about the Y-axis are used in a levitation control circuit ~f`or counteracting said rotational movements by applying levitation currents -to the combination coils.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An opto-electronic apparatus for writing and/or reading recording tracks in a recording surface of a record carrier by means of a radiation beam, such as the scanning by means of a light beam of video and/or audio recording tracks or digital data recording tracks in a reflecting recording surface of a rotating video or audio disc or a data disc respectively, which apparatus com-prises:
- a frame, - an objective holder provided with an objective having an optical axis and having a lens system for concentrating the radiation beam so as to form a radiation spot in a focussing plane, - a bearing arrangement for the objective holder, which arrangement comprises bearing means which permit move-ments of the objective holder relative to the frame in accordance with a desired number of degrees of freedom out of six theoretically possible, independent degrees of freedom and which substantially counteract those movements of the objective holder relative to the frame which are in accordance with the other, undesired degrees of freedom, and - actuator means comprising parts which are connected to the frame and to the objective holder respectively and which cooperate magnetically with each other via an air gap, for driving the objective holder relative to the frame with electromagnetic actuator forces in accordance with the said desired degrees of freedom, which actuator means comprise at least one actuator coil for each desired degree of freedom, characterized in that:
- the apparatus comprises an objective-position measuring device for continuously measuring the position of the objective holder relative to the frame in accordance with at least one of the said undesired degrees of freedom and generating a positional-error signal, - the bearing arrangement comprises bearing means com-prising parts which are connected to the frame and to the objective holder respectively and which cooperate magnetically with each other via an air gap for counter-acting undesired movements of the objective holder in accordance with at least one of said undesired degrees of freedom by means of electromagnetic levitation forces, which means comprise at least one levitation coil for the or each undesired degree of freedom in accordance with which an undesired movement of the objective holder is counteracted by electromagnetic levitation forces, and - there a levitation control circuit is provided which compares the positional-error signal from the objective-position measuring device with a reference signal of specific value and which comprises at least one output for applying an electric levitation current whose value depends on the error signal to a levitation coil for maintaining substantially constant the position of the objective relative to the frame in accordance with an undesired degree of freedom.

2. An opto-electronic apparatus as claimed in Claim 1, characterized in that all bearing means for counteract-ing movements of the objectiv~ holder in accordance with undesired degrees of freedom comprise only electromagnetic bearing means so that the objective hGlder is supported on the frame only by electromagnetic actuator forces and by electromagnetic levitation forces.

3. An opto-electronic apparatus as claimed in Claim 2, characterized in that - the objective holder is ferromagnetic and annular, - at least three ferromagnetic armatures are arranged on the objective holder at equal radial distances from the optical axis and uniformly spaced around the cir-cumference of the holder, - a radially magnetized permanent magnet is arranged between each armature and the objective holder, - that each armature comprises two armature teeth which extend in a substantially tangential direction and which are spaced from each other in a direction parallel to the optical axis of the objective, - for each armature tooth a combination coil having an elongate shape in a direction parallel to the optical axis is arranged on the frame, which coil functions both as actuator and as levitation coil and comprises turns which are disposed substantially in planes con-taining the optical axis of the objective and each have a central opening which receives the tooth which clearance, and - for translational movements along the optical axis there is provided an actuator coil whose centre lies an optical axis.

4. An opto-electronic apparatus as claimed in Claim 2, characterized in that - the objective holder is at least partly made of a ferro-magnetic material, and - all actuator coils and levitation coils are rigidly mounted on the frame: and cooperate magnetically with ferromagnetic parts of the objective holder.

5. An opto-electronic apparatus as claimed in Claim 4, characterized in that - the objective holder comprises a ferromagnetic mounting ring which is concentric with the optical axis (Z) of the objective, - for movements in accordance with all the degrees of freedom of the objective, except for rotational move-ments about the optical axis, there are arranged on the frame a plurality of coils which are situated on axially opposite sides of the mounting ring and which exert axially opposite magnetic force components on the mounting ring, and a plurality of coils which are situated on diametrically opposite sides of the mount-ing ring and which exert diametrically opposite magnetic force components on the mounting ring, which coils function as actuator coils, as levitation coils, or both as actuator coils and levitation coils.

6. An opto-electronic apparatus as claimed in Claim 1, characterized in that - the bearing arrangement permits only movements of the objective holder relative to the frame in accordance with two desired degrees of freedom in a single plane of movement, namely a first degree of freedom along a first axis (Z) of translational movement which coincides with the optical axis of the objective, and a second degree of freedom along a second axis (X) of transla-tional movement which is perpendicular to the first axis of translational movement, and - for electromagnetically driving the objective holder along the second axis of translational movement and for electromagnetically counteracting movements of the objec-tive holder in accordance with a third, undesired, degree of freedom about an axis of rotation (Y) which is perpen-dicular to the first and second axes of translational movement, there are provided combined electromagnetic means comprising at least a first and a second combina-tion coil which function. both as actuator coils and as levitation coils, which two combination coils are spaced from each other, viewed in a direction parallel to the second axis (X) of translational movement.

7. An opto-electronic apparatus as claimed in Claim 6, characterized in that - the objective holder is movable relative to the frame along the second axis (X) of translational movement over a distance of not more than a few millimetres, - two sets of combination coils are provided which are dis-posed on opposite sides of the objective holder, viewed in a direction parallel to the first axis (Z) of trans-lational movement, and - an actuator coil is provided whose centre lies substan-tially on the optical axis of the objective holder, for exerting actuator forces along the optical axis of the objective.

8. An opto-electronic apparatus as claimed in Claim 6, characterized in that - the bearing means, in order to permit movements of the objective holder only in a single plane of movement in accordance with the two said desired degrees of freedom, comprise spaced guide surfaces which are parallel to said plane of movement, and - movements of the objective holder which are not within said plane of movement (X-Z) and consequently are in accordance with the three other degrees of freedom, are substantially counteracted by the guide surfaces.

9. An opto-electronic apparatus as claimed in Claim 8, characterized in that - the objective holder is movable relative to the frame along the second axis (X) of translational movement over a distance which is sufficient for writing and/or read-ing recording tracks over the entire recording surface of a record carrier, - said first and second combination coils are spaced from each other viewed in directions parallel to said two axes (Z, X) of translational movement and also to a third axis (Y) of translational movement which is per-pendicular to said two axes, - the combination coils exert on the objective holder only electromagnetic forces which are directed parallel to the second axis (X) of translational movement - an actuator coil is connected to the objective holder for exerting actuator forces which are directed parallel to the first axis of translational movement, and - the parts of the electromagnetic actuator means and of the electromagnetic levitation means which are con-nected to the frame comprise a plurality of elongate stator portions which are parallel with the second axis of translational movement and which have a length which is at least equal to said distance of movement of the objective holder, said coils being movable relative to said elongate stator portions in a direction parallel with the second axis of translational movement in such a way that an air gap is left.

10. An opto-electronic apparatus as claimed in Claim 9, characterized in that - said first and second combination coils belong to a first set and a second set of combination coils respectively, which coils are disposed in first and second planes substantially parallel to the plane of movement of the objective holder, the coils in each set being arranged adjacent each other and overlapping each other in a direction parallel with the second axis of translational movement, - said elongate stator portions comprise elongate perma-nent-magnetic stators on the frame, which stator portions have areas of alternate north and south polarity which extend in accordance with a regular pattern in the longitudinal direction, and - commutation means are provided for commutating the electric currents to be applied to the combination coils, depending on the position and the direction of movement of the objective holder along the second axis (X) of translational movement.

11. An opto-electronic apparatus as claimed in Claim 7, characterized in that - the objective holder is provided with ferro-magnetic objective pole shoes with free ends, which shoes project from the holder in directions parallel to the second axis (X) of translational movement, - a plurality of ferromagnetic stator pole shoes are arranged on the frame, which pole shoes have free ends which are each disposed opposite the free end of an objective pole shoe to form an air gap, and - said combination coils are arranged on the stator pole shoes.

12. An opto-electronic apparatus as claimed in Claim 11, characterized in that - the apparatus comprises at least one permanent magnet for producing a permanent magnetic field in the air gaps between the stator pole shoes and the objective pole shoes, - the stator pole shoes and the objective pole shoes comprise teeth which face each other and magneti-cally cooperate with each other, via the air gap, and - the frame comprises a ferromagnetic yoke which magne-tically couples the permanent magnets to the stator pole shoes.

13. An opto-electronic apparatus as claimed in Claim 1, characterized in that the objective-position measuring device at least comprises:
a) two capacitive elements arranged in series, compris-ing facing stationary plates of an electrically conductive material on the frame and facing movable plates of an electrically conductive material on the objective holder, b) a high-frequency alternating current source (a), c) a voltage difference circuit which is inductively coupled to the high-frequency alternating current source and connected to the two capacitive elements, and d) means for applying the output signal (SL) of the voltage difference circuit to the levitation con-trol circuit.

14. An opto-electronic apparatus as claimed in Claim 1, characterized in that the apparatus comprises an opto-electronic objective-position measuring device, which device comprises:
a) a radiation source on the frame for emitting a radiation beam towards the objective holder, b) a radiation-sensitive detection system on the frame, which system comprises a plurality of detectors, each divided into at least two sub-detectors, the separating lines between the sub-detectors being parallel to each other, c) a beam-splitting optical element on the objective holder for splitting the radiation beam emitted by the radiation source in the direction of the radiation-sensitive detection system into sub-beams, the radiation-distribution among the sub-detectors being determined by the position of the optical element and thus of the objective holder relative to the frame, and d) an electronic circuit for supplying positional error signals, which circuit comprises inputs which are each individually connected to a sub-detector of the detection system.