A MULTISTAGE TRACKING SYSTEM

Abstract: A multistage tracking system for an optical disk used in recording, storing and transmitting data and the like. The tracking system using a movable sled with objective lens for coarse tracking of individual tracks on the disk and a stationary tilting mirror providing fine tracking on a selected track.

Description: The invention relates to a multistage tracking system for a rotating optical recording disc, the system comprising: a movable sled with an objective lens and fixed mirror, the sled disposed below tracks on the disk, the sled having an electrically driven drive means for moving the objective lens below and near a desired track; a stationary tilting mirror disposed adjacent the sled, the tilting mirror having an electrically driven mirror drive means for tilting the mirror; and optic means electrically connected to the mirror drive and sled drive coil and providing a light beam directed towards the tilting mirror, the tilting mirror reflecting the light towards the fixed mirror next to the objective lens and focusing the beam through the lens onto a desired track, the optical means further providing an optical generated position signal to the mirror drive means and sled drive coil through and control amplifier. Such system is inter alia known from US-A 4,571,712 or WO-A-84/01849. The known system comprises a coarse tracking loop including the sled and a fine tracking loop including the tilting mirror. The tilting mirror exhibits a resonance having a frequency which is preferably beyond the bandwidth of the fine tracking loop. In order to avoid vignetting of the light beam only very small angles of the tilting mirror are allowed. The bandwidth of the coarse tracking system therefore has to be very high. In order to meet the requirements as described hereinbefore a tracking system as defined in the opening paragraph is characterized in that the control amplifier provides a low frequency drive current directed towards the sled drive means and a high frequency current directed towards the mirror drive means. The advantages and objects of the invention will become evident from the following detailed description of the drawings when read in connection with the accompanying drawings which illustrate preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a schematic of the multistage tracking system. Fig. 2 illustrates an alternate embodiment of the multistage tracking system with parallel or series driven actuator coils. Figs. 3 and 4 illustrate frequency curves of the tilting mirror amplification and sled amplification. Figs. 5 and 6 illustrate curves illustrating the resonance frequency of the tilting mirror and sled drive coil. Fig. 7 and 7A illustrate electrical diagrams for series or parallel driven actuator coils. DETAILED DESCRIPTION OF THE DRAWINGS In Fig. 1 the multistage tracking system is designated by general reference numeral 10. The system 10 is used with a rotating optical disk 12 having a plurality of tracks 14 thereon. The disk 12 is rotated as indicated by arrow 16. A movable sled 18 is positioned above the tracks 14 and moves radially across the width of the tracks as indicated by arrow 20. The movable sled 18 includes an objective lens 22 disposed above a 45 degree fixed mirror 24. The sled 18 is moved radially by an electric motor connected to a drive coil 26. The motor is not shown in the drawings. While the electric motor and drive coil 26 are used to move the sled 18 for coarse tracking, it should be appreciated that various types of drive means could be used equally well. A stationary tilting mirror 28 is disposed adjacent the disk 12 and is connected to an electric motor and a mirror drive coil 30. While the tilting mirror 28 is driven electrically it could also be driven by a variety of different drive means. A light beam indicated by dotted lines 32 is projected from optics 34. The beam 32 may be a parallel or collimated beam which is directed towards the 45 degree stationary tilting mirror 28. The mirror 28 reflects the light toward the fixed 45 degree mirror 24 on the movable sled 18. The beam 32 is focused through the objective lens 22 onto a desired track 14 on the disk 12. The tilting of the mirror 28 is indicated by arrow 29. The multistage tracking system 10 incorporates two stages; a coarse tracking loop using the sled 18 and a fine tracking loop using the stationary tilting mirror 28. The fine tracking loop consists of an optically generated position signal E-RAD which is fed through from the optics 34 via lead 35 to a preamplifier 36 and to a fine control amplifier 38 via lead 39. The fine control amplifier 38 drives the drive coil 30 of the stationary tilting mirror 28. The coarse tracking loop is fed using an angle error signal from the fine tracking mirror 28. The angle error signal can be obtained in two ways. One way is using an angle sensor 40 connected to the mirror 28 via lead 41. When using the angle sensor 40, driving of the coarse loop can be independent of the characteristics of the fine tracking mirror. It is possible to use a mirror 28 with slide bearings and not be affected by the associated friction. The angle sensor 40 feeds the signal through a coarse control amplifier 42 via leads 43 and 45 to the sled drive coil 26. In the alternative and without using the angle sensor 40, the mirror needs to be spring suspended and the bandwidth of the coarse loop is limited by the resonance frequency of the fine tracking mirror 28. Below the resonance frequency the current from the mirror drive coil 30 is proportional to the angle of the tilting mirror 28 and the signal is fed to the coarse control amplifier 42 via leads 47 and 43. In Fig. 2 an alternate embodiment of the multistage tracking system 10 is shown using a control amplifier 48 rather than the fine control amplifier 38 and the coarse control amplifier 42. In this embodiment the control amplifier 48 feeds the two drive coils 30 and 26. The low frequency part of the drive current is directed towards the sled drive coil 26 and the high frequency part of the current directed towards the mirror drive 30. The sled drive coil 26 has a higher inductance that the mirror drive coil 30. With the addition of a high-pass element such as a capacitor 50 connected via lead 51 to the mirror drive coil 30 all the low frequency current from the control amplifier 48 will be directed to the sled drive coil 26 via lead 53. The high frequency portion of the drive current will pass over the capacitor 50 to the mirror drive coil 30 In Figs. 3 and 4, log gain of the controller is shown as a vertical line 54 and log frequency is shown as a horizontal line 56. The resonance indicated by line 58 of the tilting mirror 28 is set at a high frequency, for example a few kilohertz and beyond the bandwidth of the fine tracking loop as shown in Figs. 1 and 2. Without using the angle sensor 40, the bandwidth of the coarse loop can approach the bandwidth of the fine tracking loop. In order to avoid vignetting of the light beam 32 only very small angles of the tilting mirror 28 are allowed. The bandwidth of the coarse system therefore, has to be very high. In Fig. 3, a dotted line 60 illustrates the lead required when driving the mirror beyond its resonance. Fig. 4 illustrates the transfer of the sled control amplifier indicated by line 62. In Figs. 5 and 6 log gain is again shown as a vertical line 54 with log frequency shown as a horizontal line 56. In Fig. 5 line 64 illustrates the mirror loop gain with line 66 illustrating the sled loop gain. A total transfer of the mirror loop and sled loop is illustrated by a dotted line 68. In Fig. 6 a controller transfer is indicated by line 70 with a vertical dotted line 72 representing a crossover total loop. Figs. 7, and 7A illustrate alternate circuiting for series or parallel drive actuator coils with Fig. 7 being a parallel wiring system and Fig. 7A being an alternate series drive circuit application. From reviewing the above detailed description of the multistage tracking system 10, it can be seen that the system applies high-pass filtering for fine mirror tracking that reduces excursions and thus avoids associated adverse optical efforts thereby providing improved coarse and fine tracking used in conjunction with optical recording disks and the like.

Claims: A multistage tracking system for a rotating optical recording disc (12), the system comprising: a movable sled (18) with an objective lens (22) and fixed mirror (24), the sled disposed below tracks (14) on the disk (12), the sled (18) having an electrically driven drive means (26) for moving the objective lens below and near a desired track; a stationary tilting mirror (29) disposed adjacent the sled (18), the tilting mirror (29) having an electrically driven mirror drive means (30) for tilting the mirror; and optic means (34) electrically connected to the mirror drive means and sled drive means and providing a light beam (32) directed towards the tilting mirror (29), the tilting mirror (29) reflecting the light towards the fixed mirror (24) next to the objective lens (22) and focusing the beam (32) through the lens (22) onto a desired track, the optical means (34) further providing an optically generated position signal to the mirror drive means and sled drive means through a control amplifier (48), characterized in that the control amplifier (48) provides a low frequency drive current directed towards the sled drive means (26) and a high frequency current directed towards the mirror drive means (30). The system as described in Claim 1 further including a capacitor (50) wired between the control amplifier (48) and the mirror drive means (30).

Inventors: JANSEN, GERARDUS LUCIEN MATHILDUS, JANSSEN, PETER JOHANNES MICHIEL
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: G11B7
IPC codes: G11B7, G02B26