Focusing system for a moveable lens

Abstract: The present invention relates to a focusing system for a moveable lens comprising a moveable lens (10), moving means (15,16) for moving the lens, and a control system (20, 21, 22, 23) for supplying position control signals 24) to the moving means and for detecting when the lens is in an in-focus position. According to the invention the focusing system is characterised in that the control system comprises means for supplying a ramp position control signal to the moving means to move the lens into an in-focus position, and modulating means for modulating the ramp position control signal so that it becomes a series of pulses.

Description: The present invention relates to optical focusing systems, and to the acquisition of an in-focus state using such systems. The present invention is particularly useful for optical disk recorders. Optical disk recorders, in particular, employ objective lenses having very small size and mass. Typically, such objective lenses are slideably mounted on a moveable support such that the beam passing through the objective lens is appropriately focused and positioned with respect to a recording disk. Generally, the acquisition of an in-focus state i.e., the movement of the objective lens to an in-focus position wherein the laser or other light beam is appropriately focused at the recording level, should be reliably accomplished for preventing retrys and delays in using an optical disk recorder. Also the best focus should be quickly and accurately obtained. It is desired also to ensure that the control of the in-focus state acquisition is simplified for reducing costs of the optical system. One of the problems involved in accurately acquiring the in focus state is that of "stiction". That is, as the objective lens is urged towards the in-focus position from an out-of-focus position, the lens may be subjected to undesired friction causing undesired and erratic motions (known as "stiction") in the focus acquisition process. Such undesired changes in friction reduces the probability of rapidly and accurately acquiring focus. It is desired to provide a simple system which aviates the stiction problem and provides accurate focus acquisition at low cost. US-A- 4,733,066 shows a typical prior art focus acquisition system in which the in-focus state is reached during a so-called open loop mode. Upon detecting an in-focus condition, a focus maintenance servo is actuated for maintaining the focus of the objective lens. This specification also shows, the typical control signal having a ramp shape for moving the objective lens to the in-focus position. The arrangement in this specification requires a resilient means for supporting the focusing lens. Such resilient means is desired to be eliminated. EP-A- 209,146 shows a focus acquisition system apparently intended to overcome "stiction" and other frictional effects as discussed above. According to this published specification, the electrical current supplied to a coil for moving the objective lens towards the in-focus position is reversed for momentarily urging the objective lens to move away from the in-focus position. Apparently, the magnitude of the current for driving the objective lens towards the infocus position when it is turned on, is greater than the magnitude of the current which urges the lens to move in the reverse direction. A damper shown in this specification is apparently necessary for providing a smooth approach to the in-focus position. Column 4 of the specification, particularly the discussion beginning at line 44, describes the above system. The object of the present invention is to provide an improved focusing system for a moveable lens. The present invention relates to a focusing system for a moveable lens comprising a moveable lens, moving means for moving said lens, and a control system for supplying position control signals to said moving means and for detecting when said lens is in an in-focus position, wherein said control system comprises means for supplying a pulsed position control signal to said moving means to momentarily and repeatedly urge the lens towards the in-focus position so that the lens is allowed to coast between each successive pulse, and mpdulating means for generating the pulsed position control signal using a ramped position control signal. Such a system appears to be disclosed in JP-A-1-192021 and Patent Abstracts of Japan, Vol 13 No 482 (P-953) (3830 (1989). According to the invention the focusing system is characterised in that the pulsed position control signal is a series of pulses amplitude modulated by the ramped position control signal. In order that the invention may be more readily understood an embodiment will not be described with reference to the accompanying drawings, in which: Fig. 1 is a simplified diagrammatic illustration of a focusing system employing the present invention in an optical disk apparatus, Fig. 2 is a set of idealized wave forms used to illustrate the operation of the Fig. 1 illustrated system, Fig. 3 illustrates one form of an in-focus detection and focus acquisition system usable with the Fig. 1 illustrated system, and Fig. 4 illustrates an alternative form of the in-focus detection and focus acquisition system usable with Fig. 1 illustrated system. Referring more particularly to the drawings, like numerals indicate like parts of structural features in the various figures. A moveable objective lens 10 is used to focus a light beam 12 on disk 11 for recording or sensing information on the disk 11. The beam 12 is aligned with the optical axis of objective lens 10 in the usual manner. Optics 13, of usual design in optical disk recorders, optically couple the objective lens 10 to a laser 14 for generating the light beam that travels along path 12. A focusing coil 15, mounted about the lens 10 and preferably movable therewith, generates magnetic fields which co-act with the stationary magnetic fields from a permanent magnet 16 which is suitably mounted on a frame (not shown). A set of bearings 17 slideably support lens 10 for movement along its optical axis. It is to be understood that track seeking and track following motions of the beam 12, which are perpendicular to the optical axis of lens 10 and are used for seeking and following recording tracks on disk 11, are provided in the usual manner. Disk 11 reflects the laser 14 generated light beam back through objective lens 10 to optics 13. Optics 13 in the usual manner redirects the reflected light along path 20 to focus detector 21. Focus detector 21 supplies a focus error signal FES over electrical line 22 to focus circuits 23. Focus circuits 23 are coupled to coil 15 by a pair of signal lines 24 for supplying appropriate focus control signals, as will become apparent. Fig. 2 illustrates an open loop position control signal 30 in the shape of a ramp which is designed to move the objective lens 10 toward the disk 11 for acquiring an in-focus state of the laser 14 beam. Initially, at reverse step 31 lens 14 is moved by signal 30 to a most remote position, i.e., furthest position away from disk 11 which is a clear out-of-focus condition as indicated by the signal 33 portion of the FES signal. Ramp 30 is modulated as represented by position control pulses 32. The position control pulses 32 momentarily and repeatedly urge the lens 10 to move closer to the in-focus position, i.e., away from the position moved into as a result of the reverse step 31. The lens 10 is allowed to coast between each successive pulse 32, thereby providing a soft control of the lens 10 movement within bearings 17. The pulse control tends to overcome any "stiction" or friction which would cause lens 10 to bind and tend not to move toward disk 11. In the prior art, such stiction could cause the lens 10 to momentarily lock up. Then a position control signal which is usually in the shape of the ramp 30 would cause lens 10 to overshoot the in-focus position, requiring a focus retry. It has been determined that the position control pulses 32 which repeatedly momentarily urge the lens 10 toward disk 11 overcome the stiction. Coasting movement of the lens 10 between successive pulses 32 prevents accumulation of a difference between the actual lens 10 position and the desired position indicated by ramp 30. Pulses 32 are modulated within a power amplifier as will become apparent. The focus error signal at 33 indicates a complete out-of-focus condition and at positive excursion 34 indicates an approach to the in-focus position at point 39. The peak of signal portion 34 is detected resulting in a peak detected output signal 35. At this point, the pulses 32 are terminated and the lens is allowed to coast towards in-focus condition as represented by the signal portion 39. A stopping or reversing pulse 37 stops the lens 10 within the beam focus maintenance zone at the position represented by the signal portion 39 such that the focus maintenance servos (not shown) maintain the lens at the-in-focus position. Focus control circuits include a compensator which provides a modification of the control signal to the power amplifier as represented by numeral 36 in Fig. 2. Fig. 3 illustrates, in block diagram form, one embodiment of the focus circuits 23. The FES signal on line. 22 is supplied to compensator 40 which provides the control signal 36 shown in Fig. 2. Compensator 40 is in that portion of the focus circuits which maintains in-focus conditions. Power amplifier 41 supplies the signals over lines 24 to focus control coil 15. Electronic switch 42 is electrically actuated as will become apparent. At terminal 43, the power amplifier 41 is set to a voltage reference, such as ground reference potential. At this point, no signals are supplied over lines 24, hence the lens 10 is not moved. Terminal 44 connects the power amplifier 41 to later described focus acquisition circuits which generate the ramp 30. Terminal 45 is connected to compensator 40 and is used during focus maintenance conditions, i.e. after the lens 10 has reached the in-focus position. Control of switch 42 is indicated by dashed line 47 which is coupled to focus state control 50. On power on, the actual location of lens 10 is not known. The power on reset signal supplied over line 48 sets the off flip-flop 51 to the active state. Off flip-flop 51 sends a signal to decode circuit 52 which in turn responds by actuating switch 42 to position 43 such that lens 10 is not inadvertently moved during power up sequencing. The line 48 power on reset signal also travels through OR circuit 49 to reset in-focus indicating "IN" flip-flop 54 and also resets acquire ACQ flip-flop 53. In this machine state, lens 10 resides in an initial unknown position. To acquire focus i.e., move lens 10 to the in-focus position, a focus acquire signal on line 55 sets ACQ flip-flop 53 to the active condition for initiating focus acquire. It also supplies the signal through OR circuit 49 to again reset the in-focus condition flip-flop 54 and reset OFF flip-flop 51. Resetting IN flip-flop 54 also enables reacquiring focus in the case that focus was lost or it is desired to recalibrate and reacquire focus in error recovery procedures. As soon as ACQ flip-flop 53 is set to the active condition, three operations occur simultaneously. ACQ flip-flop 53 actuates ramp generator 60 to generate the ramp signal 30 which is initially designed to move the lens 10 to the remote out-of-focus position (by signal 31). Ramp generator 60 then supplies the ramp signal to terminal 44. Pulse generator 61 supplies pulses to decode circuit 52 to repeatedly actuate switch 42 to move the connections of power amplifier 41 input between terminals 43 and 44 thereby generating the pulses 32. Decode circuit 52 receives the ACQ flip-flop 53 active signal for passing the pulses from pulse generator 61. This is the second state of the three state machine control 50. During the pulse controlled movement of lens 10 from the remote out-of-focus position towards the in-focus position, peak detector 65 monitors the FES signal on line 22 for detecting peak 34. It is well known that the FES signal near the in-focus position 39 has a maximum peak amplitude 34 which defines a remote out-of-focus extremity of a focus maintenance linear region on which point 39 resides, i.e. straight line 38 portion of FES signal. IN flip-flop 54, when set, supplies an activating signal to decode 52 which responds to actuate switch 42 to connect terminal 45 to power amplifier 41 for maintaining the in-focus condition just acquired. IN flip-flop 54 is also connected to the reset input of ACQ 53 for holding it in the reset state while an in-focus condition is maintained. At this point in time, compensator 40 being connected provides the modification signal 36 shown in Fig. 2 which ensures good focus acquisition. Fig. 4 illustrates in block diagram form an alternative embodiment of the focus circuits 23 similar to that shown in Fig. 3 but using two electronic switches rather than a single electronic switch. Operation of the two circuits are identical. Switch 42 is replaced by two separate switches 70 and 71. Switch 70 connects ramp generator 60 through compensator 40 to power amplifier 41 as opposed to the direct connection shown in Fig. 3. This arrangement provides for better effect of compensator 40 action as represented by numeral 36 of Fig. 2. During the focus acquire mode, state control logic 50 supplies an actuating signal to electronic switch 70 for moving it to connect ramp generator 60 to compensator 40. The generation of pulses 32 is achieved by switch 71 which is operated in pulses by state control logic 50 in the same manner as switch 42 was operated in pulses between terminals 43 and 44. In the reset or initial power on state, the electronic states of switches 70 and 71 are not known. The off control state of control 50 moves switch 71 to the ground reference potential and moves switch 70 to the position shown. Upon starting the focus acquire mode, switch 70 is actuated to couple ramp generator 60 to compensator 40 and switch 71 modulates the ramp signal by momentary actuations by state control logic 50. Upon acquiring focus, as indicated by peak detector 65, state control logic 50 actuates switch 70 to the illustrated position for connecting FES line 22 directly to compensator 40 and actuates switch 71 to couple compensator 40 to power amplifier 41.

Claims: A focusing system for a moveable lens comprising a moveable lens (10), moving means (15,16) for moving said lens, and a control system (20, 21, 22, 23) for supplying position control signals (24) to said moving means and for detecting when said lens is in an in-focus position, wherein said control system comprises means (60) for supplying a pulsed position control signal (32) to said moving means to momentarily and repeatedly urge the lens towards the in-focus position so that the lens is allowed to coast between each successive pulse, and modulating means (42, 43, 44) for generating the pulsed position control signal using a ramped position control signal (30), characterised in that the pulsed position control signal is a series of pulses amplitude modulated by the ramped position control signal. A focusing system as claimed in Claim 1 wherein said control system comprises means (42) for interrupting said pulsed position control signal (32) when said lens has moved into a position a predetermined distance from said in-focus position. A focusing system as claimed in Claim 2 wherein said control system comprises means for supplying a reversing or stopping control signal to said moving means. A focusing system as claimed in Claim 2 wherein said control system comprises means (40) for supplying a compensating position control signal in combination with said reversing or stopping control signal. A focusing system as claimed in any one of the preceding claims wherein said control system comprises means for supplying an initial position control signal to move said lens into an out-of-focus position. A focusing system as claimed in any one of the preceding claims wherein said modulating means comprises at least one electrically controlled switch (42; 70,71). A focusing system as claimed in any one of the preceding claims wherein said control system comprises a peak detector for detecting when said lens is approaching said in-focus position. A method of controlling a focusing system for a moveable lens of the type comprising a moveable lens and moving means for moving said lens, said method comprising supplying a pulsed position control signal to said moving means to move said lens into an in-focus position and detecting when said lens is in an in-focus position, the pulsed position control signal being generated using a ramped signal so that it becomes a series of pulses for momentarily and repeatedly urging the lens towards the in-focus position so that the lens is allowed to coast between each successive pulse, characterised in that the pulsed position control signal is a series of pulses amplitude modulated by the ramped signal.

Inventors: FENNEMA ALAN AUGUST, ROBERTS SPENCER DONALD, FENNEMA, ALAN AUGUST, ROBERTS, SPENCER DONALD
Applicants: IBM, INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: G11B7
IPC codes: G11B7, G02B7