1. Field of the Invention
The present invention relates to an optical disk reproducing device, and, more specifically, relates to an improvement in a focusing servo mechanism for an optical disk reproducing device which eliminates a disturbance injection, permits a focusing (an automatic focus control) with a further simple circuit, and increases an S/N ratio as well as unnecessitates a readjustment even when such as an external pick up unit is exchanged.
2. Conventional Art
Recently, with regard to CD-ROM devices, the data read out speed thereof is rapidly increasing, for example, from a double speed to 32 times speed or more than that speed, for this reason, it is required to read data in a speed range from double speed to 32 times speed or more that speed. Further, also with regard to optical disk memory devices, the memory density thereof is rapidly increasing, for example, to double, four times, six times, eight times and so on.
For these sorts of the optical disk reproducing devices, a focusing servo mechanism with a very high accuracy is required. FIG. 5 is a diagram for explaining such a focusing servo mechanism.
In FIG. 5, numeral 1 is a focusing servo mechanism, numeral 2 is an optical disk therefor, numeral 3 is a rotation mechanism therefor, and numeral 4 is an optical system therefor which is constituted by a laser unit 41 for irradiating laser beams onto the optical disk 2 and a half mirror 42. Numeral 5 is a focusing servo circuit which is constituted by a photo detector (pick-up) 51, a focus position error generation circuit 52, a level detection circuit 53, a response wave form generation circuit 54, an amplitude detection circuit 55, a focus automatic adjustment controller 56, a focus servo controller 57, an offset amount adjustment circuit 58, a signal level synthesis circuit 59 performing a subtraction synthesis of the signal levels, a disturbance oscillator 60, another signal level synthesis circuit 61 performing an addition synthesis of the signal levels, a phase compensation loop filter (a so called low pass filter of CR) 62, a search wave form generation circuit 63, a change over circuit 64, a drive amplifier 65 and a focus position adjustment mechanism 66.
Herein, the tetra-section photo detector 51 uniformly divides the sensor area into four sections and generates respective detection signals of the corresponding sectioned areas. The tetra-section photo detector 51 receives reflection beams from the half mirror 42 of the optical beams from the optical disk 2, converts the received optical beam amount irradiated on the respective areas into electrical signals, generates sum signals of opposing sensor areas in different directions as detection signals A and B and sends out the detection signals A and B to the focus position error generation circuit 52.
The focus position error generation circuit 52 generates a sum signal of the detection signals A and B as a detection signal RF with regard to the total received optical beam amount (which is a detection signal representing an optical beam amount of the reflection optical beams from the optical disk received by the photo detector, and is a signal corresponding to a reflection level of the reflection optical beams from the optical disk, and which is hereinbelow called as a RF detection signal RF), as well as generates a difference signal of the detection signals A and B as a position error signal FE (FE is an abbreviation of Focus Error). Among these signals, the position error detection signal FE is applied to the phase compensation loop filter 62 via the signal level synthesis circuits 59 and 61. Then, low pass components in the signal are extracted in the phase compensation loop filter 62 and are applied to the drive amplifier 65 under a condition of an automatic focus control (hereinbelow, called as a just focus control), and the drive amplifier 65 applies an output in a direction correcting the focus error depending on the level of the low pass components to the focus position adjustment mechanism 66 to drive the laser unit 41 so that the level of the position error detection signal FE is reduced and the focusing position is automatically adjusted to a focused position.
Further, the just focus control is effected in a direction so as to eliminate the position error depending on the level of the position error detection signal FE.
On the other hand, the RF detection signal RF is applied to the level detection circuit 53 to detect whether the RF detection signal RF falls within a range of the just focus control depending on the level thereof. When the RF detection signal RF reaches the level of the just focus control, the level detection circuit 53 detects the same and the detection signal thereof is inputted to the focus servo controller 57. When the focus servo controller 57 receives the detection signal from the level detection circuit 53, the focus servo controller 57 changes over the change over circuit 64 to the side of the phase compensation loop filter 62 and sends out the same to the drive amplifier 65. Thereby, the just focus control is effected.
Further, the change over circuit 64, for the first time, is set to the side of the search wave form generation circuit 63 which is designed to generate a ramp voltage for searching a just focus point by the focus servo controller 57 (the search wave form generation circuit is a so called ramp voltage generation circuit which generates signals which cause to advance or back the focusing position continuously with respect to the disk in order to search the focusing position), and in response to the rotation drive of the optical disk 2 the search wave form generation circuit 63 is activated by the focus servo controller 57 and outputs the ramp voltage to drive the drive amplifier 65. Thereby, the drive amplifier 65 drives the focus position adjustment mechanism 66, and the focus position adjustment mechanism 66 moves a focusing lens incorporated in the laser unit 41 depending on the ramp voltage so that the focusing position of the laser beams irradiated from the laser unit 41 onto the optical disk is raised from the lowest point. Thus, the level of the RF detection signal RF increases and in response to the detection signal of the level detection circuit 53 which detects the level thereof the control moves into a condition which permits the just focus control, and at this moment the above referred to change over of the change over circuit 64 is performed by the focus servo controller 57.
Further, the pick-up unit includes the tetrasection photo detector 51, the laser unit 45 having a laser beam source and a lens disposed in an optical passage of the laser beam source and the focus position adjustment mechanism 66 which moves the lens back and forth directions, and through movement by the focus position adjustment mechanism 66 of the focusing lens which is provided in the laser unit 41 and is disposed movably along the optical axis of the laser beams from the laser beam source, the focusing position of the laser beams which are outputted from the laser beam source onto the irradiation face of the optical disk 2 can be moved in back and forth directions. Further, herein either with the drive amplifier 65 or with the focus position adjustment mechanism 66 a so called negative feed back control is formed which receives the synthesized signal from the signal level synthesis circuit 61 via the phase compensation loop filter 62 and generates an output which drives the focusing lens in the direction for reducing the position error detecting signal FE.
Accordingly, at the moment when the change over circuit 64 is changed over to the side of the phase compensation loop filter 62, the focusing servo mechanism 1 moves into the just focus control. At this instance, the focus automatic adjustment controller 56 which has received a signal from the focus servo controller 57 activates the disturbance oscillator 60. The disturbance oscillator 60 generates, for example, a saw tooth wave of about 1 kHz as a disturbance signal. The disturbance signal is added via the signal level synthesis circuit 61 to the position error detection signal FE representing the output of the focus position error generation circuit 52, the added signal is applied to the phase compensation loop filter 62 and in response thereto the focus position adjustment mechanism 66 is controlled by the drive amplifier 65, thereby, the focusing position of the laser beams irradiated from the laser unit 41 is moved finely in vertical direction and the RF detection signal RF at the moment is inputted to the response wave form generation circuit 54, in which a response wave form is generated, and the amplitude thereof is detected by the amplitude detection circuit 55 and the detection signal is inputted to the focus automatic adjustment controller 56. The focus automatic adjustment controller 56 estimates sensitivity unbalance amounts of the sensor areas divided into four sections in the tetra-section photo detector 51 and sends out a signal for generating an offset depending on the unbalance amount to the offset amount adjustment circuit 58. Then, the disturbance signal from the disturbance oscillator 60 is stopped.
The offset amount adjustment circuit 58 generates in response to the input signal an output depending on the offset amount, and the output is applied to the negative side input (xe2x88x92) of the signal level synthesis circuit 59 and is synthesized by the signal level synthesis circuit 59 into the position error detection signal FE so as to subtract the level of the position error detection signal by the offset amount (if the offset amount is negative value, the subtraction result effects as an addition). Thereby, the sensitivity unbalance amounts of the sensor areas are corrected. As a result, in the focusing servo mechanism 1 the phase compensation loop filter 62 generates an output providing the just focus position in response to the position error detection signal FE and through the output corresponding to the offset amount from the offset amount adjustment circuit 58, the detection position offset due to the sensitivity unbalance is corrected as well as the laser beams irradiated from the laser unit 41 are focused onto the optical disk 2.
FIG. 6 is a diagram for explaining the detection signals A and B of the tetra-section photo detector 51 and the offset amount and illustrates a case in which the level of detection signal A is larger than the level of detection signal B. In the drawing, the ordinate denotes voltage (V) representing the detection level and the abscissa denotes distance (xcexcm) from a reference position to a focus point.
From the drawing, it is observed that the zero cross point C of the position error detection signal FE representing a difference signal between the detection signals A and B is shifted to the right side with respect to the peak position P of the RF detection signal RF representing a sum signal of the detection signals A and B. When the relationship between the detection signals A and B is opposite as that indicated above, the zero cross point C will be shifted to the opposite direction.
Now, with the conventional focus control as has been explained above, if not the loop of the focus control enters into the just focus control, a just focusing of the optical beam source with respect to the optical disk can not be set. However, if subjected to an external vibration, the optical beam receiving position of the tetra-section photo detector 51 offsets, therefore, the automatic focusing servo loop can not be maintained with the just focus control.
Further, in order to form the automatic focusing servo loop, it is necessary to set the offset amount, for this purpose a circuit for generating a disturbance signal is indispensable,- and wherein a level adjustment of the disturbance signal is necessary so that the automatic focusing servo loop never goes out from a certain level. Therefore, the adjustment of the offset amount becomes difficult.
Still further, near the just focus point at which a signal wave form for searching a focus position is generated, the level of the response wave form with respect to the disturbance signal becomes small, therefore, a highly accurate detection is required for estimating the offset amount, in addition, since the unbalance amount depends on the tetra-section photo detector 51, readjustment is necessitated when parts are exchanged, moreover, it takes time for the readjustment.
An object of the present invention is to resolve the above mentioned conventional problems and to provide an optical disk reproducing device which eliminates a disturbance injection, permits a focusing with a further simple circuit, and increases an S/N ratio as well as unnecessitates a readjustment even when such as an external pick up unit is exchanged.
An optical disk reproducing device for achieving the above object and including a focus servo mechanism in which in response to reception of reflection beams from an optical disk by a photo detector, a received optical beam amount detection signal corresponding to the received optical beam amount and a position error detection signal with respect to a focusing position error are generated, and in response to the level of the detection signal of the received optical beam amount a focus servo controller is changed over to an automatic focus loop and through a control in a direction eliminating the position error depending on the detection signal of the position error laser beams from a laser beam optical source are automatically focused with respect to the optical disk, is characterized in that the focus servo mechanism comprises: a signal generation circuit which generates a signal for advancing or backing a focusing position of the laser beams with respect to the optical disk; a maximum value detection circuit which detects the maximum value of the detection signal of the received optical beam amount; and a level holding circuit which holds the level of the detection signal of the position error at the moment in response to the detection signal from the maximum value detection circuit, wherein the focus servo controller drives the signal generation circuit and causes to detect the maximum value through the maximum value detection circuit without changing over to the automatic focus loop, and thereafter performs a control of changing over to the automatic focus control, and after synthesizing the level of the detecting signal of the position error held in the level holding circuit and the level of the current detection signal of the position error an automatic focusing is performed in the automatic focus control.
Now, in the focus servo mechanism for an optical disk as has been explained in the section of CONVENTIONAL ART, namely, in the focus servo mechanism in which in response to reception by the photo detector of reflected optical beams from the optical disk, the RF detection signal RF and the position error detection signal FE are generated, and in response to the level of the RF detection signal RF the focus servo controller moves into the automatic focus control by changing over to the automatic focus loop and through a fine movement of the focusing position of the laser beams in response to the position error detection signal FE the optical beams from the laser beam source are automatically focused with respect to the optical disk, since the maximum peak position of the RF detection signal RF corresponds to the focusing position, if the control moves under a condition of the automatic focus control near the maximum peak position, an automatic focusing can be performed through the automatic focus loop control.
Further, at the focused position corresponding to the maximum peak position of the detection signal RF the level of the position error detection signal FE is primarily to have to assume xe2x80x9c0xe2x80x9d, therefore, the difference component from xe2x80x9c0xe2x80x9d corresponds to the offset caused by the sensitivity unbalance of the sensor areas.
Now, as a specific example of the level holding circuit, for example, a memory circuit which stores the level can be used. In such instance, in correspondence with the maximum peak position of the RF detection signal RF the level the position error detection signal FE is stored and the level stored under the condition of just focusing servo as representing the offset amount can be synthesized with the level of the position error detection signal FE. Further, the maximum value detection circuit can be modified to a detection circuit of the maximum peak value and the minimum peak value, and as a specific example of the level holding circuit a circuit which holds an average value of the maximum peak and minimum peak values can be used. In this instance, after detecting the maximum peak value and the minimum peak value of the levels of the position error detection signal FE the average value thereof is calculated, and using the average value as the offset amount the level of the position error detection signal FE is synthesized.
Since, the offset amount of the position error detection signal FE from the zero crossing point corresponds to the offset caused by the sensitivity unbalance of the sensor areas, in such case, when the zero crossing point is shifted to a positing representing the average value of the maximum peak value and the minimum peak value of the levels of the position error detection signal FE, the offset can be corrected.
As has been explained, according to the present invention, with the provision of the level holding circuit, by sampling the level of the detection signal of the focusing position error when the detection signal representing the received optical beam amount shows the maximum value as the offset amount and by holding the same in advance in the level holding circuit without changing over to the automatic focus loop, the held level is utilized under a condition of the just focusing servo to effect the automatic focusing.
In the above two cases, the storage of the offset amount based on the position error detection signal FE or the calculation of the average value is performed under a condition other than the automatic focusing servo condition. Thereby, influences due to disturbance noises are eliminated and no disturbance oscillator is required. As a result, with a further simple circuit the focusing can be performed without injecting disturbances and S/N ratio can be enhanced. Moreover, a readjustment is unnecessitated even when such as an external pick-up parts are exchanged.