JPH073696B2 - Optical pickup - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical pickup used in an optical disc reproducing apparatus.

Conventional technology Usually, in portable equipment such as a portable CD player,
Stable movement is required in any posture (vertical, horizontal, licking). In such a case, the problem with the optical pickup is that the focus lens (usually mechanically supported by the damper) that constitutes the optical pickup is affected by gravity due to the attitude difference of the device,
It may be displaced from its original position. This means that during the optical servo, the operating point of the focus lens moves with a mechanical offset, which is one of the causes of lack of operational stability. For this reason, it is usual to mechanically harden the damper that supports the focus lens to some extent so that the focus lens does not deviate much even when the posture of the device changes.

Problems to be Solved by the Invention As described above, in the conventional technology, the harder the damper is, the more the pickup is used in order to reduce the mechanical position shift of the focus lens caused by the posture difference of the device. However, there is a problem that the feedback gain of the optical servo decreases, and the servo characteristics in a normal posture deteriorate. Therefore, is the design usually made at the compromise point by balancing the amount of steady-state characteristic deterioration due to the decrease of the servo gain and the amount of shift (mechanical offset amount) of the focus lens caused by the attitude difference? Or, design the damper very hard and, instead, add a reduction compensation circuit that enhances the reduction of the servo system (including direct current) with an electronic circuit to compensate the mechanical rigidity of the damper, etc. Means are usually taken.

However, the former case only seeks a compromise between the reciprocal conditions, and the latter case loses stability to some extent at the expense of the phase margin, which is one element of the control parameter. There was a problem.

Means for Solving the Problems In order to solve the above problems, in the optical pickup of the present invention, a positive feedback loop that positively feeds back the AC component of the displacement to the actuator that drives the focus lens, and the speed of the actuator. Both the negative feedback loops for negatively feeding back the components are added.

The two feedback loops described above can be used in two directions, the focus direction and the tracking direction, but both may be used together or one of them may be used depending on the purpose.

With the above configuration, the damper becomes equivalently soft in servo operation, and sufficient servo gain can be obtained while maintaining the phase margin of the servo system, and
It is possible to realize an optical pickup that is unlikely to be affected by posture differences.

EXAMPLE FIG. 1 is a block diagram showing an example of the case where an optical servo loop (tracking servo) using the optical pickup of the present invention is formed.

In FIG. 1, 1a is a focus lens, 1b is a damper that supports the focus lens 1a, and 1c is a drive coil in the tracking direction that drives the damper 1b. The damper 1b and the drive coil 1c constitute a tracking actuator 1d.
2 is a tracking sensor for detecting a tracking error on the disc, the output of which is an amplifier 3 and a high frequency compensating circuit.
4, amplified by the adder 5 and the amplifier 6 and fed back (negative feedback) to the tracking actuator 1d. 7 is a displacement sensor (tracking direction) for detecting the mechanical displacement of the damper 1b, the output of which is an amplifier 8, a high-pass filter 9, and an adder.
5, amplified by the amplifier 6 and similarly fed back (positive feedback) to the tracking actuator 1d. Reference numeral 30 is a differentiating circuit for differentiating the displacement output of the displacement sensor 7 to detect the displacement speed of the actuator 1d, and its output is fed back (negatively fed back) to the tracking actuator 1d via the adder 5 and the amplifier 6.

That is, the tracking sensor 2, the amplifier 3, the high frequency compensation circuit
4, adder 5, amplifier 6, tracking actuator 1d,
Configure a normal optical tracking servo loop. Further, the displacement sensor 7, the amplifier 8, the high-pass filter 9, the adder 5, the amplifier 6, and the tracking actuator 1d constitute a positive feedback loop, and the displacement sensor 7, the amplifier 8, the differentiating circuit 30, the adder 5,
The amplifier 6 and the tracking actuator 1d form a velocity negative feedback loop.

This positive feedback loop lowers the value of the resonance frequency ωn of the damper 1d constituting the pickup, equivalently softens the damper and increases the low-frequency gain, while the velocity negative feedback loop functions as the positive feedback loop. It acts to suppress the peak level at the resonance point (ωn) that increases by applying

2 is a block diagram showing the configuration of FIG.
3A and 3B are Bode diagrams showing the open loop characteristics of the block diagram.

In FIG. 2, Kt is a tracking sensor 2 and an amplifier 3.
, K _A is the gain of the amplifier 6, and K p is the total gain of the displacement sensor 7 and the amplifier 8.

T ₁ and ₂ are time constants corresponding to two break points of the high frequency compensating circuit 4, T ₀ is a time constant corresponding to break points of the high-pass filter 9, T ₃ is a time constant of the differentiating circuit 30, and G ₀ Indicates the sensitivity of the tracking actuator 1d, and ζ and ωn indicate the damping of the damper 1b and the natural frequency, respectively. Reference numeral 10 is a block showing a positive feedback element for the actuator, and 31 is a block showing a speed negative feedback element.

The part 11 corresponds to the optical pickup of the present invention.

In FIG. 3a, A is the open loop gain characteristic of the tracking servo loop when a positive feedback loop and a velocity negative feedback loop are formed for the actuator of the present invention, and B in FIG. 3b shows both feedbacks. It is an open loop gain characteristic of a normal tracking servo loop. The characteristic of B corresponds to the characteristic when the positive feedback element 10 and the speed negative feedback element 31 are removed in FIG.

As is apparent from FIG. 3, in the case of A as compared with B, the gain is increased by ΔG in the range of frequencies 1 / T _{0 to} ω'n, and the servo performance is improved. Further, the phase margin φ at the gain intersection ωc, which is an index of the stability of the servo system, is almost the same as that before the positive feedback and the velocity negative feedback are given, showing that the stability is maintained. Note in the figure, Omega'n and T _'0 is a constant determined by the gain and the time constant of the high pass filter 9 can be designed to become an optimal value depending on the application.

In the case of a CD player or the like, in order to sufficiently follow the eccentricity of the disc, the range in which the ΔG gain increases may be set to about 3 to 8 Hz. That is, in order to prevent the operating point from changing due to the posture difference, the damper should be designed to be rigid, or in other words, the circuit should be designed to increase ΔG even if the value of the DC gain G of the servo loop is reduced. Due to
Performance can be maintained.

Although the tracking servo has been described as an example in the embodiment of the present invention, the same configuration and effect can be obtained for the focus servo.

In FIG. 4, a positive feedback loop and a velocity negative feedback loop are not added to the tracking servo loop as in the present invention, and instead, a low frequency compensating circuit 20 for increasing the low frequency gain is inserted in the tracking servo loop. 5 is a block diagram showing a conventional example in such a case, and the Bode plot characteristics at that time are shown in FIGS.
B shows the same characteristic as described above when no countermeasure is taken (the same as B in FIG. 3), and C shows the characteristic when low frequency compensation is performed.

As is apparent from the figure, according to this conventional method, the gain in the low frequency band can be increased by ΔG, but the resonance point (ω
It is shown that the peak level in n) increases from P to P ', which increases the risk of aggravating the transient phenomenon that occurs when the tracking servo loop is turned on and off, and the phase delay of the reduction compensation circuit is added. Therefore, the gain intersection point ωc
The phase margin at the point decreases from φ to φ ′, indicating that the stability of the servo system decreases.

As described above, according to the present invention, even if the damper for holding the focus lens is rigidly designed, a pickup in which a partial positive feedback loop and a velocity negative feedback loop for the pickup actuator are added in the optical servo loop. With the configuration, it is possible to secure the servo gain within a predetermined band without reducing the phase margin of the servo loop, so it is possible to create a device that is not affected by attitude differences and has stable servo characteristics. Can be done.

[Brief description of drawings]

1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a block diagram thereof, FIG. 3 is a Bode diagram showing its characteristics, and FIG.
FIG. 5 and FIG. 5 are a block diagram and a Bode diagram showing the configuration of the conventional embodiment. 1a ... Focus lens, 1d ... Tracking actuator, 7 ... Displacement sensor, 6,8 ... Amplifier, 9 ... High-pass filter, 30 ... Differentiation circuit.

Claims (3)

[Claims] 1. A positive feedback loop means for positively feeding back an AC component of a displacement of an actuator for driving a focus lens to the actuator itself, and a negative feedback loop means for negatively feeding back a velocity component of the actuator to the actuator itself. An optical pickup characterized by being provided. 2. The optical pickup according to claim 1, wherein both a positive feedback loop and a negative feedback loop are provided for controlling the focus direction. 3. The optical pickup according to claim 1, wherein both a positive feedback loop and a negative feedback loop are provided for controlling the tracking direction.