JPS61133046A - Tracking servo circuit - Google Patents

Abstract

PURPOSE:To shorten and stabilize especially the pull-in time in a tracking servo circuit by raising gains at the high frequency side further than usual and dropping gains at the low frequency side immediately after a servo lock is turned on and raising gains at the low frequency side as the time expires so as to improve the accuracy of the servo. CONSTITUTION:The positive side of an operational amplifier 2 is grounded, and between its negative side and an input terminal 3 a parallel circuit composed of a resistance 4 and a capacitor 5 is inserted. On a feedback path between the negative side and an output terminal 6 an active filter 10 available from connecting a fixed resistance 7, a capacitor 8 and an FET 9 as a variable resistance each other in parallel is inserted. The gate of the FET 9 is grounded and simultaneously is jointed to a minus electric power source terminal 14. When a positive voltage is given across the gate and source of the FET 9 and they are turned on with such circuit constitution, the resistance value as a variable resistance is small, whereas the folding point frequency F3 of the active filter 10 is large. Since a loop gain characteristic at this time is high in a high frequency band, the pull-in is smoothly executed, and the control accuracy of the servo is slowly improved as the time expires.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a tracking servo circuit for an optical head. For more details, for optical disc devices,
The light beam spot travels on the disk surface at a speed of 2 to 30 m/sec.
It runs at a linear speed of
It is required to perform automatic tracking in a non-contact manner while not deviating from the track with pit row spacing of about 1.6 μI. The present invention is particularly aimed at shortening and stabilizing the pull-in time in such a tracking servo circuit.

``Prior art'' Tracking servo circuits have conventionally used operational amplifiers as actuators, but at the moment of tracking lock, the actuator drive voltage changes both on the positive side and on the negative side, as shown in the characteristic (b) in Figure 3. There is also no offset on the side, so the head (1
) is preferably located in the center without deviation.

However, conventionally, when the tracking lock is applied, the actuator drive voltage has a DC offset as shown in FIG. 3(a) or (c), so the head (1) is ) becomes deviated.

"Problems to be Solved by the Invention" As mentioned above, when the actuator drive voltage has a DC offset and the head is deviated, the tracking servo is likely to come off and the retraction cannot be performed smoothly. There was a problem.

More specifically, in general, the tracking error signal when tracking is off is as shown in FIG. 5, where the dotted line represents the DC component. Given these characteristics, if the DC gain is increased to improve the control accuracy of the servo lock, even a small DC component will be greatly amplified as shown in Figure 5, and the drive signal will have the following effects as shown in Figure 6. This results in a large DC offset. therefore,
Even if an attempt is made to engage the servo lock in this state, there is a problem in that a large DC offset occurs in the actuator, as shown in FIGS. 3 and 4, and the retraction cannot be performed smoothly.

"Means for Solving the Problems" The present invention has been made to solve the problems of the conventional art as described above, and it involves connecting a capacitor and a variable resistor in parallel to the feedback path of an operational amplifier that constitutes an actuator. This is a problem caused by inserting an active filter. For example, an FET is used as the variable resistor.

"Operation" The resistance value of the variable resistor inserted in the active filter is made small immediately after the servo lock is turned on. Then,
As shown in FIG. 2, it is controlled to raise the high frequency side to make it easier to pull in the servo. Increase the resistance value of the variable resistor over time.

Then, as shown in FIG. 2, the DC gain on the low frequency side is increased, and the frequency characteristics are also changed over time to increase the accuracy of servo lock.

"Example" An embodiment of the present invention will be described below with reference to the drawings.

In FIG. 1, (2) is an operational amplifier as an actuator. The positive side of this operational amplifier (2) is grounded, and a parallel circuit of a resistor (4) and a capacitor (5) is inserted between the negative side and the input terminal (3).

Further, in the feedback path between the negative side and the output terminal (6), there is an active filter (10) in which a fixed resistor (7), a capacitor (8), and a variable resistor FET (9) are connected in parallel with each other. has been inserted. This FET (
The gate of 9) is grounded via a capacitor (11) and coupled to a negative power terminal (14) via a resistor (12) and a switch (13).

In the above circuit configuration, when a positive voltage is applied between the gate and source of FET (9) and it is turned on, the resistance value of the variable resistor is small, and the corner point of the active filter (10) at this time is The frequency (fa) is large.

Since the loop gain characteristic at this time is high in the high range as shown by the solid line characteristic (A) in FIG. 2, the pull-in is performed smoothly. As time passes, if a reverse bias is applied between the gate and source of FET (9), the corner frequency (
fb), and the DC gain characteristic becomes the dotted line characteristic (B) in Figure 2. As time passes, the corner frequency (fc) further decreases, and the DC gain characteristic becomes the dashed line characteristic (C) in Figure 2.
), thus the servo control accuracy gradually increases. Note that fl is when the resistor (4) and capacitor (5) are R1 and C1, respectively.

It is.

[Effects of the Invention] Since the present invention is configured as described above, the moment the servo is turned on, even if the servo accuracy is poor, the servo is first locked while the DC gain is small. Therefore, the offset of the actuator is small and the retraction is accurate. Once locked, as time passes,
Servo control accuracy improves by increasing the DC gain.

Therefore, disc tracking, which requires extremely high accuracy, can be performed smoothly and quickly.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram showing an embodiment of the tracking servo circuit according to the present invention, FIG. 2 is a frequency characteristic diagram, FIG. 3 is an explanatory diagram of actuator drive voltage with and without offset, and FIG. FIG. 5 is a tracking error signal waveform diagram when tracking is off, and FIG. 6 is a waveform diagram of the actuator drive voltage after amplification. (1) Head, (2) Operational amplifier as actuator, (3) Input terminal, (4) Resistor, (5) Capacitor, (6) Output terminal,
(7)...Resistor, (8)...Capacitor, (9)...
...FET, (10)...active filter.

Claims (2)

[Claims] (1) An active filter whose corner frequency can be varied by varying the time constant of a capacitor and resistor is inserted in the feedback path of the operational amplifier that constitutes the actuator, and by varying the frequency of this active filter, servo lock is achieved. A tracking servo characterized by increasing the gain on the high-frequency side and lowering the gain on the low-frequency side from the steady state immediately after turning on, and increasing the gain on the low-frequency side as time passes to improve the accuracy of the servo. circuit. (2) In claim 1, the active filter consists of a circuit in which a fixed resistor, a capacitor, and an FET are connected in parallel, and the reverse bias voltage of the FET is varied to change the resistance value of the FET, and resonance occurs. Tracking servo circuit with variable frequency.