JPH0424766B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention uses an electro-mechanical conversion element commonly called a bimorph composed of a piezoelectric element to change the mechanical height position of a rotating magnetic head. The present invention relates to a self-excited vibration suppression method for eliminating self-excited vibration of the electro-mechanical transducer in a magnetic recording/reproducing device (VTR) or the like.

Conventional configuration and its problems In a rotating magnetic head type VTR, a method of displacing the mechanical height position of the rotating magnetic head in the direction of the rotation axis using a bimorph composed of a piezoelectric element is already known. This method is applied to a method of following the curve of a recording track, and a method of having a playback head faithfully follow a recording track during special playback such as still image, slow play, and double speed playback.

As shown in FIG. 1, a bimorph 1 composed of piezoelectric elements has two piezoelectric elements 2 and 3 polarized in the direction of arrow P, bonded together with a common electrode 4 in between, and electrodes 5 and 6 on both sides. is formed. When displacing the bimorph 1 having such a configuration, a voltage is applied between the terminal 7 drawn out from the common electrode 4 and the terminal 8 drawn out from the wire electrically connecting the electrodes 5 and 6 on both sides. good. For example, when a voltage is applied so that terminal 7 is positive and terminal 8 is negative, piezoelectric element 2 extends in the longitudinal direction and piezoelectric element 3 contracts, and as a result, due to the displacement difference between piezoelectric elements 2 and 3, bimorph 1 A bend occurs in the A direction.
As is well known, the direction of this bending depends on the polarity of the voltage applied between the terminals 7 and 8 and the polarization direction of the piezoelectric elements 2 and 3.

A magnetic head movable device using a bimorph having such a configuration is shown in FIG. 10 in Figure 2
is a bimorph composed of piezoelectric elements 11 and 12, and a magnetic head 13 is fixed to one end, and the other end is fixed to a mounting member 15 by adhesive. The mounting member 15 is fixed onto a rotating disk (not shown) by screws or the like. The movable device shown in FIG. 2 thus rotates together with the rotary disk. If no voltage is applied to the piezoelectric elements 11 and 12, the magnetic head 13 will function as a conventional rotary head type VTR.
Similar to the magnetic head used in
When a voltage is applied to the magnetic head 6 from the outside through a brush (not shown) or the like, the magnetic head 13 moves in a circular motion and is displaced in the direction of arrow B (rotational axis direction).

FIG. 3 shows the frequency characteristics of the bimorph used in the magnetic head moving device. When a constant voltage sinusoidal signal is applied to the bimorph, the amount of deflection changes depending on the frequency of the applied signal as shown in A in the same figure, with positive resonance characteristics at frequency f ₁ and negative resonance characteristics at frequency f ₂ . show. On the other hand, as shown in Figure B, the phase characteristics change rapidly at the positive resonance part, and the phase lag at this time is
It is 180 degrees. Therefore, near the positive resonance part, the bimorph can be treated as a second-order system. The resonant frequency is determined by the bimorph shape, and this shape is determined by the diameter of the rotating disk, the required amount of deflection, the strength, etc. For example, if the length is 12mm, width is 8mm, and thickness is 0.4mm,
The resonant frequency f ₁ is 1KHz. Further, the Q at f ₁ at this time is usually about 10, which is a relatively large value.

When a step voltage as shown in FIG. 4a is applied to a bimorph having such characteristics, the amount of deflection of the bimorph suddenly changes as shown in FIG. 4b, causing self-excited vibration A. The self-excited vibration frequency coincides with the positive resonant frequency _f1 , and the amplitude has δt at the abrupt change part, and then gradually attenuates. Therefore, the amount of deflection of the bimorph finally settles at a displacement position δs corresponding to the applied voltage Vs.

When the resonance frequency is ωn, the secondary system transfer function G(s) can generally be expressed as in the following equation.

G(s)=ωn/S ² +2ζωnS+ωn ² ... The transient response of the system expressed by this equation changes as shown in Figure 5. The smaller the subtraction coefficient ζ, the greater the overshoot, and when ζ=0, the overshoot is gone. becomes 1, indicating the maximum value. The relationship between the damping coefficient ζ and Q is shown by the following formula, and when Q is infinite, ζ becomes zero.

Let's apply these control theories to the bimorph mentioned above. Since Q of bimorph is 10, ζ=
It becomes 0.05, and the amount of excess is almost 1. This shows that δt=2δs in FIG. 4b.

Next, a specific example of the magnetic head moving device will be explained. In a two-head VTR, which is a helical scan type VTR and has two magnetic heads A and B built into a rotating disk, the video signal for one field is recorded as one track on the magnetic tape alternately by each head, as shown in Figure 6. Obtain a recorded magnetization trajectory as shown in .
In Figure 6, T _A and T _B are respectively A head and
This is the magnetization trajectory recorded by the B head, and the magnetization trajectory
A guard band _TG in which no signal is recorded is provided between _TA and _TB . The running direction of the tape is the direction of arrow c, and the scanning direction of the rotating magnetic head is a.
When the direction is from point to point b, the still image trajectory is 20. When a still image is played back with no voltage applied to the bimorph, the head passes through the guard band _TG , resulting in a noise band on the playback screen. In order to prevent noise bands from occurring, a predetermined voltage may be applied to the bimorph so that the locus of the head becomes a straight line connecting points a and c. FIG. 7 shows the voltage waveform applied to the bimorph. Figure 7a shows the head switching signal,
The period during which the A head is in contact with the tape is t ₁ - _{t 2} ,
t ₃ −t ₄ , . . . and the period during which the B head is in contact with the tape is t ₂ − _{t 3} , t ₄ −t ₅ , . The triangular wave voltage shown in b in the same figure is the voltage applied to the bimorph for driving the A head, and the triangular wave voltage shown in c in the same figure is the voltage applied to the bimorph for driving the A head.
This is the voltage applied to the bimorph for driving the head.
When the head starts contacting the tape during still image playback, if it starts from the center point a of the recording trajectory as shown in Figure 6, the voltage at time _t1 of the triangular wave applied to the bimorph for driving head A should be set to zero. , at time _t2 , a voltage corresponding to the amount of displacement between point b and point c shown in FIG. 6 may be applied. Similarly, the voltage of the triangular wave applied to the B head driving bimorph at time _t2 may be set to zero, and at time _t3 , a voltage corresponding to the amount of displacement between point b and point c may be applied.
The same operation is repeated below. Therefore, the period during which each head is in contact with the tape corresponds to the rising period of each triangular wave, and the falling period corresponds to the period in which the mechanical height of the head returns to its initial position by the time the head next starts to contact the tape. This is the period for returning. Therefore, the falling period of each triangular wave voltage can basically be returned in any manner. For example, if we take the A head as an example, even if the waveform shown in Figure 7b is a sawtooth wave as shown in Figure 7d, if the bimorph makes the same displacement as the applied waveform, the head will come into contact with the tape. During this period, reproduction scanning is performed by faithfully on-tracking the recording track. However, in this case, the bimorph itself actually causes the above-mentioned self-excited vibration at rapid change portions such as t ₂ and t ₄ . When Q is about 10, self-excited vibrations are not sufficiently damped within one field period (1/60 seconds), so the automatic vibrations that occur at t ₂ do not converge between t ₂ - _{t 3} ,
This will also affect the time between t ₃ and _{t 4} . The amount of self-excited vibration at this time corresponds to the amount of mistrack during reproduction. In order to avoid such self-excited vibration,
In a head-type VTR, the mechanical height of the head is normally returned to its initial value gradually while the head is not in contact with the tape.

Now, consider the case where the tape is played back at a faster tape speed than during recording. The need to perform playback at a faster speed than when recording is effective in fast forwarding, especially when looking at a playback image to find a location where predetermined content is recorded. In FIG. 8, T _A and T _B are magnetization trajectories recorded at normal tape speed, and 21 is a head scanning trajectory during fast forwarding. When the tape speed during fast forwarding is 10 times that during recording, the head is 1/
You will cross 10 recording tracks in 60 seconds.
If no voltage is applied to the bimorph at this time, nine noise bands will appear on the reproduced screen, resulting in an unsightly screen. Noise bands can be avoided if a voltage is applied to the bimorph using the same concept as when playing back still images mentioned above, and the head is faithfully on-track and played back on the first recording track where the head starts contacting the tape. In this case, the required bimorph displacement is 9
This corresponds to the track pitch interval and is larger than the maximum displacement of the bimorph. The aforementioned 12 mm bimorph has a displacement limit of 3 to 4 track pitches. Therefore, when reproducing in a fast-forward state, it is effective to use the scanning trajectory 22 shown by the broken line in FIG. 8, rather than the method of on-tracking and reproducing on the same recording trajectory. The trajectory 22 is reproduced by on-tracking on a predetermined recording track for a certain period of time, and when passing through a guard band, the head is displaced in a direction perpendicular to the guard band to shorten the time it takes for the head to pass through the guard band. be. Since the direction in which the mechanical height position of the head is changed is perpendicular to the still trajectory, and the guard band is approximately parallel to the still trajectory, it is possible to displace the head in a direction perpendicular to the guard band. Therefore, if the scanning locus of the head during fast-forward playback is changed as shown by the trajectory 22, the amount of noise bands occurring on the playback screen will be extremely reduced, and a substantially satisfactory playback image can be obtained. At this time, the voltage waveform applied to the bimorph must be a sawtooth wave as shown in FIG. 9, and therefore self-excited vibrations occur due to rapid bimorph displacement. Note that the number of sawtooth waves shown in FIG. 9 is 10 required for one field period in the case of 10x speed reproduction.

In addition to the above examples, there are applications where it is necessary to rapidly displace the bimorph, and the actual change in displacement of the bimorph at this time is formed by superimposing the bimorph's self-excited vibration on the drive waveform. becomes. In order to remove the self-excited vibration of the bimorph,
Some kind of damping means is required, and conventionally, mechanical damping methods and electrical damping methods have been considered. The mechanical damping method is a method in which an elastic body such as rubber is brought into contact with a part of the bimorph, but this method has the disadvantage of reducing the amount of displacement of the bimorph, and also reduces the amount of elastic body such as rubber. There were also problems with reliability due to changes over time. On the other hand, the electrical damping method is a method in which a signal detected by some method to detect the self-excited vibration of the bimorph is fed back to the bimorph drive stage. The drawbacks are that the process becomes complicated and the equipment becomes expensive.

Purpose of the Invention The present invention solves the above-mentioned conventional drawbacks, and is a method for suppressing self-excited vibration of an electro-mechanical conversion element, which can be implemented with a simple circuit configuration and yet can effectively prevent self-excited vibration. The purpose is to obtain.

Structure of the Invention In order to achieve the above object, the method for suppressing self-excited vibration of an electro-mechanical transducer of the present invention provides a method for suppressing self-excited vibration of an electro-mechanical transducer of the present invention. The configuration is such that a desired displacement position is obtained by applying a trapezoidal wave driving voltage with a rise time equal to an integral multiple of the period T of the first self-excited vibration component of the element. According to this configuration, the electro-mechanical conversion element The self-excited vibration that occurs when a sudden change is made,
It can be removed without using a damper member or the like.

DESCRIPTION OF EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 10 shows the voltage waveform applied to the bimorph and the actual displacement change of the bimorph. In Figure a, a step waveform 23 shows the applied voltage waveform of the bimorph, and also shows the displacement change when the bimorph faithfully responds to the applied voltage.
That is, the step waveform 23 represents the amount of bimorph displacement corresponding to the applied voltage. Therefore, when a constant voltage value is applied in steps, if the bimorph responds faithfully, it will show the displacement amount of δ _S1 in steps. However, in actual bimorph displacement changes, self-excited oscillations occur at abrupt displacement change portions as shown in the bimorph response waveform 24, and these self-excited oscillations gradually attenuate, resulting in a displacement change that finally settles to δ _S1 . Note that the various self-excited vibrations shown in FIG. 10 are self-excited vibrations that are not attenuated for convenience of explanation. The period T of the self-excited vibration is a period corresponding to the positive resonant frequency of the bimorph, and the time from the rising edge of the applied voltage to the first peak value of the self-excited vibration is T/2, which is a secondary system. This can be said from the theory of transient response. Furthermore, if the Q of the bimorph is about 10, the amount of overshoot is 1, so it has already been stated that the amplitude δt ₁ of the self-excited vibration at this time is equal to 2δ _S1 . Based on the above, let's consider the response of the bimorph when the voltage waveform shown in FIG. 10 is applied. The rise time of the trapezoidal wave pulse 25 shown in FIG.
It can be created by combining the trapezoidal wave pulses 26 and 27 shown in c and d of the figure. In other words, the response characteristic to the trapezoidal pulse 25 is equal to the composite of the response characteristics of the bimorph to the trapezoidal pulses 26 and 27. 28,2
9 is a bimorph response waveform to the trapezoidal wave pulses 26 and 27, which is represented by a combination of the vibration component vector of the self-excited vibration of the bimorph and the vector of the trapezoidal wave, and is shown in FIG. 11th
In Figure a, 30 is a composite waveform of the self-excited vibration of the bimorph shown in Figure b and the trapezoidal wave shown in Figure c, which is the sum of the self-excited vibration vector component 31 and the vertical vector component 32 of the trapezoidal wave. It is expressed as The vector component 31 of self-excited vibration changes in direction and magnitude (up and down, magnitude) corresponding to the amplitude of the sine wave, whereas the vector component 32 of the trapezoidal wave changes every moment.
is always in the vertical direction at the time of rise, which corresponds to the vector component 33 of the step waveform shown in FIG. 10a. In the flat part of the trapezoidal wave, the vector component is zero. Therefore, Fig. 10 c, d
The bimorph response waveforms 28 and 29 to the trapezoidal pulses 26 and 27 shown in FIG.
Vector component 33 of the step waveform shown in Figure a
If equal to , the bimorph response waveforms 28, 29
Also, the amplitude of the self-excited vibration component is equal to δt ₁ , and the period T is also equal.

Next, let us consider the transient response of the bimorph due to the drive waveform that is a combination of the trapezoidal pulses 26 and 27. The composite waveform of the trapezoidal pulses 26 and 27 becomes a trapezoidal pulse 34 as shown in FIG. This is a composite of response waveforms 28 and 29. Since the alternating current components of the bimorph response waveforms 28 and 29 are signals having the same amplitude and period and differing in phase by 180°, the alternating current components of the combined signal cancel each other out and become zero. The DC component of the composite signal is the first
This is the value of the difference in amplitude between the amplitude of the trapezoidal wave pulse 26 shown in FIG. 0c and the amplitude of the trapezoidal wave pulse 27 shown in FIG. Therefore, the trapezoidal pulse 34 shown in FIG. 10e
The transient response waveform 35 when is applied to the bimorph becomes a waveform as shown in the figure, and after the rising portion of the period T, the displacement becomes constant at the amplitude corresponding to the trapezoidal wave pulse 34, which is the drive voltage. This means that the self-excited vibration period T of the bimorph, like the trapezoidal pulse 34,
This means that if a drive voltage with a rise time equal to is applied, the bimorph will not produce self-oscillation.

Figure 12 shows the strength of the self-excited vibration component of the bimorph when the rise time of the trapezoidal wave drive signal is changed, and the rise time t is the period T of the self-excited vibration component or its integral multiple nT (n (a positive integer), the self-excited vibration becomes weaker, and as the rise time t becomes longer, the self-excited vibration becomes weaker.

Figure 13 shows a two-head VTR corresponding to Figure 7d.
This is a bimorph drive voltage waveform during a still image.During the head contact period, a drive voltage with a rise sufficiently larger than T is applied to the bimorph, so that almost no self-excited vibration occurs. This is also clear from the explanation of FIG. On the other hand, it is necessary to return the bimorph to the initial value of the contact period in the head non-contact section, but when this needs to be done as quickly as possible, By returning to the initial value with equal falling time, it is possible to suppress the self-excited vibration of the bimorph.

Effects of the Invention As explained above, according to the present invention, by applying a trapezoidal wave having a predetermined rise time to the bimorph drive waveform, the trapezoidal waveform drive applied to the element in the middle of a field such as during fast-forward playback. The rise time of the voltage is an integral multiple of the period of the self-excited vibration component of the element.The larger the integer, the better the self-excited vibration of the bimorph can be suppressed. An extremely simple and inexpensive circuit can be used without using any members or the need to detect self-excited vibrations and apply electrical feedback to the bimorph.

[Brief explanation of drawings]

Fig. 1 is an external perspective view of a bimorph composed of piezoelectric elements, Fig. 2A is a plan view of a magnetic head movable device using a bimorph, Fig. 2B is a front view of the device, and Fig. 3
Figures A and B are explanatory diagrams of the frequency characteristics of the bimorph, Figure 4 is an explanatory diagram of the transient response characteristics of the bimorph, and Figure 5 is an explanatory diagram of the bimorph's transient response characteristics.
The figure is an explanatory diagram of the transient characteristics of the secondary system, and Figure 6 is a VTR
An explanatory diagram of the recorded magnetization locus of
An explanatory diagram of the scanning locus of the head during double speed playback, FIG. 9 is a voltage waveform diagram applied to the bimorph during 10 times speed playback in the conventional method, and FIGS. 10 and 11 are self-excitation of the bimorph in an embodiment of the present invention. Fig. 12 is an explanatory diagram of the waveform manipulation principle for suppressing vibration. Fig. 12 is an explanatory diagram of the relationship between the rise time of a trapezoidal wave and the self-excited vibration of the bimorph. Fig. 13 is an illustration of the relationship between the rise time of the trapezoidal wave and the self-excited vibration of the bimorph. It is a voltage waveform diagram applied to a bimorph. 23... Step waveform, 24, 28, 29...
...bimorph response waveform, 25, 26, 27, 34
... Trapezoidal pulse, 30 ... Composite waveform, 31 ...
Self-excited vibration vector component, 32...Vector component of trapezoidal wave, 33...Vector component of step waveform, 35...Transient response waveform of bimorph.

Claims (1)

[Claims] 1 Applying a trapezoidal wave drive voltage with a rise time equal to an integral multiple of the period of the first self-excited vibration component of this electro-mechanical conversion element to an electro-mechanical conversion element approximated by a quadratic system with a damping coefficient of 1 or less. A method for suppressing self-excited vibration of an electro-mechanical conversion element to obtain a desired displacement position by applying an electric current.