JPS6019677B2 - Method for suppressing self-excited vibration of electro-mechanical conversion elements - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a magnetic recording device in which the mechanical height position of a rotating magnetic head is changed using an electro-mechanical transducer such as a dimorph, which is composed of a piezoelectric element. Playback device (VT
This invention relates to a method for driving the electro-mechanical transducer in R), etc., and provides a method for eliminating eye-excited vibrations that occur when a bimorph undergoes a sudden change, without using a damper member or the like. It is.

In a rotary head type VTR, a method of displacing the mechanical height position of a rotary magnetic head in the direction of the rotation axis using a bimorph composed of a piezoelectric element is already known. still, slow,
This method can be applied to a method in which a playback head faithfully follows a recording track during special playback such as double-speed playback.

As shown in FIG. 1, the bimorph 1 is composed of a piezoelectric element, in which two Shoelectric elements 2 and 3 having polarization in the direction shown by the arrow P are pasted together so as to have a common electrode 4, and then The electrodes 5 and 6 are formed by a method such as vapor deposition.

In order to displace 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 following the electrodes 5 and 6 on both sides. good. For example, when a voltage is applied to terminal 7 and terminal 8, piezoelectric element 2 expands in its longitudinal direction, pressure fin element 3 contracts, and as a result, dimorph 1 undergoes a curving displacement. It is well known that it 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 moving device using a bimorph having such a configuration is shown in FIG. show.

In FIG. 2, reference numeral 9 denotes a bimorph composed of piezoelectric elements 10 and 11. A magnetic head 12 is fixed to one end by adhesive or the like, and a mounting member 14 is attached to the other end by an adhesive 13.
fixed on top. The mounting member 14 is fixed onto the rotating disk by screwing or the like. Therefore, the magnetic head movable device shown in FIG. 2 rotates together with the rotating disk. If no voltage is applied to the piezoelectric element, the magnetic head 12 only performs circular motion within a plane perpendicular to the rotation axis, similar to the magnetic head used in conventional rotary head type VTRs. When a voltage is externally applied to the lead wire 15 connected to each electrode via a brush or the like, the magnetic head 12 is displaced in the direction shown by the arrow 16 (rotation axis direction) along with the circular movement. FIG. 3 shows the frequency characteristics of a bimorph used in a magnetic head moving device.

When a sine wave signal of a constant voltage is applied to the bimorph, the amount of deflection changes depending on the frequency of the applied signal as shown in FIG. On the other hand, the phase characteristics change rapidly at the positive resonance part, as shown in b in the figure, and the phase delay at this time is 180 degrees. Therefore, near the positive resonance part, the bimorph can be treated as a second-order system. The resonant frequency is approximately determined by the shape of the bimorph, and the shape is determined by the diameter, required amount of deflection, strength, etc. of the rotating cylinder. For example, length 12
side, width 8 fences, thickness 0.4 sails, the resonant frequency f is approximately I
It is KHz. Further, the Q at f 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 causes self-excited oscillation 17 at an abrupt change point as shown in FIG. 4b.

The frequency of the eye-excited vibration coincides with the positive resonance frequency L', the amplitude has 6t at the abrupt change part, and then gradually attenuates. Therefore, the amount of deflection of the bimorph eventually settles down to a displacement amount of 6s corresponding to the applied voltage Vs. The resonant angular frequency is n
When , the transfer function G(s) of the quadratic system can generally be expressed as shown in Equation 11.

Yamana ---(1, G(S) = S2 + nS + ■Kama') The transient response of the system expressed by equation 11 changes as shown in Figure 5, and the damping coefficient is 4. The amount increases;
= 0, the overshoot amount becomes 1, indicating the maximum value.

The relationship between the attenuation coefficient and Q is expressed by the formula (2), which becomes zero when Q is infinite. Q=2 riverbank...[2] Let's apply these control theories to the bimorph mentioned above.

Since the bimorph's Q is 10, it becomes a mini 0.05, and the amount of overshoot is almost 1. This is 6t in Figure 4b.
Indicates that it is D26s. Next, a specific usage example of the magnetic head moving device will be explained. Helical scan VTRs are available in 1-head, 2-head, and 3-head types depending on the number of magnetic heads built into the rotating cylinder.
The angles at which the tape is wrapped around the rotary sill are approximately 360 degrees, 180 degrees, 120 degrees, etc., respectively.

In a two-head VTR with two heads A and B built into a rotating cylinder, each head alternately records one field's worth of video signals as one track on a magnetic tape, and the recording magnetization trajectory is as shown in Figure 6. Become. In Figure 6, T^
, T8 are magnetization trajectories recorded by the A head and B head, respectively, and a guard band TG in which no signal is recorded is provided between T^ and TB. When the running direction of the tape is the direction of arrow 18 and the scanning direction of the rotating magnetic head is from point a to point b, the still locus is 19. When reproduction is performed with no voltage applied to the bimorph, the head passes through the guard band, producing a noise band on the reproduction screen. In order to prevent noise bands from occurring, a predetermined voltage may be applied to the bimorph so that the scanning locus of the head becomes a straight line connecting points a and c. FIG. 7 shows the voltage waveform applied to the bimorph. In FIG. 7, a is a head switching signal, and the period when the A head is in contact with the tape is between ara-t4, . . . , and the period when the B head is in contact with the tape is t3. , L-t5
, ... is the period. The triangular wave voltage shown in b is the voltage applied to the A head driving bimorph, and the triangular wave voltage shown in c is the voltage applied to the B head driving bimorph. During still playback, if the point at which the head starts contacting the tape starts from point a, the center of the recording trajectory, as shown in FIG.
The voltage at this time is zero, and at time t2, a voltage corresponding to the amount of displacement between b and 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 is zero, and at time t3, the voltage is b-
It is sufficient to apply a voltage corresponding to the amount of displacement between c. Repeat the same operation below. Therefore, the period during which each head is in contact with the tape corresponds to the rising period of each triangular wave b, c, and the falling period is the period in which the mechanical height of the head is initialized before the head starts contacting the tape next time. This is the period for returning to the position of Therefore, the falling period of each of the triangular wave voltages b and c may basically be returned in any manner. For example, A
Taking the head as an example, even if the waveform in Figure 7b is a mirror-tooth wave as shown in Figure 7d, if the bimorph makes the same displacement as the applied voltage, the head is in contact with the tape. During this period, reproduction scanning is performed while faithfully on-tracking the recording track. However, in this case, actually t2, t4
The bimorph itself causes the above-mentioned self-excited oscillation in the sudden change part. If the Q is moderate, the self-excited vibration will not be sufficiently damped within one field period (1/6), and therefore the self-excited vibration that occurred at t2 will not converge between t3 and t3. -t4 will also be affected. The amount of eye excitation vibration at this time corresponds to the amount of mistrack during reproduction. In order to reduce such self-excited vibration, two-head type VT
In R, it is common to gradually return the mechanical height position of the head to its initial value while the head is not in contact with the tape. Next, let's consider still playback on a one-head VTR in which a tape is wrapped approximately 360 degrees around a rotating cylinder.

If the diameter of the rotating cylinder, tape speed, recording track width, etc. are appropriately selected, the recorded magnetization locus in a one-head VTR will have a pattern similar to the magnetization locus shown in FIG. 6 above. However, in this case, each recorded magnetization trajectory T^, TB
are all recorded with the same head. When playing stills on a one-head VTR, the head is directly on the tape, and there are very few periods when the head is not on the tape.
It is about the size of a cape. However, the day referred to here is one horizontal scanning period. Therefore, within this short period, it is necessary to return the mechanical height position of the head to the initial value when the head starts contacting the tape, so that the voltage waveform applied to the bimorph becomes as shown in FIG. 7e. However, if the voltage waveform shown in e is applied to the bimorph as it is, a satisfactory reproduced screen cannot be obtained due to the influence of the above-mentioned self-excited vibration. The need for rapid bimorph displacement is not limited to still playback in a one-head VTR, but also occurs in a two-head VTR, for example, when playing back at a tape speed faster than the tape speed during recording. The need to perform playback at a faster tape speed than during recording is effective when looking at a playback image during fast forwarding to find a location where predetermined content is recorded. In Fig. 8, the recording trajectory T^,
TB is a magnetization trajectory recorded at a normal tape speed, and is a head scanning trajectory during fast forwarding. If the tape speed during fast forwarding is set to one half of the tape speed during recording, the head will cross 10 recording tracks between 1/6th the size of the sand. If no voltage is applied to the bimorph at this time, nine 0 noise bands will occur on the playback screen, resulting in a very unsightly screen. Noise bands can be avoided by applying a voltage to the bimorph and playing back by faithfully on-tracking the first recording track where the head starts contacting the tape, using the same concept as when playing stills. In this case, the required displacement of the bimorph corresponds to a 9-track pitch interval and is larger than the maximum displacement of the bimorph.
In the aforementioned bimorph having a length of 12 legs, the displacement amount is limited to 3 to 4 track pitch intervals. Therefore, when reproducing in a fast-forward state, it is effective to use a method of taking the scanning locus indicated by the broken line 21 shown in FIG. 8, rather than the method of on-tracking and reproducing on the same 0 recording locus. The trajectory 21 is reproduced by on-tracking 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 the head passes through the guard band. It is something. The direction in which the mechanical height position of the head is changed is perpendicular to the still trajectory, and since the guard band is approximately parallel to the still trajectory, it is possible to displace the head in the direction 0 perpendicular to the guard band. Therefore, if the head scanning locus during fast forwarding is changed as shown by 21, the amount of noise bands generated 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 mirror-tooth wave shown in FIG. 9, and at this time, self-excited vibration occurs due to rapid bimorph displacement. Note that the number of sawtooth waves shown in Figure 9 is 1 per field period in the case of 1M sound speed reproduction.
Kawago is necessary. In addition to the above two examples, depending on the application, it may be necessary to rapidly displace the bimorph, and the actual displacement change of the bimorph at this time will be a form in which the self-excited vibration of the bimorph is superimposed on the drive waveform. .

In order to remove the self-excited vibrations of the bimorph, some kind of damping means is required, and so far 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 has reliability problems due to changes in the rubber etc. over time. be. The electrical damping method is a method in which a signal detected by some method from the eye-excited vibrations of the bimorph is fed back to the bimorph drive stage, but this method has the disadvantage that the means for detecting the eye-excited vibrations and the feedback circuit are complicated. . The present invention provides a method for preventing self-excited vibration with a simple circuit configuration without using a mechanical damper or a means for detecting visually excited vibration. By partially converting the waveform into a predetermined waveform, which will be described later, rapid displacement of the bimorph without visually excited vibration is realized.

．． The details of the present invention will be explained below.

FIG. 10 shows the waveform of the voltage applied to the bimorph and the actual displacement change of the bimorph.

In the figure a, a step waveform 22 shows the voltage waveform applied to the bimorph, and also shows the displacement change when the bimorph faithfully responds to the applied voltage. That is, waveform 22
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 a displacement of 6 seconds in steps. However, in the actual displacement change of the bimorph, as shown at 23, self-excited vibration occurs at a rapid displacement change portion, and the self-excited vibration gradually attenuates to a final displacement change of 6 seconds. 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 resonance 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 1, the amount of overshoot is about 1, so the amplitude 6t of the self-excited vibration at this time is equal to 263, as described above. Based on the above premise, let us consider the response of the bimorph when the haze zero pressure waveform shown in FIG. 10b is applied. The voltage value of the negative pulse waveform 24 shown in Figure b corresponds to the amount of bimorph displacement, and the pulse width corresponds to T/2. The pulse waveform 24 can be generated by combining a negative step voltage 25 and a positive step voltage 26, which correspond to the curve shown in FIG.

In other words, the response characteristic of the bimorph to the pulse voltage 24 is equal to the sum of the response characteristics of the bimorph to the step voltages 25 and 26. The response characteristics of the bimorph to step voltages 25 and 26 are 27.2
As shown in 8, the eye-excited vibration has a period T and an absolute value of the displacement amount corresponding to 63. Therefore, if we combine the signs of each eye excitation vibration shown at 27 and 28, we can obtain 2 in e of the same figure.
As shown by 9, a waveform with an amplitude of 263 and a period T is obtained, and this waveform 29 becomes the transient response of the bimorph when the pulse voltage 24 is applied. 0 Next, consider the transient response of the bimorph due to the drive waveform that is a combination of the pulse fin pressure 24 and the step voltage 22.

The composite waveform of the step waveform 22 and the pulse waveform 24 becomes a staircase wave signal 30 shown in diagram f from the above-mentioned amplitude and phase relationship, and when this signal 30 and the bimorph are applied, the bimorph's transient response is and 29. The self-excited vibrations 23 and 29 have the same amplitude and period,
Since the signals are sine wave signals with a phase difference of 180 degrees, the AC component of the combined signal is zero. Since the DC component of the composite signal is a combination of the average level 6s of the signal 23 and the average level $ of the signal 29, it takes a value of 6s. Therefore, when the step wave voltage 30 shown in the diagram f is applied, the transient response of the bimorph becomes a waveform 31 shown in the diagram g, and the rise point fall between T/a becomes - constant at a displacement of 6 s. This means that instead of applying a step voltage of 22 to the bimorph, a staircase voltage of 30
This means that self-excited vibration of the bimorph will not occur if the voltage is applied to . Next, another embodiment of the present invention will be described using FIG. 11.

Figure 11 shows a step voltage with an amplitude corresponding to a displacement of 6 s, and a step voltage with a width of T/2 and an amplitude corresponding to 0.5 seconds, and the falling part of the pulse voltage is There is a delay from the rising edge of the step voltage by a time corresponding to the period T of self-excited vibration.

The transient response of the bimorph when the voltage waveform 32 is applied is the same as the above-mentioned idea, and the eye excitation vibration 34 when the step voltage 33 shown in FIG. 11b is applied, and the pulse voltage 35 shown in FIG. 11c is applied. The resultant signal is a waveform 37 shown in figure d. Therefore, if the voltage 32 shown in Figure a is applied to the bimorph, the transient response of the bimorph will produce about one cycle of self-excited vibration, and then settle down to a predetermined displacement amount of 6 seconds. If the purpose is only to prevent self-excited vibration of the bimorph due to transient response, it is sufficient to apply the staircase wave voltage 30 shown in Fig. 10f, but there are problems in creating a waveform processing circuit, and if some of the visually excited vibrations are When using the bimorph for this purpose, a voltage waveform 32 shown in FIG. 11a may be applied to the bimorph. In this case, it is not necessary to limit the method of delaying the falling part of the pulse waveform by the time indicated by T from the rising part of the step waveform as shown in FIG. Even if it is delayed, the self-excited vibration after application of the pulse waveform can be suppressed to zero. Furthermore, although the explanation has been given in FIGS. 10 and 11 on the assumption that the eye-excited vibration is not attenuated, the same concept can be applied even when the self-excited vibration is attenuated. This is because, for example, if the step voltage 23 and the self-excited vibration 29 caused by the pulse voltage in FIG. 10 have the same amplitude at the initial one cycle and the phase is shifted by 180 degrees, the self-excited vibration will be suppressed at that point. Therefore, no eye-excited vibration occurs thereafter. That is, the degree of attenuation does not matter. Also, Figure 10b
It is assumed that the amplitude of the pulse voltage 24 shown in FIG. Furthermore, if the bimorph is causing self-excited vibration for some reason before applying the step voltage change, the pulse voltage 24 is applied to cancel the composite vibration of the self-excited vibration and the external vibration caused by applying the step voltage. It is necessary to choose the voltage value of Furthermore, since this example has been explained using an example of canceling self-excited vibration due to a positive step voltage, the self-excited vibration canceling pulse voltage 24
Although a negative pulse was used in the above, in order to cancel out the eye excitation vibration caused by a negative step voltage, a positive pulse voltage may be applied.

In other words, in order to prevent self-excited vibrations due to sudden changes, it is necessary to make the self-excited vibrations due to the changes have the same amplitude and a phase of 18
It is sufficient to apply a voltage waveform that causes eye excitation vibrations that differ by about 0 degrees. Figure 12 shows an example in which the concept of the present invention is applied to still playback of a one-head VTR.
The zero-excitation vibration of the bimorph can be prevented by changing the falling edge of the sawtooth wave shown in figure a, which was conventionally applied to the bimorph during still playback, in two steps as shown in figure b. This should be clear from the explanation so far.

Note that the period Tr of the mirror tooth wave at this time is one field period (
(approximately 16.7 msec), and the period of the staircase wave portion is m/2.
is 0.5m when the resonant frequency of the bimorph is IKHZ
seC, it is sufficiently short compared to 1 fitard period, and within the video signal missing period (approximately 1
0H), it is possible to suppress self-excited vibration. As is clear from the above explanation, according to the present invention, there is no need to use a mechanical damper section, and there is no need to detect zero self-excited vibration and apply electrical feedback to the bimorph, and the bimorph drive waveform is adjusted to a predetermined value. The bimorph's eye-excited vibration can be suppressed simply by superimposing pulse waveforms.

Brief Description of the Drawings Figure 1 is a perspective view showing the structure of a bimorph made up of piezoelectric elements, Figure 2 A is a plan view of a magnetic head moving device using the bimorph, and the mouth is a side view of the same. A diagram showing the frequency characteristics of the bimorph, FIG. 4 is a diagram to explain the transient phenomenon of the bimorph, FIG. 5 is a diagram showing the transient characteristics in a secondary system, and FIG. 6 is a diagram showing the recording magnetization locus of a VTR. Figure 7 is a voltage waveform diagram applied to the bimorph during still playback, Figure 8 is a diagram showing the head scanning locus during 1-piosonic reproduction, Figure 9 is a voltage waveform diagram applied to the bimorph during 1-piosonic reproduction, and Figure 9 is a diagram showing the voltage waveform applied to the bimorph during still reproduction. FIG. 10 is a diagram for explaining the waveform manipulation principle for suppressing self-excited vibration of bimorph according to the present invention, FIG. 11 is a waveform diagram for explaining another embodiment of the present invention, and FIG. Invention 1 - 1st figure type VTR
FIG. 3 is a voltage waveform diagram applied to the bimorph when applied during still playback.

2, 3...Piezoelectric element, 4...Common electrode, 5
, 6... side electrode.

Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 9 Figure 7 Figure 8 Figure 10 Figure 11 Figure 12

Claims (1)

[Claims] 1. In an electro-mechanical conversion element approximated by a quadratic system with an attenuation coefficient of 1 or less, the amount of mechanical conversion obtained when a first drive voltage is applied to the electro-mechanical conversion element is When obtained as a composite amount of the mechanical conversion amount corresponding to the first drive voltage and the first self-excited vibration component of the electro-mechanical conversion element, the period and amplitude of the first self-excited vibration component are A second vibration generator that generates a second self-excited vibration that is identical and has a phase difference of 180 degrees.
A method for suppressing self-excited vibration of an electro-mechanical transducer, characterized in that a drive voltage of: is superimposed on the first drive voltage. 2 When a first drive voltage that changes in a shorter time than the period T of the resonance frequency of the electro-mechanical conversion element is applied to the electro-mechanical conversion element, nT (n is an integer), the width is T/2,
A rectangular wave pulse whose amplitude is smaller than the amount of change in the first drive voltage and whose amplitude is in the opposite direction to the direction of change of the first drive voltage is superimposed on the first drive voltage. A method for suppressing self-excited vibration of an electro-mechanical conversion element according to claim 1.