JPH0124034B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a VTR that reads information from a medium (for example, a videotape) on which information is recorded electronically.
The present invention relates to an electromechanical transducer used in a device or the like to deflect a magnetic head or the like along a video track.

Conventionally, piezoelectric elements have been used as elements for this type of electromechanical conversion, but when this piezoelectric element is fired as an aggregate of microcrystals, the polarization of the single crystal faces in an arbitrary direction, so it has a high It is created by applying a voltage and performing polarization treatment. Therefore, a high voltage can be applied in this polarization direction, but if a high voltage is applied in the opposite polarization direction, the polarization structure of the piezoelectric element will be destroyed. Therefore, conventionally, as shown in Fig. 1, a diode D
was connected so that no voltage was applied in the opposite direction to the polarization direction. In the figure, A is a piezoelectric element and M is a metal plate.

However, when two oppositely polarized piezoelectric elements are used as a bimorph, a hysteresis characteristic as shown in FIG. 2 is exhibited due to the polarization and structure. Therefore, when a certain voltage is applied, the current position changes depending on the past history, such as whether the voltage was raised in the past to maintain a constant voltage, or the voltage was lowered to a constant voltage. Furthermore, when the voltage applied to one piezoelectric element is increased, the amount of displacement with respect to the changing voltage decreases due to the resistance of the intermediate electrode and the other piezoelectric element.

An object of the present invention is to apply a voltage opposite to the polarization direction to one piezoelectric element while applying voltage to the other piezoelectric element in proportion to the applied voltage, and to avoid damaging the polarization structure. It is an object of the present invention to provide an electromechanical transducer that reduces the above-mentioned drawbacks by determining the ratio of these two voltages and does not require consideration of nonlinearity in practice.

According to the present invention, a bimorph has two piezoelectric elements attached to both sides with electrodes, and two diodes have anodes or cathodes connected to each other and the other ends connected to different electrodes of the bimorph, respectively.
An electromechanical transducer is obtained which includes a capacitor whose one end is connected to the connection of two diodes and the other end is connected to the intermediate electrode of the bimorph.

The principle of the present invention is that a capacitor is formed between the electrodes of the piezoelectric element.
Now, considering the circuit shown in Figure 4 and assuming that a voltage of +E volts is applied to the input, the diode D1
Since the conducting diode D2 is non-conducting, the voltage at point a is approximately equal to E. Therefore, according to the law of conservation of charge, V ₁ =-C ₂ /C+C ₁ +C ₂ E, V ₂ =C+C ₁ /C+C ₁ +C ₂ E
Therefore, the input voltage can be distributed at a fixed ratio. Note that the distribution ratio can be changed by selecting the capacitance value of the capacitor C.

Next, the present invention will be described in detail with reference to the drawings of an embodiment of the present invention. FIG. 5 shows an example of use in an automatic track following device for a video tape recorder (VTR). In the figure, a head chip 16 is attached to the electromechanical transducer of the present invention. Here, videotape converts previously recorded information into electrical signals. This signal is amplified by a video preamplifier 17 and then converted to an RF signal by an equalizer 18. The RF signal passes through a resistor, demodulator, LPF, etc., is converted into a video signal, and is then input to the time base collector TBC21.

With a VTR, the head is not always directly above the track recorded on the videotape, and the position of the track and head may be misaligned due to tape unevenness, stretching, or other factors.
As the RF level decreases and crosstalk with neighboring tracks increases, the reproduced image will have noticeable dropouts and noise. Furthermore, since the tape speed is different during still playback, slow playback, etc. from that during normal playback, the relative speed of the tape and drum is also different, and as a result, the head traces a trajectory that straddles a plurality of tracks as shown in FIG. Therefore, when looking at the image, noise bars appear where the tracks straddle each other.

In order to eliminate the above drawbacks, the head position is changed so that the detected RF signal is maximized, and the head position is automatically changed on the track to obtain an image of only one field during slow and still playback. The device that moves it is this device. Furthermore, the microprocessor 1 receives an auto-tracking enable signal a (a signal that tells whether or not to operate this device), a capstan tack signal b (a pulse signal that indicates the number of revolutions of the capstan reel), and a vertical synchronization signal. c is input from the outside, and the RF level signal d dropout signal e is input from the RF signal to the RF level detection circuit 19 and dropout detection circuit 20, respectively.
It is input through. The microprocessor 1 generates and outputs a head position correction drive signal f, a jump, a 2-track jump, and a skip signal g from the signals (a) to (e) above. The signal g mentioned later is
It is input to TBC21. On the other hand, the above-mentioned signal f is input to the low-pass filter 2. In filter 2, approximately the first resonant frequency of bimorphs 8, 9, and 10
By reducing the components above 70% of the frequency, it alleviates the resonance of the bimorph, reduces the feedback loop of the feedback loop, and prevents oscillation due to high-order resonance frequencies. The output signal of the low-pass filter 2 is the subtraction circuit 3
After the feedback signal is subtracted by the high-voltage drive amplifier 4, the voltage is increased to the voltage required for driving the bimorph, and then applied to the electromechanical converters 5 to 10. diode 5
is provided so that an excessive voltage opposite to the polarization direction is not applied to the piezoelectric element 8. diode 6
This is also for reverse voltage protection of the piezoelectric element 10.

The capacitor 8 is attached so that when one side of the bimorph piezoelectric element is driven, a voltage in a direction opposite to the polarization is applied to the other piezoelectric element at a constant rate. This improves the nonlinearity of the bimorph.

The strain gauge 11 senses the displacement of the bimorph. This signal is sense amplifier 1
After increasing the gain with 2, apply a high pass filter.
It is input to HPF13. This is provided to reduce the frequency below the first anti-resonance frequency and prevent the influence of phase shift. The gain of the HPF output is adjusted by the gain control circuit 14, the damping coefficient at the first resonance point is increased by the differentiating circuit 15, and the displacement of the bimorph is also controlled by the closed loop including elements 3 to 15 when the signal f is sorted. Sort waveform and avoid ringing at rise and fall.

With the device configured in this way, the RF is sampled for each field, the head position is corrected, and during slow still playback, the head is always on-track, and a picture of one screen per field can be obtained without noise bars.

In the present invention, the nonlinearity of the bimorph is reduced, and the difference in the amount of change in displacement when the applied voltage is around 0 V and when the applied voltage is around αv and when it is made to swing αv is reduced.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a conventional example, FIG. 2 is a diagram showing hysteresis characteristics of a bimorph, FIG. 3 is a diagram showing an embodiment of the present invention, and FIG. 4 is a diagram for explaining the operation of the present invention. , FIG. 5 is a block diagram showing an application example of the present invention, and FIG. 6 is a diagram showing tracks recorded on a tape and head locus during still playback.

Claims (1)

[Claims] 1. A bimorph made up of two piezoelectric elements with electrodes attached on both sides pasted together, two diodes connected in series with opposite polarity and both ends connected to the outer electrodes of the bimorph, and the two diodes. An electromechanical transducer comprising: a capacitor provided between a connection point of the bimorph and an intermediate electrode of the bimorph.