JPS6242335B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a video tape recorder (hereinafter referred to as VTR).
This invention relates to a drive device for an electro-mechanical transducer that is effective when performing slow motion playback, etc. The purpose of this is to correct the gain of the driving amplifier by giving it a temperature characteristic, thereby making the output displacement amount relative to the input electrical signal stable and not changing depending on the ambient temperature.

In a VTR, a technique is known in which a rotary head for signal reproduction is moved in the axial direction of a rotary cylinder in order to realize a narrow track automatic tracking device or a variable speed reproducing device such as slow motion. That is, the rotary head is mounted on a drive element whose mechanical displacement can be varied over time,
The method of controlling the rotary head to move in a direction perpendicular to the direction of rotation by applying an appropriate electric signal to the drive element to perform automatic tracking or tracking during variable speed reproduction is well known.

Here, when tracking is performed using an electric signal applied to a drive element, there are roughly two methods.

The first is the complete feedback type, which constantly monitors the tracking status of the rotating head on the drive element and the track to be reproduced.
This is a system that feeds back error information to the drive signal of the drive element to avoid tracking errors, and since the error information includes changes in the characteristics of the drive element itself, it is not necessary to correct changes in the characteristics of the drive element. do not. However, since the drive element is required to have a fast response, it is difficult to implement this method especially when it is desired to obtain a large displacement.

The second method is an open control type or a combination of open control and average value feedback. The open control type does not have a feedback system and provides a predetermined displacement. For example, in still playback, the scanning locus of the rotary head during playback differs in angle by one track pitch from the recorded track because the tape is stopped. At this time, in order to scan the rotating head so that it perfectly matches the recording track and obtain a still image without noise, it is necessary to apply a mechanical displacement with an inclination of one track pitch during the scanning period of the rotating head. good. In order to provide this displacement, a preset slope waveform may be applied to the drive element as a drive signal voltage.

In this open control type, if the characteristics of the input signal versus output displacement of the electromechanical transducer change, it directly affects the tracking state and changes the reproduced image.
This will degrade the SN ratio. In other words, taking still playback as an example, if the sensitivity of the input signal versus output displacement of the drive element decreases due to a rise in temperature, the initially perfect tracking due to a displacement of one track pitch may change due to the rise in temperature. As a result, the displacement becomes smaller than one track pitch, resulting in poor tracking and noise on the playback screen.

Furthermore, a similar situation occurs in a combination of open control and average value feedback. This involves detecting the average value of the track deviation by some means, forming a DC voltage that provides a displacement such that the center of the track almost coincides with the center of the reproducing locus of the rotary head, and combining this DC voltage with the center of the reproducing trajectory of the rotating head. A preset gradient wave is added to drive the rotary head in addition to the electro-mechanical transducer. In this method, since average information on track deviations is fed back, the tracking condition near the track center becomes good, but it is not possible to correct even the displacement that causes the tilt displacement. Therefore, like the open control type, it is not possible to prevent tracking conditions from deteriorating due to sensitivity changes.

The present invention solves the problems of the conventional example as described above, and solves the sensitivity change due to temperature change of the electro-mechanical conversion element in a practical open control type or a combination type of open control and average value feedback. The purpose of this invention is to provide a means for effectively correcting. Hereinafter, the present invention will be explained based on illustrated embodiments. FIG. 1 is a diagram showing an example of an electro-mechanical conversion element used as a head driving element. This is a so-called piezoelectric bimorph element having a structure in which two piezoelectric bending type vibrators each having electrodes attached to both sides of rectangular piezoelectric elements 1 and 2 are bonded together. Since the thickness direction is the direction of polarization, when a voltage is supplied between the terminals 3 and 4 taken out from the electrodes attached to each piezoelectric element 1 and 2, the piezoelectric element changes depending on the magnitude of the applied voltage. will bend in the polarization direction. For example, if the left end of the piezoelectric bimorph element is fixedly supported and the right end is set as a free end (movable end), a mechanical displacement corresponding to the applied voltage can be obtained at the right end.

In general, the temperature characteristics of electromechanical transducers using piezoelectric elements are determined approximately by the material constants of the piezoelectric element.
As shown in FIG. 2, the change in displacement with respect to temperature exhibits a positive slope. In other words, if a constant voltage is applied to give a certain amount of displacement and the temperature around the element is changed, the amount of displacement will increase almost linearly with the rise in temperature, and the amount of increase will be Approximately 15% for a displacement of 20°C in the range of 0°C to 55°C
Shows change in %. If such an electromechanical conversion element is used to perform slow motion or still playback of a VTR, for example, in an open control mode, even if tracking is good at room temperature, tracking at high and low temperatures becomes poor, resulting in poor reproduction of the reproduced image. This will cause noise.

Figure 3 shows that when the above-mentioned piezoelectric bimorph element is used to drive the rotary head and variable speed playback is performed using open control, the temperature characteristics of the piezoelectric bimorph element are corrected to maintain good tracking even when the ambient temperature changes. FIG. In the figure, for example, a drive voltage for still playback (usually a triangular wave having a slope of 1 track pitch per 1/60 second) is applied to the input terminal 5.
This is amplified by a variable gain circuit consisting of an operational amplifier 6, resistors 7 and 8, and a thermistor 9, and is applied between both terminals of the piezoelectric bimorph element 10 (terminals 3 and 3 in FIG.
(equivalent to 4 hours). Since the piezoelectric bimorph element 10 bends in the vertical direction, the rotary head 11 attached to its movable end can be driven up and down to scan along a track on the magnetic tape. The gain G of the above variable gain circuit is determined by the resistance 7
When the resistance value of R ₇ is R 7 , the resistance value of resistor 8 is R ₈ , and the resistance value of thermistor 9 is R _th , it is given by the following equation.

G=1+R ₈ +R _th /R ₇ Here, R _th is a temperature-sensitive resistance, and if one with a negative temperature coefficient is selected, the gain G decreases with temperature, and the piezoelectric bimorph element The applied voltage changes as the temperature increases. This is a change in the opposite direction to the temperature characteristics of the piezoelectric bimorph element itself shown in Figure 2, and as a result, the amount of mechanical displacement according to the applied voltage is obtained at a constant value regardless of temperature changes, and the ambient temperature It is possible to realize a tracking state that is stable against changes.

Note that for the temperature coefficient of the thermistor 9, the resistance 7
It is necessary to take care to set the values of and 8 appropriately.

FIG. 4 is a circuit diagram of another embodiment of the present invention, which is a combination of open control and average value feedback described above, which is subjected to temperature correction. In the figure, an input terminal 17 is supplied with a slope wave suitable for slow and still playback modes. This means that transistors 20, 21 and 1
The signal is amplified by a differential amplifier circuit composed of 9, and further reaches an adder 25 via an amplifier 24. The adder 25 includes a control voltage generator that converts into an average DC voltage based on the output of a track deviation detector 27 that detects the track deviation between the track and the recorded track when the rotary head 11 scans. The output of device 28 is also added. That is, a voltage corresponding to the average track deviation and a preset gradient voltage according to the mode of use are added together and applied to the piezoelectric bimorph element 10 via the drive amplifier 26 to drive the rotary head 11. Here, a feedback group is constituted by the track deviation detector 27 and the control voltage generator 28, but it responds to the average deviation of the track deviation, and does not respond to changes in the sensitivity of the piezoelectric bimorph element 10. Cannot be corrected. Therefore, if the piezoelectric bimorph element 10 exhibits the temperature characteristics shown in FIG. 2, the reproduced image will deteriorate. Correction is performed by changing the gain of Load resistors 22 and 23 are connected to the collectors of the transistors 20 and 21, respectively, and the collector of a transistor 19 forming a current source is connected to both emitters of the differential amplification transistor, and the emitter is grounded via a resistor 18. Ru. It is well known that the gain of this differential amplifier circuit can be changed approximately linearly with respect to the collector current of the current source transistor. The collector current of transistor 19 is proportional to the base voltage if the voltage applied to its base is sufficiently large compared to the base-emitter voltage of the transistor. Therefore, if the base voltage of the transistor 19 is controlled to change with respect to temperature, the gain of the differential amplifier circuit will change with respect to temperature, and the temperature characteristics with respect to the sensitivity of the piezoelectric bimorph element 10 will be changed. Can be corrected.

The base of the transistor 19 is supplied with the voltage of a constant voltage source formed by a resistor 12 and a diode 13, that is, the forward voltage of the diode 13, which is DC amplified by an amplifier circuit consisting of an operational amplifier 14 and resistors 15 and 16. As is well known, the forward voltage of the silicon diode 13 is about 0.7V, and changes with temperature at a slope of 2mV/°C as shown in FIG. In FIG. 5, the vertical axis shows a scale of absolute voltage as well as a relative value when the voltage at 20° C. is taken as 100.

The temperature characteristics of the piezoelectric bimorph element shown in Fig. 2 with respect to the amount of displacement and the temperature characteristics of the forward voltage of the silicon diode shown in Fig. 5 have opposite slopes, and the relative value of the amount of change is Matches extremely well. That is, if the forward voltage of the silicon diode 13 is used to correct the temperature characteristics with respect to the displacement amount of the piezoelectric bimorph element 10,
It is possible to obtain a displacement that is almost unaffected by the ambient temperature.

Next, in the embodiment shown in FIG. 4, the correction operation will be explained by taking as an example the case where the temperature rises. As the temperature rises, the voltage across the diode 13 decreases, and the voltage applied to the base of the current source transistor 19 via the DC amplifier also decreases. Therefore, the current of the current source of the differential transistors 20 and 21 becomes small, and the gain of the differential amplifier circuit decreases. Therefore, the gradient wave applied to the input terminal 17 is applied to the adder 25 in a state where its gain is smaller than at low temperature, and drives the piezoelectric bimorph element 10. Since the mechanical displacement of piezoelectric bimorph elements tends to increase at high temperatures, temperature changes are canceled out and temperature correction is realized.

In addition, in this example, a case was explained in which a diode was used for temperature correction, but since the base-emitter voltage of a silicon transistor has similar characteristics, correction means using this can also be implemented in the same way. . Furthermore, it goes without saying that the electro-mechanical transducer is not limited to the piezoelectric bimorph element, but can be similarly corrected as long as it has temperature characteristics.

As detailed above, the present invention has an effect on the temperature characteristics of the electro-mechanical conversion element by driving the electro-mechanical conversion element through a circuit whose gain has an inverse characteristic with respect to temperature. It is possible to obtain a stable displacement that does not occur, especially when used for tracking by applying mechanical displacement to a rotating head that scans differently from the recording track, such as in slow motion playback or still playback of a VTR. It has extremely useful effects.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an example of an electro-mechanical conversion element that can be used in the present invention, FIG. 2 is a temperature characteristic diagram thereof, FIG. A configuration diagram showing another embodiment of the invention, and FIG. 5 is a temperature characteristic diagram of a silicon diode used in the same embodiment. 1, 2... Piezoelectric element, 6... Operational amplifier, 7,
8... Resistor, 9... Thermistor, 10... Piezoelectric bimorph element, 11... Rotating head, 13... Silicon diode, 19... Transistor for constant current source, 20, 21... Transistor for differential amplification , 25...Adder, 26...Drive amplifier, 27
. . . Track deviation detector, 28 . . . Control voltage generator.

Claims (1)

[Claims] 1. A circuit whose gain has an inverse characteristic with respect to the temperature characteristic of an electro-mechanical conversion element that mechanically displaces depending on the magnitude of the electric signal when an electric signal is supplied. An apparatus for driving an electro-mechanical transducer, characterized in that the electro-mechanical transducer is driven by an electric signal passed through the circuit. 2. Driving an electro-mechanical conversion element according to claim 1, wherein the circuit is an amplifier having a variable gain circuit, and the variable gain circuit includes a temperature sensitive element. Device. 3. In the description of claim 1, a piezoelectric bimorph element having a structure in which two piezoelectric ceramic elements are bonded together is used as the electro-mechanical conversion element, and the circuit is configured to control the temperature of the piezoelectric bimorph element. An electrical device comprising a circuit that corrects the displacement characteristics by either the forward voltage of a silicon diode or the base-emitter voltage of a silicon transistor, both of which have substantially opposite temperature vs. voltage characteristics. - drive device for the mechanical transducer;