GB2179212A - Drive circuit for piezoelectric bimorph - Google Patents

Abstract

A single source of high voltage 21, 22, 23 produces a predetermined output potential difference between output points 18, 19, and a control circuit responds to a low voltage control signal 38 to shift the mean level of said output voltage with respect to a datum level whilst maintaining the said potential difference between the output points constant. The datum level and output points 18, 19 are connected to a piezoelectric bimorph 3 carrying a video head (1), (Figure 1), of a video tape recorder. The circuit ensures that the voltage applied to either element of the bimorph is always in the direction of poling of that element. <IMAGE>

Description

SPECIFICATION Drive circuit This invention relates to a drive circuit and is particularly suitable for providing a floating output voltage whose mean level can be made to vary rapidly with respect to a datum level. Such a drive circuit can be used to drive a bimorph which requires the application of relatively high voltage levels. The use of bimorph drive circuits and the like requiring the application of variable level high voltages is considered to be undesirable.

The present invention seeks to provide an improved drive circuit which is suitable for driving a bimorph.

According to a first aspect of this invention a drive circuit includes a single source of high voltage which is arranged to produce a constant predetermined output voltage, and a control circuit arranged to receive a low voltage control signal and to shift the mean level of said output voltage with respect to a datum level in response thereto.

According to a second aspect of this invention, a drive circuit includes means adapted to produce a predetermined output potential difference at a pair of output points in response to the application of a single source of high voltage; a pair of amplifiers coupled to respective ones of the output points and which amplifiers are controlled in concert by a control circuit arranged to receive a low voltage control signal, such that the respective amplifiers are operative# to shift the potentials at the output points with respect to a datum level whilst maintaining the said potential difference at a constant value.

Preferably the two amplifiers receive respectively positive and negative reference potentials with respect to the datum level, and the amplifiers and the control circuit are operative such that the potential at one of said output points is always equal to or more positive than said positive reference potential and such that the potential at the other of said output points is always equal to or more negative that said negative reference level.

Conveniently, said single source of high voltage is floating, and is derived from an invertor circuit having a low voltage input.

It is well known in the art that piezoelectric devices are composed of a high dielectric constant, polycrystalline material. To produce the piezoelectric properties, the dielectric material must be subjected to a process known as polarisation in which a strong electric field is placed across the dielectric, which distorts the physical shape and electrical properties of the crystallites and creates a permanent net polar moment. The material thus becomes anisotropic, and when an electric field is applied to the material in the direction of polarisation, it contracts in the transverse direction.

If one edge of a piezoelectric element is bonded to a substrate which restrains movement, the element will bend when an electric field is applied.

If two such elements are bonded together, such that one edge of each element is free to move and the opposite edge is restrained from moving, the composite unit is known as a bimorph. When an electric field is applied to the bimorph, it bends.

The two elements may either be both poled in the same direction, or they may be poled in opposite directions.

If a high voltage is applied to a piezoelectric element in the opposite direction to that in which it has been poled, the element may become depolarised, and hence lose its piezoelectric properties.

An example of a situation in which high deflection voltages are applied to a bimorph is in the use of bimorphs in helical scan video tape recorder/ reproducers fitted for the purpose of automatic track following. In this application, the bending property of the bimorph is used to vertically position a magnetic tape reproduce head. The potential difference required in such an application is of the order of 200--300 volts.

In such, and similar applications, it is therefore necessary to ensure that neither of the bimorph elements is ever subjected to a voltage applied in the opposite direction to that in which it was poled.

The invention is now further described by way of example, with reference to the following drawings, in which: Figure 1 illustrates a video reproduce head which is mounted on a bimorph whose elements are both poled in the same direction, Figure 2 is a diagram of the preferred embodiment of the invention, and Figure 3 illustrates how the output voltages vary with the input signal.

Referring to Figure 1, a video reproduce head 1 with head winding connections 2 is mounted on a bimorph 3 comprising two piezoelectric elements 4 having a common poling direction as indicated by the arrows 5 and bonded to a common electrically conductive substrate 6 which is connected to a datum level. An electrically conductive coating, 7, acts as an upper electrode, and a similar coating 8 applied to the lower face of the lower element 4 acts as a lower electrode.

The bimorph 3 is cantilevered by means of a rigid support 9 and fixings 10.

The bimorph is coupled to the preferred embodiment of the invention as shown in Figure 2.

Referring to Figure 2, the bimorph 3 is driven by the illustrated circuit, which is in accordance with the invention.

A predetermined output potential difference at output points 18 and 19 is produced by a single source 20 of high voltage, comprising: a bridge rectifier 21; and a reservoir capacitor 22. A dc to dc converter 23 comprising a primary winding 24, and a secondary winding 25 provides an ac input to the bridge rectifier 23. This source provides a constant output floating voltage when an ac signal (not shown) derived from a constant dc source is applied to the primary winding 24. The effect of this on the bimorph, is to subject each element to a voltage in its direction of poling, which is indicated by the arrows 5. The magnitudes of these voltages are initially the same, and therefore there is no net deflection of the bimorph.

Amplifiers 26 and 27 are coupled to output points 18 and 19 respectively. Amplifier 26 comprises an NPN transistor 28; resistors 29,30; and diode 31 which is necessary to ensure a constant output voltage from the transistor 28. Preferably the circuit dimensions are chosen to draw negligible transistor current in the quiescent state.

Similarly, amplifier 27 comprises a PNP transistor 32; resistors 33 and 34, and a diode 35.

The two amplifiers 26 and 27 receive respectively positive and negative reference potentials 36,37, with respect to the datum level to which the bimorph is earthed.

The amplifiers 26, 27 receive a variable low voltage control signal from input 38.

The effect of applying the variable low voltage control signal to the amplifiers 26,27, is to shift the potential at the output points 18, 19, in a manner which is illustrated in Figure 3. This results in the voltage applied to one element of the bimorph being greater than that applied to the other, and therefore there is a new deflection of the bimorph.

The potential difference between two outputs 18, 19 remains constant. Thus by altering the low voltage control signals which is applied at input 38, the bimorph 3 may be made to deflect in the required manner.

A current limiting resistor 39 is included in the circuit. In normal operation, the transistor emitters are clamped to reference potentials 36,37. However, in the event of the control signal demanding fast slew rates, one of the diodes 31, 35 will come out of conduction enabling the resistor 39 to limit the bimorph capacitance discharge currentthrough the appropriate transistor 28,32.

The dc supplies which form the input to the dc to dc converter 23 may also be used to provide the reference potentials 36,37. Proper circuit operation will then be largely independent of supply voltage levels.

The working point of transistor 28 may be further stabilised by means of an emitter degeneration resistor which would be included in the circuit between the emitter of 28 and the junction of diode 31 and resistor 39.

Similarly, the working point of transistor 32 may be further stabilised by including an emitter degeneration resistor between the emitter of 32 and the junction of diode 35 and resistor 39.

Referring now to Figure 3, the variation of output potentials with control signal is shown. As the control signal at input 38 varies from reference potential 36 to reference potential 37 as shown by line 40, the output at point 18 increases from a value equivalent to that of reference potential 36 as shown by line 41 and the output at output 19 increases to a value equivalent to reference potential 37, as shown by line 42. The potential difference, represented by 43, remains constant, and is determined by the high voltage power supply 20.

Although the input control signal, and hence the output signals are shown in Figure 3 as varying linearly, this is merely for explanatory purposes.

Claims (8)

1. A drive circuit including a single source of high voltage which is arranged to produce a constant predetermined output voltage, and a control circuit arranged to receive a low voltage control signal and to shift the mean level of said output voltage with respect to a datum level in response thereto.

2. A drive circuit including means adapted to produce a predetermined output potential difference at a pair of output points in response to the application of a single source of high voltage; a pair of amplifiers coupled to respective ones of the output points and which amplifiers are operative to shift the potentials at the output points with respect to a datum level whilst maintaining the said potential difference at a constant value.

3. A drive circuit as claimed in claim 2 and wherein the two amplifiers receive respectively positive and negative reference potentials with respect to the datum level, and the amplifiers and the control circuit are operative, such that the potential at one of said output points is always equal to or more positive than said positive reference potential and such that the potential at the other of said output points is always equal to or more negative than said negative reference level.

4. A drive circuit as claimed in claim 2 or 3 and wherein said single source of high voltage is floating, and is derived from an invertor circuit having a low voltage input.

5. A drive circuit as claimed in claim 2,3 or 4 in which a bimorph comprising two piezoelectric elements is bonded to a common substrate such that one edge is free to move, and the opposite edge is restrained from moving, and having a common poling direction, is coupled to the pair of output points, and said common substrate is connected to datum level.

6. A drive circuit as claimed in claim 2 in which the pair of amplifiers receive a variable low voltage control signal.

7. A drive circuit as claimed in any of claims 3 to 6 in which the voltage applied to either element is always in the direction of poling of that element.

8. A drive circuit substantially as illustrated in and described with reference to the accompanying drawings.